Publisher: Elsevier   (Total: 3148 journals)

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Showing 1 - 200 of 3148 Journals sorted alphabetically
Academic Pediatrics     Hybrid Journal   (Followers: 39, SJR: 1.655, CiteScore: 2)
Academic Radiology     Hybrid Journal   (Followers: 26, SJR: 1.015, CiteScore: 2)
Accident Analysis & Prevention     Partially Free   (Followers: 106, SJR: 1.462, CiteScore: 3)
Accounting Forum     Hybrid Journal   (Followers: 28, SJR: 0.932, CiteScore: 2)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 44, SJR: 1.771, CiteScore: 3)
Achievements in the Life Sciences     Open Access   (Followers: 7)
Acta Anaesthesiologica Taiwanica     Open Access   (Followers: 6)
Acta Astronautica     Hybrid Journal   (Followers: 446, SJR: 0.758, CiteScore: 2)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 2)
Acta Biomaterialia     Hybrid Journal   (Followers: 30, SJR: 1.967, CiteScore: 7)
Acta Colombiana de Cuidado Intensivo     Full-text available via subscription   (Followers: 3)
Acta de Investigación Psicológica     Open Access   (Followers: 2)
Acta Ecologica Sinica     Open Access   (Followers: 12, SJR: 0.18, CiteScore: 1)
Acta Histochemica     Hybrid Journal   (Followers: 5, SJR: 0.661, CiteScore: 2)
Acta Materialia     Hybrid Journal   (Followers: 324, SJR: 3.263, CiteScore: 6)
Acta Mathematica Scientia     Full-text available via subscription   (Followers: 5, SJR: 0.504, CiteScore: 1)
Acta Mechanica Solida Sinica     Full-text available via subscription   (Followers: 9, SJR: 0.542, CiteScore: 1)
Acta Oecologica     Hybrid Journal   (Followers: 12, SJR: 0.834, CiteScore: 2)
Acta Otorrinolaringologica (English Edition)     Full-text available via subscription  
Acta Otorrinolaringológica Española     Full-text available via subscription   (Followers: 2, SJR: 0.307, CiteScore: 0)
Acta Pharmaceutica Sinica B     Open Access   (Followers: 2, SJR: 1.793, CiteScore: 6)
Acta Psychologica     Hybrid Journal   (Followers: 26, SJR: 1.331, CiteScore: 2)
Acta Sociológica     Open Access   (Followers: 1)
Acta Tropica     Hybrid Journal   (Followers: 6, SJR: 1.052, CiteScore: 2)
Acta Urológica Portuguesa     Open Access   (Followers: 1)
Actas Dermo-Sifiliograficas     Full-text available via subscription   (Followers: 3, SJR: 0.374, CiteScore: 1)
Actas Dermo-Sifiliográficas (English Edition)     Full-text available via subscription   (Followers: 2)
Actas Urológicas Españolas     Full-text available via subscription   (Followers: 3, SJR: 0.344, CiteScore: 1)
Actas Urológicas Españolas (English Edition)     Full-text available via subscription   (Followers: 1)
Actualites Pharmaceutiques     Full-text available via subscription   (Followers: 7, SJR: 0.19, CiteScore: 0)
Actualites Pharmaceutiques Hospitalieres     Full-text available via subscription   (Followers: 3)
Acupuncture and Related Therapies     Hybrid Journal   (Followers: 8)
Acute Pain     Full-text available via subscription   (Followers: 15, SJR: 2.671, CiteScore: 5)
Ad Hoc Networks     Hybrid Journal   (Followers: 11, SJR: 0.53, CiteScore: 4)
Addictive Behaviors     Hybrid Journal   (Followers: 18, SJR: 1.29, CiteScore: 3)
Addictive Behaviors Reports     Open Access   (Followers: 9, SJR: 0.755, CiteScore: 2)
Additive Manufacturing     Hybrid Journal   (Followers: 13, SJR: 2.611, CiteScore: 8)
Additives for Polymers     Full-text available via subscription   (Followers: 22)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 190, SJR: 4.09, CiteScore: 13)
Advanced Engineering Informatics     Hybrid Journal   (Followers: 13, SJR: 1.167, CiteScore: 4)
Advanced Powder Technology     Hybrid Journal   (Followers: 17, SJR: 0.694, CiteScore: 3)
Advances in Accounting     Hybrid Journal   (Followers: 9, SJR: 0.277, CiteScore: 1)
Advances in Agronomy     Full-text available via subscription   (Followers: 17, SJR: 2.384, CiteScore: 5)
Advances in Anesthesia     Full-text available via subscription   (Followers: 30, SJR: 0.126, CiteScore: 0)
Advances in Antiviral Drug Design     Full-text available via subscription   (Followers: 2)
Advances in Applied Mathematics     Full-text available via subscription   (Followers: 12, SJR: 0.992, CiteScore: 1)
Advances in Applied Mechanics     Full-text available via subscription   (Followers: 12, SJR: 1.551, CiteScore: 4)
Advances in Applied Microbiology     Full-text available via subscription   (Followers: 24, SJR: 2.089, CiteScore: 5)
Advances In Atomic, Molecular, and Optical Physics     Full-text available via subscription   (Followers: 15, SJR: 0.572, CiteScore: 2)
Advances in Biological Regulation     Hybrid Journal   (Followers: 4, SJR: 2.61, CiteScore: 7)
Advances in Botanical Research     Full-text available via subscription   (Followers: 1, SJR: 0.686, CiteScore: 2)
Advances in Cancer Research     Full-text available via subscription   (Followers: 35, SJR: 3.043, CiteScore: 6)
Advances in Carbohydrate Chemistry and Biochemistry     Full-text available via subscription   (Followers: 9, SJR: 1.453, CiteScore: 2)
Advances in Catalysis     Full-text available via subscription   (Followers: 5, SJR: 1.992, CiteScore: 5)
Advances in Cell Aging and Gerontology     Full-text available via subscription   (Followers: 5)
Advances in Cellular and Molecular Biology of Membranes and Organelles     Full-text available via subscription   (Followers: 14)
Advances in Chemical Engineering     Full-text available via subscription   (Followers: 29, SJR: 0.156, CiteScore: 1)
Advances in Child Development and Behavior     Full-text available via subscription   (Followers: 11, SJR: 0.713, CiteScore: 1)
Advances in Chronic Kidney Disease     Full-text available via subscription   (Followers: 11, SJR: 1.316, CiteScore: 2)
Advances in Clinical Chemistry     Full-text available via subscription   (Followers: 26, SJR: 1.562, CiteScore: 3)
Advances in Colloid and Interface Science     Full-text available via subscription   (Followers: 21, SJR: 1.977, CiteScore: 8)
Advances in Computers     Full-text available via subscription   (Followers: 14, SJR: 0.205, CiteScore: 1)
Advances in Dermatology     Full-text available via subscription   (Followers: 16)
Advances in Developmental Biology     Full-text available via subscription   (Followers: 14)
Advances in Digestive Medicine     Open Access   (Followers: 13)
Advances in DNA Sequence-Specific Agents     Full-text available via subscription   (Followers: 7)
Advances in Drug Research     Full-text available via subscription   (Followers: 26)
Advances in Ecological Research     Full-text available via subscription   (Followers: 44, SJR: 2.524, CiteScore: 4)
Advances in Engineering Software     Hybrid Journal   (Followers: 30, SJR: 1.159, CiteScore: 4)
Advances in Experimental Biology     Full-text available via subscription   (Followers: 9)
Advances in Experimental Social Psychology     Full-text available via subscription   (Followers: 51, SJR: 5.39, CiteScore: 8)
Advances in Exploration Geophysics     Full-text available via subscription   (Followers: 2)
Advances in Fluorine Science     Full-text available via subscription   (Followers: 9)
Advances in Food and Nutrition Research     Full-text available via subscription   (Followers: 68, SJR: 0.591, CiteScore: 2)
Advances in Fuel Cells     Full-text available via subscription   (Followers: 17)
Advances in Genetics     Full-text available via subscription   (Followers: 21, SJR: 1.354, CiteScore: 4)
Advances in Genome Biology     Full-text available via subscription   (Followers: 11, SJR: 12.74, CiteScore: 13)
Advances in Geophysics     Full-text available via subscription   (Followers: 8, SJR: 1.193, CiteScore: 3)
Advances in Heat Transfer     Full-text available via subscription   (Followers: 26, SJR: 0.368, CiteScore: 1)
Advances in Heterocyclic Chemistry     Full-text available via subscription   (Followers: 11, SJR: 0.749, CiteScore: 3)
Advances in Human Factors/Ergonomics     Full-text available via subscription   (Followers: 26)
Advances in Imaging and Electron Physics     Full-text available via subscription   (Followers: 4, SJR: 0.193, CiteScore: 0)
Advances in Immunology     Full-text available via subscription   (Followers: 37, SJR: 4.433, CiteScore: 6)
Advances in Inorganic Chemistry     Full-text available via subscription   (Followers: 10, SJR: 1.163, CiteScore: 2)
Advances in Insect Physiology     Full-text available via subscription   (Followers: 2, SJR: 1.938, CiteScore: 3)
Advances in Integrative Medicine     Hybrid Journal   (Followers: 6, SJR: 0.176, CiteScore: 0)
Advances in Intl. Accounting     Full-text available via subscription   (Followers: 3)
Advances in Life Course Research     Hybrid Journal   (Followers: 9, SJR: 0.682, CiteScore: 2)
Advances in Lipobiology     Full-text available via subscription   (Followers: 1)
Advances in Magnetic and Optical Resonance     Full-text available via subscription   (Followers: 8)
Advances in Marine Biology     Full-text available via subscription   (Followers: 21, SJR: 0.88, CiteScore: 2)
Advances in Mathematics     Full-text available via subscription   (Followers: 17, SJR: 3.027, CiteScore: 2)
Advances in Medical Sciences     Hybrid Journal   (Followers: 9, SJR: 0.694, CiteScore: 2)
Advances in Medicinal Chemistry     Full-text available via subscription   (Followers: 6)
Advances in Microbial Physiology     Full-text available via subscription   (Followers: 5, SJR: 1.158, CiteScore: 3)
Advances in Molecular and Cell Biology     Full-text available via subscription   (Followers: 26)
Advances in Molecular and Cellular Endocrinology     Full-text available via subscription   (Followers: 8)
Advances in Molecular Toxicology     Full-text available via subscription   (Followers: 7, SJR: 0.182, CiteScore: 0)
Advances in Nanoporous Materials     Full-text available via subscription   (Followers: 5)
Advances in Oncobiology     Full-text available via subscription   (Followers: 2)
Advances in Organ Biology     Full-text available via subscription   (Followers: 2)
Advances in Organometallic Chemistry     Full-text available via subscription   (Followers: 18, SJR: 1.875, CiteScore: 4)
Advances in Parallel Computing     Full-text available via subscription   (Followers: 7, SJR: 0.174, CiteScore: 0)
Advances in Parasitology     Full-text available via subscription   (Followers: 6, SJR: 1.579, CiteScore: 4)
Advances in Pediatrics     Full-text available via subscription   (Followers: 27, SJR: 0.461, CiteScore: 1)
Advances in Pharmaceutical Sciences     Full-text available via subscription   (Followers: 19)
Advances in Pharmacology     Full-text available via subscription   (Followers: 17, SJR: 1.536, CiteScore: 3)
Advances in Physical Organic Chemistry     Full-text available via subscription   (Followers: 10, SJR: 0.574, CiteScore: 1)
Advances in Phytomedicine     Full-text available via subscription  
Advances in Planar Lipid Bilayers and Liposomes     Full-text available via subscription   (Followers: 3, SJR: 0.109, CiteScore: 1)
Advances in Plant Biochemistry and Molecular Biology     Full-text available via subscription   (Followers: 11)
Advances in Plant Pathology     Full-text available via subscription   (Followers: 6)
Advances in Porous Media     Full-text available via subscription   (Followers: 5)
Advances in Protein Chemistry     Full-text available via subscription   (Followers: 19)
Advances in Protein Chemistry and Structural Biology     Full-text available via subscription   (Followers: 20, SJR: 0.791, CiteScore: 2)
Advances in Psychology     Full-text available via subscription   (Followers: 69)
Advances in Quantum Chemistry     Full-text available via subscription   (Followers: 7, SJR: 0.371, CiteScore: 1)
Advances in Radiation Oncology     Open Access   (Followers: 3, SJR: 0.263, CiteScore: 1)
Advances in Small Animal Medicine and Surgery     Hybrid Journal   (Followers: 3, SJR: 0.101, CiteScore: 0)
Advances in Space Biology and Medicine     Full-text available via subscription   (Followers: 7)
Advances in Space Research     Full-text available via subscription   (Followers: 430, SJR: 0.569, CiteScore: 2)
Advances in Structural Biology     Full-text available via subscription   (Followers: 6)
Advances in Surgery     Full-text available via subscription   (Followers: 13, SJR: 0.555, CiteScore: 2)
Advances in the Study of Behavior     Full-text available via subscription   (Followers: 37, SJR: 2.208, CiteScore: 4)
Advances in Veterinary Medicine     Full-text available via subscription   (Followers: 20)
Advances in Veterinary Science and Comparative Medicine     Full-text available via subscription   (Followers: 15)
Advances in Virus Research     Full-text available via subscription   (Followers: 6, SJR: 2.262, CiteScore: 5)
Advances in Water Resources     Hybrid Journal   (Followers: 57, SJR: 1.551, CiteScore: 3)
Aeolian Research     Hybrid Journal   (Followers: 6, SJR: 1.117, CiteScore: 3)
Aerospace Science and Technology     Hybrid Journal   (Followers: 394, SJR: 0.796, CiteScore: 3)
AEU - Intl. J. of Electronics and Communications     Hybrid Journal   (Followers: 8, SJR: 0.42, CiteScore: 2)
African J. of Emergency Medicine     Open Access   (Followers: 6, SJR: 0.296, CiteScore: 0)
Ageing Research Reviews     Hybrid Journal   (Followers: 12, SJR: 3.671, CiteScore: 9)
Aggression and Violent Behavior     Hybrid Journal   (Followers: 489, SJR: 1.238, CiteScore: 3)
Agri Gene     Hybrid Journal   (Followers: 1, SJR: 0.13, CiteScore: 0)
Agricultural and Forest Meteorology     Hybrid Journal   (Followers: 18, SJR: 1.818, CiteScore: 5)
Agricultural Systems     Hybrid Journal   (Followers: 32, SJR: 1.156, CiteScore: 4)
Agricultural Water Management     Hybrid Journal   (Followers: 47, SJR: 1.272, CiteScore: 3)
Agriculture and Agricultural Science Procedia     Open Access   (Followers: 4)
Agriculture and Natural Resources     Open Access   (Followers: 3)
Agriculture, Ecosystems & Environment     Hybrid Journal   (Followers: 58, SJR: 1.747, CiteScore: 4)
Ain Shams Engineering J.     Open Access   (Followers: 5, SJR: 0.589, CiteScore: 3)
Air Medical J.     Hybrid Journal   (Followers: 8, SJR: 0.26, CiteScore: 0)
AKCE Intl. J. of Graphs and Combinatorics     Open Access   (SJR: 0.19, CiteScore: 0)
Alcohol     Hybrid Journal   (Followers: 12, SJR: 1.153, CiteScore: 3)
Alcoholism and Drug Addiction     Open Access   (Followers: 12)
Alergologia Polska : Polish J. of Allergology     Full-text available via subscription   (Followers: 1)
Alexandria Engineering J.     Open Access   (Followers: 2, SJR: 0.604, CiteScore: 3)
Alexandria J. of Medicine     Open Access   (Followers: 1, SJR: 0.191, CiteScore: 1)
Algal Research     Partially Free   (Followers: 11, SJR: 1.142, CiteScore: 4)
Alkaloids: Chemical and Biological Perspectives     Full-text available via subscription   (Followers: 2)
Allergologia et Immunopathologia     Full-text available via subscription   (Followers: 1, SJR: 0.504, CiteScore: 1)
Allergology Intl.     Open Access   (Followers: 5, SJR: 1.148, CiteScore: 2)
Alpha Omegan     Full-text available via subscription   (SJR: 3.521, CiteScore: 6)
ALTER - European J. of Disability Research / Revue Européenne de Recherche sur le Handicap     Full-text available via subscription   (Followers: 11, SJR: 0.201, CiteScore: 1)
Alzheimer's & Dementia     Hybrid Journal   (Followers: 55, SJR: 4.66, CiteScore: 10)
Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring     Open Access   (Followers: 6, SJR: 1.796, CiteScore: 4)
Alzheimer's & Dementia: Translational Research & Clinical Interventions     Open Access   (Followers: 6, SJR: 1.108, CiteScore: 3)
Ambulatory Pediatrics     Hybrid Journal   (Followers: 5)
American Heart J.     Hybrid Journal   (Followers: 58, SJR: 3.267, CiteScore: 4)
American J. of Cardiology     Hybrid Journal   (Followers: 67, SJR: 1.93, CiteScore: 3)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 48, SJR: 0.604, CiteScore: 1)
American J. of Geriatric Pharmacotherapy     Full-text available via subscription   (Followers: 13)
American J. of Geriatric Psychiatry     Hybrid Journal   (Followers: 15, SJR: 1.524, CiteScore: 3)
American J. of Human Genetics     Hybrid Journal   (Followers: 40, SJR: 7.45, CiteScore: 8)
American J. of Infection Control     Hybrid Journal   (Followers: 29, SJR: 1.062, CiteScore: 2)
American J. of Kidney Diseases     Hybrid Journal   (Followers: 37, SJR: 2.973, CiteScore: 4)
American J. of Medicine     Hybrid Journal   (Followers: 50)
American J. of Medicine Supplements     Full-text available via subscription   (Followers: 3, SJR: 1.967, CiteScore: 2)
American J. of Obstetrics and Gynecology     Hybrid Journal   (Followers: 266, SJR: 2.7, CiteScore: 4)
American J. of Ophthalmology     Hybrid Journal   (Followers: 67, SJR: 3.184, CiteScore: 4)
American J. of Ophthalmology Case Reports     Open Access   (Followers: 5, SJR: 0.265, CiteScore: 0)
American J. of Orthodontics and Dentofacial Orthopedics     Full-text available via subscription   (Followers: 6, SJR: 1.289, CiteScore: 1)
American J. of Otolaryngology     Hybrid Journal   (Followers: 25, SJR: 0.59, CiteScore: 1)
American J. of Pathology     Hybrid Journal   (Followers: 32, SJR: 2.139, CiteScore: 4)
American J. of Preventive Medicine     Hybrid Journal   (Followers: 30, SJR: 2.164, CiteScore: 4)
American J. of Surgery     Hybrid Journal   (Followers: 39, SJR: 1.141, CiteScore: 2)
American J. of the Medical Sciences     Hybrid Journal   (Followers: 12, SJR: 0.767, CiteScore: 1)
Ampersand : An Intl. J. of General and Applied Linguistics     Open Access   (Followers: 7)
Anaerobe     Hybrid Journal   (Followers: 4, SJR: 1.144, CiteScore: 3)
Anaesthesia & Intensive Care Medicine     Full-text available via subscription   (Followers: 67, SJR: 0.138, CiteScore: 0)
Anaesthesia Critical Care & Pain Medicine     Full-text available via subscription   (Followers: 25, SJR: 0.411, CiteScore: 1)
Anales de Cirugia Vascular     Full-text available via subscription   (Followers: 1)
Anales de Pediatría     Full-text available via subscription   (Followers: 3, SJR: 0.277, CiteScore: 0)
Anales de Pediatría (English Edition)     Full-text available via subscription  
Anales de Pediatría Continuada     Full-text available via subscription  
Analytic Methods in Accident Research     Hybrid Journal   (Followers: 6, SJR: 4.849, CiteScore: 10)
Analytica Chimica Acta     Hybrid Journal   (Followers: 44, SJR: 1.512, CiteScore: 5)
Analytica Chimica Acta : X     Open Access  
Analytical Biochemistry     Hybrid Journal   (Followers: 216, SJR: 0.633, CiteScore: 2)
Analytical Chemistry Research     Open Access   (Followers: 13, SJR: 0.411, CiteScore: 2)
Analytical Spectroscopy Library     Full-text available via subscription   (Followers: 14)
Anesthésie & Réanimation     Full-text available via subscription   (Followers: 2)
Anesthesiology Clinics     Full-text available via subscription   (Followers: 25, SJR: 0.683, CiteScore: 2)
Angiología     Full-text available via subscription   (SJR: 0.121, CiteScore: 0)
Angiologia e Cirurgia Vascular     Open Access   (Followers: 1, SJR: 0.111, CiteScore: 0)
Animal Behaviour     Hybrid Journal   (Followers: 236, SJR: 1.58, CiteScore: 3)
Animal Feed Science and Technology     Hybrid Journal   (Followers: 8, SJR: 0.937, CiteScore: 2)
Animal Reproduction Science     Hybrid Journal   (Followers: 7, SJR: 0.704, CiteScore: 2)
Annales d'Endocrinologie     Full-text available via subscription   (Followers: 3, SJR: 0.451, CiteScore: 1)

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Similar Journals
Journal Cover
Additive Manufacturing
Journal Prestige (SJR): 2.611
Citation Impact (citeScore): 8
Number of Followers: 13  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 2214-8604 - ISSN (Online) 2214-8604
Published by Elsevier Homepage  [3148 journals]
  • Binder jetting additive manufacturing of copper foam structures
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Hadi Miyanaji, Da Ma, Mark A. Atwater, Kristopher A. Darling, Vincent H. Hammond, Christopher B. WilliamsIn binder jetting additive manufacturing (BJAM), the part geometry is generated via a binding agent during printing and structural integrity is imparted during sintering at a later stage. This separation between shape generation and thermal processing allows the sintering process to be uniquely controlled and the final microstructural characteristics to be tailored. The separation between the printing and consolidation steps offers a unique opportunity to print responsive materials that are later “activated” by temperature and/or environment. This may allow a new paradigm in multi-scale, multifunctional materials. This concept is preliminarily demonstrated using a foaming copper feedstock, such that the copper is printed, sintered and then foamed via intraparticle expansion in separate steps. The integration of foaming feedstock in BJAM could allow for creation of ultra-lightweight structures that offer hierarchical porosity, graded density, and/or tailored absorption properties. This work investigates processing protocol for copper foam structures to achieve the highest porosity. The copper feedstock was prepared by distributing copper oxides through the copper matrix via mechanical milling, and that powder was then printed into a green geometry through BJAM. The printed green parts were then heat treated using different thermal cycles to investigate the porosity evolution relative to various heating conditions. The heat treated parts were then examined for their resulting properties including porosity, microstructural evolution, and volumetric shrinkage. Parts that were initially sintered in air and then annealed in a hydrogen atmosphere led to higher porosity compared to those sintered in hydrogen alone. It was also found that the annealing of parts at 600 °C for 2 h resulted in the highest final porosity (59%) and the lowest volumetric shrinkage of 5%. Anisotropy in linear shrinkage in X, Y, and Z direction was also observed in the heat treated parts with the largest linear shrinkage occurring in the Z direction.Graphical abstractGraphical abstract for this article
       
  • Anisotropic tensile behavior of Ti-47Al-2Cr-2Nb alloy fabricated by direct
           laser deposition
    • Abstract: Publication date: Available online 25 January 2020Source: Additive ManufacturingAuthor(s): Xinyu Zhang, Chuanwei Li, Mengyao Zheng, Zhenhua Ye, Xudong Yang, Jianfeng GuAbstractAdditive manufacturing is emerging as an increasingly promising technology for TiAl alloys, which are attracting significant attention for their special microstructure and mechanical properties. In the present study, direct laser deposition was applied to fabricate single-track Ti-47Al-2Cr-2Nb alloy walls with a back-and-forth scanning strategy. The resulting alternative-band microstructure, comprising complicated microstructure bands and coarse lamellar colony bands, was characterized, and electronic back-scatter diffraction analysis showed that the lamellae in coarse colonies present a consistent orientation, nearly parallel to the substrate. The tensile properties of the as-deposited specimens varied greatly with the angle (θ) between the loading direction and the substrate. The specimens exhibited the best comprehensive tensile properties of the maximum ultimate strength and elongation (706 MPa and 0.51%) with the loading direction parallel to the substrate (θ=0°). The ultimate strength and elongation dropped to minimum values of 273 MPa and 0.16%, respectively, when the loading direction was perpendicular to the substrate (θ=90°). At the middle state of loading direction, θ=45°, the ultimate strength decreased to 358 MPa, and the elongation remained as high as 0.49%. Transmission electron microscopy observations revealed that dislocations and deformation twinning were both activated under different loading modes, and their morphologies were dramatically different. The formation mechanism of the alternative-band microstructure was proposed from the aspects of rapid solidification and the cyclic heat-treatment effect, and its possible fracture mechanism and crack propagation mode were discussed.
       
  • Scaling Up Metal Additive Manufacturing Process to Fabricate Molds for
           Composite Manufacturing
    • Abstract: Publication date: Available online 25 January 2020Source: Additive ManufacturingAuthor(s): Ahmed Arabi Hassen, Mark Noakes, Peeyush Nandwana, Seokpum Kim, Vlastimil Kunc, Uday Vaidya, Lonnie Love, Andrzej NyczABSTRACTDirect Energy Deposition (DED) systems are currently used to repair and maintain existing parts in the aerospace and automotive industries. This paper discusses an effort to scale up the DED technique in order to Additively Manufacture (AM) molds and dies used in the composite manufacturing industry. The US molds and dies market has been in a rapid decline over the last decade due to outsourcing to non-US entities. Oak Ridge National Laboratory (ORNL), Wolf Robotics and Lincoln Electric have developed a Metal Big Area Additive Manufacturing (MBAAM) system that uses a high deposition rate and a low-cost wire feedstock material. In this work we used the MBAAM system with a mild steel wire, ER70S-6, to fabricate a compression molding mold for composite structures used in automotive and mass-transit applications. In addition, the mechanical properties of the AM structure were investigated, and it was found that the MBAAM process delivers parts with high planar isotropic behavior. The paper investigates the microstructure and grain of the printed articles to confirm the roots of the observed planar isotropic properties. The manufactured AM mold was used to fabricate 50 composite parts with no observed mold deformations.
       
  • Direct-write printing for producing biomimetic patterns with self-aligned
           neurites
    • Abstract: Publication date: Available online 24 January 2020Source: Additive ManufacturingAuthor(s): Sujin Noh, Kyungha Kim, Jae-Ick Kim, Jung Hwal Shin, Hyun-Wook KangVarious techniques have been introduced to produce artificial neural constructs with aligned architectures, which have shown significantly improved neural function and regeneration. However, the techniques used to fabricate sophisticated patterns with aligned neurites show some limitations. Herein, we developed a direct-write printing process capable of producing versatile biomimetic patterns with aligned neurites using multiple cell types. Fibrin-based bio-ink was prepared for patterning with neuronal cells. After printing fixed pillars at both ends, microfibers were fabricated between the pillars using PC-12 neuronal cell/normal human dermal fibroblast (NHDF)-laden bio-ink and a direct-write printer. After two weeks of differentiation, aligned neurites were induced by the contractile force of the printed cells. We found that this self-induced alignment improved PC-12 differentiation and that neurite alignment could be adjusted by controlling the NHDF and bio-ink concentration. The bundle of cell-laden microfibers also showed uniform formation of neurites and synapse-like structures. Finally, we demonstrated the usefulness of the printing process by fabricating a Y-shaped branch and six-layered pattern. The six-layered pattern mimicking cerebral cortex tissue was produced by precise printing of two different colored cells. These results indicate that versatile biomimetic neural constructs composed of multiple cell types can be produced by our new direct-write printing process.Graphical abstractGraphical abstract for this article
       
  • Size-Dependent Stochastic Tensile Properties in Additively Manufactured
           316 L Stainless Steel
    • Abstract: Publication date: Available online 24 January 2020Source: Additive ManufacturingAuthor(s): Ashley M. Roach, Benjamin C. White, Anthony Garland, Bradley H. Jared, Jay D. Carroll, Brad L. BoyceAbstractRecent work in metal additive manufacturing (AM) suggests that mechanical properties may vary with feature size; however, these studies do not provide a statistically robust description of this phenomenon, nor do they provide a clear causal mechanism. Because of the huge design freedom afforded by 3D printing, AM parts typically contain a range of feature sizes, with particular interest in smaller features, so the size effect must be well understood in order to make informed design decisions. This work investigates the effect of feature size on the stochastic mechanical performance of laser powder bed fusion tensile specimens. A high-throughput tensile testing method was used to characterize the effect of specimen size on strength, elastic modulus and elongation in a statistically meaningful way. The effective yield strength, ultimate tensile strength and modulus decreased strongly with decreasing specimen size: all three properties were reduced by nearly a factor of two as feature dimensions were scaled down from 6.25 mm to 0.4 mm. Hardness and microstructural observations indicate that this size dependence was not due to an intrinsic change in material properties, but instead the effects of surface roughness on the geometry of the specimens. Finite element analysis using explicit representations of surface topography shows the critical role surface features play in creating stress concentrations that trigger deformation and subsequent fracture. The experimental and finite element results provide the tools needed to make corrections in the design process to more accurately predict the performance of AM components.
       
  • Additive manufacturing assisted investment casting: A low-cost method to
           fabricate periodic metallic cellular lattices
    • Abstract: Publication date: Available online 24 January 2020Source: Additive ManufacturingAuthor(s): V.H. Carneiro, S.D. Rawson, H. Puga, J. Meireles, P.J. WithersAbstractMetallic cellular solids are a class of materials known for their high specific mechanical properties, being desirable in applications where a combination of high strength or stiffness and low density are important. These lightweight materials are often stochastic and manufactured by foaming or casting. If regular (periodic) lattice structures are desired, they may be manufactured by metallic additive manufacturing techniques. However, these have characteristic issues, such as un-melted powders, porosity and heterogeneous microstructures. This study reports a novel low-cost route for producing regular lattice structures by an additive manufacturing assisted investment casting technique. Fused filament fabrication is used to produce the lattice structure pattern which is infiltrated with plaster. The pattern is then burnt off and the aluminum is cast in vacuum. In this way we can manufacture non-stochastic metallic lattices having fine struts/ribs (0.6 mm cross-section using a 0.4 mm nozzle) and relative densities down to 0.036. X-ray microcomputed tomography (μCT) showed that as-cast A356 Aluminium alloy frameworks have high dimensional tolerances and fine detail control. Frameworks based on units of six connected struts ranging from intruded (auxetic) to protruding (hexagonal) strut angles are studied. Vertical struts are finer than expected, reducing their moment of area which could impact their compressive strength. This new, low cost, route for producing high precision metallic cellular lattices offers an attractive alternative to other additive manufacturing techniques (e.g. selective laser melting or electron beam).
       
  • Evolution of Porosity and Geometrical Quality through the Ceramic
           Extrusion Additive Manufacturing Process Stages
    • Abstract: Publication date: Available online 24 January 2020Source: Additive ManufacturingAuthor(s): Kedarnath Rane, Stefano Petrò, Matteo StranoCeramic Extrusion Additive Manufacturing (CEAM) enables the die-less fabrication of small ceramic parts, with a process chain that includes four consecutive stages: the 3D printing, solvent de-binding, thermal de-binding, and sintering. The 3D printing process was implemented through Ephestus, a specially developed EAM machine for the manufacturing of parts from alumina feedstock. A test part was designed, and X-ray computed tomography (µ-CT) was used to quantify its characteristics through the processing stages of the EAM. The porosity distribution and the distribution of void size and shape were determined throughout the samples at each stage, using image analysis techniques. Furthermore, the evolution of some macroscopic quality properties was measured.The results show that both microscopic (porosity) and macroscopic (geometry, density) properties of the samples improve through the process stages. A vertical gradient of porosity is present in green and de-bound samples, with porosity decreasing with increasing sample height. After sintering, the vertical gradient of porosity disappears. The sphericity and the diameter of voids are negatively correlated and dispersed over a wide range in the green state. The sintering process has a homogenization effect on the void shape distribution. The geometrical deviation from the nominal designed dimensions and the surface quality of parts improves when moving from the green to the sintered state.Graphical Graphical abstract for this articleExperimental investigation of porosities in additive manufactured ceramics parts.
       
  • Prediction of Selective Laser Melting Part Quality Using Hybrid Bayesian
           Network
    • Abstract: Publication date: Available online 24 January 2020Source: Additive ManufacturingAuthor(s): Nathan Hertlein, Sourabh Deshpande, Vysakh Venugopal, Manish Kumar, Sam AnandAbstractAdditive manufacturing (AM) is gaining popularity because of its ability to manufacture complex parts in less time. Despite recent research involving designs of experiments (DOEs) to characterize the relationships between some AM process parameters and various part quality characteristics, to date, there seems to be no universally accepted comprehensive model that relates process parameters to part quality. In this paper, to support the goal of manufacturing parts right the first time, a Bayesian network in continuous domain is developed which relates four process parameters (laser power, scan speed, hatch spacing, and layer thickness) and five part quality characteristics (density, hardness, top layer surface roughness, ultimate tensile strength in the build direction and ultimate tensile strength perpendicular to the build direction). A machine learning algorithm is used to train the network on a database mined from a large number of publications with experimental data from parts built using 316L with selective laser melting. Using this Bayesian network, the user is able to enter a value for one or more known nodes or variables, and the network provides predictions on all the remaining nodes in the form of probability distributions. A method is developed whereby the user inputs are checked for reasonableness using an n -dimensional convex hull, and if necessary a recommendation is returned based on user-defined weights. The network is validated by retaining a subset of the training data for testing and comparing the network’s predictions to the known values. Accuracy is optimized by continually re-training the network using parts built with a specific machine of interest. The industrial relevance of this research is outlined with respect to four current challenges in AM, including the length of time to determine optimal process parameters for a new machine, ability to organize relevant knowledge, quantification of machine variability, and transfer of knowledge to new operators.
       
  • Prediction of Anisotropic Mechanical Properties for Lattice Structures
    • Abstract: Publication date: Available online 23 January 2020Source: Additive ManufacturingAuthor(s): Maxwell Munford, Umar Hossain, Shaaz Ghouse, Jonathan JeffersAdditive manufacturing methods present opportunities for structures to have tailored mechanical anisotropy by integrating controlled lattice structures into their design. The ability to predict anisotropic mechanical properties of such lattice structures would help tailor anisotropy and ensure adequate off-axis strength at an early stage in the design process. A method is described for the development of a model to predict apparent modulus and strength based on structure density and fabric, taken from CAD data. The model utilises a tensorial form of well-founded power-law relationships for these variables and is fit to mechanical test data for properties in the principal directions of manufactured titanium stochastic lattices and nylon rhombic dodecahedron structures. The results are validated against mechanical testing across at least 7 additional off-axis directions. For stochastic structures, apparent modulus is predicted in 10 directions with a mean error of 13% and strength predicted with a mean error of 10%. For rhombic dodecahedron structures apparent modulus and strength are predicted in 15 directions with mean errors of 4.2% and 5.1% respectively. This model is the first to predict the anisotropic apparent modulus and strength of structures based on lattice density and fabric tensors and will be highly useful in the mechanical design of lattice structures.Graphical abstractGraphical abstract for this article
       
  • Comparison of Microstructural Characteristics and Mechanical Properties of
           Shipbuilding Steel Plates Fabricated by Conventional Rolling versus Wire
           Arc Additive Manufacturing
    • Abstract: Publication date: Available online 23 January 2020Source: Additive ManufacturingAuthor(s): Alireza Vahedi Nemani, Mahya Ghaffari, Ali NasiriAbstractThis study aims to investigate the fabrication feasibility of a conventionally rolled low-carbon low-alloy shipbuilding steel plate (EH36) by emerging wire arc additive manufacturing (WAAM) technology using ER70S feedstock wire. Following the fabrication process, different heat treatment cycles, including air-cooling and water-quenching from the intercritical austenitizing temperature of 800 °C, were applied to both conventionally rolled and WAAM samples. Microstructural features and mechanical properties of both rolled and WAAM fabricated ship plates were comprehensively characterized and compared before and after different heat treatment cycles. Both air-cooling and water-quenching heat treatments resulted in the formation of hard martensite-austenite (MA) constituents in the microstructure of the rolled ship plate, leading to the increased hardness and tensile strength and reduced ductility of the component. On the other hand, air-cooling heat treatment was found to homogenize the microstructure of the WAAM ship plate, causing a slight decrease in the hardness and tensile strength, while the water-quenching cycle led to the formation of acicular ferrite and intergranular pearlite, contributing to the improved mechanical properties of the part. Therefore, the enhanced mechanical integrity of the water-quenched WAAM component as compared to its rolled counterpart verified the fabrication feasibility of the ship plates by the WAAM.
       
  • Pressure dependence of the laser-metal interaction under laser powder bed
           fusion conditions probed by in situ X-ray imaging
    • Abstract: Publication date: Available online 23 January 2020Source: Additive ManufacturingAuthor(s): Nicholas P. Calta, Aiden A. Martin, Joshua A. Hammons, Michael H. Nielsen, Tien T. Roehling, Kamel Fezzaa, Manyalibo J. Matthews, Jason R. Jeffries, Trevor M. Willey, Jonathan R.I. LeeLaser powder bed fusion (LPBF) additive manufacturing and laser welding are powerful metal processing techniques with broad applications in advanced sectors such as the biomedical and aerospace industries. One common process variable that can tune laser-material interaction dynamics in these two techniques is adjustment of the composition and pressure of the atmosphere in which the process is conducted. While some of the physical mechanisms that are governed by the ambient pressure are well known from the welding literature, it remains unclear how these mechanisms extend to the distinct process conditions of LPBF. In situ studies of the differences in subsurface structure and behavior are essential for understanding the effects of gas pressure and composition on the LPBF processes. This article reports the use of in situ X-ray imaging to directly probe the morphological evolution of the liquid-vapor interface during laser melting as a function of ambient pressure and oxygen partial pressure under LPBF conditions in 316 L steel, Ti-64, aluminum 6061, and Nickel 400. We observe significant changes in melt pool morphology as a function of pressure. Furthermore, similar changes in morphology occur due to an increase in oxygen partial pressure in the process atmosphere. Temperature- and composition-dependent changes in surface tension of the liquid metal drive this change in behavior, which has the potential to influence defect creation and final morphology in LPBF parts.Graphical Graphical abstract for this article
       
  • Effects of Deposition Velocity in the Presence/Absence of E6-Glass Fibre
           on Extrusion-based 3D Printed Mortar
    • Abstract: Publication date: Available online 23 January 2020Source: Additive ManufacturingAuthor(s): Pshtiwan Shakor, Shami Nejadi, Sheila Sutjipto, Gavin Paul, Nadarajah GowripalanAdditive Manufacturing (AM) technologies are widely used in various fields of industry and research. Continual research has enabled AM technologies to be considered as a feasible substitute for certain applications in the construction industry, particularly given the advances in the use of glass fibre reinforced mortar. An investigation of the resulting mechanical properties of various mortar mixes extruded using a robotic arm is presented. The nozzle paths were projected via ‘spline’ interpolation to obtain the desired trajectory and deposition velocity in the reference frame of the manipulator. Along each path, various mortar mixes, with and without chopped glass fibre, were deposited at different velocities. Tests were conducted to determine their mechanical performance when incorporated in printed structures with different layers (1, 2, 4 and 6 layers). The results are compared with those of conventional cast-in-place mortar. In this study, the mixes consist of ordinary Portland cement, fine sand, chopped glass fibres (6 mm) and chemical admixtures, which are used to print prismatic- and cubic-shaped specimens. Mechanical strength tests were performed on the printed specimens to evaluate the behaviour of the materials in the presence and absence of glass fibre. Robot end-effector velocity tests were performed to examine the printability and extrudability of the mortar mixes. Finally, horizontal and vertical line printing tests were used to determine the workability, buildability and uniformity of the mortar mix and to monitor the fibre flow directions in the printed specimens. The results show that printed specimens with glass fibre have enhanced compressive strength compared with specimens without glass fibre.Graphical abstractGraphical abstract for this article
       
  • Experimental observation of stress formation during selective laser
           melting using in situ X-ray diffraction
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Felix Schmeiser, Erwin Krohmer, Norbert Schell, Eckart Uhlmann, Walter ReimersAbstractDespite the ongoing success of metal additive manufacturing and especially the selective laser melting (SLM) technology, process-related defects, distortions and residual stresses impede its usability for fracture-critical applications. In this paper, results of in situ X-ray diffraction experiments are presented that offer insights into the strain and stress formation during the manufacturing of multi-layer thin walls made from Inconel 625. Using different measuring modes and laser scanning parameters, several experimental observations are discussed to validate and extend theoretical models and simulations from the literature. As a sample is built-up layer by layer, the stress state changes continuously up until the last exposure. The localized energy input leads to a complex stress field around the heat source that involves alternating tensile and compressive stresses. The correlation of temperature and yield strength results in a stress maximum at a certain distance to the top layer. The present study demonstrates the potential of high-energy synchrotron radiation diffraction for in situ SLM research.
       
  • Microstructure and properties of high-strength C+N austenitic stainless
           steel processed by laser powder bed fusion
    • Abstract: Publication date: Available online 23 January 2020Source: Additive ManufacturingAuthor(s): J. Boes, A. Röttger, W. TheisenAbstractIn the developing field of laser powder bed fusion (L-PBF), austenitic stainless steels, such as AISI 316 L, have gained great importance owing to their excellent processability. However, the moderate strength of these steels limits their applicability. This can be counteracted by the use of nitrogen as an alloying element to improve both strength and corrosion resistance.In this work, nitrogen-alloyed high-strength austenitic stainless steel X40MnCrMoN19-18-1 was processed by L-PBF, and the resulting microstructural and mechanical properties were investigated. The same material was also processed by hot isostatic pressing (HIP), which was used as a reference state. In the L-PBF process, argon and nitrogen were used as process gases to investigate the influence of process atmosphere on the microstructure and on changes in the chemical composition during processing. The results show a minor decrease in the nitrogen content of the steel after L-PBF, independently of the process gas, whereby argon resulted in a slightly higher specimen density. The microstructure after L-PBF processing contained small precipitates that could be removed by a short solution-annealing treatment. The tensile properties of the L-PBF-built steel are comparable to those of the steel produced by hot isostatic pressing in terms of ultimate tensile strength, but had lower elongation to fracture values. The ductility of the material was enhanced by solution annealing without significant impairment of the ultimate tensile strength. This work demonstrates that nitrogen-alloyed stainless steels can be processed by means of L-PBF and can extend the variety of appropriate steels towards applications with high requirements for the material strength and chemical resistance.
       
  • Experimental analysis of additively manufactured component and design
           guidelines for lightweight structures: a case study using electron beam
           melting
    • Abstract: Publication date: Available online 22 January 2020Source: Additive ManufacturingAuthor(s): Pan Wang, Jie Song, Mui Ling Sharon Nai, Jun WeiAbstractPowder bed fusion additive manufacturing (AM) technology, such as electron beam melting (EBM) and selective laser melting, has attracted tremendous academic and industrial interests because of its capacity to fabricate components with greater complexity compared with traditional processes, without significantly increasing the cost. It provides significantly higher design freedom to the designers and can make the built components closer to the optimum design in theory when compared with traditional processes. However, the mechanical performance of the new design fabricated by AM has not been clarified yet. Here, we report the fabrication and tensile deformation behavior of the EBM-built lightweight car suspension double wishbone for both conventional and optimized designs. EBM process is an effective method to produce a highly-dense Ti-6Al-4 V lightweight design component with good reproducibility and fine α/β duplex microstructure. A poor mechanical performance in the optimized design is observed, which results from the build thickness-dependent mechanical performance that is caused by both various microstructures and rough surfaces in the Ti-6Al-4 V parts. Notably, the rough surface plays a dominant role in premature failure when the build thickness is below 2 mm. Based on these findings, the degraded mechanical performance in the optimized design is discussed. The experimental results and analyses provide a guideline for the design of lightweight structures that are mainly comprised of thin walls and/or struts.
       
  • Measuring the spreadability of pre-treated and moisturized powders for
           laser powder bed fusion
    • Abstract: Publication date: Available online 20 January 2020Source: Additive ManufacturingAuthor(s): Laura Cordova, Ton Bor, Marc de Smit, Mónica Campos, Tiedo TingaFor AM processes—specifically Laser Powder Bed Fusion (L-PBF) processes—powder flowability is essential for the product quality, as these processes are based on a thin layer spreading mechanism. However, the available techniques to measure this flowability do not accurately represent the spreading mechanism. Hence, this paper presents two novel applicator tools specifically designed to test the spreadability of L-PBF powders in thin layer application. The results were checked by running standard tests to analyze the powder morphology, moisture content, chemical composition and flowability using the Hall-flowmeter. For this study, four common L-PBF metal powders were selected: Inconel 718, Ti6Al4V, AlSi10Mg and Scalmalloy. From the as-received state, drying (vacuum and air) and moisturizing treatments were applied to compare four humidity states and investigate the feasibility of pre-treating the powders to remove moisture, which is known to cause problems with flowability, porosity formation and enhanced oxidation. The tests reveal that AlSi10Mg is the most susceptible alloy to moisture and oxygen pick-up, considerably decreasing the spreadability and relative density on the build platform. However, the results also reveal how challenging the direct measurement of moisture levels in metal powders is.Graphical abstractGraphical abstract for this article
       
  • Effects of annealing on the structure and mechanical properties of
           FeCoCrNi high-entropy alloy fabricated via selective laser melting
    • Abstract: Publication date: Available online 18 January 2020Source: Additive ManufacturingAuthor(s): Danyang Lin, Lianyong Xu, Hongyang Jing, Yongdian Han, Lei Zhao, Fumiyoshi Minami1To widen the applications of FeCoCrNi high-entropy alloys (HEAs) fabricated via selective laser melting, their mechanical properties must be improved, and annealing plays an important role in this regard. In this study, the microstructure, residual stress, and mechanical properties of the as-printed specimen and specimens annealed at 773–1573 K for 2 h were compared. As the annealing temperature increased, the specimen structure recrystallized from all columnar grains to equiaxial grains containing numerous annealing twins. The dislocation network, which formed during the solidification process under considerable shrinkage strain, decomposed into dislocations. The residual stress, yield strength, and hardness decreased, while the plasticity and impact toughness increased. During the deformation of as-printed and low-temperature-annealed specimens, the dislocation network remained unchanged and provided resistance to the dislocations moving within it, thus strengthening the specimen. The tensile strength remained largely unchanged owing to the reduction in the residual stress during low-temperature annealing, as well as the formation of the twinning network and dislocation wall under large deformation upon high-temperature annealing. Meanwhile, the ductility greatly increased, thus increasing the potential for industrial application of HEAs.Graphical abstractGraphical abstract for this article
       
  • Inconel-steel multilayers by liquid dispersed metal powder bed fusion:
           Microstructure, residual stress and property gradients
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): S.C. Bodner, L.T.G. van de Vorst, J. Zalesak, J. Todt, J.F. Keckes, V. Maier-Kiener, B. Sartory, N. Schell, J.W. Hooijmans, J.J. Saurwalt, J. KeckesSynthesis of multi-metal hybrid structures with narrow heat affected zones, limited residual stresses and secondary phase occurrence represents a serious scientific and technological challenge. In this work, liquid dispersed metal powder bed fusion was used to additively manufacture a multilayered structure based on alternating Inconel 625 alloy (IN625) and 316L stainless steel (316L) layers on a 316L base plate. Analytical scanning and transmission electron microscopies, high-energy synchrotron X-ray diffraction and nanoindentation analysis reveal sharp compositional, structural and microstructural boundaries between alternating 60 μm thick alloys’ sub-regions and unique microstructures at macro-, micro- and nano-scales. The periodic occurrence of IN625 and 316L sub-regions is correlated with a cross-sectional hardness increase and decrease and compressive stress decrease and increase, respectively. The laser scanning strategy induced a growth of elongated grains separated by zig-zag low-angle grain boundaries, which correlate with the occurrence of zig-zag cracks propagating in the growth direction. A sharp fiber texture within the 316L regions turns gradually into a fiber texture in the IN625 regions. The occurrence of the C-like stress gradient with a pronounced surface tensile stress of about 500 MPa is interpreted by the temperature gradient mechanism model. Chemical analysis indicates a formation of reinforcing spherical chromium-metal-oxide nano-dispersoids and demonstrates a possibility for reactive additive manufacturing and microstructural design at the nanoscale, as a remarkable attribute of the deposition process. Finally, the study shows that the novel approach represents an effective tool to combine dissimilar metallic alloys into unique bionic hierarchical microstructures with possible synergetic properties.Graphical abstractGraphical abstract for this article
       
  • Fabricating an effective calcium zirconate layer over the calcia grains
           via binder-jet 3D-printing for improving the properties of calcia ceramic
           cores
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Huoping Zhao, Chunsheng Ye, Shuhua Xiong, Zitian Fan, Longzhi ZhaoAbstractTo improve the formability and properties of calcia (CaO) based ceramic core, the binder-jet 3D-printing was performed to fabricate porous CaO/caicium zirconate (CaZrO3) ceramic core composites with two nanozirconia addition methods. The effects of the nanozirconia addition method and additive amount on the properties of the 3D-printed CaO/CaZrO3 bodies were investigated. The dimensional accuracy, surface roughness, relative density, bending strength, and hydration resistance of CaO/CaZrO3 bodies printed with a nanozirconia suspension binder for deposition in the CaO powder layer were better than those of CaO/CaZrO3 bodies printed in the traditional manner of directly mixing nanozirconia in the CaO powder. Application of the nanozirconia suspension uniformly capped nanozirconia particles on the surfaces of the CaO particles and filled the pores of the CaO powder layer, which afforded denser and more uniform green bodies. After sintering at 1500 °C, the ZrO2 formed thicker and denser CaZrO3 layers with the CaO over the CaO grain surfaces, which improved the strength and hydration resistance of the sintered CaO/CaZrO3 ceramic core bodies, and certainly reduced their collapsibility.
       
  • Influence of formulation parameters on the freeform extrusion process of
           ceramic pastes and resulting product properties
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Benedikt Finke, Jutta Hesselbach, Anna Schütt, Marius Tidau, Benedikt Hampel, Meinhard Schilling, Arno Kwade, Carsten SchildeThe rheology of a ceramic paste is known to be a key factor in the process of additive manufacturing of ceramic parts via extrusion freeforming. The rheological properties of ceramic pastes can be influenced by several formulation parameters. In this study, the mutual influence between formulation parameters, printing properties and mechanical properties of ceramic pastes (Al2O3) and the resulting green bodies are investigated. Special focus is set on elucidating the origins and causes of the altered paste properties to allow targeted material development of pastes for the use in extrusion freeforming. Glycerine and nanoparticulate boehmite needles are tested as additives and they successfully improve the printability in the extrusion freeforming process of the paste and compression strength of the green body. Considerable difference in the dependency of the mechanical properties on the formulation parameters was detected after a partial sintering of the green bodies at 1000 °C. Dewatering, shrinkage during drying and a running of the deposited lines could be reduced successfully by adjusting the formulation. The impact of the formulation parameters on the printing performance could be linked to the dependency of the volume flow rate on the ratio of pressure over viscosity.Graphical abstractGraphical abstract for this article
       
  • Control of thermally stable core-shell nano-precipitates in additively
           manufactured Al-Sc-Zr alloys
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Philipp Kürnsteiner, Priyanshu Bajaj, Ankit Gupta, Markus Benjamin Wilms, Andreas Weisheit, Xiaoshuang Li, Christian Leinenbach, Baptiste Gault, Eric Aimé Jägle, Dierk RaabeLaser Additive Manufacturing (LAM) of light metals such as high-strength Al-based alloys offers tremendous potential for e.g. weight reduction and associated reduced fuel consumptions for the transportation industry. Typically, commercial Sc-containing alloys, such as Scalmalloy®, rely on precipitation hardening to increase their strength. Conventional processing involves controlled ageing during which ordered and coherent Al3Sc precipitates form from a Sc-supersaturated solid solution. Here we show how the intrinsic heat treatment (IHT) of directed energy deposition (DED) can be used to trigger the precipitation of Al3Sc already during the LAM process. High number densities of 1023 nano-precipitates per m3 can be realized through solid-state phase transformation from the supersaturated Al-Sc matrix that results from the fast cooling rate in LAM. Yet, the IHT causes precipitates to coarsen, hence reducing their strengthening effect. We implement alternative solidification conditions to exploit the IHT to form a Zr-rich shell around the Al3Sc precipitates that prevents coarsening. Our approach is applicable to a wide range of precipitation-hardened alloys to trigger in-situ precipitation during LAM.Graphical abstractGraphical abstract for this article
       
  • Grain refinement in an unalloyed tantalum structure by combining Wire+Arc
           additive manufacturing and vertical cold rolling
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): G. Marinelli, F. Martina, S. Ganguly, S. WilliamsAbstractComponents manufactured via Wire + Arc Additive Manufacturing are usually characterised by large columnar grains. This can be mitigated by introducing in-process cold rolling; in fact, the associated local plastic deformation leads to a reduction of distortion and residual stresses, and to microstructural refinement. In this research, inter-pass rolling was applied with a load of 50 kN to a tantalum linear structure to assess rolling’s effectiveness in changing the grain structure from columnar to equiaxed, as well as in refining the grain size. An average grain size of 650 μm has been obtained after five cycles of inter-pass rolling and deposition. When the deformed layer was reheated during the subsequent deposition, recrystallisation occurred, leading to the growth of new strain-free equiaxed grains. The depth of the refined region has been characterised and correlated to the hardness profile developed after rolling. Furthermore, a random texture was formed after rolling, which should contribute to obtaining isotropic mechanical properties. Wire + Arc Additive Manufacture demonstrated the ability to deposit sound refractory metal components and the possibility to improve the microstructure when coupled with cold inter-pass rolling.
       
  • Melt pool size control through multiple closed-loop modalities in
           laser-wire directed energy deposition of Ti-6Al-4V
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): B.T. Gibson, Y.K. Bandari, B.S. Richardson, W.C. Henry, E.J. Vetland, T.W. Sundermann, L.J. LoveAbstractSensing and closed-loop control are critical attributes of a robust 3D printing process, such as Directed Energy Deposition (DED), in which it is necessary to manage geometry, material properties, and residual stress and distortion. The present research demonstrates multiple modes of closed-loop melt pool size control in laser-wire based DED, a form of large-scale metal additive manufacturing. First, real-time closed-loop melt pool size control through laser power modulation was demonstrated for intralayer control of bead geometry. Aspects such as controller tuning, response time, interaction with primary process variables, and disturbance rejection are presented. Next, an interlayer trend in laser power during the printing of layered components was documented, which inspired the development of novel modes of control. A controller that modulates print speed and deposition rate on a per-layer basis was developed and demonstrated, enabling the control of either average melt pool size alone or average laser power in coordination with real-time melt pool size control. This work demonstrates that accumulated heat in components under construction can be exploited to maintain process stability as print speed and deposition rate are automatically increased under closed-loop control. This has major implications for overall production efficiency. Control modes are characterized in terms of their effect on local bead geometry, global part geometry, and interlayer effect on energy density, among other factors.
       
  • Heating and flow computations of an amorphous polymer in the liquefier of
           a material extrusion 3D printer
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): F. Pigeonneau, D. Xu, M. Vincent, J.-F. AgassantAbstractThe heating of a polymer in a liquefier of a material extrusion 3D printer is numerically studied. The problem is investigated by solving the mass, momentum, and energy conservation equations. The polymer is taken as a generalized Newtonian fluid with a dynamical viscosity function of shear rate and temperature. The system of equations is solved using a finite element method. The boundary conditions are adapted by comparison with the previous work of Peng et al. [5] showing that the thermal contact between the polymer and the liquefier is very well established. The limiting printing conditions are studied by determining the length over which the polymer temperature is below the glass transition temperature. This provides a simple relation for the inlet velocity as a function of the working parameters and the polymer properties.
       
  • Investigating the effect of metal powder recycling in Electron beam Powder
           Bed Fusion using process log data
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Sujana Chandrasekar, Jamie B. Coble, Sean Yoder, Peeyush Nandwana, Ryan R. Dehoff, Vincent C. Paquit, Sudarsanam S. BabuAbstractRecycling metal powders in the Additive Manufacturing (AM) process is an important consideration in affordability with reference to traditional manufacturing. Metal powder recyclability has been studied before with respect to change in chemical composition of powders, effect on mechanical properties of produced parts, effect on flowability of powders and powder morphology. However, these studies involve ex situ characterization of powders after many use cycles. In this paper, we propose a data-driven method to understand in situ behavior of recycled powder on the build platform. Our method is based on comprehensive analysis of log file data from various sensors used in the process of printing metal parts in the Arcam Electron Beam Melting (EBM) ® system. Using rake position data and rake sensor pulse data collected during Arcam builds, we found that Inconel 718 powders exhibit additional powder spreading operations with increased reuse cycles compared to Ti-6Al-4V powders. We substantiate differences found in in situ behavior of Ti-6Al-4V and Inconel 718 powders using known sintering behavior of the two powders. The novelty of this work lies in the new approach to understanding powder behavior especially spreadability using in situ log file data that is regularly collected in Arcam EBM® builds rather than physical testing of parts and powders post build. In addition to studying powder recyclability, the proposed methodology has potential to be extended generically to monitor powder behavior in AM processes.
       
  • Effects of inter layer time and build height on resulting properties of
           316L stainless steel processed by laser powder bed fusion
    • Abstract: Publication date: Available online 16 January 2020Source: Additive ManufacturingAuthor(s): Gunther Mohr, Simon J. Altenburg, Kai HilgenbergAbstractLaser powder bed fusion (L-PBF) is the most prominent additive manufacturing (AM) technology for metal part production. Among the high number of factors influencing part quality and mechanical properties, the inter layer time (ILT) between iterative melting of volume elements in subsequent layers is almost completely unappreciated in the relevant literature on L-PBF. This study investigates the effect of ILT with respect to build height and under distinct levels of volumetric energy density (VED) using the example of 316 L stainless steel. In-situ thermography is used to gather information on cooling conditions during the process, which is followed by an extensive metallographic analysis. Significant effects of ILT and build height on heat accumulation, sub-grain sizes, melt pool geometries and hardness are presented. Furthermore, the rise of defect densities can be attributed to a mutual interplay of build height and ILT. Hence, ILT has been identified as a crucial factor for L-PBF of real part components especially for those with small cross sections.
       
  • Magnetic Shielding Promotion via the Control of Magnetic Anisotropy and
           Thermal Post Processing in Laser Powder Bed Fusion Processed NiFeMo-based
           Soft Magnet
    • Abstract: Publication date: Available online 16 January 2020Source: Additive ManufacturingAuthor(s): Abd El-Moez A. Mohamed, Ji Zou, Richard S. Sheridan, Kai Bongs, Moataz M. AttallahThe aim of this study is to promote the magnetic shielding characteristics of laser powder bed fusion (LPBF) processed NiFeMo alloy. This was achieved via controlling the crystallographic texture of the builds to increase the grain population along the easy axis of magnetisation, as well as the use of post-process hydrogen heat treatment (HT) and hot isostatic pressing (HIP) processes. The as-fabricated microstructure typically demonstrates weak magnetic properties due to the alignment of the crystallographic orientation/spin order along the [100] hard axis of magnetisation, which is parallel to the build direction since it is also the preferred growth direction during solidification in cubic materials. Tilting the build orientation to align the easy magnetisation axes [110] and [111] along the build principal directions results in an improvement in the magnetic shielding characteristics normal and transverse to the build principal directions. Furthermore, the HT/HIP processes further promoted the soft ferromagnetic characteristics, with the best magnetic shielding properties being registered for the [111] tilted sample following both HIP and HT, demonstrating 60-100 folds improvement compared with the as-fabricated condition. The improved ferromagnetism following HIP + HT was due to several combined effects, including stress relief, consolidation of gas pores, recrystallisation, and grain growth. The post-processing sequence (HT + HIP vs. HIP + HT) appeared to affect the resulting magnetic characteristics. Finally, the tensile properties for the builds were characterised to ensure that both functional and mechanical behaviours would achieve the required performance.Graphical abstractGraphical abstract for this article
       
  • Corrigendum to “Strong and light cellular silicon carbonitride –
           Reduced graphene oxide material with enhanced electrical conductivity and
           capacitive response’’ [Addit. Manuf. 30 (2019) 100849]
    • Abstract: Publication date: Available online 15 January 2020Source: Additive ManufacturingAuthor(s): J.J. Moyano, J. Mosa, M. Aparicio, D. Pérez-Coll, M. Belmonte, P. Miranzo, M.I. Osendi
       
  • Corrigendum to “Evaluation of a thermomechanical model for prediction of
           residual stress during laser powder bed fusion of Ti-6Al-4V” [Addit.
           Manuf. 27 May (2019) 489–502]
    • Abstract: Publication date: Available online 15 January 2020Source: Additive ManufacturingAuthor(s): R.K. Ganeriwala, M. Strantza, W.E. King, B. Clausen, T.Q. Phan, L.E. Levine, D.W. Brown, N.E. Hodge
       
  • Thermal Curing Kinetics Optimization of Epoxy Resin in Digital Light
           Synthesis
    • Abstract: Publication date: Available online 14 January 2020Source: Additive ManufacturingAuthor(s): Alec Redmann, Paul Oehlmann, Thomas Scheffler, Lukas Kagermeier, Tim A. OsswaldAbstractA new class of high-performance resins are available for additive manufacturing with the introduction of Digital Light Synthesis (DLS) technology. In combination with Continuous Liquid Interface Production (CLIP), DLS uses ultraviolet light and oxygen to continuously grow objects from a pool of resin instead of printing them layer-by-layer, subsequently increasing the printing speed and the mechanical performance. For many DLS resin systems, a secondary thermal curing step is required in order to reach the final material properties after printing. This step is a major limiting factor in the production time of the DLS process, as materials may require several hours of thermal post curing. The aim of this study is to optimize this secondary curing cycle for the epoxy-based resin EPX 82 by reducing the thermal curing time while avoiding a negative influence on the final mechanical properties. Differential scanning calorimetry (DSC) was used with different heating rates and a chemical reaction model was developed. The Di Benedetto relationship was used to include diffusion control for high degrees of cure. Based on the kinetics study, an optimized curing cycle was generated using different maximum conversion rates and heating rates. The results show that it is possible to reduce the thermal curing cycle time by more than 9 hours (73 percent) compared to the supplier recommendation while maintaining the mechanical properties of the final part.
       
  • Development and validation of an energy simulation for a desktop additive
           manufacturing system
    • Abstract: Publication date: Available online 14 January 2020Source: Additive ManufacturingAuthor(s): Li Yi, Bahram Ravani, Jan C. AurichAbstractThe assessment and minimization of energy consumptions of additive manufacturing (AM) processes are currently emerging research tasks. It is evident that the energy consumption of an AM process can be one or two orders of magnitude higher than conventional manufacturing processes. For improving the sustainability performance of AM, the energy use of AM should be evaluated and optimized in the design phase for planning products and AM processes. In order to support the quantification and evaluation of the energy consumption of AM, we have developed an energy simulation of a desktop AM system by using a physical modeling approach. Moreover, experiments have been carried out to validate and confirm the simulation accuracy and reliability. The result of the experimental validation has shown that the accuracy of the developed simulation approach can be up to approximately 98%.
       
  • Influence of Post Deposition Annealing on Microstructure and Properties of
           Laser Additively Manufactured Titanium Copper Alloys
    • Abstract: Publication date: Available online 14 January 2020Source: Additive ManufacturingAuthor(s): S A Mantri, T Alam, Y Zheng, J C Williams, R BanerjeeAbstractWhile copper is a potent strengthener in titanium alloys, its use in commercial alloys has been severely restricted due to the strong tendency for segregation during solidification, leading to heterogeneous microstructures and what has often been referred to as the “beta-fleck” problem. This problem can be largely obviated by using additive manufacturing (AM) for processing Ti-Cu alloys. This study focuses on AM of a binary Ti-4Cu and a ternary Ti-4Cu-4Al alloy using the laser engineered net shaping (LENS) process. The influence of post-deposition annealing treatments and the subsequent cooling rate on the microstructure and tensile properties of these alloys has been investigated in detail. The phase fraction of the eutectoid alpha + Ti2Cu product is dependent on the cooling rate from above the beta transus temperature. Additionally, the Ti2Cu phase exhibited a far-from equilibrium composition in case of the water-quenched Ti-4Cu-4Al alloy. Both the yield stress (∼550-650 MPa) as well as the ductility (∼15-18%) were also higher in case of the ternary alloy. The high strengths exhibited by the water-quenched samples of both alloys, while maintaining appreciable tensile ductility, could be attributed to clustering of Cu within the α laths, revealed by atom probe tomography.
       
  • Microstructure refinement for superior ductility of Al–Si alloy by
           electron beam melting
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Huakang Bian, Kenta Aoyagi, Yufan Zhao, Chikatoshi Maeda, Toshihiro Mouri, Akihiko ChibaRefining the microstructure to improve the ductility of an Al‒Si alloy is challenging. In this paper, we report for the first time a novel microstructure refinement approach for AlSi10Mg (wt%) alloys using electron beam melting (EBM) technology, without the addition of any modification elements. The synergetic effect of superheating, fast cooling, and preheating contributes to a refined Si phase with a fine granular structure (0.5–2 μm) within bimodal Al grains (40 μm grains and 0.5–2 μm sub-grains). The results provide good evidence for the proposed refinement mechanism. A maximum ductility of approximately 32.7 % with a tensile strength of approximately 136 MPa was achieved for the as-built AlSi10Mg EBM alloy. After solution heat treatment and T6-like aging, nano-Si precipitates formed which strengthened the alloys. The pathway developed in this study for refining the Al–Si alloy microstructure to improve the tensile ductility will provide a feasible and fast manufacturing method for improving the microstructure and mechanical properties of other low-melting temperature alloys in the near future using EBM technology.Graphical abstractGraphical abstract for this article
       
  • Stress-based tool-path planning methodology for fused filament fabrication
    • Abstract: Publication date: Available online 13 January 2020Source: Additive ManufacturingAuthor(s): Lingwei Xia, Sen Lin, Guowei MaAbstractTool-path planning has a considerable impact on the quality of components printed by fused filament fabrication (FFF). This research proposes a path generation strategy based on the orientations of the maximum principal stresses. According to stress calculations from finite element analysis (FEA) of the components, tool-paths, which are programmed as parallel to the maximum principal stress directions, are constructed with the depth-first search (DFS) method and a connection criterion. The breakpoints in the tool-paths are then eliminated by connecting adjacent tool-paths. The Dijkstra algorithm is engaged to reduce the nozzle jump distance and shorten the production time. Stretching tests of different specimens printed with the developed path generation algorithms demonstrate that the model with the stress-based path has better mechanical performance. The digital image correlation (DIC) method and scanning electron microscopy (SEM) are employed to observe the fracture processes and fracture surfaces, respectively. Corresponding results of DIC and SEM reveal that different path filling forms exhibit variable failure patterns because of filament anisotropy. The filling fraction is calculated and indicates that the deposition quality of the advanced path is not compromised. This work provides a synthesis methodology for improving the mechanical performance of 3D printing products.
       
  • Additive manufacturing technology and business model change – a
           review of literature
    • Abstract: Publication date: Available online 13 January 2020Source: Additive ManufacturingAuthor(s): Jyrki Savolainen, Mikael CollanAbstractThis paper is a literature review on business models used in the additive manufacturing industry. We focus the investigation by categorizing the effects additive manufacturing into four classes by looking at incremental and disruptive applications in closed and in open market models. The economic feasibility of these applications is critically discussed on the background of the existing literature. Additive manufacturing business models is an emerging area of research, where tangible, case-based evidence is still rare, and the views on the business potential of additive manufacturing technologies are strongly divided.
       
  • Stereolithography (SLA) 3D printing of an antihypertensive polyprintlet:
           Case study of an unexpected photopolymer-drug reaction
    • Abstract: Publication date: Available online 13 January 2020Source: Additive ManufacturingAuthor(s): Xiaoyan Xu, Pamela Robles-Martinez, Christine M. Madla, Alvaro Goyanes, Fanny Joubert, Abdul W. Basit, Simon GaisfordThe introduction of three-dimensional (3D) printing in the pharmaceutical arena has caused a major shift towards the advancement of modern medicines, including drug products with different configurations and complex geometries. Otherwise challenging to create via conventional pharmaceutical techniques, 3D printing technologies have been explored for the fabrication of multi-layer oral dosage forms to reduce pill burden and improve patient adherence. In this study, stereolithography (SLA), a vat polymerisation technique, was used to manufacture a multi-layer 3D printed oral dosage form incorporating four antihypertensive drugs including irbesartan, atenolol, hydrochlorothiazide and amlodipine. Although successful in its fabrication, for the first time, we report an unexpected chemical interaction between a photopolymer and drug. Fourier Transform Infrared (FTIR) spectroscopy and Nuclear Magnetic Resonance (NMR) spectroscopy confirmed the occurrence of a Michael addition reaction between the diacrylate group of the photoreactive monomer and the primary amine group of amlodipine. The study herein demonstrates the importance of careful selection of photocurable resins for the manufacture of drug-loaded oral dosage forms via SLA 3D printing technology. In addition, this work highlights the potential safety-associated challenges for the successful adoption of SLA towards the development of drug delivery platforms in the pharmaceutical arena.Graphical abstractGraphical abstract for this article
       
  • Data-driven modeling of thermal history in additive manufacturing
    • Abstract: Publication date: Available online 12 January 2020Source: Additive ManufacturingAuthor(s): Mriganka Roy, Olga WodoAbstractAdditive manufacturing (AM) has the potential to construct complex geometries through the simple and highly repetitive process of layer-by-layer deposition. The process is repetitive and fully automated, but the interactions between layers during deposition are tightly coupled. To unravel these interactions, the computational models of the manufacturing process are critically needed. However, current state-of-the-art physics-based models are computationally demanding and cannot be used for any realistic optimization. To address this challenge, we built a surrogate model (SM) of thermal profiles that significantly reduced the computational cost. We built this model based on the observation that any AM process exhibits a high level of redundancy and periodicity, making it an ideal problem for machine learning and surrogate modeling. We introduced a unique geometry representation that is the key insight for this work. Rather than directly using the part geometry, we directly use the gcode and translate it into a set of features (local distances from heat sources, e.g. extruder, and sinks, e.g. cooling surfaces). This set of features is directly used as an input for the SM of thermal history. Since this set can be calculated a priori from GCode, the explicit geometry considerations are largely factored out. Moreover, we leveraged the analytical solution to the moving heat source model to determine heat influence zone (HIZ). The size of HIZ allows deciding a priori what should be the cardinality of the distance sets. We showed that for fused filament fabrication, the size of HIZ is small; thus, the number of input variables for the SM is small as well. To build the SM, we first generated the thermal data using a physics-based model and use it to train an artificial neural network model. We trained the SM and demonstrate its high predictive power and low computational cost. Specifically, we demonstrated the capabilities of our model to construct the thermal history for points at the interfaces between roads, with acceptable accuracy (error below 5.0%) in almost real time (0.034 s). With such performance, this model opens the possibility of optimization as well as process planning, and in-situ monitoring for closed-loop control.
       
  • Effects of geometry constraints and fiber orientation in field assisted
           extrusion-based processing
    • Abstract: Publication date: Available online 11 January 2020Source: Additive ManufacturingAuthor(s): Madhuparna Roy, Phong Tran, Tarik Dickens, Bryan D. QuaifeAbstractComposite manufacturing processes adapted for assisted-additive manufacturing (AM) have recently been proposed. Extrusion-based AM utilizes shear-driven alignment in producing printed structures where polymers and fibers naturally align parallel to the material flow. Convergent flow geometries become the dominant processing route for thermoplastic-melts and thermoset polymer extrusions. For rotational fibers, the phenomenon known as Jeffrey orbits poses issues during extrusion through a convergent channel, resulting in a randomized fiber architecture. Methods of minimizing Jeffrey orbits include the application of an additional external force such as a magnetic field to arrest or counteract the rotation. This work explores a combination of magnetic forces in conjunction with adjusted channel geometries using theory and experimental observations. The findings suggest the ability to alter fiber orientation in flow in a 300 cP viscosity matrix by modifying the extrusion channel geometry.
       
  • Additive Manufactured Graphene Composite Sierpinski Gasket Tetrahedral
           Antenna for Wideband Multi-Frequency Applications
    • Abstract: Publication date: Available online 11 January 2020Source: Additive ManufacturingAuthor(s): William Clower, Matthew J. Hartmann, Joshua B. Joffrion, Chester G. WilsonHere we report a pre-fractal antenna design based on the Sierpinski tetrahedron that has been developed with additive manufacturing. The Sierpinski tetrahedron-based antenna was simulated with finite element method (FEM) modeling and experimentally tested to highlight its potential for wideband communications. The Sierpinski tetrahedron-based antennas were fabricated by two methods, the first involves printing the antenna out of acrylonitrile butadiene styrene (ABS), followed by spin casting a coating of an ABS solution containing graphene flakes produced through electrochemical exfoliation, the second method involves 3D printing the antenna from graphene-impregnated polylactic acid (PLA) filament directly without any coating. Both fabrication methods yield a conductive medium to enable receiving EM signals in the low GHz frequency range with measured input return losses higher than 35 dB at resonance. These antennas incorporate the advantages of 3D printing which allows for rapid prototyping and the development of devices with complex geometries. Due to these manufacturing advantages, self-similar antennas like the Sierpinski tetrahedron can be realized which provide increased gain and multi-band performance.Graphical abstractGraphical abstract for this article
       
  • Correlations of Cracking with Scan Strategy and Build Geometry in Electron
           Beam Powder Bed Additive Manufacturing
    • Abstract: Publication date: Available online 11 January 2020Source: Additive ManufacturingAuthor(s): Y.S. Lee, M.M. Kirka, J. Ferguson, V.C. PaquitAbstractThe extension of metal additive manufacturing (AM) to non-weldable Ni-based superalloys remains a challenge for the electron beam melting process. Various cracking mechanisms, including solidification, liquation, strain-age, and ductility dip cracking, make it difficult to fabricate traditionally non-weldable Ni-based superalloys using the AM process. Because airfoil geometries are highly complicated, the correspondingly complex thermal signatures lead to various types of cracking in geometries that are under severe mechanical restraints during the printing process. This work aims to understand the correlations between cracking, scan strategy, and part geometry in airfoil geometries. Crack locations were monitored via an in-situ near-infrared camera during printing. A part-scale finite element method (FEM) was used to reveal cracking mechanisms. New scan strategies that avoided cracking were utilized in an FEM simulation. The present work demonstrates the potential for scan strategy optimization to manipulate stress distribution and the resultant microstructure of parts for industrial applications.
       
  • A New Method for Predicting Cracking at the Interface between Solid and
           Lattice Support During Laser Powder Bed Fusion Additive Manufacturing
    • Abstract: Publication date: Available online 10 January 2020Source: Additive ManufacturingAuthor(s): Hai T. Tran, Qian Chen, Jonathan Mohan, Albert C. ToAbstractFor laser powder bed fusion (L-PBF) additive manufactured (AM) metals, residual stress-induced cracking often occurs at the interface between the solid and lattice support, and hence it is important to characterize the as-built critical J-integral of the interface to prevent cracking to occur. However, the standard testing method for the critical J-integral of the interface (ASTM E1820-01) does not work well in this situation for four reasons: 1) standard test blocks consisting of half solid and half lattice support crack during the printing process; 2) even after reinforcing the block with side walls to prevent cracking, post-stress relief causes the yield strength to change significantly, which would affect J-integral significantly; 3) post-build machining processes to obtain the required standard specimen geometry release a significant amount of residual stress, which also gives incorrect J-integral value; 4) the interface is so brittle that it is very difficult to machine it to the required standard configuration. Hence a more effective method that combines printing experiments and residual stress simulation is proposed to determine the as-built critical J-integral of the interface. First, a number of rectangular block specimens with lattice supports of identical height overlaid by solids of different heights are built by L-PBF in Inconel 718 in order to determine the critical height that the block would crack. Next, the experimentally-validated modified inherent strain method is utilized to simulate residual stress and compute the critical J-integral at where the interfacial cracking occurs. The proposed method is subsequently validated using the obtained critical J-integral to predict cracking in different geometries. This method eliminates the uncertainties associated with stress relaxation by heat treatment and machining on mechanical properties, as well as sheds light on crack prediction for as-built L-PBF components.
       
  • Wire and arc additive manufacturing of a Ni-rich NiTi shape memory alloy:
           microstructure and mechanical properties
    • Abstract: Publication date: Available online 10 January 2020Source: Additive ManufacturingAuthor(s): Z. Zeng, B.Q Cong, J.P. Oliveira, W.C Ke, N. Schell, B. Peng, Z.W. Qi, F.G Ge, W. Zhang, S.S. AoAbstractWire and Arc Additive Manufacturing (WAAM) was used for fabrication of NiTi parts using a commercialy available Ni-rich NiTi wire as the feedstock material. The as-built parts are near fully austenitic at room temperature as confirmed by differential scanning calorimetry, X-ray diffraction and superelastic cycling. The as-built microstructure changed from collumnar, in the first deposited layers, to equiaxed in the last deposited ones as a result of the different thermal cycle conditions. This is the first work where WAAM NiTi parts exhibit superelastic behavior under tensile conditions, highlighting the potential use of the technique for the creation of parts shaped in a complex manner based on this material and process. The potential to use WAAM for deposition of advanced functional materials is demonstrated.
       
  • Enhanced fatigue characteristics of a topology-optimized porous titanium
           structure produced by selective laser melting
    • Abstract: Publication date: Available online 9 January 2020Source: Additive ManufacturingAuthor(s): Y.J. Liu, D.C. Ren, S.J. Li, H. Wang, L.C. Zhang, T.B. SercombeThe fatigue properties are critical considerations for porous structures, and most of the existing porous materials have unsatisfactory performances due to a lack of structural optimization. This work shows that a topology-optimized structure fabricated by selective laser melting using commercial-purity titanium (CP-Ti) exhibits excellent fatigue properties with an ultra-high normalized fatigue life of ∼0.65 at 106 cycles and at a low density of 1.3 g/cm3. The main factors affecting fatigue, i.e., material properties and a porous structure were studied. Both the factors can affect the fatigue crack initiation time, thereby affecting the fatigue life. Because of twinning that occurred during the fatigue process, the porous CP-Ti samples exhibit a high plasticity. In addition, the fatigue crack propagation rate is significantly reduced because of the high plasticity of the CP-Ti material and the occurrence of fatigue crack deflection.Graphical Graphical abstract for this article
       
  • Effects of Electron Beam Manufacturing Induced Defects on Fracture in
           Inconel 718
    • Abstract: Publication date: Available online 9 January 2020Source: Additive ManufacturingAuthor(s): D.M. Bond, M.A. ZikryAbstractThe effects of electron beam manufactured (EBM) process-induced defects on local microstructural failure initiation and propagation in IN 718 have been investigated. Predictions for transgranular fracture, based on local cleavage plane stresses, and for intergranular fracture, based on dislocation-grain boundary (GB) interactions and evolving dislocation pileups, were combined with a crystalline dislocation-density plasticity approach to understand the influence of AM process-induced defects, such as porosity, NbC precipitates, and regions of dry powder. High local stresses along the peripheries of pores caused crack nucleation, and mismatches in deformation behavior between NbC precipitates and the surrounding matrix led to local stress gradients that induced crack nucleation and decohesion at precipitate/matrix interfaces. Regions of unmelted powder had significant stress accumulations that initiated failure at low nominal strains. Failure due to high localized stresses near regions of unmelted powder was dominant over precipitate/matrix decohesion and crack nucleation near pore peripheries. Based on the predictions, the mechanical behavior of AM alloys is governed by local dislocation-density evolution near process-induced defects, which preferentially nucleate material failure. Furthermore, interactions between these different defect types can significantly accelerate failure initiation and propagation.
       
  • The effect of hot isostatic pressure on the corrosion performance of
           Ti-6Al-4V produced by an electron-beam melting additive manufacturing
           process
    • Abstract: Publication date: Available online 9 January 2020Source: Additive ManufacturingAuthor(s): Avi Leon, Galit Katarivas Levy, Tomer Ron, Amnon Shirizly, Eli Aghion
       
  • Investigation of dissolution behavior of Laves phase in Inconel 718
           fabricated by laser directed energy deposition
    • Abstract: Publication date: Available online 8 January 2020Source: Additive ManufacturingAuthor(s): Shang Sui, Jing Chen, Zuo Li, Haosheng Li, Xuan Zhao, Hua TanThe mechanical properties of Inconel 718 are closely related to the morphology and size of the Laves phase, which must be quantitatively controlled to change the effect of the Laves phase from deleterious to beneficial. In this study, post-heat treatment was used to regulate the morphology and size of the Laves phase in Inconel 718 fabricated using laser directed energy deposition, and the dissolution behavior of the Laves phase during solution heat treatment was investigated. The results indicated that the sharp corners and grooves of the Laves phase preferentially dissolved, causing the morphology of the Laves phase to change from a long-striped to granular shape during dissolution. The dissolution kinetics of the Laves phase were also investigated using the Johnson–Mehl–Avrami–Kolmogorov and Singh–Flemings models. The initial stage of dissolution was controlled by both the long-range diffusion of Nb and the interfacial reaction. However, with decreasing degree of Nb segregation, the interfacial reaction became dominant.Graphical Graphical abstract for this article
       
  • Layer by layer coating for bio-functionalization of additively
           manufactured meta-biomaterials
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): S. Amin Yavari, M. Croes, B. Akhavan, F. Jahanmard, C.C. Eigenhuis, S. Dadbakhsh, H.C. Vogely, M.M. Bilek, A.C. Fluit, C.H.E. Boel, B.C.H. van der Wal, T. Vermonden, H. Weinans, A.A. ZadpoorAdditive manufacturing has facilitated fabrication of complex and patient-specific metallic meta-biomaterials that offer an unprecedented collection of mechanical, mass transport, and biological properties as well as a fully interconnected porous structure. However, applying meta-biomaterials for addressing unmet clinical needs in orthopedic surgery requires additional surface functionalities that should be induced through tailor-made coatings. Here, we developed multi-functional layer-by-layer coatings to simultaneously prevent implant-associated infections and stimulate bone tissue regeneration. We applied multiple layers of gelatin- and chitosan-based coatings containing either bone morphogenetic protein (BMP)-2 or vancomycin on the surface of selective laser melted porous structures made from commercial pure Titanium (CP Ti) and designed using a triply periodic minimal surface (i.e., sheet gyroid). The additive manufacturing process resulted in a porous structure and met the the design values comparatively. X-ray photoelectron spectroscopy spectra confirmed the presence and composition of the coating layers. The release profiles showed a continued release of both vancomycin and BMP-2 for 2–3 weeks. Furthermore, the developed meta-biomaterials exhibited a very strong antibacterial behavior with up to 8 orders of magnitude reduction in both planktonic and implant-adherent bacteria and no signs of biofilm formation. The osteogenic differentiation of mesenchymal stem cells was enhanced, as shown by two-fold increase in the alkaline phosphatase activity and up to four-fold increase in the mineralization of all experimental groups containing BMP-2. Eight-week subcutaneous implantation in vivo showed no signs of a foreign body response, while connective tissue ingrowth was promoted by the layer-by-layer coating. These results unequivocally confirm the superior multi-functional performance of the developed biomaterials.Graphical abstractGraphical abstract for this article
       
  • Simulation study of the spatter removal process and optimization design of
           gas flow system in laser powder bed fusion
    • Abstract: Publication date: Available online 7 January 2020Source: Additive ManufacturingAuthor(s): Xiaobing Zhang, Bo Cheng, Charles TuffileAbstractAn urgent need in the laser powder bed fusion (LPBF) process is to efficiently remove emissions from or around the moving melt pool since the powder bed contamination by spatter can potentially damage fabricated part quality. The objective of this study is to propose new designs of the gas flow system in the build chamber to enhance the removability of spatter. Specifically, a Computational Fluid Dynamics (CFD) model for the LPBF gas flow system has been developed to simulate the complicated flow behavior inside the build chamber. The movement of spatter has been calculated by the Discrete Phase Model (DPM). The fully coupled CFD-DPM fluid-particle interaction method has been applied to capture the influence of gas flow on solid particles accurately. Additionally, an analytical expression is utilized to obtain the threshold velocity of inert gas flow upon the powder bed. The spatter distribution in a generic gas chamber design was studied. It was found that the Coanda effect, a gas flow downward tendency toward the substrate, can have a significant impact on the spatter removal process. With the proposed new designs, the Coanda effect is minimized, and most of the spatters can be removed from the build region without blowing up powder bed particles.
       
  • 3D BIOPRINTING OF ANISOTROPIC ENGINEERED TISSUE CONSTRUCTS WITH
           ULTRASONICALLY INDUCED CELL PATTERNING
    • Abstract: Publication date: Available online 7 January 2020Source: Additive ManufacturingAuthor(s): Parth Chansoria, Rohan ShirwaikerAs 3D bioprinting continues to evolve as a promising alternative to engineer complex human tissues in-vitro, there is a need to augment bioprinting processes to achieve the requisite cellular and extracellular organizational characteristics found in the original tissues. While the cell distribution within bioinks is typically homogeneous, incorporating appropriate cellular patterning within the bioprinted constructs is an essential first step towards the eventual formation of anisotropically organized tissue matrix essential to its biomechanical form and function. This study describes a new bioprinting technique that uses ultrasonic standing bulk acoustic waves (SBAW) to organize cells into controllable anisotropic patterns within viscous bioinks while maintaining high cell viability. First, we develop a 3D computational model to discern the SBAW pressure pattern in response to multiple ultrasonic frequencies (0.71 – 2 MHz). We then experimentally analyze the patterns and viabilities of human adipose-derived stem cells (hASC) and human osteosarcoma cells (MG63) in alginate as a function of the SBAW frequency. Computational results indicate the formation of parallel pressure strands with higher pressure amplitudes near the bottom of the deposited layer, which is corroborated by experimental images of cell patterning. The inter-strand spacing is found to be affected by the frequency (p 
       
  • Experimental and analytical study of the polymer melt flow through the
           hot-end in material extrusion additive manufacturing
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Marcin P. Serdeczny, Raphaël Comminal, David B. Pedersen, Jon SpangenbergAbstractMaterial extrusion additive manufacturing utilizes a thermoplastic polymer in the form of a solid filament as a built material. The polymer melts inside the hot-end channel and flows under the pressure generated by the filament feeding force. The flow of polymer through the hot-end is not fully understood yet, as it involves many complex phenomena, such as phase transition, shear rate and temperature dependent viscosity, as well as viscoelastic effects. In this paper, we investigate experimentally the filament feeding force, as a function of the feeding rate, for different materials (PLA and ABS), liquefier temperatures, nozzle diameters, and lengths of the liquefier. Two extrusion regimes are identified: a linear regime with a stable flow, and a non-linear regime with fluctuations in the feeding force, which are concomitant to unstable extrusion. Increasing the liquefier length and liquefier temperature are found to extend the linear extrusion regime. It is shown that the filament feeding force predicted by the analytical models available in the literature deviates from the measurements, especially at high feeding rates. A model solely based on heat transfer considerations is proposed to estimate the maximum feeding rate before the extrusion becomes unstable. The modelling results agree well with the measurements. The model can be used to select the hot-end design as well as appropriate printing parameters.
       
  • Elucidation of bubble evolution and defect formation in directed energy
           deposition based on direct observation
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Peiyu Zhang, Xin Zhou, Xing Cheng, Hongmei Sun, Haiqiang Ma, Yinghong LiAbstractSurface pore defects are always formed during directed energy deposition processes, which may stem from entrapped gas bubbles. Such defects have detrimental effects on the build quality and performance of safety-critical metal parts. Despite previous experimental and theoretical studies devoted to this subject, direct observations of the dynamic behavior of gas bubbles and elucidation of how they form surface pore defects have not yet been achieved. In this work, the relationships between surface pore defects and the bubbles originating on the melt pool surface were carefully studied using high-speed photography at up to 20,000 frames per second. This allowed us to observe the presence of two types of bubble evolution, namely bubble retention and explosion. The appearance of surface pores was a result of dynamic competition between bubble explosion and solidification of the surrounding melt, where the final location of the surface pores is determined by the melt convection and the boundary motion of the melt pool. In the case of single-track deposition, complex thermocapillary convection drives gas bubble diffusion, and pore defects cluster along the lateral edge. In the case of multi-track deposition, surface pore defects were more likely to occur on the last track due to the gravity-driven flow effect that is determined by the track path and overlap.
       
  • A 3D printing strategy for fabricating in situ topographical
           scaffolds using pluronic F-127
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): JiUn Lee, SooJung Chae, Hyeongjin Lee, Geun Hyung KimAbstractTopographical cues are one of the prerequisites for successful regeneration of muscle tissue. However, fabrication methods using three-dimensional (3D) bioprinters are limited by the simple nozzle-based extrusion or uncontrollability of photo-reactive systems. Hence, most studies on inducing topographical cues were focused on two-dimensional (2D) surface structures and based on imprinting and soft-lithography processes. Although 2D patterned surfaces provide outstanding insight into optimal patterned architectures by facilitating the analysis of various myoblast responses, it can be difficult to achieve complex 3D structures with microscale topographical cues. For this reason, we propose a new strategy for obtaining topographical cues in 3D printed synthetic biopolymers for regenerating muscle tissue. A uniaxially aligned pattern was obtained on the struts of the matrix composed of poly(ε-caprolactone) (PCL) or poly(lactic-co-glycolic acid) (PLGA), by taking advantage of the immiscible rheological properties and flow-induced force in the dispersed pluronic F-127 phase (sacrificial material) and matrix materials. To observe the effects of the topographical cues of the fabricated structures on cellular responses, including myogenic gene expression, myoblasts (C2C12 cells) were cultured for various lengths of time, and the micropatterned structure provided a rapid myotube formation, compared to the control, which did not undergo the microscale patterning process. In addition, to extend the printing technique, the alignment of primary tenocytes cultured on the uniaxial patterned structure was evaluated.
       
  • Biomechanical influence of structural variation strategies on functionally
           graded scaffolds constructed with triply periodic minimal surface
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Xiang-Yu Zhang, Xing-Chen Yan, Gang Fang, Min LiuAbstractThe rapid development of additive manufacturing technology makes it possible to fabricate parts with complex inner structures, especially for functionally graded scaffolds (FGS) in the field of bone tissue engineering. The parametric design of FGS is of great significance to the in-depth study of the effects of structural parameters of porous bone scaffolds on their mechanical properties and rehabilitation of patients. The present study proposed a parametric design method for FGS using a triply periodic minimal surface (TPMS). Graded structures were obtained by altering the Gyroid TPMS equation. Uniform and functionally graded samples were fabricated using selective laser melting of Ti-6Al-4V powder. The FGSs successfully realized flexible control of structural parameters and showed comparable mechanical properties and permeability with natural bone tissue. Furthermore, heat treatment was verified to be an effective way to improve the ductility of TPMS-FGS. The deformation process and principal strain distribution of the FGSs were elucidated using a digital image correlation method. The FGSs proposed in the present study showed great potential in orthopedic implant or bone-substituting biomaterials.
       
  • Drop-on-demand high-speed 3D printing of flexible milled carbon
           fiber/silicone composite sensors for wearable biomonitoring devices
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Elham Davoodi, Haniyeh Fayazfar, Farzad Liravi, Elahe Jabari, Ehsan ToyserkaniThree-dimensionally (3D) printed flexible piezoresistive composite sensors have provided valuable solutions for the personalized therapeutic development due to their promising capability in biomonitoring applications. Silicone rubber (SR) matrix is an important candidate to enable flexibility to the 3D printed devices. However, 3D printing of silicone inks blended with conductive fillers is limited due to the high viscosity, long curing time, and high percolation threshold. In the present study, a novel high-speed material jetting (MJ) 3D printing of high-viscosity conductive inks based on the mixture of a UV crosslinkable silicone rubber and milled carbon fibers (MCF) is demonstrated. The MCF content was optimized for printability, UV curability, and electrical conductivity. The sensors (with 30 wt. % MCF content) show high flexibility and foldability as well as a high resistance sensitivity to sever bending tests. The stretchability of 3D printed sensors was further improved by sandwiching the MCF/SR sensing layer between the SR layers. The electromechanical evaluation of the sandwiched MCF/SR sensors (S-MCF/SR) confirmed the high piezoresistive sensitivity of sensors (gauge factor in order of ∼400). Finally, the 3D printed sensors were employed for monitoring human joint motions to demonstrate the potential application in monitoring biosignals.Graphical abstractGraphical abstract for this article
       
  • Influence of processing and microstructure on the local and bulk thermal
           conductivity of selective laser melted 316L stainless steel
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Jacob C. Simmons, Xiaobo Chen, Arad Azizi, Matthias A. Daeumer, Peter Y. Zavalij, Guangwen Zhou, Scott N. SchiffresAbstractMaterial properties of parts made via selective laser melting are not the same as the well-established properties for bulk base materials, due to the unique processes used to produce the parts. Meanwhile, additive manufacturing is increasingly being used for heat exchangers and heat removal devices, which demand high thermal conductivities. The thermal properties are also important for many non-destructive testing technologies. The thermal conductivity of selective laser melted 316 L stainless steel was studied as a function of processing conditions and build orientation. The porosity and thermal conductivity were measured versus processing conditions. A critical energy density of 44.4 J/mm3 was observed below which the porosity increased and the thermal conductivity decreased. For the lowest-porosity sample, the local thermal conductivity map taken with frequency domain thermoreflectance showed a variation in the stainless steel thermal conductivity between 10.4 and 19.8 W/m-K, while the average thermal conductivity of 14.3 W/m-K from the thermal conductivity map agreed, within measurement uncertainty, with the bulk thermal conductivity measurements. The thermal conductivity trend was not fully explained by the porosity, as effective medium models fail to predict the trend. Amorphous stripes in the selective laser melted stainless steel grains were identified by transmission electron microscopy. These amorphous regions also resulted in decreased x-ray diffraction intensities with increasing porosity. The amorphous regions are hypothesized to lower the thermal conductivity at faster laser scanning speeds due to less time at elevated temperatures. We also found that in-print plane and through-print plane thermal conductivities have the same value when the energy density is greater than this critical amount. When the energy density reduces below this critical amount, the in-plane conductivity exceeds the through-plane.
       
  • Microstructure characteristics and failure mechanisms of Ti-48Al-2Nb-2Cr
           titanium aluminide intermetallic alloy fabricated by directed energy
           deposition technique
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Jiawei Wang, Qian Luo, Huaming Wang, Yu Wu, Xu Cheng, Haibo TangThe high-energy input and thermal history during additive manufacturing lead to complex phase transformations in titanium aluminide alloy. This study mostly focuses on determining the solid-state phase transformation mechanisms during laser deposition and the failure mechanisms of alloys using molecular dynamics simulations. Because of the directional temperature gradient, columnar grains with fully lamellar microstructures are formed first after solidification. A narrow region just below the melting pool is reheated to high temperatures, thus enhancing the precipitation of new equiaxed grains. Multiple thermal cycles in the α + γ phase region promote the formation of massive γ phases (γm) at the grain boundaries. Finally, a nearly lamellar microstructure of alternating columnar and equiaxed grains with γm phases is formed. The deposited titanium aluminide alloy has good room and high-temperature (760 °C) tensile properties of 545 ± 9 and 471 ± 37 MPa, with elongations of 1.50 % ± 0.47 % and 1.50 % ± 0.45 %, respectively. The room and high-temperature samples both fail in the columnar grain region. The molecular dynamics simulations suggest that the interface between α2 and γm is the weakest, especially in the case of semicoherent interfaces (6° angle in the [1–10] direction), which provides good nucleation sites for cracks. Although the equiaxed grain regions contain several γm–α2 interfaces, the samples still fail in the columnar grain regions due to the increase in the cracking distance in the equiaxed regions caused by randomly oriented α2 + γ lamellae and the comparably good plasticity of the γm phases.Graphical abstractMultiple thermal cycles in the α + γ phase region promote the formation of a massive γ phase (γm) at the grain boundaries. Finally, a nearly lamellar microstructure of alternating arrangement of columnar and equiaxed grains with γm phases is formed. Based on the relations among the orientations of the γm, γ, and α2 phases, five interface structure models can be established for the molecular dynamics simulations of TiAl alloy fabricated by directed energy deposition, which can be used to accurately predict the location of the crack nucleation sites during the tensile test. Furthermore, we revealed, for the first time, that the interface between α2 and γm is the weakest, especially in the case of semicoherent interfaces (6° angle in the [1–10] direction), which provides good nucleation sites for cracks.Graphical abstract for this article
       
  • Enabling high-performance 3D printing of Al powder by decorating with high
           laser absorbing Co phase
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Kang Geng, Yafeng Yang, Shaofu Li, R.D.K. Misra, Qingshan ZhuAbstractPure Al with high laser reflectivity is essentially incompatible with laser powder bed fusion. The retention of a large number of unmelted particles leads to inferior geometrical quality and mechanical properties of printed pure Al parts. In the present study, we propose decorating Al with a small amount of high laser absorbing Co nanoparticles on the surface of Al powders to reduce laser reflectivity and improve printability. The experimental results confirmed that the approach was very effective. The near homogenous dispersion of Co slightly modified the surface chemical composition and roughened the powder surface. This approach completely melted the particles and eliminated the internal pores, thereby favorably tuning the geometrical dimensions. Additionally, the introduction of Co provided solid solution strengthening and precipitation hardening via dispersion of second-phase Al9Co2 with a coherent interfacial relationship with the Al matrix. The tensile properties of printed Al parts were comparable to commercial medium-strength Al alloys at an optimal Co-content of 0.5 wt.%.
       
  • The influence of oxygen on the chemical composition and mechanical
           properties of Ti-6Al-4V during laser powder bed fusion (L-PBF)
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Kai Dietrich, Johannes Diller, Sophie Dubiez-Le Goff, Dominik Bauer, Pierre Forêt, Gerd WittIn Laser powder bed fusion (L-PBF), metal powders, sensitive to humidity and oxygen, like AlSi10Mg or Ti-6Al-4 V are used as starting material. Titanium-based materials are influenced by oxygen and nitrogen due to the formation of oxides and nitrides, respectively. During this research, the oxygen concentration in the build chamber was controlled from 2 ppm to 1000 ppm using an external measurement device. Built Ti-6Al-4 V specimens were evaluated regarding their microstructure, hardness, tensile strength, notch toughness, chemical composition and porosity, demonstrating the importance of a stable atmospheric control. It could be shown that an increased oxygen concentration in the shielding gas atmosphere leads to an increase of the ultimate tensile strength by 30 MPa and an increased (188.3 ppm) oxygen concentration in the bulk material. These results were compared to hot isostatic pressed (HIPed) samples to prevent the influence of porosity. In addition, the fatigue behavior was investigated, revealing increasingly resistant samples when oxygen levels in the atmosphere are lower.Graphical abstractGraphical abstract for this article
       
  • Quantifying quality of 3D printed clay objects using a 3D structured light
           scanning system
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Kwangwoo Wi, Vignesh Suresh, Kejin Wang, Beiwen Li, Hantang QinAbstractThree-dimensional (3D) printing, or additive manufacturing, has been increasingly used in many fields, including the medicine, food, sensing, metal, automotive, and construction industries. Regardless of its growing applications, there are few of methods, guidelines, and specifications for measuring and quantifying the qualities of 3D printed objects. This is particularly so for objects those are too small, too large, and/or too fragile to be handled manually. In this study, for the first time, a non-contact, and non-destructive measurement method, a 3D structured light scanning system (3D-SLSS), was employed for evaluating the printing qualities of clay objects with different levels of visual defects (e.g., roughness and distortion). 3D scanned images of these clay samples were developed using 3D-SLSS. Then, they were sliced along their sides (perpendicular to the base) to generate a number of two-dimensional (2D) plots, from which various parameters (e.g., sample total height [Htotal], outer diameter [DMouter], layer thickness [TL], layer width, [(WL], surface angle [Sα], semi-cross-sectional area [XA], and surface roughness [R]) were measured. These measurements were then compared with the designed values. The percentages of the differences between the measured and designed values were used to develop a diagnosed area of deficiency, by which the overall qualities of the printed samples were quantified. The results illustrated that all the printed samples exhibited certain differences between their measured and designed values, even for those that appeared well printed. Compared with the designed object, the printed samples generally had reduced total height, diameter, and layer thickness; increased layer width; measurable distortion; and visible surface roughness. Many of these were largely because the freshly printed clay deformed under the weight of the layers above. The distortion angle and area are two necessary parameters for quantifying the degree of distortion of a printed sample. The diagnosed area of deficiency can well describe the overall qualities of the printed samples. 3D-SLSS is a relatively simple, fast, and inexpensive characterization method. Moreover, it can be conveniently extended to various industries for quality control of diverse 3D printing products.
       
  • Development of poly(L-lactide) (PLLA) microspheres precipitated from
           triacetin for application in powder bed fusion of polymers
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Maximilian A. Dechet, Anna Demina, Lea Römling, Juan S. Gómez Bonilla, Franz J. Lanyi, Dirk W. Schubert, Andreas Bück, Wolfgang Peukert, Jochen SchmidtIn this work, the development and processing behavior of poly(L-lactide) (PLLA) particles for powder bed fusion (PBF) of polymers obtained via a green and sustainable process route are thoroughly studied. Liquid-liquid phase separation and precipitation from triacetin, a non-toxic solvent, are applied for the production of highly spherical PLLA particles of excellent flowability. Starting from the measured cloud-point diagram of the PLLA-triacetin system, appropriate temperature profiles for the precipitation process are derived. The effect of process parameters on the product properties is addressed in detail; the PLLA particles are characterized regarding their size distribution and morphology. Furthermore, material properties including thermal behavior (c.f. processing window for powder bed fusion (PBF)) and powder flowability are assessed. The spherical PLLA particles of narrow size distribution display a wide sintering window of 59 K and an excellent flowability due to the intrinsic surface roughness of the particles. Thus, tensile test bars and complex porous gyroid specimens were successfully manufactured via PBF without the need for any additional surface functionalization of the particles with flow agents. The high potential of the newly developed PLLA powders produced via an environmentally friendly approach omitting the use of halogenated or toxic solvents, as well as flowing aids, is demonstrated by mechanical testing of the printed specimens.Graphical abstractGraphical abstract for this article
       
  • Characterization of AISI 304L stainless steel powder recycled in the laser
           powder-bed fusion process
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Austin T. Sutton, Caitlin S. Kriewall, Sreekar Karnati, Ming C. Leu, Joseph W. NewkirkAbstractDuring part fabrication by laser powder-bed fusion (L-PBF), an Additive Manufacturing process, a large amount of energy is input from the laser into the melt pool, causing generation of spatter and condensate, both of which have the potential to settle in the surrounding powder-bed compromising its reusability. In this study, AISI 304 L stainless steel powder is subjected to seven reuses in the L-PBF process to assess the changes in powder properties that occur as a result of successive recycling. The powder was characterized morphologically by particle size and shape distribution measurements, chemically through inert gas fusion for evaluation of oxygen content, and microstructurally by X-ray diffraction for phase identification. The evolution in powder properties was used to explain observed performance differences obtained by the Hausner ratio and a Revolution Powder Analyzer for quantifying flowability. The results show that recycled powder coarsens and becomes more spherical, accrues oxygen, and accumulates delta ferrite as it is reused. Due to the change in powder morphology, recycled powder exhibited improved flowability in comparison to the virgin powder.
       
  • Building free-form thin shell parts using supportless extrusion-based
           additive manufacturing
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Prahar M. Bhatt, Rishi K. Malhan, Pradeep Rajendran, Satyandra K. GuptaAbstractConventional material extrusion additive manufacturing is capable of building complex structures. Overhanging features require the use of support structures. Printing the support structure requires additional time and material. Conventional processes need time to remove support material and may lead to degraded surface finish. The use of support structures can be avoided by dynamically reorienting the build-platform. This paper presents a novel approach to build accurate thin shell parts using supportless extrusion-based additive manufacturing. We describe the layer slicing algorithm, the tool-path planning algorithm, and the neural network-based compensated trajectory generation scheme to use a 3 degree of freedom build-platform and a 3 degree of freedom extrusion tool to build accurate thin shell parts using two manipulators. Such thin shell parts cannot be built without supports by previous supportless AM processes. We illustrate the usefulness of our algorithms by building several thin shell parts.
       
  • Upper bound of feed rates in thermoplastic material extrusion additive
           manufacturing
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Cheng Luo, Xiang Wang, Kalman B. Migler, Jonathan E. SeppalaAbstractIn this work, we develop a simple model to determine the upper bound of feed rates that do not cause jamming in material extrusion additive manufacturing, also known as fused deposition modeling (FDM)™ or fused-filament fabrication (FFF). We first derive a relation between the tube temperature and Péclet number for the solid portion of polymer filaments. We focus on the boundary between the solid and molten polymer in the heated portion of the tube. We find the Péclet number that corresponds to the point at which this boundary makes contact with the nozzle, and identify this as the upper bound of the feed rate. We compare our predictions to experimental results. We find good agreement for tube temperatures sufficiently above the glass-transition temperature, which is the temperature region of typical additive manufacturing.
       
  • A new approach to eliminating enclosed voids in topology optimization for
           additive manufacturing
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Yulin Xiong, Song Yao, Zi-Long Zhao, Yi Min XieAbstractTopology optimization is increasingly used in lightweight designs for additive manufacturing (AM). However, conventional optimization techniques do not fully consider manufacturing constraints. One important requirement of powder-based AM processes is that enclosed voids in the designs must be avoided in order to remove and reuse the unmelted powder. In this work, we propose a new approach to realizing the structural connectivity control based on the bi-directional evolutionary structural optimization technique. This approach eliminates enclosed voids by selectively generating tunnels that connect the voids with the structural boundary during the optimization process. The developed methodology is capable of producing highly efficient structural designs which have no enclosed voids. Furthermore, by changing the radius and the number of tunnels, competitive and diverse designs can be achieved. The effectiveness of the approach is demonstrated by two examples of three-dimensional structures. Prototypes of the obtained designs without enclosed voids have been fabricated using AM.
       
  • Effect of post-treated low-temperature plasma nitriding on the wear and
           corrosion resistance of 316L stainless steel manufactured by laser
           powder-bed fusion
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): M. Godec, Č. Donik, A. Kocijan, B. Podgornik, D.A. Skobir BalantičAbstractDue to the limited wear and corrosion properties of the austenitic stainless steel AISI 316 L, some applications require the benefits of nitriding. The aim of this work was to investigate whether the same positive effect of nitriding could be obtained for 316 L that was additive manufactured using the laser powder-bed fusion process and further solution treated at 1060 °C for 30 min, low-temperature plasma nitrided at 430 °C or both. This study was designed to better understand the additive-manufactured and solution-treated microstructures as well as developing a nitride and a diffusion layer. The comparison of the wear and corrosion resistance, the microhardness and the microstructure changes of the additive-manufactured steel in different post-treated conditions with a commercial steel was carried out. It was found that the post-treated low-temperature plasma nitriding improves the wear and corrosion resistance of the additive-manufactured samples. The obtained values are similar to the values of conventionally fabricated and nitrided 316 L. The solution treating itself (without further nitriding) did not have any significant impact on these properties. It was possible to explain the microstructure at the nano level as well as correlating the wear and corrosion properties.
       
  • On the strain-life fatigue parameters of additive manufactured plastic
           materials through fused filament fabrication process
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Soran Hassanifard, Seyed M. HashemiAbstractIn this paper, the effects of part build directions or raster orientations have been studied on the strain-life fatigue parameters of a wide range of 3D printed plastic materials. These materials have been manufactured through Fused Filament Fabrication (FFF), also known under its trademarked name Fused Deposition Modeling (FDM). To do so, precise analyses of fatigue data with the Ramberg-Osgood form of stress-strain curves were utilized through a strain-based approach to fatigue. The effects of different load ratios and types of loading on fatigue parameters have also been studied. Materials considered in this study were Ultem 9085, Polycarbonate (PC), and Polylactic Acid (PLA). Additive manufactured plastic parts that are FDM-processed exhibited large anisotropy of strain-life fatigue parameters. Hence, the upper and lower bounds for fatigue life prediction were introduced based on the strongest and weakest part build directions or raster orientations of 3D printed materials. For all materials studied in the present paper, fill densities, which seem to have significant impact on fatigue strength of 3D printed parts, have been selected based on the maximum fatigue strength of each part. Results showed that, in some build orientations, the transition fatigue life does not exist. In other orientations, in which the plastic strain components are high enough, transition fatigue lives vary roughly between 20–400 cycles. This means that if the part design in very low cycle fatigue regime is of interest, plastic strains and more complicated plasticity analysis are needed. Results show that the load ratio has no major impact on the fatigue parameters of 3D printed PC parts. In addition, changing in the loading type from tensile fatigue to rotating bending fatigue can significantly impact the fatigue strength coefficient of 3D printed PLA specimens however, it does not noticeably alter the fatigue strength exponents.
       
  • Design and characterisation of an additive manufacturing benchmarking
           artefact following a design-for-metrology approach
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Vicente M. Rivas Santos, Adam Thompson, Danny Sims-Waterhouse, Ian Maskery, Peter Woolliams, Richard LeachAbstractWe present the design and characterisation of a high-speed sintering additive manufacturing benchmarking artefact following a design-for-metrology approach. In an important improvement over conventional approaches, the specifications and operating principles of the instruments that would be used to measure the manufactured artefact were taken into account during its design process. With the design-for-metrology methodology, we aim to improve and facilitate measurements on parts produced using additive manufacturing. The benchmarking artefact has a number of geometrical features, including sphericity, cylindricity, coaxiality and minimum feature size, all of which are measured using contact, optical and X-ray computed tomography coordinate measuring systems. The results highlight the differences between the measuring methods, and the need to establish a specification standards and guidance for the dimensional assessment of additive manufacturing parts.
       
  • Polymer-derived SiOC replica of material extrusion-based 3-D printed
           plastics
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Apoorv Kulkarni, Gian Domenico Sorarù, Joshua M. PearceA promising method for obtaining ceramic components with additive manufacturing (AM) is to use a two-step process of first printing the artifact in polymer and then converting it to ceramic using pyrolysis to form polymer derived ceramics (PDCs). AM of ceramic components using PDCs has been demonstrated with a number of high-cost techniques, but data is lacking for fused filament fabrication (FFF)-based 3-D printing. This study investigates the potential of lower-cost, more widespread and accessible FFF-based 3-D printing of PDCs. Low-cost FFF machines have a resolution limit set by the nozzle width, which is inferior to the resolutions obtained with expensive stereolithography or selective laser sintering AM systems. However, to match the performance a partial PDC conversion is used here, where only the outer surface of the printed polymer frame is converted to ceramic. Here the FFF-based 3-D printed sample is coated with a preceramic polymer and then it is converted into the corresponding PDC sample with a high temperature pyrolysis process. A screening experiment is performed on commercial filaments to obtain ceramic 3-D prints by surface coating both hard thermoplastics: poly lactic acid (PLA), polycarbonate (PC), nylon alloys, polypropylene (PP), polyethylene terephthalate glycol (PETG), polyethylene terephthalate (PET), and co-polyesters; and flexible materials including: flexible PLA, thermoplastic elastomer and thermoplastic polyurethane filaments. Mass and volume changes were quantified for the soaking and pyrolysis steps to form a hollow ceramic skin. All 3-D printing materials extruded at 250 microns successfully produced hollow ceramics skins of less than 100 microns. Details on the advantages and disadvantages of the different 3-D printing polymer precursors are discussed for this processing regime. The novel results developed here can be used to choose FFF-based polymers to use for PDC processing on a wide range of applications such as heat exchangers, heat sinks, scaffoldings for bone tissue growth, chemical/ gas filters and custom scientific hardware.Graphical abstractGraphical abstract for this article
       
  • Selective laser melting of 304L stainless steel: Role of volumetric energy
           density on the microstructure, texture and mechanical properties
    • Abstract: Publication date: March 2020Source: Additive Manufacturing, Volume 32Author(s): Milad Ghayoor, Kijoon Lee, Yujuan He, Chih-hung Chang, Brian K. Paul, Somayeh PasebaniAbstractThe role of volumetric energy density on the microstructural evolution, texture and mechanical properties of 304L stainless steel parts additively manufactured via selective laser melting process is investigated. 304L is chosen because it is a potential candidate to be used as a matrix in a metal matrix composite with nanoparticles dispersion for energy and high temperature applications. The highest relative density of 99 %±0.5 was achieved using a volumetric energy density of 1400 J/mm3. Both XRD analysis and Scheil simulation revealed the presence of a small trace of the delta ferrite phase, due to rapid solidification within the austenitic matrix of 304L. A fine cellular substructure ranged between 0.4–1.8 μm, was detected across different energy density values. At the highest energy density value, a strong texture in the direction of [100] was identified. At lower energy density values, multicomponent texture was found due to high nucleation rate and the existing defects. Yield strength, ultimate tensile strength, and microhardness of samples with a relative density of 99 % were measured to be 540 ± 15 MPa, 660 ± 20 MPa and 254 ± 7 HV, respectively and higher than mechanical properties of conventionally manufactured 304L stainless steel. Heat treatment of the laser melted 304L at 1200 °C for 2 h, resulted in the nucleation of recrystallized equiaxed grains followed by a decrease in microhardness value from 233 ± 3 HV to 208 ± 8 HV due to disappearance of cellular substructure.
       
  • Additive manufacturing of high modulus steels: new possibilities for
           lightweight design
    • Abstract: Publication date: Available online 3 January 2020Source: Additive ManufacturingAuthor(s): H. Springer, C. Baron, F. Mostaghimi, J. Poveleit, L. Mädler, V. UhlenwinkelAbstractThis work demonstrates the feasibility of fabricating bulk nanostructured high modulus steels in-situ by additive manufacturing. This ideal match of novel processes and alloy concepts opens up new pathways for lightweight design by producing light, stiff, strong and ductile components with minimal geometric restraints. On the example of an Fe – Ti – B alloy, a conventional processing sequence of melting and casting pre-alloys, gas-atomisation and laser powder bed fusion (selective laser melting) led to finely dispersed metastable particle and matrix phases. A simple annealing step transformed them into the desired equilibrium constituents of ductile ferrite (matrix) and light and stiff TiB2 (particles), with only minimal changes in particle size (about 20 – 150 nm in diameter) and distribution (mainly on the matrix grain boundaries). This nano-scaled composite structure promises an extremely attractive property profile, i.e. an increased stiffness/ratio at elevated strength and without deteriorated ductility. However, the not yet optimized parameters of the laser fusion process led to the formation of few pores and cracks, which prevented the complete assessment of the property profile of the manufactured samples. Material and processing strategies for the further development of this promising lightweight design approach – including the suitability of other powder metallurgy processing routes – are outlined and discussed.
       
  • Investigation of LPBF A800H steel parts using Computed Tomography and
           Mössbauer spectroscopy
    • Abstract: Publication date: Available online 3 January 2020Source: Additive ManufacturingAuthor(s): R.R. Gainov, D. Faidel, W. Behr, G. Natour, F. Pauly, H. Willms, F.G. VagizovAbstractLaser powder bed fusion (LPBF) was applied in this study to produce a prototype of a miniaturized catalytic burner (CAB), which is a key component of high-temperature polymer electrolyte fuel cells. This prototype was characterized by its complex design with numerous channels, chambers, and thin walls. The test samples and CAB prototype were made of a heat-resistant, anti-corrodible steel called "Alloy 800H" (1.4876), a material that poses problems for welding operations and especially for the LPBF process due to its strong susceptibility to hot cracking and spatters. The effects of LPBF parameter variation on preliminary test samples were investigated by nano-focus Computed Tomography (CT) and Optical microscopy to clarify the internal structure and defects for further LPBF process optimization. Mössbauer spectroscopy points out that LPBF process does not lead to either local phase separation nor oxidation of steel, which is critical factor for use of CAB at high temperatures. The sufficient LPBF parameter sets were used to manufacture the CAB prototype, which was examined by micro-CT and optics as well. The main result of the investigation is a demonstration of the technological feasibility to decrease the number and size of defects in complex LPBF-manufactured Alloy 800H constructions without changes in phase composition at high temperatures.
       
  • On the effect of shielding gas flow on porosity and melt pool geometry in
           laser powder bed fusion additive manufacturing
    • Abstract: Publication date: Available online 3 January 2020Source: Additive ManufacturingAuthor(s): Joni Reijonen, Alejandro Revuelta, Tuomas Riipinen, Kimmo Ruusuvuori, Pasi PuukkoAbstractMetal additive manufacturing is moving from rapid prototyping to on-demand manufacturing and even to serial production. Consistent part quality and development of a wider range of available materials are key for wider adoption. This requires control and optimization of various laser and scanning parameters. Therefore, process modeling has been extensively pursued to reduce experimental runs in the search for parameters that produce dense, high-quality parts for the given alloy. However, these optimal parameters remain machine-specific if conditions defined by the machine architecture are not considered. Previous studies have shown that shielding gas flow is one such parameter that affects porosity and mechanical properties of parts produced with laser powder bed fusion. However, a lack of consensus remains regarding which phenomena are responsible for the observed decrease in quality. In this study, the effect of shielding gas flow velocity on porosity and melt pool geometry in laser powder bed fusion additive manufacturing is studied. It is shown that decreasing the gas flow velocity leads to a drastic loss of penetration of single scan tracks, leading to increased lack-of-fusion porosity at the part level. This is attributed to the obstruction of the laser beam by the process-induced vapor plume emissions of the individual tracks being scanned. As the vapor plume, and how effectively it is removed by the shielding gas flow, have a significant effect on the melt pool geometry in laser powder bed fusion, models aiming at predicting the melt pool geometry and attempts to transfer process parameters from one machine to another should consider the effect of the shielding gas flow.
       
  • Heat-treatment effects on a bimetallic additively-manufactured structure
           (BAMS) of the low-carbon steel and austenitic-stainless steel
    • Abstract: Publication date: Available online 3 January 2020Source: Additive ManufacturingAuthor(s): Md.R.U. Ahsan, A.N.M. Tanvir, Gi-Jeong Seo, Brian Bates, Wayne Hawkins, Chanho Lee, P.K. Liaw, Mark Noakes, Andrzej Nycz, Duck Bong KimAbstractA bimetallic additively-manufactured structure (BAMS) is a type of functionally-graded multi-material structure used for achieving different complementary material properties within the same structure as well as cost optimization. Wire + arc additive manufacturing (WAAM) offers the capability to fabricate the BAMS in a simultaneous or sequential way. To fully utilize the benefits of the BAMS, the interfacial joint should be strong, and each of the constituents should have reasonable mechanical integrity. For this, a BAMS of low-carbon steel and austenitic-stainless steel was fabricated using a gas-metal-arc-welding (GMAW)-based WAAM process. Then, the BAMS was heat-treated at a range of 800 °C to 1,100 °C and 30 minutes to 2 hours. This resulted in 35% and 250% increases in the ultimate tensile strength and elongation, compared to the as-deposited BAMS. After the heat-treatment, the failure location moved from the low-carbon-steel to the stainless-steel side. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAx), and the Vickers hardness test were used to characterize the BAMS. In this paper, it is experimentally validated that heat-treatment at 950 °C-1 hour is the near-optimal condition for the BAMS.
       
  • Novel in-situ residual strain measurements in additive manufacturing
           specimens by using the Optical Backscatter Reflectometry
    • Abstract: Publication date: Available online 3 January 2020Source: Additive ManufacturingAuthor(s): Shaoquan Wang, Kaspar Lasn, Christer Westum Elverum, Di Wan, Andreas EchtermeyerAbstractMaterial extrusion (MEX) is a well established production method in additive manufacturing. However, internal residual strains are accumulated during the layer-by-layer fabrication process. They bring about shape distortions and a degradation of mechanical properties. In this paper, an in-situ distributed measurement of residual strains in MEX fabricated thermoplastic specimens is achieved for the first time. This innovative measuring system consists of an Optical Backscatter Reflectometry (OBR) interrogation unit connected to a distributed fiber optic strain sensor which is embedded during the MEX process. The characteristic residual strain distribution inside 3D printed components is revealed and numerically validated. The main mechanisms of residual strain creation and the sensing principles of in-situ OBR are described. A minimum measuring range of 4 mm and a spatial resolution of 0.15 mm were experimentally demonstrated. The potential of in-situ OBR technology for detecting invisible manufacturing defects was shown by a trial experiment.
       
  • A novel method to 3D-print fine-grained AlSi10Mg alloy with isotropic
           properties via inoculation with LaB6 nanoparticles
    • Abstract: Publication date: Available online 3 January 2020Source: Additive ManufacturingAuthor(s): Qiyang Tan, Jingqi Zhang, Ning Mo, Zhiqi Fan, Yu Yin, Michael Bermingham, Yingang Liu, Han Huang, Ming-Xing ZhangMetal additive manufacturing offers a tool to bring formerly unmanufacturable, geometrically complex, engineered structures into existence. However, considerable challenges remain in controlling the unique microstructures, defects and properties that are created through this process. For the first time this work demonstrates how LaB6 nanoparticles can be used to control such features in Al alloys produced by Selective Laser Melting (SLM). A novel and efficient mechanical agitation process is used to inoculate AlSi10Mg powder with LaB6 nanoparticles which resulted in a homogenous, crack-free, equiaxed, very fine-grained as built microstructures. The substantial grain refinement is attributed to the good crystallographic atomic matching across the Al/LaB6 interfaces which facilitated Al nucleation on the LaB6 nanoparticles. The LaB6-inoculated AlSi10Mg exhibited near-isotropic mechanical properties with an improved plasticity compared with un modified AlSi10Mg.Graphical abstractGraphical abstract for this article
       
  • Correlation between forming quality and spatter dynamics in laser powder
           bed fusion
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Jie Yin, Dengzhi Wang, Liangliang Yang, Huiliang Wei, Peng Dong, Linda Ke, Guoqing Wang, Haihong Zhu, Xiaoyan ZengAbstractLaser powder bed fusion (LPBF) has broad application prospects due to its high fabrication accuracy and excellent performance, but the dynamic mechanical properties of LPBF components are relatively low due to defects of the melt track such as protrusions and depressions, whose generation mechanisms remain unclear. In this work, we investigate the correlation between the ex situ melt track properties and the in situ high-speed, high-resolution characterization. We correlate the protrusion at the starting position of the melt track with the droplet ejection behaviour and backward surging melt. We also reveal that the inclination angles of the depression walls are consistent with the ejection angles of the backward-ejected spatter. Furthermore, we quantify the vapour recoil pressure by in situ characterization of the deflection of the typical forward-ejected spatter. Our results clarify the intrinsic correlation of the melt track properties, which is important for the stable LPBF formation with few defects.
       
  • Novel design of closed-cell foam structures for property enhancement
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): M.A. Kader, P.J. Hazell, A.D. Brown, M. Tahtali, S. Ahmed, J.P. Escobedo, M. SaadatfarAbstractCellular materials, such as foams, can be used as load bearing members in civil construction and as protective energy absorbing structures for personnel and equipment. In the present study, novel lightweight closed-cell structures were designed, and their mechanical properties and collapse mechanisms were investigated through a combination of experimental validation and finite element (FE) simulations. Selected porous structure designs were manufactured from acrylonitrile butadiene styrene (ABS) using additive manufacturing technology. These 3D printed structures were subjected to quasi-static loading to determine the dependence of their elastic and plastic responses from their topological features. Deformation mechanisms were elucidated through quasi-static compression experiments and FE modelling. The appropriate distribution of the base material in the designed closed-cell structures inherits the merits of uniform stress distribution and large deformations that lead to reaching high strengths and desirable energy absorption efficiencies. The effects of relative density and cell shape were studied in detail from elastic loading through the large plastic strain densification regions. The effects of cellular architecture on deformation mechanisms and energy absorption capabilities demonstrated the possibility of enhancing energy absorption efficiencies with appropriate design criteria. Based on the experimental and numerical analyses, the most efficient energy absorbing closed-cell structure was proposed.
       
  • The microstructure evolution and tensile properties of Inconel 718
           fabricated by high-deposition-rate laser directed energy deposition
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Zuo Li, Jing Chen, Shang Sui, Chongliang Zhong, Xufei Lu, Xin LinIn order to meet the requirements for rapid manufacturing of large-scale high-performance metal components, the unique advantages of high-deposition-rate laser directed energy deposition (HDR-LDED, deposition rate ≥ 1 kg/h) technology have been attracted great attention. HDR-LDED technology significantly improves the efficiency by simultaneously increasing the mass and energy input on basis of conventional laser directed energy deposition (C-LDED, deposition rate ≤ 0.3 kg/h), which dramatically changes the solidification condition and thermal cycling effect compared to C-LDED processes. Based on this, Inconel 718 bulk samples were fabricated with a deposition rate of 2.2 kg/h and a height of 75 mm. Through experimental observation combined with finite element simulation, the precipitation morphology, thermal cycling effect and tensile properties at room temperature of the block samples at heights of 6 mm (bottom region), 37 mm (middle region) and 69 mm (top region) from the substrate were investigated. The results show that both temperature interval and incubation time satisfy the precipitation conditions of the second phases because of the intense thermal cycling effect so that δ, γ" and γ' phase are precipitated in the bottom and middle region of the as-deposited sample during the HDR-LDED process. As a result, the micro-hardness and the yield strength of the bottom region (385 HV; 745.1 ± 5.2 MPa) are similar to those of the middle region (381 HV; 752.2 ± 12.1 MPa), respectively. And they are both higher than those of the top region (298 HV; 464.7 ± 44.2 MPa). The tensile fracture mechanism is shown in both fracture and debonding of the Laves phase. The inhomogeneous microstructures and corresponding mechanical property differences of Inconel 718 fabricated by HDR-LDED along the deposition direction suggest the necessity to conduct further research of the post heat treatment in the future.Graphical abstractGraphical abstract for this article
       
  • A computationally efficient thermal model for selective laser melting
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Y. Yang, F. van Keulen, C. AyasAbstractSelective laser melting (SLM) is a widely used additive manufacturing method for building metal parts in a layer-by-layer manner thereby imposing almost no limitations on the geometrical layout of the part. The SLM process has a crucial impact on the microstructure, strength, surface quality and even the shape of the part, all of which depend on the thermal history of material points within the part. In this paper, we present a computationally tractable thermal model for the SLM process which accounts for individual laser scanning vectors. First, a closed form solution of a line heat source is calculated to represent the laser scanning vectors in a semi-infinite space. The thermal boundary conditions are accounted for by a complimentary correction field, which is computed numerically. The total temperature field is obtained by the superposition of the two. The proposed semi-analytical model can be used to simulate manufacturing geometrically complex parts and allows spatial discretisation to be much coarser than the characteristic length scale of the process: laser spot size, except in the vicinity of boundaries. The underlying assumption of linearity of the heat equation in the proposed model is justified by comparisons with a fully non-linear model and experiments. The accuracy of the proposed boundary correction scheme is demonstrated by a dedicated numerical example on a simple cubic part. The influence of the part design and scanning strategy on the temperature transients are subsequently analysed on a geometrically complex part. The results show that overhanging features of a part obstruct the heat flow towards the base-plate thereby creating local overheating which in turn decrease local cooling rate. Finally, a real SLM process for a part with an overhanging feature is modelled for validation of the proposed model. Reasonable agreement between the model predictions and the experimentally measured values can be observed.
       
  • Adding an electroactive response to 3D printed materials: Printing a
           piezoelectret
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Iain Kierzewski, Sarah S. Bedair, Brendan Hanrahan, Harvey Tsang, Liangbing Hu, Nathan LazarusAbstractElectrets have been increasingly investigated for their high piezoelectric sensitivity for sensing and energy harvesting applications, but fabricating complex 3D structures for optimum performance has remained challenging. 3D printing capabilities have likewise become a mature manufacturing technology widely used for end-user customization and rapid prototyping, but limitations on materials and geometries have complicated the incorporation of electroactive structures. In this paper, the first completely 3D printed porous piezoelectret is demonstrated. These samples were structured using standard infill patterns commonly used in 3D printing, allowing easy incorporation with current 3D printing technology. Pores generated by fused-filament fabrication (FFF) are characterized, charged, and the resultant piezoelectret activity measured. Analytical electromechanical models are used to understand and compare the measured charge density and piezoelectric coefficients. The piezoelectric coefficient is found to increase strongly with decreasing infill percentages. An average piezoelectric d33 coefficient of 87 pC N−1 is achieved for 5 % infill samples and is found to be stable for a period of at least 2 weeks, competitive with many other state-of-the-art single-pore piezoelectretic materials. These results provide a first step in using 3D printing techniques to optimize and integrate piezoelectrets into parts, allowing a useful new electroactive functionality for additive manufacturing.
       
  • Analysis of Multi-scale Mechanical Properties of Ceramic Trusses Prepared
           from Preceramic Polymers (Revision Prepared for Additive Manufacturing)
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): N.R. Brodnik, J. Schmidt, P. Colombo, K.T. FaberAbstractTo better understand the impact of complex structure on mechanical properties in additively manufactured ceramics, truss structures were 3D printed in preceramic polymer and mechanically evaluated in the pyrolyzed SiOC state. Specimens were printed using digital light processing with a siloxane polymer resin blend. Four different designs were printed: two bending-dominant Kelvin cell structures, a stretching-dominant octet structure, and a mixture of the two with geometries chosen for equivalent stiffness. Mechanical characterization was done at multiple length scales: uniaxial compression to evaluate the entire truss structure, and three-point flexure to assess individual beam elements. After pyrolysis, it was found that truss designs exhibited different shrinkages at the beam element scale despite being composed of the same preceramic polymer and exhibiting isotropic shrinkage at the macro-truss scale. This manner of nonuniform shrinkage has rarely, if ever been reported, as it is standard practice in additive manufacturing to report only bulk linear shrinkage. In uniaxial compression, Kelvin structures with thicker beams exhibited the highest strength of 10 MPa, and octet structures exhibited the lowest strength of 3.8 MPa. In beam element flexure however, the octet beams had the highest strength, 1.9 GPa, four times stronger than the Kelvin beam elements and 500 times stronger than the octet bulk structure. Lastly, the implications for interchangeable truss structures are discussed.
       
  • Fabrication of continuous carbon fiber mesh for lightning protection of
           large-scale wind-turbine blade by electron beam cured printing
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Ben Wang, Yueke Ming, Yansong Zhu, Xueling Yao, Gerhard Ziegmann, Hong Xiao, Xiaohui Zhang, Jingjing Zhang, Yugang Duan, Jinru SunWind-turbine blades are more vulnerable to lightning strikes as they lack a protection system for large-scale glass fiber reinforced polymer (GFRP) composite structures. A low-energy electron beam (EB) cured printing process for fabricating a continuous carbon fiber-reinforced thermoset resin as a non-metallic lightning protection mesh on a GFRP composite surface was carried out in this study. During the proposed process, a continuous carbon fiber mesh was printed through a Fused Filament Fabrication that integrates the rapid curing of an epoxy resin with low-energy EB irradiation. The printing process was analyzed and optimized by examining the correlation between the EB exposure dose and the printing height. Results from artificial lightning strikes showed that the printed carbon fiber mesh prevented damage, and the structure remained relatively intact with residual strength reaching 90.1 % at 100 kA maximum peak current. The protection mechanism was investigated using a high-speed camera, which revealed that the carbon fiber mesh spreads the striking current outside the laminate instead of penetrating inside.Graphical abstractGraphical abstract for this article
       
  • Implications of lattice structures on economics and productivity of metal
           powder bed fusion
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Iñigo Flores, Niklas Kretzschmar, Abdul Hadi Azman, Sergei Chekurov, David Bue Pedersen, Atanu ChaudhuriThe cost-effectiveness of metal powder bed fusion (PBF) systems in high-throughput production are dominated by the high cost of metallic powder materials. Metal PBF technologies become more competitive in production scenarios when Design for Additive Manufacturing (DfAM) is integrated to embed functionality through shape complexity, weight, and material reduction through topology optimization and lattice structures.This study investigates the value of DfAM in terms of unit cost and manufacturing time reduction. Input design parameters, such as lattice design-type, part size, volume fraction, material type and production volumes are included in a Design-of-Experiment to model their impact. The performance variables for cost and manufacturing time were assessed for two scenarios: (i) outsourcing scenario using an online quotation system, and (ii) in-house scenario utilizing a decision support system (DSS) for metal PBF.The results indicate that the size of the part and the lattice volume fraction are the most significant parameters that contribute to time and cost savings. This study shows that full utilization of build platforms by volume-optimized parts, high production volumes, and reduction of volume fraction lead to substantial benefits for metal PBF industrialization. Integration of DfAM and lattice designs for lightweight part production can decrease the unit cost of production down to 70.6% and manufacturing time can be reduced significantly down to 71.7% depending on the manufacturing scenarios and design constraints when comparing to solid infill designs. The study also provides a case example of a bracket design whose cost is reduced by 53.7%, manufacturing time is reduced by 54.3 %, and the overall weight is reduced significantly with the use of lattices structures and topology optimization.Graphical abstractGraphical abstract for this article
       
  • Optimization and re-design of a metallic riveting tool for additive
           manufacturing—A case study
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Alexander Großmann, Patrick Weis, Christof Clemen, Christian MittelstedtAbstractThis paper discusses the topology optimization and additive manufacturing (AM) specific re-design of a metallic C-frame as it is used in the riveting process in the automotive industry. The main objective of the optimization and re-design process is the reduction of the structural weight where special attention needs to be paid to the specific manufacturing process of powder bed fusion which is a powder based layerwise additive manufacturing process. The initial optimization and AM specific re-design are performed under consideration of a number of free parameters that drive the performance and weight of the C-frame, and several generated solutions are compared under special consideration of the weight, the mechanical performance and the general manufacturability using powder bed fusion. The selected optimized solution then undergoes a final detailed re-design which focusses on given manufacturing restrictions. The mechanical performance of the optimized C-frame is assessed employing detailed finite element simulations by evaluating the stress and deformation state. The general manufacturability of the optimized part by powder bed fusion is demonstrated by the manufacturing of a scaled prototype. In order to enable a comparison of the new AM solution with a classical manufacturing process, an optimized C-frame geared towards classical milling is established as well. Both solutions are compared concerning weight, mechanical performance, manufacturability and economic aspects, and it can be shown that the AM solution offers a number of advantages that cannot be exploited when employing classical means of manufacturing. This paper may serve as an introduction to the rather complex field of AM design of load bearing structures and is an illustrated case study thereof which can be of use for engineers working in this specific field that is still the topic of global academic and industrial research.
       
  • Automated non-destructive inspection of Fused Filament Fabrication
           components using Thermographic Signal Reconstruction
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Joshua E. Siegel, Maria F. Beemer, Steven M. ShepardAbstractManufacturers struggle to produce low-cost, robust and intricate components in small batches. Additive processes like Fused Filament Fabrication (FFF) inexpensively generate such complex geometries, but potential defects may limit these components’ viability in critical applications. We present a high-accuracy, high-throughput and low-cost approach to automated non-destructive testing (NDT) for FFF interlayer delamination. This Artificially Intelligent (AI) approach utilizes Flash Thermography (FT) data processed with Thermographic Signal Reconstruction (TSR). A Deep Neural Network (DNN) attains 95.4% per-pixel accuracy when differentiating four delamination severities 5 mm below the surface in PolyLactic Acid (PLA) widgets, and 98.6% accuracy in differentiating acceptable from unacceptable states for the same components. Automation supports time- and cost-efficient inspection for delamination defects in 100% of widgets, supporting FFF's use in critical and lot-size one applications.
       
  • Additive manufacturing aboard a moving vessel at sea using passively
           stabilized stereolithography (SLA) 3D printing
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Brennan T. Phillips, Josh Allder, Grady Bolan, R. Sean Nagle, Allison Redington, Tess Hellebrekers, John Borden, Nikolai Pawlenko, Stephen LichtAbstractIn this paper we investigate the use of passive stabilization to support stereolithography (SLA) printing aboard a moving vessel at sea. 3D printing is a useful technology onboard a seagoing vessel to support engineering development, shipboard maintenance, and other applications when land-based manufacturing resources are unavailable. SLA printed material is particularly suited for underwater applications requiring sealed housings, since SLA printers are capable of producing high-resolution models that are fully solid and impervious to water. Hydrostatic pressure can quickly compromise parts created using standard fused filament fabrication (FFF) 3D printing. However, the dynamic environment onboard a moving vessel could impact the ability of an SLA printer to selectively cure voxels in a liquid resin bath as it undergoes constant motion, and can cause spilling over the walls of the resin tank. Using passive stabilization platforms onboard moving research vessels, we successfully printed a number of parts with no discernable differences from those produced in a traditional land-based laboratory. As a practical demonstration of this capability, we printed at sea underwater pressure housings that remained sealed to 200 m water depth with functional integrated internal electronics. No post-print machining was required to create the sealed housings. This work lays the foundation for lithographic 3D printing in seagoing oceanographic and naval applications, and additionally presents an economical approach for producing custom waterproof pressure housings in the field.
       
  • Laser opto-ultrasonic dual detection for simultaneous compositional,
           structural, and stress analyses for wire + arc additive manufacturing
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Yuyang Ma, Zhenlin Hu, Yun Tang, Shixiang Ma, Yanwu Chu, Xin Li, Wei Luo, Lianbo Guo, Xiaoyan Zeng, Yongfeng LuAbstractThe complex, nonequilibrium physical, chemical, and metallurgical nature of additive manufacturing (AM) tends to lead to uncontrollable and unpredictable material and structural properties. Therefore, real-time monitoring of AM is of great significance. However, current AM relies on separate postprocess analyses, which are usually destructive, costly, and time-consuming. In this study, we investigated a laser opto-ultrasonic dual (LOUD) detection approach for simultaneous and real-time detection of elemental compositions, structural defects, and residual stress in aluminium (Al) alloy components during wire + arc additive manufacturing (WAAM) processes. In this approach, a pulsed-laser beam was used to excite the surfaces of Al alloy samples to generate ultrasound and optical spectra. As a result, the compositional information can be obtained from the optical spectra, while the structural defects and residual stress distributions can be extracted from the ultrasonic signals. The silicon (Si) and copper (Cu) compositions obtained from optical spectral analyses are consistent with those obtained from the electron-probe microanalyses (EPMA). The 1 mm blowhole and the residual stress distribution of the sample were detected by the ultrasonic signals in the LOUD detection, which shows consistency with the conventional ultrasonic testing (UT). Both results indicate that the LOUD detection holds the promising of becoming an effective testing method for AM processes to ensure quality control and process feedback.
       
  • Integrated laser-based powder bed fusion and fused filament fabrication
           for three-dimensional printing of hybrid metal/polymer objects
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Yuan-Hui Chueh, Chao Wei, Xiaoji Zhang, Lin LiAbstractTo produce complex functional devices while eliminating the need for assembly calls for a multi-material additive manufacturing technology. This paper presented a 3D-printing system that integrated fused filament fabrication (FFF) and laser-based powder bed fusion (PBF) to produce hybrid metal and polymer components. The design and operation procedure of the system were introduced. PBF-printed metal and FFF-printed polymer, both of which differ in material properties, were joined through PBF-printed interlocking structures, with their joining strength enhanced by laser heating. The mechanisms and scientific rationale that governed metal/polymer joining were discussed. Tensile and shear tests confirmed good joint strength of the printed metal/polymer components, which were created without adhesives. In addition, metal powder deposition onto the top of polymer substrates through laser melting was demonstrated. Layers of copper (Cu10Sn) were successfully deposited onto the top of a PLA/SS 316L composite substrate; however, their joint strength remained a problem to resolve. Finally, several 3D components consisting of hybrid stainless steel (SS 316L), copper (Cu10Sn) and polymer (PLA, PET) were successfully printed and their potential applications were discussed.
       
  • Load path visualization and fiber trajectory optimization for additive
           manufacturing of composites
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Takuya Suzuki, Shinya Fukushige, Mitsuyoshi TsunoriAbstractA methodology of fiber trajectory optimization is proposed for Additive Manufacturing of composites. The present method aligns fiber with a physically-determined load path to simultaneously increase the stiffness and strength of the composite structures. The fiber trajectories of the open-hole panel and Payload Attach Fitting (PAF) were determined. In the case of open-hole panel, the deformation and the failure index were decreased by 8 % and 55 % compared to those obtained by the unidirectional structure. In the case of PAF, the decrease in failure index was 76 %, but the reduction of deformation was not significant (6 %). The present method also identified the structural members that did not contribute to strength and rigidity, which in turn realized the appropriate weight savings and increased the specific strength and specific stiffness.
       
  • 3-D printed porous cellulose acetate tissue scaffolds for additive
           manufacturing
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Hanxiao Huang, Derrick DeanAbstractWe have printed microscale 3-dimensional tissue scaffolds using cellulose acetate (CA) for the first time and produced a range of pore sizes ranging from 99 to 608 μm that are potentially favorable for tissue engineering. In the process we have elucidated some of the formulation-fabrication-morphology relationships which enabled advancements in ink development, optimization of fabrication parameters, and morphological control. The challenges for printing very small pores were discussed and improved by adjustment of printing conditions and use of a rheological modifier. We believe this study will increase the knowledge base for additive manufacturing of CA and enable further research into the use of 3D-printed CA for tissue engineering applications. Also, our findings on printing optimization may provide some practical principles and methodologies that are applicable for the ink development using other biomaterials.
       
  • Particle shape and size analysis for metal powders used for additive
           manufacturing: Technique description and application to two gas-atomized
           and plasma-atomized Ti64 powders
    • Abstract: Publication date: Available online 23 November 2019Source: Additive ManufacturingAuthor(s): E.J. Garboczi, N. HrabeAbstractThe particle size and shape distributions of metal powders used in additive manufacturing powder bed fusion processes are of technological importance for the final built product. Current three-dimensional (3D) measurements of these distributions assume a spherical shape, while techniques that measure both size and shape are always two-dimensional (2D) measurements of particle projections. This paper describes a set of techniques using X-ray computed tomography, combined with various mathematical algorithms, to measure the 3D size, shape, and internal porosity of individual particles. Calibrated by a limited amount of visual examination of 3D images of individual particles, these techniques can classify powder particles as single near-spherical (SnS) particles, and non-spherical (NS) particles, which consist of either single highly non-spherical particles or multi-particles, where two or more smaller particles have been joined together. From this 3D data, other algorithms can generate 2D particle size and shape information to compare with the results of 2D measurement techniques. These techniques are applied to two metal powders composed of a specific alloy of titanium with aluminum and vanadium, denoted as Ti64, which is in common use as a powder for selective laser or electron beam melting powder bed additive manufacturing. One powder was made with a gas-atomization process, the other with a plasma-atomization process, both have been recycled, and both pass the specifications for additive manufacturing use. The powders differ in the fraction of NS particles and porous particles, in their size and shape distributions, and in average shape and size statistics. The SnS/NS classification enables one to show how these classes contribute to the overall particle size distributions, even for a single powder type, and is useful for comparing different sources of powder as well as studying how the size/shape distributions of a powder might change over multiple recycling events.
       
  • Selective laser melting of dual phase AlCrCuFeNix high entropy alloys:
           Formability, heterogeneous microstructures and deformation mechanisms
    • Abstract: Publication date: January 2020Source: Additive Manufacturing, Volume 31Author(s): Shuncun Luo, Chunyang Zhao, Yue Su, Qi Liu, Zemin WangPreparing dual-phase high-entropy alloys (DP-HEAs) by selective laser melting (SLM) has never been achieved owing to high crack susceptibility induced by rapid solidification. Here we design and fabricate new face-centered cubic (FCC) and body-centered cubic (BCC) DP-HEAs based on BCC AlCrCuFeNi HEA using SLM. Results show that the addition of Ni facilitates the columnar-to-near-equiaxed transition and improves the formability of the as-built AlCrCuFeNix (2.0 ≤ x ≤ 3.0) HEAs. Especially, the as-built AlCrCuFeNi3.0 HEA exhibits modulated nano-sized lamellar or cellular dual-phase structures and possesses the best combination of ultimate tensile strength (∼ 957 MPa) and ductility (∼ 14.3%). Post-deformation research reveals that the FCC phase is deformed through planar dislocation slip with {111} slip systems, and stacking faults (SFs). In the ordered BCC (B2) phases, high densities of Cr-rich nano-precipitates make B2 phase severely distort during tension, thus triggering the formation of deformation nano-twins and SFs on {112} planes. Strain-activated B2-to-FCC phase transition occurs in the B2 phase. Moreover, serrated tensile flow is first discovered in DP-HEAs due to the continual initiation and propagation of twins in the B2 phase. The uncovered synergy of various deformation modes and the underlying back stress strengthening induced by heterogeneous microstructures contribute to the high ultimate tensile strength and good ductility of the as-built AlCrCuFeNi3.0 HEA.Graphical abstractGraphical abstract for this article
       
  • Laser-based powder bed fusion of alumina toughened zirconia
    • Abstract: Publication date: Available online 18 November 2019Source: Additive ManufacturingAuthor(s): Fabrizio Verga, Mario Borlaf, Laura Conti, Kevin Florio, Marc Vetterli, Thomas Graule, Manfred Schmid, Konrad WegenerAbstractAlumina toughened zirconia (ATZ) parts were produced via a laser-based powder bed fusion technology using a conventional Nd-YAG continuous wave laser. The powder was produced using a spray drying process and the laser matter interaction was enhanced by a binder pyrolysis. A processing window led to part densities of over 90%. Thermal post-processing to further increase the part density was investigated using dilatometry. The microstructure was analysed using X-ray powder diffraction measurements. The mechanical properties were assessed using a four-point bending test on ten specimens, reaching a bending strength of 31 ± 11 MPa.
       
  • Linking pyrometry to porosity in additively manufactured metals
    • Abstract: Publication date: Available online 17 November 2019Source: Additive ManufacturingAuthor(s): John A. Mitchell, Thomas A. Ivanoff, Daryl Dagel, Jonathan D. Madison, Bradley JaredAbstractPorosity in additively manufactured metals can reduce material strength and is generally undesirable. Although studies have shown relationships between process parameters and porosity, monitoring strategies for defect detection and pore formation are still needed. In this paper, instantaneous anomalous conditions are detected in-situ via pyrometry during laser powder bed fusion additive manufacturing and correlated with voids observed using post-build micro-computed tomography. Large two-color pyrometry data sets were used to estimate instantaneous temperatures, melt pool orientations and aspect ratios. Machine learning algorithms were then applied to processed pyrometry data to detect outlier images and conditions. It is shown that melt pool outliers are good predictors of voids observed post-build. With this approach, real time process monitoring can be incorporated into systems to detect defect and void formation. Alternatively, using the methodology presented here, pyrometry data can be post processed for porosity assessment.
       
  • Extended finite element method (XFEM) modeling of fracture in additively
           manufactured polymers
    • Abstract: Publication date: Available online 14 November 2019Source: Additive ManufacturingAuthor(s): R. Ghandriz, K. Hart, J. LiAbstractThe fracture of additively manufactured polymer materials with various layer orientations is studied using the extended finite element method (XFEM) in an anisotropic cohesive zone model (CZM). The single edge notched bending (SENB) specimens made of acrylonitrile-butadiene-styrene (ABS) materials through fused filament fabrications with various crack tip/layer orientations are considered. The XFEM coupled with anisotropic CZM is employed to model the brittle fracture (fracture between layers), ductile fracture (fracture through layers), as well as kinked fracture behaviors of ABS specimens printed with vertical, horizontal, and oblique layer orientations, respectively. Both elastic and elastoplastic fracture models, coupled with linear or exponential traction-separation laws, are developed for the inter-layer and cross-layer fracture, respectively. For mixed inter-/cross- layer fracture, an anisotropic cohesive zone model is developed to predict the kinked crack propagations. Two crack initiation and evolution criteria are defined to include both crack propagation between layers (weak plane failure) and crack penetration through layers (maximum principal stress failure) that jointly determine the zig-zag crack growth paths. The anisotropic cohesive zone model with XFEM developed in this study is able to capture different fracture behaviors of additively manufactured ABS samples with different layer orientations.
       
  • Revisiting solidification microstructure selection maps in the frame of
           additive manufacturing
    • Abstract: Publication date: Available online 14 November 2019Source: Additive ManufacturingAuthor(s): P. Mohammadpour, A. Plotkowski, A.B. PhillionAbstractUnderstanding microstructural development in additive manufacturing under highly non-equilibrium cooling conditions and the consequent effects on mechanical properties of the final component is critical for accelerating industrial adoption of these manufacturing techniques. In this study, simple but effective theoretical solidification models are recalled to evaluate their ability to predict of microstructural features in additive manufacturing applications. As a case study, the resulting solidification microstructure selection maps are created to predict the stable growth modality and the columnar to equiaxed transition (CET) of an Al-10Si-0.5Mg alloy processed via Selective Laser Melting. The potential of this method in microstructural predictions for additively manufactured products, as well as outstanding challenges and limitations, are discussed.
       
  • Behavior of yttria-stabilized zirconia (YSZ) during Laser Direct Energy
           Deposition of an Inconel 625-YSZ Cermet
    • Abstract: Publication date: Available online 10 November 2019Source: Additive ManufacturingAuthor(s): Harish Rao, Richard P. Oleksak, Kory Favara, Arshad Harooni, Bhaskar Dutta, David MauriceAbstractThermal barrier coatings (TBC) are regularly used today to protect and extend the service life of several superalloys which are extensively used in high temperature applications. The existing TBCs are typically between 0.1 to 0.5 mm in thickness, are deposited on metal substrates using plasma spray or electron beam vapor deposition, and can reduce temperatures at the substrate surface by up to 300 °C. For greater temperature reductions there is a need for thicker TBCs. The building of thick TBCs has to date been stymied by poor adhesion, and cracking during deposition. It has been suggested that a functionally graded approach may reduce the residual stresses which result in these defects. To date there have been few reports on the deposition of ceramic or cermet coatings using laser AM and none have reported on the phase stability of ceramic particles post-deposition. This paper is a first report on the phase stability of ceramic particles following the compositional segregation of elements during deposition using a powder feed additive manufacturing process. Functionally graded (FG), thick TBCs (>3 mm) consisting of Inconel 625 (IN625) and yttria-partially stabilized zirconia (8YSZ) were deposited on an A516 steel substrate via laser direct energy deposition (LDED). Good interfaces were observed between the bond coat (BC) and first cermet layer and between the graded cermet layers. However, cermet layers deposited with 10 wt.% or more YSZ developed a thin layer of YSZ on the surface. The thin layer of YSZ greatly hindered additional deposition of new cermet layers. In cermet layers that did exhibit good interfaces, fine, re-solidified, YSZ particles were homogenously distributed within the Inconel 625 matrix. The YSZ particles exhibited a tetragonal lattice structure and were depleted of yttrium. In contrast, the thin YSZ layer formed on a cermet surface showed no yttrium depletion.
       
  • In-situ full-field mapping of melt flow dynamics in laser metal additive
           manufacturing
    • Abstract: Publication date: Available online 6 November 2019Source: Additive ManufacturingAuthor(s): Qilin Guo, Cang Zhao, Minglei Qu, Lianghua Xiong, S. Mohammad H. Hojjatzadeh, Luis I. Escano, Niranjan D. Parab, Kamel Fezzaa, Tao Sun, Lianyi ChenAbstractMelt flow plays a critical role in laser metal additive manufacturing, yet the melt flow behavior within the melt pool has never been explicitly presented. Here, we report in-situ characterization of melt-flow dynamics in every location of the entire melt pool in laser metal additive manufacturing by populous and uniformly dispersed micro-tracers through in-situ high-resolution synchrotron x-ray imaging. The location-specific flow patterns in different regions of the melt pool are revealed and quantified under both conduction mode and depression mode. The physical processes at different locations in the melt pool are identified. The full-field melt-flow mapping approach reported here opens the way to study the detailed melt-flow dynamics under real additive manufacturing conditions. The results obtained provide crucial insights into laser additive manufacturing processes and are critical for developing reliable high-fidelity computational models.
       
  • 3D Printed Optics with a Soft and Stretchable Optical Material
    • Abstract: Publication date: Available online 25 October 2019Source: Additive ManufacturingAuthor(s): Edidiong Nseowo Udofia, Wenchao ZhouWaveguides are important optical elements for sensing, illumination, artistic displays, integrated optical circuits, as well as teaching aids for demonstrating important optical phenomena. However, despite the high demand, most optical materials are difficult to fabricate into desired shapes using state-of-the-art manufacturing technologies. This paper presents a novel method for 3D printing customizable optics with a soft and stretchable (over 100% elastic strains) thermoplastic polymer. To showcase the versatility of this approach, several applications were demonstrated, including unique artistic illumination, caustic patterns, beam splitter and combiner on both planar and 3D conformal surfaces, and optical encoder. The printed waveguides exhibit an outstanding optical transparency of more than 98% and an optical loss of less than 0.22 dBcm-1. The simplicity of the fabrication process, low-cost, excellent optical properties, and flexibility provide an attractive pathway for fabricating integrated optical devices and new opportunities for controlling light.Graphical abstractGraphical abstract for this articleFigure: (a) Photograph showing high level of transparency of as-printed object, and (b) Caustic patterns of (a) produced by illumination from a suspended overhead green LED. (c) As-printed waveguide splitter and combiner circuit on a 3D printed dome surface, and (d) Top view of lighted circuited. (e) Pattern of our group name “AM3 Lab” on a black paper substrate, and (f) Lighted with different LEDs.
       
  • Enabling direct writing of an epoxy resin with thermo-activated organic
           thixotropes
    • Abstract: Publication date: Available online 13 October 2019Source: Additive ManufacturingAuthor(s): Ruel McKenzie, Hilmar KoernerAbstractDirect writing a thermosetting resin typically requires a rheological modifier or peripheral reaction rate-modulating equipment to enable shape fidelity during parts fabrication. This work describes the processing and characterization of low molecular weight diamide-based derivatives of hydroxystearic acid to facilitate direct writing of an epoxy resin. These low molecular weight gelators (LMWG) are thermally activated to produce sufficient yield stress for self-supporting, reactive, physical gels. These organic gelators are semi-crystalline and enable two modes of processing to produce form-stable resin formulations – cold processing and melt processing. Physical gelation occurs by assembly of the LMWG into supramolecular morphologies [1] that vary by mode of processing. Flow of the form-stable epoxy resin is induced by yielding of the physical gel structure. When the physical gel is cured at temperatures below the melt transition of the organic gelator, the network structure likely kinetically traps the organic gelator in a metastable state. Recrystallization of the kinetically trapped organic gelator is impeded when the network is post-cured above the melt transition temperature of the organic gelator. The use of low molecular weight agents that physically gel by thermal activation, generates low viscosity solution processability and suggests that this platform may be suitable for high solids loading applications amenable to direct writing.
       
 
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