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Publisher: Elsevier   (Total: 3183 journals)

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Showing 1 - 200 of 3183 Journals sorted alphabetically
Academic Pediatrics     Hybrid Journal   (Followers: 37, SJR: 1.655, CiteScore: 2)
Academic Radiology     Hybrid Journal   (Followers: 25, SJR: 1.015, CiteScore: 2)
Accident Analysis & Prevention     Partially Free   (Followers: 101, SJR: 1.462, CiteScore: 3)
Accounting Forum     Hybrid Journal   (Followers: 27, SJR: 0.932, CiteScore: 2)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 38, SJR: 1.771, CiteScore: 3)
Achievements in the Life Sciences     Open Access   (Followers: 5)
Acta Anaesthesiologica Taiwanica     Open Access   (Followers: 7)
Acta Astronautica     Hybrid Journal   (Followers: 434, SJR: 0.758, CiteScore: 2)
Acta Automatica Sinica     Full-text available via subscription   (Followers: 2)
Acta Biomaterialia     Hybrid Journal   (Followers: 27, SJR: 1.967, CiteScore: 7)
Acta Colombiana de Cuidado Intensivo     Full-text available via subscription   (Followers: 3)
Acta de Investigación Psicológica     Open Access   (Followers: 3)
Acta Ecologica Sinica     Open Access   (Followers: 11, SJR: 0.18, CiteScore: 1)
Acta Histochemica     Hybrid Journal   (Followers: 3, SJR: 0.661, CiteScore: 2)
Acta Materialia     Hybrid Journal   (Followers: 297, 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: 1, SJR: 1.793, CiteScore: 6)
Acta Poética     Open Access   (Followers: 4, SJR: 0.101, CiteScore: 0)
Acta Psychologica     Hybrid Journal   (Followers: 25, 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  
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: 17, SJR: 1.29, CiteScore: 3)
Addictive Behaviors Reports     Open Access   (Followers: 9, SJR: 0.755, CiteScore: 2)
Additive Manufacturing     Hybrid Journal   (Followers: 11, SJR: 2.611, CiteScore: 8)
Additives for Polymers     Full-text available via subscription   (Followers: 23)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 177, SJR: 4.09, CiteScore: 13)
Advanced Engineering Informatics     Hybrid Journal   (Followers: 12, 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: 16, SJR: 2.384, CiteScore: 5)
Advances in Anesthesia     Full-text available via subscription   (Followers: 29, 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: 11, SJR: 0.992, CiteScore: 1)
Advances in Applied Mechanics     Full-text available via subscription   (Followers: 11, 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: 2, SJR: 0.686, CiteScore: 2)
Advances in Cancer Research     Full-text available via subscription   (Followers: 33, 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: 28, SJR: 0.156, CiteScore: 1)
Advances in Child Development and Behavior     Full-text available via subscription   (Followers: 10, SJR: 0.713, CiteScore: 1)
Advances in Chronic Kidney Disease     Full-text available via subscription   (Followers: 10, 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: 20, 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: 15)
Advances in Developmental Biology     Full-text available via subscription   (Followers: 13)
Advances in Digestive Medicine     Open Access   (Followers: 12)
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: 29, SJR: 1.159, CiteScore: 4)
Advances in Experimental Biology     Full-text available via subscription   (Followers: 8)
Advances in Experimental Social Psychology     Full-text available via subscription   (Followers: 49, SJR: 5.39, CiteScore: 8)
Advances in Exploration Geophysics     Full-text available via subscription   (Followers: 1)
Advances in Fluorine Science     Full-text available via subscription   (Followers: 9)
Advances in Food and Nutrition Research     Full-text available via subscription   (Followers: 65, 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: 20, SJR: 1.354, CiteScore: 4)
Advances in Genome Biology     Full-text available via subscription   (Followers: 10, SJR: 12.74, CiteScore: 13)
Advances in Geophysics     Full-text available via subscription   (Followers: 6, SJR: 1.193, CiteScore: 3)
Advances in Heat Transfer     Full-text available via subscription   (Followers: 25, 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: 23)
Advances in Imaging and Electron Physics     Full-text available via subscription   (Followers: 3, SJR: 0.193, CiteScore: 0)
Advances in Immunology     Full-text available via subscription   (Followers: 36, 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: 8, 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: 20, SJR: 0.88, CiteScore: 2)
Advances in Mathematics     Full-text available via subscription   (Followers: 12, SJR: 3.027, CiteScore: 2)
Advances in Medical Sciences     Hybrid Journal   (Followers: 7, 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: 4, SJR: 1.158, CiteScore: 3)
Advances in Molecular and Cell Biology     Full-text available via subscription   (Followers: 23)
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: 4)
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: 5, SJR: 1.579, CiteScore: 4)
Advances in Pediatrics     Full-text available via subscription   (Followers: 25, SJR: 0.461, CiteScore: 1)
Advances in Pharmaceutical Sciences     Full-text available via subscription   (Followers: 17)
Advances in Pharmacology     Full-text available via subscription   (Followers: 16, SJR: 1.536, CiteScore: 3)
Advances in Physical Organic Chemistry     Full-text available via subscription   (Followers: 9, 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: 10)
Advances in Plant Pathology     Full-text available via subscription   (Followers: 5)
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: 65)
Advances in Quantum Chemistry     Full-text available via subscription   (Followers: 6, SJR: 0.371, CiteScore: 1)
Advances in Radiation Oncology     Open Access   (Followers: 1, 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: 6)
Advances in Space Research     Full-text available via subscription   (Followers: 419, SJR: 0.569, CiteScore: 2)
Advances in Structural Biology     Full-text available via subscription   (Followers: 5)
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: 36, 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: 5, SJR: 2.262, CiteScore: 5)
Advances in Water Resources     Hybrid Journal   (Followers: 53, SJR: 1.551, CiteScore: 3)
Aeolian Research     Hybrid Journal   (Followers: 6, SJR: 1.117, CiteScore: 3)
Aerospace Science and Technology     Hybrid Journal   (Followers: 375, 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: 11, SJR: 3.671, CiteScore: 9)
Aggression and Violent Behavior     Hybrid Journal   (Followers: 468, SJR: 1.238, CiteScore: 3)
Agri Gene     Hybrid Journal   (Followers: 1, SJR: 0.13, CiteScore: 0)
Agricultural and Forest Meteorology     Hybrid Journal   (Followers: 17, SJR: 1.818, CiteScore: 5)
Agricultural Systems     Hybrid Journal   (Followers: 31, SJR: 1.156, CiteScore: 4)
Agricultural Water Management     Hybrid Journal   (Followers: 44, 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: 6, 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: 11)
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: 10, SJR: 0.201, CiteScore: 1)
Alzheimer's & Dementia     Hybrid Journal   (Followers: 53, 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: 6)
American Heart J.     Hybrid Journal   (Followers: 58, SJR: 3.267, CiteScore: 4)
American J. of Cardiology     Hybrid Journal   (Followers: 63, SJR: 1.93, CiteScore: 3)
American J. of Emergency Medicine     Hybrid Journal   (Followers: 45, SJR: 0.604, CiteScore: 1)
American J. of Geriatric Pharmacotherapy     Full-text available via subscription   (Followers: 12)
American J. of Geriatric Psychiatry     Hybrid Journal   (Followers: 14, SJR: 1.524, CiteScore: 3)
American J. of Human Genetics     Hybrid Journal   (Followers: 35, 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: 35, 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: 242, SJR: 2.7, CiteScore: 4)
American J. of Ophthalmology     Hybrid Journal   (Followers: 66, 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: 30, SJR: 2.139, CiteScore: 4)
American J. of Preventive Medicine     Hybrid Journal   (Followers: 28, 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: 64, SJR: 0.138, CiteScore: 0)
Anaesthesia Critical Care & Pain Medicine     Full-text available via subscription   (Followers: 23, 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: 5, 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: 207, 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: 212, SJR: 1.58, CiteScore: 3)
Animal Feed Science and Technology     Hybrid Journal   (Followers: 6, SJR: 0.937, CiteScore: 2)
Animal Reproduction Science     Hybrid Journal   (Followers: 7, SJR: 0.704, CiteScore: 2)

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Similar Journals
Journal Cover
Additive Manufacturing
Journal Prestige (SJR): 2.611
Citation Impact (citeScore): 8
Number of Followers: 11  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 2214-8604
Published by Elsevier Homepage  [3183 journals]
  • Accelerated refilling speed in rapid stereolithography based on
           nano-textured functional release film
    • Abstract: Publication date: October 2019Source: Additive Manufacturing, Volume 29Author(s): Li Wang, Yu Luo, Zhiqiang Yang, Wanjing Dai, Xiaoyang Liu, Jingyuan Yang, Bingheng Lu, Lijia Chen Nowadays, along with the demand for new technologies and new materials, a revolution in 3D printing technology is emerging. In recent years, stereolithography 3D printing has been widely used in both academia and industry, due to its fast forming speed, high precision, and low-cost advantages. The continuous liquid interface production technology has made the printing speed even faster. However, the process of resin refilling constrains the printing speed and the printing capabilities of such technologies, since only hollow structures can be fabricated. In this study, a nano-textured hydrophobic PDMS contacting layer and an oxygen-permeable membrane were bonded together as the functional release film. The oxygen inhibition layer was successfully maintained by the molecular oxygen permeated through the composite release film, achieving rapid stereolithography, and key factors that affecting resin refilling are selectively studied by the orthogonal experiment. Crucially, according to the simulation and experimental results, the adoption of the hydrophobic nano-texture not only increased the refilling speed of the resin by two times and reduced the printing time by nearly 25%, but also improved the printing reliability by reducing the vacuum (negative pressure) caused by the original slow refilling speed. Additionally, optical simulations also demonstrated that the nano-texture would not influence the curing effect of the resin. This work proposed a promising strategy for rapid stereolithography of 3D models containing larger cross-sectional areas.Graphical abstractA nano-textured PDMS contacting layer and an oxygen-permeable membrane were bonded together as the printing substrate, providing high oxygen permeability to form an oxygen inhibition layer. The introduction of the nano-texture on PDMS not only increased the refilling speed of the resin by two times and reduced the printing time by nearly 25%, and the printing reliability of larger cross-sectional areas was remarkably improved.Graphical abstract for this article
       
  • Compressive creep of AlSi10Mg parts produced by selective laser melting
           additive manufacturing technology
    • Abstract: Publication date: Available online 17 July 2019Source: Additive ManufacturingAuthor(s): Naor Elad Uzan, Barak Ratzker, Peri Landau, Sergey Kalabukhov, Nachum Frage Compressive creep properties of AlSi10Mg parts produced by additive manufacturing selective laser melting (AM-SLM) were studied using a spark plasma sintering (SPS) apparatus capable of performing uniaxial compressive creep tests. Stress relief-treated specimens were tested under an applied stress of 100-130 MPa in the 175–225 °C temperature range. Utilizing two different configurations, the creep tests were conducted either with or without a low-density electric current (˜2.63–3.26 A/mm2) flowing through the test specimens. The results revealed that the creep rate increased under the influence of an applied electric current. The creep parameters (i.e., stress exponent n and apparent activation energy Q), were empirically determined. The stress exponent values were found to be 19.6 ± 1.2 and 16.2 ± 1.4 with and without current, respectively, while apparent activation energy was found to be 142 ± 9 kJ/mol and 150 ± 13 kJ/mol with and without current, respectively. The experimental results, together with microstructural examination of specimens, indicate that plastic deformation was controlled by dislocation activity. Furthermore, it is suggested that the annihilation process of dislocations during creep was enhanced by the electric current.
       
  • Effect of microstructure on the Charpy impact properties of directed
           energy deposition 300M steel
    • Abstract: Publication date: Available online 17 July 2019Source: Additive ManufacturingAuthor(s): Fenggang Liu, Xin Lin, Jing Shi, Yongjian Zhang, Peiying Bian, Xun Li, Yunlong Hu Direct manufacturing techniques, such as directed energy deposition (DED), are able to produce complex components efficiently. In this study, microstructure evolution and impact toughness of DED 300 M ultra-high strength steel are investigated. The results show that the microstructure of the as-deposited DED 300 M ultra-high strength steel is mainly composed of martensite and some blocky bainite. The micro-segregation of elements is observed within the interdendritic area. After heat treatment, the microstructure becomes uniform and consists of martensite and lower bainite. The impact toughness of the as-deposited DED 300 M ultra-high strength steel is 9 J/cm2, while it is significantly increased to 25 J/cm2 after heat treatment. Furthermore, it is observed that the fracture mode of the as-deposited sample is quasi-cleavage fracture. During the process of propagation, the main cracks would go across the martensite packet and deflect in the another one, and secondary cracks also deflected in the high-angle grain boundaries. By contrast, the fracture mode of heat-treated DED 300 M steel is ductile fracture.
       
  • An analytical method to predict and compensate for residual stress-induced
           deformation in overhanging regions of internal channels fabricated using
           powder bed fusion
    • Abstract: Publication date: Available online 17 July 2019Source: Additive ManufacturingAuthor(s): Amar M. Kamat, Yutao Pei Powder bed fusion (PBF) is ideally suited to build complex and near-net-shaped metallic structures such as conformal cooling channel networks in injection molds. However, warpage occurring due to the residual stresses inherent to this process can lead to shape deviation in the internal channels and needs to be minimized. In this research, a novel analytical model based on the Euler-Bernoulli beam bending theory was developed to estimate the residual stress-induced deformation of internal channels printed horizontally using PBF. The model was used to predict the shape deviation for three different shapes of channel cross sections (circular, elliptical, and diamond-shaped), and showed very good agreement with the experimentally determined shapes of nine different internal channels (three cases per cross-sectional shape). Further, the model predictions were used to compensate for the shape deviation in the design stage, resulting in a reduction in root mean square (RMS) deviation of the circular channel by a factor of 2. The proposed approach is thus expected to be a useful tool to generate design-for-AM guidelines for the additive manufacturing of overhangs and internal channels.
       
  • Cavitation erosion resistance of 316L austenitic steel processed by
           selective laser melting (SLM)
    • Abstract: Publication date: Available online 16 July 2019Source: Additive ManufacturingAuthor(s): C. Hardes, F. Pöhl, A. Röttger, M. Thiele, W. Theisen, C. Esen Every SLM-fabricated component typically possesses a process-specific microstructure that fundamentally differs from any conventionally fabricated specimen. This publication addresses the evaluation of microstructure-related influencing factors on the resistance against cavitation erosion. We exemplarily compared the findings to a cast and hot rolled reference sample. Due to careful adjustment of the process parameters, the overall cavitation erosion resistance of both SLM-processed and conventionally fabricated 316L are very much alike in the investigated case. The incubation period of intact surface areas is improved by the greater hardness and yield strength of the SLM specimen, which is attributable to an increased dislocation density and a smaller grain size. Nevertheless, processing and powder feeding during SLM-fabrication occasionally results in microstructural defects, at which pronounced mass loss during cavitation was registered. X-ray measurements of the residual stresses reveal the development of severe compressive stresses that emerge after a few seconds of cavitation. This compressive stress state delays the immediate propagation of SLM-inherent micro cracks. Moreover, investigations of the microstructure in combination with examination of the ongoing surface deformation highlighted the emergence of coarse grains that grew towards the temperature gradient. This effect leads to a temporarily high surface roughness, local stress concentrations and an increased probability of cavitation impacts. Furthermore, parallel cracks appear perpendicular to the scan tracks that are traced back to formerly protruded slip bands.
       
  • High-Throughput Characterization of Fluid Properties to Predict Droplet
           Ejection for Three-Dimensional Inkjet Printing Formulations
    • Abstract: Publication date: Available online 12 July 2019Source: Additive ManufacturingAuthor(s): Zuoxin Zhou, Laura Ruiz Cantu, Xuesheng Chen, Morgan R. Alexander, Clive J. Roberts, Richard Hague, Christopher Tuck, Derek Irvine, Ricky Wildman Inkjet printing has been used as an Additive Manufacturing (AM) method to fabricate three-dimensional (3D) structures. However, a lack of materials suitable for inkjet printing poses one of the key challenges that impedes industry from fully adopting this technology. Consequently, many industry sectors are required to spend significant time and resources on formulating new materials for an AM process, instead of focusing on product development. To achieve the spatially controlled deposition of a printed voxel in a predictable and repeatable fashion, a combination of the physical properties of the ‘ink’ material, print head design, and processing parameters is associated. This study demonstrates the expedited formulation of new inks through the adoption of a high-throughput screening (HTS) approach. Use of a liquid handler containing multi-pipette heads, to rapidly prepare inkjet formulations in a micro-array format, and subsequently measure the viscosity and surface tension for each in a high-throughput manner is reported. This automatic approach is estimated to be 15 times more rapid than conventional methods. The throughput is 96 formulations per 13.1 working hours, including sample preparation and subsequent printability determination. The HTS technique was validated by comparison with conventional viscosity and surface tension measurements, as well as the observation of droplet ejection during inkjet printing processes. Using this approach, a library of 96 acrylate/methacrylate materials was screened to identify the printability of each formulation at different processing temperatures. The methodology and the material database established using this HTS technique will allow academic and industrial users to rapidly select the most ideal formulation to deliver printability and a predicted processing window for a chosen application.
       
  • Towards additive manufacturing of magnesium alloys through integration of
           binderless 3D printing and rapid microwave sintering
    • Abstract: Publication date: Available online 11 July 2019Source: Additive ManufacturingAuthor(s): Mojtaba Salehi, Saeed Maleksaeedi, Mui Ling Sharon Nai, Manoj Gupta 3D printing (3DP) is a two-step additive manufacturing technique (AM) in which additively manufactured green parts in the first step are transformed into functional parts during the second step. 3DP could attract more interest if a new window of opportunity for its first and second steps is opened. Here we use capillary-mediated binderless 3DP as a novel method to additively manufacture green parts of Mg-5.06Zn-0.15 Zr powder. A unified perspective on the development steps of process parameters to obtain sufficient handling strength and a high level of dimensional accuracy in the green parts without compromising its chemical composition is established by using a scanning electron microscope, X-ray micro-tomography, vibrational spectroscopy, and chemical analysis. For the first time, microwave (MW) sintering is successfully used for densification of the green parts with centimeter-scale dimensions in which the primary chemical composition of the Mg-Zn-Zr powder is retrieved from the green parts, resulting in a compositionally zero-sum AM process. It is found that swelling leads to loss of shape fidelity during MW sintering of the green parts at temperatures ≥ 510 °C. As discussed in the context of thermal and non-thermal effects, MW significantly reduced sintering time by a factor of three to four times when compared to sintering in a conventional furnace. The results of this study suggest the notion of capillary-mediated binderless 3DP as well as MW sintering as a potential alternative for the first and second steps of 3DP, respectively.Graphical abstractGraphical abstract for this article
       
  • Feasibility Study of Silicone Stereolithography with an Optically Created
           Dead Zone
    • Abstract: Publication date: Available online 9 July 2019Source: Additive ManufacturingAuthor(s): Dong Sung (Danny) Kim, Jakkrit Suriboot, Melissa A. Grunlan, Bruce L. Tai Vat photopolymerization (VP) of silicone can produce better finish and higher resolution than the conventional extrusion-based method. One challenge in the current bottom-up VP processes is the separation that forms between the cured part and vat at each layer. Oxygen-inhibition is commonly adopted as a solution (i.e. “dead zone”), but it is limited by the size, material, and environment. Herein, a method to optically create the dead zone by low one photon polymerization (LOPP) is investigated. LOPP is achieved by a low-absorbance wavelength and a gradient light beam. In this study, two sets of the experiments, stationary exposure and moving exposure, were conducted with two low-absorbance wavelengths (375 nm and 385 nm) for a formulated UV-curable silicone. The first experiment measured the effect of beam power; the second experiment measured the effect of scanning speed. The results show that the lower-absorbance wavelength (385 nm) generates a larger, more stable dead zone and a smaller curing spot in both experiments, while the 375 nm wavelength produces a rapidly changed dead zone in the stationary condition and nearly no dead zone in the moving condition. The curing speed of 385 nm at the same power level was 10 times slower than 375 nm, but could be scaled up non-linearly by the beam power. A tripled light power of 385 nm can accelerate the process by a factor of 7 and be comparable to that of 375 nm. Thus, this study confirms the feasibility of an optically created dead zone and also uncovers the necessity of high-power light source for this application.
       
  • Flow analysis of the polymer spreading during extrusion additive
           manufacturing
    • Abstract: Publication date: Available online 9 July 2019Source: Additive ManufacturingAuthor(s): Jean-François Agassant, Franck Pigeonneau, Lucas Sardo, Michel Vincent The spreading of molten polymer between the moving printing head and the substrate in extrusion additive manufacturing is studied. Finite element computation and an analytical model have been used. The hypotheses of the analytical model are qualitatively justified by the results of the numerical computation. The analytical calculation is a powerful tool to rapidly evaluate the relationships between processing parameters (extrusion rate, printing head velocity, gap between the printing head and the substrate) and some characteristics of the deposition (dimensions of the deposited filament, pressure at the printing head nozzle, separating force between substrate and printing head). An isothermal hypothesis is discussed. The viscous non-Newtonian behavior is accounted for through an approximate shear thinning power law model. A printing processing window is defined following several requirements: a continuous deposit, without spreading in front of the printing head, maximum and minimum spreading pressures, an upper-limit for the separating force between head and substrate.
       
  • Direct Fabrication of Bimetallic Ti6Al4V+Al12Si Structures via Additive
           Manufacturing
    • Abstract: Publication date: Available online 8 July 2019Source: Additive ManufacturingAuthor(s): Yanning Zhang, Amit Bandyopadhyay Ti6Al4V + Al12Si compositionally graded cylindrical structures were fabricated on a Ti6Al4V substrate using laser engineered net shaping (LENS™) process. LENSTM fabricated materials had two regions of Ti6Al4V + Al12Si compositions, a pure Al12Si, and a pure Ti6Al4V area. Microstructural changes were affected by both laser power and compositional variations. In addition, TiSi2 and Ti3Al phase formations were also identified in low and high laser power processed Ti6Al4V + Al12Si sections, respectively. Moreover, the high laser power processed Ti6Al4V + Al12Si section showed the highest hardness value of 685.6 ± 10.6 HV0.1, which was caused due to the formation of new intermetallic phases. This high hardness section exhibited brittle failure modes during compression tests, while the pure Al12Si sections showed ductile deformation. The maximum compressive strengths of Ti6Al4V + Al12Si compositionally graded material was 507.8 ± 52.0 MPa. Our results show that compositionally gradient bulk structures of Ti6Al4V and Al12Si can be directly manufactured using additive manufacturing, however, performances can vary significantly based on process parameters and compositional variations.Graphical abstractGraphical abstract for this article
       
  • Real-time feedback controlled conversion in vat photopolymerization of
           ceramics: a proof of principle
    • Abstract: Publication date: Available online 8 July 2019Source: Additive ManufacturingAuthor(s): Thomas Hafkamp, Gregor van Baars, Bram de Jager, Pascal Etman Technical ceramics for high-performance applications can be additively manufactured using vat photopolymerization technology. This technology faces two main challenges: increasing ceramic product size and improving product quality. The integration of process control strategies into AM equipment is expected to play a key role in tackling these challenges. This work demonstrates the feasibility of real-time and in-situ feedback control of the light-initiated polymerization reaction that lies at the core of vat photopolymerization technology. To prove the principle, a single-layer experimental setup was developed in which the degree of conversion was measured by infrared spectroscopy. Experimental data obtained from this setup was used to develop a control-oriented process model and identify its parameters. A material perturbation was applied by adding an inhibitor and the case with and without feedback control were compared. The results show that the feedback controller successfully compensated for the material perturbation and reached the same final conversion value as the unperturbed case. This result can be considered a fundamental step towards additive manufacturing of defect-free ceramic parts using in-line process control.Graphical abstractGraphical abstract for this article
       
  • Novel microstructural features of selective laser melted lattice struts
           fabricated with single point exposure scanning
    • Abstract: Publication date: Available online 8 July 2019Source: Additive ManufacturingAuthor(s): E. Onal, A.E. Medvedev, M.A. Leeflang, A. Molotnikov, A.A. ZadpoorGraphical abstractGraphical abstract for this article Single point exposure scanning strategy is a largely unexplored selective laser melting (SLM) technique to fabricate lattice structures without an STL file. In this work, we used a series of>50 sets of processing parameters to systematically study the SLM fabrication of vertical struts made of Ti-6Al-4 V. In addition, the effects of (partial) re-melting and latent heat on hardness and developed microstructure were investigated. We demonstrate that the strut dimensions could be controlled by the processing parameters with higher energy inputs, resulting in larger strut diameters up to a saturation point (i.e. 520 µm corresponding to 0.5 J energy). The single point exposure method yielded keyhole shaped pores that were up to 4 times smaller than those observed, typically when the hatching and contour technique is used. Transmission electron microscopy (TEM) images revealed ultrafine (˜200-300 nm) and homogeneous microstructure in the double-melted specimens as compared to the classic microstructure of SLM Ti6Al4V observed in the single melted strut: hierarchical α'-laths with varying sizes (0.1-1 μm). Finally, we investigated the texture and identified the retained β phase, which enabled us to check the Burgers orientation relationship (BOR) between α' and β phases. This is the first attempt to systematically study the microstructural features resulting from the single point exposure SLM technique for lattice structures made from Ti6Al4V, and exhibits the processability window that could be used to engineer the microstructure of such structures.
       
  • Multi-Scale Computational Modeling of Residual Stress in Selective Laser
           Melting with Uncertainty Quantification
    • Abstract: Publication date: Available online 8 July 2019Source: Additive ManufacturingAuthor(s): Daniel Moser, Michael Cullinan, Jayathi Murthy Selective Laser Melting (SLM) is a powder-based additive manufacturing technique which creates parts by fusing together successive layers of powder with a laser. The quality of produced parts is highly dependent on the proper selection of processing parameters, requiring significant testing and experimentation to determine parameters for a given machine and material. Computational modeling could potentially be used to shorten this process by identifying parameters through simulation. However, simulating complete SLM builds is challenging due to the difference in scale between the size of the particles and laser used in the build and the size of the part produced. Often, continuum models are employed which approximate the powder as a continuous medium to avoid the need to model powder particles individually. While computationally expedient, continuum models require as inputs effective material properties for the powder which are often difficult to obtain experimentally. Building on previous works which have developed methods for estimating these effective properties along with their uncertainties through the use of detailed models, this work presents a part scale continuum model capable of predicting residual thermal stresses in an SLM build with uncertainty estimates. Model predictions are compared to experimental measurements from the literature.
       
  • Non-destructive testing for wire + arc additive manufacturing of aluminium
           parts
    • Abstract: Publication date: Available online 5 July 2019Source: Additive ManufacturingAuthor(s): João B. Bento, Ana Lopez, Inês Pires, Luísa Quintino, Telmo G. Santos Wire + Arc Additive Manufacturing (WAAM) has already proven to be successful for the production of large metal parts. However, there are still no specific standards available to label the quality requirements of the parts produced by WAAM and this is preventing a more widespread adoption of the technique.A crucial step towards the quality assurance of WAAM parts will be the development of Non-Destructive Testing (NDT) systems capable of identifying defects while parts are being produced. In this regard, Eddy Current Testing (ECT) can play a significant role, by allowing the inspection of both ferromagnetic and non-ferromagnetic materials, with high speeds and without contact with the material surface. The limitation here is that commercial ECT targets only the inspection of surface and subsurface defects.This study is focused on the development of a NDT system which includes customized ECT probes for the inline layer-by-layer detection of defects in aluminium WAAM samples. Results revealed that the developed EC probes were able to locate artificial defects: at depths up to 5 mm; with a thickness as small as 350 µm; with the probe up to 5 mm away from the inspected sample surface.The developed ECT probes proved to surpass the limitation of commercial ones. Also, these probes were able to overcome the limitations caused by the surface roughness of the samples and the high temperatures involved in the deposition process. These preliminary results represent an important step for the development of NDT systems for WAAM.
       
  • High-power laser-matter interaction during laser powder bed fusion
    • Abstract: Publication date: Available online 3 July 2019Source: Additive ManufacturingAuthor(s): Jie Yin, Langliang Yang, Xu Yang, Haihong Zhu, Dengzhi Wang, Linda Ke, Zemin Wang, Guoqing Wang, Xiaoyan Zeng Laser powder bed fusion (LPBF) additive manufacturing (AM) is developing with the goal of fabricating parts with high performance and high efficiency. Laser power is the key factor to the efficiency, microstructure and performance in LPBF. However, there are limited reports regarding the laser-matter interaction in LPBF under high-power conditions. In this work, the molten pool characteristics and spatter behavior in LPBF with a high power and a wide process window (from 350 W to 1550 W) are studied based on high-speed high-resolution imaging. The results show that the molten pool characteristics and spatter behavior depend on the laser input energy. The average ejection velocity and ejection angle increase with the laser power. The droplet column ejection and large spatters are prone to occur with a high-power laser. Furthermore, the times at which the vapor depression and the protrusion in the molten pool first occur decrease dramatically with an increase in the laser input energy. When the laser mode and spot size are kept constant, the laser power determines the amount of time required for melting, the vapor depression and the protrusion in LPBF to occur, while the laser scan velocity determines whether the laser dwell time is sufficient for these phenomena to form.
       
  • Neutron diffraction residual stress determinations in Fe3Al based iron
           aluminide components fabricated using wire-arc additive manufacturing
           (WAAM)
    • Abstract: Publication date: Available online 2 July 2019Source: Additive ManufacturingAuthor(s): Chen Shen, Mark Reid, Klaus-Dieter Liss, Zengxi Pan, Yan Ma, Dominic Cuiuri, Stephen van Duin, Huijun Li The Wire-Arc Additive Manufacturing (WAAM) process is an increasingly attractive method for producing porosity-free metal components. However, the residual stresses and distortions resulting from the WAAM process are major concerns as they not only influence the part tolerance but can also cause premature failure in the final component during service. The current paper presents a method for using neutron diffraction to measure residual stresses in Fe3Al intermetallic wall components that have been in-situ additively fabricated using the WAAM process with different post-production treatments. By using averaging methods during the experimental setup and data processing, more reliable residual stress results are obtained from the acquired neutron diffraction data. In addition, the present study indicates that the normal residual stresses are significant compared to normal butt/fillet welding samples, which is caused by the large temperature gradient in this direction during the additive layer depositions.
       
  • Development of a Hot-Melt Extrusion (HME) process to produce drug loaded
           Affinisol™ 15LV filaments for Fused Filament Fabrication (FFF) 3D
           printing
    • Abstract: Publication date: Available online 2 July 2019Source: Additive ManufacturingAuthor(s): Elke Prasad, Muhammad T. Islam, Daniel J. Goodwin, Andrew J. Megarry, Gavin W. Halbert, Alastair J. Florence, John Robertson The aim of the present work was to develop a pilot scale process to produce drug-loaded filaments for 3D printing of oral solid dose forms by fused filament fabrication (FFF). Using hot melt extrusion, a viable operating space and understanding of processing limits were established using a hydrophilic polymer (hydroxypropyl methylcellulose (HPMC) - Affinisol™ LV15). This was then extended to formulate paracetamol (PCM) loaded Affinisol™ 15LV filaments across a wide range of compositions (5 - 50 wt% drug). From the process development work, challenges in achieving a pilot scale process for filament production for pharmaceutical applications have been highlighted. 3D printing trials across the range of compositions demonstrated limitations concerning the ability to print successfully across all compositions. Results from characterisation techniques including thermal and mechanical testing when applied to the formulated filaments indicated that these techniques are a useful predictive measure for assessing the ability to print a given formulation via filament methods. Oral solid dosage forms of variable surface area to mass ratios printed from suitable filament compositions demonstrated the ability to modify the release rates of drug for fixed formulations across substantial timescales.
       
  • Integrated manufacture of polymer and conductive tracks for real-world
           applications
    • Abstract: Publication date: Available online 2 July 2019Source: Additive ManufacturingAuthor(s): Barbara Urasinska-Wojcik, Neil Chilton, Peter Todd, Christopher Elsworthy, Myles Bates, Gethin Roberts, Gregory J. Gibbons The present study demonstrates for the first time a unique UK-designed and built Additive Manufacturing (AM) hybrid system that combines polymer based structural deposition with digital deposition of electrically conductive elements. This innovative manufacturing system is based on a multi-planar build approach to improve on many of the limitations associated with AM, such as poor surface finish, low geometric tolerance and poor robustness. Specifically, the approach involves a multi-planar Material Extrusion (ME) process in which separated build stations with up to 5 axes of motion replace traditional horizontally-sliced layer modelling. The construction of multi-material architectures also involved using multiple print systems in order to combine both ME and digital deposition of conductive material. To demonstrate multi-material 3D Printing (3DP) we used three thermoplastics to print specimens, on top of which a unique Ag nano-particulate ink was printed using a non-contact jetting process, during which drop characteristics such as shape, velocity, and volume were assessed using a bespoke drop watching system. Electrical analysis of printed conductive tracks on polymer surfaces was performed during mechanical testing (static tensile and flexural testing and dynamic fatigue testing) to assess robustness of the printed circuits. Both serpentine and straight line patterns were used in the testing of Ag particle loaded ink and they showed very similar resistance changes during mechanical exposure. Monitored resistance and stress changed as a function of strain exhibiting hysteresis with more prominent residual strain during stretching and compression cycles and 3-point bending flexural tests of PA and CoPA substrates. Bare and encapsulated tracks exhibited low electrical resistivity (1-3*10-6Ω*m), and its change was more rapid on ABS and minor on PA and CoPA when increasing tensile and flexural strain up to 1.2% and 0.8%, respectively. Resistance of Ag tracks on ABS also increased rapidly during fatigue testing and the tracks easily fractured during repeated stretching-compression cycles at 1% and 1.2% strain. No resistance changes of Ag tracks printed on PA and CoPA were observed at lower strain amplitudes whereas at higher strain amplitudes these changes were the lowest for conductive tracks on CoPA. Thermal analyses were conducted to determine the printed material’s glass transition temperature (Tg), stability and degradation behavior to find the optimum annealing conditions post printing. The novel AM printer has the ability to fabricate fully functional objects in one build, including integrated printed circuitry and embedded electronics. It enables product designers and manufactures to produce functional saleable electronic products. This new technology also gives the opportunity for designers to improve existing products, as well as create new products with the added advantages of geometrically unconstrained 3DP.
       
  • The effect of texture on the anisotropy of thermophysical properties of
           additively manufactured AlSi10Mg
    • Abstract: Publication date: Available online 29 June 2019Source: Additive ManufacturingAuthor(s): Einat Strumza, Ori Yeheskel, Shmuel Hayun The process of additive manufacturing (AM) has rapidly developed over the past two decades and is now addressing the needs of industry for fast production of samples with tailored properties and complex geometries. One of the most common alloys fabricated from powder using the Laser Powder Bed Fusion (L-PBF) method is AlSi10Mg. The effects of the inherent anisotropy and existing porosity in AM AlSi10Mg were investigated in terms of thermophysical properties, namely thermal conductivity, diffusivity, heat capacity and thermal expansion. These properties were measured in the two principal directions, namely parallel and perpendicular to the printing direction (i.e., in the Z- and X-directions, respectively). In both cases, the sample showed abnormal thermal expansion and conductivity, as compared to a conventionally fabricated sample. After heat treatment, macro- and microstructure analysis confirmed that thermally induced porosity (TIP) had occurred. The anisotropic behaviors of thermal conductivity, diffusivity and thermal expansion were found to be related to the texture, preferred orientation and pore distribution of the aluminum grains in the L-PBF-treated samples.
       
  • The Formation and Kr-ion Irradiation Behaviour of New Microstructural
           Features in Additively Manufactured Titanium Aluminium Alloy
    • Abstract: Publication date: Available online 29 June 2019Source: Additive ManufacturingAuthor(s): Hanliang Zhu, Yan Ma, Tao Wei, Huijun Li, Robert Aughterson, Gregory LumpkinABSTRACTNew microstructural features were found in the TiAl alloy manufactured using the gas tungsten arc welding-based additive manufacturing technology. The ion-irradiation response of the new microstructure features were investigated in-situ via irradiation with 1 MeV Kr2+ ions at room and 873 K. Examination of the microstructure showed that the typical lamellar microstructure consisting of α2-Ti3Al and γ-TiAl phases formed α2/γ lamellar interfaces and γ/γ twin boundaries. Apart from this, the γ lamellae were also found to form γ/γ lamellar boundaries with the two γ lamellae in the same orientation or the  // orientation relationship. This is not observed in the TiAl alloys fabricated using traditional alloy fabrication methods. Kr ion-irradiation at room and elevated temperatures resulted in no significant difference in the morphologies of most radiation-induced defects in the orientated γ lamellae and the orientated γ lamellae. However, the areas of the new boundaries exhibited different damage morphologies in comparison with the traditional γ/γ twin boundaries. The formation mechanisms of the new microstructural features formed in the additive manufacturing process and their irradiation behaviour are investigated and discussed.Graphical Graphical abstract for this article
       
  • The Effects of Material Anisotropy on Secondary Processing of Additively
           Manufactured CoCrMo
    • Abstract: Publication date: Available online 28 June 2019Source: Additive ManufacturingAuthor(s): Patxi Fernandez-Zelaia, Vinh Nguyen, Hayley Zhang, Arkadeep Kumar, Shreyes N. Melkote Components produced by near net shape additive manufacturing processes often require subsequent subtractive finishing operations to satisfy requisite surface finish and geometric tolerances. It is well established that the microstructure and properties of the as-built component are sensitive to the additive processing history. Therefore, downstream secondary processes may be affected by the as-built components’ mechanical behavior. In this work we study the sensitivity of secondary machining operations on CoCrMo samples produced via selective laser melting. Utilizing novel high-throughput mechanical testing, microstructure characterization, and a rigorous statistical analysis we investigate the degree of material anisotropy present in the as-built material. We then study the effects of this anisotropy on secondary processing via slot milling experiments. Our results indicate that mechanical anisotropy is driven by both the morphology of the microstructure as well as crystallographic texture. The machining force response is correspondingly sensitive to these sources of anisotropy, which has the potential to impact how manufacturers finish additively built parts.
       
  • Effect of Ultrasonic Shot Peening on the Surface Defects of Thin Struts
           Built by Electron Beam Melting: Consequences on Fatigue Resistance
    • Abstract: Publication date: Available online 28 June 2019Source: Additive ManufacturingAuthor(s): Théo Persenot, Alexis Burr, Emeric Plancher, Jean-Yves Buffière, Rémy Dendievel, Guilhem Martin Ti-6Al-4 V thin struts, built by selective electron beam melting have been submitted to hot isostatic pressing and further processed with ultrasonic shot peening to investigate the effect of this surface treatment on surface defects. The consequence of those surface defects on the fatigue resistance of thin struts has been evaluated before and after treatment. X-ray microtomography has been used to track the defect population with repeated scans before and after ultrasonic shot peening as well as after fatigue tests conducted to failure. Our results show that the fatigue resistance of surface-treated struts (measured at 105 cycles) is doubled compared to the fatigue resistance of struts with an as-built surface. This enhancement in fatigue resistance is attributed to a refinement of the sub-surface microstructure, to the introduction of local compressive residual stresses and to a significant surface smoothing effect induced by ultrasonic shot peening. Ultrasonic shot peening fails to “heal” the most tortuous surface defects observed in the as-built condition, leaving residual defects in the sub-surface region. The presence of those residual surface defects limits the fatigue resistance when compared to struts with a machined surface.Graphical abstractGraphical abstract for this article
       
  • On process-step parallel computability and linear superposition of
           mechanical responses in additive manufacturing process simulation
    • Abstract: Publication date: Available online 26 June 2019Source: Additive ManufacturingAuthor(s): Dirk Munro, Can Ayas, Matthijs Langelaar, Fred van Keulen We study linearity assumptions in the transient macroscale mechanical aspect of additive manufacturing (AM) process simulation. Linearity assumptions are often resorted to in combination with calibrated inelastic deformation components to arrive at computationally tractable yet reasonably accurate AM process models. We point out that linearity assumptions permit the independent computation of the response increment in each step of the AM process, and the total mechanical response is the superposition of all the process-step increments. In effect, process-step increments are computed with respect to the stress-free reference configuration in each step. The implication is that the mechanical response increment in each linearised AM process step may be computed in parallel. Trivial process-step-wise parallel computability breaks down, however, if nonlinearity (i.e. geometric or material) is modelled. In our investigation the influence of geometric nonlinearity on part distortion is small (but this is of course part-geometry specific), and more realistic stresses are obtained by imposing a nonlinear elastoplastic material law after the parallel computation and superposition of the linear AM response increments. It is demonstrated that simulation wall-clock time is reduced by exploiting process-step parallel computability in the linear regime. Moreover, numerical experiments suggest that process-step parallelization scales better (in wall-clock time) than conventional parallelization in the sequential computation of each response increment.
       
  • Modeling of solidification microstructure evolution in laser powder bed
           fusion fabricated 316L stainless steel using combined computational fluid
           dynamics and cellular automata
    • Abstract: Publication date: Available online 26 June 2019Source: Additive ManufacturingAuthor(s): Yi Zhang, Jing Zhang This work presents a novel modeling framework combining computational fluid dynamics (CFD) and cellular automata (CA), to predict the solidification microstructure evolution of laser powder bed fusion (PBF) fabricated 316 L stainless steel. A CA model is developed which is based on the modified decentered square method to improve computational efficiency. Using this framework, the fluid dynamics of the melt pool flow in the laser melting process is found to be mainly driven by the competing Marangoni force and the recoil pressure on the liquid metal surface. Evaporation occurs at the front end of the laser spot. The initial high temperature occurs in the center of the laser spot. However, due to Marangoni force, which drives high-temperature liquid flowing to low-temperature region, the highest temperature region shifts to the front side of the laser spot where evaporation occurs. Additionally, the recoil pressure pushes the liquid metal downward to form a depression zone. The simulated melt pool depths are compared well with the experimental data. Additionally, the simulated solidification microstructure using the CA model is in a good agreement with the experimental observation. The simulations show that higher scan speeds result in smaller melt pool depth, and lack-of-fusion pores can be formed. Higher laser scan speed also leads to finer grain size, larger laser-grain angle, and higher columnar grain contents, which are consistent with experimental observations. This model can be potentially used as a tool to optimize the metal powder bed fusion process, through generating desired microstructure and resultant material properties.
       
  • Experimental Validation of Thermo-mechanical Part-Scale Modeling for Laser
           Powder Bed Fusion Processes
    • Abstract: Publication date: Available online 25 June 2019Source: Additive ManufacturingAuthor(s): Michael Gouge, Erik Denlinger, Jeff Irwin, Chao Li, Pan Michaleris Numerical simulation has been posited as a key tool to reduce the unwanted distortion which occurs during laser powder bed fusion additive manufacturing, yet the scale and speed of the process makes traditional moving source modeling impractical. In this work a part-scale model is validated for the distortion modeling of laser powder bed fusion manufacture. The model uses an automatically generated adaptive voxel mesh which reduces the size of the finite element mesh and thus the computational time required to simulate large and complex additive geometries. The approach address several of the major challenges of effects part-scale modeling, namely the addition of new material into the simulation, the voxel meshing of thin or detailed geometry sections, and the efficient meshing of thick cross sections. The part-scale simulation uses input from a detailed small-scale analysis as part of a multi-scale approach. However, the part-scale approach could also be applied as part of a traditional experimentally calibrated inherent strain modeling approach. Validation is achieved by comparing the three-dimensional scans of three manufactured parts, a small thin walled Inconel 625 Compliant Cylinder, a small Inconel 718 build with both very thin and very thick sections, and an industrial scale part formed from AlSi10Mg. The models show a maximum peak distortion error of 13% and a minimum Correlation of 90.5% for comparisons made at selected points. A comparison of build versus simulation time shows that the adaptive meshing allows the model to run in much less time than it takes for the parts to be constructed. A presentation of thermal and mechanical results demonstrates the part-scale model's ability to capture the complex evolution of the thermo-mechanical behavior of components formed using powder bed fusion.
       
  • Controlling the extent of atomic ordering in intermetallic alloys through
           additive manufacturing
    • Abstract: Publication date: Available online 24 June 2019Source: Additive ManufacturingAuthor(s): Andrew B. Kustas, Chris M. Fancher, Shaun R. Whetten, Daryl J. Dagel, Joseph R. Michael, Donald F. Susan Control of the atomic structure, as measured by the extent of the embrittling B2 chemically ordered phase, is demonstrated in intermetallic alloys through additive manufacturing (AM) and characterized using high fidelity neutron diffraction. As a layer-by-layer rapid solidification process, AM was employed to suppress the extent of chemically ordered B2 phases in a soft ferromagnetic Fe-Co alloy, as a model material system of interest to electromagnetic applications. The extent of atomic ordering was found to be insensitive to the spatial location within specimens and suggests that the thermal conditions within only a few AM layers were most influential in controlling the microstructure, in agreement with the predictions from a thermal model for welding. Analysis of process parameter effects on ordering found that suppression of B2 phase was the result of an increased average cooling rate during processing. AM processing parameters, namely interlayer interval time and build velocity, were used to systematically control the relative fraction of ordered B2 phase in specimens from 0.49 to 0.72. Hardness of AM specimens was more than 150% higher than conventionally processed bulk material. Implications for tailoring microstructures of intermetallic alloys are discussed.
       
  • Binder Jetting: A Review of Process, Materials, and Methods
    • Abstract: Publication date: Available online 22 June 2019Source: Additive ManufacturingAuthor(s): Mohsen Ziaee, Nathan B Crane Binder Jet printing is an additive manufacturing technique that dispenses liquid binding agent on powder. Layers are formed repeatedly to build up a physical article. Binder jetting (BJ) can be adapted to almost any powder with high production rates. The BJ process utilizes a broad range of technologies including printing methods, powder deposition, dynamic binder/powder interaction, and post-processing methods. A wide variety of materials have been demonstrated including polymers, metals, and ceramics, but a common challenge is developing printing and post-processing methods that maximize part performance. This article presents a broad review of technologies and approaches that have been applied in Binder Jet printing and points towards opportunities for future advancement.
       
  • Investigations on nozzle geometry in Fused Filament Fabrication
    • Abstract: Publication date: Available online 19 June 2019Source: Additive ManufacturingAuthor(s): Vinzenz Nienhaus, Kevin Smith, Dieter Spiehl, Edgar Dörsam This investigation focuses on geometric parameters of nozzles used in Fused Filament Fabrication. They are mainly responsible for the extrusion force. Therefore, the influence of the geometric parameters was investigated. Typical nozzles are made of brass and feature a decrease in diameter from an entry channel to a capillary with a conical section in between. At a fixed entry and capillary diameter, variable parameters are the angle of the conical section and the length of the capillary. Commercially available and custom nozzles with various of these parameters were investigated on a test stand using Polylactic Acid (PLA) filament. All nozzles exhibit a common behavior. The extrusion force rises linearly with increasing filament feed velocity. At a certain point the forces fluctuate and increase rapidly. Here, unmolten plastic reaches the nozzle. This characteristic is dependent on extrusion temperature and geometric parameters of the nozzles. A work envelope was defined; the lowest extrusion forces were found for a 56° conical section, but differences in force between the angles from 30° to 118° are low in comparison to the total extrusion force. Different capillary lengths were used to determine the entry pressure loss at different filament feed velocities. The material and coating of the nozzles had no significant influence on extrusion force. A higher thermal mass, two conical sections or two entry channels have a positive effect on extrusion forces and maximum filament feed velocities, thus maximal build rate.
       
  • Effects of variable gravity conditions on Additive Manufacture by Fused
           Filament Fabrication using Polylactic Acid Thermoplastic Filament
    • Abstract: Publication date: Available online 19 June 2019Source: Additive ManufacturingAuthor(s): Aidan Cowley, Jeremy Perrin, Alexandre Meurisse, Antoine Micallef, Miranda Fateri, Louis Rinaldo, Nathan Bamsey, Matthias Sperl The capability to manufacture items in space is an exploration enabling advancement, and will be crucial for sustainable human exploration as we progress beyond Earth orbit. The extrusion based Fused Filament Fabrication (FFF) method using thermoplastics represents a robust and simple methodology applicable to printing parts for both current and future human spaceflight exploration missions. Understanding the performance and behaviour of the FFF process under varying gravity loads is therefore an important knowledge gap that needs to be addressed in order to fully appreciate the characteristics of space manufactured elements. At present, it is not well understood how gravity can influence the characteristics of such elements fabricated in variable gravity environments. In this study, we detail an experiment conducted on a parabolic flight campaign (PFC) wherein we produced a number of FFF polylactic acid (PLA) polymer test articles and compared them to terrestrially fabricated articles. We report on the methodology and the operational parameters used, as well as presenting an analysis of the samples via optical microscopy and tomography. Compressive, tensile and other technical properties are reported herein. A number of explanations are presented to explain the variance in specimens relative to terrestrial reference samples.
       
  • Effects of Nb and Mo on the Microstructure and Properties of 420 Stainless
           Steel Processed by Laser-Powder Bed Fusion
    • Abstract: Publication date: Available online 18 June 2019Source: Additive ManufacturingAuthor(s): Subrata Deb Nath, Emma Clinning, Gautam Gupta, Vincent Wuelfrath-Poirier, Gilles L’Espérance, Ozkan Gulsoy, Martin Kearns, Sundar V. Atre Niobium (Nb) and molybdenum (Mo) are conventionally added to stainless steels to improve their mechanical and corrosion properties. However, the effects of Nb and Mo addition on the processing and properties in laser-powder bed fusion (L-PBF) have not been well investigated, especially in the context of 420 stainless steel. In this study, 420 stainless steel pre-alloyed with Nb (1.2 wt.%) and Mo (0.57 wt.%) was processed by L-PBF and characterized in terms of its physical, mechanical and corrosion properties as well as microstructure. The addition of Nb and Mo did not significantly affect the densification of 420 stainless steel when printed over an energy range of 28 to 75 J/mm3 and a maximum density of 99.3 ± 0.02 % theoretical at 63 J/mm3 was achieved. In mechanical tests, L-PBF 420 stainless steel specimens exhibited higher mechanical properties in the presence of Nb and Mo. After heat treatment, the UTS of 420 stainless steel with Nb and Mo improved to 1750 ± 30 MPa and elongation to 9.0 ± 0.2 %, much higher than previously reported properties achieved in L-PBF and exceeding wrought 420 stainless steel. The tempering of martensite phases as well as the presence of nanoscale NbC were found to correlate with improved mechanical properties. In electrochemical tests, 420 stainless steel exhibited slightly better corrosion properties with the addition of Nb and Mo.
       
  • Microstructure and nanoindentation creep behavior of CoCrFeMnNi
           high-entropy alloy fabricated by selective laser melting
    • Abstract: Publication date: Available online 17 June 2019Source: Additive ManufacturingAuthor(s): Zhenlin Xu, Hui Zhang, Weihuo Li, Aiqin Mao, Lin Wang, Guangsheng Song, Yizhu He Selective laser melting (SLM) was used to fabricate an equiatomic CoCrFeMnNi high-entropy alloy (HEA). The SLM-fabricated CoCrFeMnNi HEA samples were studied with X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), electron backscatter diffraction (EBSD) and nanoindentation techniques to characterize the microstructure and creep behavior. It was found that the HEA comprised a single face-centered cubic (fcc) structure. Due to the fast solidification and high temperature gradients of the molten pool during the SLM process, the microstructure comprised cellular subgrains with grain boundary angles lower than 5°. Based on energy dispersive spectrometry (EDS) maps, the constituent elements of the SLM-fabricated HEA were homogenously distributed. Moreover, the effect of the peak holding load on the nanoindentation creep deformation of the SLM-fabricated HEA was investigated using a Berkovich indenter. The results of this study indicated that the creep was mainly dominated by deformation controlled by dislocation motion.Graphical abstractGraphical abstract for this article
       
  • Effect of Post-Treatments under Hot Isostatic Pressure on Microstructural
           Characteristics of EBM-built Alloy 718
    • Abstract: Publication date: Available online 16 June 2019Source: Additive ManufacturingAuthor(s): Sneha Goel, Anumat Sittiho, Indrajit Charit, Uta Klement, Shrikant Joshi Electron beam melting (EBM) has emerged as an important additive manufacturing technique. In this study, Alloy 718 produced by EBM was investigated in as-built and post-treated conditions for microstructural characteristics and hardness. The post-treatments investigated were hot isostatic pressing (HIP) and combined HIP + heat treatment (HIP + HT) carried out as a single cycle inside the HIP vessel. Both the post-treatments resulted in significant decrease in defects inevitably present in the as-built material. The columnar grain structure of the as-built material was found to be maintained after post-treatment, with some sporadic localized grain coarsening noted. Although HIP led to complete dissolution of δ and γ′′ phase, stable NbC and TiN (occasionally present) particles were observed in the post-treated specimens. Significant precipitation of γ′′ phase was observed after HIP + HT, which was attributed to the two-step aging heat treatment carried out during HIP + HT. The presence of γ′′ phase or otherwise was correlated to the hardness of the material. While the HIP treatment resulted in drop in hardness, HIP + HT led to ‘recovery’ of the hardness to values exceeding those exhibited by the as-built material.
       
  • Optimization and comparison of porosity rate measurement methods of
           Selective Laser Melted metallic parts
    • Abstract: Publication date: Available online 15 June 2019Source: Additive ManufacturingAuthor(s): Thibaut de Terris, Olivier Andreau, Patrice Peyre, Frédéric Adamski, Imade Koutiri, Cyril Gorny, Corinne Dupuy The systematic manifestation of porosities inside selective laser melted (SLM) parts is a well-known issue. In order to improve the density of SLM parts, it is important not only to assess the physical origin of the different types of porosities, but also to be able to estimate as precisely as possible the porosity rate so that one may select the optimum manufacturing parameters.Considering 316 L steel parts built with different input energies, the current paper aims to (1) present the different types of porosities generated by SLM and their origins, (2) compare different methods to characterize parts density and propose optimal procedures. After a preliminary optimization step, three methods were used for quantifying porosity rate: the Archimedes method, the helium pycnometry and micrographic observations.The Archimedes method shows that results widely depend on the fluid nature and temperature, but also on the sample volume and its surface roughness. The best results were obtained on samples without surface modifications, and with demineralized water. The optimal analyzed volume was 2 cm3.During the micrographic observations, various magnifications were studied combined with a different number of micrographs. A magnification of at least x 100 was required to provide reliable results. The analysis area, and thus the number of micrographs needed to estimate the porosity amount was found to depend on the porosity amount itself. For low porosity amounts (
       
  • Microstructure, fatigue, and impact toughness properties of additively
           manufactured nickel alloy 718
    • Abstract: Publication date: Available online 15 June 2019Source: Additive ManufacturingAuthor(s): Mageshwari Komarasamy, Shivakant Shukla, Sarah Williams, Kumar Kandasamy, Shawn Kelly, Rajiv S. Mishra A complete understanding of processing-structure-property-performance relationship of additively manufactured (AM) components are critical from an application standpoint. Therefore, in the current investigation, a comprehensive microstructural characterization and mechanical properties (tensile, fatigue and impact toughness) evaluation of nickel alloy 718 AM by the laser powder bed fusion (L-PBF) technique have been performed. AM builds were made from powders manufactured via different atomization conditions. Although the standard post-heat treatment procedure led to the removal of severe interdendritic segregation both grain boundary and intra-grain precipitation of δ phase occurred. Regardless of δ phase presence, axial fatigue properties of both the AM builds were similar to design handbook wrought fatigue data. However, due to the δ phase, impact toughness properties were comparable to the wrought material conditions that exhibited δ phase. Fractured surfaces of Charpy impact samples exhibited crack propagation extensively along the boundaries decorated by δ precipitates.Graphical abstractGraphical abstract for this article
       
  • Viscoelastic Properties of Fused Filament Fabrication Parts
    • Abstract: Publication date: Available online 15 June 2019Source: Additive ManufacturingAuthor(s): José Luis Colón Quintana, Alec Redmann, Gerardo A. Mazzei Capote, Angel Pérez-Irizarry, Abrahan Bechara, Tim A. Osswald, Roderic Lakes Parts made by fused filament fabrication differ in their mechanical properties from the parent material. To investigate the effect of the manufacturing process on the mechanical properties of 3D-printed parts, a series of experiments including Dynamic Mechanical Analysis (DMA) and ultrasonic wave propagation were conducted. For this purpose, printed parts were made from custom ABS filament and were printed using a rectangular bead shape to minimize porosity. The main properties investigated included the elastic, loss and storage moduli, and the material loss tangent (tan δ). Results indicate that the elastic modulus of the printed material was somewhat lower than that of the parent material, about 2 GPa for frequencies 0.1 Hz to 100 Hz. Furthermore, tan δ was largest for the parent material at about 0.03 compared with 0.01 to 0.02 for the printed material. Ultrasonic longitudinal wave measurements at 1 MHz on printed specimens with bead angles of 0°, 45° and 90°, revealed minimal anisotropy and, consistent with DMA results, tan δ was also largest for the parent material.
       
  • Layer geometry control for the fabrication of lattice structures by wire
           and arc additive manufacturing
    • Abstract: Publication date: Available online 11 June 2019Source: Additive ManufacturingAuthor(s): Takeyuki Abe, Hiroyuki Sasahara Wire and arc additive manufacturing (WAAM) is an additive manufacturing (AM) technology that uses wire-form materials and arc discharges as the energy source. AM techniques can fabricate complicated shapes that cannot be obtained via conventional processing. Building lattice structures inside components enables weight reduction while maintaining high strength. Strut shapes must be constructed to form these lattice structures using WAAM. For fabricating strut shapes with high accuracy, the process parameters should be optimized. However, the relationship between layer geometry and process parameters is not clear. Therefore, in this study, struts were fabricated under various process conditions to investigate the influences of process parameters on the built object geometry. The results showed that fabrication of strut shapes depends on the heat input condition. Moreover, it was found that the arc discharge time had the highest influence on the layer height and diameter. The inclination angle of an overhanging shape had little influence on the dimensional accuracy of the built object. In addition, computer-aided manufacturing (CAM) system was developed for the fabrication of lattice structures, and the lattice structures were successfully built using WAAM. The build accuracy was measured using an x-ray computed tomography (CT) scanner; the deviation in the structures designed using the CAM system and the actual fabricated structures measured using the CT scanner was lower than approximately ±2.3 mm.
       
  • Additive Manufacturing Standards for Space Resource Utilization
    • Abstract: Publication date: Available online 11 June 2019Source: Additive ManufacturingAuthor(s): Hunter Williams, Evan Butler-Jones
       
  • Heterogeneities Dominate Mechanical Performance of Additively Manufactured
           Metal Lattice Struts
    • Abstract: Publication date: Available online 11 June 2019Source: Additive ManufacturingAuthor(s): Amber D. Dressler, Elliott W. Jost, John C. Miers, David G. Moore, Carolyn C. Seepersad, Brad L. Boyce Architected structural metamaterials, also known as lattice, truss, or acoustic materials, provide opportunities to produce tailored effective properties that are not achievable in bulk monolithic materials. These topologies are typically designed under the assumption of uniform, isotropic base material properties taken from reference databases and without consideration for sub-optimal as-printed properties or off-nominal dimensional heterogeneities. However, manufacturing imperfections present throughout the lattices and their constituent struts create significant variability in mechanical properties and part performance. This study utilized a customized tensile bar with a gauge section consisting of five parallel struts loaded in a stretch (tensile) orientation to examine the impact of manufacturing heterogeneities on quasi-static deformation of the struts, with a focus on ultimate tensile strength and ductility. The customized tensile specimen was designed to prevent damage during handling, despite the sub-millimeter thickness of each strut, and to enable efficient, high-throughput mechanical testing. The strut tensile specimens and reference monolithic tensile bars were manufactured using a direct metal laser sintering (also known as laser powder bed fusion) process in a precipitation hardened stainless steel alloy, 17-4PH, with minimum feature sizes ranging from 0.5-0.82 mm, comparable to minimum allowable dimensions for the process. Over 70 tensile stress-strain tests were performed revealing that the effective mechanical properties of the struts were highly stochastic, considerably inferior to the properties of larger as-printed reference tensile bars, and well below the minimum allowable values for the alloy. Pre- and post-test non-destructive analyses revealed that the primary source of the reduced properties and increased variability was attributable to heterogeneous surface topography with stress-concentrating contours and commensurate reduction in effective load-bearing area.
       
  • Bridging Additive Manufacturing and Sand Casting: Utilizing Foundry Sand
    • Abstract: Publication date: Available online 11 June 2019Source: Additive ManufacturingAuthor(s): Kevin J. Hodder, Richard J. Chalaturnyk Although additive manufacturing technology is available for the direct fabrication of metal parts, the process is still in a juvenile state compared to older metal fabrication methods such as sand casting. Therefore, limited standards are available stipulating the use of additively-manufactured parts in critical service conditions such as extreme environments or safety components. However, since sand casting is suited for multiple units of parts, the time and resources needed to produce a single part through sand casting is not ideal for a competitive market. Although additive manufacturing or “3D printing” has been combined with metal casting in the past through “rapid casting” to fabricate sand molds directly, the sand used is stipulated by the 3D printer. The use of specialized sand may result in changes to infrastructure and large amounts of additional sand required to be stored on location. The main question we sought to answer was if traditional foundry sand or “non-standard” sand could be used within a 3D printing system' We report herein that the although the increase in surface roughness may be tolerable, the use of foundry sand within a 3D printer produces molds with less than optimal results, mainly due to the absence of compaction. Binder bleeding via the liquid binder jetting process also contributes to a loss in dimensional quality.
       
  • Nozzle-Integrated Pre-Deposition and Post-Deposition Heating of Previously
           Deposited Layers in Polymer Extrusion Based Additive Manufacturing
    • Abstract: Publication date: Available online 7 June 2019Source: Additive ManufacturingAuthor(s): Darshan Ravoori, Hardikkumar Prajapati, Viswajit Talluru, Ashfaq Adnan, Ankur Jain The adhesion and merging of adjacent filaments in polymer extrusion based additive manufacturing (AM) plays a key role in determining the thermal and mechanical properties of the built part. It is well-known that maintaining the deposited filaments at a high temperature aids in the process of adhesion and merging. While external mechanisms such as laser and infrared heating have been used in the past to heat up deposited filaments, this paper presents a simpler, less invasive and in situ mechanism for heating of previously deposited layers using a hot metal block integrated with and rastering together with the filament-dispensing nozzle. Infrared thermography based quantitative measurement of temperature field along the raster line is carried out for two configurations – a preheater and a postheater traveling ahead of or behind the nozzle respectively. In each case, significant temperature rise in the deposited filaments is shown. A configuration comprising both preheater and postheater is shown to result in additional thermal benefits. The measured temperature rise is shown to be a function of process parameters such as raster speed and heater-to-base gap. Experimental measurements are also shown to agree well with theoretical and simulation models. Cross-section imaging of samples printed with preheating and without preheating clearly show significant improvement in neck growth and filament-to-filament merging compared to the baseline case. Improvement in thermal and structural performance of printed samples is also demonstrated. Compared to other techniques proposed in the past, the heating approach presented in this work is passive and requires minimal additional costs or complexity. The improved temperature field and consequently enhanced filament adhesion reported here may help design and build parts with superior thermal and mechanical properties using polymer AM.
       
  • 3D printed poly(lactic acid) composites with grafted cellulose nanofibers:
           effect of nanofiber and post-fabrication annealing treatment on composite
           flexural properties
    • Abstract: Publication date: Available online 7 June 2019Source: Additive ManufacturingAuthor(s): Ju Dong, Changtong Mei, Jingquan Han, Sunyoung Lee, Qinglin Wu Poly(lactic acid) (PLA) and PLA grafted cellulose nanofibers (PLA-g-CNFs) mixture were extruded into filaments, and subsequently 3D printed into composites. As-3D printed composites were then thermally annealed at a temperature above PLA glass transition temperature (Tg). Dynamic mechanical analysis, including temperature ramp, frequency sweep, and creep for annealed composites, confirmed the enhanced responses to various viscoelastic factors. Such enhancements were ascribed to the presence of PLA crystalline regions containing both ɑ and ɑʹ phases, which were induced and developed through the annealing treatment. After 3-point bending test at 70 °C, unannealed composites were partially damaged, while annealed composites preserved the originally well-integrated layer structures. Experimental creep and recovery data essentially fitted to the Burger’s model and Weibull’s distribution function, respectively. The calculated parameters (e.g., moduli) from numerical fitting curves demonstrated the synergetic effect of PLA-g-CNFs and annealing treatment on the enahncement of flexural properties for 3D printed PLA composites.Graphical abstractGraphical abstract for this article
       
  • Clinical, Industrial, and Research Perspectives on Powder Bed Fusion
           Additively Manufactured Metal Implants
    • Abstract: Publication date: Available online 7 June 2019Source: Additive ManufacturingAuthor(s): Morgan Lowther, Sophie Louth, Amy Davey, Azad Hussain, Paola Ginestra, Luke Carter, Neil Eisenstein, Liam Grover, Liam Grover For over ten years, metallic skeletal endoprostheses have been produced in select cases by additive manufacturing (AM) and increasing awareness is driving demand for wider access to the technology. This review brings together key stakeholder perspectives on the translation of AM research; clinical application, ongoing research in the field of powder bed fusion, and the current regulatory framework. The current clinical use of AM is assessed, both on a mass-manufactured scale and bespoke application for patient specific implants. To illuminate the benefits to clinicians, a case study on the provision of custom cranioplasty is provided based on prosthetist testimony. Current progress in research is discussed, with immediate gains to be made through increased design freedom described at both meso- and macro-scale, as well as long-term goals in alloy development including bioactive materials. In all cases, focus is given to specific clinical challenges such as stress shielding and osseointegration. Outstanding challenges in industrialisation of AM are openly raised, with possible solutions assessed. Finally, overarching context is given with a review of the regulatory framework involved in translating AM implants, with particular emphasis placed on customisation within an orthopaedic remit. A viable future for AM of metal implants is presented, and it is suggested that continuing collaboration between all stakeholders will enable acceleration of the translation process.
       
  • Interrupted fatigue testing with periodic tomography to monitor porosity
           defects in wire + arc additive manufactured Ti-6Al-4V
    • Abstract: Publication date: August 2019Source: Additive Manufacturing, Volume 28Author(s): Romali Biswal, Xiang Zhang, Muhammad Shamir, Abdullah Al Mamun, Mustafa Awd, Frank Walther, Abdul Khadar Syed Porosity defects remain a challenge to the structural integrity of additive manufactured materials, particularly for parts under fatigue loading applications. Although the wire + arc additive manufactured Ti-6Al-4 V builds are typically fully dense, occurrences of isolated pores may not be completely avoided due to feedstock contamination. This study used contaminated wires to build the gauge section of fatigue specimens to purposely introduce spherical gas pores in the size range of 120–250 micrometres. Changes in the defect morphology were monitored via interrupted fatigue testing with periodic X-ray computed tomography (CT) scanning. Prior to specimen failure, the near surface pores grew by approximately a factor of 2 and tortuous fatigue cracks were initiated and propagated towards the nearest free surface. Elastic-plastic finite element analysis showed cyclic plastic deformation at the pore root as a result of stress concentration; consequently for an applied tension-tension cyclic stress (stress ratio 0.1), the local stress at the pore root became a tension-compression nature (local stress ratio −1.0). Fatigue life was predicted using the notch fatigue approach and validated with experimental test results.
       
  • Multi-Material 3D Printing of a Soft Pressure Sensor
    • Abstract: Publication date: Available online 4 June 2019Source: Additive ManufacturingAuthor(s): Md Omar Faruk Emon, Faez Alkadi, Daryl George Philip, Da-Hye Kim, Kyung-Chang Lee, Jae-Won Choi The emergence of smart technologies is spurring the development of a wider range of applications for stretchable and conformable sensors, as the design flexibility offered by additive manufacturing may enable the production of sensors that are superior to those produced by conventional manufacturing techniques. In this work, a multi-material 3D printing system with three extrusion heads was developed to fabricate a stretchable, soft pressure sensor built using an ionic liquid (IL)–based pressure-sensitive layer that was sandwiched between carbon nanotube (CNT)–based stretchable electrodes and encapsulated within stretchable top and bottom insulating layers. The sensor materials were modified in order to achieve 3D printable characteristics. The capability of the system was tested by printing structures made from three materials and a multilayer sensor via an extrusion-based direct-print process. Multi-material 3D printing of the sensor was successfully realized, as the sensing material retained its functionality once the printing process was complete.
       
  • In-situ characterization and quantification of melt pool variation under
           constant input energy density in laser powder-bed fusion additive
           manufacturing process
    • Abstract: Publication date: Available online 1 June 2019Source: Additive ManufacturingAuthor(s): Qilin Guo, Cang Zhao, Minglei Qu, Lianghua Xiong, Luis I. Escano, S. Mohammad H. Hojjatzadeh, Niranjan D. Parab, Kamel Fezzaa, Wes Everhart, Tao Sun, Lianyi Chen Size and shape of a melt pool play a critical role in determining the microstructure in additively manufactured metals. However, it is very challenging to directly characterize the size and shape of the melt pool beneath the surface of the melt pool during the additive manufacturing process. Here, we report the direct observation and quantification of melt pool variation during the laser powder-bed fusion additive manufacturing process under constant input energy density by in-situ high-speed high-energy x-ray imaging. We show that the melt pool can undergo different melting regimes and both the melt pool dimension and melt pool volume can have orders-of-magnitude change under a constant input energy density. Our analysis shows that the significant melt pool variation cannot be solely explained by the energy dissipation rate proposed previously. We found that energy absorption changes significantly under a constant input energy density, which is another important cause of melt pool variation. Our further analysis reveals that the significant change in energy absorption originates from the separate roles of laser power and scan speed in depression zone development. The results reported here are important for understanding the laser powder-bed fusion additive manufacturing process and guiding the development of better metrics for processing parameter design.Graphical abstractGraphical abstract for this article
       
  • 3D printing of highly-loaded slurries via layered extrusion forming:
           parameters optimization and control
    • Abstract: Publication date: Available online 1 June 2019Source: Additive ManufacturingAuthor(s): Shiyan Tang, Li Yang, Guanjin Li, Xinwang Liu, Zitian Fan The optimization of slurry compositions and processing parameters has significant potential for layered extrusion forming, a novel slurry-based additive manufacturing method. The optimal slurry composition was composed of 50vol.% Al2O3 loading, 1.5wt.% acetic acid as dispersant and 2wt.% methylcellulose solution as binder. The processing parameters including layer height, print speed and nozzle diameter significantly influenced the fabrication quality. The orthogonal experiment showed that the print speed of 15mm/s, nozzle diameter of 0.40mm and layer height set as 70% of nozzle diameter was the optimized processing conditions. The lattice structure constructed under the optimized conditions exhibited uniform and well-shaped morphology before and after sintering. The solid-infilled ceramic specimen prepared via optimized parameters exhibited uniform structure and the surface roughness was 0.75μm, which greatly improved the surface quality.
       
  • Metallic clay
    • Abstract: Publication date: Available online 31 May 2019Source: Additive ManufacturingAuthor(s): Sander Leeflang, Shahram Janbaz, Amir A. Zadpoor From pottery to clay tablets and building materials, clay easily qualifies as one of the most versatile materials in the history of human civilization. Clay owes this versatility to the distinct properties it exhibits before and after firing. Soft, unfired clay can morph into complex shapes, while fired clay offers a fixed shape and higher stiffness. Despite several potential applications, thus far, no designer materials with similar properties have been demonstrated. Here, we introduce the concept of metallic clay: a designer material that mimics the two-state behavior of clay. Metallic clay could initially morph into arbitrarily complex shapes owing to numerous degrees-of-freedom that its various kinematic (moving) and compliant (deformable) joints afford. Once the desired shape is achieved, shape-locking mechanisms (kinematic or compliant) lock the shape of the metallic clay. The fabrication of metallic clay requires novel designs of joints and locking mechanisms that are compatible with metal 3D printing (additive manufacturing) techniques such that metallic clay can be fabricated through a single-step, non-assembly, and self-supporting 3D printing process. We designed with 3D printing 17 prototypes using selective laser melting from a medical grade high strength titanium alloy (Ti-6Al-4 V) to demonstrate the various aspects of metallic clay.Graphical abstractGraphical abstract for this article
       
  • Effect of process parameters on non-modulated Ni-Mn-Ga alloy manufactured
           using powder bed fusion
    • Abstract: Publication date: Available online 29 May 2019Source: Additive ManufacturingAuthor(s): Frans Nilsén, Iñigo Flores Ituarte, Mika Salmi, Jouni Partanen, Simo-Pekka Hannula Increasingly, metal parts made by additive manufacturing are produced using powder bed fusion (PBF). In this paper we report upon the combined effects of PBF parameters, including power and scan speed, in layer-by-layer manufacturing of gas atomized non-modulated (NM) Ni-Mn-Ga alloy. The effects of process parameters upon PBF is studied by applying nine different parameter sets in the as-printed state and after homogenization and ordering. The chemical composition of the samples is analyzed using EDX attached to an SEM, and the crystal structures are determined by X-ray diffraction. The phase transformation temperatures are measured using a low-field ac susceptibility measurement system and the magnetic properties are measured with a vibrating sample magnetometer (VSM). Before the heat-treatment, all as-printed samples showed paramagnetic behavior with low magnetization and no phase transformations could be observed in the susceptibility measurements. After annealing, the samples recovered the ferromagnetic behavior with comparable magnetization to annealed gas atomized powder. The as-printed samples were composed of a mixture of different crystal structures. However, after annealing the original NM structure with a = b = 5.47 Å and c = 6.66 Å with a c/a -ratio of 1.22 was recovered and crystallographic twins could be observed in an SEM.
       
  • In-Process Monitoring of Porosity During Laser Additive Manufacturing
           Process
    • Abstract: Publication date: Available online 29 May 2019Source: Additive ManufacturingAuthor(s): Bin Zhang, Shunyu Liu, Yung C. Shin This paper describes a deep-learning-based method for porosity monitoring in laser additive manufacturing process. High-speed digital cameras were mounted coaxially to the process laser beam for in-process sensing of melt-pool data, and convolutional neural network models were designed to learn melt-pool features to predict the porosity attributes in deposited specimens during laser additive manufacturing. With the image processing tools developed in this paper, the extraction of porosity information from raw quality inspection data, such as cross-section images and tomography data sets, can be automated. The CNN models with a compact architecture, part of whose hyperparameters were selected through cross-validation analysis, achieved a classification accuracy of 91.2% for porosity occurrence detection in the direct laser deposition of sponge Titanium powders and presented predictive capacity for micro pores below 100 µm. For local volume porosity prediction, the model also achieved a root mean square error of 1.32% and exhibited high fidelity for both high porosity and low porosity specimens.Graphical abstractGraphical abstract for this article
       
  • Microstructure and Mechanical Properties of Zr-modified Aluminum Alloy
           5083 Manufactured by Laser Powder Bed Fusion
    • Abstract: Publication date: Available online 29 May 2019Source: Additive ManufacturingAuthor(s): Le Zhou, Holden Hyer, Sharon Park, Hao Pan, Yuanli Bai, Katherine P. Rice, Yongho Sohn Aluminum alloy (AA), AA5083 and AA5083 alloyed with 0.7 wt.% Zr (hereafter denoted as AA5083+Zr) were manufactured by laser powder bed fusion (LPBF) using gas atomized powders. Parametric investigation was performed by using laser powers of 200 W and 350 W with various scan speeds. AA5083 alloy was difficult to manufacture by LPBF due to formation of excessive pores or solidification cracking or compositional deviation from evaporation at all processing parameters examined, i.e., poor buildability/printability. However, with the addition of 0.7 wt.% Zr, the AA5083+Zr alloy was manufactured with near-full density, without solidification cracks and with proper composition. The as-built AA5083+Zr alloy exhibited outstanding tensile properties with yield strength of 212 MPa, tensile strength of 317 MPa and elongation of 22.3%. After heat treatment at 400 °C for 2 hours, the yield and tensile strength of LPBF AA5083+Zr alloy increased to 319 MPa and 392 MPa, respectively, although the elongation reduced to 14.1%. Microstructural analyses revealed a dramatic difference in grain size and distribution between the as-built AA5083 and AA5083+Zr alloys. The primary, pro-peritectic Al3Zr particles were observed at the melt boundaries, which would refine the grains in the as-built AA5083+Zr alloy, and help eliminate the solidification cracking. Furthermore, a significant amount of nano-scale (˜3 nm) Al3Zr precipitates were observed within the grains after the heat treatment of AA5083+Zr alloy, which would contribute to the increase in strength observed. A qualitative processing map correlating the porosity/cracks and laser power/scan speed is proposed and discussed with respect to the effect of Zr on the refinement of microstructure and improvement of buildability/printability.
       
  • Automated Metrology and Geometric Analysis of Additively Manufactured
           Lattice Structures
    • Abstract: Publication date: Available online 29 May 2019Source: Additive ManufacturingAuthor(s): Davis J. McGregor, Sameh Tawfick, William P. King Additive manufacturing (AM) enables the fabrication of complex lattice structures, for which a single part may have hundreds or thousands of individual geometric features. Conventional methods for measuring part geometry and performing quality control, which typically use a small number of low-dimensional measurements, are not well suited for lattice structures. This paper describes a method for scanning and automatically extracting individual features of the lattice and applies this method to characterize AM lattice structures in both two-dimensional and three-dimensional lattices. The research measured 94 lattice parts fabricated from 3 materials in 9 different designs using either a high-resolution document scanner or X-ray computed tomography (CT). A statistical analysis considered manufacturing variances as a function of material type and part design on a subset of the data, comprising the size and location of over 15,000 individual features. We studied the geometric variations of these struts in uniform, hierarchical and gradated parts. For a single design and material, the standard deviation of lattice feature size is quite small. For example, a lattice strut with thickness 0.5 mm has a standard deviation of 30 µm. However, when the same process is used to manufacture multiple parts having different designs and from different materials, the standard deviation of feature size can be larger by 2X or more. This type of automated measurement and analysis may allow for rigorous monitoring, qualification and control of AM lattice parts in production.
       
  • Microstructure, mechanical, and tribological properties of M3:2 high-speed
           steel processed by selective laser melting, hot-isostatic pressing, and
           casting
    • Abstract: Publication date: Available online 28 May 2019Source: Additive ManufacturingAuthor(s): Karina Geenen, Arne Röttger, Florian Feld, Werner Theisen In this work, the influence of different manufacturing techniques of M3:2 high-speed steel on the resulting microstructure and the associated material properties was investigated. Therefore, microstructure as well as the mechanical and tribological properties of cast steel (with subsequent hot-forming) and steel powder processed by two techniques: hot-isostatic pressing (HIP) and selective laser melting (SLM) were compared. A detailed SLM parameter analysis revealed that the porosity of SLM specimens can be decreased towards a smaller point distance and a longer exposure time (high energy input). A rise in preheating temperature is associated with a reduction in the crack density or the complete avoidance of cracks. In this context, the high-speed steel showed outstanding densification behavior by SLM, even though this steel is considered to be hardly processable by SLM due to its high content of carbon and hard phase-forming elements. In addition, the reusability of steel powder for SLM processing was investigated. The results indicated that multiple reuse is possible, but only in combination with powder processing (mechanical sieving) after each SLM cycle. The microstructure of SLM-densified high-speed steel consists of a cellular, fine dendritic subgrain segregation structure (submicro level) that is not significantly affected by preheating the base plate. The mechanical and tribological properties were examined in relation to the manufacturing technique and the subsequent heat treatment. Our investigations revealed promising behavior with respect to hardness tempering (position of the secondary hardness peak) and tribology of the M3:2 steel processed by SLM compared to the HIP and cast conditions.
       
  • Powder bed fusion of poly(phenylene sulfide) at bed temperatures
           significantly below melting
    • Abstract: Publication date: Available online 28 May 2019Source: Additive ManufacturingAuthor(s): Camden A. Chatham, Timothy E. Long, Christopher B. Williams In this paper, the authors present evidence of printing poly(phenylene sulfide) (PPS), a high-performance polymer, via powder bed fusion (PBF) using a bed temperature of 230 °C, which is significantly below both its observed melting temperature (Tm ˜ 285 °C) and its observed onset temperature of crystallization (Tc ˜ 255 °C). This contradicts existing material screening guidelines for PBF, which suggest maintaining bed temperature above the observed onset of crystallization. The authors believe that printing PPS at a comparatively-low bed temperature is beneficial for minimizing the occurrence of PPS side-reactions (e.g., chain extension, branching, and crosslinking) during printing and for enabling processing of a high-temperature polymer on “standard” PBF printers, which typically have maximum build temperatures below 250 °C. Existing methods for theoretically determining processing bounds were used to predict a range of energy densities at which PPS can be printed. One combination of process parameter values was selected based on machine constraints imposed by typical PBF machines not designed to print high-temperature polymers and used to fabricate multilayer, complex parts. The presented process parameters result in final part density upwards of 1.3 g/cm3 and ultimate tensile strength and elongation of 62 MPa and 3.3 %, respectively. Hypotheses on the generalizability of low-temperature PBF printing of high-performance polymers, and steps towards updating materials and process parameter selection guidelines for PBF, are also presented.
       
  • Numerical simulations of the mesostructure formation in material extrusion
           additive manufacturing
    • Abstract: Publication date: Available online 24 May 2019Source: Additive ManufacturingAuthor(s): Marcin P. Serdeczny, Raphaël Comminal, David B. Pedersen, Jon Spangenberg A computational fluid dynamics model is used to predict the mesostructure formed by the successive deposition of parallel strands in material extrusion additive manufacturing. The numerical model simulates the extrusion of the material onto the substrate. The model takes into account the effect of the presence of the previously extruded material on the shape of the subsequently deposited strands. The simulated mesostructures are compared to optical micrographs of the mesostructures of 3D-printed samples, and the predictions agree well with the experiments. In addition, the influence of the layer thickness, the strand-to-strand distance, and the deposition configuration (with aligned or skewed layers) on the formation of the mesostructure is investigated. The simulations provide detailed information about the porosity, the inter- and intra-layer bond line densities, and the surface roughness of the mesostructures, which potentially can be used in a model-based slicing software.
       
  • Infiltration studies of additive manufacture of WC with Co using binder
           jetting and pressureless melt method
    • Abstract: Publication date: August 2019Source: Additive Manufacturing, Volume 28Author(s): Corson L. Cramer, Peeyush Nandwana, Richard A. Lowden, Amy M. Elliott Additive manufacturing (AM) of tungsten carbide-cobalt (WC-Co) is explored starting with WC preforms shaped with binder jet additive manufacturing (BJAM) followed by melt infiltration of Co. The research objective is to demonstrate the ability to net-shape WC-Co composites through BJAM of a WC preform followed by backfilling with cobalt via pressureless infiltration. This method also has the potential to minimize shrinkage and grain growth compared to other AM techniques. The effects of sintering, Co content, and infiltration time on the net shaping and properties of processed composites are shown. The best shaped material had an average grain size of 5.1 μm, 32 vol.% Co, density of 98.54% theoretical, fracture toughness of 23.2 MPa m1/2, and hardness of 9.0 GPa. Data presented illustrates that the proposed approach results in favorable ceramic-metal (cermet) properties and is viable for fabricating cermets of other material combinations. Successful AM of cermets provides complex geometries, high throughout, and low costs.
       
  • The Effect of Process Parameters on Residual Stress Evolution and
           Distortion in the Laser Powder Bed Fusion of Ti-6Al-4V
    • Abstract: Publication date: Available online 23 May 2019Source: Additive ManufacturingAuthor(s): N.C. Levkulich, S.L. Semiatin, J.E. Gockel, J.R. Middendorf, A.T. DeWald, N.W. KlingbeilABSTRACTLaser powder bed fusion (LPBF) is a popular additive manufacturing (AM) process that has shown promise in fabricating novel components that can be utilized for a wide variety of applications. However, one of the main drawbacks of LPBF is that it produces large thermal gradients and fast cooling rates during the solidification of each layer, which can lead to large levels of residual stress/distortion, sometimes resulting in build failure/rejection. In the present work, several experimental techniques (x-ray diffraction, hole drilling, contour method, and laser line profilometry) were utilized to establish the effect of LPBF process parameters (scan speed, laser power, build height, build plan area, and substrate condition) on residual stress evolution and distortion. X-ray diffraction and hole-drilling measurements were performed on the surfaces of the LPBF deposits and substrates, while bulk residual stresses were measured using the contour method. In addition, a laser line profilometer was used to measure the distortion after fabrication. The results obtained by the non-destructive and destructive measurement techniques suggested that process parameters greatly influence the development of residual stress and distortion throughout the LPBF deposit and the substrate. Furthermore, the experimental results in this work provide a valuable foundation for future modeling and simulation of the evolution of residual stress and distortion.
       
  • In situ Monitoring of Material Jetting Additive Manufacturing Process via
           Impedance Based Measurements
    • Abstract: Publication date: Available online 22 May 2019Source: Additive ManufacturingAuthor(s): Logan D. Sturm, Mohammed I. Albakri, Pablo A. Tarazaga, Christopher B. WilliamsABSTRACTIn this paper, the authors explore the use of impedance-based monitoring techniques for in-situ detection of additive manufacturing build defects. By physically coupling a piezoceramic (PZT) sensor to the part being fabricated, the measured electrical impedance of the PZT can be directly linked to the mechanical impedance of the part. It is hypothesized that one can detect build defects in geometry or material properties in-situ by comparing the signatures collected during printing of parts with that of a defect-free control sample. In this paper, the authors explore the layer-to-layer sensitivity for both PZT sensors embedded into printed parts and for a fixture-based PZT sensor. For this work, this concept is evaluated in context of material jetting. A set of control samples is created and used to establish a baseline signature. (e.g., internal voids) are fabricated and their layer-to-layer signatures are compared to a control sample. Using this technique, the authors demonstrate an ability to track print progress and detect defects as they occur. For embedded sensors the defects were detectable at 2.28% of the part volume (95.6 mm3) and by fixture-based sensors when it affected 1.38% of the part volume.
       
  • Fabrication and durability testing of a 3D printed façade for desert
           climates
    • Abstract: Publication date: Available online 22 May 2019Source: Additive ManufacturingAuthor(s): Giulia Grassi, Sonia Lupica Spagnolo, Ingrid Paoletti The use of 3D printing in architecture has grown tremendously over the last decade thanks to its strong reputation as a versatile, cheap and fast technology. However, its ability to maintain its initial performances over time cannot be taken for granted. Its durability, in fact, depends on several factors (above all design accuracy, quality of materials and environmental aggressiveness), which may lead or contribute to rapid performance decay over time. With this in mind, the paper describes the design-to-production process for a façade shading system using additive manufacturing and the associated testing campaign to assess the feasibility of the design and durability of materials. The aim of the project was to create a second skin for an overlay pavilion at Expo 2020 in Dubai with a façade design that evokes desert dunes. Horizontal lamellas, with a complex curved geometry, were generated using computational design optimised for additive manufacturing. In order to select the most suitable 3D-printable material, tests were conducted on different polymers in a climatic chamber at Politecnico di Milano to monitor material performances over time at high temperatures such as the ones in Dubai. The data gathered from these tests was crucial to the correct design of the façade manufacturing process.
       
  • Direct Ink Writing of Three Dimensional Ti2AlC Porous
           Structures
    • Abstract: Publication date: Available online 20 May 2019Source: Additive ManufacturingAuthor(s): Hamada Elsayed, Anna Chmielarz, Marek Potoczek, Tobias Fey, Paolo Colombo In this work, the Direct Ink Writing (DIW) technique was used to produce three-dimensional Ti2AlC ceramic components with high, uniform porosity. Suitable formulations were developed, with appropriate rheological properties for extruding thin filaments through a nozzle with a diameter of 810 µm. The main rheological properties of the inks were investigated to evaluate their behavior and flowability during the printing process. Porous Ti2AlC lattices were fabricated, in selected conditions, with uniform pore size and good interconnectivity, and sintered at 1400 °C in Ar. Total porosity ranged from ∼44 to ∼63 vol%, and the mechanical strength ranged from ∼43 to 83 MPa. The influence of the ink composition and heat-treatment conditions on the phase composition of the 3D porous structures was also evaluated.
       
  • Reinforcement of material extrusion 3D printed polycarbonate using
           continuous carbon fiber
    • Abstract: Publication date: Available online 20 May 2019Source: Additive ManufacturingAuthor(s): M.N. Jahangir, K.M.M. Billah, Y. Lin, D.A. Roberson, R.B. Wicker, D. Espalin Additive manufacturing (AM) technologies are capable of fabricating custom parts with complex geometrical shapes in a short period of time relative to traditional fabrication processes that require expensive tooling and several post processing steps. Material extrusion AM, known commercially as Fused Filament Fabrication (FFF) technology, is a widely used polymer AM process, however, the effects of inherent porosity on mechanical strength continues to be researched to identify strength improvement solutions. To address the effect of porosity and layer adhesion on mechanical properties (which can sometimes result in 27-35 % lower ultimate tensile strength when compared to plastic injection molding), an approach was employed to reinforce 3D printed polycarbonate (PC) parts with continuous carbon fiber (CF) bundles. ASTM D638 Type I specimens were fabricated with printing interruptions to manually place and embed CF bundles. Specimens contained either one, two, or three layers of embedded CF bundles. Results demonstrated a maximum of 77 % increase in tensile yield strength when PC was reinforced with three CF bundles and micrographs showed multiple regions with zero porosity due to the CF inclusion. PC with three bundles of CF (modulus of 3.36 GPa) showed 85 % higher modulus of elasticity than the neat PC specimens (modulus of 1.82 GPa). The manual placement of CF and its impact on mechanical properties motivated the development of an automated selective deposition method using an ultrasonic embedding apparatus. Substantial technology development towards the embedding process of continuous carbon fiber bundles using ultrasonic energy was achieved in an automated fashion which is complementary of digital manufacturing and novel when compared to other existing processes.
       
  • An Inherent Strain Based Multiscale Modeling Framework for Simulating
           Part-scale Residual Deformation for Direct Metal Laser Sintering
    • Abstract: Publication date: Available online 20 May 2019Source: Additive ManufacturingAuthor(s): Qian Chen, Xuan Liang, Devlin Hayduke, Jikai Liu, Lin Cheng, Jason Oskin, Ryan Whitmore, Albert C. To Residual distortion is a major technical challenge for laser powder bed fusion (LPBF) additive manufacturing (AM), since excessive distortion can cause build failure, cracks and loss in structural integrity. However, residual distortion can hardly be avoided due to the rapid heating and cooling inherent in this AM process. Thus, fast and accurate distortion prediction is an effective way to ensure manufacturability and build quality. This paper proposes a multiscale process modeling framework for efficiently and accurately simulating residual distortion and stress at the part-scale for the direct metal laser sintering (DMLS) process. In this framework, inherent strains are extracted from detailed process simulation of micro-scale model based on the recently proposed modified inherent strain model. The micro scale detailed process simulation employs the actual parameters of the DMLS process such as laser power, velocity, and scanning path. Uniform but anisotropic strains are then applied to the part in a layer-by-layer fashion in a quasi-static equilibrium finite element analysis, in order to predict residual distortion/stress for the entire AM build. Using this approach, the total computational time can be significantly reduced from potentially days or weeks to a few hours for part-scale prediction. Effectiveness of this proposed framework is demonstrated by simulating a double cantilever beam and a canonical part with varying wall thicknesses and comparing with experimental measurements which show very good agreement.
       
  • Extrusion Control for High Quality Printing on Big Area Additive
           Manufacturing (BAAM) Systems
    • Abstract: Publication date: Available online 19 May 2019Source: Additive ManufacturingAuthor(s): Phillip Chesser, Brian Post, Alex Roschli, Charles Carnal, Randall Lind, Michael Borish, Lonnie Love Big Area Additive Manufacturing (BAAM) is a large format additive manufacturing (AM) process. The size scale has enabled many new applications for AM. However, at this scale, lack of high print resolution and extruder flowrate control lead to potentially significant geometric deviations in the printed part. This paper examines strategies to improve geometric quality in BAAM parts. Multi-resolution printing, extrusion diversion, and feedforward extruder control are examined herein. These methods were all found to be effective in mitigating phenomena detrimental to geometric part quality on the BAAM process.
       
  • New scanning strategy to reduce warpage in additive manufacturing
    • Abstract: Publication date: Available online 18 May 2019Source: Additive ManufacturingAuthor(s): Davi Leão Ramos, Fawzi Belblidia, Johann Sienz This paper proposes a novel geometric based scanning strategy adopted in the selective laser melting (SLM) manufacturing technology aimed at reducing the level of residual stresses generated during the build-up process. A set of computer simulations of the build, based on different scans strategies, including temperature dependent material properties, and a moving heat flux, were performed. The research novelty explores intermittent scan strategies in order to analyze the effect of reduction on heat concentration on the residual stress and deformation. Coupled thermal-structural computations revealed a significant stress and warpage reduction on the proposed scanning scheme. Different powder material properties were investigated and the computational model was validated against published numerical and experimental studies.
       
  • Method to optimize an additively-manufactured functionally-graded lattice
           structure for effective liquid cooling
    • Abstract: Publication date: August 2019Source: Additive Manufacturing, Volume 28Author(s): Akihiro Takezawa, Xiaopeng Zhang, Masaki Kato, Mitsuru Kitamura The development of cooling devices is important for many industrial products, and the lattice structure fabricated by additive manufacturing is expected to be useful for effective liquid cooling. However, lattice density should be carefully designed for an effective arrangement of coolant flow. In this research, we optimize the lattice density distribution using a lattice structure approximation and the gradient method. Fluid flow is approximated by deriving effective properties from the Darcy–Forchheimer law and analyzing the flow according to the Brinkman–Forchheimer equation. Thermal conduction and convection are also approximated as a weakly coupled problem. We use a simple basic lattice shape composed of pillars, optimizing only its density distribution by setting the pillar diameter as the design variable. Steady-state pressure and temperature reductions are treated as multi-objective functions. Through 2D and 3D numerical studies, we discuss the validity and limitations of the proposed method. Although observable errors in accuracy exist between the results obtained from the optimization and full scale models, relative performance optimization was considered successful.
       
  • Spatially resolved acoustic spectroscopy for integrity assessment in
           wire–arc additive manufacturing
    • Abstract: Publication date: August 2019Source: Additive Manufacturing, Volume 28Author(s): Paul Dryburgh, Don Pieris, Filomeno Martina, Rikesh Patel, Steve Sharples, Wenqi Li, Adam T. Clare, Stewart Williams, Richard J. Smith Wire–arc additive manufacturing (WAAM) is an emergent method for the production and repair of high value components. Introduction of plastic strain by inter-pass rolling has been shown to produce grain refinement and improve mechanical properties, however suitable quality control techniques are required to demonstrate the refinement non-destructively. This work proposes a method for rapid microstructural assessment of Ti–6Al–4V, with limited intervention, by measuring an acoustic wave generated on the surface of the specimens. Specifically, undeformed and rolled specimens have been analysed by spatially resolved acoustic spectroscopy (SRAS), allowing the efficacy of the rolling process to be observed in velocity maps. The work has three primary outcomes (i) differentiation of texture due to rolling force, (ii) understanding the acoustic wave velocity response in the textured material including the underlying crystallography, (iii) extraction of an additional build metric such as layer height from acoustic maps and further useful material information such as minimum stiffness direction. Variations in acoustic response due to grain refinement and crystallographic orientation have been explored. It has been found that the limited α-variants which develop within prior-β grains lead to distinctive acoustic slowness surfaces. This allowed prior-β grains to be resolved. A basic algorithm has been proposed for the automated measurement, which could be used for in-line closed loop control. The practicality and challenges of applying this approach in-line with fabrication are also discussed.Graphical abstractGraphical abstract for this article
       
  • Heat Retention Modeling of Large Area Additive Manufacturing
    • Abstract: Publication date: Available online 15 May 2019Source: Additive ManufacturingAuthor(s): Kyosung Choo, Brian Friedrich, Tim Daugherty, Austin Schmidt, Clark Patterson, Martin A. Abraham, Brett Conner, Kirk Rogers, Pedro Cortes, Eric MacDonald Large Area Additive Manufacturing now enables the fabrication of structures that are dramatically more substantial than those produced with standard 3D printing. Because the use of support structure is generally not appropriate when printing at these scales, understanding the limits of overhanging feature angles is necessary to establish the economic case for the use of large 3D printing. Additionally, understanding the physics of the process is paramount to avoiding expensive failed prints. Rapid sequential layers can result in slumping as the structure retains excessive heat when the next layer is printed. In this study, the thermal profiles of large printed parts have been experimentally measured and mathematically modeled in order to assist in the decision of whether a delay between layers is required to avoid slumping of overhanging features. The model can be used to insert additional dwell times after each layer so that the next layer of printing initiates after the previous layer is sufficiently cool such that the existing structure is appropriately solidified. Inputs to the model include the feedstock material, the number of beads in the overhanging wall, the angle of overhang and the threshold of failure represented as out-of-plane displacement from the intended geometry. The proposed thermal model can then be used with slicing software to insert pauses a priori, or can be leveraged during the print in conjunction with infrared imaging in order to provide in situ process control to improve quality and yield.
       
  • Quality control of AlSi10Mg produced by SLM: metallography versus CT scans
           for critical defect size assessment
    • Abstract: Publication date: Available online 15 May 2019Source: Additive ManufacturingAuthor(s): S. Romano, A. Abel, J. Gumpinger, A.D. Brandão, S. Beretta While the adoption of metal additive manufacturing (AM) is growing exponentially owing to its wide range of potential applications, its application to safety-critical and structural parts is significantly impeded by the lack of standards. Quality assessment of AM products is a crucial requirement, as the AM process induces internal defects that can have detrimental effects on the fatigue resistance.By evaluating the defect distribution, it is possible to perform a fracture mechanics assessment to estimate the fatigue strength and service lifetime of AM materials. This strategy has been successfully applied to selective laser-melted AlSi10Mg by performing X-ray micro-computed tomography (µCT) and applying suitable statistical methods (i.e., statistics of extremes). However, it remains unclear whether complex and expensive nondestructive inspection methods (e.g., µCT) are necessary and whether simpler and more conventional approaches (i.e., microscopy of polished sections (PSs), as prescribed by ASTM E2283) would provide equivalent information for the estimation of internal defects.In this study, the size of the most detrimental defect was estimated by performing both light microscopy on PSs and µCT on three batches of fatigue specimens characterized by different internal porosities. The results showed that both techniques were able to pinpoint a significant difference in the prospective largest defect in a material volume corresponding to the gauge section of a specimen. However, extrapolation of the critical defect size for fatigue failure using PS data was less accurate and less conservative than that using CT data. An evaluation of the techniques with regard to time and cost indicated that µCT allowed the investigation of larger sample volumes and the reduction of both man hours and cost.Graphical abstractGraphical abstract for this article
       
  • Single-operation, multi-phase additive manufacture of electro-chemical
           double layer capacitor devices
    • Abstract: Publication date: Available online 14 May 2019Source: Additive ManufacturingAuthor(s): Lukas Fieber, Jack D. Evans, Chun Huang, Patrick S. Grant additive manufacturing (AM) may offer a flexible, cost-effective approach to address conventional manufacturing limitations, such as time-consuming, high work-in-progress, multi-step assembly. In principle AM can also allow more novel geometric or even bespoke designs of structural and functional products. However, in terms of energy storage devices such as batteries and supercapacitors, the benefits of AM have not yet been explored to any significant extent. In this paper, a hybrid-AM system, combining low-cost (FDM) and direct ink writing (DIW) techniques, has been designed to fabricate supercapacitors (electro-chemical double layer capacitors, EDLCs) in a single, automated operation. The inherent flexibility of the AM process provided an opportunity to address restrictions in geometric form factor associated with conventional planar supercapacitor manufacturing approaches. Functioning, ring-shaped EDLC devices were manufactured in a single, multi-material operation comprising symmetric activated carbon electrodes in a 1M (KOH) electrolyte hydrogel. Gravimetric and areal electrode capacitances were 116.4±0.6 Fg−1 and 599.2±0.30mF cm−2 at 10mVs−1, with a columbic efficiency of 99.6±0.4% in the as-printed condition. The work aims to accelerate progress towards monolithic integration of energy storage devices in product manufacture, offering an alternative fabrication process for applications with irregular volume/shape and mass-customization requirements.Graphical abstractGraphical abstract for this article
       
  • Resolution, energy and time dependency on layer scaling in finite element
           modelling of laser beam powder bed fusion additive manufacturing
    • Abstract: Publication date: Available online 11 May 2019Source: Additive ManufacturingAuthor(s): Wenyou Zhang, Mingming Tong, Noel M. HarrisonABSTRACTThe Laser Beam Powder Bed Fusion (PBF-LB) category of Additive Manufacturing (AM) is currently receiving much attention for computational process modelling. Major challenges exist in how to reconcile resolution, energy and time in a real build, with the practical limitations of resolution (layer height and mesh resolution), energy (heat format and magnitude) and time (heating and cooling step times) in the computational space. A novel thermomechanical PBF-LB process model including an efficient powder-interface heat loss mechanism was developed. The effect of variations in layer height (layer scaling), energy and time on the temperature and stress evolution was investigated. The influence of heating step time and cooling step time was characterised and the recommended ratio of element size to layer scaling was presented, based on a macroscale 2D model. The layer scaling method was effective when scaling up to 4 times the layer thickness and appropriately also scaling the cooling step time. This research provides guidelines and a framework for layer scaling for finite element modelling of the PBF-LB process.
       
  • Internal pores in DED Ti-6.5Al-2Zr-Mo-V alloy and their influence on crack
           initiation and fatigue life in the mid-life regime
    • Abstract: Publication date: Available online 10 May 2019Source: Additive ManufacturingAuthor(s): Xiaofan He The present work aims to investigate the mechanism of crack initiation induced by internal pores, which are inevitable in additive manufacturing (AM), and the influence of internal pores on the fatigue performance of directed energy deposited (DED) Ti-6.5Al-2Zr-Mo-V. After fatigue test under constant amplitude alternating stress at three stress levels, thirty-one pieces of DED Ti-6.5Al-2Zr-Mo-V specimens were found that cracks initiating from internal pores. Scanning electron microscope (SEM) and its accessories, such as energy dispersive spectrometry (EDS) and electron backscattered diffraction (EBSD), were used to analyze the characteristics of pore defects and clarify the mechanism of crack initiation. The results show that the specificity of the microstructure affected by the DED process and pore defects, such as segregation of Al and the existence of incomplete grain boundaries, are the main causes of crack initiation. Then, the crack initiation modes were divided into three types, and a classification model was established that can make the effect of pore defects on fatigue life clearer and more intuitive.
       
  • Biodegradation-affected fatigue behavior of additively manufactured porous
           magnesium
    • Abstract: Publication date: Available online 10 May 2019Source: Additive ManufacturingAuthor(s): Y. Li, H. Jahr, X-Y. Zhang, M.A. Leeflang, W. Li, B. Pouran, F.D. Tichelaar, H. Weinans, J. Zhou, A.A. Zadpoor Additively manufactured (AM) biodegradable metals with topologically ordered porous structures hold unprecedented promise as potential bone substitutes. The first reports on this type of biomaterials have just recently appeared in the literature. There is, however, no information available in the literature regarding their mechanical performance under cyclic loading or the interactions between biodegradation and cyclic loading. We therefore used selective laser melting (SLM) to fabricate porous magnesium alloy (WE43) scaffolds based on diamond unit cells. The microstructure of the resulting material was examined using electron back-scattered diffraction, scanning transmission electron microscopy, and X-ray diffraction. The fatigue behaviors of the material in air and in revised simulated body fluid (r-SBF) were evaluated and compared. Biodegradation decreased the fatigue strength of the porous material from 30% to 20% of its yield strength. Moreover, cyclic loading significantly increased its biodegradation rate. The mechanistic aspects of how biodegradation and cyclic loading interacted with each other on different scales were revealed as well. On the micro-scale, cracks initiated at biodegradation pits and propagated transgranularly. In addition, dislocations became more tangled after the fatigue tests. On the macro-scale, cracks preferred initiating at the strut junctions where tensile stress concentrations were present, as revealed by the finite element analysis of the porous material under compressive loading. Most of the cracks initiated in the struts were positioned on the periphery of the specimens. Furthermore, the biodegradation pattern was found to be location-dependent with more localized biodegradation occurring in the center of the specimens. Further improvements in the biodegradation-affected fatigue performance of the AM porous Mg alloy may therefore be realized by optimizing both the topological design of the porous structure and the laser-processing parameters that determine the microstructure of the SLM porous material.
       
  • Bagasse – a major agro-industrial residue as potential resource for
           nanocellulose inks for 3D printing of wound dressing devices
    • Abstract: Publication date: Available online 10 May 2019Source: Additive ManufacturingAuthor(s): Gary Chinga-Carrasco, Nanci V. Ehman, Daniel Filgueira, Jenny Johansson, María E. Vallejos, Fernando E. Felissia, Joakim Håkansson, María C. Area Sugarcane bagasse, an abundant residue, is usually burned as an energy source. However, provided that appropriate and sustainable pulping and fractionation processes are applied, bagasse can be utilized as a main source of cellulose nanofibrils (CNF). We explored in this study the production of CNF inks for 3D printing by direct-ink-writing technology. The CNF were tested against L929 fibroblasts cell line and we confirmed that the CNF from soda bagasse fibers were found not to have a cytotoxic potential. Additionally, we demonstrated that the alginate and Ca2+ caused significant dimensional changes to the 3D printed constructs. The CNF-alginate grids exhibited a lateral expansion after printing and then shrank due to the cross-linking with the Ca2+. The release of Ca2+ from the CNF and CNF-alginate constructs was quantified thus providing more insight about the CNF as carrier for Ca2+. This, combined with 3D printing, offers potential for personalized wound dressing devices, i.e. tailor-made constructs that can be adapted to a specific shape, depending on the characteristics of the wound healing treatment.
       
  • Corrosion behavior of additively manufactured Ti-6Al-4V parts and the
           effect of post annealing
    • Abstract: Publication date: Available online 9 May 2019Source: Additive ManufacturingAuthor(s): Ali Hemmasian Ettefagh, Congyuan Zeng, Shengmin Guo, Jonathan Raush This paper evaluates the corrosion behavior of Ti-6Al-4V alloy parts produced by laser-based powder bed fusion additive manufacturing (AM). The effect of post annealing heat treatment on the corrosion resistance of AM parts is studied by comparing the heat treated samples with cold rolled commercial titanium alloy samples. The results obtained via corrosion tests show that the corrosion rate of as-fabricated AM parts is almost sixteen times worse than the commercial grade samples. The accelerated rate was due to the presence of non-equilibrium phases and can be ameliorated by a proper post heat treatment process at 800°C for 2 hours. The proposed heat treatment makes the corrosion behavior of AM parts comparable to the commercial grade samples, due to the stress relief of the martensitic phase and formation of BCC phase of β Ti-6Al-4V which has a higher corrosion resistance. CALculation of PHAse Diagrams (CALPHAD) method was used to identify the equilibrium phases.
       
  • Process Development and Impact of Intrinsic Heat Treatment on the
           Mechanical Performance of Selective Laser Melted AISI 4140
    • Abstract: Publication date: Available online 9 May 2019Source: Additive ManufacturingAuthor(s): James Damon, Robin Koch, Daniel Kaiser, Gregor Graf, Stefan Dietrich, Volker Schulze The low alloy steel AISI 4140 (German grade 42CrMo4) is one of the most frequently used Quench&Tempering (Q&T) steels with a wide range of applicability. Until now, commercially available iron powders for additive manufacturing can be summed up by their low amount of carbon. Fusion welding of Q&T steels often leads to cracks due to brittle martensitic transformation and the associated volume change. Therefore, the selection of appropriate process parameters in laser powder bed fusion (LPBF) plays a key role for the final material properties and is achieved through utilization of a new process development strategy and evaluation of microstructural features of test cubes. In this work tensile specimens were successfully produced with optimal process parameters and mechanical tests of additively built samples indicate mechanical performance comparable with a 450°C tempered state of conventionally cast material. By correlating the measured mechanical properties of LPBF samples to those of a conventional Q&T state, an estimation of the intrinsic heat treatment during LPBF was carried out using an inverse transient Hollomon-Jaffe approach. This analysis indicates that a rapid reheating rate of 103 − 105°C/s to ≈700 − 800°C of the previous built layers is the determinant for the low hardness found in the LPBF process of AISI 4140. This is also in accordance with the finely dispersed carbide precipitates in the as built condition. Furthermore, the effect of bed pre-heating on the final material tempering state was found to be negligible. This shows the importance of a balanced match between LPBF process parameters and subsequent application demands as well as necessary postprocessing steps.
       
  • On microstructural homogenization and mechanical properties optimization
           of biomedical Co-Cr-Mo alloy additively manufactured by using electron
           beam melting
    • Abstract: Publication date: Available online 8 May 2019Source: Additive ManufacturingAuthor(s): Daixiu Wei, Ainiwaer Anniyaer, Yuichiro Koizumi, Kenta Aoyagi, Makoto Nagasako, Hidemi Kato, Akihiko Chiba The electron beam melting (EBM), a layer-by-layer additive manufacturing (AM) technique, has been recently utilized for fabricating metallic components with complex shape and geometry. However, the inhomogeneity in microstructures and mechanical properties are the main drawbacks constraining the serviceability of the EBM-built parts. In the present study, we found remarkable microstructural inhomogeneity along build direction in the EBM-built Co-based alloy, owing to the competitive grain growth and subsequent isothermal γ-fcc → ε-hcp phase transformation, which affects the corresponding tensile properties significantly. Then, we succeeded in eliminating the inhomogeneities, modifying the phase structures and refining grain sizes via comprehensive post-production heat treatment regimes, which provides a valuable implication for improving the reliabilities of AM-built metals and alloys. The Co-based alloy can be selectively transformed into predominant ε or predominant γ phase by the regime, and the grains were refined to 1/10 of the initial sizes by repeated heat treatment. Finally, we investigated the tensile properties and fracture behaviors of the alloy before and after each heat treatment. The γ → ε strain-induced martensitic transformation is the major deformation mode of the γ phase, meanwhile the formation of stripped ε phase at {111}γ habit planes contributed to a good combination of strength and ductility. Nevertheless, the ε phase was deformed mainly by (0001)ε ε basal and {1 1¯00 }ε ε prismatic slip systems, exhibiting very limited ductility and strength. In addition, the ε grains act as secondary hardening factor in the samples consisting of dual γ/ε phase, leading to a non-uniform deformation behavior.Graphical abstractGraphical abstract for this article
       
  • Room Temperature Extrusion 3D Printing of Polyether Ether Ketone Using a
           Stimuli-Responsive Binder
    • Abstract: Publication date: Available online 6 May 2019Source: Additive ManufacturingAuthor(s): Chang-Uk Lee, Johanna Vandenbrande, Adam E. Goetz, Mark A. Ganter, Duane W. Storti, Andrew J. Boydston We report our efforts toward 3D printing of polyether ether ketone (PEEK) at room temperature by direct-ink write technology. The room-temperature extrusion printing method was enabled by a unique formulation comprised of commercial PEEK powder, soluble epoxy-functionalized PEEK (ePEEK), and fenchone. This combination formed a Bingham plastic that could be extruded using a readily available direct-ink write printer. The initial green body specimens were strong enough to be manipulated manually after drying. After printing, thermal processing at 230 °C resulted in crosslinking of the ePEEK components to form a stabilizing network throughout the specimen, which helped to preclude distortion and cracking upon sintering. A final sintering stage was conducted at 380 °C. The final parts were found to have excellent thermal stability and solvent resistance. The Tg of the product specimens was found to be 158 °C, which is 13 °C higher than commercial PEEK as measured by DSC. Moreover, the thermal decomposition temperature was found to be 528 °C, which compares well against commercial molded PEEK samples. Chemical resistance in trifluoroacetic acid and 8 common organic solvents, including CH2Cl2 and toluene, were also investigated and no signs of degradation or weight changes were observed from parts submerged for 1 week in each solvent. Test specimens also displayed desirable mechanical properties, such as a Young’s modulus of 2.5 GPa, which corresponds to 63% of that of commercial PEEK (reported to be 4.0 GPa).
       
  • Reducing residual stress by selective large-area diode surface heating
           during laser powder bed fusion additive manufacturing
    • Abstract: Publication date: Available online 6 May 2019Source: Additive ManufacturingAuthor(s): John D. Roehling, William L. Smith, Tien T. Roehling, Bey Vrancken, Gabriel M. Guss, Joseph T. McKeown, Michael R. Hill, Manyalibo J. Matthews High residual stresses are typical in additively manufactured metals and can reach levels as high as the yield strength, leading to distortions and even cracks. Here, an in situ method for controlling residual stress during laser powder bed fusion additive manufacturing was demonstrated. By illuminating the surface of a build with homogeneously intense, shaped light from a set of laser diodes, the thermal history was controlled thereby reducing the residual stress in as-built parts. 316L stainless steel bridge-shaped parts were built to characterize the effect of in situ annealing on the residual stress. A reduction in the overall residual stress value of up to 90% was realized without altering the as-built grain structure (no grain growth). Some annealing effects on the cellular-dendritic solidification structure (patterns of higher solute content) occurred in areas that experienced prolonged exposure to elevated temperature. A comparison of the in situ process to conventional post-build annealing demonstrated equivalent stress reduction compared to rule-of-thumb thermal treatments. Use of this method could reduce or remove the need for post processing to remove residual stresses.Graphical abstractGraphical abstract for this article
       
  • Inter-layer bonding characterisation between materials with different
           degrees of stiffness processed by fused filament fabrication
    • Abstract: Publication date: Available online 4 May 2019Source: Additive ManufacturingAuthor(s): A. Khudiakova, F. Arbeiter, M. Spoerk, M. Wolfahrt, D. Godec, G. Pinter One of the main benefits of material extrusion additive manufacturing, also known as fused filament fabrication (FFF) or 3D printing, is the flexibility in terms of printing materials. Locally reinforced components can be easily produced by selectively combining reinforced with unfilled tough thermoplastics. However, such multi-material composites usually lack sufficient weld strength in order to be able to withstand operation loads. The present study attempts to close this gap by characterising the cohesion between the strands of two materials with different stiffness, namely neat PLA and short carbon fibre reinforced PLA (CF-PLA), produced by FFF using advanced fracture mechanical techniques. The full set of engineering constants of both materials were obtained under the assumption of transverse isotropy from tensile tests in combination with digital image correlation. Double cantilever beam (DCB) and cracked round bar (CRB) tests were used to determine the critical energy release rate (GIc). Both tests were in good correlation with each other and revealed that the interlayer PLA/CF-PLA bonding was at least as tough as the interlayer CF-PLA/CF-PLA bonding.
       
  • 3D printing of high density polyethylene by fused filament fabrication
    • Abstract: Publication date: Available online 4 May 2019Source: Additive ManufacturingAuthor(s): Carl G. Schirmeister, Timo Hees, Erik H. Licht, Rolf Mülhaupt Polyolefin thermoplastics like high density polyethylene (HDPE) are the leaders in terms of world-scale plastics’ production, environmentally benign polymerization processes, recycling, and sustainability. However, additive manufacturing of HDPE by means of fused deposition modeling (FDM) also known as fused filament fabrication (FFF) has been problematic owing to its massive shrinkage, voiding and warpage problems accompanied by its poor adhesion to common build plates and to extruded HDPE strands. Herein we overcome these problems and improve Young’s modulus, tensile strength and surface quality of 3D printed HDPE by varying 3D printing parameters like temperature and diameter of the nozzle, extrusion rate, build plate temperature, and build plate material. Both nozzle diameter and printing speed affect surface quality but do not impair mechanical properties. Particularly, an extrusion rate gradient prevents void formation. For the first time additive manufactured HDPE and injection-molded HDPE exhibit similar mechanical properties with exception of elongation at break. Excellent fusion of the extruded polymer strands and the absence of anisotropy are achieved, as verified by microscopic imaging and measuring the tensile strength parallel and perpendicular to the 3D printing direction.
       
  • FAILURE SURFACE DEVELOPMENT FOR ABS FUSED FILAMENT FABRICATION PARTS
    • Abstract: Publication date: Available online 4 May 2019Source: Additive ManufacturingAuthor(s): Gerardo A. Mazzei Capote, Natalie M. Rudolph, Paul V. Osswald, Tim A. Osswald Fused Filament Fabrication (FFF) is the most widely available Additive Manufacturing technology. Offering the possibility of producing complex geometries in a compressed product development cycle and in a plethora of materials, it comes as no surprise that FFF is attractive to multiple industries, including the automotive and aerospace segments. However, the high anisotropy of parts developed through this technique implies that failure prediction is extremely difficult -a requirement that must be satisfied to guarantee the safety of the final user. This work applies a criterion that incorporates stress interactions to define a 3D failure envelope that could prove an invaluable tool in formalizing the embrace of FFF in industry. Tensile, compressive and torsion tests were executed on coupons developed in a traditional FFF printer, as well as a customized, 6-axis robotic printer necessary to produce specimens in out of ordinary orientations. These tests were used to calculate the parameters of the mathematical function that describe the failure envelope. Mechanical tests clearly showed significant difference between tensile, compressive and shear strengths. The calculated envelope shows strong interactions between axial loads, and a considerable interaction between shear stresses and loads applied in directions parallel and perpendicular to the beads.
       
  • The Re-usability of Heat-exposed Poly (ethylene terephthalate) Powder for
           Laser Sintering
    • Abstract: Publication date: Available online 4 May 2019Source: Additive ManufacturingAuthor(s): Hao Gu, Zahir Bashir, Lanti Yang Selective laser sintering, also called laser sintering (LS), is an additive manufacturing process that requires micronized plastic powder. Recently, we showed poly (ethylene terephthalate (PET) powder is a suitable material for LS, with a comparable printing performance as the current front-runner, polyamide 12 (PA12). However, the LS process, by its nature, leaves unused powder that has been exposed to heat for prolonged time, and this powder may not be fully re-usable due to degradation.In this work, the re-use potential of heat-exposed PET powder is established. This is a matter of crucial importance as powders suitable for LS are very expensive, and the powder left after a building episode has to be re-used. Heat-exposed PA12 has to be blended or refreshed with virgin powder, to avoid printing defects. In contrast, heat-exposed PET powder, after 96 h at 210 °C, could be used, without refreshing with a portion of virgin powder. The printed articles from heat-exposed powders were as good as those from the fresh powder. There was no cross-linking and there was only a minor increase in the molecular weight of the powder after 96 h, at 210 °C.
       
  • 3D printing of nano-scale Al2O3-ZrO2 eutectic ceramic: Principle analysis
           and process optimization of pores
    • Abstract: Publication date: August 2019Source: Additive Manufacturing, Volume 28Author(s): Shuai Yan, Yunfei Huang, Dake Zhao, Fangyong Niu, Guangyi Ma, Dongjiang Wu Pores are common defects in the process of directed laser deposition (DLD) which not only greatly reduce the fracture toughness of ceramic materials, but also lead to the failure of shaped parts. In this paper, the formation mechanism of pores was analyzed and the effects of laser power, feeding rate, scanning speed and ultrasonic power on pores were investigated. Transmission electron microscope, scanning electron microscopy observation and X-ray diffraction analysis were carried out for sample microstructure and phase composition respectively. The relative density of samples was measured by the progressive focused ion beam and the porosity was calculated by image processing software Image. The results show that the pores are divided into gas holes and shrinkage cavities. The appearance of circular gas holes with smooth inner walls are caused by the feeding method by gas forced blowing, the gas mixed with powder itself, and the gas in the molten pool formed by gasification of low-melting impurities and alumina/zirconia during laser processing. The gas holes are evenly distributed in the cross-section of the thin-walled specimen parallel to the scanning speed. As the temperature changes drastically, the material around the melt solidifies first, the melt will be attached to the solidified material to shrink, so that the melt can not be filled as a solid and finally the shrinkage cavities are formed. Generally the shrinkage cavities are irregular and the pore wall is relatively rough, mainly concentrated on the top of thin-walled samples. The laser power has the greatest influence on the pores, which has the greatest effect on the porosity but little effect on the shrinkage cavities. After attaching the ultrasound, the gas holes are mainly distributed on the top of the sample, and the shrinkage cavities almost disappear due to acoustic streaming effect of ultrasonic. When the ultrasonic power is 180 W, the porosity reaches a minimum of 0.1±0.05% and the relative density is 99.9±0.1%.
       
  • Design of laser parameters for selectively laser melted maraging steel
           based on deposited energy density
    • Abstract: Publication date: Available online 2 May 2019Source: Additive ManufacturingAuthor(s): Asuka Suzuki, Ryoya Nishida, Naoki Takata, Makoto Kobashi, Masaki Kato In this study, the effects of laser power and scan speed on the relative density, melt pool depth, and Vickers hardness of selectively laser melted (SLM) maraging steel were systematically investigated. The change in these structural parameters and hardness could not always be clarified by the volumetric energy density, which is widely used in the SLM processes. The deposited energy density, wherein the thermal diffusion length is used as a heat-distributed depth, can express the change in these structural parameters and the hardness with one curve. To clarify the effect of the laser parameters, the deposited energy should be used instead of the volumetric energy density. Thus, this study provides a new insight on the selection of the laser condition for SLM-fabricated materials.Graphical abstractGraphical abstract for this article
       
  • Additively manufactured calcium phosphate reinforced CoCrMo alloy:
           Bio-tribological and biocompatibility evaluation for load-bearing implants
           
    • Abstract: Publication date: Available online 2 May 2019Source: Additive ManufacturingAuthor(s): Amit Bandyopadhyay, Anish Shivaram, Murat Isik, Jose D. Avila, William S. Dernell, Susmita Bose Cobalt-chromium-molybdenum (CoCrMo) alloys are widely used in load-bearing implants; specifically, in hip, knee, and spinal applications due to their excellent wear resistance. However, due to in vivo corrosion and mechanically assisted corrosion, metal ion release occurs and accounts for poor biocompatibility. Therefore, a significant interest to find an alternative to CoCrMo alloy exists. In the present work we hypothesize that calcium phosphate (CaP) will behave as a solid lubricant in CoCrMo alloy under tribological testing, thereby minimizing wear and metal ion release concerns associated with CoCrMo alloy. CoCrMo-CaP composite coatings were processed using laser engineered net shaping (LENS™) system. After LENS™ processing, CoCrMo alloy was subjected to laser surface melting (LSM) using the same LENS™ set-up. Samples were investigated for microstructural features, phase identification, and biocompatibility. It was found that LSM treated CoCrMo improved wear resistance by 5 times. CoCrMo-CaP composites displayed the formation of a phosphorus-based tribofilm. In vitro cell-material interactions study showed no cytotoxic effect. Sprague-Dawley rat and rabbit in vivo study displayed increased osteoid formation for CoCrMo-CaP composites, up to 2 wt.% CaP. Our results show that careful surface modification treatments can simultaneously improve wear resistance and in vivo biocompatibility of CoCrMo alloy, which can correlate to a reduction of metal ion release in vivo.Graphical abstractGraphical abstract for this article
       
  • Direct comparison of additively manufactured porous Titanium and Tantalum
           implants towards in vivo osseointegration
    • Abstract: Publication date: Available online 1 May 2019Source: Additive ManufacturingAuthor(s): Amit Bandyopadhyay, Indranath Mitra, Anish Shivaram, Nairanjana Dasgupta, Susmita Bose Material properties of implants such as volume porosity and nanoscale surface modification have been shown to enhance cell-material interactions in vitro and osteoconductivity in vivo. Porous tantalum (Ta) and titanium (Ti) coatings are widely used for non-cemented implants, which are fabricated using different processing routes. In recent years, some of those implants are being manufactured using additive manufacturing. However, limited knowledge is available on direct comparison of additively manufactured porous Ta and Ti structures towards early stage osseointegration. In this study, we have fabricated porous Ta and Ti6Al4V (Ti64) implants using laser engineered net shaping (LENS™) with similar volume fraction porosity to compare the influence of surface characteristics and material chemistry on in vivo response using a rat distal femur model for 5 and 12 weeks. We have also assessed whether surface modification on Ti64 can elicit similar in vivo response as porous Ta in a rat distal femur model for 5 and 12 weeks. The harvested implants were histologically analyzed for osteoid surface per bone surface. Field emission scanning electron microscopy (FESEM) was done to assess the bone-implant interface. The results presented here indicate comparable performance of porous Ta and surface modified porous Ti64 implants towards early stage osseointegration at 5 weeks post implantation through seamless bone-material interlocking. However, a continued and extended efficacy of porous Ta is found in terms of higher osteoid formation at 12 weeks post-surgery.Graphical abstractGraphical abstract for this article
       
  • On Microstructure and Early Stage Corrosion Performance of Heat Treated
           Direct Metal Laser Sintered AlSi10Mg
    • Abstract: Publication date: Available online 29 April 2019Source: Additive ManufacturingAuthor(s): Mehran Rafieazad, Mohsen Mohammadi, Ali M. Nasiri This study examines the impact of low-temperature heat-treatment on the microstructure and corrosion performance of direct metal laser sintered (DMLS)-AlSi10Mg alloy. Differential scanning calorimetry (DSC) was used to determine the phase(s) transition temperatures in the alloy. Two exothermic phenomena were detected and associated with the Mg2Si precipitation and Si phase precipitation in the as-printed alloy. Based on DSC results, thermal-treatments including below and above the active Si precipitation temperature at 200 °C and 300 °C, respectively, and 350 °C as an upper limit temperature for 3 h were applied to the as-printed samples. Scanning electron microscopy and X-ray diffraction analysis confirmed that heat-treatment from 200 °C to 350 °C promotes the homogeneity of the microstructure, characterized by uniform distribution of eutectic Si in α-Al matrix. To investigate the impact of the applied heat-treatment cycles on corrosion resistance of DMLS-AlSi10Mg at early stage of immersion, anodic polarization testing and electrochemical impedance spectroscopy were performed in aerated 3.5 wt.% NaCl solution. The results revealed more uniformly distributed pitting attack on the corroded surfaces by increasing the heat-treatment temperature up to 300 °C, attributed to the more protective nature of the spontaneously air-formed passive layer on the surface of the alloy at initial immersion time. Further increase of the heat treatment temperature to 350 °C induced severe localized corrosion attacks near the coarse Si particles, ascribed to the increased potential difference between the coalesced Si particles and aluminum matrix galvanic couple. In comparison, the corrosion of the as-printed and 200 °C heat treated samples was characterized by a penetrating selective attack along the melt pool boundaries, leading to a higher corrosion current density and an active surface at early exposure, associated with the weakness of the existing passive film on their surfaces.
       
  • Technology cost drivers for a potential transition to decentralized
           manufacturing
    • Abstract: Publication date: Available online 27 April 2019Source: Additive ManufacturingAuthor(s): Jaime Bonnín Roca, Parth Vaishnav, Rianne E. Laureijs, Joana Mendonça, Erica R.H. Fuchs Popular dialogue around additive manufacturing (AM) often assumes that AM will cause a move from centralized to distributed manufacturing. However, distributed configurations can face additional hurdles to achieve economies of scale. We combine a Process-Based Cost Model and an optimization model to analyze the optimal location and number of manufacturing sites, and the tradeoffs between production, transportation and inventory costs. We use as a case study the commercial aviation maintenance market and a titanium jet engine bracket as an exemplar of a class of parts that are not flight-critical. We run our analysis for three different scenarios, one corresponding to the current state of the technology, and two which represent potential improvements in AM technology. Our results suggest that the cost-minimizing number of manufacturing locations does not vary significantly when taking into account a range of plausible improvements in the technology. In this case, distributed manufacturing is only favorable for a set of non-critical components that can be produced on the same equipment with minimal certification requirements and whose annual demand is in the tens of thousands. Distributed manufacturing is attractive at lower volumes for components that require no hot isostatic pressing.
       
  • Magnetically Navigable 3D Printed Multifunctional Microdevices for
           Environmental Applications
    • Abstract: Publication date: Available online 25 April 2019Source: Additive ManufacturingAuthor(s): Roberto Bernasconi, Elena Carrara, Marcus Hoop, Fajer Mushtaq, Xiangzhong Chen, Bradley J. Nelson, Salvador Pané, Caterina Credi, Marinella Levi, Luca Magagnin Microrobotic prototypes for water cleaning are produced combining stereolithography 3D printing and wet metallization. Different metallic layers are deposited on 3D printed parts using both electroless and electrolytic deposition to impart required functionalities. In particular, by exploiting the flexibility and versatility of electrolytic codeposition, pollutants photodegradation and bacteria killing are for the first time combined on the same device by coating it with a composite nanocoating containing titania nanoparticles in a silver matrix. The microstructure of the microrobots thus obtained is fully characterized and they are successfully actuated by applying rotating magnetic fields. From the water cleaning point of view, devices show evident photocatalytic activity towards water pollutants and antimicrobial activity for gram negative bacteria.
       
  • Measurement of Actual Powder Layer Height and Packing Density in a Single
           Layer in Selective Laser Melting
    • Abstract: Publication date: Available online 24 April 2019Source: Additive ManufacturingAuthor(s): Tim Marten Wischeropp, Claus Emmelmann, Milan Brandt, Aaron Pateras For a detailed numerical analysis of laser–material interactions and melt pool dynamics in selective laser melting (SLM), it is important to consider correct powder layer height and packing density in a single layer. Thus far, most experts assume that the powder layer height, which is equal to the leveling height of the build platform divided by the packing density of the powder bed, reaches a steady state after several layers. However, this assumption neglects the fact that a certain amount of powder is deposited (e.g., by spatter), and therefore, does not contribute to the molten powder layer height and that the packing density in a single layer is smaller than in “bulk.” To determine the actual powder layer height and packing density in a single layer, experiments are conducted using two different materials (SS 17-4 PH and Ti6Al4V) and layer heights (30 and 50 µm). The results reveal that the powder layer height is between 4 and 5.5 times the leveling height of the build platform and is, therefore, significantly larger than that assumed thus far. This is an important finding to consider when one investigates the details of the laser–material interaction and melting process in SLM, e.g., by numerical simulations. The measured packing density varies by approximately 50%.
       
  • Non-equilibrium Microstructure, Crystallographic Texture and Morphological
           Texture Synergistically Result in Unusual Mechanical Properties of 3D
           Printed 316L Stainless Steel
    • Abstract: Publication date: Available online 22 April 2019Source: Additive ManufacturingAuthor(s): Sumit Bahl, Sumeet Mishra, K.U. Yazar, Immanuel Raju Kola, Kaushik Chatterjee, Satyam Suwas Mechanisms underlying the evolution of texture and microstructure during selective laser melting (SLM) and their combined effects on the mechanical response of 316 L stainless steel are presented. Long columnar grains with a fiber texture build direction (BD) evolved in the SLM printed material. Fiber texture was stronger in the horizontal build compared to the vertical build. Use of bidirectional scanning strategy enforced epitaxial growth of grains across melt pools present within a single printed layer. BD texture evolved as a consequence of maintaining the balance between epitaxy and growth of [100] along maximum thermal gradient. High dislocation density and not grain size effect of the ultra-fine cellular structure, imparted high strength to 316L. Lower average Schmid factor and smaller effective grain size in the horizontal build by virtues of crystallographic and morphological textures, respectively, imparted higher yield strength than the vertical build. The horizontal build demonstrated higher strain hardening rate in the early stages of deformation compared to the vertical build due to higher crystallographic texture dependent twinning. However, the higher rate of dislocation annihilation led to a continuous decline in the strain hardening rate of the horizontal build. In contrast, a stable strain hardening rate was maintained in the vertical build, which led to higher ductility than the horizontal build. In summary, the roles of non-equilibrium microstructure and texture (crystallographic and morphological) in regulating mechanical properties elucidated here, can be utilized in designing additively manufactured structural components of 316 L stainless steel.
       
  • On The Development Of Powder Spreadability Metrics And Feedstock
           Requirements For Powder Bed Fusion Additive Manufacturing
    • Abstract: Publication date: Available online 22 April 2019Source: Additive ManufacturingAuthor(s): Zackary Snow, Richard Martukanitz, Sanjay Joshi Existing powder feedstock metrics for powder bed fusion (PBF) additive manufacturing (AM) are related to packing efficiency and flowability, and newer techniques, such as powder rheometry and dynamic avalanche testing, have received recent attention in the literature. To date, however, no powder characterization technique is able to predict the spreadability of AM feedstock. In fact, no such spreadability metrics exist. This study endeavored to establish viable powder spreadability metrics through the development of a spreadability testing rig that emulates the recoating conditions present in commercial PBF AM systems. As no metrics for spreadability currently exist, four potential metrics were evaluated in a 3∙23 split plot experimental design. These four metrics were: (1) the percentage of the build plate covered by spread powder, (2) the rate of powder deposition, (3) the average avalanching angle of the powder, and (4) the rate of change of the avalanching angle. Three samples of gas atomized, Al-10Si-0.5 Mg PBF powder representing differing degrees of quality were used as the levels of the powder quality input variable. As no powder quality metrics have been shown to be indicative of powder spreadability in PBF, various bulk powder characteristics were used as the powder quality indicator during ANOVA. Of the four metrics tested, the average avalanching angle, while statistically dependent of the powders angle of repose, showed poor correlation with experimental data. The remaining three metrics, however, were all found to have a statistically significant dependence on the angle of repose. Increasing the angle of repose resulted in significantly worse powder spreading, i.e. poor build plate coverage and powder clumping, as measured by the viable spreading metrics. Other processing parameters, such as the recoating speed and the recoater blade material were shown to also influence the spread quality.
       
  • Approach to Qualification using E-PBF In-situ Process Monitoring in
           Ti-6Al-4V
    • Abstract: Publication date: Available online 20 April 2019Source: Additive ManufacturingAuthor(s): S. Yoder, P. Nandwana, V. Paquit, M. Kirka, A. Scopel, R.R. Dehoff, S.S. Babu Traditional design and qualification methodologies for parts manufactured by traditional methods are being applied to Additive Manufacturing (AM) without understanding the nuances of the machines. While mapping process variables and tracking build data is helpful, some variables such as build geometry, support structure, and part melt order have not been researched in depth. Changing these variables can result in significant variations in material properties and defect structure such that the process appears to be unreliable compared to traditional manufacturing. Therefore, this research focuses on the need to understand the effects of overlooked variables such as melt order and nested geometry on the distribution of defects and bulk material properties in Ti-6Al-4 V alloy builds manufactured using the Arcam AB ® electron beam powder bed fusion process1. This study collected and analyzed process log data and near infrared (NIR) images for every layer to correlate trends in porosity formation and mechanical performance. The location of pores, while naturally stochastic, is heavily influenced by the cross-sectional area as detected by NIR images and correlates with the failure sites from uniaxial testing.
       
  • Cellulose Nanocrystals Support Material for 3D Printing Complexly Shaped
           Structures via Multi-Materials-Multi-Methods Printing
    • Abstract: Publication date: Available online 17 April 2019Source: Additive ManufacturingAuthor(s): Vincent Chi-Fung Li, Xiao Kuang, Craig M. Hamel, Devin Roach, Yulin Deng, H. Jerry Qi To fabricate highly complex structures, sacrificial support material is usually needed. However, traditional petroleum-based support materials are un-sustainable, non-recyclable, and difficult to be completely removed from the target structure after 3D processing. Instead, cellulose nanocrystals (CNC) gel could serves as an interesting 3D printing support material due to its sustainability, renewability, and potential recyclability. Since CNCs are highly dispersible in water as nanoparticles and are also not UV sensitive, it has less absorption or bondability with other UV curable polymer matrices. This allows them to be completely washed out by water, which offers a green and efficient method to remove the CNC support material during post processing. In addition, with increasing needs for more intricate structures, combining different 3D printing strategies into a hybrid 3D printing platform can be highly beneficial. In this work, a multi-materials-multi-methods (M4) printer with dual direct-ink-write (DIW) and DIW-inkjet printing capability was used to fabricate various complex structures while using CNC as support material. After 3D printing, water was used to remove the CNC support structure. Even in a highly confined environment, such as the inside of a balloon structure, CNC support material was still easily removed. The potential of using sustainable CNC support material and M4 hybrid 3D printing strategies to fabricate different complex structures was demonstrated. Since CNC gel is derived from forestry products and is entirely water based, the 3D printing process was also made more environmentally friendly, sustainable, and potentially recyclable.
       
  • Fatigue Behavior of Additive Manufactured 316L Stainless Steel Parts:
           Effects of Layer Orientation and Surface Roughness
    • Abstract: Publication date: Available online 16 April 2019Source: Additive ManufacturingAuthor(s): Rakish Shrestha, Jutima Simsiriwong, Nima Shamsaei The effects of layer orientation and surface roughness on the mechanical properties and fatigue life of 316 L stainless steel (SS) fabricated via a laser beam powder bed fusion (LB-PBF) additive manufacturing process were investigated. Quasi-static tensile and uniaxial fatigue tests were conducted on LB-PBF 316 L SS specimens fabricated in vertical and diagonal directions in their as-built surface condition, as well as in horizontal, vertical, and diagonal directions where the surface had been machined to remove any effects of surface roughness. In the machined condition, horizontally built LB-PBF specimens possessed higher fatigue resistance, followed by vertically built specimens, while the lowest fatigue resistance was obtained for diagonal specimens. Similarly, in the as-built condition, vertical specimens demonstrated better fatigue resistance when compared to diagonal specimens. Furthermore, the detrimental effects of surface roughness on fatigue life of LB-PBF 316 L SS specimens was not significant, which may be due to the presence of large internal defects in the specimens. Anisotropy of LB-PBF 316 L SS specimens was attributed to the variation in layer orientation, affecting defects’ directionality with respect to the loading direction. These defect characteristics can significantly influence the stress concentration and, consequently, fatigue behavior of additive manufactured parts. Therefore, the elastic-plastic energy release rates, a fracture mechanics-based concept that incorporates size, location, and projected area of defects on the loading plane, were determined to correlate the fatigue data and acceptable results were achieved.
       
  • Role of hierarchical microstructure of additively manufactured AlSi10Mg on
           dynamic loading behavior
    • Abstract: Publication date: Available online 16 April 2019Source: Additive ManufacturingAuthor(s): Amir Hadadzadeh, Babak Shalchi Amirkhiz, Akindele Odeshi, Jian Li, Mohsen Mohammadi Microstructure of an additively manufactured AlSi10Mg through direct metal laser sintering (DMLS) process is studied using multi-scale characterization techniques including scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The microstructure of DMLS-AlSi10Mg consists of hierarchical characteristics, spanning three order of magnitude, where nanometer sized to sub-millimeter scaled features exists in the structure. These characteristics included grain and cell structures, nanoscale Si precipitates and pre-existing dislocation networks. Dynamic mechanical behavior of the material is studied using a Split Hopkinson Pressure Bar apparatus over a range of strain rates varying between 800 s-1 and 3200 s-1. Investigation of the deformed microstructures reveals the role of hierarchical microstructure on the dynamic behavior of the material. The high strain-rate deformation is accommodated by dynamic recovery (DRV) process, where low angle grain boundaries evolve due to the generation of dislocations, evolution of dislocation networks, and annihilation of dislocations. Both cell walls and Si precipitates contribute to impeding the dislocation motion and development of dislocation networks. At high strain rates, dislocation networks evolve in the nanoscale DRVed subgrains.
       
  • Silicone rheological behavior modification for 3D printing: evaluation of
           yield stress impact on printed object properties
    • Abstract: Publication date: Available online 15 April 2019Source: Additive ManufacturingAuthor(s): Edwin-Joffrey Courtial, Clément Perrinet, Arthur Colly, David Mariot, Jean-Marc Frances, René Fulchiron, Christophe Marquette Silicone-based materials are commonly used in medical applications such as pre-surgery models or implants, leading to interesting biomimetic mechanical properties. Emergence of 3D printing and particularly liquid deposition modelling (LDM) has shown that specific rheological behaviors, particularly yield stress characters, were required to achieve efficient LDM. Unfortunately, standard silicone formulations seldom present such behaviors and are then proved to have low applicability in LDM-based 3D printing.In the present study, polyethylene glycol of different lengths were added as yield stress agents in a bi-component silicone and were demonstrated to operate a drastic improvement of the material rheological behaviors, without significant impact on the final mechanical properties of the material. An interesting relationship was demonstrated between dynamic yield stress values and reachable 3D geometries (the higher σys, the more complex the 3D printed shape can be) but the study also revealed that it is not the only key factor to ensure the printability of viscoelastic materials when highly complex geometries are seek; tack and melt strength have also to be investigated.
       
  • Print time vs. elapsed time: A temporal analysis of a continuous printing
           operation for additive constructed concrete
    • Abstract: Publication date: Available online 13 April 2019Source: Additive ManufacturingAuthor(s): Brandy Diggs-McGee, Eric Kreiger, Megan Kreiger, Mike Case In additive construction, ambitious goals to fabricate a concrete building in less than 24 hours are attempted. In the field, this goal relies on a metric of print time to make this conclusion, which excludes rest time and delays. The task to complete a building in 24 hours was put to the test with the first attempt at a fully continuous print of a structurally reinforced additively constructed concrete (ACC) building. A time series analysis was performed on the events during the construction of a 512 ft2 (16’x32’x9.25’) building to explore the effect of delays on the completion time. This analysis included a study of the variation in comprehensive layer print times, expected trends and forecasting for what is expected in future prints of similar types. Furthermore, the study included a determination and comparison of print time, elapsed time and construction time, as well as a look at the effect of environmental conditions on the delay events. Upon finishing, the analysis concluded that the 3D-printed building was completed in 14-hours of print time, 31.2-hours elapsed time, or a total of 5 days of construction time. This emphasizes that reports on newly 3D-printed constructions need to provide a definition of time that includes all possible duration periods to communicate realistic capabilities of this new technology.
       
  • A review of the fatigue behavior of 3D printed polymers
    • Abstract: Publication date: Available online 6 April 2019Source: Additive ManufacturingAuthor(s): Lauren Safai, Juan Sebastian Cuellar, Gerwin Smit, Amir A. Zadpoor As additive manufacturing of polymeric materials is becoming more prevalent throughout industry and research communities, it is important to ensure that 3D printed parts are able to withstand mechanical and environmental stresses that occur when in use, including the sub-critical cyclic loads that could result in fatigue crack propagation and material failure. There has so far been only limited research on the fatigue behavior of 3D printed polymers to determine which printing or material parameters result in the most favorable fatigue behavior. To better understand the effects of the printing technique, printing materials, and printing parameters on the fatigue behavior of 3D printed materials, we present here an overview of the data currently available in the literature including fatigue testing protocols and a quantitative analysis of the available fatigue data per type of the AM technology. The results of our literature review clearly show that, due to the synergism between printing parameters and the properties of the printed material, it is challenging to determine the best combination of variables for fatigue resistance. There is therefore a need for more experimental and computational fatigue studies to understand how the above-mentioned material and printing parameters affect the fatigue behavior.
       
  • 3D printing for construction based on a complex wall of polymer-foam and
           concrete
    • Abstract: Publication date: Available online 2 April 2019Source: Additive ManufacturingAuthor(s): Benoit Furet, Philippe Poullain, Sébastien Garnier The objective of this paper is to present a new advanced Additive Manufacturing (AM) process for the construction of concrete structures: Batiprint3dTM. The proposed advanced technology consists of creating a complex wall of 3D-printed materials using a mobile and polyarticulated robot: two polymer-foam printed walls are used to encase a subsequent third wall made of concrete. Once the walls were in place, the foam is maintained to provide both an internal and external insulation to the house without requiring thermal bridges. This technique of the complex wall with 3D-printed composite foam/concrete material is similar to the use of expansive-foam formwork (FW) filled by concrete or Insulated Concrete Forms (ICF) but in that case printed directly on site. By using 3D printing for the foam and extrusion of the concrete with the same robotic system, the technique creates jointly both the structure and thermal elements of the building. In the first part of this paper the composite foam/concrete 3D printing method and optimized process parameters are présented. Polyurethane (PU) foam has weak mechanical properties and the filling of the internal void with concrete can yield in high deformations and even failure of the FW, it is therefore necessary to control this phenomenon. For that, an experimental study has been conducted to determine a filling procedure capable of minimizing the deformations. The results show that spacers between the two foam walls can allow for wall heights of poured concrete up to 50 cm. The problems solved, it was decided to experiment in full scale this new walls 3D printing method with the construction of YhnovaTM, a real 95m² social housing. This technology Batiprint3dTM have been used, it is possible now to propose a synthesis of the impacts of this new advanced technology for construction.
       
 
 
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