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Publisher: Elsevier   (Total: 3160 journals)

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Showing 1 - 200 of 3160 Journals sorted alphabetically
A Practical Logic of Cognitive Systems     Full-text available via subscription   (Followers: 9)
AASRI Procedia     Open Access   (Followers: 15)
Academic Pediatrics     Hybrid Journal   (Followers: 34, SJR: 1.655, CiteScore: 2)
Academic Radiology     Hybrid Journal   (Followers: 23, SJR: 1.015, CiteScore: 2)
Accident Analysis & Prevention     Partially Free   (Followers: 96, 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: 412, 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: 2)
Acta de Investigación Psicológica     Open Access   (Followers: 3)
Acta Ecologica Sinica     Open Access   (Followers: 10, SJR: 0.18, CiteScore: 1)
Acta Haematologica Polonica     Free   (Followers: 1, SJR: 0.128, CiteScore: 0)
Acta Histochemica     Hybrid Journal   (Followers: 3, SJR: 0.661, CiteScore: 2)
Acta Materialia     Hybrid Journal   (Followers: 258, 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: 3, 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: 28, 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: 6, SJR: 0.19, CiteScore: 0)
Actualites Pharmaceutiques Hospitalieres     Full-text available via subscription   (Followers: 3)
Acupuncture and Related Therapies     Hybrid Journal   (Followers: 6)
Acute Pain     Full-text available via subscription   (Followers: 14, 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: 8, SJR: 0.755, CiteScore: 2)
Additive Manufacturing     Hybrid Journal   (Followers: 10, SJR: 2.611, CiteScore: 8)
Additives for Polymers     Full-text available via subscription   (Followers: 22)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 154, SJR: 4.09, CiteScore: 13)
Advanced Engineering Informatics     Hybrid Journal   (Followers: 11, SJR: 1.167, CiteScore: 4)
Advanced Powder Technology     Hybrid Journal   (Followers: 17, SJR: 0.694, CiteScore: 3)
Advances in Accounting     Hybrid Journal   (Followers: 8, SJR: 0.277, CiteScore: 1)
Advances in Agronomy     Full-text available via subscription   (Followers: 14, SJR: 2.384, CiteScore: 5)
Advances in Anesthesia     Full-text available via subscription   (Followers: 28, 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: 10, 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: 14, 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: 4)
Advances in Cellular and Molecular Biology of Membranes and Organelles     Full-text available via subscription   (Followers: 12)
Advances in Chemical Engineering     Full-text available via subscription   (Followers: 27, 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: 29, SJR: 1.562, CiteScore: 3)
Advances in Colloid and Interface Science     Full-text available via subscription   (Followers: 19, 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: 14)
Advances in Developmental Biology     Full-text available via subscription   (Followers: 12)
Advances in Digestive Medicine     Open Access   (Followers: 9)
Advances in DNA Sequence-Specific Agents     Full-text available via subscription   (Followers: 5)
Advances in Drug Research     Full-text available via subscription   (Followers: 24)
Advances in Ecological Research     Full-text available via subscription   (Followers: 44, SJR: 2.524, CiteScore: 4)
Advances in Engineering Software     Hybrid Journal   (Followers: 28, 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: 46, 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: 58, SJR: 0.591, CiteScore: 2)
Advances in Fuel Cells     Full-text available via subscription   (Followers: 16)
Advances in Genetics     Full-text available via subscription   (Followers: 16, SJR: 1.354, CiteScore: 4)
Advances in Genome Biology     Full-text available via subscription   (Followers: 8, 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: 23, SJR: 0.368, CiteScore: 1)
Advances in Heterocyclic Chemistry     Full-text available via subscription   (Followers: 12, SJR: 0.749, CiteScore: 3)
Advances in Human Factors/Ergonomics     Full-text available via subscription   (Followers: 22)
Advances in Imaging and Electron Physics     Full-text available via subscription   (Followers: 2, SJR: 0.193, CiteScore: 0)
Advances in Immunology     Full-text available via subscription   (Followers: 35, SJR: 4.433, CiteScore: 6)
Advances in Inorganic Chemistry     Full-text available via subscription   (Followers: 8, 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: 7, 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: 18, SJR: 0.88, CiteScore: 2)
Advances in Mathematics     Full-text available via subscription   (Followers: 11, SJR: 3.027, CiteScore: 2)
Advances in Medical Sciences     Hybrid Journal   (Followers: 6, SJR: 0.694, CiteScore: 2)
Advances in Medicinal Chemistry     Full-text available via subscription   (Followers: 5)
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: 3)
Advances in Oncobiology     Full-text available via subscription   (Followers: 1)
Advances in Organ Biology     Full-text available via subscription   (Followers: 1)
Advances in Organometallic Chemistry     Full-text available via subscription   (Followers: 17, 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: 24, SJR: 0.461, CiteScore: 1)
Advances in Pharmaceutical Sciences     Full-text available via subscription   (Followers: 12)
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: 8, 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: 18)
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: 64)
Advances in Quantum Chemistry     Full-text available via subscription   (Followers: 6, SJR: 0.371, CiteScore: 1)
Advances in Radiation Oncology     Open Access   (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: 399, 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: 11, SJR: 0.555, CiteScore: 2)
Advances in the Study of Behavior     Full-text available via subscription   (Followers: 34, SJR: 2.208, CiteScore: 4)
Advances in Veterinary Medicine     Full-text available via subscription   (Followers: 17)
Advances in Veterinary Science and Comparative Medicine     Full-text available via subscription   (Followers: 13)
Advances in Virus Research     Full-text available via subscription   (Followers: 5, SJR: 2.262, CiteScore: 5)
Advances in Water Resources     Hybrid Journal   (Followers: 46, SJR: 1.551, CiteScore: 3)
Aeolian Research     Hybrid Journal   (Followers: 6, SJR: 1.117, CiteScore: 3)
Aerospace Science and Technology     Hybrid Journal   (Followers: 345, 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: 456, 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: 41, SJR: 1.272, CiteScore: 3)
Agriculture and Agricultural Science Procedia     Open Access   (Followers: 3)
Agriculture and Natural Resources     Open Access   (Followers: 3)
Agriculture, Ecosystems & Environment     Hybrid Journal   (Followers: 57, 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: 11, SJR: 1.153, CiteScore: 3)
Alcoholism and Drug Addiction     Open Access   (Followers: 10)
Alergologia Polska : Polish J. of Allergology     Full-text available via subscription   (Followers: 1)
Alexandria Engineering J.     Open Access   (Followers: 1, 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: 9, SJR: 0.201, CiteScore: 1)
Alzheimer's & Dementia     Hybrid Journal   (Followers: 51, SJR: 4.66, CiteScore: 10)
Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring     Open Access   (Followers: 4, SJR: 1.796, CiteScore: 4)
Alzheimer's & Dementia: Translational Research & Clinical Interventions     Open Access   (Followers: 4, SJR: 1.108, CiteScore: 3)
Ambulatory Pediatrics     Hybrid Journal   (Followers: 6)
American Heart J.     Hybrid Journal   (Followers: 50, SJR: 3.267, CiteScore: 4)
American J. of Cardiology     Hybrid Journal   (Followers: 54, 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: 10)
American J. of Geriatric Psychiatry     Hybrid Journal   (Followers: 14, SJR: 1.524, CiteScore: 3)
American J. of Human Genetics     Hybrid Journal   (Followers: 34, SJR: 7.45, CiteScore: 8)
American J. of Infection Control     Hybrid Journal   (Followers: 28, 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: 47)
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: 216, 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: 28, 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: 38, 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: 62, SJR: 0.138, CiteScore: 0)
Anaesthesia Critical Care & Pain Medicine     Full-text available via subscription   (Followers: 17, SJR: 0.411, CiteScore: 1)
Anales de Cirugia Vascular     Full-text available via subscription  
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: 42, SJR: 1.512, CiteScore: 5)
Analytical Biochemistry     Hybrid Journal   (Followers: 181, SJR: 0.633, CiteScore: 2)
Analytical Chemistry Research     Open Access   (Followers: 12, SJR: 0.411, CiteScore: 2)
Analytical Spectroscopy Library     Full-text available via subscription   (Followers: 11)
Anesthésie & Réanimation     Full-text available via subscription   (Followers: 2)
Anesthesiology Clinics     Full-text available via subscription   (Followers: 23, 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: 199, SJR: 1.58, CiteScore: 3)

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Journal Cover
Additive Manufacturing
Journal Prestige (SJR): 2.611
Citation Impact (citeScore): 8
Number of Followers: 10  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 2214-8604
Published by Elsevier Homepage  [3160 journals]
  • Reactive material jetting of polyimide insulators for complex circuit
           board design
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Fan Zhang, Ehab Saleh, Jayasheelan Vaithilingam, You Li, Christopher J. Tuck, Richard J.M. Hague, Ricky D. Wildman, Yinfeng He Polyimides are a group of high performance thermal stable dielectric materials used in diverse applications. In this article, we synthesized and developed a high-performance polyimide precursor ink for a Material Jetting (MJ) process. The proposed ink formulation was shown to form a uniform and dense polyimide film through reactive MJ utilising real-time thermo-imidisation process. The printed polyimide film showed a permittivity of 3.41 and degradation temperature around 500 °C, both of which are comparable to commercially available polyimide films. Benefiting from the capability of being able to selectively deposit material through MJ, we propose the use of such a formulation to produce complex circuit board structures by the co-printing of conductive silver tracks and polyimide dielectric layers. By means of selectively depositing 4 μm thick patches at the cross-over points of two circuit patterns, a traditional double-sided printed circuit board (PCB) can be printed on one side, providing the user with higher design freedom to achieve a more compact high performance PCB structure.Graphical abstractGraphical abstract for this article
       
  • First Demonstration of Additive Manufacturing of Cutting Tools using
           Directed Energy Deposition System: Stellite™-Based Cutting Tools
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Kellen D. Traxel, Amit Bandyopadhyay Machine-tool concepts are becoming increasingly complex to meet the demanding requirements of advanced applications. This raises per-tool costs for manufacturers and end users, motivating the development of novel, innovative fabrication methods for these tools. Our objective herein is to investigate laser-based additive manufacturing to fabricate application-optimized machine-tools that perform comparably to commercially-available products. To demonstrate this technology, multi-layer Stellite™ (Co-Cr-W superalloy) structures were deposited on a stainless-steel substrate via directed energy deposition technique to be used as a tool for cutting applications requiring high-temperature strength and ductility, an area where conventional carbide and high-speed steel tools are challenged. The as-printed structures were free of large-scale defects and voids, and were further characterized and compared to commercial Blackalloy 525 barstock (B525), a Co-Cr-W alloy tool of similar composition. The Stellite™ contained mostly Co-rich (α-phase) dendrites, as well as inter-dendritic Cr7C3 and Cr23C6 phases. The B525 composition consisted of a range of lamellar-eutectic microstructure comprised of Co-phase with W6C reinforcement. In reciprocating wear testing, Stellite™ 6 maintained a steady-state COF within 20% of B525 (0.36 ± 0.01 vs. 0.30 ± 0.01), and final wear rate as low as 38% difference from B525 (5.14*10−6 ± 4.58*10−7Nmm3 vs. 3.20*10−6 ± 4.99*10-7Nmm3). During a turning operation of SS304L, the Stellite™ 6 tool demonstrated consistent chip formation and more consistent rake-face and cratering wear in comparison to the B525 tool, indicating its adequacy for service in this application. Our results demonstrate for the first time that directed-energy-deposition can be utilized to fabricate advanced cutting tool concepts for job-specific applications.Graphical abstractGraphical abstract for this article
       
  • Hybrid manufacturing—Locating AM hubs using a two-stage facility
           location approach
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Danielle Strong, Michael Kay, Brett Conner, Thomas Wakefield, Guha Manogharan Hybrid Manufacturing is defined as the integration of Additive Manufacturing (AM), specifically metal AM, with traditional manufacturing post-processing such as heat treatment and machining. Hybrid AM enables Small and Medium Enterprises (SME) who can offer post-processing services to integrate into the growing AM supply chain. Most near-net metal AM parts require heat treatment processes (e.g. residual stress relieving/annealing) before machining to achieve final engineering specification. This research investigates a two-stage facility model to optimize the locations and capacities for new metal AM hubs which require two sequential post-processing services: heat treatment and machining. Using North American Industry Classification System (NAICS) data for machine shops and heat treatment facilities in the U.S., a p-median location model is used to determine the optimal locations for AM hub centers based on: (1) geographical data, (2) demand and (3) fixed and operational costs of hybrid-AM processing. Results from this study have identified: (a) candidate US counties to locate metal AM hubs, (b) total cost (fixed, operational and transportation), (c) capacity utilization of the AM hubs and (d) demand assignments across machine shops – heat treatment facilities – AM hubs. It was found that 2-stage p-Median model identified 22 A M hub locations as the initial sites for AM hubs which grows to 35 A M hubs as demand increases. It was also found that relatively fewer number of heat treatment facilities than machine shops resulted in a more concentrated locations of AM hubs. In addition, transportation costs were not adversely affected by the inclusion of as-build plates and showed that including heat treatment facilities as part of the hybrid AM supply chain will be mutually beneficial to all stakeholders of metal hybrid AM supply chain, i.e. AM → Heat treatment → Machining.Graphical abstractGraphical abstract for this article
       
  • Mesoscale multi-physics simulation of rapid solidification of Ti-6Al-4V
           alloy
    • Abstract: Publication date: Available online 7 December 2018Source: Additive ManufacturingAuthor(s): Dehao Liu, Yan Wang Powder bed fusion is a recently developed additive manufacturing (AM) technique for alloys, which builds parts by selectively melting metallic powders with a high-energy laser or electron beam. Nevertheless, there is still a lack of fundamental understanding of the rapid solidification process for better quality control. To simulate the microstructure evolution of alloys during the rapid solidification, in this research, a mesoscale multi-physics model is developed to simultaneously consider solute transport, phase transition, heat transfer, latent heat, and melt flow. In this model, the phase-field method simulates the dendrite growth of alloys, whereas the thermal lattice Boltzmann method models heat transfer and fluid flow. The phase-field method and the thermal lattice Boltzmann method are tightly coupled. The simulation results of Ti-6Al-4 V show that the consideration of latent heat is necessary because it reveals the details of the formation of secondary arms and provides more realistic kinetics of dendrite growth. The proposed multi-physics simulation model provides new insights into the complex solidification process in AM.Graphical abstractGraphical abstract for this article
       
  • Experiments and simulations on solidification microstructure for Inconel
           718 in powder bed fusion electron beam additive manufacturing
    • Abstract: Publication date: Available online 5 December 2018Source: Additive ManufacturingAuthor(s): G.L. Knapp, N. Raghavan, A. Plotkowski, T. DebRoy Previous research on the powder bed fusion electron beam additive manufacturing of Inconel 718 has established a definite correlation between the processing conditions and the solidification microstructure of components. However, the direct role of physical phenomena such as fluid flow and vaporization on determining the solidification morphology have not been investigated quantitatively. Here, we investigate the transient and spatial evolution of the fusion zone geometry, temperature gradients, and solidification growth rates during pulsed electron beam melting of the powder bed with a focus on the role of key physical phenomena. The effect of energy density during pulsing, which relates to the amount of heating of the build area during processing, on the columnar-to-equiaxed transition of the solidification structure was studied both experimentally and theoretically. Predictions and evaluation of the role of heat transfer and fluid flow was established using existing solidification theories combined with transient, three-dimensional numerical heat transfer and fluid flow modeling. Metallurgical characteristics of the alloy’s solidification are extracted from the transient temperature fields, and microstructure is predicted and validated using optical images and electron backscattered diffraction data from the experimental results. Simulations show that the pure liquid region solidified quickly, creating a large two-phase, mushy region that exists during the majority of solidification. While conductive heat transfer dominates in the mushy region, both the pool geometry and the solidification parameters are affected by convective heat transfer. Finally, increased energy density during processing is shown to increase the time of solidification, lowering temperature gradients and increasing the probability of equiaxed grain formation.Graphical abstractGraphical abstract for this article
       
  • A Discrete Source Model of Powder Bed Fusion Additive Manufacturing
           Thermal History
    • Abstract: Publication date: Available online 4 December 2018Source: Additive ManufacturingAuthor(s): Edwin J. Schwalbach, Sean P. Donegan, Michael G. Chapman, Kevin J. Chaput, Michael A. Groeber Significant attention has been focused on modeling of metallic additive manufacturing (AM) processes, with the initial aim of predicting local thermal history, and ultimately structure and properties. Existing models range greatly in physical complexity and computational cost, and the implications of various simplifying assumption often go unassessed. In the present work, we first formulate a fast acting Discrete Source Model (DSM) capable of handling the complex processing often encountered in metal powder bed fusion AM. We then assess implications of the source representation, details of the numeric implementation, as well as effects of boundary conditions and thermophysical parameters. We verify the DSM implementation against simple numerical thermal predictions, calibrate it with single track deposit experiments, validate outputs against multitrack deposits, and finally quantify the scaling performance. The DSM is an effective means of quickly generating an estimate of the local thermal history induced by complex scan strategies when combined with arbitrary component geometry. While a number of approximations limit its quantitative accuracy, the inexpensive nature and ability to treat complex processing plans suggests it will be useful for screening and identification of regions experiencing anomalous thermal history. Such a capability is necessary to direct usage of higher fidelity, more expensive models and experimental resources.
       
  • Molten Pool Behavior and Effect of Fluid Flow on Solidification Conditions
           in Selective Electron Beam Melting (SEBM) of a Biomedical Co-Cr-Mo Alloy
    • Abstract: Publication date: Available online 4 December 2018Source: Additive ManufacturingAuthor(s): Yufan Zhao, Yuichiro Koizumi, Kenta Aoyagi, Daixiu Wei, Kenta Yamanaka, Akihiko Chiba Selective electron beam melting (SEBM) is a type of additive manufacturing (AM) that involves multiple physical processes. Because of its unique process conditions compared to other AM processes, a detailed investigation into the molten pool behavior and dominant physics of SEBM is required. Fluid convection involves mass and heat transfer; therefore, fluid flow can have a profound effect on solidification conditions. In this study, computational thermal-fluid dynamics simulations with multi-physical modeling and proof-of-concept experiments were used to analyze the molten pool behavior and resultant thermal conditions related to solidification. The Marangoni effect of molten metal primarily determines fluid behavior and is a critical factor affecting the molten pool instability in SEBM of the Co–Cr–Mo alloy. The solidification parameters calculated from simulated data, especially the solidification rate, are sensitive to the local fluid flow at the solidification front. Combined with experimental analysis, the results presented herein indicate that active fluid convection at the solidification front increase the probability of new grain formation, which suppresses the epitaxial growth of columnar grains.
       
  • Invited review article: Where and how 3D printing is used in teaching and
           education
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Simon Ford, Tim Minshall The emergence of additive manufacturing and 3D printing technologies is introducing industrial skills deficits and opportunities for new teaching practices in a range of subjects and educational settings.In response, research investigating these practices is emerging across a wide range of education disciplines, but often without reference to studies in other disciplines. Responding to this problem, this article synthesizes these dispersed bodies of research to provide a state-of-the-art literature review of where and how 3D printing is being used in the education system. Through investigating the application of 3D printing in schools, universities, libraries and special education settings, six use categories are identified and described: (1) to teach students about 3D printing; (2) to teach educators about 3D printing; (3) as a support technology during teaching; (4) to produce artefacts that aid learning; (5) to create assistive technologies; and (6) to support outreach activities. Although evidence can be found of 3D printing-based teaching practices in each of these six categories, implementation remains immature, and recommendations are made for future research and education policy.
       
  • Qualification of channels produced by laser powder bed fusion: Analysis of
           cleaning methods, flow rate and melt pool monitoring data
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): T. Kolb, A. Mahr, F. Huber, J. Tremel, M. Schmidt Laser powder bed fusion, is an additive manufacturing technology that is used in industry for rapid prototyping and manufacturing of aftermarket products, molds and special machine parts. Quality assurance and process stability still require improvement until this technology is ready for large scale serial production. Scan strategies and parameter sets for manufacturing are often fixed when certification processes are finished. Thus, it is important to test the manufacturability of specific design features such as inner channels. In the following we will present the qualification of inner channels in different test parts for the aluminum alloy AlSi10Mg and the stainless steel 1.4542. The testing includes different cleaning methods and air flow rate measurements. Additionally, we will compare such parts and LPBF specific problems to observations with a coaxial melt pool monitoring system.
       
  • Mechanical vibration bandgaps in surface-based lattices
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Waiel Elmadih, Wahyudin P. Syam, Ian Maskery, Dimitrios Chronopoulos, Richard Leach In this paper, the phonon dispersion curves of several surface-based lattices are examined, and their energy transmission spectra, along with the corresponding bandgaps are identified. We demonstrate that these bandgaps may be controlled, or tuned, through the choice of cell type, cell size and volume fraction. Our results include two findings of high relevance to the designers of lattice structures: (i) network and matrix phase gyroid lattice structures develop bandgaps below 15 kHz while network diamond and matrix diamond lattices do not, and (ii) the bandwidth of a bandgap in the network phase gyroid lattice can be tuned by adjusting its volume fraction and cell size.
       
  • Mechanical and corrosion properties of CoCrFeNiTi-based high-entropy alloy
           additive manufactured using selective laser melting
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Tadashi Fujieda, Meichuan Chen, Hiroshi Shiratori, Kosuke Kuwabara, Kenta Yamanaka, Yuichiro Koizumi, Akihiko Chiba, Seiichi Watanabe The effectiveness of applying selective laser melting (SLM) to a CoCrFeNiTi-based high-entropy alloy was compared with that of using electron beam melting (EBM). The higher solidification rate during SLM promoted a fine uniform microstructure with no visible segregation, which led to superior tensile properties (yield strength: 773.0 ± 4.2 MPa, ultimate tensile strength: 1178.0 MPa, elongation: 25.8 ± 0.6%) and a higher pitting potential (0.88 ± 0.03 V versus Ag/AgCl in a 3.5% NaCl solution at 353 K) in comparison to the EBM specimens (743.4 ± 11.6, 932.2 ± 4.8 MPa, 4.0 ± 0.2%, and 0.50 ± 0.04 V versus Ag/AgCl, respectively). The effect of a solution treatment on the tensile properties and pitting-corrosion resistance of the SLM specimens was then examined. The effect of the solution treatment on these properties depended on the cooling method used during the treatment. In particular, the tensile properties and pitting-corrosion resistance improved as a result of water quenching. On the other hand, the properties of the solution-treated specimens depended on the size and volume fraction of very fine-ordered particles, with diameters of tens of nanometers that contained Ni and Ti. The as-built and solution-treated SLM specimens exhibited excellent tensile strength and exceptional pitting-corrosion resistance; they had higher tensile strength and pitting-corrosion resistance than the conventional high-corrosion-resistant alloys.Graphical abstractGraphical abstract for this article
       
  • Computational mechanical characterization of geometrically transformed
           Schwarz P lattice tissue scaffolds fabricated via two photon
           polymerization (2PP)
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Adi Z. Zabidi, Shuguang Li, Reda M. Felfel, Kathryn G. Thomas, David M. Grant, Donal McNally, Colin Scotchford Schwarz P unit cell-based tissue scaffolds comprised of poly(D,L-lactide-co- ε -caprolactone)(PLCL) fabricated via the additive manufacturing technique, two-photon polymerisation (2 P P) were found to undergo geometrical transformations from the original input design. A Schwarz P unit cell surface geometry CAD model was reconstructed to take into account the geometrical transformations through CAD modeling techniques using measurements obtained from an image-based averaging technique before its implementation for micromechanical analysis. Effective modulus results obtained from computational mechanical characterization via micromechanical analysis of the reconstructed unit cell assigned with the same material model making up the fabricated scaffolds demonstrated excellent agreement with a small margin of error at 6.94% from the experimental mean modulus (0.69 ± 0.29 MPa). The possible sources for the occurrence of geometrical transformations are discussed in this paper. The inter-relationships between different dimensional parameters making up the Schwarz P architecture and resulting effective modulus are also assessed and discussed. With the ability to accommodate the geometrical transformations, maintain efficiency in terms of time and computational resources, micromechanical analysis has the potential to be implemented in tissue scaffolds with a periodic microstructure as well as other structures outside the field of tissue engineering in general.Graphical abstractGraphical abstract for this article
       
  • Enriched analytical solutions for additive manufacturing modeling and
           simulation
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): John C. Steuben, Andrew J. Birnbaum, John G. Michopoulos, Athanasios P. Iliopoulos Recent developments in additive manufacturing (AM) technologies involving heat and mass deposition have exposed the need for computationally efficient modeling of thermal field histories. This is due to the effect of such histories on resulting morphologies and quantities of interest, such as micro- and meso-structure, residual strains and stresses, as well as on material and structural properties and associated functional performance at the macro-scale. Limiting undesirable manifestations of these phenomena has motivated the development of both feed-forward and feedback loop control methodologies. However, up to now the computational cost of existing methods for predicting thermal fields and associated aspects, have allowed only feed-forward control methods. Consequently, in this paper, analytic solutions are enriched and then used to model the thermal aspects of AM, in a manner that demonstrates both high computational performance and fidelity required to enable “in the loop” use for feedback control of AM processes. It is first shown that the utility of existing analytical solutions is limited due to their underlying assumptions, some of which are their derivation based on a homogeneous semi-infinite domain and temperature independent material properties among others. These solutions must therefore be enriched in order to capture the actual thermal physics associated with the relevant AM processes. Enrichments introduced herein include the handling of strong nonlinear variations in material properties due to their dependence on temperature, finite non-convex solution domains, behavior of heat sources very near domain boundaries, and mass accretion coupled to the thermal problem. The enriched analytic solution method (EASM) that implements these enrichments is shown to produce results equivalent to those of numerical methods (such as Finite Elements and Finite Differences) that require six orders of magnitude greater computational cost.
       
  • Predictive process parameter selection for Selective Laser Melting
           Manufacturing: applications to high thermal conductivity alloys
    • Abstract: Publication date: Available online 3 December 2018Source: Additive ManufacturingAuthor(s): Priyanshu Bajaj, Jonathan Wright, Iain Todd, Eric A. Jägle There is growing interest in Laser Powder Bed Fusion (L-PBF) or Selective Laser Melting (SLM) manufacturing of high conductivity metals such as copper and refractory metals. SLM manufacturing of high thermal conductivity metals is particularly difficult. In case of refractory metals, the difficulty is amplified because of their high melting point and brittle behaviour. Rapid process development strategies are essential to identify suitable process parameters for achieving minimum porosities in these alloys, yet current strategies suffer from several limitations. We propose a simple approach for rapid process development using normalized process maps. Using plots of normalized energy density vs. normalized hatch spacing, we identify a wide processability window. This is further refined using analytical heat transfer models to predict melt pool size. Final optimization of the parameters is achieved by experiments based on statistical Design of Experiments concepts. In this article we demonstrate the use of our proposed approach for development of process parameters (hatch spacing, layer thickness, exposure time and point distance) for SLM manufacturing of molybdenum and aluminium. Relative densities of 97.4% and 99.7% are achieved using 200 W pulsed laser and 400 W continuous laser respectively, for molybdenum and aluminium, demonstrating the effectiveness of our approach for SLM processing of high conductivity materials.Graphical abstractGraphical abstract for this article
       
  • Additive laser metal deposition onto silicon
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Arad Azizi, Matthias A. Daeumer, Scott N. Schiffres By employing selective laser melting (SLM), we demonstrate how Sn3Ag4Ti alloy can robustly bond to silicon via additive manufacturing. With this technology, heat removal devices (e.g., vapor chamber evaporators, heat pipes, micro-channels) can be directly printed onto the electronic package without using thermal interface materials. This has the advantage of keeping the current microprocessor about 10 °C cooler by eliminating two thermal interface materials. This reduces operating temperature, saving power and reducing electronic-waste. The bonding of common metal alloys used in additive manufacturing onto silicon is relatively weak and generally possesses high contact angles (poor wetting and interfacial strength). By using the proper interlayer material, wettability and reactivity with the silicon substrate increase drastically. Unlike conventional dissimilar material brazing that can take tens of minutes to form a strong bond, this study demonstrates how this kinetic limitation can be overcome to form a good bond in sub-milliseconds via intense laser heating. The mechanism for rapid bonding lies in using an alloy that can form a strong intermetallic bond to the substrate at a low temperature, and exposing the sample multiple times to give sufficient diffusion time for a strong bond. Bonding of Sn3Ag4Ti to silicon occurs through the formation of a thin (∼μm) titanium-silicide interfacial layer that makes the silicon wettable to the Sn3Ag4Ti. These printed parts are mechanically resistant to thermal cycling, with no mechanical failures visible after over a week of continuous thermal cycling (−40 °C and 130 °C).Graphical abstractGraphical abstract for this article
       
  • Prediction of lack of fusion porosity in selective laser melting based on
           melt pool monitoring data
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Sam Coeck, Manisha Bisht, Jan Plas, Frederik Verbist Selective laser melting is already established as a commercial production technique. Some high-end users are, however, struggling with the complexity, consistency and cost associated with the qualification of high-end products built using the technique. In-situ process monitoring is a promising means to accommodate this issue, but quantitative correlations between monitoring signals and actual part defects have been lacking. In this paper, results are presented that have been obtained with an off-axis melt pool monitoring system on a 3D Systems ProX DMP 320 using Ti-6Al-4 V ELI. The focus is on the development of a method for predicting the presence and location of lack of fusion porosities as they can have a large impact on part quality and are not always easily detected post-build. The processed signals from the monitoring system are shown to have a high degree of correlation with the presence of lack of fusion porosities as measured by CT scans. A prediction sensitivity of 90% for lack of fusion events in the range of pores having a volume greater than 0.001 mm3, roughly equivalent to 160 μm in diameter, was obtained.
       
  • New aspects about the search for the most relevant parameters optimizing
           SLM materials
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Tatiana Mishurova, Katia Artzt, Jan Haubrich, Guillermo Requena, Giovanni Bruno While the volumetric energy density is commonly used to qualify a process parameter set, and to quantify its influence on the microstructure and performance of additively manufactured (AM) materials and components, it has been already shown that this description is by no means exhaustive. In this work, new aspects of the optimization of the selective laser melting process are investigated for AM Ti-6Al-4V. We focus on the amount of near-surface residual stress (RS), often blamed for the failure of components, and on the porosity characteristics (amount and spatial distribution). First, using synchrotron x-ray diffraction we show that higher RS in the subsurface region is generated if a lower energy density is used. Second, we show that laser de-focusing and sample positioning inside the build chamber also play an eminent role, and we quantify this influence. In parallel, using X-ray Computed Tomography, we observe that porosity is mainly concentrated in the contour region, except in the case where the laser speed is small. The low values of porosity (less than 1%) do not influence RS.
       
  • Areal topography measurement of metal additive surfaces using focus
           variation microscopy
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Lewis Newton, Nicola Senin, Carlos Gomez, Reinhard Danzl, Franz Helmli, Liam Blunt, Richard Leach In this work, the performance of a focus variation instrument for measurement of areal topography of metal additive surfaces was investigated. Samples were produced using both laser and electron beam powder bed fusion processes with some of the most common additive materials: Al-Si-10Mg, Inconel 718 and Ti-6Al-4V. Surfaces parallel and orthogonal to the build direction were investigated. Measurement performance was qualified by visually inspecting the topographic models obtained from measurement and quantified by computing the number of non-measured data points, by estimating local repeatability error in topography height determination and by computing the value of the areal field texture parameter Sa. Variations captured through such indicators were investigated as focus variation-specific measurement control parameters were varied. Changes in magnification, illumination type, vertical resolution and lateral resolution were investigated. The experimental campaign was created through full factorial design of experiments, and regression models were used to link the selected measurement process control parameters to the measured performance indicators. The results indicate that focus variation microscopy measurement of metal additive surfaces is robust to changes of the measurement control parameters when the Sa texture parameter is considered, with variations confined to sub-micrometre scales and within 5% of the average parameter value for the same surface and objective. The number of non-measured points and the local repeatability error were more affected by the choice of measurement control parameters. However, such changes could be predicted by the regression models, and proved consistent once material, type of additive process and orientation of the measured surface are set.
       
  • Laser-matter interactions in additive manufacturing of stainless steel
           SS316L and 13-93 bioactive glass revealed by in situ X-ray imaging
    • Abstract: Publication date: December 2018Source: Additive Manufacturing, Volume 24Author(s): Chu Lun Alex Leung, Sebastian Marussi, Michael Towrie, Jesus del Val Garcia, Robert C. Atwood, Andrew J. Bodey, Julian R. Jones, Philip J. Withers, Peter D. Lee Laser-matter interactions in laser additive manufacturing (LAM) occur on short time scales (10−6–10−3 s) and have traditionally proven difficult to characterise. We investigate these interactions during LAM of stainless steel SS316L and 13-93 bioactive glass powders using a custom built LAM process replicator (LAMPR) with in situ and operando synchrotron X-ray real-time radiography. This reveals a wide range of melt track solidification phenomena as well as spatter and porosity formation. We hypothesise that the SS316L powder absorbs the laser energy at its surface while the trace elements in the 13-93 bioactive glass powder absorb and remit the infra-red radiation. Our results show that a low viscosity melt, e.g. 8 mPa s for SS316L, tends to generate spatter (diameter up to 250 μm and an average spatter velocity of 0.26 m s−1) and form a melt track by molten pool wetting. In contrast, a high viscosity melt, e.g. 2 Pa s for 13-93 bioactive glass, inhibits spatter formation by damping the Marangoni convection, forming a melt track via viscous flow. The viscous flow in 13-93 bioactive glass resists pore transport; combined with the reboil effect, this promotes pore growth during LAM, resulting in a pore size up to 600 times larger than that exhibited in the SS316L sample.
       
  • Three-dimensional grain growth during multi-layer printing of a
           nickel-based alloy Inconel 718
    • Abstract: Publication date: Available online 28 November 2018Source: Additive ManufacturingAuthor(s): H.L. Wei, G.L. Knapp, T. Mukherjee, T. DebRoy Heterogeneous grain structure is a source of the inhomogeneity in structure and properties of the metallic components made by multi-layer additive manufacturing (AM). During AM, repeated heating and cooling during multi-layer deposition, local temperature gradient and solidification growth rate, deposit geometry, and molten pool shape and size govern the evolution of the grain structure. Here the effects of these causative factors on the heterogeneous grain growth during multi-layer laser deposition of Inconel 718 are examined by a Monte Carlo method based grain growth model. It is found that epitaxial columnar grain growth occurs from the substrate or previously deposited layer to the curved top surface of the deposit. The growth direction of these columnar grains is controlled by the molten pool shape and size. The grains in the previously deposited layers continue to grow because of the repeated heating and cooling during the deposition of the successive layers. Average longitudinal grain area decreases by approximately 80 % when moving from the center to the edge of the deposit due to variable growth directions dependent on the local curvatures of the moving molten pool. The average horizontal grain area increases with the distance from the substrate, with a 20 % increase in the horizontal grain area in a short distance from the third to the eighth layer, due to competitive solid-state grain growth causes increased grain size in previous layers.Graphical abstractGraphical abstract for this article
       
  • Characterisation of porosity, density, and microstructure of directed
           energy deposited stainless steel AISI 316L
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Zhi’En Eddie Tan, John Hock Lye Pang, Jacek Kaminski, Helene Pepin Directed Energy Deposition (DED) was used to form a Stainless Steel AISI 316 L steel block component on a Mild Steel S235JR substrate. Porosity, density, and defect were characterised at 4 localities within the DED component by microscopy and x-ray tomography. Three-dimensional (3D) reconstruction of the x-ray tomographic image sequences focused at select porosities is presented. The element composition and Vickers microhardness measurements were taken at the fusion lines and track body locations to characterise the differences in materials and mechanical properties at the 2 locations. Lastly, an element mapping analysis was conducted to determine the solidification mode for the DED component. Sources for defects were proposed based on the characteristics of the porosity analysis and conclusions were made about the solidification behaviour of the DED component.
       
  • Designing for Big Area Additive Manufacturing
    • Abstract: Publication date: January 2019Source: Additive Manufacturing, Volume 25Author(s): Alex Roschli, Katherine T. Gaul, Alex M. Boulger, Brian K. Post, Phillip C. Chesser, Lonnie J. Love, Fletcher Blue, Michael Borish Additive manufacturing (AM), more commonly referred to as 3D printing, is revolutionizing the manufacturing industry. With any new technology comes new rules and guidelines for the optimal use of said technology. Big Area Additive Manufacturing (BAAM), developed by Cincinnati Incorporated and Oak Ridge National Laboratory’s Manufacturing Demonstration Facility, requires a host of new design parameters compared to small-scale 3D printing to create large-scale parts. However, BAAM also creates new possibilities in material testing and various applications in the manufacturing industry. Most of the design constraints of small-scale polymer 3D printers still apply to BAAM. Beyond those constraints, new rules and limitations exist because BAAM’s large-scale system significantly changes the thermal properties associated with small-scale AM. This work details both physical and software-related design considerations for additive manufacturing. After reading this guide, one will have a better understanding of slicing software’s capabilities and limitations, different physical characteristics of design and how to apply them appropriately for AM, and how to take the inherent nature of AM into consideration during the design process.
       
  • Interference fit of material extrusion parts
    • Abstract: Publication date: Available online 22 November 2018Source: Additive ManufacturingAuthor(s): L. Bottini, A. Boschetto Material extrusion is an Additive Manufacturing process able to fabricate a physical object directly from a virtual model using layer by layer deposition of a thermoplastic filament extruded by a nozzle. The fabrication of functional components implies the need for the assembly with other parts with different properties in terms of material and surface quality. One of the most used assembly method involving plastic materials is the interference fit. It consists of fastening elements in which the two parts are pushed together, by means of a fit force, and no other fastener is necessary. It requires the accurate design of the interference, typically carried out by the designers through diagrams and theoretical formulations supplied by the material manufacturers. At present no theory has been provided for material extrusion parts due to the anisotropic behavior: the mesostructure, the surface roughness and the dimensional deviations mainly depend upon the build orientation.In this work the effects of the surface morphology and the interference grade on the assembly and disassembly forces in an interference fit joint are investigated. For the purpose, a design of experiment with a factorial plan has been carried out. The coupling behavior and the maximum forces are discussed. A new variable namely the real interference has been introduced and a relationship between this variable and the assembly force has been found. Through this model it is possible to know in advance the force necessary to assemble a material extrusion part with an assigned interference grade.
       
  • Microstructure modelling of laser metal powder directed energy deposition
           of Alloy 718
    • Abstract: Publication date: Available online 22 November 2018Source: Additive ManufacturingAuthor(s): Chamara Kumara, Andreas Segerstark, Fabian Hanning, Nikhil Dixit, Shrikant Joshi, Johan Moverare, Per Nylén A multi-component and multi-phase-field modelling approach, combined with transformation kinetics modelling, was used to model microstructure evolution during laser metal powder directed energy deposition of Alloy 718 and subsequent heat treatments. Experimental temperature measurements were utilised to predict microstructural evolution during successive addition of layers. Segregation of alloying elements as well as formation of Laves and δ phase was specifically modelled. The predicted elemental concentrations were then used in transformation kinetics to estimate changes in Continuous Cooling Transformation (CCT) and Time Temperature Transformation (TTT) diagrams for Alloy 718. Modelling results showed good agreement with experimentally observed phase evolution within the microstructure. The results indicate that the approach can be a valuable tool, both for improving process understanding and for process development including subsequent heat treatment.
       
  • Deformation of honeycomb cellular structures manufactured with Laser
           Engineered Net Shaping (LENS) technology under quasi-static loading:
           experimental testing and simulation
    • Abstract: Publication date: Available online 19 November 2018Source: Additive ManufacturingAuthor(s): Paweł Baranowski, Paweł Płatek, Anna Antolak-Dudka, Marcin Sarzyński, Michał Kucewicz, Tomasz Durejko, Jerzy Małachowski, Jacek Janiszewski, Tomasz Czujko The paper presents a methodology investigation of honeycomb cellular structures deformation process in quasi-static compression tests. Two honeycomb topologies with different elementary cells were designed and manufactured from Ti-6Al-4 V alloy powder with the use of Laser Engineered Net Shaping (LENS) system and compressed using a universal strength machine. To simulate the deformation process with LS-Dyna software, the mechanical properties of the material were assessed and correlated. An elasto-visco-plastic material model (Mat_Plasticity_With_Damage) was used for predicting the material behavior. The results of experimental tests and numerical simulations were compared. A reasonable agreement between deformation, failure and force histories was obtained. Additionally, both the topologies were compared for their energy absorption capabilities. The validated numerical modelling with the adopted constitutive model will be used in the further studies to analyze different cellular structures topologies subjected to dynamic loading.
       
  • Visible light 3D printing with epoxidized vegetable oils
    • Abstract: Publication date: Available online 19 November 2018Source: Additive ManufacturingAuthor(s): Diego Savio Branciforti, Simone Lazzaroni, Chiara Milanese, Marco Castiglioni, Ferdinando Auricchio, Dario Pasini, Daniele Dondi Stereolithography is a 3D printing technique in which a liquid monomer is photopolymerized to produce a solid object. The most widely used materials usually belong to the family of acrylate monomers, and photopolymerization occurs through a radical pathway. Photoinitiators can absorb UV or (less often) visible light, producing radicals for direct decomposition or hydrogen abstraction. Due to the toxicity of acrylates, vegetable oil-derived monomers were used in this study. In fact, vegetable oils contain unsaturations, and thus, they can be exploited as monomers. In particular, linseed oil, tung oil or edible oils (soybean, sunflower or corn) could be good candidates as raw materials. Unfortunately, the photoinduced radical polymerization of these oils either does not occur or is too slow for 3D printing applications. For this reason, the oils were modified as epoxides. Epoxides are monomers that are more reactive than natural oils, and they can be polymerized via a cationic mechanism. The aim of this work was to exploit visible light generated by a common digital projector (like those used in classrooms) as a light source. Since the tested photoacid generators working under visible light are ineffective for the polymerization of epoxidized oils, a multi-component photo-initiating mixture was used.Graphical abstractGraphical abstract for this articleVegetable oil epoxides, together with curcumin and visible light could replace acrylates from 3D printing
       
  • Novel Plasma Treatment for Preparation of Laser Sintered Nanocomposite
           parts
    • Abstract: Publication date: Available online 19 November 2018Source: Additive ManufacturingAuthor(s): Alaa Almansoori, Kerry J. Abrams, Ammar D. Ghali Al-Rubaye, Candice Majewski, Cornelia RodenburgABSTRACTPolymer Laser Sintering (LS) is a well-known Additive Manufacturing process, capable of producing highly complex geometries with little or no cost penalty. However, the restricted range of materials currently available for this process has limited its applications. Whilst it is common to modify the properties of standard LS polymers with the inclusion of fillers e.g. nanoclays, achieving effective dispersions can be difficult. The work presented here investigates the use of plasma treatment as a method of enhancing dispersion with an expectation of improving consistency and surface quality of laser sintered nanocomposite parts. To enable the preparation of polyamide 12 nanocomposite powder for applications in LS, plasma surface modification using Low Pressure Air Plasma Treatment was carried out on two nanoclays: Cloisite 30B (C30B) and Nanomer I.34TCN (I.34TCN). Plasma treatment strongly reduced the aggregation of the nanoclay (C30B and I.34TCN) particles, and powders displayed higher decomposition temperatures than those without plasma treatment. LS parts from neat polyamide 12, untreated I.34TCN and plasma treated I.34TCN composites were successfully produced with different complex shapes. The presence of well dispersed plasma treated nanoclays was observed and found to be essential for an improved surface quality of LS fabricated which was achieved only for plasma treated I.34TCN. Likewise, some mechanical properties could be improved above that of PA12 by incorporation of treated I.34TCN. For example, the elastic modulus of plasma treated composites was higher than that of polyamide 12 and the untreated composite. In the case of the ultimate strain, the plasma treated composite performed better than untreated and results had a reduced variation between samples. This illustrates the feasibility of the use of plasma treatments on nanoclays to improve the properties of LS parts, even though further studies will be required to exploit the full potential.
       
  • The Influence of Forced-Air Cooling on a 3D Printed Part Manufactured by
           Fused Filament Fabrication
    • Abstract: Publication date: Available online 15 November 2018Source: Additive ManufacturingAuthor(s): Chun-Ying Lee, Chung-Yin Liu The dimensional quality and mechanical properties of a fused filament fabrication (FFF)-printed 3D model are influenced by several process parameters. A forced-air cooling system that moves along with the print head was designed and installed on a commercial 3D FFF printer to control the cooling of the printed model. The quality of the printed polylactide (PLA) model, including the dimensions and mechanical properties, was investigated for different cooling air velocities. It was found that the cooling air velocity had different influences on the dimensional quality and mechanical strength of the printed model. More specifically, higher cooling speeds generated better geometric accuracy but lower mechanical strength. With the highest and lowest cooling air speeds of 5 m/s and 0 m/s, respectively, the tensile strengths of the printed models differed by 4-fold. In order to determine a suitable cooling air velocity setting for each specific printing material, a design model was proposed. The determined printing parameters were employed in the fabrication of a Rubik’s cube, as an example. The assembled cube demonstrated satisfactory performance both in the dimensional quality and in the mechanical function. Therefore, the cooling air velocity can be employed as an additional control parameter in 3D printing for a specified model.
       
  • Selective laser melting of typical metallic materials: An effective
           process prediction model developed by energy absorption and consumption
           analysis
    • Abstract: Publication date: Available online 13 November 2018Source: Additive ManufacturingAuthor(s): Y.H. Zhou, Z.H. Zhang, Y.P. Wang, G. Liu, S.Y. Zhou, Y.L. Li, J. Shen, M. Yan Selective laser melting (SLM) is a laser-based additive manufacturing technique that can fabricate parts with complex geometries and sufficient mechanical properties. However, the optimal SLM process windows of metallic materials are difficult to predict, especially when exploring new metallic materials. In this paper, a universal and simplified model has been proposed to predict the energy density suitable for SLM of a variety of metallic materials including Ti and Ti alloys, Al alloy, Ni-based superalloy and steel, on the basis of the relationship between energy absorption and consumption during SLM. Several important but easily overlooked factors, including the surface structure of metallic powder, porosity of powder bed, vaporization and heat loss, were considered to improve the accuracy of the model. Results show that, to achieve near-full density parts, the energy absorption (Qa) by the local powder bed should be approximately 3–8 times greater than the energy consumption (Qc), and this finding applies to all materials investigated. The value of Qa/Qc highly depends on material properties, particularly laser absorptivity, latent heat of melting and specific heat capacity. Experiments on high-entropy alloy (CrMnFeCoNi) and Hastelloy X alloy, new metallic materials for SLM, have been further conducted to verify the model. Results confirm that the model can predict suitable laser energy densities needed for processing the various metallic materials without tedious trial and error experiments. Indications and uncertainty of the model have also been analyzed.Graphical abstractGraphical abstract for this article
       
  • Geometrical effects on residual stress in selective laser melting
    • Abstract: Publication date: Available online 25 September 2018Source: Additive ManufacturingAuthor(s): L.A. Parry, I.A. Ashcroft, R.D. Wildman Selective laser melting is an increasingly attractive technology for the manufacture of complex and low volume / high value metal parts. However, the inevitable residual stresses that are generated can lead to defects or build failure. Due to the complexity of this process, efficient and accurate prediction of residual stress in large components remains challenging. For the development of predictive models of residual stress, knowledge on their generation is needed. This study investigates the geometrical effect of scan strategy on residual stress development. It was found that the arrangement of scan vectors due to geometry, heavily influenced the thermal history within a part, which in turn significantly affected the transverse residual stresses generated. However, irrespective of the choice of scanned geometry and the thermal history, the higher magnitude longitudinal stresses had consistent behaviour based on the scan vector length. It was shown that the laser scan strategy becomes less important for scan vector length beyond 3 mm. Together, these findings, provide a route towards optimising scan strategies at the meso-scale, and additionally, developing a model abstraction for predicting residual stress based on scan vectors alone.
       
  • The Effect of SLM Process Parameters on Density, Hardness, Tensile
           Strength and Surface Quality of Ti-6Al-4V
    • Abstract: Publication date: Available online 5 September 2018Source: Additive ManufacturingAuthor(s): Amir Mahyar Khorasani, Ian Gibson, Umar Shafique Awan, Alireza Ghaderi In this paper, we printed Ti-6Al-4 V SLM parts based on Taguchi design of experiment and related standards to measure and compare hardness with different mechanical properties that were obtained in our previous research such as density, strength, elongation, and average surface. Then the effect of process parameters comprising laser power, scan speed, hatch space, laser pattern angle coupling, along with heat treatment as a post-process, in relation to hardness was analysed. The relation of measured factors with each other was also studied and related mechanisms were discussed in depth. The original contribution in this paper is in producing a large and precise dataset and the comparison with mechanical properties. Another contribution is related to the analysis of process parameters in relation to hardness and explaining them by rheological phenomena. The results showed an interesting similarity between hardness and density which is highly related to the formation of the melting pool and porosities within the process.
       
  • Using Machine Learning to identify In-Situ Melt Pool Signatures Indicative
           of Flaw Formation in a Laser Powder Bed Fusion Additive Manufacturing
           Process
    • Abstract: Publication date: Available online 11 November 2018Source: Additive ManufacturingAuthor(s): Luke Scime, Jack Beuth Because many of the most important defects in Laser Powder Bed Fusion (L-PBF) occur at the size and timescales of the melt pool itself, the development of methodologies for monitoring the melt pool is critical. This works examines the possibility of in-situ detection of keyholing porosity and balling instabilities. Specifically, a visible-light high speed camera with a fixed field of view is used to study the morphology of L-PBF melt pools in the Inconel 718 material system. A scale-invariant description of melt pool morphology is constructed using Computer Vision techniques and unsupervised Machine Learning is used to differentiate between observed melt pools. By observing melt pools produced across process space, in-situ signatures are identified which may indicate flaws such as those observed ex-situ. This linkage of ex-situ and in-situ morphology enabled the use of supervised Machine Learning to classify melt pools observed (with the high speed camera) during fusion of non-bulk geometries such as overhangs.
       
  • Recycled polypropylene blends as novel 3D printing materials
    • Abstract: Publication date: Available online 8 November 2018Source: Additive ManufacturingAuthor(s): Nicole E. Zander, Margaret Gillan, Zachary Burckhard, Frank Gardea Consumer-grade plastics can be considered a low-cost and sustainable feedstock for fused filament fabrication (FFF) additive manufacturing processes. Such materials are excellent candidates for distributed manufacturing, in which parts are printed from local materials at the point of need. Most plastic waste streams contain a mixture of polymers, such as water bottles and caps comprised of polyethylene terephthalate (PET) and polypropylene (PP), and complete separation is rarely implemented. In this work, blends of waste PET, PP and polystyrene (PS) were processed into filaments for 3D printing. The effect of blend composition and styrene ethylene butylene styrene (SEBS) compatibilizer on the resulting mechanical and thermal properties were probed. Recycled PET had the highest tensile strength at 35 ± 8 MPa. Blends of PP/PET compatibilized with SEBS and maleic anhydride functionalized SEBS had tensile strengths of 23 ± 1 MPa and 24 ± 1 MPa, respectively. The non-compatibilized PP/PS blend had a tensile strength of 22 ± 1 MPa. PP/PS blends exhibited reduced tensile strength to ca. 19 ± 1-3 MPa with the addition of SEBS. Elongation to failure was generally improved for the blended materials compared to neat recycled PET and PS. The glass transition was shifted to higher temperatures for all of the blends except the 50-50 wt. % PP/PET blend. Crystallinity was decreased for the blends, but was highest in the 75-25 wt. % PP/PS and the 50-50 wt. % PP/PET blend with SEBS-maleic anhydride. Solvent extraction of the dispersed phase revealed polypropylene was the matrix phase in both the 50-50 wt. % PP/PET and PP/PS blends.Graphical abstractGraphical abstract for this article
       
  • terial_Spec_Sheets/MSS_PJ_PJMaterialsDataSheet_051Digital Design and
           Nonlinear Simulation for Additive Manufacturing of Soft Lattice Structures
           
    • Abstract: Publication date: Available online 6 November 2018Source: Additive ManufacturingAuthor(s): O. Weeger, N. Boddeti, S.-K. Yeung, S. Kaijima, M.L. Dunn Lattice structures are frequently found in nature and engineering due to their myriad attractive properties, with applications ranging from molecular to architectural scales. Lattices have also become a key concept in additive manufacturing, which enables precise fabrication of complex lattices that would not be possible otherwise. While design and simulation tools for stiff lattices are common, here we present a digital design and nonlinear simulation approach for additive manufacturing of soft lattices structures subject to large deformations and instabilities, for which applications in soft robotics, healthcare, personal protection, energy absorption, fashion and design are rapidly emerging. Our framework enables design of soft lattices with curved members conforming to freeform geometries, and with variable, gradually changing member thickness and material, allowing the local control of stiffness. We model the lattice members as 3D curved rods and using a spline-based isogeometric method that allows the efficient simulation of nonlinear, large deformation behavior of these structures directly from the CAD geometries. Furthermore, we enhance the formulation with a new joint stiffening approach, which is based on parameters derived from the actual node geometries. Simulation results are verified against experiments with soft lattices realized by PolyJet multi-material polymer 3D printing, highlighting the potential for simulation-driven, digital design and application of non-uniform and curved soft lattice structures.
       
  • Evidence of austenite by-passing in a stainless steel obtained from laser
           melting additive manufacturing
    • Abstract: Publication date: Available online 6 November 2018Source: Additive ManufacturingAuthor(s): Michella Alnajjar, Frédéric Christien, Krzysztof Wolski, Cedric Bosch Microstructural characterization was carried out on AISI 17-4 PH stainless steel fabricated by selective laser melting (SLM) in an argon environment. Conventionally, this steel exhibits a martensitic structure with a small fraction of δ ferrite. However, the combined findings of x-ray diffraction and electron backscatter diffraction (EBSD) proved that SLM-ed 17-4 PH steel has a fully ferritic microstructure, more specifically  δ ferrite. The microstructure consists of coarse ferritic grains elongated along the build direction, with a pronounced solidification crystallographic texture. These results were associated to the high cooling and heating rates experienced throughout the SLM process that suppressed the austenite formation and produced a “by-passing” phenomenon of this phase during the numerous thermal cycles. Furthermore, the energy-dispersive X-ray spectroscopy (EDS) measurements revealed a uniform distribution of elements without any dendritic structure. The extremely high cooling kinetics induced a diffusionless solidification, resulting in a homogeneous elemental composition. It was also found that the ferritic SLM-ed material can be transformed to martensite again by re-austenitization at 1050 °C followed by quenching.Graphical abstractGraphical abstract for this article
       
  • Aging effects of polyamide 12 in selective laser sintering: Molecular
           weight distribution and thermal properties
    • Abstract: Publication date: Available online 6 November 2018Source: Additive ManufacturingAuthor(s): Katrin Wudy, Dietmar Drummer The material aging in selective laser sintering SLS of polyamide 12 is one challenge, which has to be overcome for implementation of this technique in serial production. High temperatures and along going processing times lead to chemical and physical aging effects of the supporting partcake material. Resulting aging mechanisms are not understood up to now. The investigations in this study aims at the influence of processing time and temperature on molecular changes and thermal properties of polyamide 12 partcake material in selective laser sintering. The focus of the investigations lays on the global heat exposure of the of the bulk material und thus on global material changes. Gel permeation chromatography analysis was used to determine the molecular weight distribution and changes of polymer structure. The Mark-Houwink plot exhibits a linear chain growth, which is a hint for a resulting solid state polycondensation. With increasing build time and build chamber temperature the average molecular weight is rising, whereby the influence of build time is more significant. The rise of chain length leads to a reduction of crystallization temperature, which was detected by DSC.Graphical abstractGraphical abstract for this article
       
  • Effects of Cold Plasma Treatment on Interlayer Bonding Strength in FFF
           Process
    • Abstract: Publication date: Available online 6 November 2018Source: Additive ManufacturingAuthor(s): Chin-Cheng Shih, Matthew Burnette, David Staack, Jyhwen Wang, Bruce L. Tai Fused Filament Fabrication (FFF) is the most popular additive manufacturing method because of its numerous capabilities and relatively low cost. This comes with a trade off as FFF printed parts are typically weak in the layer deposition direction due to insufficient interlayer bonding. This research adopts the method of cold plasma treatment and investigates the potential enhancement of interlayer bonding by altering the printed surface prior to the deposition of the next layer. Polylactic acid (PLA) is used as the printing material, due to its ubiquity in industry. The bonding strength is measured by the shear bond strength test. The results show that bond strength improved over 100% with 30 s of treatment and over 50% with 300 s of treatment. A mechanically polished surface is also included in the comparison for the high surface wettability, but the result shows no improvement. This indicates that wettability may not be the dominant mechanism for enhanced bonding after treatment.
       
  • Reducing the Roughness of Internal Surface of an Additive Manufacturing
           Produced 316 Steel Component by Chempolishing and Electropolishing
    • Abstract: Publication date: Available online 6 November 2018Source: Additive ManufacturingAuthor(s): Pawan Tyagi, Tobias Goulet, Christopher Riso, Robert Stephenson, Nitt Chuenprateep, Justin Schlitzer, Cordell Benton, Francisco Garcia-Moreno Surface roughness of an as produced AM component is very high, which prohibits the direct utilization of additively manufactured (AM) components for the intended applications. Reducing surface roughness is exponentially more challenging for the internal surfaces of an AM component. This paper reports our research in the area of postprocessing of interior surfaces of an AM component. We have investigated electropolishing and chemical polishing (chempolishing) methods to reduce the surface roughness of the internal surface. We found that chempolishing was effective in simultaneously reducing the internal and external surface roughness of 316 steel AM components. Chempolishing is found suitable for any complicated AM shape and geometry. Our electropolishing methodology was effective in reducing the surface roughness of the internal or external surfaces provided that a counter electrode could be positioned in the proximity of the surface to be polished. We have performed optical profilometry, scanning electron microscopy, and contact angle measurement study to investigate the difference between electropolishing and chemical polishing methods.
       
  • Temperature-dependent sintering of two viscous particles
    • Abstract: Publication date: December 2018Source: Additive Manufacturing, Volume 24Author(s): Caroline Balemans, Nick O. Jaensson, Martien A. Hulsen, Patrick D. Anderson Selective laser sintering (SLS) is a promising additive manufacturing technique, where powder particles are fused together under the influence of a laser beam. To obtain good material properties in the final product, the powder particles need to form a homogeneous melt during the fabrication process. On the one hand, you want to give the particles enough time to fuse, such that the original shape is no longer visible, and interdiffusion of polymers can take place. On the other hand, you want the process to be as fast as possible. This is contradictory, thus choosing the right conditions is not trivial. We developed a computational model based on the finite element method to study the material and process parameters concerning the melt flow of the powder particles. In this work, we restrict ourselves to varying the temperature-dependent viscosity, the process parameters, and the convective heat transfer coefficient of the sintering of two polymer (polyamide 12) particles. The simulations allow for a quantitative analysis of the influence of the different material and processing parameters. From the simulations follows that an optimal sintering process has a low ambient temperature, a narrow beam width with enough power to heat the particles only a few degrees above the melting temperature, and a polymer of which the viscosity decreases significantly within these few degrees.
       
  • Mechanical Properties of Hexagonal Lattice Structures Fabricated Using
           Continuous Liquid Interface Production Additive Manufacturing
    • Abstract: Publication date: Available online 3 November 2018Source: Additive ManufacturingAuthor(s): Davis J. McGregor, Sameh Tawfick, William P. King Additive manufacturing (AM) is a key enabler for architectured lattice materials, because of the geometric complexity of parts that can be produced. Recent advancements in AM have enabled rapid production speeds, high spatial resolution, and a variety of engineering polymers. An open question remains whether production grade AM can accurately and repeatably produce lattice parts. This study presents design, production, and mechanical property testing of hexagonal lattice parts manufactured using continuous liquid interface production (CLIP) based AM. We printed and tested 84 parts, in three polymer materials having relative density ranging from 0.06 to 0.23. Lattice wall structures were reliably printed when truss aspect ratio was in the range 5 to 20 and wall thicknesses are 0.35 or 0.5 mm. The printed lattice parts, each comprising hundreds of slender walls, were measured using high resolution optical scanning. The images were analyzed to evaluate the difference between the printed parts and their designs, and the effect of geometric deviations on the mechanical behavior. The measured elastic moduli of the printed parts are close to the values expected from the materials specifications. The strength of all printed parts deviates by 7% from the behavior predicted from the scanned geometry. The failure mode of the printed structures depends upon the material and part geometry. To our knowledge, this is the largest study on the accuracy and performance of AM lattice parts, and the first study of its type for lattice parts made using CLIP.
       
  • Evolution of 316L Stainless Steel Feedstock Due to Laser Powder Bed Fusion
           Process
    • Abstract: Publication date: Available online 2 November 2018Source: Additive ManufacturingAuthor(s): Michael J. Heiden, Lisa A. Deibler, Jeff M. Rodelas, Josh R. Koepke, Dan J. Tung, David J. Saiz, Bradley H. Jared
       
  • Investigation on the mode of failures and fatigue life of laser-based
           powder bed fusion produced stainless steel parts under variable amplitude
           loading conditions
    • Abstract: Publication date: Available online 2 November 2018Source: Additive ManufacturingAuthor(s): Sagar Sarkar, Cheruvu Siva Kumar, Ashish Kumar Nath Additive Manufacturing (AM) is a method of joining metal/non-metals or composites layer by layer using different energy sources. Among the various AM processes, laser-based powder bed fusion (LPBF) is very popular, in which geometrically complex structures can be manufactured directly from CAD models. One of the least investigated areas in LPBF is the fatigue property of LPBF produced stainless steel parts, which find a variety of engineering and medical applications. In actual service conditions, many engineering components undergo variable cyclic loadings. Therefore, in order to widen industrial applications of LPBF process, effects of variable amplitude loading under both zero and tensile mean stresses on the fatigue life of LPBF produced 15-5 precipitation hardened stainless steel parts have been examined in the present study. Further, different modes of failure, effects of load sequences on fatigue life and the cumulative damage during the process have also been studied. For a typical case under tensile mean stress, results showed that the number of cycles to failure with low to high loading sequence was almost double of that with the sequence reversed. Also, the cumulative damage was more in the first case than that of the second case. Fracture surfaces were studied using Scanning Electron Microscopy to investigate the mode of failures and completely different fracture surface morphologies for these two cases explain the observed difference in number of cycles to failure with the reversal of the load sequence.
       
  • Implications of modeling approaches on the fatigue behavior of cellular
           solids
    • Abstract: Publication date: Available online 2 November 2018Source: Additive ManufacturingAuthor(s): Gianpaolo Savio, Stefano Rosso, Andrea Curtarello, Roberto Meneghello, Gianmaria Concheri According to recent studies, a new paradigm in the geometric modeling of lattice structures based on subdivision surfaces for additive manufacturing overcomes the critical issues on CAD modeling highlighted in the literature, such as scalability, robustness, and automation. In this work, the mechanical behavior of the subdivided lattice structures was investigated and compared with the standard lattices. Five types of cellular structures based on cubic cell were modeled: struts based on squared or circular section, with or without fillets and cell based on the subdivision approach. Sixty-five specimens were manufactured by selective laser sintering technology in polyamide 12 and tensile and fatigue tests were performed. Furthermore, numerical analyses were carried out in order to establish the stress concentration factors.Results show that subdivided lattice structures, at the same resistant area, improve stiffness and fatigue life and reduce stress concentration while opening new perspectives in the development of lattice structures for additive manufacturing technologies and applications.Graphical abstractGraphical abstract for this article
       
  • Planar Deposition Flow Modeling of Fiber Filled Composites in Large Area
           Additive Manufacturing
    • Abstract: Publication date: Available online 1 November 2018Source: Additive ManufacturingAuthor(s): Blake P. Heller, Douglas E. Smith, David A. Jack The rapid transition of the Fused Filament Fabrication (FFF) Additive Manufacturing (AM) process from small scale prototype models to large scale polymer deposition has been driven, in part, by the addition of short carbon fibers to the polymer feedstock. The addition of short carbon fibers improves both the mechanical and thermal properties of the printed beads. The improvements to the anisotropic mechanical and thermal properties of the polymer feedstock are dependent on the spatially varying orientation of short carbon fibers which is itself a function of the velocity gradients in the flow field throughout the nozzle and in the extrudate during deposition flow. This paper presents a computational approach for simulating the deposition flow that occurs in the Large Area Additive Manufacturing (LAAM) process and the effects on the final short fiber orientation state in the deposited polymer bead and the resulting bead mechanical and thermal properties. The finite element method is used to evaluate Stokes flow for a two-dimensional planar flow field within a Strangpresse Model 19 LAAM polymer deposition nozzle. A shape optimization method is employed to compute the shape of the polymer melt flow free surface below the nozzle exit as the bead is deposited on a moving print platform. Three nozzle configurations are considered in this study. Fiber orientation tensors are calculated throughout the fluid domain using the Folgar-Tucker fiber interaction model. The effective bulk mechanical properties, specifically the longitudinal and transverse moduli, and the coefficient of thermal expansion, are also calculated for the deposited bead based on the spatially varying fiber orientation tensors. Fiber orientation is found to be highly aligned along the deposition direction of the resulting bead and the computed properties through the thickness of the bead are found to be affected by nozzle height during deposition.
       
  • In situ defect detection in selective laser melting via full-field
           infrared thermography
    • Abstract: Publication date: Available online 1 November 2018Source: Additive ManufacturingAuthor(s): Jamison L. Bartlett, Frederick M. Heim, Yellapu V. Murty, Xiaodong Li Selective laser melting (SLM) has become one of the most commonly utilized processes in metal additive manufacturing (AM). Despite its widespread use and capabilities, SLM parts are still being produced with excessive volumetric defects and flaws. The complex dependence of defect formation on process parameters, geometry, and material properties has inhibited effective quality assurance in SLM production. Exacerbating these issues are the difficulties thus far in accurately detecting and identifying defects in-process so that parts may be qualified without destructive testing. Some of the most detrimental defects produced during SLM processing are lack of fusion (LoF) defects, which are frequently found to be in excess of 100 μm in size, thus these defects are of critical importance to detect and remove. In this work, we have developed and demonstrated the capabilities of a novel in situ monitoring system using full-field infrared (IR) thermography to monitor AlSi10Mg specimens during SLM production. Using layerwise relative surface temperature measurements, subsurface defects were identified via their retained thermal signature at the surface; transient thermal modeling was performed, which supported these observations. Parts were characterized using ex situ scanning electron microscopy (SEM) to validate data identified defects and, critically, to estimate detection success. The IR defect detection method was highly effective in identifying defects, with an 82% total success rate for LoF defects; detection success improved with increasing defect size. The method was also used statistically to analyze the presence of systematic process errors during SLM production, expanding the capabilities of IR monitoring methods. This unique analysis method and simple integration for in situ IR monitoring can immediately improve non-destructive qualification methods in SLM processing.
       
  • Plastic Anisotropy of Additively Manufactured Maraging Steel: Influence of
           the Build Orientation and Heat Treatments
    • Abstract: Publication date: Available online 31 October 2018Source: Additive ManufacturingAuthor(s): Barry Mooney, Kyriakos I. Kourousis, Ramesh Raghavendra This experimental study investigates the combined effect of the three primary Additive Manufacturing (AM) build orientations (0∘, 45∘, and 90∘) and an extensive array of heat treatment plans on the plastic anisotropy of maraging steel 300 (MS1) fabricated on the EOSINT M280 Direct Metal Laser Sintering (DMLS) system. The alloy's microstructure, hardness, tensile properties and plastic strain behaviour have been examined for various strengthening heat-treatment plans to assess the influence of the time and temperature combinations on plastic anisotropy and mechanical properties (e.g. strength, ductility). A comprehensive visual representation of the material's overall mechanical properties, for all three AM build orientations, against the various heat treatment plans is offered through time - temperature contour maps. Considerable plastic anisotropy has been confirmed in the as-built condition, which can be reduced by aging heat-treatment, as verified in this study. However, it has identified that a degree of transverse strain anisotropy is likely to remain due to the AM alloy's fabrication history, a finding that has not been previously reported in the literature. Moreover, the heat treatment plan (6h at 490∘C) recommended by the DMLS system manufacturer has been found not to be the optimal in terms of achieving high strength, hardness, ductility and low anisotropy for the MS1 material. With the use of the comprehensive experimental data collected and analysed in this study, and presented in the constructed contour maps, the alloy's heat treatment parameters (time, temperature) can be tailored to meet the desired strength/ductility/anisotropy design requirements, either for research or part production purposes.
       
  • Laser metal deposition of compositionally graded TiZrNbTa refractory
           high-entropy alloys using elemental powder blends
    • Abstract: Publication date: Available online 31 October 2018Source: Additive ManufacturingAuthor(s): Henrik Dobbelstein, Evgeny L. Gurevich, Easo P. George, Andreas Ostendorf, Guillaume Laplanche In the present study, laser metal deposition (LMD) was used to produce compositionally graded refractory high-entropy alloys (HEAs) for screening purposes by in-situ alloying of elemental powder blends. A compositional gradient from Ti25Zr50Nb0Ta25 to Ti25Zr0Nb50Ta25 is obtained by incrementally substituting Zr powder with Nb powder. A suitable strategy was developed to process the powder blend despite several challenges such as the high melting points of the refractory elements and the large differences in melting points among them. The influence of the LMD process on the final chemical composition was analyzed in detail and the LMD process was optimized to obtain a well-defined compositional gradient. Microstructures, textures, chemical compositions and mechanical properties were characterized using SEM, EBSD, EDX, and microhardness testing, respectively. Compositions between Ti25Zr0Nb50Ta25 and Ti25Zr25Nb25Ta25 were found to be single-phase bcc solid solutions with a coarse grain microstructure. Increasing the Zr to Nb ratio beyond the equiatomic composition results in finer and harder multiphase microstructures. The results shown in the present study clearly show for the first time that LMD is a suitable processing tool to screen HEAs over a range of chemical compositions.
       
  • In situ multi-elemental analysis by laser induced breakdown spectroscopy
           in additive manufacturing
    • Abstract: Publication date: Available online 31 October 2018Source: Additive ManufacturingAuthor(s): Vasily N. Lednev, Pavel A. Sdvizhenskii, Roman D. Asyutin, Roman S. Tretyakov, Mikhail Ya. Grishin, Anton Ya. Stavertiy, Sergey M. Pershin The feasibility of in situ quantitative multi-elemental analysis during the additive manufacturing process has been demonstrated for the first time using laser induced breakdown spectroscopy (LIBS). The coaxial laser cladding technique was utilized for the production of highly wear-resistant coatings (nickel alloy reinforced with tungsten carbide grains). High-quality production as well as gradient composition coating synthesis required an online technique for quantitative elemental analysis. A low-weight, compact LIBS probe was designed to equip the laser cladding head installed at an industrial robot. Hot solidified clad as well as a melt pool surface was sampled by the LIBS probe but meaningful analytical results were achieved only for the latter case due to non-uniform distribution of tungsten carbide grains in the upper surface layer. No effect was observed for the laser ablation at the melt pool on the clad properties by optical microscopy and scanning electron microscopy studies. On-line LIBS quantitative analysis of key components (carbon and tungsten) was achieved during the synthesis of highly wear-resistant coatings and obtained results were in good agreement with offline analysis obtained by electron energy dispersive X-ray spectroscopy, X-ray fluorescence spectroscopy, and the combustion infrared absorption method.
       
  • Influence of printing parameters on the stability of deposited beads in
           fused filament fabrication of poly(lactic) acid
    • Abstract: Publication date: Available online 31 October 2018Source: Additive ManufacturingAuthor(s): Shahriar Bakrani Balani, France Chabert, Valérie Nassiet, Arthur Cantarel Fused filament fabrication (FFF) is one of the various types of additive manufacturing processes. Similar to other types, FFF enables free-form fabrication and optimised structures by using polymeric filaments as the raw material. This work aims to optimise the printing conditions of the FFF process based on reliable properties, such as printing parameters and physical properties of polymers. The selected polymer is poly(lactic) acid (PLA), which is a biodegradable thermoplastic polyester derived from corn starch and is one of the most common polymers in the FFF process. Firstly, the maximum inlet velocity of the filament in the liquefier was empirically determined according to process parameters, such as feed rate, nozzle diameter and dimensions of the deposited segment. Secondly, the rheological behaviour of the PLA, including the velocity field, shear rate and viscosity distribution in the nozzle, was determined via analytical study and numerical simulation. Our results indicated the variation in the shear rate according to the diameter of the nozzle and the inlet velocity. The shear rate attained its maximum value near the internal wall at high inlet velocities and smaller diameters. Finally, the distribution of the viscosity along the radius of the nozzle was obtained. At high inlet velocity, several defects appeared at the surface of the extrudates. At the highest shear rates, the extrudates underwent severe deformation. The defects predicted via numerical simulation were reasonably consistent with that observed from an optical microscope. Hence, these results are effective for selecting the printing parameters (i.e. nozzle diameter, feed rate and layer height) to improve the quality of the manufactured parts.
       
  • Transient Development of Residual Stresses in Laser Beam Melting - A
           Neutron Diffraction Study
    • Abstract: Publication date: Available online 31 October 2018Source: Additive ManufacturingAuthor(s): F. Bayerlein, F. Bodensteiner, C. Zeller, M. Hofmann, M.F. Zaeh Additive Manufacturing (AM) is increasingly used for the production of functional parts. In order to ensure product reliability in challenging load cases and environments, a valid knowledge of the residual stress state is crucial. Since typical, complex AM geometries necessitate simulative efforts for this prediction, suitable validation data are essential. This study presents results from neutron diffraction measurements on different stages of a build-up of a simple cuboid structure by laser beam melting. Strains are generally measured in three perpendicular directions, coinciding with the symmetry axes of the structure, so that the stresses in these directions can be calculated using Hooke's Law. The strain-free reference is obtained from measurements on small matchstick geometries cut from an analogously manufactured cuboid at the respective measurement spots. By providing quasi-transient data of the evolution of residual stresses in both the base plate and the part, simulation models can be investigated towards their structural validity. Lastly, at two positions, strain measurements were obtained from six directions in order to determine an estimation of the full strain tensor. Results indicate that the assumption of negligible shear strains may not be justifiable.
       
  • Mechanical performance of polymer powder bed fused objects – FEM
           simulation and verification
    • Abstract: Publication date: Available online 30 October 2018Source: Additive ManufacturingAuthor(s): Anders Lindberg, Johan Alfthan, Henrik Pettersson, Göran Flodberg, Li Yang Additive manufacturing (3D printing) enables the designing and producing of complex geometries in a layer-by-layer approach. The layered structure leads to anisotropic behaviour in the material. To accommodate anisotropic behaviour, geometrical optimization is needed so that the 3D printed object meets the pre-set strength and quality requirements. In this article a material description for polymer powder bed fused also or selective laser sintered (SLS) PA12 (Nylon-12), which is a common 3D printing plastic, was investigated, using the Finite Element Method (FEM). The Material Model parameters were obtained by matching them to the test results of multipurpose test specimens (dumb-bells or dog bones) and the model was then used to simulate/predict the mechanical performance of the SLS printed lower-leg prosthesis components, pylon and support. For verification purposes, two FEM designs for a support were SLS printed together with additional test specimens in order to validate the used Material Model. The SLS printed prosthesis pieces were tested according to ISO 10328 Standard. It was found that the FEM simulations, together with the Material Model, gave good estimations for the location of a failure and its load. It was also noted that there were significant variations among individual SLS printed test specimens, which impacted on the material parameters and the FEM simulations. Hence, to enable reliable FEM simulations for the designing of 3D printed products, better control of the SLS process with regards to porosity, pore morphology and pore distribution is needed.
       
  • Powder bed fusion metrology for additive manufacturing design guidance
    • Abstract: Publication date: Available online 28 October 2018Source: Additive ManufacturingAuthor(s): Jared Allison, Conner Sharpe, Carolyn Conner Seepersad Design for additive manufacturing (DFAM) guidelines are important for helping designers avoid iterations and leverage the design freedoms afforded by additive manufacturing (AM). Comprehensive design guidelines should incorporate a variety of features of interest to designers, and given the wide variety of AM processes and their associated capabilities and limitations, those guidelines may need to be process- or even machine-specific. One way to generate detailed DFAM guidelines is to implement a metrology study focused on a strategically designed test part. This paper describes how quantitative design guidelines are compiled for a polymer selective laser sintering (SLS) process via a metrology study. As part of the metrology study, a test part is designed to focus specifically on geometric resolution and accuracy of the polymer SLS process. The test part is compact, allowing it to be easily inserted into existing SLS builds and therefore eliminating the need for dedicated metrology builds. To build a statistical foundation upon which design guidelines can be compiled, multiple copies of the test part are fabricated within existing commercial builds in a factorial study with materials, build orientations, and locations within the build chamber as control factors. Design guidelines are established by measuring and analyzing the as-built test parts. The guidelines are summarized in this paper and documented in a publicly accessible, online web tool.
       
  • Continuous wave vs pulsed wave laser emission in selective laser melting
           of AlSi10Mg parts with industrial optimized process parameters:
           microstructure and mechanical behaviour
    • Abstract: Publication date: Available online 28 October 2018Source: Additive ManufacturingAuthor(s): C.A. Biffi, J. Fiocchi, P. Bassani, A. Tuissi Currently, selective laser melting (SLM) is among the most widespread of the additive manufacturing (AM) technologies. Commercially available SLM systems can offer both continuous wave (CW) and pulsed wave (PW) emissions of the laser power. It has been demonstrated that relative density and geometric features can be affected by the laser emission parameters, but their effects on the material properties have not yet been investigated. In this study, specimens were produced from the same AlSi10Mg powder using two commercially available SLM machines operating with CW and PW emissions. Optimal process conditions were used, as indicated by the SLM suppliers. This choice was made to provide the most effective comparison between the material performances obtained using different industrial SLM systems. The specimen microstructures were investigated using scanning electron microscopy coupled with electron backscatter diffraction analysis. Moreover, thermal analysis, microhardness measurements, and compression tests were performed to investigate the thermal and mechanical properties. It was revealed that a slightly finer microstructure was obtained using the PW laser, while the increased thermal load during CW laser melting resulted in larger liquid pools, enhanced phase modifications, and better mechanical properties.Graphical abstractGraphical abstract for this article
       
  • Photopolymer Formulation to Minimize Feature Size, Surface Roughness, and
           Stair-Stepping in Digital Light Processing-Based Three-Dimensional
           Printing
    • Abstract: Publication date: Available online 28 October 2018Source: Additive ManufacturingAuthor(s): Kavin Kowsari, Biao Zhang, Sahil Panjwani, Zaichun Chen, Hardik Hingorani, Saeed Akbari, Nicholas X. Fang, Qi Ge Achievement of optimized lateral and vertical resolution is a key factor to obtaining three-dimensional (3D) structural details fabricated through digital light processing (DLP)-based 3D printing technologies which exploit digitalized ultraviolet (UV) or near-UV light to trigger localized photopolymerization forming solid patterns from liquid polymer resins. Many efforts have been made to optimize printing resolution through improving the optical systems. However, researchers have paid comparatively little attention to understand the influences of polymer formulation on the printing resolution and surface quality. Here, we report an investigation on the effects of in-house formulated (meth)acrylate-based photopolymer constituent types and concentrations on the resolution and quality of structures printed on a bottom-exposure DLP-based 3D printing system. We examined a wide variety of resin formulations to determine optimal formulations that yield best printing resolution and surface quality over a reasonably broad range of mechanical properties. We demonstrated the controlled fabrication of sub-pixel conical and aspherical smooth features, whereby the shape and dimensions could be prescribed with the resin formulation and process parameters. Such features hold promising implications in micro-optic and microfluidic fabrication using the DLP-based 3D printing technique. Additionally, we devised di(meth)acrylate-based resin formulations which could exclusively produce optically-clear layers in contrast to opaque, rough surfaces resulting from commonly-used diacrylate-based resins including those available commercially. Use of this solution minimized the ‘stair-stepping’ effect in components printed in a layer-by-layer manner. We also showed that the maximum lateral and vertical resolution attainable using our present system were 7 µm and 4 µm, respectively, while maintaining uniform width and height. Taken together, the present findings provide a basis for optimized photopolymer resin formulations that retain maximum vertical and lateral resolutions and minimal surface roughness and layering artifacts for a versatile range of mechanical and rheological properties suited to novel applications in 3D printing of smooth free-form solids, micro-optics, and direct fabrication of microfluidic platforms with functional surfaces.
       
  • Numerical Study of Temperature and Cooling Rate in Selective Laser Melting
           with Functionally Graded Support Structures
    • Abstract: Publication date: Available online 28 October 2018Source: Additive ManufacturingAuthor(s): Jie Song, Youxiang Chew, Lishi Jiao, Xiling Yao, Seung Ki Moon, Guijun Bi Support structures are critical to the successful printing of the overhang structures in selective laser melting. The heat transfer performance of support structures has significant influence on the temperature distribution and cooling rate within the overhang structures which in turn determine the microstructure and residual stress. In the present study, functionally graded support structures have been proposed and their thermal performance has been numerically investigated, with the consideration of different materials, cooling times and gradedness values. It has been found that functionally graded support structures can maintain a higher temperature level than the conventional uniform support structure at the bottom of overhang, which is equivalent to an extra pre-heating effect. The temperature fluctuation and cooling rate at the bottom of overhang can also be reduced by adopting the functionally graded support structures.
       
  • In-situ Monitoring of Laser-based PBF via off-axis Vision and Image
           Processing Approaches
    • Abstract: Publication date: Available online 28 October 2018Source: Additive ManufacturingAuthor(s): Yingjie Zhang, Jerry Y H Fuh, Dongsen Ye, Geok Soon Hong With the development of powder bed fusion (PBF) additive manufacturing technique for functional parts production, process monitoring and diagnosis is highly demanded to ensure its process reliability and repeatability. An off-axis vision monitoring method using high-speed camera is proposed in this paper. An optical filter with 350 nm-800 nm cut-off was used to enhance the image contrast between the plume and the melt pool. A new image processing method was designed to extract features from the melt pool, plume and spatters, respectively. Kalman filter tracking was used to pinpoint the exact melt pool position, and image segmentation algorithm was developed to segment the melt pool, plume and spatters from each other; a new tracking method was utilized to remove the spatters generated in the previous frame. After image processing, the features of melt pool intensity, plume area, plume orientation, spatter number, spatter area, spatter orientation and spatter velocity were extracted and their correlations with the scanning quality were investigated. The results indicated that these features were potential indicators for scanning quality assessment. The proposed method could be used to further study the characteristics of plume and spatter and to explore the diagnosis performance based on the fusion of melt pool, plume and spatter information. It provides a promising means for in-situ monitoring and control of PBF process.
       
  • Additive manufacturing of heat-sensitive polymer melt using a pellet-fed
           material extrusion
    • Abstract: Publication date: Available online 28 October 2018Source: Additive ManufacturingAuthor(s): Zuoxin Zhou, Iulia Salaoru, Peter Morris, Gregory J. Gibbons The most common method for Additive Manufacturing (AM) of polymers is melt extrusion, which normally requires several pre-processing steps to compound and extrude filament feedstock, resulting in an overall long melt residency time. Consequently a typical melt extrusion-based AM process is time/cost consuming, and limited in the availability of materials that can be processed. Polyvinyl alcohol (PVOH) is one of the heat-sensitive polymers demonstrating a thermal decomposition temperature overlapping its processing window. This study proposed to use a pellet-fed material extrusion technique to directly process PVOH granules without the necessity of using any pre-processing steps. The approach essentially combined compounding, extrusion and AM, allowing multi-material printing with minimum exposure to heat during the process. The processing parameters were determined via thermal and rheological characterisation of PVOH. Effects of processing temperature and time on the thermal decomposition of PVOH were demonstrated, which further affected the tensile properties and solubility. An increase in Young’s modulus, stress at 2% strain, and Ultimate Tensile Stress were 98 ± 4%, 40 ± 5%, and 20 ± 2%, respectively, was observed when PVOH residency time was reduced from 25 min to 14 min. The pellet-fed material extrusion technology demonstrated good 3D printability, multi-material printing capability, and great versatility in processing polymer melts.
       
  • High strain rate compressive deformation behavior of an additively
           manufactured stainless steel
    • Abstract: Publication date: December 2018Source: Additive Manufacturing, Volume 24Author(s): Brandon McWilliams, Brahmananda Pramanik, Andelle Kudzal, Josh Taggart-Scarff In this work the effect of manufacturing strategy and post process treatment on the high strain rate (HSR) compressive deformation behavior of additively manufactured powder bed fusion 17-4PH stainless steel is studied. Specimens were fabricated using three different laser vector path strategies to impart different thermal histories and resulting microstructures in the material. The effect of post processing in the form of hot isostatic pressing and heat treatment and their effect on HSR compressive deformation response of the material was studied. Defect characteristics were quantified using x-ray micro computed tomography. HSR behavior was characterized using split Hokinson bar testing at rates from 1000–4000 s−1. It was found that the laser vector strategy had a strong influence on the development of microstructure and defect characteristics and spatial distribution in the materials which strongly influence the HSR response and the HSR compressive flow stresses of the materials varied by as much as 43% in the regimes tested.
       
  • Influence of powder characteristics and additive manufacturing process
           parameters on the microstructure and mechanical behaviour of Inconel 625
           fabricated by Selective Laser Melting
    • Abstract: Publication date: December 2018Source: Additive Manufacturing, Volume 24Author(s): Christopher Pleass, Sathiskumar Jothi Selective Laser Melting (SLM) as an additive manufacturing process can fabricate near to net shape metallic components directly from Computer aided design models, which may be difficult to fabricate using conventional manufacturing methods. In this work, the powdered metals used as the raw material feedstock in the Selective Laser Melting (SLM) process were studied. SLM manufacturing processibility of nickel based super alloy, powders related to the particle Size Distribution (PSD), flow ability, mechanical properties and microstructures was investigated. Different powder characterisation methods were also investigated to establish which might be most useful for SLM application. Three different Inconel 625 (IN625) powder feedstock materials have been accounted for this study. Firstly, three different IN625 powders were fully characterised for chemical composition, particle size distribution and flow ability using different types of characterisation techniques. It has been found that the presence of any significant proportion of powder particles smaller than 10-μm diameter, leads to severe agglomeration and make SLM processing difficult. Secondly, coupons were manufactured using SLM from each powder with different process parameter, which were analysed for porosity and mechanical behaviour. Next, the scanning electron microscopy (SEM), electron back scattering diffraction (EBSD) are employed to investigate the microstructures. Finally, data analysis was employed on the data collected by metal powders characterization, SLM manufacturing, SEM/EBSD study and mechanical properties of the IN625. It has been observed that the powder characteristics, as well as SLM process parameters influences on the quality of the IN625 fabricated.
       
  • Feasibility study on additive manufacturing of recyclable objects for
           space applications
    • Abstract: Publication date: December 2018Source: Additive Manufacturing, Volume 24Author(s): Miranda Fateri, Ali Kaouk, Aidan Cowley, Stefan Siarov, Manel Vera Palou, Fernando Gobartt González, Romain Marchant, Samantha Cristoforetti, Matthias Sperl Future exploration missions beyond low-Earth orbit would significantly benefit from a closed loop recyclable Additive Manufactured capability, allowing the production of general purpose tools and items in a time and cost effective manner. To realize this ambition, we present a feasibility study of a Solvent-Cast Direct-Write method using Polyvinyl Alcohol as biodegradable material. Process parameters such as solution viscosity, evaporation rate, print pressure and scan speed are optimized in order to achieve a consistent and reliable print outcome. We demonstrate the process by fabricating test complex geometries of sample specimens. Moreover, we report on the mechanical properties of printed geometries as well as the recyclability aspects. Physical and chemical properties of the printable solutions were investigated and the viability of the method for space is discussed.
       
  • Printing of complex free-standing microstructures via laser-induced
           forward transfer (LIFT) of pure metal thin films
    • Abstract: Publication date: December 2018Source: Additive Manufacturing, Volume 24Author(s): Matthias Feinaeugle, Ralph Pohl, Ton Bor, Tom Vaneker, Gert-willem Römer A combined approach of laser-induced forward transfer (LIFT) and chemical etching of pure metal films is studied to fabricate complex, free-standing, 3-dimensional gold structures on the few micron scale. A picosecond pulsed laser source with 515 nm central wavelength is used to deposit metal droplets of copper and gold in a sequential fashion. After transfer, chemical etching in ferric chloride completely removes the mechanical Cu support leaving a final free-standing gold structure. Unprecedented feature sizes of smaller than 10 μm are achieved with surface roughness of 0.3 to 0.7 μm. Formation of interfacial mixing volumes between the two metals is not found confirming the viability of the approach.Graphical abstractGraphical abstract for this article
       
  • Acoustic resonance testing of additive manufactured lattice structures
    • Abstract: Publication date: Available online 25 October 2018Source: Additive ManufacturingAuthor(s): Y. Ibrahim, Z. Li, C.M. Davies, C. Maharaj, J.P. Dear, P.A. HooperABSTRACTAdditive manufacturing (AM) allows engineers to design and manufacture complex weight saving lattice structures with relative ease. These structures, however, present a challenge for inspection. A non-destructive testing and evaluation method used to assess material properties and quality is the focus of this paper, namely acoustic resonance (AR) testing. For this research, AR testing was conducted on weight saving lattice structures (fine and coarse) manufactured by powder bed fusion. The suitability of AR testing was assessed through a combined approach of experimental testing and FE modelling. A sensitivity study was conducted on the FE model to quantify the influence of element coarseness on the resonant frequency prediction and this needs to be taken into account in the application and analysis of the technique. The analysis was extended to extract effective modulus values for the lattice structures and the solid materials from every detected overtone, allowing for multiple measurements from a single AR test without the need to carefully isolate the fundamental. The AR and FE modelling modulus of elasticity values were validated using specimens of known properties. There was fair agreement between the FE and compression test extracted values of effective modulus for the coarse lattice. For the fine lattice, there was agreement in the values of effective modulus extracted from AR, 3-point bend, and compression experimental tests carried out. It was found that loose powder fusing from AM resulted in the fine lattice structure having a higher density (at least 1.5 times greater) than calculated due to the effect of loose powder adhesion. This effect resulted in an increased stiffness of the fine lattice structure. AR can be used as a measure of determining loose powder adhesion and other unique structural characteristics resulting from AM.
       
  • Epoxy Infiltrated 3D Printed Ceramics for Composite Tooling Applications
    • Abstract: Publication date: Available online 23 October 2018Source: Additive ManufacturingAuthor(s): Michael Maravola, Brett Conner, Jason Walker, Pedro Cortes The use of additive manufacturing (AM) provides an opportunity to fabricate composite tooling molds in a rapidly and cost effectively manner. This work has shown the use of a polymer based infiltrated ceramics produced via binder jetting for producing composite tooling molds. Here, molds based on silica sand as well as zircon sand have been printed on a S-Max 3D printer unit and subsequently impregnated with an epoxy system for yielding functional molds in the range of autoclave temperatures around 150-177 °C. The mechanical properties of the infiltrated 3D printed materials have been investigated and it was observed that the polymer-infiltrated systems resulted in a compressive and flexural strength one order of magnitude higher than the non-infiltrated printed ceramic material. A thermal analysis was also performed on both the infiltrated and non-infiltrated printed samples, and it was recorded that the incorporation of the polymer resulted in a larger coefficient of thermal expansion on the infiltrated systems. Here, a carbon fiber reinforced composite was manufactured with the infiltrated composite tooling molds printed in the S-Max unit, and it was observed that the assembled molds are capable of producing a successful composite material. The present work has demonstrated that a binder jetting process, is a feasible technology for producing thermostable low cost composite tooling molds.
       
  • 3D printing of discontinuous and continuous fibre composites using
           stereolithography
    • Abstract: Publication date: Available online 23 October 2018Source: Additive ManufacturingAuthor(s): Yukako Sano, Ryosuke Matsuzaki, Masahito Ueda, Akira Todoroki, Yoshiyasu Hirano 3D printing technology has revolutionized the field of machinery, aerospace, and electronics. To address the shortcomings of previous studies on improving the poor mechanical properties of the resin used in 3D printers, this study presents a technology for fabricating short fibres or a continuous fibre-composite material using stereolithography 3D printing. Glass powder and fibreglass fabric were used as the discontinuous and continuous fibre reinforcement of light-cured resin material. The tensile strength and Young’s modulus showed a marked increase: these were 7.2 and 11.5 times higher than those of the resin specimen, respectively.
       
  • Non-destructive evaluation of additively manufactured polymer objects
           using X-ray interferometry
    • Abstract: Publication date: December 2018Source: Additive Manufacturing, Volume 24Author(s): Omoefe J. Kio, Jumao Yuan, Adam J. Brooks, Gerald L. Knapp, Kyungmin Ham, Jinghua Ge, Denis Van Loo, Leslie G. Butler X-ray interferometry provides a dark-field image, essentially a small-angle X-ray scattering image, of the voids and print defects in an additively manufactured polymer object. The interferometers used were tuned to scattering length 2–5 μm and configured to measure scattering along both vertical and horizontal directions. The samples studied included Stanford Bunnies, fabricated from acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA), and a quadratic test object fabricated from PLA. The dark-field projection images show orientation-dependent X-ray scattering which is due to anisotropic voids and gaps at the filament-to-filament interface in these fused deposition modeling additive manufacturing objects. SEM corroborates the existence of gaps between filaments.The absorption and dark-field volumes are used to correlate printhead trajectory with print defect density. The absorption volume is used to generate perimeter points slice-by-slice, and from these points, the 2D curvature is calculated. There is a slight increase in X-ray scattering, hence print defect density, at regions with high curvature.Two X-ray interferometry techniques were used: stepped-grating and single-shot. As currently developed, stepped-grating has the larger field-of-view—examination of an entire test object—whilst single-shot has the potential for real-time, in situ measurement of the printing process within 1 mm of the printhead.
       
  • Effect of post processing on the creep performance of laser powder bed
           fused Inconel 718
    • Abstract: Publication date: Available online 22 October 2018Source: Additive ManufacturingAuthor(s): Z. Xu, J.W. Murray, C.J. Hyde, A.T. Clare In this study, the creep performance of laser powder bed fusion manufactured Inconel 718 specimens is studied in detail and compared with conventional hot-rolled specimens alongside as-built then heat-treated and as-built then hot-isostatic pressed specimens. Hot-rolled specimens showed the best creep resistance, while the hot-isostatic pressed specimens yielded the worst performance, inferior to the as-built condition. Creep testing of all samples showed increased secondary creep rate was consistently correlated with a reduced life. Fractography revealed intergranular fracture was the primary failure mode for all as-built samples. Preferential intergranular precipitation in the case of the hot-isostatic pressed specimens during hot-isostatic pressing extensive intergranular cracking as the primary failure mechanism. Heat-treated specimens possessed only sparse intergranular precipitates, thereby explaining an improved creep lifetime. The hot-rolled specimens, having smallest grain size, showed the least extensive cracking, particularly in locations of finest grains, explaining avoidance of intergranular fracture as a key creep mechanism, thereby explaining the ductile creep fracture surfaces in the case of the hot-rolled samples.
       
  • The Design of an Additive Manufactured Dual Arm Manipulator System
    • Abstract: Publication date: Available online 22 October 2018Source: Additive ManufacturingAuthor(s): Bradley S. Richardson, Randall F. Lind, Peter D. Lloyd, Mark W. Noakes, Lonnie J. Love, Brian K. Post Additive manufacturing (AM), commonly referred to as 3D printing, was originally used for rapid prototyping. However, research into new technologies has allowed AM to become applicable far beyond prototype fabrication. Oak Ridge National Laboratory (ORNL), sponsored by the Office of Naval Research, has designed and developed an anthropomorphic seven degree-of-freedom (DOF) dual arm hydraulic manipulator using metal AM technologies. The titanium manipulators are designed for subsea use. All electrical and fluidic passageways are printed into each arm. The novel, cam-based design uses low-flow, energy-efficient valves. The hydraulic power unit is built into the base of the hydraulic arms’ mount. This article will detail the novel AM design of the hydraulic manipulator system. It will cover the manipulators’ pitch and rotary link designs, custom valves, hydraulic power unit, and the motivation for a dual arm design. This article will also describe lessons learned throughout the project and draw conclusions for future applications.
       
  • Turn-Key Use of an Onboard 3D Printer for International Space Station
           Operations
    • Abstract: Publication date: Available online 22 October 2018Source: Additive ManufacturingAuthor(s): William J. O’Hara, Jason M. Kish, Mary J. Werkheiser Current 3D printing capabilities onboard the International Space Station (ISS) are classified as experimental payloads. As payloads the products of these printers are returned to the ground for testing and analysis. However, it has long been thought that 3D printing must one day become a tool of space operations much like the electrical diagnostic equipment, and the soldering iron. This paper explores a case study in the use of one of the payload 3D printers to manufacture a device to be used by the crew as part of nominal ISS Operations. The nature of this attempt represented a collaboration between the ISS payload community and the systems operations community to capitalize on the strengths of both to accomplish a step in what may one day become a common and cost-savings capability in spaceflight operations. The path from concept development through onboard printing and crew inspection will be described. The lessons learned from this process are reviewed as constructive feedback on how existing processes can be expanded to enable this capability in the future. This experience will be carried forward into the development of a new process which will open the door for future use of 3D printing onboard the ISS.
       
  • Material-property evaluation of magnesium alloys fabricated using
           wire-and-arc-based additive manufacturing
    • Abstract: Publication date: Available online 21 October 2018Source: Additive ManufacturingAuthor(s): Hisataka Takagi, Hiroyuki Sasahara, Takeyuki Abe, Hiroki Sannomiya, Shinichiro Nishiyama, Shuichiro Ohta, Kunimitsu Nakamura Material properties, such as porosity, tensile strength, and microstructure, of magnesium-alloy components fabricated using wire-and-arc-based additive-manufacturing techniques, which essentially represent a form of arc-welding technology have been examined. In the proposed method, the wire material is melted by arc discharge, and the molten metal is subsequently solidified and accumulated. Magnesium wire developed in this study facilitated fabrication of magnesium-alloy components using the said additive-manufacturing process. Subsequently, combinations of fabrication conditions, such as the welding current, torch feed speed, and cross feed of the torch, were explored, and suitable conditions for realizing a solid structure with fewer weld defects compared to those observed when using die-casting and other manufacturing methods, were determined. Tensile tests and microstructure observations were also performed to elucidate mechanical properties of magnesium alloy components fabricated via the said wire-and-arc-based technique. It was demonstrated that the fabricated object possesses sufficient tensile strength compared to the observed standard value of the bulk material. Furthermore, results from microstructure observations demonstrated that the higher the torch feed speed, the finer is the microstructure. Moreover, the observed microstructure at the boundary between the substrate and fabricated object was finer compared to that at the top layer.Graphical abstractGraphical abstract for this article
       
  • Effect of Initial Filament Moisture Content on the Microstructure and
           Mechanical Performance of ULTEM® 9085 3D Printed Parts
    • Abstract: Publication date: Available online 18 October 2018Source: Additive ManufacturingAuthor(s): R.J. Zaldivar, T.D. Mclouth, G.L. Ferrelli, D.N. Patel, A.R. Hopkins, D. Witkin This study investigates the moisture absorption characteristics of the ULTEM® 9085 filament and how the uptake concentration affects the quality of material extrusion manufactured 3-D parts. The filament was exposed to humidity conditions to achieve various moisture concentrations (0, 0.05, 0.1, 0.16, 0.4 and 0.8 wt.%) and corresponding printed parts were evaluated for mechanical performance using tensile test dogbones. The rate of transport was modeled by Fickian diffusion and diffusion coefficients were obtained for various exposure conditions. Moduli, strain to failure and ultimate strength were evaluated in the XY (flat horizontal) and ZX(vertical) direction relative to the build plate orientation. Image analyses of cross-sections as well as their corresponding fracture surfaces were evaluated for consolidation, porosity distribution and failure behavior. Mechanical test data showed a significant decrease in tensile strength (> 60%) and failure strain (> 50%) over the range of filament moisture levels investigated. A decrease in failure strain of 41% was observed with moisture levels as low as 0.16%. This degradation was especially sensitive in parts printed in the vertical direction, which resulted in an ultimate failure strain of only 1%. The changes in mechanical performance are believed to be due to a combination of entrapped volatiles resulting in increased porosity at higher moisture levels as well as moisture induced pseudo-crosslinking at lower concentrations. Optical micrographs showed that specimens with 0.16% moisture or greater were filled with observable porosity and increased surface roughness. The rheological behavior of extruded material indicated plasticization as evidenced by melt flow index measurements and changes in the flow characteristics of moisture-exposed extrudate. DMA data show a distinct decrease in Tg with increased moisture content, which is consistent with plasticization. The absorption characteristics at room temperature lab conditions indicate that the material will reach an unacceptable level within one hour of room-temperature exposure. This investigation emphasized the need for awareness of the moisture sensitivities of ULTEM® 9085 when manufacturing high-quality material extrusion processed structures.
       
  • Thermal conductivity of sintered copper samples prepared using 3D
           printing-compatible polymer composite filaments
    • Abstract: Publication date: Available online 16 October 2018Source: Additive ManufacturingAuthor(s): Navid Dehdari Ebrahimi, Y. Sungtaek Ju Metal-filled polymers containing micro-powders of highly conductive metals can serve as a starting material to fabricate complex metal structures using economic filament extrusion-based 3D printing and molding methods. We report our measurements of the thermal conductivity of copper samples prepared using these methods before and after a thermal treatment process. Sintering the samples at 980 ℃ leads to an order of magnitude improvement in thermal conductivity when compared with as-printed or as-molded samples. Thermal conductivity values of approximately 30 W/mK are achieved using commercially available polymer-copper composite filaments with a copper volume fraction of 0.4. Over-sintering the samples at 1080 ℃ further enhances the thermal conductivity by more than two folds, but it leads to uncontrolled shrinkage of the samples. The measured thermal conductivities show a modest decrease with increasing temperatures due to increased electron-phonon scattering rates. The experimental data agree well with the thermal conductivity models previously reported for sintered porous metal samples. The measured electrical conductivity, Young’s modulus and yield strength of the present sintered samples are also reported.
       
  • A study of thermal expansion coefficients and microstructure during
           selective laser melting of Invar 36 and stainless steel 316L
    • Abstract: Publication date: Available online 13 October 2018Source: Additive ManufacturingAuthor(s): Mostafa Yakout, M.A. Elbestawi, Stephen C. Veldhuis This paper presents an experimental study on the metallurgical issues associated with selective laser melting of Invar 36 and stainless steel 316 L and the resulting coefficient of thermal expansion. Invar 36 has been used in aircraft control systems, electronic devices, optical instruments, and medical instruments that are exposed to significant temperature changes. Stainless steel 316 L is commonly used for applications that require high corrosion resistance in the aerospace, medical, and nuclear industries. Both Invar 36 and stainless steel 316 L are weldable austenitic face-centered cubic crystal structures, but stainless steel 316 L may experience chromium evaporation and Invar 36 may experience weld cracking during the welding process. Various laser process parameters were tested based on a full factorial design of experiments. The microstructure, material composition, coefficient of thermal expansion, and magnetic dipole moment were measured for both materials. It was found that there exists a critical laser energy density for each material, EC, for which selective laser melting process is optimal for material properties. The critical laser energy density provides enough energy to induce stable melting, homogeneous microstructure and chemical composition, resulting in thermal expansion and magnetic properties in line with that expected for the wrought material. Below the critical energy, a lack of fusion due to insufficient melt tracks and discontinuous beads was observed. The melt track was also unstable above the critical energy due to vaporization and microsegregation of alloying elements. Both cases can generate stress risers and part flaws during manufacturing. These flaws could be avoided by finding the critical laser energy needed for each material. The critical laser energy density was determined to be 86.8 J/mm3 for Invar 36 and 104.2 J/mm3 for stainless steel 316 L.
       
  • Pore analysis and mechanical performance of selective laser sintered
           objects
    • Abstract: Publication date: Available online 10 October 2018Source: Additive ManufacturingAuthor(s): Göran Flodberg, Henrik Petterson, Yang Li In this work, systematic studies were carried out on SLS (selective laser sintering) printed samples, with two different geometries, standard test samples dumb-bells (dog bones) and tubes (Ø 30 mm and 150 mm long), consisting of two different materials, viz. PA12 (polyamide) with and without the addition of carbon fibres (CFs). These samples were tested according to their respective ISO standards. The standard test samples exhibited relatively small differences with regards to printing directions when PA12 was used alone. Their tensile strengths (σm) were approx. 75%-80% of the injection-moulded sample. The addition of carbon fibres significantly enhanced the tensile strengths, namely 50% greater for the vertically printed test sample and more than 100% greater for the horizontally printed samples, compared to the respective objects consisting of PA12 alone. The strong difference in printing directions can be attributed to the orientation of the carbon fibres. Mechanical tests on the SLS printed tubes confirmed the trends that were found in the standard test samples. Porosity and pore structure inside the SLS printed tubes were studied by combining optical microscopy and X-ray microtomography with image analysis. It was found that porosity was a general phenomenon inside the SLS printed samples. Nevertheless, there were significant differences in porosity, which probably depended on the properties of the materials used, both with and without carbon fibres, thus causing significant differences in light absorption and heat conductivity. The printed samples made of PA12 alone possessed quite a high level of porosity (4.7%), of which the size of the biggest pore was hundreds of microns. The twenty biggest pores with an average size of 75*104 μ m3 accounted for 43% of the total porosity. However, the porosity of the printed samples made from PA12 + CF was only 0.68%, with the biggest pore being only tens of microns. The corresponding average pore size of the 20 biggest pores was 72*103 μ m3, which was one order of magnitude smaller than the printed samples made from PA12 alone. Pores inside the SLS printed samples were probably responsible for a spread in the mechanical properties measured, e.g. tensile strengths, tensile (Young’s) modulus, strain at break, etc. The ratios of their standard deviations to their corresponding mean values in the standard test samples could probably be used as an indicator of porosity, i.e. the bigger the ratio, the higher the porosity.
       
  • Modelling flow-enhanced crystallisation during fused filament fabrication
           of semi-crystalline polymer melts
    • Abstract: Publication date: Available online 10 October 2018Source: Additive ManufacturingAuthor(s): C. McIlroy, R.S. Graham Achieving better control in fused filament fabrication (FFF) relies on a molecular understanding of how thermoplastic printing materials behave during the printing process. For semi-crystalline polymers, the ultimate crystal morphology and how it develops during cooling is crucial to determining part properties. Here crystallisation kinetics are added to a previously-developed model, which contains a molecularly-aware constitutive equation to describe polymer stretch and orientation during typical non-isothermal FFF flow, and conditions under which flow-enhanced nucleation occurs due to residual stretch are revealed. Flow-enhanced nucleation leads to accelerated crystallisation times at the surface of a deposited filament, whilst the bulk of the filament is governed by slower quiescent kinetics. The predicted time to 10% crystallinity, t10, is in quantitative agreement with in-situ Raman spectroscopy measurements of polycaprolactone (PCL). The model highlights important features not captured by a single measurement of t10. In particular, the crystal morphology varies cross-sectionally, with smaller spherulites forming in an outer skin layer, explaining features observed in full transient crystallisation measurements. Finally, exploitation of flow-enhanced crystallisation is proposed as a mechanism to increase weld strength at the interface between deposited filaments.
       
  • Tailoring Green and Sintered Density of Pure Iron Parts using Binder
           Jetting Additive Manufacturing
    • Abstract: Publication date: Available online 9 October 2018Source: Additive ManufacturingAuthor(s): Issa Rishmawi, Mehrnaz Salarian, Mihaela Vlasea Binder jetting additive manufacturing (BJAM) is a comparatively low-cost process that enables manufacturing of complex and customizable metal parts. This process is applied to low-cost water-atomized iron powder with the goal of understanding the effects of printing parameters and sintering schedule on maximizing the green and sintered densities of manufactured samples respectively. The powder is characterized by using scanning electron microscopy (SEM) and particle size analysis (Camsizer X2). In the AM process, the effects of powder compaction, layer thickness and liquid binder level on green part density are investigated. Post-process heat treatment is applied to select samples, and suitable debinding parameters are studied by using thermo-gravimetric analysis (TGA). Sintering at various temperatures and durations results in densities of up to 91.3%. Image processing of x-ray computed tomography (μCT) scans of the samples reveals that porosity distribution is affected by powder spreading, and gradients in pore distribution in the sample are largely reduced after sintering. The resulting shrinkage ranges between 6.7 ± 3.0% and 25.3 ± 2.8%, while surface roughness ranges between 11.6 ± 5.0 μm and 32.1 ± 3.4 μm. The results indicate that the sintering temperature and time might be tailored to achieve target densities anywhere in the range of 64% and 91%, with possibly higher densities by increasing sintering time.Graphical abstractGraphical abstract for this article
       
  • Spatter and oxide formation in laser powder bed fusion of Inconel 718
    • Abstract: Publication date: Available online 9 October 2018Source: Additive ManufacturingAuthor(s): A.N.D. Gasper, B. Szost, X. Wang, D. Johns, S. Sharma, A.T. Clare, I.A. Ashcroft In laser powder bed fusion (PBF-LB), material is continuously ejected from the melt pool, commonly called spatter, and is distributed throughout the build chamber. There is a lack of understanding of the nature of this spatter and the effect it may have on the integrity of the final part and the quality of any recycled powder. This work reports a detailed investigation of spatter metallurgy for Inconel 718. It is seen that the spatter created during processing produces powder that is significantly different to the virgin material, with particles up to 6 times larger. Oxidation, predominantly in the form of spots or films of Al2O3 and TiO2 was observed on the surface of some of the spatter particles. It is established that this oxide formation occurs at the melt pool surface before ejection of the spatter from the melt pool, and also that this issue is generic to PBF-LB process and certain alloys. The characteristics of different types of spatter are identified and are linked to spatter generation mechanisms. The vaporisation of material during processing produces clusters of nano particles whose composition indicate a preferential vaporisation of Cr from the bulk. The results of this study highlight that oxidation and issues presented by spatter particles dissimilar from the virgin material are unavoidable and greater consideration is needed for the generation and effect of spatter on part and powder quality.Graphical abstractGraphical abstract for this article
       
  • Development of an automated laser control system for improving temperature
           uniformity and controlling component strength in selective laser sintering
           
    • Abstract: Publication date: Available online 9 October 2018Source: Additive ManufacturingAuthor(s): Tim Phillips, Scott Fish, Joseph Beaman It has been shown that quality of components built using selective laser sintering (SLS) are strongly affected by the thermal history of the building process. Temperature variations of a few degrees across the powder surface can alter the mechanical properties of components and render them unsuitable for their intended purpose. Therefore, to improve the quality of SLS components and ease their adoption into the marketplace, temperature fluctuation issues must be addressed. Some success has been demonstrated in the past at reducing temperature non-uniformity by improving the heater system that pre-heats the polymer powder prior to sintering with the laser. This paper will cover a complimentary approach of actively controlling laser fluence on the powder surface based on infrared temperature measurements. By controlling the amount of energy input by the laser, a high level of control over the final part temperature can be achieved and uniformity can be improved. This paper will cover development of the feed-forward control system and will present results showing that for constant cross-section specimens, a 45% improvement in ultimate flexural strength standard deviation was achieved.
       
  • Reactive-deposition-based additive manufacturing of Ti-Zr-BN composites
    • Abstract: Publication date: Available online 8 October 2018Source: Additive ManufacturingAuthor(s): Kellen D. Traxel, Amit Bandyopadhyay Reactive-deposition additive manufacturing was employed to manufacture titanium-based metal matrix composites for improving the wear resistance and temperature capability of commercially pure titanium (CPTi); a standard material in the aerospace, biomedical, and marine industries, among others. Composites were manufactured by leveraging in situ high-temperature reactions between CPTi, zirconium (Zr), and boron nitride (BN) powders during laser-based directed-energy-deposition (DED) 3D-printing. The effect of Zr and BN on the processability, phase formation(s), surface wear, and mechanical properties of 3D-printed titanium was studied by printing commercially-pure titanium with premixed additions of 20 wt% Zr and 10 wt% BN using Laser Engineered Net Shaping (LENS™). In the as-printed BN-containing structures, phase analysis revealed reinforcing ceramic phases TiN, TiB, and TiB2, whose presence was substantiated through first-principles analysis. The combined addition of Zr and BN produced a Ti-Zr alloy matrix with BN-particle and in situ phase-reinforced microstructure with 450% higher hardness (from 318 ± 26 HV0.1/15 to 1424 ± 361 HV0.5/15), a stabilized sliding-COF within 50 m of reciprocating wear testing, and 9x lower final wear rate in comparison to LENS™ deposited titanium. Zr-addition alone revealed a combined alloyed and particle-reinforced composite with 12% higher hardness, 23% higher compressive yield strength, and an 11% decrease in final wear rate compared to LENS™-produced titanium. Our results demonstrate that reactive-deposition based additive manufacturing can be exploited to create unique coatings and net-shape alloyed structures to enhance the surface and bulk properties of standard engineering materials such as titanium.Graphical abstractGraphical abstract for this article
       
  • Multi-functional ULTEM™1010 Composite Filaments for Additive
           Manufacturing Using Fused Filament Fabrication (FFF)
    • Abstract: Publication date: Available online 7 October 2018Source: Additive ManufacturingAuthor(s): Hao Wu, Michael Sulkis, James Driver, Amado Saade-Castillo, Adam Thompson, Joseph H. Koo This paper investigates the development of a novel high temperature polymer composite material by modifying polyetherimide (PEI) ULTEM™ 1010 with the addition of functional additives and processing it into filaments for Fused Filament Fabrication (FFF). Through twin-screw extrusion, four different formulations were obtained using combinations of hollow glass microspheres, nanoclay, and non-halogenated flame-retardant additives. These additives were designed to create a material that exhibits low density, high char yield, and low flammability. Filament quality was characterized and reported. Thermal and flammability characterization results indicated that the formulation consisting of 10 wt.% glass bubbles, 5 wt.% nanoclay, and 10 wt.% flame-retardant additives exhibited the best char yield at 62.2% and the lowest heat release capacity (HRC) of 119 J/g-1 K-1, an 10.7% improvement in char yield and 52% reduction in HRC compared to the neat polymer.
       
  • An efficient statistical approach to design 3D-printed metamaterials for
           mimicking mechanical properties of soft biological tissues
    • Abstract: Publication date: Available online 7 October 2018Source: Additive ManufacturingAuthor(s): Jialei Chen, Kan Wang, Chuck Zhang, Ben Wang Using 3D printed, patient-specific medical phantoms has become increasingly popular for use in biomedical applications including medical device testing, medical education, and surgical planning, etc. To overcome the inherent differences in mechanical properties between biological tissues and printable polymers, metamaterials are being introduced to mimic the mechanical response of the biological tissues. However, the existing trial-and-error approaches for finding the geometric parameters of the metamaterial result in time-consuming trials, which cannot meet the urgent needs for medical applications. We addressed this issue by proposing an optimization-based statistical approach with an easy-to-evaluate surrogate model to guide the design process and reduce the design time. In this paper, several validation tests were reported, including a biomedical application of mimicking the mechanical response of human articular cartilage. The proposed approach achieves excellent accuracy both visually and quantitatively. In addition, we provide an analysis of mimicking different stress-strain curves using different metamaterials. This data-driven approach demonstrates efficacy and flexibility in building the surrogate model even when no obvious physical trends can be extracted. With the proposed statistical approach, we can efficiently design the metamaterial and 3D-print mechanically accurate phantoms for sophisticated engineering applications.
       
  • A Multi-scale Convolutional Neural Network for Autonomous Anomaly
           Detection and Classification in a Laser Powder Bed Fusion Additive
           Manufacturing Process
    • Abstract: Publication date: Available online 6 October 2018Source: Additive ManufacturingAuthor(s): Luke Scime, Jack Beuth In-situ detection of processing defects is a critical challenge for Laser Powder Bed Fusion Additive Manufacturing. Many of these defects are related to interactions between the recoater blade, which spreads the powder, and the powder bed. This work leverages Deep Learning, specifically a Convolutional Neural Network (CNN), for autonomous detection and classification of many of these spreading anomalies. Importantly, the input layer of the CNN is modified to enable the algorithm to learn both the appearance of the powder bed anomalies as well as key contextual information at multiple size scales. These modifications to the CNN architecture are shown to improve the flexibility and overall classification accuracy of the algorithm while mitigating many human biases. A case study is used to demonstrate the utility of the presented methodology and the overall performance is shown to be superior to that of methodologies previously reported by the authors.
       
  • High-Temperature Mechanical Properties of AlSi10Mg Specimens Fabricated by
           Additive Manufacturing Using Selective Laser Melting Technologies (AM-SLM)
           
    • Abstract: Publication date: Available online 6 October 2018Source: Additive ManufacturingAuthor(s): Naor Elad Uzan, Roni Shneck, Ori Yeheskel, Nachum Frage Mechanical properties (tensile strength and creep) of AlSi10Mg specimens fabricated by selective laser melting (SLM) in the Z-direction were investigated in the 25-400 °C temperature range. Specimens were tested after stress relief treatment. The results revealed that yield stress (YS) significantly decreases and the elongation increases at temperatures higher than 200 °C. The ultimate tensile stress (UTS) continuously decreases with temperature. The creep parameters, namely stress exponent n and apparent activation energy Q, were found to be 25 ± 2 and 146 ± 20 kJ/mole, respectively. It was shown that plastic deformation during creep is governed by dislocation movements in primary aluminum grains. The tested material is actually an aluminum composite reinforced by sub-micron Si particles. The creep resistance of AlSi10Mg alloy fabricated by selective laser melting is close to that for aluminum matrix particles reinforced composites.
       
  • Simulation of the multi-component process gas flow for the explanation of
           oxidation during laser cladding
    • Abstract: Publication date: Available online 5 October 2018Source: Additive ManufacturingAuthor(s): Florian Wirth, Konrad Wegener Usually the process gas flow rates and the process gas types are not regarded as the primary process parameters of the laser cladding process. Herein it is shown, how the melt pool surface oxidation can be significantly reduced by the change of the carrier gas type, by a reduced carrier gas flow rate and by minor changes in the powder nozzle design. However, the absorptivity may decrease concurrently by up to 15%. A simulation model for the gas flow and the powder particle flow between the powder nozzle and the melt pool surface has been developed, which reveals the volume percentage of different gas types and so the quality of the shield gas atmosphere. Additionally, the powder particle distribution and the attenuation of the laser beam by the powder particles can be simulated. The simulation results are confirmed by experimental measurements of the powder particle density distribution in the working plane, by measurements of the oxygen volume percentage at the workpiece surface, by high-speed camera images of the melt pool surface and by absorptivity measurements, which show the effect of oxidation on the process.
       
  • Laser metal deposition of a refractory TiZrNbHfTa high-entropy alloy
    • Abstract: Publication date: Available online 5 October 2018Source: Additive ManufacturingAuthor(s): Henrik Dobbelstein, Evgeny L. Gurevich, Easo P. George, Andreas Ostendorf, Guillaume Laplanche Refractory elements have high melting points and are difficult to melt and cast. In this study it is successfully demonstrated for the first time that laser metal deposition can be used to produce TiZrNbHfTa high-entropy alloy from a blend of elemental powders by in-situ alloying. Columnar specimens with a height of 10 mm and a diameter of 3 mm were deposited with a pulsed Nd:YAG laser. The built-up specimen has near-equiatomic composition, nearly uniform grain size, equiaxed grain shape, is bcc single phase and exhibits a high hardness of 509 HV0.2.
       
  • Finite element analysis of in-situ distortion and bulging for an
           arbitrarily curved additive manufacturing directed energy deposition
           geometry
    • Abstract: Publication date: Available online 5 October 2018Source: Additive ManufacturingAuthor(s): M. Biegler, A. Marko, B. Graf, M. Rethmeier With the recent rise in the demand for additive manufacturing (AM), the need for reliable simulation tools to support experimental efforts grows steadily. Computational welding mechanics approaches can simulate the AM processes but are generally not validated for AM-specific effects originating from multiple heating and cooling cycles. To increase confidence in the outcomes and to use numerical simulation reliably, the result quality needs to be validated against experiments for in-situ and post-process cases. In this article, a validation is demonstrated for a structural thermomechanical simulation model on an arbitrarily curved Directed Energy Deposition (DED) part: at first, the validity of the heat input is ensured and subsequently, the model’s predictive quality for in-situ deformation and the bulging behaviour is investigated. For the in-situ deformations, 3D-Digital Image Correlation measurements are conducted that quantify periodic expansion and shrinkage as they occur. The results show a strong dependency of the local stiffness of the surrounding geometry. The numerical simulation model is set up in accordance with the experiment and can reproduce the measured 3-dimensional in-situ displacements. Furthermore, the deformations due to removal from the substrate are quantified via 3D-scanning, exhibiting considerable distortions due to stress relaxation. Finally, the prediction of the deformed shape is discussed in regards to bulging simulation: to improve the accuracy of the calculated final shape, a novel extension of the model relying on the modified stiffness of inactive upper layers is proposed and the experimentally observed bulging could be reproduced in the finite element model.
       
  • On Morphological Surface Features of the Parts Printed by Selective Laser
           Melting (SLM)
    • Abstract: Publication date: Available online 5 October 2018Source: Additive ManufacturingAuthor(s): Milad Hamidi Nasab, Dario Gastaldi, Nora Francesca Lecis, Maurizio Vedani Among the most popular additive manufacturing processes for metals, Powder bed fusion technology involves a layer by layer manufacturing approach utilizing a high power source, such as a laser or an electron beam, interacting with the metal powder on selected surfaces. Beam-powder interaction brings up a handful of phenomena affecting the quality of the final part in its volume and surface. In this study, different surface features generated by Selective Laser Melting of an Al-Si7-Mg alloy are investigated and interpreted based on their morphology, microstructure and hardness to improve the general understanding of defect genesis. Ballings, spatter particles and partially melted metal powders are distinguished by their morphology, size and microstructure. It is shown that these differences arise from different cooling rates during their generation. Ballings share the same microstructure with the bulk material both experiencing cooling in conduction mode. Spatters and partially melted powders show coarser microstructure driven by solidification mainly ruled by convection and radiation during their flight in the inert atmosphere of the process chamber.
       
  • Curvilinear variable stiffness 3D printing technology for improved
           open-hole tensile strength
    • Abstract: Publication date: Available online 5 October 2018Source: Additive ManufacturingAuthor(s): Sadben Khan, Kazem Fayazbakhsh, Zouheir Fawaz, Mahdi Arian Nik Fused Filament Fabrication (FFF), one of the most popular processes of 3D printing, offers flexibility in manufacturing and introduces anisotropic properties to the final parts. With the use of Curvilinear Variable Stiffness (CVS) 3D printing technology, mechanical properties of the manufactured products can be further improved and optimized. In this work, we demonstrate how CVS design can improve open-hole tensile strength and failure strain of the manufactured specimens per ASTM D5766. In addition, the ratio of the specimen width to the hole diameter is considered as a design parameter and investigated. It is found that CVS design improves the failure strength by 38.0% for a larger hole diameter configuration (from 48.0 MPa to 66.2 MPa), while the improvement in failure strain (from 0.0125 mm/mm to 0.0130 mm/mm) is limited to only 4.0%. On the other hand, for a smaller hole diameter case, a substantial improvement of 52.5% in failure strain is obtained with the use of CVS design (from 0.0141 mm/mm to 0.0215 mm/mm), while 16.7% improvement in failure stress (76.0 MPa to 88.6 MPa) is less pronounced.
       
  • Rheological Behavior of PDMS Silicone Rubber for 3D Printing of Medical
           Implants
    • Abstract: Publication date: Available online 4 October 2018Source: Additive ManufacturingAuthor(s): Jan Stieghorst, Theodor Doll The diagnosis and treatment of patients suffering from neurological diseases with patient-individualized silicone rubber-based implants is one of the most promising and challenging approaches to improve treatment outcome. Therefore, medical additive manufacturing techniques are developed for fabrication of such implants, but currently do not achieve the required printing resolution. This is caused by intensive droplet spreading of the initially liquid silicone rubber on the printing substrate. While empirical optimization approaches for the droplet spreading are intensive in cost and time, we develop a mathematical optimization approach to calculate the optimal printing parameters for minimal droplet spreading. Since the viscosity profile of thermal curing silicone rubber is the main reason for the droplet spreading, we implemented a rheology model for calculation of the optimal heat curing parameters. A Dual-Arrhenius equation was used to correlate the temperature-time-profile of the curing process with the curing-related viscosity rise and the temperature-related viscosity fall of the liquid silicone rubber. Two commonly used silicone rubbers were characterized with a rheometer at different isothermal and anisothermal curing profiles. High correlation between the calculated and the measured viscosity profiles were observed, giving the ability to optimize the curing process parameters to the rheological behaviour of the used silicone rubber.
       
  • Interlayer bonding improvement of material extrusion parts with
           polyphenylene sulfide using the Taguchi method
    • Abstract: Publication date: Available online 4 October 2018Source: Additive ManufacturingAuthor(s): Emily R. Fitzharris, Ian Watt, David W. Rosen, Meisha L. Shofner Material extrusion additive manufacturing (MEAM) and other additive manufacturing methods provide part design options that would be difficult or impossible to realize with conventional manufacturing methods. However, the mechanical properties of parts produced with MEAM are lower than bulk material properties because of the interfaces between roads and layers inherent to the additive build technique of MEAM. In addition, the success of the MEAM process and the resulting part quality depend on the proper selection of the many settings and variables present in MEAM. The effects of material dependent MEAM process parameters on the interlayer bonding and percent crystallinity of MEAM parts fabricated with polyphenylene sulfide (PPS) were examined in this study using a design of experiments technique known as the Taguchi method. The MEAM parameters studied were print temperature, heat-treatment time, and heat-treatment temperature. MEAM parts were tested perpendicular to the layers in order to characterize the interlayer bonding. Heat-treatment temperature was shown to be the most influential parameter on all the studied properties. Utilizing heat-treatments on MEAM parts increased the ultimate tensile strength (UTS) from 52% of the PPS film UTS to 80%. Similar increases were seen in the Young’s modulus, from 57% of the PPS film Young’s modulus to 72%. The study showed that utilizing post-processing heat-treatments on MEAM parts could improve the interlayer bonding in these parts. The use of these heat-treatments could be applied to other materials in order to increase the use of MEAM parts in end use applications.Graphical abstractGraphical abstract for this article
       
  • Process driven strengthening mechanisms in electron beam melted Ti-6Al-4V
    • Abstract: Publication date: Available online 26 September 2018Source: Additive ManufacturingAuthor(s): Wes Everhart, Joseph Dinardo Additive Manufacturing (AM) has significantly increased the design freedom available for metal parts and provides significant flexibility within each build to produce multiple components of varying size and shape. In order to obtain the highest build efficiency, it is ideal to print multiple parts together spanning the entire plate with as little spacing as possible between the parts. Work has been performed to characterize the variance of materials properties as a function of location within the build volume as well as component density on the build plate. This work utilizes mechanical, chemical, and microstructural analysis techniques to expand on previous work by statistically evaluating the impact of build location, and nearest neighbor proximity on tensile performance in Electron Beam Melted (EBM) Ti-6Al-4 V. Mechanical results are then correlated to structural phenomenon and the effectiveness of various strengthening mechanisms are determined. Results show that properties span a small range regardless of build design and that interstitial strengthening and lath spacing are the driving factors for mechanical strength.
       
  • Measurement and modeling of filament temperature distribution in the
           standoff gap between nozzle and bed in polymer-based additive
           manufacturing
    • Abstract: Publication date: Available online 26 September 2018Source: Additive ManufacturingAuthor(s): Hardikkumar Prajapati, Darshan Ravoori, Ankur Jain Dispensing of a polymer filament above its glass transition temperature is a critical step in several polymer-based additive manufacturing techniques. While the nozzle assembly heats up the filament prior to dispense, it is important to minimize cooling down of the filament in the standoff distance between the nozzle tip and bed. While heat transfer processes within the nozzle assembly, such as filament melting, and on the bed, such as thermally-driven filament-to-filament adhesion, have been well studied, there is a lack of work on heat transfer in the filament in the standoff region. This paper presents infrared thermography based measurement of temperature distribution in the filament in the standoff region, and an analytical model for heat transfer in this region. The analytical model based on a balance between thermal advection and convective/radiative heat loss predicts an exponentially decaying temperature distribution, the nature of which is governed by the characteristic length, a parameter that combines multiple process parameters such as mass flowrate, filament diameter, heat capacity and cooling conditions. Experimental data in a wide range of process parameters are found to be in very good agreement with the analytical model. The thermal design space for ensuring minimal temperature drop in the standoff region is explored based on the analytical model. Experimental data and theoretical modeling presented here improve our fundamental understanding of heat transfer in polymer additive manufacturing, and may contribute towards design tools for thermal optimization of polymer based additive manufacturing processes.
       
  • Novel Method for Additive Manufacturing of Metal-Matrix Composite by
           Thermal Decomposition of Salts
    • Abstract: Publication date: Available online 24 September 2018Source: Additive ManufacturingAuthor(s): Mahdi Yoozbashizadeh, Parviz Yavari, Behrokh Khoshnevis Very limited Additive Manufacturing (AM) processes have been developed for production of Metal Matrix Composites (MMCs) reinforced by ceramic. Most of these processes use different mixing techniques to mix metal and ceramic powder particles in order to be used in an existing AM process such as Selective Laser Melting (SLM) process. The current AM techniques for MMCs fabrication have limitations due to material mixing and the AM process limitations itself. This paper introduces a novel AM method for fabrication of MMCs by Thermal Decomposition of Salts (TDS). In this method inorganic salts are printed on metal powder bed to fabricate green part. The green part undergoes bulk sintering. During bulk sintering the printed inorganic salts are decomposed to fine ceramic particles to form MMC. This process is capable of generating MMC structures with uniformly distributed and dispersed ultra-fine ceramic particles in the metal matrix with less limitations and lower cost compared to other existing AM techniques. In this paper, bronze-alumina MMC was fabricated and studied by the TDS process to validate the proposed process. It was also shown that the TDS process can be used to fabricate other types of MMCs besides bronze-alumina due to the nature of the process. Design of Experiments methodology was used to study and model the effects of sintering parameters on the properties of the bronze-alumina fabricated by the TDS process. Due to MMCs unique properties combined with AM benefits, this novel method will be of great interest to various industries such as aerospace applications.Graphical abstractGraphical abstract for this article
       
  • Nondestructive Evaluation Method for Standardization of Fused Filament
           Fabrication based Additive Manufacturing
    • Abstract: Publication date: Available online 24 September 2018Source: Additive ManufacturingAuthor(s): Jeong K. Na, Erin K. Oneida In the current investigation, an ultrasonic imaging system originally developed for visualization of microstructures in sheet metals, with capabilities of generating plane two-dimensional images at spatial resolutions between 1 and 200 microns, was used to quantitatively evaluate a Fused Filament Fabrication (FFF) processed 3D test part. For the ultrasonic system, a custom software program was written to control all components of the inspection schemes in a continuous scan mode, including the movement of three orthogonal translational stages, as well as display a live ultrasonic image during scanning and provide tools for advanced post-processing of the recorded ultrasonic signals. Prior to collecting ultrasonic data for a selected test specimen, an optical flat reference standard was used to characterize the ultrasonic probes and to quantify the system’s mechanical stability, repeatability, and accuracy when measuring the physical dimensions of features. Ultrasonic data collected at different spatial resolutions were used to characterize a part’s surface flatness, internal defects, and fusion conditions; and to measure the physical dimensions of intended features. To validate the accuracy of the ultrasonic internal characterization, one side panel of the test specimen was removed for visual confirmation, and additional ultrasonic data was collected and compared to the original data. Finally, a suggestion is made for adopting a process to qualify or certify FFF based additive manufacturing machines in the market by applying a reliable NDE validation method to a standardized part with various features of different shapes and physical dimensions.
       
  • Characterization of In-Situ Measurements based on Layerwise Imaging in
           Laser Powder Bed Fusion
    • Abstract: Publication date: Available online 24 September 2018Source: Additive ManufacturingAuthor(s): Fabio Caltanissetta, Marco Grasso, Stefano Petrò, Bianca Maria Colosimo The layerwise production paradigm entailed in laser powder bed fusion (LPBF) offers the opportunity to acquire a wide range of information about the process stability and the part quality while the part is being manufactured. Different authors pointed out that high-resolution imaging of each printed layer combined with image segmentation methods can be used to detect powder recoating errors together with surface and geometrical defects. The paper presents the first study aimed at characterizing the accuracy of in-situ contour identification in LPBF layerwise images by means of a measurement system performance characterization. Different active contours segmentation methods are compared, and the sources of variability of the resulting measurements are investigated in terms of repeatability, part-to-part and build-to-build variability. The study also analyses and compares the sensitivity of in-situ measurements to different lighting conditions and laser scan directions. The results show that, by combining appropriate image pre-processing and segmentation algorithms with suitable lighting configurations, a high measurement repeatability can be achieved, i.e., a pure error that is up to one order of magnitude lower than the total measurement variability. This performance enables the detection of major geometric deviations and it paves the way to the design of statistical in-situ quality monitoring tools that rely on layerwise image segmentation.
       
  • Optimized heterogeneous plates with holes using 3D printing via vat
           photo-polymerization
    • Abstract: Publication date: Available online 24 September 2018Source: Additive ManufacturingAuthor(s): Linda M. Leben, Johanna J. Schwartz, Andrew J. Boydston, Royan J. D’Mello, Anthony M. Waas New advancements in 3D printing enable manufacturing a solid part with spatially controlled and varying material properties; this research seeks to establish techniques for finding optimal designs that use this new technology for the greatest structural benefit. We describe the use of a sequential quadratic programming based optimization solver to find an optimal distribution of material properties that minimize strain energy gradients, as calculated using finite element analysis. This design method is applied to the case of a flat thin plate with a hole, and has been proven to successfully reduce strain energy gradients and therefore stress concentrations. The optimally designed plates are 3D printed using a novel technology that uses vat polymerization technology. The computational model is validated with experiments. Enabling design engineers to customize material properties around geometric discontinuities will provide greater flexibility in reducing stress concentrations without modifying geometry or adding additional supports.
       
  • Combining cure depth and cure degree, a new way to fully characterize
           novel photopolymers
    • Abstract: Publication date: Available online 21 September 2018Source: Additive ManufacturingAuthor(s): C. Hofstetter, S. Orman, S. Baudis, J. Stampfl Bottom-up stereolithography has become a common lithography-based additive manufacturing technology (L-AMT) to fabricate parts with high feature resolution for biomedical applications. Novel vinyl ester based photopolymers, with their good biocompatibility and biodegradation behavior, showed a promising capacity as bone replacement materials. Due to further tuning of the mechanical properties, those biophotopolymers exhibit reduced curing speed in comparison to highly crosslinked resins e.g. acrylates. The slow structuring of the polymer network results in difficulties at the printing process. The Jacobs working curve characterizes the cure- and penetration depth of resins, but gives no information about the mechanical properties of the cured layer. The information of cure depth and the mechanical properties of the cured layer (cure degree) is desired. In this work, we simulated the conditions at L-AMT during the structuring process with a real-time near-infrared photorheometer to evaluate the cure degree of a cured layer at constant cure depth. Therefore, we investigated the curing behavior of mixtures with variable amount of photoinitiator (PI) and light absorber (LA) of vinyl ester based biophotopolymers. We found, that a high amount of LA is crucial for good mechanical properties at constant cure depth. Moreover, we present a technique how to optimize a resin formulation regarding the content of PI and LA.
       
  • Experimental validation of a numerical model for the strand shape in
           material extrusion additive manufacturing
    • Abstract: Publication date: Available online 20 September 2018Source: Additive ManufacturingAuthor(s): Marcin P. Serdeczny, Raphaël Comminal, David B. Pedersen, Jon Spangenberg We investigate experimentally and numerically the influence of the processing conditions on the cross-section of a strand printed by material extrusion additive manufacturing. The parts manufactured by this method generally suffer from a poor surface finish and a low dimensional accuracy, coming from the lack of control over the shape of the printed strands. Using optical microscopy, we have measured the cross-sections of the extruded strands, for different layer heights and printing speeds. Depending on the processing conditions, the cross-section of the strand can vary from being almost circular to an elongated rectangular shape with rounded edges. For the first time, we have compared the measurements of strands’ cross-sections to the numerical results of a three-dimensional computational fluid dynamics model of the deposition flow. The proposed numerical model shows good agreement with the experimental results and is able to capture the changes of the strand morphology observed for the different processing conditions.
       
  • Characterizing surface finish and fatigue behavior in binder-jet
           3D-printed nickel-based superalloy 625
    • Abstract: Publication date: Available online 10 September 2018Source: Additive ManufacturingAuthor(s): Amir Mostafaei, S. Harsha Vardhan, R. Neelapu, Cameron Kisailus, Lauren M. Nath, Tevis D.B. Jacobs, Markus Chmielus In this study, the fatigue properties of binder-jet 3D-printed nickel-base superalloy 625 were evaluated. Standard fatigue specimens were printed and sintered, then half of the samples were mechanically ground, while the other half were left in their as-sintered state. They were then characterized using micro-computed x-ray tomography, metallographic sample examination, and optical and stylus profilometry for surface topography. The micro-computed tomography observations showed that density of the as-printed sample was ~50%, while the sintered sample neared full densification (98.9 ± 0.3%) upon sintering at 1285 °C for 4 h in a vacuum atmosphere. The metallographic examination showed equiaxed grains. The roughness of the as-sintered samples was significant with an RMS roughness of Rq = 1.39 ± 0.20 μm as measured over a line-scan of 5 mm, but this was reduced to Rq = 0.47 ± 0.02 μm after mechanical grinding. All samples were tested to failure in fatigue, under fully-reversed tension-compression conditions. While the as-sintered samples showed poor fatigue properties compared to prior reports on cast and milled parts, the ground samples showed superior performance. Scanning electron microscopy observation was conducted on the fractured surfaces and showed that the samples underwent transgranular crack initiation, followed by intergranular crack growth and final failure. In the mechanically ground sample, hardness increased nearly two-fold up to 75 µm beneath the sample’s surface and X-ray diffraction indicated an in-plane compressive stress, grain refinement, and micro-strain on the mechanically ground sample. The surface hardening and compressive stress resulted most likely in increasing fatigue life of the binder-jetted alloy 625.
       
  • The Effect of Interlayer Cooling on the Mechanical Properties of
           Components Printed via Fused Deposition
    • Abstract: Publication date: Available online 7 September 2018Source: Additive ManufacturingAuthor(s): Nicolas G. Morales, Trevor J. Fleck, Jeffrey F. Rhoads This paper investigates the effect of interlayer cooling on the mechanical properties of acrylonitrile butadiene styrene (ABS) structures that are 3D printed using fusion based material extrusion. Two different types of samples were prepared, one designed to measure the compressive strength of the structural material, and the other designed to measure the shear strength of the structural material. Both types of samples were printed with various interlayer wait times by pausing for an allotted amount of time to allow for additional cooling before printing the sequential layer. The samples were then compressed using a Mark-10 ESM 1500 Tension and Compression Tester in accordance with ASTM D695-15. As the wait time in between layers was increased, the effective yield strength was decreased for both types of samples. Temperature data was collected from the top layer of the structures after each successive layer deposition. This data revealed significant cooling over the wait times being considered. These trends prove that additional care needs to be taken when selecting the print settings for structural components that are manufactured using fused filament fabrication. This study shows that printing processes that require additional time (i.e. larger parts, finer geometries, etc.) will inherently lead to a reduction in the mechanical strength of the printed structure.
       
 
 
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