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Publisher: Elsevier   (Total: 3162 journals)

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Showing 1 - 200 of 3162 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: 33, SJR: 1.655, CiteScore: 2)
Academic Radiology     Hybrid Journal   (Followers: 23, SJR: 1.015, CiteScore: 2)
Accident Analysis & Prevention     Partially Free   (Followers: 95, SJR: 1.462, CiteScore: 3)
Accounting Forum     Hybrid Journal   (Followers: 25, SJR: 0.932, CiteScore: 2)
Accounting, Organizations and Society     Hybrid Journal   (Followers: 36, 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: 411, 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: 249, SJR: 3.263, CiteScore: 6)
Acta Mathematica Scientia     Full-text available via subscription   (Followers: 5, SJR: 0.504, CiteScore: 1)
Acta Mechanica Solida Sinica     Full-text available via subscription   (Followers: 9, SJR: 0.542, CiteScore: 1)
Acta Oecologica     Hybrid Journal   (Followers: 12, SJR: 0.834, CiteScore: 2)
Acta Otorrinolaringologica (English Edition)     Full-text available via subscription  
Acta Otorrinolaringológica Española     Full-text available via subscription   (Followers: 2, SJR: 0.307, CiteScore: 0)
Acta Pharmaceutica Sinica B     Open Access   (Followers: 1, SJR: 1.793, CiteScore: 6)
Acta Poética     Open Access   (Followers: 4, SJR: 0.101, CiteScore: 0)
Acta Psychologica     Hybrid Journal   (Followers: 27, 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: 16, SJR: 1.29, CiteScore: 3)
Addictive Behaviors Reports     Open Access   (Followers: 8, SJR: 0.755, CiteScore: 2)
Additive Manufacturing     Hybrid Journal   (Followers: 9, SJR: 2.611, CiteScore: 8)
Additives for Polymers     Full-text available via subscription   (Followers: 22)
Advanced Drug Delivery Reviews     Hybrid Journal   (Followers: 148, 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: 12, 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: 23, 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: 32, SJR: 3.043, CiteScore: 6)
Advances in Carbohydrate Chemistry and Biochemistry     Full-text available via subscription   (Followers: 8, 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: 3)
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: 15)
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: 25)
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: 7)
Advances in Experimental Social Psychology     Full-text available via subscription   (Followers: 44, 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: 21, 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: 36, 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: 6, SJR: 0.176, CiteScore: 0)
Advances in Intl. Accounting     Full-text available via subscription   (Followers: 3)
Advances in Life Course Research     Hybrid Journal   (Followers: 8, SJR: 0.682, CiteScore: 2)
Advances in Lipobiology     Full-text available via subscription   (Followers: 1)
Advances in Magnetic and Optical Resonance     Full-text available via subscription   (Followers: 9)
Advances in Marine Biology     Full-text available via subscription   (Followers: 17, 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: 22)
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: 11)
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: 9)
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: 62)
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: 395, 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: 10, SJR: 0.555, CiteScore: 2)
Advances in the Study of Behavior     Full-text available via subscription   (Followers: 33, 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: 340, 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: 448, 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: 42, 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: 9)
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: 52, 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: 34, SJR: 2.973, CiteScore: 4)
American J. of Medicine     Hybrid Journal   (Followers: 46)
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: 208, SJR: 2.7, CiteScore: 4)
American J. of Ophthalmology     Hybrid Journal   (Followers: 63, 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: 43, SJR: 1.512, CiteScore: 5)
Analytical Biochemistry     Hybrid Journal   (Followers: 175, SJR: 0.633, CiteScore: 2)
Analytical Chemistry Research     Open Access   (Followers: 11, 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: 192, 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: 9  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 2214-8604
Published by Elsevier Homepage  [3162 journals]
  • 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
       
  • Enriched Analytical Solutions for Additive Manufacturing Modeling and
           Simulation
    • Abstract: Publication date: Available online 9 October 2018Source: Additive ManufacturingAuthor(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.
       
  • 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.
       
  • Additive Manufacturing of Silicone Structures: A Review and Prospective
    • Abstract: Publication date: Available online 4 October 2018Source: Additive ManufacturingAuthor(s): Farzad Liravi, Ehsan Toyserkani Additive Manufacturing (AM) has the potential to facilitate the limitless design and fast fabrication of silicone structures with controllable internal features and heterogeneous properties. The challenging task of developing AM systems able to handle viscous thermosetting silicones is reviewed in this work, with a focus on their use for biomedical applications. Moreover, the development of silicone tailored for AM is reviewed by examining three areas of curing mechanisms, rheological properties, and mechanical performance.
       
  • 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.
       
  • 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: Available online 25 September 2018Source: Additive ManufacturingAuthor(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.
       
  • 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.
       
  • 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
       
  • Printing of Complex Free-standing Microstructures via Laser-induced
           Forward Transfer (LIFT) of Pure Metal Thin Films
    • Abstract: Publication date: Available online 24 September 2018Source: Additive ManufacturingAuthor(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.
       
  • 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.
       
  • Additive Laser Metal Deposition on Silicon
    • Abstract: Publication date: Available online 24 September 2018Source: Additive ManufacturingAuthor(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 chip about 10 °C cooler by eliminating two thermal interface materials, thus reducing operating temperature, saving power and reducing electronic-waste. Bonding of common metal alloys used in additive manufacturing and 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 (−35 °C and 140 °C).Graphical abstractGraphical abstract for this article
       
  • 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.
       
  • When CFRTP products are manufactured with a 3D printer
    • Abstract: Publication date: Available online 22 September 2018Source: Additive ManufacturingAuthor(s): Ryosuke Matsuzaki, Taishi Nakamura, Kentaro Sugiyama, Masahito Ueda, Akira Todoroki, Yoshiyasu Hirano, Yusuke Yamagata The present study investigated the effects of set radius of curvature and fiber bundle size on the precision of the radius of curvature during continuous carbon fiber three-dimensional (3D) printing. First, individual circles with various radii using various sizes of fiber bundles were printed with a 3D printer. It was demonstrated that with a larger fiber bundle size or a smaller set radius, the printed radius would be lower than the set value. Based on the printing experiments, twist and path length difference models were constructed to predict the actual printed radius. Comparison between the models and experimental values verified that the twist model provides the upper bound on the printed radius and the path length difference model provides the lower bound.
       
  • 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.
       
  • Microstructure Characterization and Grain Morphology of Alloy 625 with 0.4
           wt% Boron Modification Manufactured by Laser Wire Deposition
    • Abstract: Publication date: Available online 21 September 2018Source: Additive ManufacturingAuthor(s): Y. Tiana, N. Chekira, X. Wanga, A. Nommeots-Nomma, R. Gauvina, M. Brochua
       
  • Standardized X-ray tomography testing of additively manufactured parts: a
           round robin test
    • Abstract: Publication date: Available online 20 September 2018Source: Additive ManufacturingAuthor(s): Anton du Plessis, Stephan G. le Roux Micro computed tomography (microCT) allows non-destructive insights into the quality of additively manufactured parts and the processes that produce them. MicroCT has been used widely in this industry but the use of this technique is often time consuming and costly which reduces its potential impact and the benefits associated with its use. By using standardized test procedures, the analysis time and cost can be minimized and confidence in obtained results increased. A round robin test was conducted as follows: a series of standard test procedures (part sizes and shapes and test protocols) were applied - using one microCT system - to identical parts produced on a variety of metal additive manufacturing systems (specifically laser powder bed fusion systems). These are simple parts: a 10 mm cube, a 15 mm diameter vertical-built cylinder and a basic topology optimized example part – a bracket. The 15 mm diameter cylinder acts as witness specimen for the build of the complex part. All these were produced in Ti6Al4V, and in some cases parts were provided with variations in process parameters or manufacturing conditions which led to different types of intentional manufacturing flaws or defects. Various intentional and unintentional flaws were identified and quantified. The major result shown is that the analysis of a simple 10 mm cube clearly identifies incorrect process parameters even for very low levels of porosity, with unique porosity distributions and characteristics. It is found that generally this porosity extends to larger, more complex parts. The witness specimen (15 mm cylinder) allows clear identification of layered stop-start flaws, at a resolution better than a complex part built alongside it, allowing to identify defective builds. The results indicate a successful first step at standardized microCT analysis procedures for improvement of processes and quality control in additive manufacturing.
       
  • 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.
       
  • Feasibility study on additive manufacturing of recyclable objects for
           space applications
    • Abstract: Publication date: Available online 19 September 2018Source: Additive ManufacturingAuthor(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.
       
  • The Effects of Hot Isostatic Pressing on Parts Fabricated by Binder
           Jetting Additive Manufacturing
    • Abstract: Publication date: Available online 18 September 2018Source: Additive ManufacturingAuthor(s): Ashwath Yegyan Kumar, Yun Bai, Anders Eklund, Christopher B. Williams Hot Isostatic Pressing (HIP) is a technique of applying high pressures through a fluid medium at high temperatures to enclosed powders, castings and pre-sintered metal parts to eliminate porosity. Due to uniform volumetric shrinkage expected from this process, it can be a useful post-processing technique for complex-geometry parts fabricated using Additive Manufacturing (AM) techniques. In order for the technique to work effectively, parts are typically required to have a minimum density of 92%, where surface porosity is closed. While HIP has been used in conjunction with powder bed fusion AM processes, its use for parts made using Binder Jetting (BJ) has not been investigated in detail due to the limitations of BJ in fabricating sufficiently high-density parts without infiltration. In this work, detailed investigations on the effect of HIP on BJ parts printed from three different powder configurations, which led to varying levels of porosity, are performed. The effects of HIP on the density, microstructure, tensile strength, and ductility of the resulting parts is reported. A maximum density of 97.32% was achieved by HIP of printed and sintered parts created via bimodal powders. Both the tensile strength and ductility were found to improve following HIP, which suggests that the reduction in porosity is predominant compared to the detrimental effects of grain coarsening.
       
  • High strain rate compressive deformation behavior of an additively
           manufactured stainless steel
    • Abstract: Publication date: Available online 15 September 2018Source: Additive ManufacturingAuthor(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.
       
  • Development and characterization of multilayer laser cladded high speed
           steels
    • Abstract: Publication date: Available online 14 September 2018Source: Additive ManufacturingAuthor(s): N. Ur Rahman, L. Capuano, A. van der Meer, M.B. de Rooij, D.T.A. Matthews, G. Walmag, M. Sinnaeve, A. Garcia-Junceda, M. Castillo, G.R.B.E. Römer Two high speed steel (HSS) alloys were laser cladded on 42CrMo4 steel cylindrical substrate by using a 4 kW Nd:YAG laser source. After optimization of the laser material processing parameters for single layers, multilayered clads were produced. Microstructural characterization of the laser deposits constitutes studies of the carbides and matrix, which was done by using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Electron Backscattered Diffraction (EBSD) and High Resolution Transmission Electron Microscopy (HRTEM).The strengthening mechanism of LC1 (Fe-Cr-Mo-W-V) was comprised of a martensitic matrix and retained austenite along with networks of VC and Mo2C eutectic carbides. Cr enriched fine carbides (Cr7C3 and Cr23C6) were embedded within the matrix. During laser cladding of the multilayer deposits, cladding of subsequent layers had a detrimental effect on the hardness of previously cladded layers, which was due to tempering of existing lath martensite. To overcome the hardness drop, a new alloy LC2 (Febal−x-Cr-Mo-W-V-Cox) was blended by addition of 3-5 % of Co in LC1. The addition of Co resulted in an overall increase in hardness and a reduction in the hardness drop during sequential layer cladding; the latter was due to the presence of Co in the solid solution with Fe.HRTEM was performed to characterize the nanometer-sized precipitates evolved during the re-heating. These carbides were either enriched with V and W or formed from a complex combination of V, Mo, W and Cr with lattice spacings of 0.15 nm to 0.26 nm.
       
  • Laser Peening: A Tool for Additive Manufacturing Post-processing
    • Abstract: Publication date: Available online 14 September 2018Source: Additive ManufacturingAuthor(s): Lloyd Hackel, Jon R. Rankin, Alexander Rubenchik, Wayne E. King, Manyalibo Matthews Additive manufacturing (AM) is rapidly moving from research to commercial applications due to its ability to produce geometric features difficult or impossible to generate by conventional machining. Fielded components need to endure fatigue loadings over long operational lifetimes. This work evaluates the ability of shot and laser peening to enhance the fatigue lifetime and strength of AM parts. As previously shown, peening processes induce beneficial microstructure and residual stress enhancement; this work takes a step to demonstrate the fatigue enhancement of peening including for the case of geometric stress risers as expected for fielded AM components. We present AM sample fatigue results with and without a stress riser using untreated baseline samples and shot and laser peening surface treatments. Laser peening is clearly shown to provide superior fatigue life and strength. We also investigated the ability of analysis to select laser peening parameters and coverage that can shape and/or correctively reshape AM components to a high degree of precision. We demonstrated this potential by shaping and shape correction using our finite element based predictive modeling and highly controlled laser peening.
       
  • Multiple modulus silicone elastomers using 3D extrusion printing of low
           viscosity inks
    • Abstract: Publication date: Available online 13 September 2018Source: Additive ManufacturingAuthor(s): Sijia Zheng, Michael Zlatin, Ponnambalam Ravi Selvaganapathy, Michael A. Brook Silicone elastomers are of commercial interest in a number of areas because of their distinctive properties. Current 3D-printing (additive manufacturing) technologies for silicones mainly rely on the extrusion of high-viscosity pre-elastomer inks of one or two parts. Some of the challenges presented by high viscosity materials, for instance, difficulties in mixing and changing inks to create devices from more than one type of silicone, could be overcome by use of lower viscosity inks. Here we describe a family of rapidly curing (shape holding within
       
  • Embedding electronics in 3D printed structures by combining fused filament
           fabrication and supersonic cluster beam deposition
    • Abstract: Publication date: Available online 11 September 2018Source: Additive ManufacturingAuthor(s): Andrea Bellacicca, Tommaso Santaniello, Paolo Milani We present an integrated additive manufacturing approach for the rapid prototyping of objects with embedded electric circuits. Our approach relies on the combined use of standard fused filament fabrication (FFF) for the production of thermoplastic 3D freeform components, and supersonic cluster beam deposition (SCBD) for the fabrication of embedded electrical conducting lines and resistors with tailored conductivity. SCBD is an additive fabrication technique based on the deposition of neutral metallic clusters carried in a highly collimated supersonic beam. SCBD produces strongly adherent conducting layers onto polymeric substrates with electrical resistance depending only on the thickness of the cluster-assembled film. A multi-step fabrication procedure alternating FFF and SCBD was developed and optimized allowing the fabrication of conductive 3D oblique paths, bridging vias, and sockets for standard electronic components fitting. This resulted in the simplification of the topology of planar electric circuits by enabling out-of-plane connections, minimizing the implementation of bulky passive electrical components and avoiding the use of soldering and conductive adhesives for the integration of active electronic components. A dark-activated light sensor was produced as a demonstrator.
       
  • Influence of layer thickness and post-process treatments on the fatigue
           properties of CoCr scaffolds produced by laser powder bed fusion
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): Antonio Cutolo, Bram Neirinck, Karel Lietaert, Charlotte de Formanoir, Brecht Van Hooreweder Over the last years, additive manufacturing (AM) techniques such as laser powder bed fusion (L-PBF) have been frequently adopted for efficiently producing biomedical implants. L-PBF offers the advantage of low material waste and high accuracy enabling the production of complex and highly personalized geometries. However, when manufacturing time is considered, the L-PBF production rate is relatively low compared to conventional production techniques. The aim of this paper is to present the impact of layer thickness on static and fatigue properties of CoCr scaffolds produced by means of L-PBF. An increased layer thickness (from 30 μm to 60 μm) leads to an improvement in terms of production rate of 40 to 50% without affecting the final geometry of the structure. A fatigue test campaign was conducted on both 30 μm to 60 μm layer thickness samples in “as-built” condition. The analysis of the test results with a local stress method highlighted no significant differences in terms of fatigue performances. In addition, the effect of post-process treatments, such as hot isostatic pressing (HIP) and chemical etching on static and fatigue properties were investigated. It is shown that HIP does not affect the fatigue properties of the scaffolds whilst chemical etching is capable of improving fatigue resistance when the local stress approach is considered.Graphical abstractGraphical abstract for this article
       
  • 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.
       
  • Depth-sensing time-dependent response of additively manufactured Ti-6Al-4V
           alloy
    • Abstract: Publication date: Available online 9 September 2018Source: Additive ManufacturingAuthor(s): Muztahid Muhammad, Mohammad Masoomi, Brian Torries, Nima Shamsaei, Meysam Haghshenas Depth-sensing (instrumented) indentation testing technique is a robust, reliable, convenient and non-destructive characterization method to study small-scale mechanical properties and rate-dependent plastic deformation in metals and alloys at ambient and elevated temperatures. In the present paper, depth-sensing indentation creep behavior of an additively manufactured, via laser powder bed fusion (L-PBF) method, Ti-6Al-4 V alloy is studied at ambient temperature. Indentation creep tests were performed through a dual-stage scheme (loading followed by a constant load-holding and unloading) at different peak loads of 250 mN, 350 mN, and 450 mN with holding time of 400 s. Creep parameters including creep rate, creep stress exponent, and indentation size effect were analyzed, according to the Oliver and Pharr method, at different additive manufacturing scan directions and scan sizes. To assess processing parameter/ microstructure/ creep property correlations in the additively manufacture Ti-6Al-4 V alloy, microstructural quantitative analyses (i.e. optical microscopy and scanning electron microscopy) were performed as well. The findings of this study, according to stress exponent values, showed that the controlling mechanism of the creep at ambient temperature for the examined L-PBF Ti-6Al-4 V is mainly glide-controlled dislocation creep. These findings were compared against traditionally processed Ti-6Al-4 V as well.
       
  • Machining of functionally graded Ti6Al4V/ WC produced by Directed Energy
           Deposition
    • Abstract: Publication date: Available online 8 September 2018Source: Additive ManufacturingAuthor(s): Olusola Oyelola, Pete Crawforth, Rachid M’Saoubi, Adam T. Clare Additive manufacturing (AM) technologies offer new processing routes for functionally graded materials. At present, parts built using these processes often require additional processing as a result of the characteristic surface finish limitations synonymous with AM processes. A difficulty thus arises in the post processing of these components as volumes within the part have differing material properties by definition and will therefore exhibit variable machinability.In this study, machining of functionally graded Ti6Al4V/ WC components consisting of a metal matrix composite (MMC) region and a single alloy region produced via direct energy deposition using commercially available tooling is explored. The influence of post processing on surface integrity is investigated and reported. The effect of material variation on cutting forces and tool response along the component is also analysed and reported. Cutting forces within the MMC region are found to increase by as much as 40% which has been subsequently related to the periodic changes in microstructure generated by the layer by layer build strategy. Tool wear mechanisms are investigated and the influence of material pull out on surface integrity of both MMC and single material regions is explored. This study provides an insight into how the layer building strategies, particularly with multiple materials and the resulting variation in microstructure influences the machining of resulting components.
       
  • Heterogeneous microstructures and corrosion resistance of biomedical
           Co-Cr-Mo alloy fabricated by electron beam melting (EBM)
    • Abstract: Publication date: Available online 8 September 2018Source: Additive ManufacturingAuthor(s): Daixiu Wei, Yuichiro Koizumi, Akihiko Chiba, Kosuke Ueki, Kyosuke Ueda, Takayuki Narushima, Yusuke Tsutsumi, Takao Hanawa We have investigated the spatial distribution of microstructures of a Co-Cr-Mo alloy rod fabricated by Electron Beam Melting (EBM) method along built height. The topside of the rod is rich in γ-fcc phase and consists of fine grains with high local distortion density. The bottom part has an ε-hcp single phase and consists of relatively coarser grains with low local distortion density. The middle part of the rod consisted of the mixture of both phases. The mean grain size increases from 56 μm (at the top of the rod) to 159 μm (at the bottom), and is accompanied by a decrease in the γ-fcc phase fraction. On the other hand, a large number of precipitates including the main M23X6 phase and minor phases (η-phase and π-phase) were observed. The area fraction of the precipitates increases gradually from 5.26 % (at the top) to 8.73 % (at the bottom), and the relative proportion of each phase fluctuates at different positions. The hardness of the top side is lower than that of the bottom side. As a result, the hardness of the samples, as well as the area fraction of precipitates formed in the samples, increases gradually from top to bottom of the rod, while corrosion resistance is uniformly high throughout the rod almost independently of the location. The mechanism behind the formation of phase distribution is discussed in terms of thermodynamic phase stability and kinetics of phase transformation accompanying the thermal history during the post-solidification process.
       
  • 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.
       
  • Repercussions of powder contamination on the fatigue life of additive
           manufactured maraging steel
    • Abstract: Publication date: Available online 6 September 2018Source: Additive ManufacturingAuthor(s): A. Gatto, E. Bassoli, L. Denti A wide range of materials is suitable for processing by powder bed fusion (PBF) techniques. Among the latest formulations, maraging steel 18Ni-300, which is a martensite-hardenable alloy, is often used when both high fracture toughness and high strength are required, or if dimensional changes need to be minimised. In direct tooling, 18Ni-300 can be successfully employed in numerous applications, for example in the production of dies for injection moulding and for casting of aluminium alloys; moreover, it is particularly valuable for high-performance engineering parts.Even though bibliographic data are available on the effects that parameters, employed in PBF processes, have on the obtained density, roughness, hardness and microstructure of 18Ni-300, there is still a lack of knowledge on the fatigue life of PBF manufactured parts. This paper describes the fatigue behaviour of 18Ni-300 steel manufactured by PBF, as compared by forging. Relevant negative effects of the cross-contamination of the raw material are originally identified in this paper, which emphasizes the inadequacy of current acceptability protocols for PBF powders. In the absence of contamination, endurance achieved by PBF is found equal to that by forging and consistent with tooling requirements as set out by industrial partners, based on injection moulding process modelling.Graphical abstractGraphical abstract for this article
       
  • Embedding Anti-counterfeiting Features in Metallic Components via Multiple
           Material Additive Manufacturing
    • Abstract: Publication date: Available online 6 September 2018Source: Additive ManufacturingAuthor(s): Chao Wei, Zhe Sun, Yihe Huang, Lin Li The aerospace, automotive and medical industries are suffering from significant number of counterfeited metallic products that not only have caused financial losses but also endanger lives. The rapid development of additive manufacturing technologies makes such a situation even worse. In this investigation, we successfully applied a novel hybrid powder delivery selective laser melting (SLM) approach to embed dissimilar tagging material (Cu10Sn copper alloy) safety features (e.g. QR code) into metallic components made of 316 L stainless steel. X-ray imaging was found to be a suitable method for the identification of the embedded safety features up to 15 mm in depth. X-ray fluorescence was used for the chemical composition identification of the imbedded security tagging material. A criterion for the selection of tagging material, its dimensions and imbedding depth is proposed. The multiple material SLM technology was shown to offer the potential to be integrated into metallic component production for embedding anti-counterfeiting features.
       
  • Effect of chemical composition on microstructure, strength and wear
           resistance of wire deposited Ni-Cu alloys
    • Abstract: Publication date: Available online 6 September 2018Source: Additive ManufacturingAuthor(s): O.O. Marenych, D. Ding, Z. Pan, A.G. Kostryzhev, H. Li, S. van Duin Two Ni-Cu alloys (Monel K500 and FM 60) having various Mn, Fe, Al, Ti and C contents were deposited on a Monel K500 plate at three different speeds using wire arc additive manufacturing technique. Microstructure characterisation, in particular a detailed study of precipitates, was carried out using optical and scanning electron microscopy. Mechanical properties were assessed using hardness, tensile and wear testing. For similar deposition conditions, Monel K500 has exhibited smaller secondary dendrite arm spacing and higher number density of Ti-rich particles, although the Ti concentration in FM 60 was higher. Finer microstructure and Ti precipitation led to superior hardness, tensile and wear resistance of Monel K500 compared to FM 60. The variation in microstructure-properties relationship with alloy composition is discussed.Graphical abstractGraphical abstract for this article
       
  • Numerical tools to investigate mechanical and fatigue properties of
           additively manufactured MS1-H13 hybrid steels
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): Alireza Ebrahimi, Mohsen Mohammadi Additive manufacturing (AM) has been recently used to deposit metal powder on top of conventional metals. Of particular interest is hybrid additively manufactured steels which were found to be a suitable solution to benefit from features of each metal at different spots of a mechanical component. Due to its superior mechanical characteristics, maraging steel (MS1) has recently attracted tremendous attention for additive manufacturing applications mainly in aerospace, tool and die, and marine industries or to be 3D printed on top of other metals as a hybrid product using different techniques such as Direct Metal Laser Sintering (DMLS). In this paper a predictive finite element (FE) model and a combined analytical-numerical framework were developed to evaluate the mechanical performance of hybrid additively manufactured components and facilitate the prediction of hardness and fatigue life of these parts. The proposed tools were employed in two scopes: First to simulate the indentation hardness test of hybrid DMLS-MS1-H13 steels; and second to calculate fatigue crack nucleation life of maraging steel including defects (i.e. welding residual stresses). Parameters such as local and global displacements, changes in Young’s modulus, and hardness, high cycle fatigue life, welding temperature distribution, and residual stress were investigated. The hardness experiments were carried out to improve the reported data found in similar studies, which were used as the main resource to validate the proposed numerical framework. The capabilities of the presented frameworks enable this work to target existing ambiguities in additively manufactured mechanical components.Graphical abstractGraphical abstract for this article
       
  • An investigation on mechanical and microstructural properties of 316LSi
           
    • Abstract: Publication date: Available online 31 August 2018Source: Additive ManufacturingAuthor(s): Meysam Akbari, Radovan Kovacevic A robotized laser/wire direct metal deposition system was utilized to fabricate 316LSi coupons. The mechanical and microstructural properties were then characterized. Two types of coupons, thin-walled and block, of short and long inter-layer time intervals were considered. It was found that different thermal histories caused by different inter-layer time intervals have significant impact on mechanical and microstructural properties. The thin-walled samples with lower cooling rates showed coarser columnar grains, lower ultimate tensile strength, and lower hardness compared to the block samples. The melt pool was monitored in real-time. An empirical correlation between the melt pool area and cooling rate was achieved that could enable control of scale of the final solidification structure by maintaining the melt pool size in real-time. Further, to study the anisotropic behavior, tensile samples were loaded in parallel and perpendicular directions with respect to the deposition direction. The results indicated that samples in the perpendicular direction had lower UTS and elongation for both coupon types, revealing a weaker bonding at inter-layer/bead interface due to the existence of lack-of-fusion pores.
       
  • Starch-Hydroxyapatite Composite Bone Scaffold Fabrication Utilizing a
           Slurry Extrusion-Based Solid Freeform Fabricator
    • Abstract: Publication date: Available online 29 August 2018Source: Additive ManufacturingAuthor(s): Caitlin Koski, Bonny Onuike, Amit Bandyopadhyay, Susmita Bose Significant efforts have been made to treat bone disorders through the development of composite scaffolds utilizing calcium phosphate (CaP) through additive manufacturing techniques. However, the incorporation of natural polymers with CaP during 3D printing is difficult and remains a formidablechallenge in bone and tissue engineering applications. The objective of this study is to understand the use of a natural polymer binder system in ceramic composite scaffolds using a ceramic slurry-based solid freeform fabricator (SFF). This was achieved through the utilization of naturally sourced gelatinized starch with hydroxyapatite (HA) ceramic in order to obtain high mechanical strength and enhanced biological properties of the green part without the need for cross-linking or post processing. The parametric effects of solids loading, polycaprolactone (PCL) polymer addition, and designed porosity on starch-HA composite scaffolds were assessed through mechanical strength, microstructure, and in vitro biocompatibility utilizing human osteoblast cells. It was hypothesized that starch incorporation would improve the mechanical strength of the scaffolds and increase proliferation of osteoblast cells in vitro. Starch loading was shown to improve mechanical strength from 4.07 ± 0.66 MPa to 10.35 ± 1.10 MPa, more closely resembling the mechanical strength of cancellous bone. Based on these results, a reinforcing mechanism of gelatinized starch based on interparticle and apatite crystal interlocking is proposed. Morphological characterization utilizing FESEM and MTT cell viability assay showed enhanced osteoblast cell proliferation in the presence of starch and PCL. Overall, the utilization of starch as a natural binder system in SFF scaffolds was found to improve both green strength and in vitro biocompatibility.Graphical abstractGraphical abstract for this articlea) Ceramic slurry preperation of starch and hydroxyapaitite (HA) utilized for fabrication of bone scaffolds without the need for post processing. b) Schematic of Solid Freeform Fabricator. c) Reprensentation of scaffold model utilizing solidworks file and CURA program and final scaffold prints d) In vitro cell work regarding the proliferation of osteoblast cells utilizing starch based composite HA scaffolds, ultimately aquiring sufficient mechanical integrity and enhanced biactivity to be utilized in bone repair.
       
  • Additive manufacturing of pharmaceuticals for precision medicine
           applications: A review of the promises and perils in implementation
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): Megha Trivedi, Joann Jee, Suzanila Silva, Carmel Blomgren, Vasco M. Pontinha, Dave L. Dixon, Benjamin Van Tassel, Michael J. Bortner, Christopher Williams, Eric Gilmer, Alexander P. Haring, Justin Halper, Blake N. Johnson, Zhenyu Kong, Matthew S Halquist, Paul F Rocheleau, Timothy E. Long, Thomas Roper, Dayanjan S. Wijesinghe Precision medicine is an emerging field in healthcare that seeks to tailor preventive and therapeutic strategies to the unique physiology, biochemistry, lifestyles, and genetics of individual patients. There are several technologies that are key to the successful delivery of precision medicine including pharmacogenomics, pharmacometabolomics, improved point-of-care testing, and therapeutically-tailored medications. The inclusion of additive manufacturing (AM) technology, more commonly known as 3D printing, in the manufacture of oral dosage forms such as tablets, provides an avenue for the implementation of precision medicine in current healthcare practice via the prescription of specific dosage forms and drug combinations tailored to individual needs. Widespread commercialization of AM of pharmaceuticals has the potential to disrupt the supply chain used by the healthcare industry worldwide with the cost-saving potential of minimizing waste related to unused, expired medications. Despite the potential of this technology, many clinical and regulatory challenges will need to be addressed prior to large-scale implementation of AM fabricated therapeutics for precision medicine applications. This review investigates both the potential and the challenges of delivering AM fabricated medications for therapeutic use in precision medicine applications.
       
  • Limitations of the inherent strain method in simulating powder bed fusion
           processes
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): Matteo Bugatti, Quirico Semeraro Process optimization has always been a crucial step for effective usage of metal additive manufacturing (AM) processes: it consists in establishing quantitative relations between final part's characteristics and process parameters to find their optimal combination and obtain a fully functional mechanical component. Experimental investigation techniques are usually employed for this purpose but they can be extremely expensive and time-consuming, especially when the output of the process depends on a large number of parameters, like for AM. Numerical simulation could represent an alternative solution: by reproducing the real process characteristics, a simulation could provide useful insights, allowing to evaluate the performance of the process for different parameter combinations without relying exclusively on expensive experimental campaigns.In this work, a finite element AM simulation based on the inherent strain (IS) method was developed and the prediction performance in terms of part's residual deformation was evaluated by comparing the numerical results with the measurements carried out on an experimental campaign. A new model calibration approach for prediction improvement was also implemented and it allowed to discover an unexpected behaviour of the model that strongly affects the validity of this method for AM simulation.
       
  • Mechanical and corrosion properties of AlCoCrFeNi high-entropy alloy
           fabricated with selective electron beam melting
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): Kosuke Kuwabara, Hiroshi Shiratori, Tadashi Fujieda, Kenta Yamanaka, Yuichiro Koizumi, Akihiko Chiba Additive manufacturing is expected to be the manufacturing method for components made with high-entropy alloys (HEAs). In this study, the mechanical and electrochemical behaviors were investigated for equi-molar HEA (AlCoCrFeNi) obtained with selective electron beam melting (SEBM). The mechanical properties of SEBM products were improved compared with those of a cast specimen. Electrochemical measurements in artificial seawater revealed the corrosion behaviors of HEA (AlCoCrFeNi). The pitting potential of SEBM specimens (0.112 V vs. Ag/AgCl) was lower than that of a cast specimen (0.178 V vs. Ag/AgCl). The mechanical and electrochemical properties of SEBM products were influenced by the phase morphologies formed during the SEBM process.Graphical abstractGraphical abstract for this article
       
  • Micro-scale finishing of the surface and form of a Ti-6Al-4V lightweight
           rotor obtained by laser powder bed fusion used for air bearing
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): Claude Sanz, Romain Gerard, Paul Morantz, Ahmed Chérif, Paul Shore, Hélène Mainaud-Durand, Alexander J.G. Lunt The European Organisation for Nuclear Research, CERN, is in the process of designing and testing parts for the next generation of linear accelerators. In order to operate the experiments, the pre-alignment precision of the components of the two opposing accelerating complexes has placed increased demands on part tolerances, which are now approaching the micrometre. In order to meet these demanding requirements, improvements are necessary to the build processes, machining parameters and post-manufacture characterisation stages. One of the most promising methods for the production of these parts is Laser Powder Bed Fusion, and as such, this paper focuses on the manufacture of the lightweight air bearing rotor component and the micro-scale tolerance machining required by this part. The results demonstrate that despite being able to initially machine the part to a form tolerance approaching 2 μm, subsequent notch cutting and the release of residual stresses from the part obtained by Laser Powder Bed Fusion induces an 18 μm part misalignment which is larger than the tolerance limits of 5 μm required for operation. This demonstrates that further minimisation and understanding of the residual stresses induced during machining are required to facilitate the effective manufacture of high precision components of this type.Graphical abstractGraphical abstract for this article
       
  • Embedded electrical tracks in 3D printed objects by fused filament
           fabrication of highly conductive composites
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): J.C. Tan, H.Y. Low The incorporation of electrical components into 3D printed products such as sensors or printing of circuits requires the use of 3D printable conductive materials. However, most conductive materials available for fused filament fabrication (FFF) have conductivities of less than 1000 S/m. Here, we describe the study of conductive thermoplastic composites comprising either nylon – 6 or polyethylene (PE) matrix. The fillers used were nickel and Sn95Ag4Cu1, a low melting point metal alloy. The combination of nickel metal particles and tin alloy allows for higher metal loading at lower melt viscosity, compared to composites of nickel metal particles alone. Conductivities of 31,000 S/m were achieved, and 30 vol. % metal loading was processable by a single screw extruder. Embedded conductive tracks of various geometries were easily printed via FFF. Electrical conductivity of embedded conductive track has been investigated as a function of geometrical variation, where conductive tracks printed along a horizontal axis show resistance of ≤ 1 Ω. Porosity of the printed track is shown to increase with prints along the vertical axis, leading to a reduction in electrical conductivity of more than two orders of magnitude.
       
  • A New Photopolymer Extrusion 5-Axis 3D Printer
    • Abstract: Publication date: Available online 25 August 2018Source: Additive ManufacturingAuthor(s): Muhammad Asif, Joo Hyun Lee, Mikyla J. Lin-Yip, Simone Chiang, Alexis Levaslot, Tim Giffney, Maziar Ramezani, Kean Chin Aw Popular 3D printing techniques such as fused deposition modelling (FDM) and stereolithography (SLA) have certain limitations and challenges. Although printing multi-material functional parts combining smart and conventional materials is a promising area, existing printers are not ideally suited to this, with FDM printers typically requiring high operating temperatures and SLA using a tank containing one single material. Common 3D printers also require the deposition of additional “support” material to hold the shape of an object when printing overhang structures. The concept of adding additional rotational axes to the system to eliminate this problem has shown promising results, but such systems still lack the capability to print complex structures without supports. To overcome these limitations there is a need to develop a new 3D printing techniques that combine the strengths of existing methods. A photopolymer extrusion 3D printing technique, which combines the strengths of FDM and UV assisted 3D printing technology is demonstrated in this paper. By using photopolymer extrusion in combination with two additional rotational axes, the printer developed in this work not only allows the traditional layer upon layer printing, but is also capable of free form printing. Fumed silica is used as a filler in order to control the material viscosity for proper extrusion and curing. Mechanical tests were conducted on objects printed using different concentrations of filler in the photopolymer to understand its effect and determine the range of suitable filler concentration. Then, printing of free-form and self-supported structures is successfully demonstrated.
       
  • Reporting fidelity in the literature for computer aided design and
           additive manufacture of implants and guides
    • Abstract: Publication date: Available online 25 August 2018Source: Additive ManufacturingAuthor(s): Hanna E. Burton, Sean Peel, Dominic Eggbeer The aim of this study was to critically evaluate the nature and reporting fidelity of literature about applications of computer aided design (CAD) and metal additive manufacture (AM) to surgical guides and implants. Increasingly, non-specialist designers such as surgeons or prosthetists are partaking in some or all of the design process. To comply with local regulations, it is imperative that quality is ensured during the design process, yet it is rare for literature to report on the design process of medical devices with sufficient detail to allow proper evaluation or reproduction.This study reviewed the CAD/AM literature for implant and guide design, focussing on detailed justifications for design decisions, economic impacts, and production methods. This review showed that the fidelity of reporting in the literature was low; with opportunities to report crucial design decisions, engineering parameters, and how these relate to clinical results being frequently missed.This research proposes the low fidelity in reporting is likely due to a combination of: reporting for different specialisms, resulting in a lack of expert knowledge in certain areas and assumed knowledge in others; commercial sensitivity of design and manufacturing methods; low volume of clinical cases; and a large gap in translating research to clinical applications. This study concluded that higher fidelity in reporting methods are required when discussing the design of AM medical implants, which would allow comparisons between studies, provide evidence to support design quality, and enable evidence-based decision-making.
       
  • Microstructure evolution of 316L produced by HP-SLM (high power selective
           laser melting)
    • Abstract: Publication date: Available online 25 August 2018Source: Additive ManufacturingAuthor(s): Maria L. Montero-Sistiaga, Miguel Godino-Martinez, Kurt Boschmans, Jean-Pierre Kruth, Jan Van Humbeeck, Kim Vanmeensel New generation of selective Laser Melting (SLM) machines are evolving towards higher power lasers as well as multi laser systems in order to increase the productivity. The increase in laser power and the modification of the laser power distribution leads to microstructural and mechanical property variations that are still not well understood.This work aims at better understanding the interaction of a 1 kW top-hat power distribution laser on a well know material, 316 L stainless steel. The influence of texture and microstructure on relative density and crack density, when varying scan rotation, was evaluated. The high power (HP) laser and low power (LP) laser were compared with respect to microstructure and mechanical properties. HP leads to an increase in morphological and crystallographic texture together with a coarsening of the cell structure in contrast to the more random and finer cells found in LP processed material. Hot isostatic pressing was applied as a post-process treatment in order to close remaining pores and cracks. This helped in achieving higher elongations for LP and HP processed materials, while competitive mechanical properties to the 316 L material specifications were obtained in both cases.
       
  • Dual Process Monitoring of Metal-Based Additive Manufacturing Using Tensor
           Decomposition of Thermal Image Streams
    • Abstract: Publication date: Available online 25 August 2018Source: Additive ManufacturingAuthor(s): Mojtaba Khanzadeh, Wenmeng Tian, Aref Yadollahi, Haley R. Doude, Mark A. Tschopp, Linkan Bian Additive manufacturing (AM) processes are subject to lower stability compared to their traditional counterparts. The process inconsistency leads to anomalies in the build, which hinders AM’s broader adoption to critical structural component manufacturing. Therefore, it is crucial to detect any process change/anomaly in a timely and accurate manner for potential corrective operations. Real-time thermal image streams captured from AM processes are regarded as most informative signatures of the process stability. Existing state-of-the-art studies on thermal image streams focus merely on in situ sensing, feature extraction, and their relationship with process setup parameters and material properties. The objective of this paper is to develop a statistical process control (SPC) approach to detect process changes as soon as it occurs based on predefined distribution of the monitoring statistics. There are two major challenges: 1) complex spatial interdependence exists in the thermal images and current engineering knowledge is not sufficient to describe all the variability, and 2) the thermal images suffer from a large data volume, a low signal-to-noise ratio, and an ill structure with missing data. To tackle these challenges, multilinear principal component analysis (MPCA) approach is used to extract low dimensional features and residuals. Subsequently, an online dual control charting system is proposed by leveraging multivariate T2 and Q control charts to detect changes in extracted low dimensional features and residuals, respectively. A real-world case study of thin wall fabrication using a Laser Engineered Net Shaping (LENS) process is used to illustrate the effectiveness of the proposed approach, and the accuracy of process anomaly detection is validated based on X-ray computed tomography information collected from the final build offline.
       
  • A Modified Method for Estimating Inherent Strains from Detailed Process
           Simulation for Fast Residual Distortion Prediction of Single-Walled
           Structures Fabricated by Directed Energy Deposition
    • Abstract: Publication date: Available online 25 August 2018Source: Additive ManufacturingAuthor(s): Xuan Liang, Lin Cheng, Qian Chen, Qingcheng Yang, Albert To Predicting residual distortion in metal additive manufacturing (AM) is important to ensure quality of the fabricated component. The inherent strain method is ideal for this purpose, but has not been well developed for AM parts yet. In this paper, a modified inherent strain model is proposed to estimate the inherent strains from detailed AM process simulation of single line depositions on top of each other. The obtained inherent strains are employed in a layer-by-layer static equilibrium analysis to simulate residual distortion of the AM part efficiently. To validate the model, depositions of a single wall and a rectangular contour wall models with different number of layers deposited by a representative directed energy deposition (DED) process are studied. The proposed model is demonstrated to be accurate by comparing with full-scale detailed process simulation and experimental results. To make the method practical, a small-scale detailed simulation model is proposed to extract the mean inherent strains. Based on this approach, simulation results applied to the rectangular contour wall structures of different heights show that the modified inherent strain method is quite efficient, while the residual distortion of AM parts can be accurately computed within a short time. The improvement of the computational efficiency can be up to 80 times in some specific cases.
       
  • Assessment of the effect of isolated porosity defects on the fatigue
           performance of additive manufactured titanium alloy
    • Abstract: Publication date: Available online 23 August 2018Source: Additive ManufacturingAuthor(s): Romali Biswal, Abdul Khadar Syed, Xiang Zhang Studies on additive manufactured (AM) materials have shown that porosity reduces the fatigue strength. However, the quantitative impact is not well understood. This paper presents a mechanistic approach to quantify the influence of size, location and shape of gas pores on the fatigue strength of AM Ti-6Al-4 V. Ideal spherical and oblate spherical pore geometries were used in the finite element (FE) analysis. The FE results showed a stress concentration factor of 2.08 for an internal spherical pore, 2.1 for a surface hemispherical pore and 2.5 for an internal oblate spherical pore. Subsurface pores within a distance of the pore diameter from the free surface were found to be most critical. The material’s constitutive relation under the cyclic load was modelled by a mixed non-linear hardening rule that was calibrated with published literature on selective laser melted Ti-6Al-4 V. The cyclic plasticity effect caused a local mean stress relaxation, which was found to be dependent on the pore geometry, the applied stress amplitude and the stress ratio. Fatigue life was predicted by using the FE calculated local strain amplitude and maximum stress in the strain-life relationship proposed by Smith-Watson-Topper. The methodology was validated by published literature with crack initiation at gas pores of known size, location, and shape. Parametric study showed that for internal pores, fatigue performance is more sensitive to the shape and location of the pore than the size. An S-N curve was proposed by the parametric study to account for the fatigue strength reduction due to internal gas pores.
       
  • Alpha texture variations in additive manufactured Ti-6Al-4V investigated
           with neutron diffraction
    • Abstract: Publication date: Available online 18 August 2018Source: Additive ManufacturingAuthor(s): M. Neikter, R. Woracek, T. Maimaitiyili, Ch. Scheffzük, M. Strobl, M.-L. Antti, P. Åkerfeldt, R. Pederson, C. Bjerkén Variation of texture in Ti-6Al-4 V samples produced by three different additive manufacturing (AM) processes has been studied by neutron time-of-flight (TOF) diffraction. The investigated AM processes were electron beam melting (EBM), selective laser melting (SLM) and laser metal wire deposition (LMwD). Additionally, for the LMwD material separate measurements were done on samples from the top and bottom pieces in order to detect potential texture variations between areas close to and distant from the supporting substrate in the manufacturing process. Electron backscattered diffraction (EBSD) was also performed on material parallel and perpendicular to the build direction to characterize the microstructure. Understanding the context of texture for AM processes is of significant relevance as texture can be linked to anisotropic mechanical behavior. It was found that LMwD had the strongest texture while the two powder bed fusion (PBF) processes EBM and SLM displayed comparatively weaker texture. The texture of EBM and SLM was of the same order of magnitude. These results correlate well with previous microstructural studies. Additionally, texture variations were found in the LMwD sample, where the part closest to the substrate featured stronger texture than the corresponding top part. The crystal direction of the α phase with the strongest texture component was [112¯3].
       
  • On Anisotropy, Strain Rate and Size Effects in Vat Photopolymerization
           Based Specimens
    • Abstract: Publication date: Available online 18 August 2018Source: Additive ManufacturingAuthor(s): Dayakar L. Naik, Ravi Kiran The influence of build orientation, layer thickness, strain rate and size effect on the Young’s modulus, ultimate tensile strength and fracture strains in vat photopolymerization based additively manufactured specimens is investigated. Mechanical testing and subsequent scanning electron microscopy tests on additively manufactured specimens are conducted. Anisotropy in mechanical behavior is only observed in specimens fabricated in different planes. An increase in layer thickness and decrease in strain rate resulted in lower strength, stiffness and higher fracture strains. No significant size effect on strength and failure strains is observed. Cure kinetics is found to have significant influence on mechanical properties of additively manufactured specimens.
       
  • Real-Time Acoustic Emission Monitoring of Powder Mass Flow Rate for
           Directed Energy Deposition
    • Abstract: Publication date: Available online 18 August 2018Source: Additive ManufacturingAuthor(s): Justin Whiting, Adam Springer, Federico Sciammarella In order to ensure a reliable and repeatable additive manufacturing process, the material delivery rate in the directed energy deposition (DED) process requires in situ monitoring and control. This paper demonstrates acoustic emission (AE) sensing as a method of monitoring the flow of powder feedstock in a powder fed DED process. With minimal calibration, this signal closely correlates to the actual mass flow rate. This article describes the fabricated mass flow monitoring system, documents various conditions in which the actual flow rate deviates from its set value, and details situations that highlight the system’s utility. While AE mass flow monitoring is not free of concerns, its features make it an attractive measurement technique in the powder-fed DED process. The work presented here highlights the results obtained and illustrates that accurate monitoring of powder flow in real-time regardless of environmental conditions within the build chamber is possible.
       
  • Electron beam additive manufacturing of Inconel 718 alloy rods: Impact of
           
    • Abstract: Publication date: Available online 18 August 2018Source: Additive ManufacturingAuthor(s): Shi-Hai Sun, Yuichiro Koizumi, Tsuyoshi Saito, Kenta Yamanaka, Yun-Ping Li, Yujie Cui, Akihiko Chiba Inconel 718 alloy rods were fabricated by electron–beam melting (EBM), where the cylindrical axes (CAs) deviated from the build directions (BD) by 0°, 45°, 55°, and 90°. The microstructures and high-temperature tensile properties of the rods were investigated by taking into account the effect of the BD. Columnar grain structures or mixtures of columnar and equiaxed grains were obtained in the rods. The direction of the grains tended to be oriented not only in the BD but also in the two horizontal EB scanning directions (SDs), which were perpendicular to each other. As a result, the crystal orientation of the rods could be controlled by appropriate choice of the CA. For instance, the rods oriented along the  ,  , directions could be fabricated by choosing their CAs parallel to the BD, face diagonal, and space diagonal of the cubic space defined by the BD and the two SDs, respectively. The highest strength was obtained for the oriented rod. The dependence of strength on the rod orientation could be explained in terms of the anisotropies in the crystal orientation, columnar grain structure, and arrangement of the precipitate particles.
       
  • 3D Printing a Mechanically-Tunable Acrylate Resin on a Commercial DLP-SLA
           Printer
    • Abstract: Publication date: Available online 18 August 2018Source: Additive ManufacturingAuthor(s): Joseph Borrello, Philip Nasser, James Iatridis, Kevin D. Costa Multi-material 3D printing with several mechanically distinct materials at once has expanded the potential applications for additive manufacturing technology. Fewer material options exist, however, for additive systems that employ vat photopolymerization (such as stereolithography, SLA, and digital light projection, DLP, 3D printers), which are more commonly used for advanced engineering prototypes and manufacturing. Those material selections that do exist are limited in their capacity for fusion due to disparate chemical and physical properties, limiting the potential mechanical range for multi-material printed composites. Here, we present an ethylene glycol phenyl ether acrylate (EGPEA)-based formulation for a polymer resin yielding a range of elastic moduli between 0.6 MPa and 33 MPa simply by altering the ratio of monomer and crosslinker feedstocks in the formulation. This simple chemistry is also well suited to form seamless adhesions between mechanically dissimilar formulations, making it a promising candidate for multi-material DLP 3D printing. Preliminary tests with these polymer formulations indicate that variability due to molecular differences between hard and soft formulations is near net shape and less than 3% of the prescribed dimensions, comparable to existing commercial DLP and SLA resins, with unique advantages of a wide range of elastomer stiffness and seamless fusion for 3D printing of structurally detailed and mechanically heterogeneous composites.
       
  • Structural evolutions in 3D-printed Fe-based metallic glass fabricated by
           selective laser melting
    • Abstract: Publication date: Available online 18 August 2018Source: Additive ManufacturingAuthor(s): Di Ouyang, Wei Xing, Ning Li, Yicheng Li, Lin Liu The mechanisms of microstructural evolution in amorphous alloys during the selective laser melting process are crucial for modulating the properties of 3D-printed parts. Here, a map is constructed that illustrates the effect of laser energy density on structural evolution. Experiments combined with finite element method simulations reveal that high energy density leads to substantial crystallization. A mathematical model based on the phase transformation curves has been put forward to roughly predict the amorphous content in 3D-printed bulk metallic glasses (BMGs) under various process parameters. The present results provide guidance to optimize the process parameters to achieve desirable microstructures and properties of 3D-printed BMGs.
       
  • Comparison of the Effects of a Sulfuric Acid Environment on Traditionally
           Manufactured and Additive Manufactured Stainless Steel 316L Alloy
    • Abstract: Publication date: Available online 18 August 2018Source: Additive ManufacturingAuthor(s): Jacob T. Miller, Holly J. Martin, Edward Cudjoe The effects on the surface and mechanical properties of stainless steel AISI316L dogbones created using either traditional manufacturing (TM) or laser powder bed fusion (LPBF) exposed to 0.75 M sulfuric acid solution over 2184 hours were studied. General corrosion was not a major form of corrosion, based on surface feature changes, surface roughness, and mass loss for either method. No change to the mechanical properties occurred for the TM samples. Both tensile stress and strain decreased for the LPBF samples. The decrease was caused by hydrogen embrittlement, due to the formation of large brittle particles, as demonstrated by scanning electron microscopy.
       
  • Silica coated titanium using Laser Engineered Net Shaping for enhanced
           wear resistance
    • Abstract: Publication date: Available online 17 August 2018Source: Additive ManufacturingAuthor(s): Bryan Heer, Amit Bandyopadhyay Laser Engineered Net Shaping (LENSTM) was utilized to create novel silica (SiO2) coatings onto commercially-pure titanium (Cp-Ti). It was hypothesized that if silica could be deposited as a coating via laser surface engineering, high hardness and wear resistance could be added to existing Cp-Ti material. Post-deposition heat-treatments in the form of laser passes (LP) and a furnace residual stress-relief were completed on the coatings and mechanical/material properties were subsequently evaluated. Titanium silicide (Ti5Si3) formation and related dendritic microstructures were identified throughout the coating by x-ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopic (SEM) analysis, and appeared more ordered after stress-relief heat treatment. High hardness values of approximately 1500 HV were measured at the coating’s topmost surface while specific wear rates showed a maximum 98% reduction from 346.2 × 10-6 mm3/N-m in the Cp-Ti substrate to 7.1 × 10-6 mm3/N-m in the heat treated 1 L P coating. In situ tribofilm formation was observed during wear, which indicated self-healing properties from the material and likely aided further in wear reduction. Our results show that silica coating on titanium via laser surface engineering could be used as a suitable manufacturing practice to create hard, Ti5Si3-reinforced ceramic coatings with high wear resistance and self-healing properties for applications ranging from biomedical to aerospace.Graphical abstractGraphical abstract for this article
       
  • Properties of copper modified polyamide 12-powders and their potential for
           the use as laser direct structurable electronic circuit carriers
    • Abstract: Publication date: Available online 17 August 2018Source: Additive ManufacturingAuthor(s): Sandra Balzereit, Friedrich Proes, Volker Altstädt, Claus Emmelmann Three-dimensional molded interconnect devices (3D-MID) are commercially produced by the laser direct structuring (LDS)-method with a market share of over 50 %. The process chain of this method starts with injection molding. The polymer of this part is functionalized with LDS-additives which allow the part to be laser structured subsequently. This technique is less suitable for prototypes and small-scale productions of 3D-MIDs because of its properties. Contrary to the injection molding process, the additive manufacturing (AM), such as powder bed based manufacturing processes, e.g. selective laser sintering (SLS), is a constantly emerging processing technology for the fabrication of prototypes and small-scale productions. It enables the tool-free manufacturing of highly complex parts. Unmodified polyamide 12 (PA 12), e.g. PA2200 (supplier: EOS GmbH) is most commonly used for the SLS of polymer parts. The LPKF Laser & Electronics AG in Garbsen, Germany, transferred the LDS-method to SLS-process. A standard SLS-polymer part is coated with a special paint, that contains the necessary LDS-additives. Once coated and dried, these parts can be laser direct structured similar to standard 3D-MIDs. In this study, the authors use copper particles in order to functionalize a standard polyamide 12 powder for laser activation and selective metallization. The study shows, that the addition of copper particles enables the laser direct structuring of polyamide 12. SLS-demonstrators were successfully laser activated and selectively metallized. Furthermore, the copper particles enhance the mechanical properties as well as the heat conductivity of polyamide 12.
       
  • An interlaboratory comparison of X-ray computed tomography measurement for
           
    • Abstract: Publication date: Available online 16 August 2018Source: Additive ManufacturingAuthor(s): Andrew Townsend, Radu Racasan, Richard Leach, Nicola Senin, Adam Thompson, Andrew Ramsey, David Bate, Peter Woolliams, Stephen Brown, Liam Blunt This paper presents the results of the CT-STARR (CT-Surface Texture for Additive Round Robin) Stage 1 interlaboratory comparison. The study compared the results obtained for the extraction of areal surface texture data per ISO 25178-2 from five X-ray computed tomography (CT) volume measurements from each of four laboratories. Two Ti6Al4V ELI (extra low interstitial) components were included in each of the CT acquisitions. The first component was an additively manufactured (AM) cube manufactured using an Arcam Q10 electron beam melting (EBM) machine. Surface texture data was extracted from CT scans of this part. The values of selected parameters per ISO 25178-2 are reported, including Sa, the arithmetic mean height, for which the values from the Nikon MCT 225 metrology CT measurements were all within 0.5% of the mean reference focus variation measurement. CT resolution requirements are discussed. The second component was a machined dimensional test artefact designed to facilitate independent analysis of CT global voxel scaling errors and surface determination errors. The results of mathematical global scaling and surface determination correction of the dimensional artefact data is reported. The dimensional test artefact errors for the XT H 225 commercial CT for length, outside diameter and inside diameter reduced from -0.27%, -0.83% and -0.54% respectively to less than 0.02% after performing mathematical correction. This work will assist the development of surface texture correction protocols, help define surface-from-CT measurement envelope limits and provide valuable information for an expanded Stage 2 interlaboratory comparison, which will include a more diverse range of CT systems and technologies, further expanding the surface-from-CT knowledge base.
       
  • Effect of oxygen content in new and reused powder on microstructural and
           mechanical properties of Ti6Al4V parts produced by Directed Energy
           Deposition
    • Abstract: Publication date: Available online 10 August 2018Source: Additive ManufacturingAuthor(s): Jean Nicolas Rousseau, Alexandre Bois-Brochu, Carl Blais Some AM processes such as directed energy deposition (DED) have typical powder usage efficiencies ranging between 40 and 80 %. Since, for a given alloy, powder cost is proportional to its purity, choosing a less expensive powder or reusing powders is interesting for economical and environmental reasons. The work summarized below studied the effect of oxygen content in Ti6Al4V powders on mechanical properties of AM parts fabricated by DED. Three different powders with increasing oxygen content were used to produce specimens and characterize its effect on microstructure and tensile properties before and after heat treatment. Our results show that no clear oxygen pick up was measured from either powder recycling or during printing of parts. Only coarsening of the particle size distribution and the presence of fragmented particles was observed for the recycled powder. Comparing the chemistry of parts vs that of powder feedstock it was determined that for all the tests, the Al content was slightly lower in the parts and that no significant loss of vanadium was noted when printing with new (fresh) powders. On the other hand, V loss was significant in parts made with recycled powders, although still leaving them within acceptable chemistry to respect their original grade 5 classification.
       
  • Additive manufacturing of ceramic nanopowder by direct coagulation
           printing
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): Tucker J. Hensen, Trevor G. Aguirre, Corson L. Cramer, Austin S. Wand, Kaka Ma, David A. Prawel, John D. Williams, Troy B. Holland This work investigates the feasibility of a binderless, extrusion-based additive manufacturing approach to fabricate alumina (Al2O3) parts from nanopowder. Traditional manufacture of ceramics with subtractive methods is limited due to their inherent hardness and brittleness, inevitably leading to ceramic parts with less-than-optimal geometries for the specific application. With an additive manufacturing approach, ceramic parts with complex 3D geometries, including overhangs or hollow enclosures, become possible. These complex ceramic parts are highly valuable in heat exchanger, condenser, biomedical implant, chemical reactant vessel, and electrical isolation applications. This research employed direct coagulation of alumina nanopowder slurries with the polyvalent salt tri-ammonium citrate providing the solidification mechanism in an extrusion-based printing process. The viscosity of the slurries was adjusted from ∼35 Pa-s to ∼1000 Pa-s by adjusting pH from ∼9 to ∼4, resulting in a paste that is suitable for extrusion, which retains near-net geometry. It was shown that the direct coagulation approach can be used to create a suspension with tuneable flow characteristics and coagulation rate, and a mechanism describing the process was proposed. The direct coagulation printing (DCP) method is described in detail, including how slurry is extruded, solidified, and printed in complex geometries, and sintered to full density. Parts were printed with a sintered resolution of 450 μm and green densities as high as 65%. After sintering at 1550 °C for up to 2 h, parts were shown to be fully dense (>97%) with an average grain size of ∼2 μm. Mechanical properties were characterized with a comparison to different materials and methods from literature, showing hardness and flexural modulus up to ∼1800 HV and 400 GPa, respectively.Graphical abstractGraphical abstract for this article
       
  • laser sintering process oF Ceramic powders: the effect of particle size on
           the mechanical properties of sintered layers
    • Abstract: Publication date: Available online 9 August 2018Source: Additive ManufacturingAuthor(s): Daniele Sofia, Diego Barletta, Massimo Poletto Selective Laser Sintering (SLS) of ceramic powders is studied in order to understand how the initial material properties and the process conditions affect the degree of sintering/melting and the mechanical properties of the sintered material. Unimodal powder samples of different narrow particle size distributions between 16 and 184 μm were sintered with a 40 W CO2 laser, using laser scan speeds of either 50 or 100 mm s-1 and, in both cases, a scanning energy of 160 J m-1. The sintered material was studied by means of optical and SEM microphotography and characterized in terms of bulk density and tensile strength. The Rumpf approach to relate interparticle forces to the strength of powder agglomerates was used in this work to estimate the average strength of the sintered interparticle contacts starting from the tensile strength of specimens. In turn, the average strength of the neck contact was used to estimate the size of the neck of fused material between two sintered particles. These data coupled with the Frenkel model for particle sintering allowed an estimate of the sintering temperature for the different experimental conditions tested. The temperatures found are consistent with the glass transition temperature of the material used. The effect of particle size and scanning speed is assessed and discussed.
       
  • An implementation of real-time feedback control of cured part height in
           Exposure Controlled Projection Lithography with in-situ interferometric
           measurement feedback
    • Abstract: Publication date: Available online 9 August 2018Source: Additive ManufacturingAuthor(s): Xiayun Zhao, David W. Rosen Exposure Controlled Projection Lithography (ECPL) is an in-house additive manufacturing process that can cure microscale photopolymer parts on a stationary transparent substrate with a time sequence of patterned ultraviolet beams delivered from underneath. An in-situ interferometric curing monitoring and measurement (ICM&M) system has been developed to measure the ECPL process output of cured height profile. This study develops a real-time feedback control system that utilizes an empirical process model and an online ICM&M feedback to automatically and accurately cure a part with targeted height. Due to the nature of photopolymerization, the total height of an ECPL cured part is divided into exposure cured height and dark cured height. The exposure cured height is controlled by a closed-loop feedback on-off controller. The dark cured height is compensated by an empirical process model obtained from the ICM&M measurements for a series of cured parts. A parallel computing software application is developed to implement the real-time measurement and control simultaneously. The experimental results directly validate the ICM&M system’s real-time capability in capturing the process dynamics and in sensing the process output. Meanwhile, it evidently demonstrates the feedback control system’s satisfactory performance in achieving the setpoint of total height, despite the presence of ECPL process uncertainties, ICM&M noises and computing interruptions. A comprehensive error analysis is reported, implying a promising submicron control with enhanced hardware. Generally, the study establishes a paradigm of improving additive manufacturing with a real-time closed-loop measurement and control system.
       
  • Texture Analysis of Additively Manufactured Ti-6Al-4V using Neutron
           Diffraction
    • Abstract: Publication date: Available online 9 August 2018Source: Additive ManufacturingAuthor(s): A. Pesach, E. Tiferet, S.C. Vogel, M. Chonin, A. Diskin, L. Zilberman, O. Rivin, O. Yeheskel, E.N. Caspi This study presents a detailed characterization of room temperature bulk microstructure and texture of additively manufactured Ti-6Al-4 V alloy samples with the neutron time-of-flight diffractometer HIPPO. A comparison is made between samples that were manufactured by two different methods utilizing selective laser melting and electron beam melting. Analysis of the orientation distribution function shows a dependency upon the particular fabrication technique used as well as on the location within the built body and orientation relative to the build direction. It is shown that the texture components strength in the hexagonal phase depends on the relative tilt angle between the build direction and that the overall texture of samples prepared with the electron beam method is weaker than those prepared with the selective laser melting. Such knowledge on the bulk microstructure allows to optimize additive manufacturing process parameters.
       
  • Nozzle Clogging Factors During Fused Filament Fabrication of Spherical
           Particle Filled Polymers
    • Abstract: Publication date: Available online 7 August 2018Source: Additive ManufacturingAuthor(s): Tobias Beran, Tom Mulholland, Frank Henning, Natalie Rudolph, Tim A. Osswald Fused filament fabrication with reinforced or filled polymers provides improved material properties compared to ordinary feedstock. A current limitation of these materials is the occurrence of nozzle clogging at higher filler contents. In this work, an experiment is designed to identify the factors causing nozzle clogging. Glass sphere-filled polycarbonate is investigated by varying nozzle and filler diameters, the resin viscosity, the filler content, and the extrusion pressure. Equations identifying nozzle clogging and intermittent clogging conditions are provided. Based on these results, a model for the clogging of sphere-filled polymers is proposed. Last, a mathematical model is derived, which approximates the printability of filled polymers without the preparation of composites. This model is based on the nozzle geometry, the filler type and content, the resin viscosity, and the printer’s maximum extrusion force.
       
  • Simulation of Buckling of Internal Features during Selective Laser
           Sintering of Metals
    • Abstract: Publication date: Available online 7 August 2018Source: Additive ManufacturingAuthor(s): Luis S. Santos, Satyandra K. Gupta, Hugh A. BruckABSTRACTAdditive manufacturing (AM) processes, such as Selective Laser Sintering (SLS), have enabled the fabrication of geometrically complicated designs. However, undesired distortions due to thermally-induced residual stresses may lead to loss of tolerance or failure of the part. One potential failure mode is buckling, particularly when realizing high aspect ratio features, like for infill, to minimize weight. In this paper, we address distortions and part failures due to buckling by using a finite element model to predict residual stress distributions and sintering induced distortions. Initially, we conduct a transient thermal simulation to determine the Heat Affected Zone (HAZ), which is then used in the thermomechanical simulation. In addition, we imposed perturbations on the mechanical mesh based on the buckling eigenmodes. Finally, a thermomechanical viscoplastic analysis was performed layer-by-layer to obtain the final residual stress state and subsequent distortions that occur after cooling down to ambient temperature. A model was used to describe the evolution of porosity due to laser sintering, and then a model of the effects of porosity on the viscoplastic constitutive properties of the sintered material was used in the thermomechanical simulation. Modeling results are compared against experimental specimens using a Durelli (aka, Theta) specimen geometry fabricated with a 3D Systems ProX 200 Selective Laser Sintering (SLS) machine. The geometry of the specimen represents an internal feature with a high aspect ratio that is prone to buckling, and the dimensions were modified based on the simulation results to confirm the ability of the modeling approach to provide accurate mitigation of buckling-induced distortions.
       
  • Investigation of process-structure-property relationships in polymer
           extrusion based additive manufacturing through in situ high speed imaging
           and thermal conductivity measurements
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): Darshan Ravoori, Lorenzo Alba, Hardikkumar Prajapati, Ankur Jain Additive manufacturing has gained significant research attention due to multiple advantages over traditional manufacturing technologies. A fundamental understanding of the relationships between process parameters, microstructure and functional properties of built parts is critical for optimizing the additive manufacturing process and building parts with desired properties. This is also critical for a multi-functional part where the process needs to be optimized with respect to disparate performance requirements such as mechanical strength and thermal conductivity. This paper presents in situ high speed imaging and build-direction thermal conductivity measurements of polymer extrusion based additively manufactured parts in order to understand the effect of process parameters such as raster speed, infill percentage and layer height on build-direction thermal conductivity. Measurements of thermal conductivity using a one-dimensional heat flux method are correlated with in situ process images obtained from a high speed camera as well as cross section images of the built part. Results indicate strong dependence of build-direction thermal conductivity on raster speed, layer thickness and infill percentage, which is corroborated by high speed imaging of the printing process at different values of these process parameters. Key trade-offs between process throughput and thermal properties are also identified. In addition to enhancing our fundamental understanding of polymer extrusion based additive manufacturing and its influence on thermal properties of built parts, results presented here may facilitate process optimization towards parts with desired thermal and multi-functional properties.
       
  • The influence of post-production heat treatment on the multi-directional
           properties of nickel-aluminum bronze alloy fabricated using wire-arc
           additive manufacturing process
    • Abstract: Publication date: Available online 6 August 2018Source: Additive ManufacturingAuthor(s): Chen Shen, Zengxi Pan, Donghong Ding, Lei Yuan, Ning Nie, Dongzhi Luo, Dominic Cuiuri, Stephen van Duin, Huijun Li In this paper, a nickel-aluminum bronze alloy component is built using wire-arc additive manufacturing process. In order to investigate the influence of anisotropy introduced by the wire-arc additive manufacturing process, the layer-by-layer manufactured components with different post-production heat treatments are characterized by optical and scanning electron microscopy morphologies, X-ray diffraction and mechanical tests in longitudinal, transverse and normal directions. The experimental results show that the deposit exhibits higher strengths in the longitudinal and transverse direction than in the normal direction. Also, the ductility of the alloy is significantly improved with the designed quenching and tempering method, and competitive mechanical properties are achieved when tempering temperature reaches 650 °C. In addition, the anisotropy in the additively manufactured alloy can be effectively modified by the quenching and tempering heat treatments.Graphical abstractGraphical abstract for this article
       
  • Microstructure and mechanical properties of X65MoCrWV3-2 cold-work tool
           steel produced by selective laser melting
    • Abstract: Publication date: Available online 4 August 2018Source: Additive ManufacturingAuthor(s): J. Boes, A. Röttger, C. Mutke, C. Escher, W. Theisen In this study, martensitic cold-work tool steel X65MoCrWV3-2 was processed by selective laser melting (SLM) by varying the laser scanning parameters and baseplate preheating temperatures. Porosity as well as crack density of the SLM-densified steel were determined by quantitative image analysis. The resulting microstructure and the associated local mechanical properties were characterized, and the hardness-tempering behavior of the SLM-densified steel was compared to the behavior of the conventionally manufactured X65MoCrWV3-2 steel in the cast and hot-formed condition. Regardless of the preheating temperature, SLM-densified X65MoCrWV3-2 possesses a porosity of less than 0.5 vol.-%. The crack density was reduced significantly by means of a higher preheating temperature. The microstructure after SLM densification shows a fine, equiaxed cellular-dendritic subgrain structure, superimposed by lath- or needle-like martensite. The martensite morphology appeared to be finer at a lower preheating temperature, whereas the observed subgrain structure did not seem to be influenced by the preheating temperatures. Microhardness measurements indicated tempering effects in first solidified layers caused by the densification of subsequently deposited layers. Peak hardness after tempering of the SLM-densified steel was found to be higher compared to the maximum hardness in the X65MoCrWV3-2 steel in the cast condition.
       
  • Effects of heat accumulation on microstructure and mechanical properties
           of Ti6Al4V alloy deposited by wire arc additive manufacturing
    • Abstract: Publication date: Available online 4 August 2018Source: Additive ManufacturingAuthor(s): Bintao Wu, Zengxi Pan, Donghong Ding, Dominic Cuiuri, Huijun LiABSTRACTComplex thermal behaviour during fabrication plays an import role in the geometrical formation and mechanical properties of Ti6Al4V components manufactured using Wire Arc Additive Manufacturing (WAAM) technology. In this study, through in-situ temperature measurement, the heat accumulation and thermal behaviour during the gas tungsten wire arc additive manufacturing (GT-WAAM) process are presented. The effects of heat accumulation on microstructure and mechanical properties of additively manufactured Ti6Al4V parts were studied by means of optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and standard tensile tests, aiming to explore the feasibility of fabricating Ti6Al4V parts by GT-WAAM using localized gas shielding. The results show that due to the influences of thermal accumulation, the layer’s surface oxidation, microstructural evolution, grain size, and crystalline phase vary along the building direction of the as-fabricated wall, which creates variations in mechanical properties and fracture features. It has also been found that it is necessary to maintain the process interpass temperature below 200 °C to ensure an acceptable quality of Ti6Al4V part fabricated using only localized gas shielding in an otherwise open atmosphere. This research provides a better understanding of the effects of heat accumulation on targeted deposition properties during the WAAM process, which will benefit future process control, improvement, and optimization.
       
  • Energy absorption characteristics of metallic triply periodic minimal
           surface sheet structures under compressive loading
    • Abstract: Publication date: Available online 4 August 2018Source: Additive ManufacturingAuthor(s): Lei Zhang, Stefanie Feih, Stephen Daynes, Shuai Chang, Michael Yu Wang, Jun Wei, Wen Feng Lu Designing metallic cellular structures with triply periodic minimal surface (TPMS) sheet cores is a novel approach for lightweight, multi-functional structural applications. Different from current honeycombs and lattices, TPMS sheet structures are composed of continuous and smooth shells, allowing for large surface areas and continuous internal channels. In this paper, we investigate the mechanical properties and energy absorption abilities of three types of TPMS sheet structures (Primitive, Diamond, and Gyroid) fabricated by selective laser melting (SLM) with 316 L stainless steel and classify their failure mechanisms and printing accuracy with the help of numerical analysis. The results reveal that the properties and deformation mechanisms strongly depend on the unit cell geometry. TPMS sheet structures are found to exhibit superior stiffness, plateau stress and energy absorption ability compared to body-centred cubic lattices, with Diamond-type sheet structures performing best. Linear and post-yielding mechanical behaviour of TPMS sheet structures as predicted by explicit finite element models is in good agreement with experimental results. The simulation results also show that Diamond and Gyroid sheet structures display relatively uniform stress distributions across all lattice cells under compression, leading to stable collapse mechanisms and desired energy absorption performance. In contrast, P-type structures display rapid diagonal shear band development followed by localized wall buckling. Lastly, an energy absorption diagram is developed to facilitate a systematic way to select optimal densities of TPMS structures for energy absorbing applications.
       
  • On the influence of laser defocusing in Selective Laser Melting of 316L
    • Abstract: Publication date: Available online 4 August 2018Source: Additive ManufacturingAuthor(s): Jitka Metelkova, Yannis Kinds, Karolien Kempen, Charlotte de Formanoir, Ann Witvrouw, Brecht Van Hooreweder Despite its many benefits, Selective Laser Melting's (SLM) relatively low productivity compared to deposition-based additive manufacturing techniques is a major drawback. Increasing the laser beam diameter improves SLM's build rate, but causes loss of precision. The aim of this study is to investigate laser beam focus shift, or “defocus”, using a dynamic focusing unit, in order to increase the laser spot size. When applied to the SLM process, focus shift can be integrated into a “hull-core” strategy. This involves scanning the core with a high productivity parameter set using defocus while enabling return to the focused smaller spot size position for hull scanning. To assess the process stability, single line scans were made from 316L stainless steel powder. The consolidated melt pool morphology was analyzed and correlated with the process parameters comprising laser power, scanning speed and defocus distance. In order to link the melt pool morphology with the heat input, Volumetric Energy Density, Normalized Enthalpy and Rosenthal equation were considered. The suitability of using the Normalized Enthalpy as a design parameter to predict the melt pool depth and Rosenthal equation to predict its width was highlighted. This study shows that within a single laser setup, implementing defocus can lead to a potential productivity increase by 840 %, i.e. to 18.8 mm3/s.Graphical abstractGraphical abstract for this article
       
  • Columnar to equiaxed transition during direct metal laser sintering of
           AlSi10Mg alloy: effect of building direction
    • Abstract: Publication date: Available online 2 August 2018Source: Additive ManufacturingAuthor(s): Amir Hadadzadeh, Babak Shalchi Amirkhiz, Jian Li, Mohsen Mohammadi In the current study, cylindrical samples of AlSi10Mg alloy were fabricated using direct metal laser sintering (DMLS) technique in vertical and horizontal directions. The microstructure of the samples was analyzed using scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. It was observed that, by changing the building direction from vertical to horizontal, columnar to equiaxed transition (CET) occurred in the alloy. While 75% of the grains in the vertical sample were columnar, by changing the direction to horizontal, 49% of the grains evolved with columnar shape and 51% of them were equiaxed. Moreover, the texture of DMLS-AlSi10Mg alloy changed due to CET. While {001} fiber texture evolved in the vertical sample, the direction tilted away from the building direction in the horizontal one. Using the fundamentals of solidification and constitutional undercooling, the solidification behavior of AlSi10Mg alloy during DMLS process was modeled. It was observed that, the determinant parameter in CET during DMLS of AlSi10Mg alloy is the angle between the nominal growth rate and direction of the growing dendrite, which is controlled by the geometry and building direction of the sample. Further TEM studies confirmed that, CET alters the shape and coherency of Si precipitates and dislocation density inside the α-Al dendrites in DMLS-AlSi10Mg alloy.
       
  • 3D printed hierarchically-porous microlattice electrode materials for
           exceptionally high specific capacity and areal capacity lithium ion
           batteries
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): Mohammad Sadeq Saleh, Jie Li, Jonghyun Park, Rahul Panat Despite the enormous potential of additive manufacturing in fabricating three-dimensional battery electrodes, the structures realized through this technology are mainly limited to the interdigitated geometries due to the nature of the manufacturing process. This work reports a major advance in 3D batteries, where highly complex and controlled 3D electrode architectures with a lattice structure and a hierarchical porosity are realized by 3D printing. Microlattice electrodes with porous solid truss members (Ag) are fabricated by Aerosol Jet 3D printing that leads to an unprecedented improvement in the battery performance such as 400% increase in specific capacity, 100% increase in areal capacity, and a high electrode volume utilization when compared to a thin solid Ag block electrode. Further, the microlattice electrodes retain their morphologies after 40 electrochemical cycles, demonstrating their mechanical robustness. These results indicate that the 3D microlattice structure with a hierarchical porosity enhances the electrolyte transport through the electrode volume, increases the available surface area for electrochemical reaction, and relieves the intercalation-induced stress; leading to an extremely robust high capacity battery system. Results presented in this work can lead to new avenues for improving the performance of a wide range of electrochemical energy storage systems.Graphical abstractGraphical abstract for this article
       
  • Micromechanical analysis of the effective properties of lattice structures
           in additive manufacturing
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): José Souza, Alexander Großmann, Christian Mittelstedt Lightweight design is an area of mechanical engineering that becomes increasingly important in many industries, as they pursue reduced mass and more efficient parts. A special class of materials for load-bearing structures are metallic cellular materials with cubic unit cells, which can be manufactured conveniently through laser beam melting (LBM). Such materials exhibit a rather complex microstructure and can be analysed using analytical and numerical methods wherein the determination of properties such as relative density, effective elastic and yield strength properties is of special interest. This paper addresses closed-form analytical methods based on beam theories for the determination of the effective properties of additively manufactured microstructures such as lattices, and a comparison with experimental results [1], [2] which leads to excellent agreements for relative densities lower than 40%, although results reveal a great dependency on the manufacturing strategy. Lastly, a classification concerning the topology of the cellular units is presented as well in order to help the engineer choose appropriate geometries for specific application purposes. In conclusion, this structural concept may be applied in many fields such as bioengineering and in functional graded materials as they are applied in lightweight engineering.
       
  • Chemical Compatibility of Fused Filament Fabrication-based 3-D Printed
           Components with Solutions Commonly Used in Semiconductor Wet Processing
    • Abstract: Publication date: Available online 30 July 2018Source: Additive ManufacturingAuthor(s): Ismo T.S. Heikkinen, Christoffer Kauppinen, Zhengjun Liu, Sanja M. Asikainen, Steven Spoljaric, Jukka V. Seppälä, Hele Savin, Joshua M. Pearce 3-D printing shows great potential in laboratories for making customized labware and reaction vessels. In addition, affordable fused filament fabrication (FFF)-based 3-D printing has successfully produced high-quality and affordable scientific equipment, focusing on tools without strict chemical compatibility limitations. As the additives and colorants used in 3-D printing filaments are proprietary, their compatibility with common chemicals is unknown, which has prevented their widespread use in laboratory chemical processing. In this study, the compatibility of ten widely available FFF plastics with solvents, acids, bases and solutions used in the wet processing of semiconductor materials is explored. The results provide data on materials unavailable in the literature and the chemical properties of 3-D printable plastics that were, are in line with literature. Overall, many 3-D printable plastics are compatible with concentrated solutions. Polypropylene emerged as a promising 3-D printable material for semiconductor processing due to its tolerance of strongly oxidizing acids, such as nitric and sulfuric acids. In addition, 3-D printed custom tools were demonstrated for a range of wet processing applications. The results show that 3-D printed plastics are potential materials for bespoke chemically resistant labware at less than 10% of the cost of such purchased tools. However, further studies are required to ascertain if such materials are fully compatible with clean room processing.
       
  • Strengthening mechanisms in direct metal laser sintered AlSi10Mg:
           Comparison between virgin and recycled powders
    • Abstract: Publication date: Available online 30 July 2018Source: Additive ManufacturingAuthor(s): Amir Hadadzadeh, Carter Baxter, Babak Shalchi Amirkhiz, Mohsen Mohammadi Rod shaped samples of AlSi10Mg additively manufactured using recycled powder through direct metal laser sintering (DMLS) process showed higher quasi-static uniaxial tensile strength in both horizontal and vertical build directions than those of cast counterpart alloy. In addition, they offered mechanical properties within the range of other additively manufactured counterparts. TEM showed that the microstructure of the as-built samples comprised of cell-like structures featured by dislocation networks and Si precipitates. HRTEM studies revealed the semi-coherency characteristics of the Si precipitates. After deformation, the dislocation density increased as a result of generation of new dislocations due to dislocation motion. The dislocations bypassed the precipitates by bowing around them and penetrating the semi-coherent precipitates. Strengthening of recycled DMLS-AlSi10Mg alloys manufactured in both directions was attributed to Orowan mechanism (due to existence of Si precipitates), Hall-Petch effect (due to eutectic cell walls), and dislocation hardening (due to pre-existing dislocation networks). Due to the slightly different microstructure, the contribution of each strengthening mechanism was slightly different in identical samples made with virgin powder.
       
  • Microstructure and micro-texture evolution of additively manufactured
           β-Ti alloys
    • Abstract: Publication date: Available online 26 July 2018Source: Additive ManufacturingAuthor(s): S.A. Mantri, R. Banerjee This study focuses on the microstructural evolution in additively manufactured (AM) β titanium alloys due to solid-state phase transformations occurring during the reheating of previously deposited layers, directly influencing the uniformity of microstructure across the entire build. During the AM of titanium alloys of a wide variety of compositions, including α + β alloys such as Ti-6Al-4 V, and β alloys, when the laser or electron beam hits the sample, the grains in the previously deposited topmost layers either re-melt or transform into the β phase. Subsequently, during the cooling cycle, depending on the alloy composition, second-phase precipitation may occur within these layers via solid-state precipitation. The present study compares two binary β -Ti alloys, Ti-12Mo and Ti-20 V, that have been processed using laser engineered net shaping (LENS™), a directed energy deposition technique for AM. Compared to Ti-V, which exhibited grains of only the β phase in the as-built condition, the less β stabilized Ti-Mo had extensive second-phase α precipitation within the build. The location within the LENS™ build played a pivotal role in determining the size scale, area fraction, and morphology of the α precipitates. These changes have been attributed to the different thermal cycles experienced during the deposition process. Irrespective of the alloy composition, columnar grains were observed in the depositions with a strong [001]β texture along the build direction. In the Ti-12Mo alloy, wherein second phase α precipitation takes place, there was no significant α texturing, with all twelve variants forming.
       
  • Large-format Polymeric Pellet-Based Additive Manufacturing for the Naval
           Industry
    • Abstract: Publication date: Available online 25 July 2018Source: Additive ManufacturingAuthor(s): Daniel Moreno Nieto, Victor Casal López, Sergio Ignacio Molina Polymeric Pellet-Based Additive Manufacturing (PPBAM) systems are increasing in the field of 3D printing as a result of the evolution of additive technologies as their development process consolidates and expands. New opportunities for industrial integration of additive manufacturing (AM) technologies are identified, including AM of large polymeric parts. The PPBAM process consists of adapting a pellet-fed extrusion mechanism to a displacement system, either a Cartesian mechanism or a robotic arm system, building parts in a multi-layered approach. This use is justified by the extruded filament sizes required and the material costs when facing large-format prints. In this article, a pellet extrusion based printer prototype is presented together with a case study. The case study consists of the development of a two cubic meter capacity plastic part for the naval industry with a topology optimization design approach and material selection and validation methodology for a large-volume pellet based extrusion system. Two functional prototypes were developed with the selected materials from the explained methodology a PLA and a flame retardant ABS, and post processed to full fill the actual product´s specifications.
       
  • Thickness dependency of mechanical properties of laser-sintered polyamide
           lightweight structures
    • Abstract: Publication date: October 2018Source: Additive Manufacturing, Volume 23Author(s): David Tasch, Anna Mad, Robert Stadlbauer, Martin Schagerl Laser sintering (LS), as an additive manufacturing process for production of polymer structures, provides the possibility of directly manufacturing personalized, structural motorcycle components for motor sports. To create such lightweight structures, the wall thickness and position limits of the LS systems need to be investigated in detail. Appearing process-related flaws such as different amounts of crystallinity, surface roughness, and defects such as pores exhibit dimensions similar to the wall thickness. To study the process-related effects on the mechanical properties of 450 tensile test specimens in z-direction, the build areas of two LS systems were screened and a detailed wall thickness investigation was conducted. In addition, dynamic mechanical analysis, differential scanning calorimetry, and scanning electron microscopy for several wall thicknesses similar to the spot size were conducted. The investigations showed that the Young's moduli and ultimate tensile strengths of the produced specimens of the two commercial EOS systems, P396 and P770, are similar and evenly distributed. However, distinct differences were found in elongation at break. The scattering of mechanical properties is more in the specimens produced by P770 than in those produced by P396. The Poisson's ratio does not vary between thin- and thick-walled structures. Furthermore, structures with a thickness below 1 mm showed distinctive losses in stiffness, ultimate tensile strength, and elongation at break.
       
  • An Overview of Functionally Graded Additive Manufacturing
    • Abstract: Publication date: Available online 11 July 2018Source: Additive ManufacturingAuthor(s): Giselle Hsiang Loh, Eujin Pei, David Harrison, Mario D. Monzón Functionally Graded Additive Manufacturing (FGAM) is a layer-by-layer fabrication process that involves gradationally varying the material organisation within a component to achieve an intended function. FGAM establishes a radical shift from contour modelling to performance modelling by having the performance-driven functionality built directly into the material. FGAM can strategically control the density and porosity of the composition or can combine distinct materials to produce a seamless monolithic structure. This paper presents a state-of-art conceptual understanding of FGAM, covering an overview of current techniques that can enable the production of FGAM parts as well as identify current technological limitations and challenges. The possible strategies for overcoming those barriers are presented and recommendations on future design opportunities are discussed.
       
  • Multi-Material 3D Printing: the Relevance of Materials Affinity on the
           Boundary Interface Performance
    • Abstract: Publication date: Available online 11 July 2018Source: Additive ManufacturingAuthor(s): L.R. Lopes, A.F. Silva, O.S. Carneiro Multi-material extrusion in 3D printing is gaining attention due to a wide range of possibilities that it provides, specially driven by the commercial availability of a large variety of non-conventional filament materials. As a result, one can print models that are not limited to aesthetics purposes but can now also provide larger functionality, and therefore with mechanical performance tuned according to their purpose. With this in mind, this paper addresses the mechanical performance of multi-material printed objects, specially focused on the interface zone generated between the different materials at their geometrical boundaries. Tensile test specimens were designed and printed in three types: (A) a single-material specimen printed by a single extrusion head; (B) a single-material but multi-section specimen printed in a zebra-crossing structure by two extrusion heads; and (C) a multi-material specimen printed with two materials in a zebra-crossing pattern. The materials considered were PLA, TPU and PET. The comparison of the mechanical performance between Type-A and -B specimens demonstrated the negative influence of the presence of a geometrical boundary interface between the same material. On the other hand, the comparison between Type-B and -C demonstrated how the previous performance loss was yet more drastic when the lack of chemical affinity between the materials was present. The methodology proposed to assess the quality of the pairs of materials selected is innovative, and enabled to depict the importance of the boundary design in multi-material printing techniques.
       
  • Selective laser melting of rare earth element Sc modified aluminum alloy:
           Thermodynamics of precipitation behavior and its influence on mechanical
           properties
    • Abstract: Publication date: Available online 10 July 2018Source: Additive ManufacturingAuthor(s): Han Zhang, Dongdong Gu, Jiankai Yang, Donghua Dai, Tong Zhao, Chen Hong, Andres Gasser, Reinhart Poprawe The interest of selective laser melting (SLM) Al-based alloys for lightweight applications, especially the rare earth element Sc modified Al-Mg alloy, is increasing. In this work, high-performance Al-Mg-Sc-Zr alloy was successfully fabricated by SLM. The phase identification, densification behavior, precipitate distribution and mechnical properties of the as-fabricated parts at a wide range of processing parameters were carefully characterized. Meanwhile, the evolution of nanoprecipitation behavior under various scan speeds is revealed and TEM analysis of precipitates shows that a small amount of spherical nanoprecipitates Al3(Sc,Zr) were embedded at the bottom of the molten pool using a low scan speed. While no precipitates were found in the matrix using a relatively high scan speed due to the combined effects of the variation of Marangoni convection vector, ultrashort lifetime of liquid and the rapid cooling rate. An increased hardness and a reduced wear rate of 94 HV0.2 and 1.74 × 10-4 mm3N-1 m-1 were resultantly obtained respectively as a much lower scan speed was applied. A relationship between the processing parameters, the surface tension, the convection flow, the precipitation distribution and the resultant mechanical properties has been well established, demonstrating that the high-performance of SLM-processed Al-Mg-Sc-Zr alloy could be tailored by controlling the distribution of nanoprecipitates.
       
  • Determination of the Effect of Scan Strategy on Residual Stress in Laser
           Powder Bed Fusion Additive Manufacturing
    • Abstract: Publication date: Available online 9 July 2018Source: Additive ManufacturingAuthor(s): J. Robinson, I. Ashton, P. Fox, E. Jones, C. Sutcliffe Any literature investigation of Laser Powder Bed Fusion (L-PBF) manufacturing of metal parts would reveal that the development of internal stresses is a serious limitation in the application of this technology. Researchers have used a variety of different methods to quantify this stress and investigate scanning strategies aimed at reducing or distributing this stress more evenly in the part. The most common methods used to assess the levels of stress in parts are deflection based. These techniques provide a rapid method to give a quantitative comparison of scan strategies and parameters. Although studies have calculated the levels of stress relieved by the measured deflection, these studies often neglect the stresses that remain in the part after release. This study shows that these stresses can still be considerable. Non-destructive diffraction based methods can be used to calculate the profile of stress in a part but these are often prohibitively expensive or difficult to use on a large scale. This study presents a methodology which combines deflection based methods with either the hole drilling or contour methods. Results show that these experiments can be completed in a cost effective manner, with standard lab based equipment to generate a through thickness measurement of residual stress.
       
 
 
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