Localized Changes of Stainless Steel Powder Characteristics During Selective Laser Melting Additive Manufacturing
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METAL additive manufacturing allows for the manufacture of complex, near net-shape parts. Powder bed additive manufacturing processes require a metal powder feedstock, as well as a substrate on which to build, and employ either an electron beam or laser heat source to melt the powder in a succession of stacked and fused layers. This research focuses on identifying and describing local powder bed degradation during the selective laser melting (SLM) process. It is well known in literature that laser parameter selection can influence the microstructure of SLM-processed material.[1–3] However, one of the economic advantages often attributed to powder bed additive manufacturing is that unmelted powder can be reused as
D. GALICKI is with the University of Tennessee Knoxville, 1512 Middle Drive, Knoxville, TN. Contact e-mail: [email protected] F. LIST is with the Oak Ridge National Laboratory, Oak Ridge, TN. S.S. BABU and A. PLOTKOWSKI are with the University of Tennessee Knoxville, 1512 Middle Drive, Knoxville, TN and also with the Oak Ridge National Laboratory, Oak Ridge, TN. H.M. MEYER III is with the Materials Science and Technology Division, Oak Ridge, TN. R. SEALS and C. HAYES are with the Y12-CNS Nuclear Complex, Oak Ridge, TN. Manuscript submitted April 3, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
feedstock for future builds. In reality, the recyclability of unmelted powders is not well understood for industrial usage due to the uncertainty over how powder degradation occurs and affects the properties of additively manufactured parts. As a result, the economic benefit from reusing powder is often avoided in industrial additive manufacturing processing facilities. Powder recyclability studies have been performed in an attempt to understand and quantitatively verify the physiochemical changes in unmelted powder as a function of powder reuse. Tang et al. and Nandwana et al. have studied recycled Ti-6Al-4V powders during electron beam powder bed processing.[4,5] Tang’s study incorporated 21 reuse cycles, with no change in part geometry or process conditions, and with periodic additions of fresh powder to the original reused powder bed. Nandwana’s study restricted the blending of new powders with reused powder and achieved six reuse cycles. Both studies incorporated the following measurements to characterize the bulk properties of the recycled powder beds: Hall flow; powder size distribution (PSD); elemental chemical analysis; and surface roughness. These studies revealed an increase in oxygen content within the Ti-6Al-4V powder and parts as a function of reuse cycles. The authors attributed the oxygen increase to the high oxygen solubility of titanium and the repeated exposure of the powder to atmospheric
conditions during part unloading, powder recovery, and loading efforts. Interestingly, studies on powder recyclability during selective laser melting are limited. Liu et al. studied stainless steel 316L spatter in selective laser melting and its effect on the macroscopic properties of tensile specimens
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