Investigation of Powder Recyclability and Liquid-Solid-Gas Interactions during Powder Bed Selective Laser Melting of Stainless Steel 316L

This dissertation pertains to the fundamental understanding of powder degradation and the dissolution of gas, specifically interstitials such as oxygen, into metal alloy powders and parts during selective laser melting (SLM) additive manufacturing (AM) and their impact on defect generation, microstructure, and mechanical properties of parts built using this process. A powder recyclability experiment was conducted to investigate the effect that powder re-use has on bulk powder and build properties. Mechanical testing and analysis of parts produced during the experiment showed higher than normal yield strengths and provided evidence of powder re-use affecting the ductility of SLM parts. A heat affected zone experiment was conducted to investigate the local property changes in unmelted powder. Local oxidation in unmelted powder near melt zones was observed and characterized. The size of the melt zone adjacent to the unmelted powder bed was shown not to be a contributing factor to the extent of local oxidation. A variable oxygen environment experiment was conducted to investigate the effect of elevated levels of atmospheric oxygen on stainless steel 316L builds. Mechanical testing results indicate that the mechanical properties of additively manufactured 316L parts are insensitive to significant variation of cover gas O₂ concentration in the build chamber during processing. A cooling rate study was conducted in an effort to use laser processing parameters to manipulate the microstructure/mechanical properties of as-built parts. High-speed infrared (IR) data was analyzed and representative cooling rates of the parts and the surrounding powder bed as a function of build height were obtained. Finally, stainless steel 316L spatter material was observed to solidify as single-crystal ferrite. This is unusual behavior considering that the composition of stainless steel 316L usually causes it to solidify as austenite. The single-crystal nature of the ferritic spatter particles is particularly interesting and suggests that the particles solidify at rapid velocities and that conditions exist that allow for the nucleation of ferrite to outcompete that of the austenite phase.