Localized Changes of Stainless Steel Powder Characteristics During Selective Laser Melting Additive Manufacturing

In laser powder bed additive manufacturing processes, feedstock materials are often recycled after each build. Currently, a knowledge gap exists regarding powder reuse effects on powder size distribution, morphology, and chemistry as a function of part geometry and processing conditions. It was found during selective laser melting (SLM) of 316 stainless steel that a significant amount of (0.100 wt pct) oxygen pickup can occur in molten material (spatter) ejected from the powder bed surface. This value was significantly larger than the oxygen content of the as-received powder feedstock (0.033 wt pct). Furthermore, the powders in the heat-affected-zone regions, adjacent to molten pool, also exhibit oxygen pickup (≥ 0.043 wt pct). The oxygen content in unmelted 316L powder was found to vary as a function of its spatial position in the powder bed, relative to the heat source. Interestingly, the volume of melted material (i.e., thin vs thick walls) did not correlate well with the extent of oxygen pickup. Possible mechanisms for oxygen pickup in the powder during SLM, such as adsorption and breakdown of water, oxygen solubility, spatter re-introduction, and solid-state oxide growth, are discussed.