Understanding the effects of metal powder feedstock heterogeneity on the laser powder bed fusion process

Abstract

Powder reuse is important to reduce the cost and improve the sustainability of laser powder bed fusion (PBF-LB) additive manufacturing. Several powder reuse strategies involve the blending of unused feedstock powder with used powder, which assume that the bulk properties of blends are sufficient knowledge for decision making. Here we consider how potential chemical heterogeneity within a blend may occur locally in the dispenser (e.g., a relatively high fraction of one component of the blend compared to the expected ratio). This becomes particularly important when the usage histories of the constituent powders in the blend have significant differences. A set of experiments was designed to introduce controlled heterogeneities in the dispenser and assess the effects on the spreading process and printed parts. Specific layer-wise heterogeneities were created by switching back and forth between powder feedstocks (IN718 and CoCrMo) during a build, as an analogous but more easily measurable situation compared to mixing and tracking reused powders of the same alloy. The Co concentration was spatially mapped parallel to the build height for lightly sintered powder capture capsules and solidified parts to determine how these heterogeneities manifest in the process before and after laser melting. The melting process in PBF-LB was determined to cause significant elemental redistribution as opposed to the initial powder spreading process, which had little contribution. In every case, the starting inhomogeneity diluted in intensity but increased in spatial size to more than twice the programmed layer thickness.