Influence of powder recycling on 316L stainless steel feedstocks and printed parts in laser powder bed fusion

In Laser Powder Bed Fusion (L-PBF) of metallic materials, costs and material yield strongly depend on the ability to reuse powder efficiently, as a significant amount is not solidified as part by the laser beam. However, some of the powder is nevertheless exposed to high temperatures during the manufacturing process resulting in an alteration of the feedstock properties if reused. Therefore, there is a need to study and understand powder degradation during the L-PBF process and its direct effects on the printed parts. In this study, gas-atomized 316 L stainless steel powder was used, recovered, sieved and reused up to 15 times in order to produce successive L-PBF prints without adding any virgin powder. Both recycled powders and elaborated parts were fully characterized at each iteration to investigate changes in particles (morphology, rheology, microstructure and chemical composition) as well as printed parts (porosity, microstructure, microhardness and tensile properties). Recycled powder exhibited larger particle size and an improved flowability. A gradual increase in oxygen content was observed, along with the presence of colored and oxidized particles, as well as magnetic particles. Parts density slightly decreased with powder reuse and their microstructure featured more numerous and finer grains along reuse cycles. On the other hand, no significant difference was found on the microhardness and the tensile properties of the L-PBF components.