Influence of build characteristics and chamber oxygen concentration on powder degradation in laser powder bed fusion

International audiencePowder reuse is essential in laser powder bed fusion (L-PBF) to limit material waste and improve the process sustainability. However, complex laser-material interactions result in an inevitable alteration of the attributes of the recovered powder at the end of the fabrications. Powder degradation is influenced by numerous factors including the processing parameters but also the build topology. In this work, various build characteristics were studied to develop a better understanding of the effects on powder degradation, with the aim of minimizing it. LPBF prints with different melted volume fractions, part spacings and fabrication heights were conducted using stainless steel 316L (SS316L) material. Powders recovered on the build platform, as well as in zones of interest directly adjacent to solidified areas were characterized to investigate the changes in particles characteristics, with special regards to powder oxidation. In combination with these geometrical features, the effect of the oxygen concentration in the build chamber was also studied. The results allow to identify the relation between the oxygen content of recovered powder and the volume fraction of printed parts. It also highlights the presence of heat-affected zones in the near vicinity of the parts with a strong oxygen pick-up in a 500 mu m wide zone around the consolidated material. The results also show the significance of the oxygen concentration in the process atmosphere, with a less severe oxidation and fewer ejecta in the powder recovered after printing under 300 wppm O2 compared to 1000 wppm O2