Oxygen balance during laser powder bed fusion of Alloy 718

The generation of spatters during laser powder bed fusion (L-PBF) of Alloy 718 is known to be detrimental for the re-use of the feedstock powder and the quality of the final product. In this study, dedicated powder sampling from different positions within the build chamber and powder bed was performed. The results clearly indicate the importance of the surface-to-volume ratio of the built components on powder degradation, as demonstrated by the analysis of the powder using capsules filled with dense lattice structures. Extensive formation of Al- and Cr-rich oxides on the entrained powder deposited on the gas inlet and outlet was detected. Significant oxygen pick-up by spatter particles compared to the virgin powder was measured (>300 ppm O2); while the as-built material experienced a slight loss (~30 ppm O2). The change in the microstructure of spatter particles in comparison to the virgin powder, namely the primary dendrite arm spacing, indicates significantly higher cooling rate during spatter solidification, estimated to be of about 108 K/s, compared to around 106 K/s for the virgin powder and 107 K/s for the L-PBF component. These findings allows to evaluate the extent of powder degradation during L-PBF and establish the oxygen balance of the process. [Display omitted] •An important oxygen transfer from the virgin powder and from the environment to the spatters occurs•Some oxygen is lost from the virgin powder to the component (~30 ppm) and preferentially transferred to spatters•Degradation of Alloy 718 powder is observed as an increasing coverage of Al-/Cr-rich oxide features•Spatters ejected from the melt pool experience higher cooling rates (~108 K/s) than entrained particles (~106 K/s)•Components with high surface-to-volume ratio promote powder degradation and oxygen pick-up of the powder bed