Spatter oxidation during laser powder bed fusion of Alloy 718: Dependence on oxygen content in the process atmosphere

Artikel i vetenskaplig tidskrift, 2021

In laser powder bed fusion (L-PBF), powder degradation is mainly driven by the accumulation of highly oxidized spatter particles in the powder bed. Although the amount of spattering can be controlled by the melt pool stability, spatter formation is an unavoidable characteristic of PBF processes. Oxidized spatter risks defect formation in the printed components. However, the factors influencing the level of spatter oxidation during L-PBF processing are not yet fully understood. Herein, the residual oxygen in the process atmosphere was reduced from the traditionally applied 1000–20 ppm using an oxygen partial pressure control system to process Alloy 718 powder. Spatter particles accumulated on the gas inlet were further analyzed to reveal the effect of the oxygen content in the process atmosphere on the spatter oxidation by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Increasing the residual oxygen in the process atmosphere increased surface coverage by oxide phases rich in Al and Cr. The XPS analysis confirmed that the surface of Alloy 718 spatter particles were covered with Al- and Cr-based oxides, whose thickness increased with the oxygen content in the process atmosphere. The bulk oxygen content in the spatter powder showed the same trend with approximately thrice the oxygen content in spatters generated at 1000 ppm O2 (608 ppm O in the sample) compared to spatters generated with oxygen at 20 ppm (206 ppm O in the sample). Thermodynamic simulations demonstrate a transition from thick Al- and Cr-based mixed corundum and spinel-type oxides to Al-based corundum oxide with decreasing oxygen partial pressure, consistent with the XPS findings.