Abstract
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 O 2 (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. • The spatter oxidation decreased with the residual oxygen content in the L -PBF atmosphere. • Significant changes in the surface oxide chemistry of spatter particles occurred as the residual oxygen varied. • The thickness of oxide phases decreased with the residual oxygen content. • The surface oxide was significantly enriched with Al and Cr as the oxygen level in the L -PBF atmosphere increased. • Thermodynamic calculations confirm preferential Al oxidation at lower oxygen levels compared to Cr.
Highlights
A paradigm shift is expected in the manufacturing industry due to the potential advancement granted by additive manufacturing (AM), as it allows the production of complex designs with limited material waste
Powder degradation during PBF has been evidenced for various alloy systems, during electron beam powder bed fusion (EB-PBF) because of the long-term exposure to high temperatures (> 1000 ◦C), despite the vacuum [1,3], and during laser powder bed fusion (L-PBF) with the formation of oxidized spatters [4,5,6,7,8]
The results highlight a significant in crease in oxygen content from the virgin powder to the spatter and a consistent oxygen gain with increasing residual oxygen in the process atmosphere, leading to an increase of up to 450 ppm of oxygen or a fourfold increase compared with the original powder feedstock
Summary
A paradigm shift is expected in the manufacturing industry due to the potential advancement granted by additive manufacturing (AM), as it allows the production of complex designs with limited material waste. It was established that characterization of the spatters at the gas inlet and outlet enabled tracking of the increase in the powder bed degradation, as these typically redeposit on the process area This indicates a potential to reduce the oxidation of spatter par ticles by reducing the partial pressure of oxygen in the L-PBF chamber using atmosphere purity control systems as presented elsewhere [6,17, 18]. A systematic approach was adopted to under stand the spatter oxidation mechanism and dominating oxidizing ele ments in relation to the residual oxygen content in the L-PBF chamber atmosphere For this purpose, the oxygen partial pressure in the process chamber was reduced from the commonly used 1000–20 ppm O2, and spatter samples were collected from the gas inlet at 1000, 800, 600, 400, and 20 ppm oxygen partial pressure. The analysis was further complemented by highresolution scanning electron microscopy and bulk chemical analysis
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