Surface chemical analysis of spatter particles generated in laser powder bed fusion of Hastelloy X in process atmospheres with high and low oxygen content
Artikel i vetenskaplig tidskrift, 2023
Additive manufacturing, particularly laser powder bed fusion (LPBF), has received much attention in recent years because of its multiple benefits over traditional manufacturing. One of the key factors affecting the repeatability and performance of the materials is the existence of defects. Defects can be driven by process by-products called spatters, which consist of particles covered by an oxide layer formed during their travel time in the process atmosphere. As a standard process atmosphere consists of argon containing a residual oxygen level of around 0.1%, one possible way of addressing spatter-driven defect formation is by reducing the oxygen level, thereby reducing the oxidation of the spatter particles. In this study, Hastelloy X powder was processed by means of LPBF in an argon atmosphere containing 1000-ppm O2 or 50-ppm O2. Spatter particles were collected in a controlled manner, allowing sampling particles of different sizes, which were analyzed in terms of their surface chemical composition by means of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). By combining these two tools, a comprehensive assessment of the surface chemical composition was conducted, taking advantage of XPS for the evaluation of the overall surface chemistry and of Auger nanoprobe analysis for high lateral analytical resolution combined with depth profiling. It is shown that tighter oxygen control will both limit the overall oxidation and affect the surface chemical composition. At regular O2 level in the process atmosphere (1000 ppm), spatter particles are covered by a 70-nm-thick oxide layer, on average. The thickness is substantially greater than that measured in spatter particles collected from the process atmosphere containing 50-ppm oxygen, which averages 6 nm and is comparable with that of the virgin powder, thus revealing a potential for defect mitigation through control of the process atmosphere. Nonetheless, substantial differences in the surface chemical composition were identified between spatters and virgin powder, notably with the appearance of Al- and Ti-oxides on spatter particles, revealing the influence of the manufacturing process on surface characteristics.