Toward the Understanding of CoAl2O4 Additions on the Formation of Microstructure in Alloy 718 Processed by Laser Powder Bed Fusion

Abstract

This investigation rationalizes the impact of oxide particles on the melt pool and subsequent formation of microstructure in superalloy Inconel 718 (Alloy 718) processed by laser powder bed fusion (L-PBF). A mixture of Alloy 718 powder with 0.2 wt pct of CoAl2O4 was processed without scan rotation. It was found that the physical presence of oxides appeared to increase the degree of melt pool penetration by inhibiting the heat transfer. Without favorable thermal conditions to trigger heterogeneous nucleation, a more noticeable tendency to form columnar grains was characterized in samples with CoAl2O4. It is believed that the presence of oxide particles in the melt pool contributed to decreasing the cooling rates during solidification and retarded the columnar-to-equiaxed transition. A comparatively deeper degree of melt pool penetration also enhanced the relative density of samples with CoAl2O4. Despite the occurrence of the balling phenomenon, the reduced Marangoni convection also contributed to less micro-humping and better continuity of the melt tracks that decreased the surface roughness by at least 16.7 pct. The addition of CoAl2O4 to the powder feedstock significantly impacted the processing window of L-PBF processed Alloy 718 and may potentially be used as a means to engineer the microstructure.