The nature of oxide films in process-induced lack-of-fusion defects on laser powder bed fusion-fabricated Hastelloy X Ni-based alloy

Abstract

This study investigates the oxide films formed on the inner walls of lack-of-fusion (LOF) defects in Hastelloy X Ni-based alloy produced by laser powder bed fusion (LPBF) and their influence on corrosion behavior. Using X-ray CT and high-resolution transmission electron microscope, we revealed that these oxide films, located at the pore-matrix interfaces, exhibit an uneven thickness and play a significant structural role relative to the voids. The oxide film consists of a 6.3 nm NiO inner layer and a 1.2 nm amorphous outer layer, with the inner layer maintaining a coherent orientation with the matrix, minimizing interface strain. Electrochemical analyses showed that the existence of oxide films would reduce the diffusion coefficient of point defects and the concentration of dissolved metal cations in the passive film formed on the LOF defects while also boosting the outward diffusion rate of cations therein. This process diminishes the rate of dissolution associated with surface activity for LPBF Hastelloy X Ni-based alloy in corrosive mediums, ultimately promoting the repassivation process and improving the corrosion resistance properties.