Coarse grain structure induced by laser surface melting for enhanced oxidation resistance of IN625

This work examined the microstructure and high temperature oxidation characteristics of laser surface melted IN625. A 400 W continuous laser at various scanning speeds was used for laser surface melting. Isothermal oxidation experiments were conducted at time intervals of 2 h, 10 h, 25 h, 50 h and 100 h at 900 °C in air. The microstructure evolution was analysed by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). It was shown that bulky Nb-rich precipitates were refined and evenly distributed within the γ matrix after laser surface melting. Mixed oxides, consisting of Cr₂O₃ and (Ni, Fe)Cr₂O₄ spinel oxides, were formed in the as-cast IN625 but a dense Cr₂O₃ scale was formed in the laser surface melted IN625. It was found that a semi-continuous Nb₂O₅ band was developed in the laser surface melted IN625 after oxidation. It was further revealed that grain growth occurred after laser surface melting, forming a coarse grain region near the surface. The coarse grain structure in IN625 suppresses grain boundary diffusion, resulting in less oxide growth compared to fine-grained counterparts, and enhances the oxidation resistance.