Subsurface grain refinement in electron beam-powder bed fusion of Alloy 718: Surface texture and oxidation performance

Subsurface grains of Alloy 718 additively manufactured via electron beam-powder bed fusion technique were refined using shot peening to improve the surface texture and oxidation performance. Oxidation of the specimens was performed at 650 and 800 °C in ambient air. Due to plastic deformation upon shot peening, compressive residual stress and high microstrain were generated in the subsurface region within a depth of approximately 50 μm. The shot-peened specimen exhibited lower surface roughness, finer subsurface grains, and higher hardness compared to the as-built specimen. Shot peening, coupled with hot isostatic pressing and heat treatment (HIP-HT), yielded superior oxidation performance with substantially low oxidation kinetics at 800 °C. The smooth surface, as well as dense and refined subsurface microstructure resulting from shot peening, facilitated the formation of a continuous, protective, and adherent Cr-rich oxide scale.