Effects of ultrasonic shot peening process on the microstructure and mechanical properties of nickel-based superalloys formed by selective laser melting

Selective laser melting (SLM) is an attractive additive manufacturing technique for fabricating high-performance superalloys engineering components. Unfortunately, these as-built parts generally exhibit unsatisfied mechanical properties because of their natural thermal residual stress and non-equilibrium microstructures. Ultrasonic shot peening (USP) can effectively inhibit defect expansion and even close pores, thereby improving the mechanical properties of alloys. In this study, the internal defects, residual stress and microhardness redistribution, tensile properties, as well as microstructural response of the GH3230 alloy treated by different ultrasonic peening time are systematically studied. USP treatment induces a significant carbides fragmentation and synthesizes surface gradient heterogeneous structure, which displays a microstructure gradient in grain size, carbides size and dislocation density from surface to core. Long-time USP treatment induces work softening of GH3230 alloys. The work softening mechanism of the GH3230 alloys after long-time USP treatment is ascribed to the increase of surface temperature and high stored strain energy. Both the microhardness and residual compressive stress in the softened region of GH3230 alloys are reduced. Work softening effect increases the surface plasticity of the GH3230 sample, which induces deeper gradient structure. The various strengthening mechanisms of gradient heterogeneous structures, as well as the multiple effects of hetero deformation induced (HDI) hardening, and densification strengthening are responsible for achieving strength-ductility synergy. The research findings provide new insights based on USP for improving the mechanical properties of heterogeneous superalloys engineering components.