Laser-power-dependent surface strengthening effect of additive manufactured ZGH4736 superalloy

In this study, nickel-based superalloy ZGH4736 was fabricated by laser powder bed fusion (LPBF) and subsequently subjected to surface strengthening (SS) with shot peening to enhance its surface properties. A comprehensive analysis was conducted to evaluate the effects of SS on the residual stress, microstructure, and microhardness of LPBF-fabricated ZGH4736 alloy specimens under varying laser powers. The findings reveal that SS induces substantial compressive residual stress (CRS) in the surface layer, with the maximum residual stress occurring in the subsurface region. Higher laser power increases the magnitude of this maximum residual stress but reduces the depth at which it occurs. Additionally, a gradient deformation layer with nanocrystalline grains forms near the surface after SS, extending to a depth of approximately 150 μm. The grain size is smallest at the outermost surface and gradually increases with depth toward the base material. Conversely, micro-strain and dislocation density follow an inverse trend, decreasing progressively with depth. Additionally, deformation twins and stacking faults in the surface layer restrict dislocation motion, while intersections of dislocations in different directions create Lomer-Cottrell (L-C) dislocation locks, further impeding dislocation movement. As a result, the surface microhardness of the ZGH4736 alloy is significantly improved.