Homogenization of local microstructure and mechanical properties in friction stir welded Al-Cu alloy joint achieved through laser shock peening

Friction stir welding (FSW) is widely used to join large-size Al alloy components in advanced manufacturing industries with high requirements for mechanical properties. However, due to the huge differences in microstructure and local mechanical properties between different regions of FSW Al alloy joint, early yielding and strain localization tend to occur in low-strength regions, limiting the further improvement of the tensile properties. In this study, laser shock peening (LSP) was applied as a post-weld surface treatment to FSW Al-Cu alloy joints, resulting in increases of 52 %, 10 %, and 21 % in yield strength, tensile strength, and elongation, respectively. The effects of LSP on local mechanical properties, microstructure and overall mechanical properties were systematically studied. Compared to the stirring zone LSP induced more significant grain refinement and higher dislocation density in the thermo-mechanically affected zones (TMAZs) due to the lower initial dislocation density and fewer precipitates presented in the TMAZs. LSP generated more significant grain boundary and dislocation strengthening on TMAZs to obtain high strength improvement and achieve local mechanical property homogenization. The homogenization of local microstructure and mechanical properties hindered strain localization, facilitating the full utilization of the dislocation storage capacity of each region to accommodate plastic deformation, thereby enhancing the overall elongation of the joint. The research findings provide new insights based on LSP for improving the tensile properties of heterogeneous welded joints.