Effect of Laser Shock Peening on Dissimilar AA6061-T6/Pure Copper Friction Stir Spot Welded Joints

Abstract

Friction Stir Spot Welding (FSSW) is a solid-state route for joining dissimilar aluminum-copper assemblies, but welding-induced softening and the brittle Al-Cu intermetallic layer limit joint reliability. This work investigates the integrated use of FSSW and Laser Shock Peening (LSP) as a post-weld surface treatment for 3 mm thick AA6061-T6/pure copper lap joints. A Taguchi L9 array with ANOVA was used to optimize rotational speed, dwell time, and tool geometry, identifying tool shape as the dominant factor; the optimum joint was then treated by LSP with a Q-switched Nd:YAG laser and compared with the as-welded baseline. The joint exhibited an asymmetric response: the aluminum side recovered its HAZ softening and exceeded its base-metal hardness, while the copper side softened slightly through laser-induced thermal recovery, yet the joint showed a net strengthening. XRD and FTIR supported grain refinement and the absence of thermal-oxidation damage. The integrated FSSW + LSP route is thus proposed as a viable post-weld strengthening strategy for thick dissimilar Al/Cu joints.