Laser shock peening of 6061-T6 aluminum alloy friction stir welding joints: Microstructure evolution, fracture morphology and strengthening mechanism

Enhancing the performance of Friction Stir Welding (FSW) joints is essential for improving the mechanical strength and service life of critical components in aerospace and automotive industries. To overcome the bottleneck of refining welding parameters alone, this study applies Laser-Shock Peening (LSP) as a post-weld treatment to 6061-T6 Al alloy FSW joints and delivers a systematic evaluation of the resulting property improvements. Multi-scale characterization—including Electron Back Scatter Diffraction (EBSD), X-ray Diffraction (XRD), mechanical testing, depth-resolved residual-stress profiling, in situ Scanning Electron Microscopy (SEM) fracture observation, and tensile property evaluation—was employed to correlate mechanical response with microstructural evolution. High-amplitude LSP shock waves introduced a high-density dislocation structure and Compressive Residual Stress (CRS) into the surface layer: surface residual compression reached −70.6 MPa and persisted to a depth of 180 μm, completely cancelling the 104.7 MPa tensile stress originally present in the as-welded heat-affected zone (HAZ). EBSD revealed that subsurface grains were refined from 21.6 μm to 6.8 μm, the length fraction of high-angle grain boundaries doubled relative to low-angle boundaries, dislocation density increased, and the average Schmid factor decreased. Consequently, yield strength rose by 34.36 % and HAZ micro-hardness increased by up to 23.6 %. The work establishes a quantitative “stress–microstructure–property” relationship for LSP-strengthened FSW joints and demonstrates that LSP simultaneously enhances strength and fracture resistance, offering a readily implementable post-weld strengthening route for high-strength AL welded assemblies.