Microstructural evolution, precipitate redistribution, and fracture behavior of optimized friction stir welded 7055-T6 aluminum alloy joints

This study investigates the microstructural evolution, precipitate behavior, hardness distribution, and fracture characteristics of friction stir welded 7055-T6 aluminum alloy joints under optimized parameters. EBSD revealed ultrafine equiaxed grains (∼3.3 μm) with 75.8% high-angle boundaries in the nugget zone, confirming complete recrystallization. SEM–EDS and XRD indicated precipitate coarsening in the HAZ but uniform reprecipitation in the WNZ, producing a characteristic ‘W-shaped’ hardness profile with partial recovery (∼145 HV). The joint achieved a peak tensile strength of 475.2 MPa, exceeding reported values. Fractography confirmed ductile fracture by microvoid coalescence near the HAZ–TMAZ interface, with dimples nucleated at fine precipitates. The integration of grain refinement, precipitate redistribution, and favorable texture evolution explains the improved strength and toughness, establishing a clear process–structure–property–fracture relationship for aerospace-grade aluminum alloy joints.