Process–structure–property assessment of dissimilar friction-stir-welded AA7070-T6/AA6013-T6 joints: Softening, residual stress, and coupled performance

Dissimilar friction stir welding of precipitation-hardened aluminum alloys is strongly affected by alloy mismatch, asymmetric material flow, weld-zone softening, and residual-stress development. In this study, the process–structure–property relationship of dissimilar AA7070-T6/AA6013-T6 friction-stir-welded joints was experimentally investigated under three welding conditions. The rotation speed was kept constant at 2000 rpm, while the traverse speed and effective alloy-position arrangement were varied. The joints were characterized using Vickers microhardness mapping, transverse tensile testing, X-ray diffraction residual-stress analysis, SEM fractography, EBSD, and TEM/STEM-EDS microstructural examination. In addition, the Softening Index and Coupled Performance Index were used as comparative indicators to evaluate hardness degradation and combined joint performance. The results revealed an asymmetric hardness distribution across the dissimilar joints, with localized softening in the weld-affected region. Among the investigated conditions, Case-2 exhibited the best overall mechanical response, achieving the highest ultimate tensile strength and elongation, and approximately 70% joint efficiency relative to the stronger AA7070-T6 base alloy. Residual-stress analysis showed that the longitudinal component was dominant over the transverse component, and Case-2 produced the lowest tensile longitudinal residual-stress peak and the smoothest stress transition across the weld. EBSD confirmed substantial grain refinement in the nugget zone, while TEM/STEM-EDS revealed a chemically heterogeneous interfacial region containing redistributed Zn/Mg-rich and Cu-rich precipitate-related features. SEM fractography further indicated that Case-2 exhibited the most ductile fracture response. Overall, Case-2 provided the most favorable balance between hardness retention, tensile strength, ductility, residual-stress control, weld-line homogeneity, and fracture behavior within the investigated experimental matrix. The proposed indices are useful for comparative assessment of the studied welding conditions; however, broader design-of-experiments validation is required before general optimization can be claimed.