Effect of FSW parameters on residual stress, microstructure and mechanical performance of dissimilar 7075/2017A Al joints

This study investigates the effects of friction stir welding (FSW) parameters on residual stresses, microstructural evolution, hardness distribution, and tensile behaviour of dissimilar Al 7075-T651/Al 2017A-T451 joints. Three welding conditions with different welding ratios were analysed to clarify the role of heat input and material flow on joint performance. Residual stress measurements revealed asymmetric distributions typical of dissimilar FSW, with “ M-shaped ” longitudinal profiles and tensile stresses concentrated in the nugget zone. Lowest welding ratio (1.66 rev/mm) resulted in reduced tensile residual stresses and improved stress uniformity. Microstructural analyses showed sound weld formation with refined nugget-zone grains (∼2.5 – 3.5 μm), high-angle grain boundaries, and high dislocation density structures, indicating dynamic recrystallization dominated by severe plastic deformation. Thermally stable Al-Fe–Mn–Si-rich dispersoids persisted in the nugget zone, whose microstructure closely resembled that of the AA2017A base material, consistent with the dominance of the advancing-side alloy within the nugget zone. The optimal welding condition achieved the highest strength and ductility ( Rp 0.2 = 304 ± 3 MPa, R m = 421 ± 2 MPa, A t = 8.4 ± 0.6%,) and the 3-4 times lower weld-line heterogeneity Ψ compared to other conditions (e.g., Ψ – R m : 0.98% vs. 3.47%), confirming enhanced mechanical uniformity. These results demonstrate that appropriate FSW parameter selection and precipitation stability are critical for producing structurally reliable dissimilar aluminium joints.