Effect of Zn and Sn interlayers on microstructure and mechanical properties of dissimilar AA7075–AZ31B friction stir welds

This study investigates the influence of Zn and Sn interlayers on the microstructure and mechanical properties of dissimilar friction stir welds between AA7075-T651 aluminum alloy and AZ31B magnesium alloy. Joints produced without an interlayer exhibited poor material mixing, interfacial cracking, and root defects. The use of a Sn interlayer produced a homogeneous lamellar stir zone with fine, discontinuous Mg₂Sn intermetallic layer (thickness <1.2 μm), effectively suppressing continuous brittle Al–Mg intermetallic phases. This resulted in the highest tensile strength (76.4 MPa, ∼29.6% joint efficiency), highest peak hardness (154.4 HV), and improved ductility, with fracture shifting to the stir zone (SZ) / thermomechanically affected zone (TMAZ) interface region on the Mg side. In contrast, the Zn interlayer offered only limited improvement (53.81 MPa) due to incomplete dispersion, residual defects, and localized MgZn₂ formation. No continuous intermetallic layers were observed in any joints, and fracture surfaces predominantly showed brittle morphology associated with intermixing defects and localized intermetallics. The findings demonstrate that interlayer effectiveness in Al–Mg dissimilar friction stir welding (FSW) is governed primarily by material flow compatibility rather than chemical barrier effects, providing new directions for interlayer selection and design in dissimilar friction stir welding.