Effect of a Zn interlayer on the Microstructure and Properties of 7075 Aluminium Alloy Resistance Spot Welding Joint

Resistance spot welding (RSW) was carried out on a 3 mm thick 7075 aluminium alloy. A 50 μm thick Zn interlayer was added to improve the weld quality and mechanical properties of the joint. The macroscopic morphology, microstructure, precipitated phase, microhardness, and mechanical properties of the RSW joint before and after the addition of the Zn interlayer were comparatively analyzed. After adding the Zn interlayer, the microstructure and mechanical properties of the welding joints were considerably improved, and the effective range of the welding current was expanded. The grain size of the welding joint was clearly refined, the columnar crystal zone narrowed, and the equiaxed crystal zone widened. The precipitated phase changed from a reticular or long-strip-like to point- or short-strip-like and decreased in size. The number of precipitated phases increased, and the phase distribution became more uniform. The number of defects in the welding joint clearly decreased, and the joint microhardness increased. The maximum tensile shear force sustained by the Zn-RSW joint was 14.29 kN, which was 51.5 % greater than that without the Zn interlayer. The addition of the Zn interlayer changed the fracture mode from interfacial fracture to button fracture, and obvious plastic deformation occurred before the fracture. The Zn-RSW joint could absorb 245 % more energy than the RSW joint.