In this study, 7075 aluminum alloy with a thickness of 10 mm was successfully welded with no obvious defects by pulsed variable polarity plasma arc (PVPPA) welding. The mechanical properties of PVPPA welded joints have been researched by post weld heat treatment (PWHT). The results indicate that the heat treatment strongly affects the mechanical properties of the welded joints. The tensile strength and the microhardness of the welded joints gradually improved with the increase of the solution temperature. With the increase of the solution time, the tensile strength, and microhardness first dramatically increased and then decreased slightly. The best tensile strength of 537.5 MPa and the microhardness of 143.7 HV were obtained after 490 °C × 80 min + 120 °C × 24 h, and the strength was nearly 91.2% of that of the parent metal, and increased about 35% compared with as-welded. The improvement of strength and microhardness was mainly due to the precipitation of η′ phase.

A 7075 aluminum alloy was successfully welded by pulsed variable polarity plasma arc welding (PVPPAW) and the single-stage aging behavior of the 7075 aluminum alloy PVPPAW joint was systematically investigated. The results demonstrated that the tensile strength of the welded joints initially increased and then decreased with the increase of the single-stage aging temperature and time. After single-stage aging at 490 °C for 80 min and at 130 °C for 24 h, the tensile strength of the welded joint was 551 MPa, which was increased by 38.5% compared to the as-welded joint. Moreover, the conductivity was 25% international annealed copper standard (IACS) at room temperature, and the resistance to stress corrosion was improved. The main strengthening phases of the weld center were η′ and η phase. The average precipitate size slightly increased with the increase of the single-stage aging temperature, but no obvious change was observed with the increase of the single-stage aging time. The area fraction was initially increased and then decreased with the increase of the single-stage aging temperature and time.

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.