Effects of Heat Treatment Pulse on Microstructure and Mechanical Properties of 7075 Aluminum Alloy Resistance Spot Welding Joints

In response to the presence of joint defects and occurrence of softening in aluminum alloys resulting from resistance spot welding (RSW), heat treatment pulse has been used to improve welded joints. The effects of applying a heat treatment current on the microstructure, mechanical properties, precipitated phase, and fracture characteristic of the resistance spot welding joints were studied. The microstructures of the welded joints were analyzed by optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Moreover, the mechanical properties of the welded joints were analyzed by tensile and microhardness tests. The results showed that the heat treatment pulse increased the uniformity of the element distributions and grain sizes of the welded joints and eliminated the intergranular crack in the columnar crystal zones of these joints. Compared with the single pulse, the mechanical properties of the welded joint improved by applying a heat treatment pulse. The tensile shear force of the welded joint increased by 11.2% after applying an appropriate heat treatment current. The hardness of the joints decreased slightly, the toughness increased, and the fracture modes changed from ductile-brittle fracture to ductile fracture. This method improved the easy softening of the joint during aluminum alloy RSW and avoided the formation of defects.