Effect of heat treatment on cracks and properties of self-piercing riveted joint in aluminum alloy

By considering the influence of cracks in self-piercing riveted joints of aluminum alloys on the static mechanical properties, an annealing process was applied to heat-treat 2024 aluminum alloy base plates before riveting. Additionally, solution treatment and aging treatment were used to heat-treat multiple groups of self-piercing riveted specimens composed of 2024 aluminum alloy and 6061 aluminum alloy. Combined with tensile-shear tests and scanning electron microscopy (SEM) characterization, the joint cracks and mechanical properties after different heat treatment processes were analyzed, and the evolution mechanism of failure modes was explored. The results have shown that riveting 2024 aluminum alloy base plates after annealing treatment can effectively reduce cracks at the joint bottom. The solid solution + artificial aging treatment significantly increases the joint load by 23.97%, and the solid solution + natural aging treatment significantly increases the joint load by 20.29% and thus can be used as a heat treatment process to improve the joint load. Affected by different heat treatment processes, the failure mode of 6061 alloy group joints changes from upper plate fracture to upper plate fracture + partial rivet pull-out and then to upper plate fracture + complete rivet pull-out. The failure mode of the 2024 alloy group joints transforms from upper plate fracture to upper plate fracture + partial rivet pull-out. The shift from a single failure mode to a mixed failure mode disperses stress, delays crack propagation, and improves safety. Additionally, the fracture mechanism changes from ductile fracture or brittle fracture to ductile-brittle mixed fracture. The solution + aging treatment endows the joints with both certain toughness and strength, resulting in wide adaptability.