Fatigue failure mechanism and estimation of aluminum alloy self-piercing riveting at different load levels

In order to investigate the mechanical properties of self-piercing riveting (SPR) joints in low ductility aluminum alloys, the study utilized three different types of aluminum alloy sheets: 7075, 6061, and 5754. These sheets were joined together using self-piercing riveting technology. The research compared the geometric parameters, microhardness, quasi-static mechanical properties, and fatigue properties formed with the joints of the three aluminum alloys. Additionally, the fatigue life of the self-piercing riveting joint in the 7075 aluminum alloy was analyzed using the Paris law and its corollary. The findings revealed that effective connections were established in all three aluminum alloys, although the expansion of the rivet was comparatively lower in the 7075 alloy. Plastic deformation resulted in an increase in microhardness of joined sheets. The hardness of the joined sheets followed the order: 7075 > 5754 > 6061. Furthermore, the 7075 alloy exhibited superior static mechanical and fatigue properties due to its inherent material. However, despite its lower material strength, the 5754 alloy outperformed the 6061 alloy at low fatigue load levels as a result of work hardening. The failed joints exhibited black oxide, and fretting damage under fatigue loading exacerbated the fatigue failure of the joint, serving as the primary cause. In the case of the 7075 alloy, three types of failure occurred due to fretting damage, which was influenced by the unique nature of the lower die. Fretting between the lower sheet and the rivet leg primarily caused lower sheet fracture and rivet failure, while fretting wear between the rivet head and the upper sheet or between the two sheets mainly led to upper sheet fracture failure. The fatigue life estimation method employed in the study, based on the Paris law, demonstrated good agreement with the experimental results.