Effects of Hole Sizes and Stack Thicknesses on Mechanical Properties and Failure Behavior of Pre-holed Self-piercing Riveted Steel-Aluminum Joints

The conventional self-piercing riveting (C-SPR) process is appropriate for joining similar or dissimilar materials. However, the insufficient penetration and upsetting deformation of the rivet pose significant challenges to the application of the C-SPR process due to the increasing material strength and sheet thickness. In this paper, a new process of pre-holed self-piercing riveting (PH-SPR) is adopted to join the multilayer high-strength steel/aluminum hybrid sheets, and the effects of hole sizes and stack thicknesses on forming quality, mechanical properties, and failure behavior of PH-SPR joints are explored. The comparisons of microstructure and riveting stress between the C-SPR joint and PH-SPR joint are discussed experimentally and numerically. The results show that the larger hole size can improve effective thickness and energy absorption, while small influences are found on undercut and joint strength. For the three-layer PH-SPR joints, the decreasing stack thickness and middle sheet thickness results in better effective thickness and mechanical properties. Two types of failure modes of SPR joints are observed under quasi-static tests, including rivet fracture in C-SPR joints and rivet pulled out from the bottom sheet in PH-SPR joints. The larger hole size and thinner stack thickness of PH-SPR joints result in increased sheet warpage and reduced tearing volume of the bottom aluminum sheet.