Numerical investigation of joinability and forming quality improvement on self-piercing riveting process with varying sheet stack combinations

Reducing complexity and increasing efficiency of the self-piercing riveting (SPR) process in joining varying sheet stack combinations by reducing the required rivet-die combinations have attracted great attention in the automobile industry. In this paper, the SPR joinability and the joining ranges of material strength and thickness of three types of stack combinations (e.g., steel-aluminium (SA), aluminium-aluminium (AA), and aluminium-steel (AS)) are investigated. In particular, the forming quality improvement process is developed to solve the forming quality defects generated in the riveting process. Two-dimensional axisymmetric finite element (FE) models are developed for each stack combination to simulate the SPR process, and the effects of rivet hardness, die type, sheet material and sheet thickness on the SPR joinability are studied numerically. The results show that the SPR joinability decreases with the increase in the thickness ratio, and the SPR joinability approaches zero when the thickness ratio exceeds 1.47. The rivet H4 and rivet H6 with a flat die can achieve higher SPR joinability compared to rivet H1. Specifically, the rivet H4 is suitable for joining the AA stack, and the rivet H6 is suitable for joining the SA and AS stacks. The average undercut ratio increases with the increase in rivet hardness, while a decreasing tendency is observed with the increase in thickness ratio. The rivet stiffness and hardness can be adjusted to solve the forming quality parameter defects and overcome upsetting deformation defects. Switching the riveting direction for the large thickness ratios (1.47 and 2.08) has proven to be an effective method to achieve successful joining.