Forming of asymmetric rivet joints through a versatile self-piercing riveting process with tumbling kinematics

Abstract

Conventional self-piercing riveting (SPR) typically results in rotationally symmetric joints with uniform mechanical behavior. However, increasing demands for material-efficient lightweight design, complex load paths and hybrid material systems require versatile joining solutions. Asymmetrical rivet joints offer the potential to tailor joint properties to specific load directions by deliberately introducing geometric anisotropy, such as controlled undercuts or shaft deformations. This enables direction-dependent load-bearing capacities, enhanced energy absorption in crash-relevant applications and improved formability when joining dissimilar materials. Building on previous findings in material flow control, this study investigates a versatile tumbling self-piercing riveting (T-SPR) approach using adapted tumbling strategies to intentionally generate non-rotationally symmetric joint geometries to extend process limits beyond those of conventional SPR. The objective is to evaluate joint feasibility and the controlled adjustment of geometric joint properties through parameter combinations. Force-displacement curves, 3D surface measurements and metallographic cross-sections characterize rivet tilting, rivet head geometry, undercut and residual sheet thickness. Asymmetric angle combinations lead to direction dependent geometric features that combine characteristics of symmetric reference cases. Hardness measurements reveal locally increased strain hardening and an asymmetric hardness distribution throughout the rivet, indicating anisotropic load-bearing behavior. Overall, the findings demonstrate that T-SPR enables tailored asymmetric joints.