A novel electromagnetic self-pierce upsetting riveting with flat die for joining ultra-high strength steel and aluminum structures

Abstract

Ultrahigh-strength steel (UHSS) is a promising material that can decrease sheet thickness and increase automobile safety. However, joining UHSS with aluminum alloys poses significant challenges to existing joining processes, such as self-piercing riveting (SPR), which often leads to rivet upsetting owing to the high strength of UHSS. This study proposes a novel joining process called electromagnetic self-pierce-upsetting riveting (ESP-UR). This method combines the piercing of a semi-hollow rivet leg in SPR with the upsetting of a rivet shank in traditional riveting, utilizing a novel rivet designed for high-strength and low-ductility steel. The effectiveness of the ESP-UR process was validated through riveting experiments involving 1.5-mm-thick 22MnB5 UHSS and 2.0-mm-thick 5052 aluminum sheets. A numerical experimental analysis and microstructural characterization were performed to characterize the joint formation mechanism. The results indicated that small protrusions created by the short rivet leg embedded in the lower sheet were compensated for by rivet cavities, resulting in a flat bottom surface of the joint. Furthermore, the hole diameter positively influenced the strength of the ESP-UR joint, with joints featuring a hole diameter of 5.8 mm exhibiting an 8.5 % increase in shear strength compared to those with a 5.6-mm diameter under a stroke of 5.0 mm. Additionally, the failure modes were analyzed and discussed. These findings provide theoretical guidance for optimizing stroke and hole diameter designs in future engineering applications.