Research on the formation mechanism and mechanical properties of refill friction stir riveting (RFSR) joints of aluminium alloy AA7075-T6

Abstract

In this study, a novel refill friction stir riveting (RFSR) process was proposed to achieve hybrid solid-state bonding and mechanical interlocking of high-strength aluminium alloy AA7075-T6 sheets. The size of mechanical interlock is predefined by the depth of groove designed on the rivet shank, and realized through sheet material refilling. High-quality solid-state bonding is also generated at the sheet faying surface under the high pressure and strong friction stir from the rivet. The quality characteristics, microstructure, heat generation, microhardness distribution and mechanical properties were systematically investigated to uncover the joint formation mechanism and verify the effectiveness of the RFSR process. The results reveal that a mechanical interlock with 82% of the predefined value was successfully achieved by sheet material refilling into the groove on the rivet shank. An extra mechanical interlock, which was formed between the rivet shank and trapped bottom sheet, further enhanced the joint strength. Continuous solid-state bonding with a width of 0.39 mm was also generated between the two sheets. Strengthened by the multiple connection features, the peak lap-shear and cross-tension forces reached up to 9.09 ± 0.24 kN and 6.82 ± 0.11 kN respectively, and the corresponding failure modes were rivet shank fracture and pull-out of rivet shank from the bottom sheet.