Formation mechanism and mechanical strength evaluation of hybrid riveted/solid-state bonded aluminium alloy joint

In this research, an improved friction self-piercing riveting (F-SPR) process with internally threaded rivet and flat die was proposed to achieve hybrid riveted/solid-state bonded connection for low-ductility aluminium alloys. Instead of relying on rivet shank flaring to form mechanical interlock in the existing F-SPR process, a reliable threaded connection was established between the rivet shank and sheets by rapid filling of sheet material into the rivet thread grooves. A perfectly flat joint bottom was formed without inducing large-scale sheet distortion. A series of experimental tests were performed to clarify the joint quality characteristics, formation mechanism and mechanical performance. The results revealed that the quality of newly formed five aluminium threads demonstrated a decline trend from bottom to top. Two kinds of thread formation mechanisms were identified: The mechanism I is solely composed of Filling step and applied to upper four threads; The mechanism II consists of Filling-Fracturing-Welding steps and is applied to the lowest thread. Discontinuous solid-state bonding was formed at the sheet interface around the rivet shank periphery, whilst a bowknot-liked solid-state bonding zone with a weak welded zone and an unwelded zone was formed at the joint centre. Rivet pull-out failure from the bottom sheet was observed for the studied AA7075-T6 joints under both lap-shear and cross-tension loads. The peak loads reached 9.79±0.23 kN and 5.41±0.15 kN in the lap-shear and cross-tension tests, and the corresponding energy absorption was 13.82±0.59 J and 16.42±0.87 J.