JOINT FORMATION AND MECHANICAL PERFORMANCE OF FRICTION SELF PIERCING RIVETED ALUMINUM ALLOY AA7075-T6 JOINTS

AA7xxx series aluminum alloys have great potentials in mass saving of vehicle bodies due to pretty high specific strength. However, the use of these high strength materials poses significant challenges to traditional self-piercing riveting (SPR) process. To address this issue, friction self-piercing riveting (F-SPR) was applied to join aluminum alloy AA7075-T6 sheets. F-SPR is realized by feeding a high speed rotating steel rivet to aluminum alloy sheets to form a dissimilar material joint. The effects of spindle speed and rivet feed rate on F-SPR joint cross-section geometry evolution, riveting force and energy input were investigated systematically. It was found that the rivet shank deformation, especially the buckling of the shank tip before penetrating through the top sheet has significant influence on geometry and lap-shear failure mode of the final joint. A medium rivet feed rate combined with a high spindle speed was prone to produce a defect free joint with sound mechanical interlocking. F-SPR joints with the failure mode of rivet shear fracture was observed to have superior lap-shear peak load and energy absorption over the joints with mechanical interlock failure. The optimized F-SPR joint in this study exhibited 67.6% and 13.9% greater lap-shear peak load compared to, respectively, SPR and refill friction stir spot welding joints of the same sheets. This research provides a valuable reference for further understanding the F-SPR process.