Bonding mode and mechanical properties of keyhole-free friction stir rivet welded aluminum alloy joints

A novel method called friction stir rivet welding (FSRW) is proposed to combine mechanical interlocking and metallurgical joining, and is expected to improve such mechanical properties and eliminate the keyhole defects in traditional friction stir spot welded (FSSW) joints. The FSRWed joint forms metallurgical joining between the upper and lower sheets, while mechanical interlocking between the rivet and the surrounding plasticized materials. The spindle force, bonding mode, mechanical properties and failure mode of three joints are comparatively investigated to clarify the strengthening mechanism of FSRW method. Results indicate that the keyhole defects are successfully eliminated in FSRWed joints, and the mechanical properties of the joints are considerably improved by using the threaded rivet FSRW (T-FSRW). The spindle force requirement of T-FSRW exhibit greater than that of cylindrical rivet FSRW (C-FSRW). Similarly, compared with the FSSWed joint, the T-FSRWed joint has a 65% higher shear tension strength and 170% increase in cross tension strength, with the corresponding extension at fracture increased by 19% and 220%, respectively. Rivet geometry plays a crucial role on the joints’ mechanical properties, and determines the transverse section and joining parameters. The strengthening mechanism of the FSRW method is assumed from the combination of mechanical interlocking and metallurgical joining. In addition, analysis of the fracture morphologies demonstrate that the shear tension failures of the FSRWed joints mainly exhibit neck, shear fracture and upper sheet tensile fracture, while the cross tension failures exhibit neck fractures and rivet pull-outs.