Influence of Plunge Depth on Microstructural Evolution and Mechanical Performance of Al/Steel Friction Stir Riveted Joints

A novel friction stir rivet welding process was employed to join SS304 and AA6061-T6 plates. The process utilizes the Steel-over-Al lap configuration and pinless tool, which enhances the mechanical properties of the joint while eliminating keyhole defects typically found in conventional friction stir spot welding. The influence of plunge depth on the joint macro- and microstructure was investigated. Furthermore, the mechanical properties and failure modes under varying plunge depths were compared to elucidate the joint strengthening mechanism. The results indicate that under the combined effect of axial force and frictional heating, the steel side of the joint comprises three distinct regions: the large deformation zone (LDZ), small deformation zone (SDZ), and heat affected zone (HAZ). Pronounced dynamic recrystallization occurs in both the LDZ and SDZ. At the Al/steel interface, Fe 4 Al 13 phase forms, and the thickness of the intermetallic compound layer increases from 2.7 μm to 6.7 μm with greater heat input. As the plunge depth increases, the size of the countersunk rivet head formed in the joint enlarges, while the effective bearing plate thickness decreases. Tensile-shear tests reveal that at the plunge depth of 0.4 mm, the rivet head size and bearing plate thickness reach a suitable match, yielding a maximum shear load of 9008 N. All joints failed via a pull-out shear fracture. Specifically, the fracture mode is mixed, involving brittle fracture of the intermetallic compound layer at the Al/steel interface and ductile fracture within the countersunk rivet head.