The Joint Formation Mechanism, Microstructure, and Mechanical Performance of Resistance Rivet-Welded Mg/Steel Joints

Dissimilar combination of Mg and steel was joined by resistance rivet welding (RRW) based on the semi-tubular rivet. The formation process, microstructure evolution, and mechanical performance of the RRW Mg/steel joints were studied in detail. The stages of the Mg alloy pierced by rivet, the formation, and growth process of hollow sphere weld were analyzed. The investigation of microstructure demonstrated that the hollow sphere weld was characterized by the Mg sphere in the center around by the steel nugget. The microstructure of steel nugget consisted of martensite, and the Mg sphere and steel nugget were distinct from each other. Moreover, the dot-shaped and dendrite-shaped defects could be found in the steel nugget near the interface of Mg sphere/steel nugget, and the farther away from the interface, the fewer dot-shaped defects. The formation mechanisms of dot-shaped and dendrite-shaped defects were elucidated. The mechanical performance of the joint micro-zones was calculated based on the elasto-plastic model by the nanoindentation. Tensile-shear results showed that the maximum peak load could reach 9627 N at 9 kA-200 ms with the fracture mode of button pullout fracture.