Nugget Formation and Performance of Resistance- Rivet-Welded Al/Steel Joints

Resistance rivet welding (RRW) is a method developed in recent years for the joining of dissimilar materials. In this study, RRW with solid rivets was used to join 6016 aluminum alloy and CR1180T ultrahigh strength steel. The nugget formation process was investigated in terms of dynamic resistance, microstructure analysis, and microhardness measurement. The mechanical properties and failure mode of RRW joints were also investigated. The results showed that the Al under the rivet leg gradually melted from the middle to both sides during the piercing stage. Molten Al was squeezed into the space under the rivet cap. A mushroom-shaped weld nugget was first formed inside the steel sheet and gradually grew toward the rivet. The microstructure of the weld nugget was coarse martensite. A layer of uniform intermetallic compounds was formed between the rivet leg and the Al sheet. Three failure modes — interface failure, pullout failure (PF), and Al sheet failure —were identified. The strain distribution of the joints with three failure modes in the tensile process was compared. Under the optimal parameters, the failure mode of the RRW joint was PF, and the average maximum tensile-shear force of the joints reached 11.4 kN.