Microstructural and mechanical characterization of steel-DP780/Al-5052 joints formed using resistance element welding with concealed rivet cover

Resistance element welding (REW) with a concealed rivet cover (Q235) was used to join steel-DP780 and Al-5052. The macroscopic morphology and the microstructure were observed using optical microscopy and scanning electron microscopy. The mechanical properties of the joint were also tested. The results show that the intermetallic compound (IMC) formed at the Q235/Al interface is FeAl3, while the IMCs formed at the DP780/Al interface are Fe2Al5 and FeAl3. The welding current has a significant influence on the nugget size, peak load, average hardness of aluminum heat-affected zone, and the bearing area of the aluminum plate. There are three fracture models of the countersunk rivet REW joint under a tensile-shear force: the interface failure fracture model, the fusion pullout failure fracture model, and the heat-affected zone failure fracture model. At a welding current of 18 kA, the nugget pullout failure mode is observed, and the best mechanical properties of the joint are obtained. The shear resistance of the joint can reach up to 5712 N.