Microstructure and Mechanical Properties of Resistance Element Welded Joints for DP780 Steel and 6061 Aluminum Alloy

DP780 steels and 6061‐T6 aluminum alloys are joined using the resistance element welding (REW) process with a flat Q235 rivet. The macrostructure, microstructure, and mechanical properties of the REW joints are investigated. The nugget zone and upper‐critical heat‐affected zone (UGHAZ) microstructure exhibit lath martensite. The inter‐critical heat‐affected zone (ICHAZ) microstructure consists of ferrite and martensite. Two types of intermetallic compound (IMC) layers are formed at the steel/Al interface. A tongue‐like Fe2Al5 layer is found adjacent to the steel side, while a needle‐like Fe4Al13 layer is found adjacent to the aluminum alloy side. The welding current has a significant influence on the nugget size, peak load, and energy absorption of joints. The fracture behavior analysis of the REW joints is performed using the in situ digital image correlation (DIC) method. There are two failure modes of the REW joint under different welding currents: pull‐out failure (POF) and the base material pull‐out failure (BPF). At a welding current of 17 kA, the maximum tensile‐shear value of the REW joints is 6.27 kN with a nugget diameter of 5.37 mm. The failure mode of the REW joints is mainly BPF mode that exhibits superior lap‐shear performance.