Abstract This paper presents the resistance spot welding process test and numerical simulation analysis of medium manganese steel TRIP steel (7MnSteel) and DP590 steel plate. The results show that the failure mode is PF-TT failure, and IF and PF-TT are investigated. At the same time, based on the numerical simulation under the typical welding parameters, the welding plate has the stress concentration at the electrode edge at the beginning of the welding. Along with the welding, there is the stress concentration in the heat-affected zone of the medium manganese TRIP steel, which may also be the reason for the failure of the joint here. The current density is concentrated at the contact surface between the electrode and the weld plate, and the error in the diameter of the nugget measured by simulation and experiment is within 7%, which verifies the accuracy and reliability of the model.

A single-pulse spot welding test was performed on 0.13C-6.98Mn medium manganese TRIP steel. The test results showed that the joint performance increases with a rise in welding heat input before splashing occurs. Owing to the fusion zone (FZ) being closest to the heat source, its austenite content was completely transformed and its performance was worse than that of the heat affected zone comprising austenite. The segregation bands of C and Mn observed in the FZ were considered to be the main reason for the deterioration of grain boundary strength. These characteristics of the weld zone led to evident intergranular cracking and transgranular fracture in the tensile test, and all samples in this test showed partial or complete interfacial fractures.

AZ31B magnesium alloy/AA6061 aluminium alloy welded structures have been widely used in the lightweight design of automobile bodies. To improve the joint strength, the ultrasonic spot welding (USW) solid state connection method is used in this paper. The results show that at a welding energy of 1540 J, there is evidence of melting in the weld joint. As the welding energy increases, the intermetallic compounds (IMCs) grow rapidly and form a reaction layer approximately 17 μm thick. The IMCs are mainly composed of the γ-Mg17Al12+β-Mg2Al3 phase, where the β-Mg2Al3 phase hardness is as high as 6.5 GPa and shows a typical pop-in phenomenon and the average hardness of the γ-Mg17Al12 cubic structure phase is 3.4 GPa. When the welding energy reaches 1540 J, the β phase rapidly grows, and the joint performance decreases rapidly. The fracture morphology indicates a cleavage fracture and is accompanied by a large number of secondary cracks.