Fatigue Characteristics of DP780 Steel Spot Welding Joints with Different Static Fracture Modes

Two specimens made of DP780 dual phase steels were produced by resistance spot welding and the quality was evaluated by the static tension-shear test and low cycle fatigue test. The fracture characteristics, macrostructure, microstructure, microhardness distribution and fatigue morphology were studied. The results show that the nugget size and microhardness gradient in the heat affected zone affect the static load-bearing performance and the fracture mode. Meanwhile, applied load, microhardness gradient in the heat affected zone and inclined angle on faying surface caused by macro deformation have influence on the low cycle fatigue life. Compared with the specimen which has high static load-bearing capacity, samples with lower static load capacity has a higher fatigue life and plastic deformation characteristics under the same applied load of 40% of its static load-bearing capacity, which indicates that the applied load is the controlling factor in the low cycle fatigue test.