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 loadbearing 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.

Herein, 6016 aluminum alloy is joined to HC420LA high‐strength steel by resistance spot welding technology with two kinds of cover plates. The morphology and microstructure of the joint are observed by optical microscopy and scanning electron microscopy. The formation mechanism and control methods of solidification defects such as shrinkage cavities and cracks are discussed. The results show that independent nuggets form on aluminum alloy and high‐strength steel side, respectively, when two kinds of cover plates are added. In some special conditions, the aluminum/steel interface presents a double‐peak shape because of the imbalance between the stress from molten aluminum and that from applied force. Meanwhile, a large shrinkage cavity and longitudinal crack originate from the shrinkage cavity form in the nugget on the aluminum side. All these defects generate in the process of solidification, and a model is established to explain the formation mechanism of solidification defects. Moreover, the control methods to prevent the solidification defects are discussed. Based on the model and control methods, a good joint without solidification defects is produced.