Characteristics of the Resistance Spot Welding Joints in Dissimilar Thickness Dual-phase Steels

Abstract

The characteristics of dissimilar thickness dual-phase steel DP780/DP600 resistance spot welding joints have been investigated. The experimental results reveal that the weld nugget of welded joint forms as-cast structure and consists of lath martensite with little ferrite, while the heat affected zone (HAZ) near the base metal of welded joint softened. The failure location of the welded joint initiates from the DP780 in the tensile-shear test, and the proper criterion of nugget diameter to ensure the pullout failure mode should be equal to 5.5 (where t is the average thickness of both base metals in mm). The dual-phase steel spot-welding is prone to expulsion. Expulsion at the electrode/workpiece interfaces affects the surface quality; expulsion from the faying surface doesn’t decrease peak load, but increases the indentation rate of the welded joints. Compared to welding time and electrode force, expulsion is more sensitive to welding current.