Investigation of Resistance Spot Welded Joints Made on Ultra-high-Strength Steel Sheets

In the automotive industry there is an increasing demand for the reduction of CO2 emissions, which requires new generations of high strength steels. The high yield strength is originated with the combination of alloying elements, rolling and heat treatment. In the ultra-high strength steel (UHSS) category the microstructure is contain more and more martensite which results higher strength but less elongation. During welding, these properties irreversibly changes by the heat cycle, for example in the heat-affected zone (HAZ) strength reduction can be expected. Because these problems necessary to investigate the weldability of these UHSS sheets. In the automotive industry the most frequently used welding process is the resistance spot welding for thin sheet welding (e.g., in case of chassis elements). This is a precise pressure welding process so it uses pressure force and welding current, which means it has a thermomechanical effect on the base material. Therefore, in this case it has more chance to optimise the parameters for these welding sensitive materials. The present research work aims to investigate the mechanical properties of resistance spot welded joints of 1200 and 1400 MPa tensile strength steel sheets. The heat-affected zone softening and other weldability problems are investigated too. Very important to know how can the mechanical properties improve by technological parameter optimisation, so in this article it is examined and the results are written too. Previously the resistance spot welded joints properties were investigated in case of lower strength steel sheets like DP1000, DP800 and DP600. In this article a comparison is made between resistance welded joints of steel sheets from 600 MPa to 1400 MPa joint properties. This comparison basically focuses on weldability, mechanical properties, and welding technological modifications, and according to these, some designing conclusions are written.