Resistance spot weldability of interstitial free steel and high strength low alloy steel

Abstract

Interstitial free steels and high strength low alloy steels are two of the most popular grades in automobile sector owing to their excellent formability and high strength-to-weight ratio. This also necessitates the joining of these two dissimilar materials. Resistance spot welding remains the widely used joining technique due to its high process efficiency, cleanliness and low cost. Despite these advantages, for successful implementation of such dissimilar metal welds in applications it is a-priori necessary to determine the optimum welding parameters; however, it is a challenge in case of these two dissimilar materials. Thus, this investigation primarily aims to determine the optimum process parameters for resistance spot welds between extra interstitial free (EIF) steel and a high strength low alloy (HSLA) steel sheets of thickness 1.2 mm. The mechanical properties of the spot welds were determined using universal testing machine (Instron) following tensile shear configuration. Besides, the macrostructure, microstructure, hardness profiles, modes of failure and location of failure were studied for various combinations of welding current and time. It was observed that the maximum load bearing capacity of spot welds under quasi-static loading increased with an increase in nugget diameter. The nugget diameter increased significantly with the decrease in mean dynamic contact resistance between the two sheets during welding. With an increase in heat input and nugget diameter, there is a transition in failure mode from interfacial to pull out mode before eventually facing the prospect of expulsion. In all the cases, crack initiated from EIF steel sheet, particularly at the interface of base metal / heat affected zone. At a welding current of 10 kA, defective welding took place due to flow of molten metal from the weld pool. Thus the optimum welding parameter for the present set of materials and sheet thickness was considered to be 9 kA current and 300 ms time at 2.6 kN electrode force.