An examination of the resistance spot welding process of the ultra-high-strength steel with experimental and numerical methods

Since ultra-high-strength steels are being preferred in the automotive industry, the weldability of these steels has been gaining importance. When the scientific literature on the resistance spot weldability of UHSS is investigated, it is seen that additional studies are needed to fully understand this process. So, the motivation of this study is to get more information through numerical, and experimental methods about the resistance spot welding process of UHSS. In this study, the finite element model of the RSW process was generated as an axisymmetric and fully electrical-thermal-mechanical coupled via MSC. Marc software. MS1500 ultra high strength steel is selected as workpieces, and this process included the joining of the workpieces with the same sheet thicknesses (1.2 mm) and the same material in this study. This study provided a prediction of the quality, shape of the weld nugget, and the predicted temperature distribution via finite element simulations with the variations of process parameter which is current values. Firstly, two main simulation were generated with 8 kA ideal sinusoidal alternative current, and alternative current with phase shift to convergence of the finite element models for the real RSW process. As a result, the heat generation amount for the ideal sinusoidal alternative current is higher than the phase shifted current one during welding time. Comparing the simulation results with physical experiments, the alternative current with phase shift was preferable to constructing the FE model of the RSW process. When comparing the experimental results and the results of generated simulation with 8 kA phase shifted current, there is a relatively minor 2% error in the diameter length and 40% error in nugget thickness at the last welding cycle. The nugget shape in the experimental result was an ellipse, however, the nugget shape in the simulation result was a flat ellipse. After that, all simulation model were generated with the phase shifted current values of 7; 7.5; 8.5; 9; 9.5 and 10 kA for different simulation models. Based on the simulation results, the nugget diameter increases as the phase shifted current value increases. However, as the current value increases, temperature values between workpiece/workpiece and workpiece/electrode have increased because the energy input in the welding cycles increases. As a consequence, material melting between workpiece/electrode was visible in the generated simulation with 8.5; 9; 9.5 and 10 kA phase shifted current.