High-strength steel sheets of HT980-class are beginning to be used for automobile bodies in order to reduce body-weight and improve fuel-efficiency. Resistance spot welding is an important method in joining of high-strength steel sheets. The microstructure of welds becomes martenstic structure and high tensile residual stress is generated. It is important to ensure the performance of resistance spot welds in order to use high-strength steels more widely in automobiles. Therefore, the evaluation of characteristics of resistance spot welds is quite important.
In this study, residual stress of resistance spot welds of HT980 sheets were investigated by numerical simulation and X-ray stress measurement. First, resistance spot welded test pieces were fabricated and nugget formation process was observed. Residual stress distribution in the welds of the test pieces was measured by X-ray diffraction method with a two-dimensional detector. High tensile residual stress was measured in the spot welds. The measurement results for the three test pieces were comparable. Second, residual stress distribution in the resistance spot welds was calculated by current-temperature-microstructure-mechanics coupled simulation. Comparison between the experiment and the numerical simulation proved that nugget formation process was reproduced by the simulation. Residual stress distribution obtained by the simulation was in good accordance with the measurement results when initial residual stress in HT980 sheet before welding was taken into consideration.
