Temperature and torque in FSSW of steel sheets: experimental measurements and modelling

The torque and temperature evolutions with process parameters in friction stir spot welding (FSSW) of steels were determined and modelled in this work. Four base materials, including two mild steels (DC01 and DC05) and two high-strength steels (HC420 and DP600), were welded using tungsten carbide pinless tools. The relationship between the thermo-mechanical conditions developed during welding and the process parameters was analysed by performing temperature and torque measurements. Tool damage was assessed and related with the welding time. It was found that both the tool diameter and the rotational speed have a strong influence on the torque and temperature in FSSW. Additionally, a maximum threshold temperature of 1100 °C was registered for all the steels tested. It was also observed that, while for the lower strength steels, the temperature evolves with the process parameters, for the higher-strength steels, the welding temperature remains close to the threshold value, independent of the process parameters. This shows the important influence of the base material properties on the heat generation. Analytical models to predict the torque and temperature from FSSW process parameters were developed, calibrated and validated using the experimental data.