Shared mechanism between flow drill screw and friction stir welding and its impact on failure prediction of steel-aluminum joints

Severe plastic deformation and heat generation during the flow drill screw (FDS) process led to profound changes in the microstructures and mechanical properties of the heat-affected zone (HAZ). Microhardness measurements, electron backscattered diffraction (EBSD), and forming process simulation results showed that the HAZ was located within 1.0 mm from the thread tip. According to the peak temperature, temperature distribution, and element size in the thread region, the HAZ was divided into four subzones. The challenge in the numerical simulation of FDS joints lied in how to accurately acquire the mechanical properties of HAZ with limited size. In the present study, large-sized equivalent specimens were prepared with the friction stir welding (FSW) process. Based on the mechanical testing results of the equivalent specimens, a three-dimensional numerical model was established. Simulation results revealed that considering the material softening of HAZ was crucial for accurately predicting the mechanical behaviour of the joints under the tensile loading condition. The prediction accuracy of the peak force and failure displacement was improved by 7.0% as compared with that obtained using the numerical model without considering the material softening of HAZ. Under shear and mixed loading conditions, the error difference of predicted peak force and failure displacement with the two models was less than 3.0%. The deformation and failure characteristics of the joints could be accurately predicted by both the models.