Dislocation density based constitutive model for ultrasonic vibration assisted friction stir welding of dissimilar Al/Mg alloys

The establishment of appropriate acoustic plastic constitutive model is fundamental for the prediction of temperature and strain/strain rate histories which are key process variables determining the weld quality in the ultrasonic vibration assisted friction stir welding (UVaFSW) process of dissimilar Al/Mg alloys. In this study, a double-internal-variable dislocation based constitutive model is proposed to describe thermomechanical behaviors without/with ultrasonic vibration (UV) more suitably. Combined with computational fluid dynamics model, the constitutive equation is applied to simulate the UVaFSW process of dissimilar Al/Mg alloys, and the effects of UV on the heat transfer, strain/strain rate and material flow are quantitatively studied. The results indicate that the ultrasound increases the probability of dislocation annihilation and reduces the immobile dislocation density in the plastic deformation area, leading to a significant decrease in material flow stress. Besides, the calculation results under different heat inputs indicate that a reasonable heat input can maximize the beneficial effects of ultrasound in UVaFSW. Compared with the experimental data, the results simulated by the developed constitutive equation is validated with a high prediction accuracy.