Mechanism of ultrasonic effects on thermal-stress field in Cu/Al-FSW process

This paper examined the mechanism of ultrasonic depressive effects on Al/Cu intermetallic compounds (IMCs) during friction stir welding (FSW) process by combining numerical simulations and macro/microstructural evolution analyses. The coupling process of ultrasonic field and the multienergy field for Al/Cu-FSW was simulated for the first time. The ultrasonic mechanisms of the acoustic thermal effect, acoustic anti-friction effect and acoustic softening were considered in the constitutive model and thermal-stress boundary conditions. Based on our research, ultrasonic vibration (UV) could homogenize the Al/Cu plastic difference and the Al/Cu rheological properties in the mixed zone. The modeling results showed that the difference in flow stress between Al and Cu was reduced on the two sides of the weld. The gradient of the phase-volume fraction in the weld cross section was reduced under UV, which reduced the power of Al/Cu interdiffusion and inhibited the generation of IMCs via diffusion. In addition, the acoustic softening and antifriction effects reduced the contact interfacial heat flux and the peak temperature. The heat generation within the nugget zone decreased by approximately 12.8 %. The UV reduced the absolute value of the effective heat of formation (EHF) for Al2Cu and Al4Cu9, which indicated that ultrasound could inhibit the generation of IMCs to some extent. The decrease of heat and concentration gradient weakened the atomic diffusion and growth rate of IMCs. Therefore, the SEM images of the Al/Cu interfaces showed that the total thickness of the Al/Cu-IMCs decreased by approximately 30.5 %, the distribution of IMCs was more uniform, and the boundaries were straighter under ultrasonic effects. Under ultrasonic action, more fine Cu particles entered the Al side, and the Cu particles were scattered, showing a dispersed distribution. Furthermore, acoustic softening enhanced the plastic flow and increased the number of layered staggered structures at the Al/Cu interface, which enhanced the mechanical interlocking of the Al/Cu composite. The tensile strength improved 21.3 % under the ultrasonic treatment.