Unveiling the mechanism of ultrasonic vibration on suppression of intermetallic compound growth in Al/Mg dissimilar friction stir welding

The formation of hard and brittle intermetallic compounds (IMCs) is a critical challenge in the friction stir welding (FSW) of Al/Mg dissimilar alloys, severely deteriorating joint integrity and mechanical performance. Although novel ultrasonic vibration-enhanced friction stir welding (UVeFSW) has been shown to reduce IMCs thickness, the underlying suppression mechanism remains unrevealed. In this study, the role of ultrasonic vibration (UV) in mitigating IMCs formation during the FSW of Al/Mg dissimilar alloy was systematically studied using both experimental and numerical methods. TEM was employed to characterize the IMC layer thickness and grain morphology at the Al/Mg interface within the weld nugget zone. In-situ temperature measurements revealed negligible differences in average welding temperatures between conventional FSW and UVeFSW, with both remaining well below the eutectic temperature. This confirms that IMC formation primarily proceeds through solid-state atomic diffusion. EBSD, micro-XRD, and TEM analyzes consistently showed that UV significantly reduces GND density and overall dislocation content in the interfacial region. It was found that UV suppresses short-circuit diffusion by decreasing dislocation density at the bonding interface, thereby lowering the Al/Mg atomic diffusion rate and effectively reducing the overall IMCs thickness. These findings offering a transformative approach to enhancing the interfacial integrity and mechanical performance of Al/Mg dissimilar welded joints.