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.

The composite structures/components made by friction stir lap welding (FSLW) of Mg alloy sheet and Al alloy sheet are of wide application potentials in the manufacturing sector of transportation vehicles. To further improve the joint quality, the ultrasonic vibration (UV) is exerted in FSLW, and the UV enhanced FSLW (UVeFSLW) was developed for making Mg-to-Al dissimilar joints. The numerical analysis and experimental investigation were combined to study the process mechanism in Mg/Al UVeFSLW. An equation related to the temperature and strain rate was derived to calculate the grain size at different locations of the weld nugget zone, and the effect of grain size distribution on the threshold thermal stress was included, so that the prediction accuracy of flow stress was further improved. With such modified constitutive equation, the numerical simulation was conducted to compare the heat generation, temperature profiles and material flow behaviors in Mg/Al UVeFSLW/FSLW processes. It was found that the exerted UV decreased the temperature at two checking points on the tool/workpiece interface from 707/671 K in FSLW to 689/660 K in UVeFSLW, which suppressed the IMCs thickness at Mg-Al interface from 1.7 µm in FSLW to 1.1 µm in UVeFSLW. The exerted UV increased the horizontal materials flow ability, and decreased the upward flow ability, which resulted in the increase of effective sheet thickness/effective lap width from 2.01/3.70 mm in FSLW to 2.04/4.84 mm in UVeFSLW. Therefore, the ultrasonic vibration improved the tensile shear strength of Mg-to-Al lap joints by 18 %.