Elucidating the interface microstructure and local strain distribution during dissimilar ultrasonic vibration assisted friction stir welding of Al/Mg alloys

The interface microstructures and strain distribution of dissimilar friction stir welding (FSW) of AA7075/AZ31B joints with or without ultrasonic vibration were examined. The results reveal that the ultrasonic vibrations and tool offset greatly influence the dynamic recovery towards the Mg side by reducing the non-indexing region at the interface. Moreover, the grain recrystallization near the interface differed from the other areas due to severe deformation. Continuous dynamic recrystallization (CDRX) occurs at the AA7075 side, whereas the Mg side exhibits discontinuous dynamic recrystallization (DDRX). The higher net strain component was observed for ultrasonic-assisted FSW (UVaFSW). The strain component was changed significantly towards the AA7075 side compared with the AZ31B side. It occurred because of different recrystallization mechanisms. The strain component was well spread towards the AA7075 side, whereas the dispersed strain component was observed towards the AZ31B. Along the plate thickness direction, the intermetallic compound layer (IMC) thickness first increases, then decreases, reaching a maximum value of 1.5 to 1.8 mm from the top. The ultrasonic vibration reduced the overall thickness of intermetallic in the joint interface, regardless of the tool offset conditions.