Clarification of microstructure and texture evolution of friction stir-welded dissimilar AA7075/AZ31B using ultrasonic assistance

The trade-off between welding and strength loss during dissimilar metals joining has been important for decades. Strength loss minimization is essential for industries. The current study investigates the microstructure clarification in the nugget zone (NZ) of AA7075/AZ31B dissimilar friction stir welded joints with or without ultrasonic vibration (UV) at different tool offset conditions. The reduction of incomplete dynamic recrystallization towards the AA7075 side and its transformation into complete dynamic recrystallization (DRX) were observed through the analysis of the grain microstructure, which showed that ultrasonic vibration improved dislocations and grain boundaries. Texture analysis discloses high-strain texture towards the AA7075 side, whereas less complex stresses texture for the AZ31 side under UV. Moreover, the grain refinement on both sides of the interface also affects the joint tensile strength. The maximum strength was obtained with ultrasonic vibration. The Schmid factor results reveal that the UV has a minor influence on the slip system towards the Al side. Conversely, it activates the twining system towards the AZ31 side.