Influences of ultrasonic on friction stir welding of Al/Ti dissimilar alloys under different welding conditions

Ti6Al4V and 6061-T6 Al alloys were joined by ultrasonic-assisted friction stir welding (UAFSW) at different rotational velocities and pin offsets to explore the influencing mechanisms of the ultrasonic. Results indicate that the positive or negative influence of ultrasonic depends on the welding condition. Applying ultrasonic increases the joint tensile strength at 450 rpm ~ 850 rpm rotational velocity and 1.2 mm pin offset because it does not only decreases the void defect and the size of Ti6Al4V fragments but also promotes the diffusion bonding at Al/Ti interface. When a large pin offset of 1.6 mm is used, a thick intermetallic compounds (IMCs) layer is formed at the Al/Ti interface and large Ti6Al4V fragments appear in the stir zone. Applying ultrasonic at the large pin offset induces cracks initiating in the thick IMCs layer and the formation of large void defect in the stir zone. This investigation provides a reference for the reasonable application of UAFSW technique.