Effect of rotational speed on microstructure and mechanical properties of Al-Ti friction stir welds with ultrasonic vibrations

Ultrasonic vibration-enhanced friction stir welding (UVFSW) has been conducted on Al and Ti alloys at various rotational speeds with the aim of improving the microstructure and mechanical properties of the resultant welds. The application of UVFSW in Al-Ti welding could limit the formation of intermetallic compounds at Al-Ti interface as well as improve the weld's appearance and microstructure. Ultrasonic vibrations reduced the sizes of flash and wave, indicating the improved material flowability in UVFSW. At the UVFSW interface, Ti substrate was partly fragmented, forming Ti particles at the interface, while the presence of Ti particles decreased with the increase of rotational speed. In lap shear tests, the ultimate load subjected to UVFSW welds was 4.40 kN and achieved at 1000 rpm, whereas the ultimate load of FSW welds was 2.52 kN at 1200 rpm. Shear dimple was the dominant characteristic of the fracture of UVFSW welds, whereas FSW welds mainly exhibited faceted characteristics in fracture.