Interface enhancement mechanism induced by nanoscale interface structures in Al/Ti dissimilar joints using ultrasonic assisted trailing support friction stir welding

A high-quality bonding interface between Al and Ti is a challenge in trailing support friction stir welding (TSFSW). In this study, ultrasound was applied to the Al/Ti bottom interface. The Al/Ti joints obtained by ultrasonic assisted TSFSW (UA-TSFSW) and TSFSW were analyzed and compared. Ultrasound regulated the peak temperature via acoustic anti-friction and acoustic softening effects, while the welding axial load was influenced by thermal and acoustic softening effects. At moderate (500 mm/min) and cold (700 mm/min) parameters, ultrasound increased the peak temperature and decreased the axial load, but the opposite results were observed at the hot parameter of 300 mm/min. The main interface consisted of metallurgical bonding (intermetallic compound (IMC) layers), hybrid bonding (tearing and mixed layers) and mechanical interlocking structures. Ultrasound influenced the fragmentation and distribution of original oxide films, thereby affecting the interface IMC structure. Ultrasonic vibration induced more complete fragmentation of oxide films in the regions with poor material flow. A nanoscale TiAl₃ layer replaced the oxygen-containing amorphous layer at the main interface of 700 mm/min and the Ti-protrusion interface of 500 mm/min. The nanoscale TiAl₃ layer provided a stronger bonding interface than that of the oxygen-containing amorphous layer. Compared with TSFSW, the maximum joint strength and interface strength of UA-TSFSW were increased to 237 MPa and 274 MPa, which reached 77.8 % and 89.8 % of the Al base material, respectively. UA-TSFSW offers an effective approach to enhancing the Al/Ti interface strength.