Effects of rotating speed on microstructure and mechanical properties of Al/Ti friction stir welding joints with trailing support

The property differences between Al and Ti pose challenges in achieving a high-quality bonding interface during friction stir welding (FSW). Optimizing the bottom heat behavior in Al/Ti FSW is crucial for improving interface bonding. Trailing support FSW (TSFSW) technology replaced the traditional back support with a trailing support column, and prevented huge heat loss from the joint bottom. The temperature in the bottom weld was similar to or higher than that in the middle weld. The ranges of Al-SZ and Al-TMAZ at the bottom weld were both enlarged. Plasticized Al metal achieved a cycle flow depending on three flow routes, while Ti metal had two local and independent flow routes. The horizontal and vertical bonding interfaces of Al and Ti were generated because of unique material flow. The tearing zone, mixed zone, local IMCs block and continuous IMCs layer were gradually formed in the vertical interface as heat input increased. The mixed zone characterized as TiAl₃/Ti laminated pattern was the main interface structure, while the continuous IMCs layer including TiAl₃ and TiAl phases formed only at excessive heating (650 rpm). The joints obtained by 450 rpm and 650 rpm both fractured at the Al/Ti interface, leading to low tensile and interface strength. The maximum joint strength and interface strength of 223 MPa and 251 MPa were achieved at 500 rpm. These two strength values reached 73 % and 82 % of Al-BM strength, respectively.