Elucidation of welding speed on the microstructure and mechanical properties of medium-thick dissimilar Al/Ti double-side friction stir welded joint

Conventional friction stir welding requires a large tool for welding medium-thick plates, which makes it chal-lenging to obtain high-property medium-thick Ti/Al joints due to relatively high heat generation and temper-ature near the shoulder. Therefore, double-side friction stir welding (DS-FSW) which employs a relatively small tool was used to join dissimilar medium-thick plates of TC4 titanium alloy and 2024-T4 aluminium alloy. The influence of welding speed on the weld formation, microstructure, welding loads, temperature, and performance of joints was studied systematically. It reveals that the welding distortion occurs after welding the first-side weld, which results in a lower temperature and welding loads for the second-side weld than those for the first-side weld. Furthermore, a sound joint can be obtained together with a diffusion-type interface under the welding speed of 75 mm/min. When the welding speed is reduced to 30 mm/min, the typical intermetallic compounds (IMCs) consisting of TiAl and TiAl3 are generated at the interface. In addition, the hook structure located in the center of the joint interface extends far to the SZ in the medium-thick Ti/Al DS-FSW dissimilar joint, which results in the formation of cracks on both sides of the hook structure. By contrast, the better mechanical per-formance of the joint can be obtained when the welding speed is relatively high. It implies that the welding speed severely determines the joint performance by affecting the joint microstructure and the welding distortion after the first-side weld in the DS-FSW of medium-thick Ti/Al dissimilar alloys.