Interface characterization and formation mechanism of Al/Ti dissimilar joints of refill friction stir spot welding

In recent years, lightweight structures have been widely used in aerospace field. Refill friction stir spot welding has been widely used in dissimilar material joining field in recent years because it can eliminate keyhole defects and reduce welding heat input. In this paper, the Al/Ti-welded joint without keyhole defects was obtained by refill friction stir spot welding. The interfacial layer of joint consists of the solid solution layer and the supersaturated intermetallic compound layer. Aluminum and titanium atoms at the interface diffuse to form a solid solution layer during welding. Furthermore, the supersaturated atoms precipitated from the solid solution layer to produce TiAl3. Thermodynamic analysis proved that TiAl3 is produced because it has the lowest Gibbs free energy. The 50-nm-thick interfacial layer is formed instantaneously during the welding process. Firstly, the basic conditions of reaction temperature compound formation were proved by numerical simulation results. In addition, the numerical simulation results show that intense strain occurs in the material near the interface during the welding process. The resulting dislocation acts as a channel for atomic diffusion and accelerates atomic diffusion.