*3.1. Microstructural Characteristics of the As-Welded Joint*

The morphology of Ti3Al after LFW can be observed in Figure 4. At the welding interface, under the action of pressure and friction forces, the interfacial temperature increased, then the Ti3Al at the interface reached its molten state, and thus Ti3Al was extruded and formed a flash. After being cooled in air, the flash was grayish white with the same direction. During the LFW process, different materials have flashes with different morphologies, which is highly related to the material physical properties. More specifically, it is easier to form flashes that are extruded as whole bodies from materials with high plasticity and fluidity. As for materials with poor plasticity, during extrusion, the flash separates and curls toward both sides. The morphology of the flash formed by LFW on the Ti3Al alloy is similar to that of ordinary Ti-based alloys, such as TC4, TC11, and TC17, and exhibits integral extrusion (Figure 4). In the linear friction welding process of Ti3Al, the welding temperature can reach over 1200 ◦C, as illustrated in Figure 5.

**Figure 4.** Morphology of the Ti3Al joint after LFW.

**Figure 5.** Temperature curve measured during Ti3Al LFW.

In Figure 6, it can be seen that there were no defects such as voids, cracks, and unwelded regions at the welded interface of the Ti3Al LFW joint. The Ti3Al LFW joint can be divided into three zones: the BM zone, the TMAZ, and the WZ. The width of the WZ was about 80 μm and the entire width of the joint was 700 μm. During the LFW process of Ti3Al, heat was generated at the contact surface under the action of pressure and vibration friction, and then was conducted towards the base metal, which formed a high-to-low temperature gradient from the contact surface to the base material. Meanwhile, under the action of friction pressure, the metal at the interface flowed and extruded from the interface, resulting in a greater deformation of the microstructure. Therefore, the Ti3Al LFW joint was characterized by a microstructure with gradient variation formed at different temperatures and deformation degrees. The microstructure in the different zones of the joint was quite different. In the BM zone, the morphology, distribution, and quantity of the α2, O, and β phases demonstrated had not obvious change, as shown in Figure 6a. In the TMAZ near the side of the BM zone, the lamellar O and β phases deformed and after cooling formed a metastable structure, while the morphology of the α<sup>2</sup> phase was not significantly deformed, as shown in Figure 6e. At regions closer to the joint center, the α<sup>2</sup> phase began to deform and the metastable portion increased as the metal was elongated along the direction of the flow, as shown in Figure 6d. In the TMAZ, close to the side of WZ the α<sup>2</sup> phase increased severely and the length-width ratio increased, almost parallel to the joint center, as shown in Figure 6c. At regions closer to the central part of joint's interface, the α<sup>2</sup> phase further reduced, as shown in Figure 6b. In the WZ, the α2, O, and β phases all transformed to the metastable β phase, except a small amount of deformed a2 phase, as shown in Figure 6a.

**Figure 6.** Microstructures in different zones of the as-welded Ti3Al LFW joint: (**a**) in WZ, (**b**) in TMAZ close to WZ, (**c**) in TMAZ, (**d**) in TMAZ close to BM, (**e**) in BM close to TMAZ, and (**f**) in the BM.
