*3.2. Effects of Heat Treatment on Joint Microstructure*

The Ti3Al LFW joints were heat treated at 700 ◦C, 750 ◦C, 800 ◦C, 850 ◦C, and 900 ◦C, respectively, and the corresponding microstructures after heat treatment are shown in Figure 7. After being heat treated at 700 ◦C, the central zone of the joint was composed of small equiaxed grains with a size of around 10 μm. There was a wider grain boundary and dot-like structures less than 1 μm precipitated inside the grains, as shown in Figure 7a. Mount of the dot-like structures (white color point) appeared in the grain and grain boundary reduced in the width, as shown in Figure 7b,c. Under higher temperature, the dot-like structures inside the equiaxed grains began to grow and transform to short acicular structures at 850 ◦C, as shown in Figure 7a. When the temperature reached 900 ◦C, the size of the short acicular structures inside the equiaxed grains was close to 3 μm, and exhibited a trend of becoming lamellar, as shown in Figure 7e.

**Figure 7.** *Cont.*

**Figure 7.** The Ti3Al LFW joint microstructures in the WZ after heat treatments at different temperatures: (**a**) after heat treatment at 700 ◦C, (**b**) after heat treatment at 750 ◦C, (**c**) after heat treatment at 800 ◦C, (**d**) after heat treatment at 850 ◦C, and (**e**) after heat treatment at 900 ◦C.

The TMAZ of the as-welded joint was mainly composed of deformed α<sup>2</sup> phase and metastable structures. After heat treatment at 700 ◦C, the morphology of the residual deformed α<sup>2</sup> phase did not change significantly compared to that in the as-welded joint. The dot-like structures precipitated between α<sup>2</sup> phases, while short acicular structures appeared locally, as shown in Figure 8a. As the temperature increased, the size of the dot-like structures increased, while the size of the α<sup>2</sup> phase decreased, which was consistent with the variation in the dot-like structures inside the equiaxed grains at the center of the joint, as shown in Figure 8b. More specifically, at 800 ◦C, the morphology of the dot-like structure began to transform into short acicular structure, as shown in Figure 8c. It began to exhibit lamellar structure and the size of the α<sup>2</sup> phase decreased further at 850 ◦C, as shown in Figure 8d. The average length of lamellar structures is up to 5 μm, as shown in Figure 8e.

**Figure 8.** The Ti3Al LFW joint microstructures in the TMAZ after heat treatments at different temperatures: (**a**) after heat treatment at 700 ◦C, (**b**) after heat treatment at 750 ◦C, (**c**) after heat treatment at 800 ◦C, (**d**) after heat treatment at 850 ◦C, and (**e**) after heat treatment at 900 ◦C.

The microstructure variation in the TMAZ close to the side of the base metal was different than that in the other two zones. The morphology of the α<sup>2</sup> phase did not change significantly with temperature, while the heat treatment led to larger impact on the microstructures between the α<sup>2</sup> phases. At 700 ◦C, the microstructures between the α<sup>2</sup> phases exhibited dot distribution, as shown in Figure 9a. After heat treatment at 750 ◦C, the dot-like structures transformed to acicular structures, and then grew rapidly with the increase in temperature, as shown in Figure 9b. At 800 ◦C, the length of the acicular structures α<sup>2</sup> reached 3 μm but still exhibited a narrow shape, as shown in Figure 9c. At 850 ◦C, the acicular structures α<sup>2</sup> is larger than that in the 800 ◦C sample, as shown in Figure 9d. After heat treatment at 900 ◦C, the length reached approximately 5 μm, as shown in Figure 9e

**Figure 9.** The Ti3Al LFW joint microstructures in the BM zone after heat treatments at different temperatures: (**a**) after heat treatment at 700 ◦C, (**b**) after heat treatment at 750 ◦C, (**c**) after heat treatment at 800 ◦C, (**d**) after heat treatment at 850 ◦C, and (**e**) after heat treatment at 900 ◦C.

In the LFW process, under the combined action of axial and friction forces, the contact surface of the Ti3Al alloy undergoes friction, deformation, heat generation, and extrusion of the interface metal. Therefore, the microstructure of the Ti3Al LFW joint is affected by welding process parameters, such as friction pressure, welding amplitude, welding frequency, and cooling rate. The welding temperature of Ti-based alloys during LFW can reach temperatures above 1200 ◦C. Since the cooling rate after welding is relatively fast, martensite structures can form in TC4 titanium alloys, while metastable β phase can form in Ti17 titanium alloys and near-b titanium alloy [19,32]. From the phase diagram of Ti3Al-based alloys [33], it can be seen that the Ti3Al material has three phase regions: the β + O phase region, the α<sup>2</sup> + β + O phase region, and the β phase region. The microstructures of the as-welded Ti3Al alloy LFW joint indicated that the joint forming between the base material and the interface contains different microstructures, including an α<sup>2</sup> +O+ β three-phase region, an α<sup>2</sup> + metastable β dual-phase region, and a metastable β phase region. In the welding process and under the action of friction pressure, deformation occurs in the aforementioned four phase regions ranging from the base metal to the weld interface.

The degree of deformation is related to the amplitude and extent of the softening and flow of the Ti3Al alloy. The closer to the weld interface, the more severe the deformation. In the welding process, two main transitions occur: O phase → β phase and α<sup>2</sup> phase → β phase. In the weld zone, the temperature, which is over 1200 ◦C, leads to O phase → β and α<sup>2</sup> phase → β phase transitions, but the LFW process is short, so the α<sup>2</sup> phase → β phase transition is incomplete. Thus, there is a

small amount of deformed α<sup>2</sup> phase and the extrusion of more softened alloy, which makes the weld spacing narrower after the upsetting force formation process of LFW. The zone is mainly composed of β phase and a small amount of deformed α<sup>2</sup> phase. Due to the higher cooling rate of LFW, the disordered β phase transits into an ordered β phase, while the α<sup>2</sup> phase remains. As the distance from the weld zone increases, the maximum welding temperature decreases, so the corresponding temperature range is located in the α<sup>2</sup> + β dual-phase region. Within this temperature region, the O phase transforms to β phase, while the partial α<sup>2</sup> phase transforms to β phase. During the cooling process, the residual deformed α<sup>2</sup> phase can remain until room temperature, therefore the α<sup>2</sup> portion in the WZ is higher than that in the TMAZ close to near the WZ. For the TMAZ close to the BM, the welding temperature is relatively low so the high temperature region is located at the α<sup>2</sup> + β + O phase region and the β + O phase region. Within this temperature region, the α<sup>2</sup> phase is relatively stable and basically does not transform into β phase. Due to the fact that the O phase has poor stability, the transition of O phase → β phase occurs easily during the welding process. The higher the temperature, the more complete the transition and the lesser the O phase. Therefore, in the TMAZ near the base metal of the as-welded joint under thermo-mechanical action, the O, β, and α<sup>2</sup> phases in the deformed microstructure do not transform to β phase. The difference in the relationship between temperature and stress/strain in the different regions of the material during the LFW process has a great and large-area impact on the microstructure (morphology, size, and volume fraction of the β, α2, and O phases) at the different joint regions. After heat treatment, the metastable β phase decomposes into α2, O, and β phases. The significant differences in the microstructure of each zone of the as-welded joint are responsible for the large differences in the microstructure of each zone of the joint after heat treatment. The more obvious impact is that of the different heat treatment temperatures on the size and morphology of the α<sup>2</sup> phase formed by the decomposition of the metastable β phase.
