**1. Introduction**

As a type of Ti-Al series intermetallic compound, Ti3Al-based alloys have better high-temperature performance, oxidation resistance, creep resistance, and higher service temperature than ordinary Ti alloys. Compared with the Ni-based alloys, the density of Ti3Al-based alloys is lower and about half that of Ni-based alloys, which is beneficial for the reduction in equipment weight. As a high-temperature structural material that can fill the temperature gap between the serving temperatures of Ti alloys and Ni-based superalloys, Ti3Al has a good application prospect in the aerospace field, such as to substitute for structural materials with lower strength, to reduce the weight of engines of various vehicles as well as the vehicle's own weight, and to enhance the specific thrust and efficiency of engines. Owing to the above advantages, Ti3Al-based alloys have broad application prospects, since they can meet the urgent needs of future aerospace component structures for light structural materials with high specific strength, high specific modulus, and excellent overall performance [1–4]. In the application of Ti3Al, it is important to solve the jointing problem between the same or different materials that will directly affect its application [5,6].

The main problem in the welding of Ti3Al alloys is that the joint has low room-temperature plasticity and is very sensitive to cracks during solidification. The transition from β phase to α phase is one of the main factors affecting the welding of Ti3Al. During the welding process of Ti3Al alloys, the cooling rate effect on the joint performance plays a decisive role, i.e., as the cooling rate increases, the β phase is cooled rapidly, and the main phase in the microstructure of the welded joint is metastable β phase [7]. The β phase is relatively soft and has a lower fracture toughness, and it is thermodynamically unstable, thus, the transformation from β phase to α<sup>2</sup> phase will occur in high temperature applications. At a slightly lower cooling rate, a very hard α<sup>2</sup> (martensite) phase easily forms in the joint and exhibits very high hardness and enhanced brittleness [8,9]. Another problem in the fusion welding of Ti3Al is that the thermo-mechanically affected zone (TMAZ) is overheated, and thus, the grains are prone to undergo irreversible coarsening and exhibit significantly increasing brittleness. In addition, hot cracks occur in the joints when a thicker slab is welded. However, the potential advantage of solid-phase welding is to avoid casting structures, gas pores, cracks, deformations, and residual stresses caused by material melting during fusion welding.

As one of the solid-phase welding technologies, weld joints are formed on the contact surface through plastic deformation, surface activation, diffusion, recrystallization, and interaction between rubbing bodies. Linear friction welding (LFW) has a series of advantages, including that the joint is a forged structure, the grain-refined structure is dense, the joint quality is high, there is no dust and weld spatter during the welding process, no filler material and gas protection is needed, and there is less material loss and fewer weld defects [10]. Due to the aforementioned advantages, LFW has been widely used in the welding of carbon steel and stainless steel [11–13], aluminum alloys [14,15], titanium alloys [16–19], and nickel-based alloys [20–22], and has become the key technology in the manufacture and repair of Ti-alloy integral discs of aero-engines [23,24]. The LFW process is a thermo-mechanical coupling process that occurs under the action of axial and friction forces, heat generation, phase transformation, and deformation at the welding interface. The phase composition and microstructural characteristics of different materials lead to significant differences in the deformation mechanism and phase transition characteristics in the interface and their vicinity under the thermo-mechanical coupling during LFW. In the LFW process, the main structures of the lamellar-equiaxed dual-state α + β type TC4 Ti alloy in the TMAZ are composed of deformed α phase and acicular martensite, while the weld nugget is composed of martensite [25,26]. In the basket weave structure of TC17 LFW joint, the microstructure in the TMAZ is composed of smaller deformed β grains that formed under the effects of force and temperature duing LFW, and in which residual acicular α phase and metastable β phase are scattered, while the WZ is composed of metastable β phase with fine grains [27,28]. The microstructure of the Ti-Al series of Ti2AlNb alloy includes O phase and α phase. In the LFW joint, the microstructure in the TMAZ is metastable β phase, residual O phase, and α phase, where the O phase structure disappears before the α phase. The main phase in the weld nugget zone is metastable β phase with fine grains [29].

Compared to the above materials, the Ti3Al alloy has a distinguishable microstructure, since it is mainly composed of β, α2, and O phases. The structural and physical properties of β, α2, and O phase are significantly different [30]. In the LFW process, the thermo-mechanical action may have great impact on the deformation and phase transition of those three phases. However, the deformation differences of the three phases in the joint and the effect of microstructure variation caused by heat treatment on the performance of the joint have been rarely reported in the literature [31]. In this study, Standard heat treatment of the Ti3Al alloy is 980 ◦C (solution treatment) + 800 ◦C (aging treatment). LFW was performed on Ti3Al alloy, and then the joints were subjected to different heat treatments. The microstructural characteristics of the Ti3Al alloy LFW joint and the effect of heat treatment on the microstructure and mechanical properties of the joints were specifically analyzed.
