*2.2. Experimental Methods*

The LFW test was carried out on an LFW-20T equipment, self-developed by AVIC Manufacturing Technology Institute (Beijing, China). The equipment is shown in Figure 2a and the welding principle of the LFW process is shown in Figure 2b. One specimen of a pair to be welded was held by a clamp with a reciprocating motion and the other was held by a clamp having a horizontal motion. During welding, one specimen started to reciprocate with high frequency and small amplitude under the action of the exciting force, while the other gradually moved toward the reciprocating specimen under the action of thrust. Once the specimens come into contact, the convex portions of the interface rub each other. As the frictional pressure increased, the actual contact area increased, and then the frictional force increased rapidly. Accordingly, the interface temperature increased and the friction interface was gradually covered by a layer of high-temperature visco-plastic metal. Under the action of upset pressure, the metal in the welding zone underwent plastic flow and the extruded metal formed a flash. The metal components on the two sides were firmly welded together through mutual diffusion and recrystallization of the metal in the welding zone, and the entire welding process was completed.

**Figure 2.** (**a**) The LFW-20T equipment; (**b**) Basic mechanism of the LFW process. The parameters of the Ti3Al LFW process are listed in Table 2.


**Table 2.** Welding process parameters.

The cooling rate is very fast after LFW, and the joint is very narrow with severe plastic deformation. So the range from 700 ◦C–900 ◦C was used to research the evolution of microstructure and Properties of the Ti3Al joint. After the LFW process, the joints were heated in vacuum at 700 ◦C, 750 ◦C, 800 ◦C, 850 ◦C, and 900 ◦C for four hours and were cooled in a furnace. The specimen for microstructure characterization was taken along the direction perpendicular to the welded surface, and then the sample was ground and polished. Aqueous solutions of hydrofluoric acid and nitric acid were adopted successively to etch the sample. Subsequently, the sample was observed using a Zeiss field-emission scanning electron microscope (Jena, Germany) at the test center of the China Aviation Manufacturing Technology Research Institute. The specimen for microhardness measurement was subjected to several processes, including inlaying, polishing, and etching. The microhardness characterization was performed on an Tukon2500 microhardness tester (Wolpert Wilson, IL, USA), where the microhardness of an as-welded joint and a welded joint after heat treatment was measured at a load of 0.2 kg for 15 s. Based on the distance from the center of the joint, at least five points were selected to measure the microhardness of each zone, among the base metal (BM), the TMAZ, and the weld zone (WZ) of the Ti3Al joint. The point-to-point spacing was greater than 0.2 mm. The position for the microstructure and microhardness measurements is shown in Figure 3a. A tensile test piece was prepared according to the HB5143-96 standard, in order to measure the tensile properties of the joints at room-temperature, as shown in Figure 3b. The center line of the joint remained at the center of the tensile test sample and three test pieces were taken from the specimens under different heat treatment conditions.

**Figure 3.** (**a**) Schematic diagram of the intercepting process for the metallographic specimen; (**b**) Tensile specimen dimensions.
