**Xiaohong Li 1,2, Jianchao He 2, Tiancang Zhang 2, Jun Tao 2, Ju Li <sup>2</sup> and Yanhua Zhang 1,\***


Received: 3 March 2019; Accepted: 8 April 2019; Published: 10 April 2019

**Abstract:** Heat treatment at different temperatures was carried out on a Ti3Al linear friction welding joint. The characteristics and evolution of the microstructure in the weld zone (WZ) and the thermo-mechanically affected zone (TMAZ) of the Ti3Al LFW joint were analyzed. Combined with the heat treatment after welding, the effect of the heat treatment temperature on the joint was discussed. The test results indicated that the linear friction welding (LFW) process can accomplish a reliable connection between Ti3Al alloys and the joint can avoid defects such as microcracks and voids. The weld zone of the as-welded Ti3Al alloy joint was mainly composed of metastable β phase, while the TMAZ was mainly composed of deformed α<sup>2</sup> phase and metastable β phase. After being heat treated at different temperatures, the WZ of the Ti3Al LFW joint exhibited a significantly different microstructure. After heat treatment at 700 ◦C, dot-like structures precipitated and the joint microhardness increased significantly. Subsequently, the joint microhardness decreases with the increase in temperature. Under heat treatment at temperatures above 850 ◦C, the formed structure was acicular α<sup>2</sup> phase and the joint microhardness after heat treatment was lower than that of the as-welded joint.

**Keywords:** intermetallic compound; linear friction welding; microstructure; heat treatment
