Microstructure evolution and mechanical properties of a linear friction welded TC4-DT titanium alloy joint

In this research, linear friction welding of TC4-DT titanium alloy with Widmannstatten microstructure characteristic was conducted. The microstructure evolution, mechanical properties and fracture mechanism of this joint were carried out in detail. Results reveal that the joint could be divided into four zones according to their various microstructure: base metal (BM), heat-affected zone (HAZ), thermo-mechanically affected zone (TMAZ) and weld zone (WZ). The HAZ showed coarsening of the β phase, while the TMAZ showed α colony deformation and precipitation of the fine secondary α phase. In WZ, complete transformation to α' martensite structure occurred. An “M” shaped hardness profile developed across the weld, with maximum hardness in the TMAZ due to work hardening and precipitation strengthening effects. The WZ also showed higher hardness than the base metal because of the α' martensite strengthening. The average ultimate tensile strength, average yield strength and average elongation measured were 855.5 MPa, 745.1 MPa and 7.6 % as well, all satisfying the TC4-DT standard. Fracture testing showed that failure occurred in the base metal rather than the weld line, as the weld joint had higher strength. The crack propagation was not in a straight line, following the lamellar α/β boundaries, which is associated with the damage tolerant characteristics of TC4-DT with a Widmannstatten microstructure.