Effects of microstrcture composition and orientation on micro mechanical properties of linear friction welding joint weld zone for heteromorphic TC17 titanium alloy

In this research, the microstructure and micro mechanical property of linear friction welded joint for heteromorphic TC17 titanium alloy were investigated by nanoindentation testing and electron backscatter diffraction technique. The results show that the special microstructure in the weld zone (WZ) of the joint has great effects on its micro mechanical property. The WZ is composed of the transformed beta phase, which exists in the form of equiaxed substructure, and the microstructure of the WZ can be divided into two parts along the centerline of the weld based on its main orientation. The WZ of the TC17 (alpha + beta) side has the main orientation < 112 > and < 111 >, while the TC17 (beta) side has the main orientation < 101 >. Meanwhile, more high angle grain boundaries (HAGB) are found in the WZ of the TC17 (alpha + beta) side. These microstructure characteristics have a significant impact on the micro mechanical property of the WZ. Due to more HAGB are in the WZ of the TC17 (alpha + beta) side than the TC17 (beta) side, the interface strengthening effect is a slightly stronger in the TC17 (alpha + beta) side, which leads to better elastic modulus, hardness, and fracture toughness. While the TC17 (alpha + beta) side microstructure has the {101} < 111 > and {112} < 111 > slip modes that are easier activated under RD direction force, so the microstructure in the WZ of the TC17 (alpha + beta) side has a smaller yield strength and a larger hardening exponent than that in the WZ of the TC17 (beta) side. In addition, it was also found that the area in the edge of the WZ near thermo-mechanically affected zone of the TC17 (alpha + beta) side (TC17 (alpha + beta)-TMAZ) has the minimum micro mechanical property and is the weak area of the WZ, which is related to the lower degree of recrystallization compared to other areas of the WZ. This area has the smallest elastic modulus (146.4 MPa), hardness (3.4 GPa), yield strength (904 MPa), and fracture toughness (0.173 MPa*m1/2), while the hardening exponent is 0.107. The macro tensile fracture location also matches the weak area in the WZ of the joints. Besides, it was also found that the average yield strength and tensile strength of the joint are 1016 MPa and 1065 MPa, while the elastic modulus and elongation are 111.2 GPa and 2.1%. And the microhardness distribution of the joint presents a "w" shape, which is due to lower phase alpha/beta interface content in the WZ of the LFW joint for heteromorphic TC17 alloy and leads to the minimum mechanical property.