Microstructure evolution and mechanical behavior of Ti-43.5Al-4Nb-1Mo-0.1B alloy joints produced by linear friction welding

Linear friction welding (LFW) is considered a potential manufacturing process for aero-engine TiAl alloy blades; however, the interaction mechanism between welding parameters and the microstructure-properties of the joint is insufficiently understood. In this work, the effect of welding pressure (100 MPa, 150 MPa, 200 MPa) on the microstructural evolution and mechanical properties of welded joints was investigated by LFW on a Ti-43.5Al-4Nb-1Mo-0.1B alloy (named TNM alloy). The study reveals that the coupling effect of high temperature and severe mechanical pressure in the LFW induces a transformation from the dual-phase structure with thicker lamellae in the base material (BM) to a near-lamellar structure with equiaxed γ-grains and finer α + γ lamellae in the welding zone (WZ). High-pressure-induced mechanical fragmentation and thermo-strain-induced DRX mechanisms result in significant grain refinement. The highest DRX fraction of 91.74 % is obtained at WZ within the joint under 100 MPa. However, the finest average grain size with 2.61 μm is observed in WZ under 200 MPa due to the strong mechanical fragmentation effect. Furthermore, the strong thermo-mechanical effects promote dislocation multiplication, and the α-phase slip systems are preferentially activated, forming a strong basal plane texture with <0001>_α parallel to the vibration direction. The joint exhibited a comparable tensile strength to BM at room temperature with a typical cleavage fracture mode due to the coupled effects between the strengthening induced by grain boundary and dislocation, and the weakening caused by high γ-phase fraction and DRX grains. Moreover, the average ultimate tensile strength (UTS) of the LFWed joints obtained under different welding pressures exceeded 757 MPa, which could reach over 95 % of the BM's UTS. These findings, based on microstructure-property interactions of TiAl alloy joints using LFW, can ideally serve as the basis for future TiAl blade manufacturing.