Study on the longitudinal superplastic properties of TC4 titanium alloy laser welding joint with rare earth elements

Aiming at the problem of low superplasticity of titanium alloy laser welded joints, this paper proposes to add rare earth Yb element in the form of Yb₂O₃ in the weld, and analyzes the effects of Yb₂O₃ content on the peak flow stress, elongation and microstructure of laser welded joint of TC4 titanium alloy at high temperature. The results show that Yb₂O₃ can reduce the β grain size of the weld and the superplastic deformation flow stress of the joint, increase the elongation, to promote the equiaxed process of the deformation process. With the increase of Yb₂O₃ content, the stress-strain curve of the joint moves down, the peak flow stress decreases first and then increases, while the elongation increases first and then decreases. In the process of superplastic deformation the lowest peak flow stress is 11.9 MPa and the elongation is 572.3% when the content of Yb₂O₃ is 6 wt.%. The minimum grain size of β in the weld center is 127 μm, which is 62.3 % finer than the 337 μm of the β grain size without Yb₂O₃. At this time, the martensitic structure undergoes phase transformation, the morphology changes into lamellar, the degree of equiaxed structure in the weld area is the highest, and the superplasticity of the weld is the best.