Research On The Microstructure And Property Of High-niobium TiAl/Ti600 Electron Beam Welded Joints

TiAl alloys and high temperature titanium alloy are an attractive alternative candidates for the rocket engine thrust chamber because of its low density and high specific strength, especially maintained sufficient strength and stiffness at elevated temperatures. Owing to TiAl alloy welding severe cold cracking tendency, it affect s the joint performance seriously, so the joint crack formation mechanism and control measures of 2mm thick high-niobium TiAl/Ti600 alloy electron beam welding(EBW) were studied.Through the EBW of high-niobium TiAl/Ti600 alloy, this article found that the joint forms cold cracking easily. The main causes include microstructure embrittlement and welding residual stresses. In terms of microstructure, large acicular α/α2 phases were formed in the fusion zone due to the fast cooling. Fine acicular microstructures increase the joint’s brittleness and produce cracks easily. In terms of residual stress in the joint, the residual stress field by the finite element simulation shows that the joint’s residual stresses exceed the room temperature yield strength of high-niobium TiAl alloys on account of fast cooling rates of EBW, such joints are prone to generate micro-cracks at low stress, which greatly affect the joints the use of performance.Considering of Ti3 Al alloy weldability analysis and the cooling rate impacting on the microstructure and stress, this article proposed to adopt electron beam deflection welding and electron beam compensation welding to control the generation of joint cracks. When using welding of electron beam deflection, the numbers of joints crack decreased with increasing beam deflection distance. However, the mechanical properties were decreased owing to the enhancive α phase. Especially when the offset distance reached 0.5 mm, the incomplete fusion defect was appeared. When electron beam compensation welding has been adopted, fine aciculars and basket-weave a(14)a(17) ncreased the toughness, crack-free joint was obtained. With an increasing in the number of compensation, the joint’s mechanical properties increased significantly. In addition, the hybrid welding of beam deflection and compensation have also been studied, the results showed that: hybrid welding could also be obtained sound joints, but the joint’s properties decreased compared with the beam compensation welding, so beam compensation welding was an effective measures to control cracks.To verify the effect of residual stress on the joint’s crack, the results for the finite element simulation of high-niobium TiAl/Ti600 alloy EBW revealed that: longitudinal residual stress of the center-welding fusion zone reached 480 MPa, exceeding its yield strength. After adopting beam deflection welding, the longitudinal stress of fusion zone was almost constant. When beam compensation weld was employed, the cooling rate drastically reduced in the fusion zone and its longitudinal stress diminished gradually as the numbers of compensation increased. The results indicated that longitudinal stress in the fusion zone helped to inhibit the cracks formation.