Research On The Property And Stress Of Electron Beam Welding Joint In TC4Titanium Alloy Spherical Shell

In this paper, electron beam welding experiment of the TC4titanium flat with thethickness of60mm was carried out. After the welding, the two different heat-treatmentswere taken to the welded joints. And the changes in the microstructure and themechanical properties, especially the changes of the fatigue performance of the jointsbefore and after the welding or the heat treatment were investigated. Combining withthe numerical simulation, analysis of the stress about the titanium ball shell structurewas taken.After the welding, the cross section of the electron beam welded joints of the largethickness TC4titanium alloy was "nail" shaped, weld zone was composited by theneedle-like ’ martensite. And the microstructure and properties of the ’ martensite werequite different with the base metal which combined with the (+β) phase, making thejoint’s hardness and strength higher than the base metal, but plastic of the joint wasworse than the base metal. In the S-N curve, fatigue strength of the joint was12%higher than the base metal, and in the low stress level, the fatigue life of the joint wasbetter than the base metal. Organizational analysis and the results of the performancetests show that using the electron beam welding method to weld the TC4titanium alloyof60mm thickness could get the full penetration weld joints with a good processperformance.The results of the welding temperature field numerical simulation show that in theelectron beam welding process of the large thickness material, it a big difference in thethickness direction of the heat receiving. In the direction of the penetration, the jointreduced absorption of heat heat, and the time of the bottom joint in the state of themolten pool was lower the the upper joint, making the performances of EBW jointsmore uneven than the joints of thin plate and its plastic gradually become poor in thethickness direction.making. In the direction of the penetration the grains become fine,and the organization gradient dDi/dx-X got increased. It’s the main reason which madethe elongation、bend angle and the fatigue life of the joint at different thickness havesignificant changes. After the solution and aging treatment, the organization distribution of the jointbecome more uniform and stable. The heat treatment eased the hardness distribution andeliminated the mutation in the junction between the weld zone and the heat affectedzone, weakened the unevenness of the joint in the direction of penetration, andincreased the hardness and strength of the joint. But the ductility、toughness and thefatigue performances of the joint got deteriorate; After the stress relief annealing, thegrains in the various regions of the EBW joint got refined, and the distribution of theorganization was adjusted, and the slow cooling made the internal relaxation occur inthe joint which can eliminate the harmful residual stress effectively. The heat treatmentimproved the ductility、toughness and the the fatigue performances of the jointsignificantly, and at the same time made the joint maintain a certain strength andhardness.Numerical simulation results in the stress field of the TC4titanium ball shellstructure show that with the movement of the electron beam, there was an obviouscompressive stress area in the advance of the heat source constrained by the surroundingmetal. While the joint weld area had gradually evolved into a tensile stress from thecompressive stress state; And restricted by the sphere shell shape, the residual stressafter welding was mainly characterized by tensile stress, but near the weld zone was thecompressive stress, the tensile stress and compressive stress in the cross section were inbalance. After the stress relief annealing, the residual tensile stress of the ball shellstructure was reduced, the residual stress distribution of the entire structure becomeuniform.