Numerical Simulation Study On Continuous/Pulsed Electron Beam Welding Of TC2 Titanium Alloy Sheet

With the rapid development of space technology,the construction of large spacecraft and the long-term operation and maintenance of aircraft have raised the demand for space welding.Space welding is limited by the energy supply,and it is necessary to obtain a larger penetration under lower energy input conditions.The Electron Beam Welding method has the advantages of high energy utilization rate,good space applicability,and at the same time,the pulse effect can effectively increase the penetration depth.Therefore,this paper takes the TC2 titanium alloy material commonly used in space structure as the research object to carry out the numerical simulation study of Continuous/Pulsed Electron Beam Welding of 2mm thick TC2 titanium alloy sheet.Through Continuous and Pulsed Electron Beam Welding flat plate butt tests on the TC2 titanium alloy sheet,it was found that when the average power of the Continuous Electron Beam Welding and the Pulsed Electron Beam Welding was the same,the titanium alloy sheet can be penetrated under different welding parameters.The welding seam of Continuous Electron Beam Welding was well formed.The surface of the welding seam has smooth,regularly arranged and dense fish scales.The cross-sectional shape of the welding seam is funnel-shaped.Under the action of the Pulsed Electron Beam Welding,the width of the weld surface decreases,the crosssectional shape changes from a nail shape to a wedge shape,and the weld surface appears concave,the back welding spatter increases,and the continuity becomes poor.Therefore,the Computational Fluid Dynamics numerical simulation method was used to study the influence mechanism of Pulsed Electron Beam Welding parameters on weld formation.In Pulsed Electron Beam Welding,the energy density of the pulse peak is higher than that of Continuous Electron Beam Welding,and the reaction force of the metal vapor is enhanced,so that a small hole is formed on the surface of the molten pool,the fluid is discharged to the surroundings,and the liquid at the bottom of the hole flows downward The energy was brought to the bottom of the molten pool,which reduces the width of the weld surface.For Continuous Electron Beam Welding,the reaction force of the metal vapor is relatively weak,and the metal in the molten pool flows outward under the effect of the surface tension gradient,which increases the width of the weld surface and decreases the aspect ratio.In Continuous Electron Beam Welding,the width of the upper surface of the weld was 2.3mm,and the width of the upper surface of the Pulsed Electron Beam Welding with a frequency of 100 Hz is 2.1mm.When the pulse frequency was increased to 500 Hz,the width of the upper surface of the weld is further reduced to 1.3 mm.Numerical simulation of the temperature field of Continuous/Pulsed Electron Beam Welding of TC2 titanium alloy,the calculation results show that,compared with continuous electron beam welding,the thermal cycle curve of Pulsed Electron Beam Welding is sawtooth,and the temperature of the molten pool is fluctuatingly rising and falling.The peak temperature of Pulsed Electron Beam Welding with frequencies of 100 Hz,300Hz and 500 Hz reached 4274 K,4254K and 4369 K respectively;while the peak temperature under Continuous Electron Beam Welding was only 3509 K.In Continuous Electron Beam Welding,the longitudinal residual stress in the longitudinal direction of the weld appears as tensile stress,and in the middle of the weld,the stress level is stable,and a stable region with a higher amplitude appears,and the stress amplitude is close to the yield strength of the material,reaching 517.3MPa,while the transverse residual stress,the middle part of the weld shows tensile stress,the two ends of the weld show compressive stress,and the compressive stress value is much higher than the tensile stress value,the compressive stress value can reach 509.1MPa,while the tensile stress value is only 122.8MPa.In the direction of the vertical weld,the longitudinal residual stress is tensile stress in the fusion zone and heat-affected zone,and the compressive stress in the base material area.The distribution trend of residual stress in Pulsed Electron Beam Welding joints is the same as that of Continuous Electron Beam Welding,but the width of the weld and heat affected zone is reduced due to the pulse effect,and the distribution area of residual tensile stress is reduced.In the weld and heat affected zone the average value of longitudinal residual tensile stress decreases.In Continuous Electron Beam Welding,the average residual tensile stress in the weld zone is 485.7 MPa,while in the same area,the average residual tensile stress at frequencies of 100 Hz,300 Hz,and 500 Hz are 451.9MPa,348.9MPa and 328.6MPa.The transverse stress amplitudes of different welding parameters vary greatly.The transverse residual tensile stress in the middle of the weld is 129 MPa at a frequency of 500 Hz,but only 83 MPa at 300 Hz,a decrease of 35.7%.