Investigation On Electron Beam Welding Of Ti₂AlNb Alloy And Dissimilar Metals Between Ti₂AlNb Alloy And TC4 Alloy

As a new type high-temperature stuructural material,Ti₂AlNb alloy has low density,high specific strength and high specific modulus,and good antioxidation and creep resistance,etc.It has potential to substitute for the high temperature nickel-based alloy in the fabrication of aeroengine components.During practical application,the welding of Ti₂AlNb alloy and the welding of dissimilar metals between Ti₂AlNb alloy and TC4 titanium alloy are often needed.Due to the special chemical compositions and microstructure of Ti₂AlNb alloy,their ductility is lower at room temperature.As a result,the welding defects such as crack are easily generated during fusion welding.Compared with other fusion welding methods,electron beam welding?EBW?has the advantages of high-density energy,small welding deformation,high welding efficiency and good protective atmosphere.It is suitable for the welding of Ti₂AlNb alloy.Based on this,the welding of Ti₂AlNb alloy and dissimilar metals between Ti₂AlNb alloy and TC4 alloy were carried out respectively in this paper.The main research contents are as follows.MSC.Marc software is used to simulate the electron beam welding process of Ti₂AlNb alloy.The welding parameters are optimized on the basis of the results of numerical analyses.The function of electron beam oscillation is studied and the microstructural evolution of welded joint is predicted.Electron beam welding of Ti₂AlNb alloy is carried out.The relationship between microstructure and mechanical properties of the welded joint is investigated,and the effect of different welding parameters on microstructure and mechanical properties of the joints are discussed.Electron beam welding of Ti₂AlNb/TC4 dissimilar metals is also carried out.It is contrasted to both the similar Ti₂AlNb alloy joint and similar TC4 alloy joint.The microstructure,mechanical properties and distribution of alloying elements for dissimilar joints are discussed.The effect of electron beam deviation on the dissimilar joints is investigated.The simulation results of EBW process show that the effect of different parameters on weld morphology is ranked as follows:electron beam current,acceleration voltage and welding speed.During welding,electron beam scanning can decrease the peak temperature and cooling rate of molten pool,and the weld thermal cycle curve presents sawtooth-like pattern.Compared with that without electron beam scanning,the peak temperature is slightly decreased.The calculated value of cooling rate in the weld center is 600?/s,which is much higher than the critical cooling rate of B2 phase transition?120?/s?.During the EBW of Ti₂AlNb alloy,it is predicted that the fusion zone?FZ?consists of B2 phase owing to the rapid cooling rate.Based on these,the welding experiments of Ti₂AlNb alloy are conducted,and the simulation results are verified.Results show that the simulated molten pool profile is consistent with the actual weld morphology.Metallographic observation indicates that the FZ of Ti₂AlNb alloy consists of single B2 phase.The heat-affected zone?HAZ?is successively composed of B2 single phase region,two-phase region containing B2 phase and?₂ phase,and three-phase region containg B2 phase,?₂phase and O phase.Tests of mechanical properties show that the average microhardness of FZ is higher than that of HAZ and base metal?BM?.The tensile strength of welded joint is close to that of BM,but the plasticity of joint is decreased to a certain extent.The tensile fracture of joints ruptured in FZ presented the mixed mode of ductile-brittle fracture,while the tensile fracture of joints ruptured in HAZ was the mode of quasi-cleavage fracture.During welding,the plasticity and strength of welded joint can be improved to a certain degree by the addition of circular electron beam scaning.The optimized parameters for the welding of Ti₂AlNb alloy are as follows:electron beam current of the first-bead wlding and the modification welding are 12mA and 9.5mA respectively,welding speed300mm/min,acceleration voltage 50kV,and the frequency of circular electron beam scaning is500Hz.The welding of dissimilar metals between Ti₂AlNb alloy and TC4 alloy is carried out.Results show that The FZ of dissimilar joint consists of B2 phase and??phase.At the Ti₂AlNb side,HAZ near FZ consists of B2 phase and a small amount of?₂ phase,while HAZ near BM consists of B2phase,?₂ phase and O phase.At the TC4 side,HAZ near FZ is mainly made of martensite??phase,while HAZ near BM consists of?phase and?phase.Mechanical properties of dissimilar joints indicate that the microhardness distribution of joint is“M”type,and the microhardness near fusion line is the highest.The strength and plasticity of Ti₂AlNb/TC4 dissimilar joint are those of between Ti₂AlNb alloy similar joint and TC4 alloy similar joint.The tensile fracture of dissimilar joint presents the mixed mode of ductile-brittle fracture which is dominated by the quasi-cleavage fracture.Compared with the EBW procedure which electron beam aimed at the weld center,when electron beam was deviated to TC4 alloy with the distance of 0.15mm,the molten area of TC4 alloy is increased during welding.Therefore,the variation of microhardness in weld zone can be reduced and the tensile strength of welded joint is improved.