Research On Mechanism And Suppression Method Of Crack Of Electron Beam Welded TiAl Alloy

TiAl alloy is a new high temperature structural material with light weight and high strength.It is considered as a new candidate for nickel-based superalloy owing to its high specific strength and specific modulus.However,due to the poor plasticity of the TiAl alloy at room temperature,it is very sensitive to welding cracks.In this paper,electron beam welding?EBW?of TiAl alloy is investigated.A new interlayer designed by simulating the fusion line is used to realize crack-free joining of TiAl alloy.The weld is composed of brittle?₂ phase,block?phase and?₂/?lamella during direct welding of TiAl alloy.The peak temperature of heat affected zone?HAZ?reaches the?transformation point,under the high temperature of welding heat cycle.Thus,the secondary phase transformation occurs in HAZ and a large number of?₂phases are generated,which significantly increases the crack sensitivity of the joint.The weld and HAZ are always in a high stress state during the cooling process.Under the combine effect of the brittle microstructure and high-level residual stress,cracks generate and expand in the direction perpendicular to the weld.Electron beam welding of TiAl alloy was carried out by utilizing a pure V interlayer.Though the microstructure of the weld is improved to some extent,the cracks are not eliminated.The microstructure of the weld is consisting mainly of Ti-V solid solution and?₂-Ti₃Al phase.The addition of V element cannot completely inhibit the formation of?₂ brittle phase in the weld,and the joint is still sensitive to cracks.The V element cannot significantly improve the properties of the joint and the average tensile strength of the joints was 387 MPa.A new interlayer designed by simulating the fusion line is proposed on the basis of the above research.The Y-shaped interlayers are processed according to the shape of the fusion line.And the Y-shaped groove of TiAl base metal is prepared accordingly.This method significantly reduced the volume fraction of molten TiAl base metal and realized the crack-free welding of TiAl alloy.The weld is mainly composed of the?'martensite and residual?phase with good plasticity.During the welding process,melting connection is realized between the interlayer and micro-melting base metal.A spatter-like TiAl reaction layer,a TiAl+Ti₃Al transition reaction layer and a Ti₃Al reaction layer are sequentially formed between the micro-melting base metal and the liquid metal in the weld.When the thickness of the interlayer is 1.7 mm,the average hardness value of the weld is about 375 HV,which is much lower than that of the joint obtained by direct weld and the plasticity of the weld is obviously improved.The average tensile strength of the joint is 553MPa,which is 88%of the TiAl base metal.