Study On Interface Microstructure Evolution And Mechanical Properties Regulation Mechanism Of TiAl Alloy Brazed Joints

As typical TiAl alloys,TiAl-based and Ti₃Al-based alloys have the advantages of low density,high specific strength,excellent corrosion resistance,good high temperature mechanical properties and oxidation resistance.They are believed to be ideal lightweight high temperature structural materials,which are expected to gradually replace the traditional superalloys in the most advanced technology of aerospace.At the same time,TiAl-based and Ti₃Al-based alloys have their own characteristics,respectively,thus they are connected with each other in complex practical applications.In this paper,similar brazing of TiAl-based alloy（Ti-47Al-2Nb-2Cr-0.15B（at.%））,similar brazing of Ti₃Al-based alloy（Ti-22Al-15Nb（at.%））and dissimilar brazing of TiAl-based alloy and Ti₃Al-based alloy were realized by using homemade Ti H₂-24Ni（at.%）powders filler metal.The interfacial microstructure and formation mechanism of the brazed joints were analyzed.The effect of brazing temperature（1060～1120℃）and holding time（0～30 min）on the microstructure of the brazed joints and its internal relationship with mechanical properties were studied.Meanwhile,the fracture mechanism of the brazed joints was clarified and the optimum brazing process parameters were determined.The vacuum brazing of TiAl-based alloy with Ti H₂-24Ni filler metal was successfully brazed.The TiAl brazed joint is mainly composed of three interfacial reaction layers:solid diffusion layer I,isothermal solidification layer II and central brazing layer III.The typical structure of the brazing seam is TiAl/Al₃Ni Ti₂+Ti₃Al/Ti₃Al/Ti₃Al+Ti₂Ni/Ti₃Al/Al₃Ni Ti₂+Ti₃Al/TiAl.With the increase of brazing temperature（1060～1120℃）or holding time（0～30min）,layer I gradually increases,layer II gradually decreases until it disappears and the interfacial structure of layer III first refines but then coarsens.The brazing process parameters reach a maximum value of 379 MPa at 1080℃/10 min.Affected by the interfacial structure of layer III,the shear strength of TiAl brazed joints firstly increased and then decreased.The fracture mode of TiAl brazed joints obtained with different brazing process parameters shows brittle fracture.The fracture mainly occurs in the continuous Ti₂Ni phase aggregation in the central brazing layer III,and few cracks expand and extend to the layer I.The main factors affecting the mechanical properties of TiAl brazed joints are the aggregation of Ti₂Ni and the coarsening of Al₃Ni Ti₂.The typical interfacial microstructure of Ti₃Al-based alloy brazed joints is Ti₃Al/B2+Ti₃Al/Ti₂Ni/B2+Ti₃Al/Ti₃Al.The solid diffusion layer I is composed with B2 and Ti₃Al and the central brazing layer II is consistent with continuous Ti₂Ni layer.The increasing brazing temperature（1060～1100℃）or holding time（0～30 min）broadens the mixing region of B2 phase and B2+Ti₃Al phase and decreases the thickness of continuous Ti₂Ni layer,when the former is beneficial to the joint performance but the latter is opposite,resulting in the shear strength of Ti₃Al brazed joint improved.The maximum shear strength reaches 374 MPa at 1100℃/20 min.However,the overhigh brazing temperature（1120℃）or too long holding time（30 min）makes the interfacial structure of brazed joints coarsen and the shear strength decrease.The Ti₃Al brazed joints obtained under different brazing process parameters after mechanical test fractures in the central brazing layer II composed of continuous Ti₂Ni,which is the weakest zone of Ti₃Al brazed joints.The fracture mode of Ti₃Al brazed joints shows brittle fracture.The typical structure of TiAl/Ti₃Al brazed joint is TiAl/Al₃Ni Ti₂+Ti₃Al/Ti₃Al/Ti₂Ni+Ti₃Al/B2+Ti₃Al/Ti₃Al.The evolution of interfacial microstructure of TiAl/Ti₃Al brazed joint with brazing process parameters is similar to that of TiAl brazed joint and Ti₃Al brazed joint.The shear strength at room temperature also increases first and then decreases with the increase of brazing temperature or holding time.The brazing process parameters reach a maximum value of 361 MPa at 1100℃/10 minutes.The only difference is that the isothermal solidification layer II of TiAl/Ti₃Al alloy brazed joints does not disappear with the increase of brazing temperature and holding time,which also leads to continuous Ti₃Al layer become another factor affecting the quality of TiAl/Ti₃Al brazed joints.When the continuous brittle Ti₂Ni layer decreases to a certain thickness with the change of brazing process parameters,the fracture occurs in the isothermal solidification layer II composed of continuous Ti₃Al.The fracture mode of TiAl/Ti₃Al brazed joints obtained under different brazing process parameters also shows brittle fracture.