Research On Brazing Interface Behavior And Bonding Mechanism Of TiAl Alloy

TiAl alloy has a wide application prospect in aerospace due to its low density,high specific strength and excellent creep resistance at high temperatures.However,processing large complex integral components of TiAl alloy is costly and difficult.Therefore,joining of TiAl alloy and ceramics or other metals plays an important role in reducing the weight of aircraft and widening its application.In this work,the joining technology,the formation mechanism of interface reaction products and the mechanical properties of TiAl with Al2O3 ceramics,Ti matrix composites(TMCs)and GH4169 Ni based superalloy were studied.Firstly,the new AgCu filler with Nb particle additions has been designed to join TiAl alloy and Al2O3 ceramics.The effect of brazing temperature and Nb additions on the interfacial microstructure and mechanical properties of the joint was studied.It can be found that the typical microstructure of the brazed joint was TiAl/AlCuTi phase+AlCu2Ti phase/Ag(s,s)+Cu(s,s)+Nb particles+AlCu(Ti,Nb)/Ti3(Cu,Al)3O/Al2O3.When brazed at 810℃/10min,macrodefects can be found in the brazing seam.With increasing brazing temperature,the reaction between the liquid filler and the substrate was intensified.And the thickness of AlCu2Ti reaction layer at TiAl alloy side and Ti3(Cu,Al)3O phase at the Al2O3 ceramics side increases.The maximum shear strength of 228MPa was reached when it was brazed at 870℃/10min using AgCu+10wt.%Nb brazing filler.The residual stress in the brazing joint was studied by finite element modeling.The results showed that the maximum stress was reduced about 12.6%after adding 10wt.%Nb particles.The overall residual stress of the joint decreases.TiAl alloy and TMCs were brazed with Ti-28Ni filler and TiZrNiCu amorphous filler,respectively.The wettability of the two filler alloys on the substrate was studied.And the effects of brazing temperature on the microstructure and mechanical properties of TiAl/TMCs brazed by different filler alloys were also studied.When Ti-28Ni filler was used,the thickness of reaction layer at TiAl side and the volume fraction of Ti2Ni phase decrease with increasing brazing temperature.At 1010℃,the continuous Ti2Ni phase disappears.The typical microstructure of the brazed joint was TiAl/α2-Ti3Al+Al3NiTi2/α2-Ti3Al/Ti(s,s)+Ti2Ni+TiC/TMCs.The maximum shear strength of 469.5MPa for the joint brazed at 1010℃ was reached.The typical microstructure of the joint for TiZrNiCu amorphous filler is TiAl/α2-Ti3Al+Ti(s,s)/(Ti,Zr)2(Cu,Ni)+Ti2Cu+Ti(s,s)/TMCs.At 900℃,a large number of continuous(Ti,Zr)2(Cu,Ni)phases were formed in the joint.When the brazing temperature increases to 960℃,moreα2-Ti3Al phase was formed at TiAl alloy substrate side,which inhibits further diffusion of Al into the brazing joint.That results in decreasing Al content in the middle of the brazing joint.At 990℃,the maximum shear strength of TiAl/TiZrNiCu/TMCs joints is about 230MPa.Based on this,the influence of the two brazing filler alloy on the TiAl alloy and TMCs joints was discussed.TiAl alloy and GH4169 superalloy were also brazed with Ti-28Ni filler and TiZrNiCu amorphous filler alloys.The wettability of the filler alloys on the surface of high Nb-TiAl alloy is better than that of GH4169 superalloy.When Ti-28Ni was used for brazing,the typical microstructure of the joint is TiAl/α2-Ti3Al+Al3NiTi2/Ti2Ni+Ti(s,s)+α2-Ti3Al/Cr(Cr,Ni)ss+Ni(Cr,Ni)ss/GH4169.The shear strength of the joint first increases and then decreases with the brazing temperature.The maximum shear strength for the joint brazed at 1010℃ is about 183.7MPa.At 980℃,a thinner reaction layer was formed.The joint fractured in the thin reaction layer.When the brazing temperature exceeds 1040℃,the elements diffusion was intensified and cracks can be found in the brazing seam,which would deteriorate the mechanical properties.When TiZrNiCu amorphous filler was used,the typical microstructure in the brazing joint is TiAl/α2-Ti3Al+Al3(Ni,Cu)Ti2+Al(Ni,Cu)2Ti/AlCu2(Ti,Zr)+(Ti,Zr)(Ni,Cu)+Ti2(Ni,Cu)/Cr(Ni,Cr,Fe)ss+Ni(Ni,Cr,Fe)ss/GH4169.The increase of brazing temperature leads to the thickening of the reaction layer.However,higher brazing temperature would also lead to the coarsening of Al(Ni,Cu)2Ti and Ti2(Ni,Cu)phases,which is adverse to the mechanical properties of the joint.At 960℃,the maximum shear strength of 241.9MPa was reached.By comparing the effects of two filler metals,the factors,which can influence the mechanical properties of TiAl alloy with GH4169 superalloy joints,were obtained.The Gibbs energies of the phase at the reaction layer and the mixing enthalpies of the elements were studied.Combined with the microstructure evolution,the formation mechanism of the brazing joints was clarified.The dissolution model of different filler metal to TiAl substrate was set up and the growth kinetic equation of the reaction layer at TiAl side was also obtained.