Diffusion Bonding Behavior Of β-γ High Nb-TiAl Alloy By Spark Plasma Sintering

TiAl based alloys have broad application prospects due to low density,high specific strength and specific stiffness,good creep resistance and high-temperature oxidation resistance.Welding is one of the important technical ways for forming Ti Al alloy parts.They are often needed to bond with themselves and titanium alloys,superalloys,ultra high temperature structure materials in the process of engineering application.At present,there have been few reports on diffusion bonding of TiAl based alloys by spark plasma sintering.In the early stage,our research group successfully developed the β-γ high Nb-TiAl alloy,which has been granted the national invention patent and successfully deformed without can.The diffusion bonding of β-γ high Nb-TiAl alloy by spark plasma sintering has not been reported yet so far.In this paper,spark plasma sintering technology was used to join β-γ high Nb-TiAl alloy with similar and dissimilar materials.The influence of the joining process on the microstructure and properties of joint was systematically studied.The joining mechanism of joint,growth kinetics of diffusion layer were explained.The innovative results are obtained as following:(1)For the direct joining of β-γ high Nb-TiAl alloy with similar materials,it is found that the pulse current mode can significantly improve the discontinuous recrystallization nucleation and growth rate of y-TiAl phase in the joint,inhibit the formation of α2-Ti3Al phase in the joint at higher temperature and improve the joint shear strength with non pulse current mode.The optimal process parameters are the roughness of mating surface Sa=0.543 μm and 1150 ℃×20 min×10 MPa,the joint shear strength is 395 MPa.The nearly lamellar microstructure is obtained,and the shear strength is increased to 463 MPa after heat treatment.(2)With the Ti foil used as the interlayer for diffusion bonding of β-γ high NbTiAl with similar material,the typical phase composition of the joint is β-γ high Nb-TiAl‖β/B2‖α2-Ti3al‖α-Ti+β-Ti‖α2-Ti3Al‖β/B2‖β-γ high Nb-TiAl.When the bonding temperature increases,the thickness of β/B2 diffusion layer increases while that of α2 diffusion layer decreases.The thicknesses of β/B2 and α2 diffusion layers increase with the bonding time extending.The optimal processing parameters are 1000 ℃×10 min×10 MPa,and the joint shear strength is 308 MPa.The joint strength increases compared with that of direct diffusion bonding joint at the same bonding temperature and pressure.The shear fracture mainly occurs along the interface and in the interior of β/B2 diffusion layer.The shear strength is increased to 458 MPa after heat treatment.(3)For the joining of β-γ high Nb-TiAl with TC4 alloys,the typical phase composition of the joint is β-γ high Nb-TiAl‖β/B2‖α2-Ti3Al‖TC4.The thicknesses of diffusion layers increase with the increase of bonding temperature and time.With the increase of bonding pressure,the thicknesses of diffusion layers remain unchanged.The activation energy of β/B2+α2 diffusion layers is lower than that of reported joint by hot press sintering diffusion bonding.The optimal process parameters are 850℃×20 min×5 MPa,and joint shear strength is 310 MPa.The shear fracture mainly occurs along the interface of β/B2 diffusion layer and β-γ high Nb-TiAl alloy.(4)The result of joining of β-γ high Nb-TiAl and IN718 alloys shows that the typical phase composition of the joint by direct diffusion bonding is β-γ high NbTiAl‖β/B2+τ3-NiAl3Ti2‖τ3-NiAl3Ti2‖τ4-Ni2AlTi+σ-FeCr‖Cr/Fe/Nb rich η-Ni3Ti+α-Cr+Cr rich γ-Ni‖IN718.The thicknesses of diffusion layers increase with the increase of bonding temperature.Molecular dynamics calculations show that the dislocation lines with 1/2<1 10>burgers vector serve as the main atomic diffusion dislocation channels at the beginning of holding stage.When the bonding temperature is 850 ℃,the maximum joint shear strength is 257 MPa,which is higher than that reported by direct diffusion bonding between TiAl and K418 alloy.The typical phase composition of the joint is β-γ high Nb-TiAl‖β/B2+τ3NiAl3Ti2‖α2-Ti3Al‖α-Ti+δ-NiTi2+β-Ti‖δ-NiTi2‖β2-(Ni,Fe)Ti‖ Fe/Cr rich η-Ni3Ti+η-Ni3Ti+α-Cr+δ-Ni3Nb‖η-Ni3Ti+γ-Ni+δ-Ni3Nb‖IN718 with Ti foil used as interlayer.Except α+δ+β diffusion layer,the thickness of α2 diffusion layer decreases while that of other diffusion layers increases with the increase of bonding temperature,and the thickness of diffusion layers increases with the extension of bonding time.The optimal process parameters are 850℃×20 min×10 MPa,the joint shear strength is 326 MPa.The shear fracture occurs in the diffusion layers on the side of IN718 alloy.With Ni foil used as interlayer for diffusion bonding,it is found that the recrystallization of y-Ni occurs in the interface of IN718/Ni,the typical phase composition between β-γ high Nb-TiAl alloy and Ni foil is β-γ high Nb-TiAl‖β/B2+τ3-NiAl3Ti2‖τ3-NiAl3Ti2‖τ4-Ni2AlTi‖γ’-Ni3(Al,Ti)‖γ-Ni.The thick-nesses of diffusion layers increase with the increase of bonding temperature and bonding time.With the increase of bonding pressure,the thicknesses of diffusion layers remain unchanged.The optimal process parameters are 850℃×20 min×30 MPa,the shear joint strength is 340 MPa.Shear fracture occurs in the brittle diffusion layers formed in the interface of β-γ high Nb-TiAl and Ni foil.(5)The result of joining of β-γ high Nb-TiAl and Nb-Si alloys shows that the typical phase composition of the joint by direct diffusion bonding is β-γ high NbTiAl‖β/B2+α2-Ti3Al‖δ-(Nb,Ti)3Al+σ-(Nb,Ti)2Al+Ti rich γ-(Nb,Ti)5Si3)‖Nb-Si.The thicknesses of diffusion layers increase with the increase of bonding temperature.When the bonding temperature is 1050℃,the maximum shear strength of the joint is 269 MPa.The shear fracture occurs along the interface and in the interior of diffusion layer on the side of Nb-Si alloy.With Ti foil used as interlayer,the typical phase composition of the joint is β-γ high Nb-TiAl‖α2Ti3Al‖α-Ti+β-Ti‖Ti rich β-(Nb,Ti)5Si3+Ti rich(Nb,Ti)ss‖Nb-Si.Except α+βdiffusion layer,the thickness of α2 diffusion layer decreases while that of other diffusion layers increases with the increase of bonding temperature,and the thicknesses of diffusion layers increase with the extending of bonding pressure.The optimal process parameters are 1000℃×10 min×10 MPa,and the joint shear strength is 385 MPa.Shear fracture occurs along the interface and in the interior ofβ/B2 diffusion layer.With Nb foil used as interlayer for diffusion bonding,it is found that the recrystallization of β-Nb occurs in the interface of Nb-Si/Nb,the typical phase composition between β-γ high Nb-TiAl alloy and Nb foil is β-γ high Nb-TiAl‖β/B2+α2-Ti3Al‖σ-(Nb,Ti)2Al+δ-(Nb,Ti)3Al‖β-Nb.The thicknesses of diffusion layers increase with the increase of bonding temperature.When the bonding temperature is 1100℃,the maximum joint shear strength is 260 MPa.