Research On Ti-Zr-Cu-Ni Filler And The Brazing Technology For Brazing TiAl Alloy And GH536 Alloy

TiAl alloy is one of the lightweight high temperature structural materials with the most potential to replace Ni-based superalloy,due to its low density,excellent oxidation resistance,great creep resistance,high specific strength and stiffness.However,due to its high price,TiAl alloy is often combined with Ni-based superalloy to obtain outstanding comprehensive performance with a lower cost.In this work,a novel low melting point Ti-based filler was developed to braze TiAl alloy and GH536 superalloy.The brazing process characteristics of the novel filler were investigated and the corresponding brazing process parameter was optimized.The effects of brazing parameters on the microstructures of TiAl alloy and GH536 alloy joints were investigated and the evolution law of microstructure during brazing was expounded.The effects of brazing parameters under various testing temperature on the mechanical properties of brazed joints and the fracture mechanism of brazed joints were studied.Crystalline and amorphous Ti-27.5Zr-22.5Cu-22.5Ni(wt.%)filler alloys were developed,whose microstructures,melting characteristics and wetting spreadability were analyzed by SEM?EDS?XRD?TEM and DSC.Compared with crystalline filler,amorphous filler has more uniform structure and element distribution,lower solidus temperature and narrower melting range.And the spreading areas of Ti-based filler were observed to increase with test temperature and holding time.The typical interfacial microstructure of brazed joint under brazing parameters of1110?/10 min include isothermal solidification layer?(Ni-rich(Ni,Cr,Fe)_ss,Cr-rich(Cr,Ni,Fe)_ss,Ti Ni₃,Al Ni₂Ti),continuous reaction layer?(Al₃Ni Ti₂)and continuous diffusion layer?(B₂)from GH536 alloy side to TiAl alloy side.When the joints brazed at low temperature(1070?/10 min,1090?/10 min)with short holding time(1110?/0 min),the mutual diffusion between the parent metal alloy elements and filler alloy elements is insufficient,and residual fillers remain in the brazing seam.The interfacial microstructure of isothermal solidification layer I in brazed joints are Cr-rich(Cr,Ni,Fe)ss,Cr-rich(Cr,Ni,Fe,Ti)ss,Al(Ni,Cu)2(Ti,Zr)and(Ti,Zr)(Cu,Ni,Al).With the increase of brazing temperature or holding time,the thickness of brazing seam and isothermal solidification layer I increase,while continuous reaction layer II and continuous diffusion layer III basically maintain the thickness.The room-temperature shear strength of the brazed joints increases firstly and then decreases with the increase of brazing temperature or brazing time.The maximum shear strength of the brazed joints reaches 279 MPa at 1110?for 10 min.The analysis of fracture morphology shows that all the fracture of the brazed joints appears to be cleavage fracture.In the room-temperature shear test,when the brazing time is 10 min with the brazing temperature?1090?or the brazing temperature is 1110?with the brazing holding time<5min,the fracture path of brazed joints is along with the intermetallic compounds of Al(Ni,Cu)₂(Ti,Zr)and(Ti,Zr)(Cu,Ni,Al)in the isothermal solidification layer I,while some cracks pass through the isothermal solidification layer?and extend to the continuous reaction layer?.According to Bramfitt's lattice mismatch theory and mismatch degree factor formula,the phase interface between Al₃Ni Ti₂ and B₂ is semi-coherent,and the phase interface between Al Ni₂Ti and Al₃Ni Ti₂ is non-coherent.Therefore,when the brazing holding time is 10 min with the brazing temperature?1110?or the brazing temperature?1110?with the brazing holding time?5min,the cracks in brazed joints originated between Al Ni₂Ti and Al₃Ni Ti₂ phases under the action of shear stress,and then gradually spread to the isothermal solidification layer?,continuous reaction layer?and continuous diffusion layer?.With the increase of test temperature,the high-temperature shear strength of TiAl/GH536joints brazed at 1110?/10 min decreases firstly,then increases,and finally decreases.Heterogeneous brazed joints show excellent mechanical properties(shear strength up to 156MPa)and favourable oxidation resistance at 600?.The fracture morphology of joints appears to be brittle cleavage.In addition,in the high temperature shear test,the cracks originated between Al Ni₂Ti and Al₃Ni Ti₂ phases,and then gradually spread to isothermal solidification layer?,continuous reaction layer?and continuous diffusion layer?.