Research On Brazing Process And Mechanism Of Ti60/Ti₂AlNb Alloy

Ti₂AlNb alloys and titanium alloys are emerging lightweight,high-strength materials to replace nickel-based alloys and are used in critical structural components of aerospace vehicles.For the lack of ductility of Ti₂AlNb alloy,its processing technology is complicated and its wide application is limited.Connecting the Ti₂AlNb alloy with a ductile high-temperature titanium alloy is favorable for the preparation of complex high-temperature structural parts in the aircraft.In this paper,Ti₂AlNb alloy and Ti60alloy were brazed with Cu75Pt filler metal,and the best connection process parameters were obtained.The wetting and spreading behavior of Cu75Pt brazing filler metal on the surface of the base metal was studied firstly,and the wetting mechanism of these two wetting systems was analyzed.Brazing mechanism of Ti₂AlNb alloy and Ti60 alloy with Cu75Pt filler metal was clarified.Cu75Pt filler metal used in this paper is mainly composed of Cu（s,s）and Pt（s,s）.Its melting range is 1127°C-1170°C.The wettability of Cu75Pt/Ti60 and Cu75Pt/Ti₂AlNb was studied by the sessile drop method.The results indicated that they were both contact reaction wetting,and the wetting process was realized based on the eutectic liquid phase formed by the contact reaction between the filler metal and the base metal.In Cu75Pt/Ti60 system,Ti-Cu eutectic liquid phase was formed by contact reaction and the contact reaction in Cu75Pt/Ti₂AlNb system formed Ti-Al-Cu eutectic liquid phase.The wetting interface of the two systems was mainly composed of intermetallic compounds.Among them,the main products of Cu75Pt/Ti60 wetting interface wereβ-Ti,Ti₂Cu,and Ti₃Pt;and the wetting interface products of Cu75Pt/Ti₂AlNb were（Ti,Nb）₃Pt,（Ti,Nb）₃Al,AlCu₂Ti and AlCuTi.The interfacial microstructure of Ti60/Ti₂AlNb joints brazed by Cu75Pt filler metal was analyzed and it showed that the brazing joint could be obtained by generating a Ti-Cu eutectic liquid phase preferentially with the reaction of Cu75Pt on the Ti60 side.The microstructure of the brazing joint was different from that of the wetting interface.The typical microstructure of the brazing joint was Ti60/（α+β）+β-Ti/Ti₃Pt+α-Ti/B2+O/Ti₂AlNb.The study on the evolution of brazing joint microstructure with different brazing techniques（including brazing temperature and holding time）found that increasing the brazing temperature or prolonging the holding time,the diffusion among the elements was intensified while the thickness of Ti₃Pt reaction layer decreased and the thickness of the diffusion zones increased.By studying the mechanical properties of brazed joints with different process parameters,it showed that the strength of the joints was related to the thickness of Ti₃Pt reaction layer and the microstructure of diffusion zone on Ti60 side.The fracture location of the joints occurred mainly in Ti₃Pt compound layer.As the thickness of Ti₃Pt compound decreased,the strength of the brazing joint increased.However,the Ti-Pt eutectoid microstructure in diffusion zone I and the coarse stripeα-Ti on the Ti60 side weakened the bond strength between the reaction layer and the diffusion layer,then the strength of the joints decreased.The fracture pattern of the brazing joint changed from the cleavage fracture to the intergranular fracture.The best brazing process parameters in this study was1120°C/10min/10KPa,and the room temperature and high temperature（600°C）strength of the obtained brazing joints were 183MPa and 149MPa,respectively.The high-temperature strength of this joint was higher than that obtained by other methods which indicated that it was of great significance for realizing high-temperature application of Ti60/Ti₂AlNb alloy by brazing with Cu75Pt filler metal.