Microstructure Formation And Evolution Mechanism And Mechanical Contorl Of TC4/TNM Alloy Brazed Joints

As one of the most widely used titanium alloys,TC4 is gradually moving from the cold end（structural bearing）of the aircraft to the hot end（engine）in the aerospace field.Ti Al-based alloys are considered to be the most promising alternative alloys for Ni-based superalloys due to their excellent oxidation resistance,high specific strength and low density.Using lightweight TC4 alloy to prepare engine cold end components and Ti Al-based alloy to replace Ni-based superalloy to prepare engine hot.It can reduce engine mass and improve thrust-weight ratio.It plays an extremely important role in improving the maneuverability of spacecraft..Brazing is a connection method with strong operability,low cost and mature process.Many scholars have successfully realized the connection of heterogeneous alloys by brazing,but the research results of brazing TC4/Ti Al are rarely reported.The purpose of this project is to realize the vacuum brazing connection between TC4 and Ti-43.5Al-4Nb-1Mo alloy（TNM）by using Ti-25.65Zr-13.3Cu-12.35Ni-3Co-2Mo（wt.%）amorphous foil filler metal.The mechanism of brazing process parameters and post-weld heat treatment on the interface microstructure and mechanical properties of TC4/TNM brazed joints was studied.At the same time,the interface microstructure of the brazed joints was optimized by adjusting the brazing process parameters and heat treatment process parameters to improve the mechanical properties of the joints.The typical interfacial microstructure of TC4/TNM brazed joint with 900°C/10 min brazing parameters was studied.The results show that the brazing seam is mainly composed of the interfacial reaction layer I on the TC4 side,the central brazing layer II and the interfacial reaction layer III on the TNM side.Due to the driving force of concentration,Zr,Cu,Ni and other elements in the solder diffuse to the base metal on both sides,and Ti,Al and other base metal elements dissolve into the solder.The main phase of the interfacial reaction layer I is（α+β）-Ti,the main phase of the central brazing layer II is the mixed color phaseα-Ti+γ-（Ti,Zr）₂（Cu,Ni）andγ-（Ti,Zr）₂（Cu,Ni）containing a small amount ofα-Ti,and the interfacial reaction layer III is composed of continuous fine lamellarα₂-Ti₃Al.The brazing temperature plays a decisive role in the dissolution degree of TC4and TNM in the high temperature liquid phase and the content of Ti,Al,Cu,Ni,Zr and other elements in the brazing seam,which is the main influencing factor of the interface microstructure of TC4/TNM brazed joint.When the brazing time is 10 min and the brazing temperature is 900-980°C,the interface microstructure of the joint is TC4/fine basket-like（α+β）-Ti/γ-（Ti,Zr）₂（Cu,Ni）+α-Ti/Ti₃Al/TNM,and the interface microstructure of the joint increases with the increase of brazing temperature.The hard and brittleγphase in the brazing seam decreases and the ductileα-Ti increases.When the brazing temperature is 1000°C,due to exceeding theβtransition temperature of TC4 base metal alloy,a large number of coarse needle-like（α+β）-Ti are formed in the interfacial reaction layer I,and the interfacial reaction layer of the joint changes from three layers to two layers.The corresponding phases are coarse needle-like（α+β）-Ti and Ti₃Al with poor toughness,and the coarse needle-like（α+β）-Ti further coarsens with the increase of temperature.The brazing time mainly regulates the interface microstructure of TC4/TNM brazed joint by prolonging the metallurgical reaction time in the joint,but when the brazing time is 10 min,the atomic diffusion and metallurgical reaction in the brazed joint reach relative saturation.Therefore,further prolonging the brazing time is not conducive to the improvement of the strength of the brazed joint.Under the parameters of 700°C/1 h/2 h/3 h,the microstructure of the interface of the samples after post-weld heat treatment was obviously refined,and the lamellar martensite composed of fine needle-likeα-Ti andβ-Ti appeared in the joint.However,when the heat treatment temperature was increased to 800°C,the microstructure of the joint was coarsened,and the fine needle-likeα-Ti was transformed into the thicker lath-likeα-Ti.The shear test of TC4/TNM brazed joints shows that the shear strength of brazed joints increases first and then decreases with the increase of brazing temperature and brazing time.At 980°C/10 min,the shear strength of the brazed joint reaches a maximum of 494 MPa.The maximum shear strength of the brazed joint after heat treatment is 519 MPa.At the heat treatment temperature of 700°C,the strength of the brazed joints obtained by all different parameters is higher than 494MPa,that is,the heat treatment temperature of 700°C is the most suitable heat treatment temperature for TC4/TNM brazed joints.All the TC4/TNM brazed joints after shear test are brittle fracture.The fracture starts from the central brazing layer II with a large number of continuous intermetallic compoundsγ-（Ti,Zr）₂（Cu,Ni）,but with the increase of brazing temperature,there are secondary cracks extending to the side of TNM base metal alloy.