Study On The Brazing Connection Process And Mechanism Of TC4/316

Titanium alloy has broad application prospects in important fields such as aerospace,nuclear industry and power due to the advantages of high specific strength,good corrosion resistance,high thermal strength and low temperature performance.However,the poor weldability and processability of titanium alloys and high production costs severely limit its further application.The stainless steel has the advantages of excellent mechanical properties,good weldability,low production and processing costs,while its corrosion resistance and proportion are large.To connecting the titanium alloy and stainless steel to form a composite component is beneficial to give full play to the respective advantages of the titanium alloy and stainless steel,thereby improving the performance of the component and reducing economic costs.In the nuclear industry,titanium alloy dissolution tank and stainless steel pipe are connected together;in the pipeline of aerospace satellite propulsion system in the aerospace field,the titanium alloy conduit and the stainless steel conduit are connected.However,due to the huge difference in physical and chemical properties of Ti and Fe,when welding using fusion welding,diffusion welding,friction stir welding and other welding methods,the number of brittle and hard Ti-Fe intermetallic compounds in the joint and joint stress are not easily controlled,the mechanical properties of the joint are difficult to meet the design requirements.Therefore,the brazing is used to connect the titanium alloy and stainless steel.In this paper,Ag70.5Cu27.5Ti2 and Ti57Zr13Cu21Ni9 were used for vacuum brazing connection of TC4 titanium alloy and 316L stainless steel.Scanning electron microscope(SEM)and energy dispersive spectrometer(EDS)were used to characterize and analyze the morphology of the joint;X-ray diffractometer(XRD)was used to analyze the phase composition of the joint;the microhardness tester and universal testing machine were used to characterize and analyze the microhardness of the joint and the shear strength at room temperature.Besides,the connection mechanism of vacuum brazing Ag70.5Cu27.5Ti2 and Ti57Zr13Cu21Ni9 solder to connect TC4 titanium alloy and 316L stainless steel was investigated.When vacuum brazing of TC4 titanium alloy and 316L stainless steel with Ag70.5Cu27.5Ti2 solder,the interface between the solder and the two sides enables effective connection.The research indicates that,the structure of the joint interface with Ag70.5Cu27.5Ti2 solder is TC4 titanium alloy/diffusion layer(α-Ti+Ti2Cu)/reaction layer(including TiCu,Ti2Cu)/weld center area/Ti-Fe layer/316L stainless steel.When the welding temperature is 810℃,the shear strength at room temperature is only 98.90MPa.When the brazing temperature is 840℃,the shear strength rises to 142.3MPa.When the temperature further increases to 870℃,the shear strength reaches the highest,up to 190.2MPa.As the temperature increases further,the shear strength shows a decreasing trend.When the temperature is 900℃ and 930℃,the shear strength drops to 113.6MPa and 53.3MPa respectively.With the increase of temperature,the shear strength of the joints at room temperature tends to increase first and then decrease,the hardness of joints shows an increase trend with increasing temperature.When the welding temperature is 870℃ and the holding time is 5min,10min,15min,the average shear strength of the brazed joint at room temperature is 134.9MPa,190.2MPa and 170.1MPa,respectively.With the extension of the holding time,the average shear strength at high room temperature first increases and then decreases,and the micro-hardness of the joint increases with the extension of the holding time.The optimal welding parameters in the Ag70.5Cu27.5Ti2 solder system are:the welding temperature is 870℃,the holding time is 15min,and the maximum shear strength at room temperature is 190.2MPa.The connection process of the Ag70.5Cu27.5Ti2 solder system can be divided into the following five stages:(1)Solder melting stage;(2)The interdiffusion stage of the brazing filler metal and the base material;(3)The stage in which diffusion layer and reaction layer on titanium alloy side and the Ti-Fe layer on the stainless steel side are formed;(4)Stages where the diffusion zone and reaction zone become wider and the Ag-Cu eutectic structure decreases;(5)The stage in which Ag-based solid solution is gradually replaced by Ti-Cu intermetallic compounds.Vacuum brazing connection between TC4 titanium alloy and 316L stainless steel can be realized Using Ti57Zr13Cu21Ni9 solder as an intermediate layer.The typical interface structure of the system is TC4 titanium alloy/diffusion layer/weld center/Ti-Fe/3 16L stainless steel.The main phases in the diffusion layer are Ti(s,s)phase and Ti-Fe phase.There are three phases of white,gray and black contrast in the center of the weld,white and gray are mainly Ti-Fe phase and(Ti,Zr)2(Ni,Cu)phase and black area is mainly Ti(s,s)phase and Ti-Fe phase.When the welding temperature is 920℃,the average shear strength at room temperature is 81.5MPa.When the welding temperature is 940℃,the average shear strength increases to 108.6MPa.When the welding temperature is 960℃,the average shear strength of the sample reaches the maximum,which is 133.98MPa.To further increase the welding temperature to 980℃,the average shear strength decreases to 67.3MPa.As the welding temperature increases,the average shear strength at room temperature first increases and then decreases.When the welding temperature is 940℃ and the holding time is 5min,15min and 30min,the shear strength at room temperature is 112.85MPa,133.98MPa and 109.62MPa respectively.The best performance joints are obtained under the welding temperature of 940℃ and the holding time of 15min.The average shear strength at room temperature is 133.98MPa.When using Ti57Zr13Cu21Ni9 solder to braze TC4 titanium alloy and 316L stainless steel,the connection process can be divided into the following five stages:(1)Solder melting stage;(2)Interdiffusion stage of brazing filler metal and base metal;(3)The formation stage of the diffusion layer on the titanium alloy side and the Ti-Fe layer on the stainless steel side;(4)The stage of forming gray structure at the center of the weld(960℃);(5)The stage of black tissue growth at the center of the weld(980℃).Compared Ag70.5Cu27.5Ti2 solder system joint with Ti57Zr13Cu21Ni9 solder system joint,the former has the higher shear strength and lower hardness;the fracture morphology of the Ag70.5Cu27.5Ti2 solder system joint has the ductile fracture characteristics at low temperatures,the fracture morphology of Ti57Zr13Cu21Ni9 solder system joints is all the brittle fracture.