Study On Microstructures And Properties Of 316L Stainless Steel Joint Vacuum Brazing With BNi5 Nickel-based Filler Metal

Stainless steel joints brazed with nickel-based solders are widely used in automotive EGR systems,heat exchangers,medical devices and other fields because of their excellent mechanical properties,high temperature performance and corrosion resistance.The vacuum brazing technology has many advantages,such as fine assembly,uniform heat input,low cost and high efficiency.The addition of Si and P elements in the brazing filler metal can reduce the brazing temperature and improve the fluidity of the brazing filler metal,but the brittle compound phase will be formed in the center of the brazing joint,which will affect properties of the joint.Therefore,it is important how to obtain the brazing joints with good properties.The experiment was carried out at 1150? and the holding time was 10 min,20min and 30 min respectively.The gap size was 30?m and 100?m.The interface microstructures of the brazing joint were analyzed by Scanning Electron Microscopy(SEM)and the element composition was measured by Energy Dispersion Spectrum(EDS).Hardness distribution and shear strength of joints were obtained by microhardness tester and universal tensile testing machine.Galvanic corrosion performance and intergranular corrosion sensitivity of brazing joints were tested by CHI660 E electrochemical workstation.The microstructure of the brazing joint can be clearly divided into four regions: base metal(BS),diffusion affected zone(DAZ),isothermal solidification zone(ISZ)and athermally solidification zone(ASZ).According to the calculation of Thermo-Calc software,ISZ is composed of Fe-Cr-Ni solid solution and intermetallic compound,and DAZ contains eutectic structure and brittle silicide.With the gap decreasing and the holding time extending,the ASZ zone becomes narrower and the ISZ zone becomes wider.The hardness distribution is low on both sides and high in the center because the hardness of eutectic structure and silicide is higher,while the hardness of solid solution phase is lower.The brittle compound phase is easy to form micropore under shear stress,which leads to joint cracking.Because the eutectic structure is brittle,the crack propagates along the eutectic structure,and the river pattern can be seen on the fracture.When the crack extends to the continuous solid solution phase,fracture morphology is dimples.The potentiodynamic polarization curves show that the brazing process has little effect on the corrosion potential of the joints,but has great influence on the corrosion rate.Eutectic structure and brittle compounds can accelerate the corrosion rate of joint,while the corrosion resistance of nickel-based solid solution is close to that of base metal,and the addition of Fe and Cr elements is beneficial to the formation of passive film.The results of intergranular corrosion experiments show that the base metal is still insensitive to intergranular corrosion after thermal cycling.Continuous brittle compounds are corroded by oxalic acid,showing corrosion ditches,but insensitive to intergranular corrosion.When the gap size is 100? m and holding time is 30 min,the solid solution phase in the isothermal solidification zone shows a certain sensitivity to intergranular corrosion,because the continuous compounds disperse on the grain boundary,but are not completely dissolved.