Microstructure and mechanical properties of 304 stainless steel joints brazed by using BNi82CrSiB-<italic>x</italic>Cu filler metals

To overcome the drawback of the brittle borides formed in the stainless steel joint brazed using Ni-based filler metals and further improve the mechanical properties of the joint， a small amount of Cu powders are added into the BNi-2 filler metal to suppress the formation of brittle borides in the joints. The 304 stainless steels are brazed using the BNi-2 and BNi-2 with Cu as filler metals， respectively. The influences of holding time， brazing temperature， and Cu content on the microstructure and mechanical properties of the brazed joints are investigated. The results indicate that for the BNi-2 filler metal， increasing the brazing temperature and time are favorable for reducing the boride content formed in the joint center. In addition， increasing the brazing temperature and time can accelerate the intergranular diffusion of borides， thereby leading to a degradation of joint shear strength. For the BNi-2 filler metal with Cu， when the Cu content is 1%（mass fraction，the same below）， the borides in the joint center are inhibited remarkably， forming a full Ni-based solid solution in the joint center. The joint shear strength obtained is （713.9±16.4） MPa， which is 19% higher than that of the joint without Cu additive. The joint failure occurs at the base metal/filler metal interface， recognized as a ductile fracture. However， when the Cu content increases to 5%， the borides reprecipitate in the joint center， resulting in the reduction of shear strength. In this case， the joint failure evolves to a composite mode with the co-existence of ductile and brittle fracture.