Investigation of intergranular penetration behavior in CrMnFeCoNi HEA/304 SS dissimilar brazing joints

Abstract

As-rolled and annealed CrMnFeCoNi high-entropy alloys (HEAs) were selected to braze 304 SS by using Ag50Cu as the filler metal (FM) in a vacuum furnace. Intergranular penetration was detected in each joint. To explore the evolvement mechanism of microstructure and performance, three different series of dissimilar joints were prepared and the microstructure and mechanical properties of the dissimilar joints were observed and tested. The penetration of liquid FM into the grain boundaries (GBs) of HEA was attributed to the imbalance between GB energy and solid–liquid interface energy. Intergranular penetration accelerated interdiffusion between FM and HEA, and diffusion induced further penetration. The mechanism of interdiffusion was studied, and the effects of brazing temperature, Ag50Cu content and Mn concentration in HEA on atomic immigration during brazing were discussed. Intergranular penetration inhibited grain growth in the penetration zone (PZ) of HEA. The physical difference between HEA and penetration phase generated residual stress. A layer of σ (Cr, Fe) phase formed at the 304 SS/FM interfaces, and increased the embrittlement of the dissimilar joints brazed at high temperatures. However, the tensile strength of the joints is improved by the immigration of a huge amount of Mn (a solution-strengthened element of Ag–Cu alloy) into FM and the increasing contact area induced by penetration. The tensile strength of 304 SS and the as-rolled HEA reached 434.7 MPa, which is higher than that of the joint brazed using BNi2. The annealed HEA processed low GB density and low driving force for grain growth, decreasing the difference in grain sizes in PZ, reducing penetration and ensuring interdiffusion between HEA and FM. The tensile strength of the joint of annealed HEA reached 487.1 MPa, indicating that controlling parameters of brazing and changing conditions of HEA are effective to reduce penetration.