Microstructure evolution and shear strength in CoCrFeNi/AISI 304 joint by vacuum brazing with BNi-2 filler metal

Abstract

CoCrFeNi high entropy alloy (HEA) is endowed with a series of outstanding properties, such as excellent tensile performance, good resistance to cryogenic and corrosion, due to its unique chemical composition and crystal structure. The joining of CoCrFeNi HEA alloy with AISI 304 stainless steel is of vital significance for producing complex engineering components available to extreme conditions. In this work, the brazing of CoCrFeNi HEA and AISI 304 steel was conducted by BNi-2 filler metal in a vacuum of 10−4Pa, and a CoCrFeNi/AISI 304 joint was obtained. The interfacial microstructure evolution, brazing mechanism, and mechanical properties of the joints were evaluated. The results indicate that three zones are formed in the brazing seam between CoCrFeNi and AISI 304, including an athermal solidification zone (ASZ) with CrB precipitates distributed in the center of the joint, an isothermal solidification zone (ISZ) with nickel-based solid solutions, and a diffusion affected zone (DAZ) near the base metals. The widths of ISZ and DAZ increase with the increase of brazing temperature. The shear test results show that excellent bonding occurs in the CoCrFeNi/AISI 304 joint with a maximum shear strength of 237 MPa by controlling brazing temperature at 1120 °C. At the same time, B in BNi-2 filler metal diffuses to the two base metals, resulting in a fine-grained structure and improving improve the interfacial bonding.

Keywords

• CoCrFeNi high-entropy alloys
• AISI 304 stainless steel
• Vacuum brazing
• Interfacial microstructure
• Shear strength