Diffusion brazing using powder mixtures of the CoCrFeNi MEA and low melting additives

Conventional nickel-based filler alloys for brazing commonly require low melting elements like boron or phosphorus to reduce the required brazing temperature. However, these elements often promote the formation of brittle intermetallic compounds (IMC), which can significantly reduce the joint strength. The reduction in strength is strongly dependent of the gap width as it influences the ratio of melting point depressing elements content to the faying surface area. This leads to a limitation of the maximum applicable joint gap. High entropy alloy (HEA) filler alloys could be an innovative engineering solution to overcome the technical restriction and to produce strong joints even with wider mounting gaps. This can be attributed to the tendency of HEA to form stable solid solutions that feature a high thermal and chemical stability instead of forming brittle intermetallic compounds. Therefore, it is expected that a HEA microstructure in the brazed metal leads to an improvement of the mechanical properties of the brazed joint. Unfortunately, HEA tend to have high melting temperatures, which impedes their application as brazing filler alloy. Therefore, this study presents a novel approach, in which a medium entropy alloy (MEA) consisting of pre alloyed CoCrFeNi-powder is mixed with various melting point depressants (MPDs), which melts at the brazing temperature and forms a HEA microstructure during isothermal solidification. The influence of different MPD concentrations and brazing temperatures on the resulting joint microstructure were analyzed and evaluated.