Diffusion brazing of IN718/AISI 316L dissimilar joint: Microstructure evolution and mechanical properties

In this study, diffusion brazing was employed to join IN718 superalloy and AISI 316 L stainless steel using MBF20 amorphous interlayer. The effect of interlayer thickness (50-150 mu m) was studied on the microstructure and mechanical properties of the joints. The joints were characterized by using optical and scanning electron microscopes, energy dispersive spectroscopy and X-ray diffraction analysis. Microhardness and shear tests were used to assess the mechanical properties of the joints. The microstructural investigations revealed that eutectic phases including gamma solid solution, Ni3B, CrB, Cr2B3 and Ni6Si2B were formed at the joint centerline of the sample with incomplete isothermal solidification during holding at the brazing temperature. Fine Ni3Si particles also precipitated via a solid state transformation during cooling in the vicinity of eutectic phases. Mechanism of microstructure evolution and the solidification behavior of the centerline intermetallic compounds were discussed. In addition, Cr-, Mo-, Nb-rich borides precipitated in the DAZ of IN718 side and the precipitates formed in the DAZ of AISI 316 L side were Cr-, Mo-rich and Fe-, Cr-rich borides. The maximum shear strength of 440 MPa was obtained for the sample with complete isothermal solidification. Fractography of the samples after the shearing test indicated that the sample with complete isothermal solidification exhibits a ductile failure mode, however a brittle fracture happens in the samples containing hard eutectic constituents at the joint centerline.