Effect of Substrate's Heat Treatment on Microstructure and Mechanical Properties TLP Bonding of Dissimilar X-45/FSX-414 Cobalt Based Superalloys

Abstract

In this study, a dissimilar joint of cobalt based superalloys X-45 and FSX-414 has been created under the standard heat treatment conditions with the nickel-based BNi-9 interlayer with the thickness of 50 μm by the transient liquid phase bonding process. Solution heat treatment (1150 °C/4 h) was used for bonding and the aging heat treatment (980 °C/4 h) for the homogenization of the bond. Optical metallography, Field Emission Scanning Electron Microscope with Energy Dispersive X-ray Spectroscopy, Differential scanning calorimetry, micro-hardness test and shear strength test were used for the characterization of microstructural and mechanical evolution in base metals and bonds. The solution heat treatment leads to the dissolution of a part of the M23C6 and M6C carbides in the substrate of superalloys, which due to the aging heat treatment secondary and fine carbides M23C6 precipitate in the substrate and help strengthen the superalloys. The diffusion of the boron from the molten interlayer to the base metals resulted in the complete isothermal solidification and a nickel-based single-phase solid solution has been developed. Cobalt-chromium-tungsten-molybdenum-based carboborides precipitation with high hardness in the vicinity of the isothermal solidification zone and in base metals results in the non-homogeneity of microhardness profile along the bonding area. Homogenization heat treatment did not have an effect on the omission of these precipitations, but by more uniform redistribution of the alloying elements strengthen solid solution that could increase shear strength from 655 to 688 MPa, which is somewhat equivalent to the shear strength of the 45-X alloy and also 93% of the shear strength of the FSX-414 alloy.