Effect of TLP bonding atmosphere on microstructural and mechanical characteristics of Hastelloy x superalloy joints

Important variables such as temperature, time, surface quality, atmosphere, chemical composition, and interlayer thickness affect the quality of the transient liquid phase (TLP) bonding. The mentioned factors have an essential role in the behavior of the created joints by affecting the formation of intermetallic phases. In this research, the TLP joints of Hestalloy X (HX) superalloy were prepared by a Bni–2 interlayer with a thickness of 80 µm and at a bonding temperature (T_b) of 1070 °C. The joining process was done in different atmospheres, including air, argon, and vacuum (10⁻⁵ torr) for 40 min. Field-emission scanning electron microscope (FESEM), X-ray diffraction analysis (XRD), microhardness, and shear tests were employed to check the samples’ mechanical and metallurgical aspects. The results of microstructural investigations showed that joints prepared under argon and air atmospheres contain holes and porosities due to the partial oxidation of the joint-base metal (BM) interface. The results of mechanical tests prove that the joint made in the vacuum has the best shear strength (about 80% of the strength of the BM). This is attributed to the diffusion of the boron element into the BM and the reduction of harmful intermetallic borides in the bonding region.