Phase formation during dissimilar transient liquid phase (TLP) bonding of IN939 to IN625 using a Ni-Cr-Fe-Si-B interlayer

Abstract

An effort was made to assess the applicability of using transient liquid phase bonding technique to join a nickel-based precipitation-hardened superalloy (Inconel 939) to a solid-solution-hardened one (Inconel 625) to avoid the limitations of other joining techniques such as fusion welding. Microstructural development and phase formation mechanism of the joint region were studied in Inconel 939/Ni-Cr-B-Fe-Si/Inconel 625 system. The chemical composition of the isothermally solidified zone in each side of the joint was governed by the chemical composition of the adjacent base alloy. Eutectic-γ solid solution, chromium-rich boride and nickel-rich boride phases were formed at the centerline of the joints with insufficient holding times due to the segregation of chromium and boron elements during the athermal solidification. Increasing the holding time resulted in the decrement of athermally solidified zone width. In the holding time of 30 min, the isothermal solidification was finished and a bond without detrimental centerline-eutectics was produced. Hard and brittle chromium-rich and chromium-molybdenum-rich boride particles were formed by a solid-state precipitation phenomenon in the diffusion affected zone of Inconel 939 and Inconel 625, respectively. Chromium depletion was observed in the vicinity of these precipitates. The holding time had almost no influence on the width of the joint area. However, higher concentrations of solute elements were detected at the joints’ centerline in longer holding times.