Microstructural characteristics of a cast IN718 superalloy bonded by isothermal solidification

Abstract

Isothermal solidification is a key feature of transient liquid phase bonding which prevents the formation of deleterious intermetallic phases in the joint centerline and results in bonds with improved mechanical performance. This paper discusses the metallurgical characteristics and mechanical properties of an as-cast IN718 superalloy bonded by diffusion-induced isothermal solidification of Ni-7Cr-4.5Si-3.2B-3Fe (wt%) filler metal. After transient liquid phase bonding of as-cast IN718 at 1000 °C for 60 min, a bond exhibiting a solid solution microstructure with joint efficiency of 72% in terms of shear strength was obtained. The joining process was effectively able to prevent the formation of hard and brittle nickel and chromium borides, which typically lead to critical problems in brazing. The formation of Nb-rich Laves phase, which is well known as a major issue in the fusion welding of IN718, was not observed. The bonding time, which governs the extent of isothermal solidification, was a critical parameter for controlling the mechanical properties of the joints in terms of shear strength and hardness distribution across the bond.