Microstructure Evolution of Athermal Solidification Zone and its Effect on Mechanical Properties in Dissimilar Tlp Bonded Joint of In718 Ni-Based Alloy/ 316ln Steel

Transient liquid phase (TLP) bonding could be used for repairing large cracks or worn areas on aerospace components. TLP bonding of IN718 Ni-based alloy and 316LN steel was carried out using BNi-2 Ni-based foil at 1050℃ for different holding time. The formation and evolution of the athermal solidification zone (ASZ) were studied. The effect of ASZ on the mechanical properties of joints was discussed. The analysis shows that there are four kinds of intermetallic phases in ASZ, which are Ni23B6, CrB, Ni-Si-B, and Ni3Si. The formation of ASZ is divided into four stages: (1) isothermal solidification of γ(Ni) solid solution, (2) formation of ternary eutectic of γ(Ni) + Ni23B6 + CrB, (3) formation of γ(Ni) + Ni23B6 + CrB + Ni-Si-B, and (4) Ni3Si via solid-state transformation. In the experiment of mechanical properties, the eutectic structure in ASZ causes great stress concentration, thus the joint has a brittle fracture, and the fracture path passes directly through the intermetallic phase in ASZ. The analysis of mechanical properties and fracture microstructure showed that with the decrease of the width of ASZ and the content of hard-brittle eutectic structure in ASZ, the shear strength and tensile strength were greatly increased. Correspondingly, the fracture mechanism of the joint changed from brittle fracture to ductile fracture