Microstructure Evolution of Athermal Solidification Zone and Its Effect on Mechanical Properties in Dissimilar Transient Liquid Phase Bonded Joint of IN718 Ni-Based Alloy/316LN Steel

Transient liquid phase (TLP) bonding of IN718 Ni-based alloy and 316LN steel was carried out using BNi-2 alloy foil at 1050 °C for diverse bonding time. The formation process of intermetallics in the athermal solidification zone (ASZ) and the effects of intermetallics on the mechanical properties of joints and how they break are discussed. The effect of ASZ on the mechanical properties of joints was discussed. The analysis showed four kinds of intermetallic phases in ASZ, which are Ni 23 B 6 , CrB, Ni-Si-B, and Ni 3 Si. Besides, the formation of precipitates in ASZ was summarized in the following four stages: (1) γ (Ni) solid solution, (2) γ (Ni) + Ni 23 B 6 + CrB, (3) γ (Ni) + Ni 23 B 6 + CrB + Ni-Si-B, and (4) Ni 3 Si via solid-state transformation. The experimental results of mechanical properties showed that the intermetallic phases formed in ASZ cause greater stress concentration, so the mechanical properties of this kind of TLP joint are not excellent. After the shear test, the joint with a large number of intermetallic compounds at the centerline of the joint showed the characteristic of brittle fracture. In addition, SEM images of the cross section of TLP joints showed that the crack nucleates in ASZ and propagates along the centerline of the joints. When the content of intermetallics at the centerline of the joint decreases, the joint shows ductile fracture.