Current-Assisted Tlp Joints: Microstructure and Mechanical Properties in Inconel 718 Alloys Using Bni-2 Filler

Abstract

This study utilized a novel method of current-assisted transient liquid phase (TLP) bonding, using BNi-2 brazing material to join Inconel 718 effectively. Joule heating was used in the bonding process to rapidly heat the joint and maintain it for a while, followed by natural cooling. Compared to traditional furnace TLP bonding, this method significantly improved bonding efficiency, reduced energy consumption, and minimized the thermal impact on the base metal. The current-assisted TLP joint can complete isothermal solidification in a short time. The distribution of borides in the diffusion affected zone (DAZ) was related to the direction of the current, and the grain size of the base metal in the DAZ region significantly reduced due to the influence of element diffusion. Increasing current density enhanced the diffusion capability of the elements, leading to a significant increase in the width of the DAZ region and blurring the boundary with the isothermally solidified zone (ISZ), resulting in a more uniform structure. By observing the microstructure of specimens at different bonding times and COMSOL temperature field simulation, an evolution model of the current-assisted TLP bonding was established. Element diffusion was affected not only by the concentration gradient but also by the current. If there was not enough time to complete isothermal solidification, brittle eutectic would precipitate on the positive side of the brazing seam, severely weakening the mechanical properties of the joint. In addition, the hardness of the base metal decreased after current-assisted TLP bonding and increased after furnace TLP bonding, which is related to the decomposition and formation of the strengthening phase during the bonding process. Extending the bonding time changed the fracture mechanism of the joint, causing many step-like cracks to appear on the fracture surface, slowing down the cracks expansion speed. The shear strength of the current-assisted TLP joints after completing isothermal solidification was comparable to that of the furnace TLP joints, with a transparent plastic deformation process before fracture, but the numerical fluctuation was slightly higher.

Keywords: Transient liquid phase bonding, Current-assisted TLP, Electromigration effect, Inconel 718 alloy, Microstructure evolution, Mechanical properties