Phase transformations in TLPB process for Inconel-718/Al and Ni/Al

POLISERPI MARIANA; BOERI ROBERTO; BUZOLIN RICARDO; POLETTI CECILIA; SOMMADOSSI SILVANA

Congreso; Calphad 2019; 2019

High temperature welds are a complex research field that requires the optimization of bonding process and investigation of phase transformation during its fabrication. Ni based superalloys and Al-Ni intermetallics, belong to those materials which still represent some challenges related to joining process. Inconel 718 is one of the most used superalloy in high temperature environment such as aerospace and power generation industry, due to the combination of solid solution and precipitation strengthening mechanisms. Because of its microstructural complexity, some difficulties arise when they are subject to brazing, fusion welding and diffusion welding techniques. In this context, the transient liquid phase bonding, TLPB, or diffusion brazing is an attractive alternative. TLPB was developed by Paulonis et al. and emerged from those problems during conventional welding processes. TLPB involves four main stages: melding of filler metal, substrate dissolution, isothermal solidification and homogenization if the connexion zone [1]. Although there are several publications on TLPB, fundamental mechanisms of diffusion-reaction processes for commercial alloys, microstructure evolution and the effects of the joining process on technological properties are still unclear. Knowledge of equilibrium phase diagrams, kinetics and diffusion are required to understand phase transformations, thermal stability and phase formation sequence during this process. Recent investigations show that it is feasible to use TLPB to obtain defects-free and reduced thickness joints in superalloys and Al-Ni intermetallics [2-4]. The aim of this work is to study the interconnetion zone of Inc718 / Al / Inc718 couples bonded by TLPB at 800-1100ºC, focus in the phase stability by means of SEM-EDS/EBSD and in-situ analysis using synchrotron X-ray source. Composition and nano and micro-hardness profiles were obtained. Additionally, phase evolution in the interconnection zone was monitored by changes in layers width and grain size. Cr,Nb,Fe-rich phases were identified next to the base material due to nickel outward diffusion from base metal and a thick AlNi layer and secondary phases in grain boundary towards joint centreline. Spheroidal nano-precipitates were found inside the large AlNi grains.