Effect of post-bond heat treatment on microstructural evolution and mechanical properties of brazed ultrathin-walled structure

A nickel-based superalloy Inconel 718 ultrathin-walled structure was manufactured by high-temperature brazing using Ni-Cr-Si filler metal. The microstructure in the brazed fillet was analysed, and the solidification behaviour and the formation of the eutectic island in the brazed fillet were investigated. The effects of post-bond heat treatment (PBHT) with different temperatures and durations on the microstructural evolution and mechanical properties of the brazed ultrathin-walled structure were investigated in detail. The results indicate that PBHT at a relatively high temperature significantly affects the mechanical properties of the ultrathin-walled structure, which is attributed to the formation of the precipitates at a deeper location in the base metal. In addition, the mechanical performance is enhanced by the generation of δ- and γ''-phases under relatively low temperatures (980 °C and 740 °C). On the other hand, the decrease in mechanical performance is caused by the formation of network-like precipitates and the Ni-Si-Nb phase at the grain boundary over extended ageing times. By analysing the fracture mode evolution and the propagation of cracks in the brazing fillet, the effect of PBHT on fracture morphology evolution is determined primarily by the proportion of brittle fractures and the formation of the strengthening phase in the base metal.