Study on mechanical properties and microstructure development of Inconel 718 ultrathin-walled capillary-and-plate brazed structure using BNi-5 filler metal

The fuel indirect precooled turbine combined cycle engine is the promising power to realise more green environmental protection intercontinental navigation and hypersonic aerospace transportation in the future. The compact, lightweight precooler arranged in the front of the inlet is a critical component of the advanced heat management system, which could improve the fuel efficiency and flight range of the turbine engine. This paper mainly focuses on the high-temperature brazing process of Inconel 718 ultrathin-walled capillary-and-plate brazed structure using the BNi-5 brazing filler metal. In this investigation, a testing method for tensile strength of ultrathin-walled structures was proposed; the effects of different brazing parameters on the mechanical properties and microstructure evolution of different types of brazed structures were studied to investigate the particularity of ultrathin-walled capillary-and-plate brazed structure. The brazed joint was mainly composed of γ-Ni solid solution, Ni₅Si₂, G-phase, Ni₃Si, and Cr₃Ni₅Si₂. The results showed that the higher brazing temperature and longer holding time were beneficial to improve the mechanical properties of the conventional lap brazed structure; however, the mechanical properties of the ultrathin-walled capillary-and-plate structure decrease significantly. The optimised brazing process for the ultrathin-walled structure was 1150 °C for 3 min; dissolution behaviour of the ultrathin-walled capillary and the deeper position of the precipitates induced by the acute diffusion phenomenon play an important role in the tensile strength of the ultrathin-walled structure. And the failure mechanism was analysed in detail; the result indicated that ultrathin-walled capillary-and-plate brazed structure was mainly affected by the eutectic structure of the brazing fillet; the Nb-rich precipitates in the diffusion affected zone and the dissolution behaviour of the ultrathin-walled base metal.