Microstructure evolution and mechanical properties of brazing joint for ultra-thin-walled Inconel 718 considering grain size effect and brazing temperature

The systematic investigation of the mechanical properties and microstructure evolution process of ultra-thin-walled Inconel 718 capillary brazing joints is of great significance because of the exceptionally high demands on its application. To achieve this objective, this study investigates the impact of three distinct brazing temperatures and five typical grain sizes on the brazed joints’mechanical properties and microstructure evolution process. Microstructural evolution analysis was conducted based on Electron Back Scatter Diffraction (EBSD), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HRTEM), and Focused Ion Beam (FIB). Besides, the mechanical properties and fracture behavior were studied based on the uniaxial tension tests and in-situ tension tests. The findings reveal that the brazing joint’s strength is higher for the fine-grain capillary than the coarse-grain one, primarily due to the formation of a dense branch structure composed of G-phase in the brazing seam. The effects of grain size, such as pinning and splitting, are amplified at higher brazing temperatures. Additionally, micro-cracks initiate around brittle intermetallic compounds and propagate through the eutectic zone, leading to a cleavage fracture mode. The fracture stress of fine-grain specimens is higher than that of coarse-grain due to the complex micro-crack path. Therefore, this study contributes significantly to the literature by highlighting the crucial impact of grain size on the brazing properties of ultra-thin-walled Inconel 718 structures.