Method of the low temperature brazing connection between graphite electrode and copper conductor

A method of graphite medium temperature active metallization and low temperature soft brazing was designed to realize the connection between graphite electrode and enamelled copper conductor in the conductivity sensor. Ag-Cu-Ti alloy solder paste was used to realize vacuum active metallization of graphite surface, and then Sn-Ag alloy flux-cored wire was used to braze graphite metal layer and enameled copper wire at low temperature. The microstructure, phase composition and element diffusion of graphite-metal layer and graphite-metal layer/oxygen-free copper reaction interface were analyzed by means of metallographic microscope, scanning electron microscope and energy dispersive spectrometer, and the bonding strength of graphite - oxygen - free copper brazing joint was measured by universal stretcher. The result shows that Ag-Cu-Ti solder paste is combined with the graphite surface obviously, and the reaction layer of TiC compound is formed.The thickness of the solder layer left on the surface of graphite is about 60 μm. The metal layer between the Sn-Ag alloy solder and graphite presents good wettability, the wetting angle is 16°, the intermediate reaction layer is dominated by Ag₃Sn intermetallic compound. The bonding layer between solder and wire is composed of Cu₃Sn (ε phase) and Cu₆Sn₅ (η phase). The tensile strength of the brazed joint is 39 MPa, and the mechanical properties meet the reliability requirements of the connection between graphite electrode and enamelled copper wire.