Metallurgical and mechanical examinations of molybdenum/graphite joints by vacuum arc pressure brazing using Ti-Zr filler materials

The aim of this work was to join Molybdenum (Mo) and a graphite to obtain laminated structure using a high-temperature and hot press welding method with a Ti-Zr foil as the binder in a vacuum condition. The microstructure and composition changes of the interface were characterized by scanning electron microscopy and X-ray diffraction, respectively. The properties of the interface were studied using shear tests and hardness measurements. The optimal processing conditions for Mo and graphite joints involved a pressure of 0.5-1 MPa, holding time of 10-30 s, and brazing time of 5 min with a melting current of less than 150 A under a 1.3 × 10−3 Pa vacuum condition. A transition zone with a width of approximately 0.3 mm and an average micro-hardness reaching 565 HV was formed, which is much higher than that of graphite and Mo. The main reason is the formation of different carbide and solid solutions such as MoC, Mo9Ti4, and Ti2C0.06. The shear bond strength of the graphite and molybdenum connection is 15.2 MPa, which is higher than that of graphite. The connection mechanism of Mo and graphite was the interlayer reacting with the Mo or graphite substrate at a high temperature to form compounds and solid solutions, respectively, covering the surface of the substrate. Therefore, an effective connection of Mo and graphite was successfully achieved. A model of the Mo and graphite connection structure was introduced.