Grade 2 Ti-CP was gas tungsten arc welded using flux cored (FC) wires and flux pastes that contained various MgF 2 contents. The effects of MgF 2 on bead morphology, chemical composition and hardness of weld bead were investigated and interpreted. With an increase of MgF 2 content in the flux paste, depth/width ratio of weld bead increased gradually with little variation in interstitial element contents and hardness. Weld bead made with cold FC wire feed showed even deeper and narrower bead, indicating the greater effectiveness of wire feed than flux paste on weld penetration. While the 50% MgF 2 FC wire produced complete slag coverage and smooth weld bead surface, 85% MgF 2 wire resulted in incomplete slag coverage and rough weld surface. Arc spectroscopy revealed that the 50% MgF 2 FC wire produced plasma spectrum with atomic and ionised titanium peaks, which is an indication of a high temperature arc and a larger amount of flux vapours in the arc. Therefore, it is believed that deep weld penetration associated with high MgF 2 fluxes in this experiment is caused by arc constriction, resulting from the greater amount of flux vapours owing to high arc temperature.

The interracial reactions and kinetics of the reaction layer growth in Cu-Zr/AlzO3 system were investigated and compared with those in Cu-Ti/A1203 system. Thermodynamic analysis showed that the interfacial reaction can proceed until the activity of aluminum in the Cu-5 at.% Zr filler metal reaches about 0.27 at 1373 K. Growth of reaction layer, ZrO2, was controlled by the diffusion of oxygen through ZrO: layer. The activation energy for ZrO2 growth was 126 kJ/mol. Instead of using a complex redox reaction, a simple oxidation model between reactive metal and oxygen was found to adequately describe the reaction layers growth in ceramic/metal systems. The parabolic rate constant could be expressed in terms of oxygen vacancy concentration at the reactive metal/reactive metal oxide interface. The slower growth of TiO, in comparison with ZrO2, can be rationalized using the parabolic rate constant. The lower diffusion coefficient of oxygen vacancy and a less negative free energy change of TiO formation have a dominant effect over the higher oxygen vacancy concentration at the Ti/TiO interface which resulted in slower TiO growth.