INTERFACIAL REACTION BETWEEN ALUMINA AND CU-TI FILLER METAL DURING REACTIVE METAL BRAZING

If a chemical reaction at the interface can influence significantly the properties of a brazed joint using reactive filler metals, it is very important to understand which reaction occurs and how the reaction products grow at the interface. In this research, the interfacial reaction and the kinetics of reaction products growth in alumina brazing utilizing Cu-Ti filler metals were investigated. Cu-Ti filler metals reduced Al2O3 to form TiO at the interface. Thermodynamically, reduction of Al2O3 is possible through the dissolution of the aluminum by the filler metal. At 1300 K, for example, interfacial reaction can proceed until the activity of aluminum reaches about 0.02 in Cu-20 at.-% Ti filler metal. With time, the TiO layer grew toward the center of the brazing filler metal following a parabolic rate law, at the cost of another complex oxide, presumably Ti3Cu3O, which formed next to the TiO. The activation energy of TiO growth was 208 kJ/mol (50 kcal/mol), which corresponds to the activation energy of oxygen diffusion in the TiO. Therefore, it appears likely that the growth of TiO is controlled by oxygen diffusion.