Microstructural and mechanical properties of CFC composite/Ti6Al4V joints brazed with Ag-Cu-Ti and refractory metal foils

Carbon fiber-reinforced carbon composite (CFC)/Ti6Al4V alloy brazing has been performed with Ag-Cu-Ti braze and Nb (or W) foil. Satisfactory bonding is achieved at the interfaces among substrates, refractory metal layer and fillers. The joining region consists of filler I, refractory metal layer and filler II. The fillers I and II are composed of Ag-based solid solution, TiCu and Cu-based solid solution. A diffusion layer (comprising Ti-based solid solution and Ti2Cu) and a thin TiC reaction layer develop adjacent to Ti6Al4V and CFC substrates, respectively. Regarding the joining with Nb foil, slight dissolution and diffusion occur between Nb and Ti in the fillers upon brazing. However, neither inter-diffusions nor reactions between W and fillers are involved in CFC/Ti6Al4V joining with W foil. The average shear strengths of joints with Nb and W foils are about 200% higher than those without refractory metal foil, indicating that the thermal mismatch in the joint can be relieved by the introduced refractory metal foils with appropriate coefficients of thermal expansion. Moreover, Nb foil with high ductility is beneficial for the inhibition of both the micro-crack propagation and the brittle-phase formation in the joint. W foil can act as a hard barrier to adjust the joint stress distribution and to decrease the joint stress concentration.