Graphite-reinforced AgCuTi/Cu foam composite filler to braze Al2O3 ceramic and 304 stainless steel

Al2O3 ceramic and 304 stainless steel were brazed with AgCuTi filler in this paper. Two kinds of composite filler were prepared by adding Cu foam and Cu foam covered with graphite (Gr-Cu foam) to relieve the residual stress of the joints. The effects of Cu foam pore size on the joints were investigated in detail, and the evolution mechanism of the joints was analyzed. Graphite-reinforced Cu foam was fabricated by spraying graphite for the first time. Results showed that the extremely low coefficient of thermal expansion of graphite and the good energy absorption performance of Cu foam were both conducive to reducing the thermal residual stress during the cooling process. Gr-Cu foam with pre-filled pores was used to solve the problems of structural dissolution of Cu foam and unfilled solder in Cu foam pores to a certain extent. And it played a key role in preventing the diffusion of metal atoms. The graphite on the surface of the Cu foam generated TiC in the brazing seam. The TiC and Cu foam promoted the formation of the TiCu strengthening phase. Furthermore, when the pore size of the Gr-Cu foam was 150 μm, the maximum shear strength of the joint was 246.84 MPa. This value was about 40% higher than that of Cu foam with the same pore size. The residual stresses of joint with and without graphite-reinforced were calculated, respectively, for both conditions. The reduction of residual stress was found after adding the graphite during brazing.