Reliable brazing of SiBCN ceramic and TC4 alloy using AgCuTi filler with the assist of laser melting deposited FGM layers

When the SiBCN ceramic and TC4 alloy were brazed using AgCuTi filler, the coefficient of thermal expansion (CTE) mismatch between the SiBCN and TC4, as well as the formation of brittle CuTi due to the vigorous reaction of TC4 and AgCuTi filler, produced high residual stress in the joint. To relieve the residual stress, TC4 was coated by SiC and TiB reinforced TC4-based composite layers (named as SiC Layer and TiB Layer) through laser melting deposition technology before the brazing process. The composite layers could be treated as functionally graded materials layers (FGM layers). Afterward, the brazing of the SiBCN and TC4 coated with the FGM layers was investigated. The FGM layers induced the CTE to be graded from the TC4 to the SiBCN, minimizing the residual stress in the joint. The SiC in the SiC Layer reduced the contact area of AgCuTi and TC4. The reaction of AgCuTi and TC4 was restrained, so that the CuTi volume in the brazing part decreased. Therefore, the residual stress generated in the SiBCN was relieved, and the joint strength increased. With the increase of the brazing temperature, the interfacial reaction was promoted. The reaction layer adjacent to the SiBCN grew, and the CuTi volume in the joint enlarged. When the joint was brazed with the assist of the FGM layers at 850 °C for 5 min, the maximum joint strength reached 38 MPa.