Interfacial Stress Analysis And Structural Design For Composite And Cu Joints

C/C composites have excellent properties, such as low density, high thermal conductivity, high thermal shock, especially low neutron activated energy, high melting point and sublimation temperature. Due to these characteristics, the joint of C/C composites and Cu is considered as one of the candidate materials for the facing materials in the ITER. However, the mismatch of thermal expansion coefficient (CTE) between Cu and C/C composites causes great residual thermal stress, and consequently results in the degradation of joint during the cooling process. In order to reduce the residual thermal stress, the interface was designed as flat surfaces with Nb interlayer and conical surfaces. In this paper, interfacial stress was studied by the finite element method (FEM). In addition, the results of finite element analysis were proved by experiments.As for the16mm×16mm flat interface, the stress σz without Nb interlayer is apparently concentrated in the edge of the joint, and reaches the maximum value at C/C side,1.15mm from the interface. Whereas, the maximum stress σzmax in joints with Nb interlayer is transferred to Nb side near Nb/Cu interface. The simulation of FEM displays that σzmax decreases with the increasing of the Nb interlayer thickness, and then remains steady when the thickness of Nb interlayer is1mm. The experimental results shows that the strength of the joint is improved as the Nb interlayer thickness increases, which is consistence to the FEM results. When the thickness is lmm, the average value of the strength reaches37.39MPa. There’re five kinds of reaction products in joints, which are respectively Nb2C, TiC, Ag(S.S), Cu(S.S) and Cu4Ti3. In particular, a continuous TiC layer formed at the C/C composites/braze interface, can improve the wettability of Cu on C/C composites.The conical interfacial structure is designed to relief residual stress, as well. The residual stress in C/C composites adjacent to the cone can be reduced effectively by transferring most residual stress to AgCuTi braze filled in the cone. When the height and diameter of cone are2mm and3.2mm respectively, the average value of strength is52MP,2.57times higher than that of the flat interface. While, the maximum value of the strength is63MPa,2.67times higher than that of the flat interface. Compared to Nb interlayer, an appropriate conical interfacial shape is more beneficial to decrease the residual thermal stress, which can promote the performance of connection between C/C composites and Cu.