Study On Microstructure And Mechanical Properties Of C/C Composite And TC4 Brazed Joint

In the recent development of new materials, C/C composite has been considered to be an ideal high temperature material with potential application in aerospace field because of its low density, high thermal conductivity, high thermal shock and fatigue resistance, especially its high mechanical properties at high temperature. It has been successfully used as aircraft brakes materials, armour materials for the divertor and nozzle materials. The nozzle component made of C/C composite and TC4 can greatly decrease the weight and increase the efficient of rocket engine. In this paper, the interface reaction mechanism was analyzed, and the relationship between microstructures and mechanical properties was investigated when the C/C composite and TC4 were brazed with AgCuTi and TiZrNiCu (adding to interlayer) filler metals.The interface structure was C/C composite/TiC/TiCu/Ag(s.s)+Ti₃Cu₄+TiCu/ Ti₃Cu₄/TiCu/Ti₂Cu/Ti(s.s)+Ti₂Cu/TC4 with AgCuTi filler metal. The kinds of the reaction products were not changed but the thicknesses of the reaction layers were changed with different brazing parameters. With increased brazing temperature and bonding time, the thicknesses of the TiC/TiCu and Ti₃Cu₄/ TiCu/Ti₂Cu/Ti(s.s)+Ti₂Cu increased, while the Ag(s.s)+Ti₃Cu₄+TiCu in the middle of brazing seam decreased as well as the amount of TiCu+Ti₃Cu₄ decreased. When the brazing temperature was 1183K for 600s, the optimum shear strength was 25MPa. The shear strength decreased at other brazing parameters. The fracture surface analysis showed that the fracture location was related to the orientation of carbon fiber. When carbon fiber was parallel to the joined surface, the joints were fractured in the composites. When carbon fiber was vertical to the joined surface, the joints were fractured at TiC layer at low and middle brazing parameters and the joints were fractured at TiC layer or TiCu/Ti₂Cu interface at high brazing parameters.To release residual stress and increase the mechanical properties at high temperature, C/C composite and TC4 were brazed with TiZrNiCu filler metal and Cu/Mo composite interlayer. The interface structure was Cu/Cu+(51)Zr₁₄/Ti₂(Cu,Ni) +Ti(Cu,Ni)+TiCu+Cu₂TiZr/TiC/C/C composite at low brazing parameters. With the increased brazing parameter, TiCu and Cu₂TiZr disappeared, and Ti(Cu,Ni)2 appeared. The interface structure was Cu/Cu₅₁Zr₁₄/Ti₂(Cu,Ni)+Ti(Cu,Ni)+ Ti(Cu,Ni)2/TiC/C/C composite. At high brazing parameter, the interface structure was Cu/Cu₅₁Zr₁₄/Cu(s.s)+Ti(Cu,Ni)2/TiC/C/C composite. The thicknesses of Cu₅₁Zr₁₄ and TiC reaction layers increased with the increased brazing temperature and holding time. The maximum shear strength was 21MPa at room temperature and 27MPa at 923K when the brazing parameter was 1173K for 300s, which can meet the performance requirement. The shear strength gradually increased with the increased thickness of Cu, but the maximum shear strength was no more than 23MPa. The fracture surface analysis showed that the situation of fracture was related to the orientation of carbon fiber. When carbon fiber was parallel to the joined surface, the joints were fractured in the composites. When carbon fiber was vertical to the joined surface, the joints were fractured at TiC at low and middle brazing parameters and the joints were fractured at TiC and TiCu/Ti₂Cu interface at high brazing parameters.The distribution of residual stress of the C/C composite/TC4 brazed joint was simulated by FEM calculation. Combining the analyses of fracture, it was shown that the mechanical properties and fracture location were mainly related to the shear stressÏ„_xy. The shear stressÏ„_xy at the edge of the C/C composite/filler metal interface using AgCuTi filler metal was lower than that of TiZrNiCu filler metal. Using TiZrNiCu filler metal, the residual stress of the joint would be released with interlayers. The releasing the residual stress of Cu/Mo composite interlayer was better than that of Cu or Mo single interlayer. With the increased of thickness of interlayer, the distribution of shear stressÏ„_xy was not changed, but the shear stressÏ„_xy gradually decreased.Based on the Gibbs energies of formation and diffusion theory, the reaction phases and thickess of reaction layers were studied. The kinetic equations of C/C composite/filler metal/TC4 (or Cu) interface were set up. The kinetic equation parameters were achieved which can predict the microstructure and mechanical properties of the joint.