Research On Process And Mechanism Of Brazing C_f/LAS Composite To TC4 Alloy

In the recent development of new materials, C_f/LAS composite has been considered to be an ideal high temperature material with potential application in precise optical instrument and aerospace field, especially in the applications requiring dimensional stability and size precision, because of its low or even nearly zero coefficient of thermal expansion, high chemical stability, good mechanical properties and excellent thermal shock resistance. Connected C_f/LAS composite with TC4 alloy to made structures, can greatly broaden the application of composition materials. In this paper, the interface reaction mechanism was analyzed, and the relationship between microstructures and mechanical properties was investigated when the C_f/LAS composite and TC4 alloy were brazed with AgCuTi and AgCu filler metals.The interface structure was C_f/LAS composites / TiSi2+TiC / Ti3Cu4 / Cu(s,s) / Ag(s,s) / TiCu / Ti2 Cu / Ti2Cu+Ti(s,s) /TC4 alloy 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 TiSi2 + TiC / Ti3Cu4 and Ti2 Cu / Ti2 Cu + Ti(s,s) increased, while the Ti3Cu4 / TiCu and Ag(s,s) + Cu(s,s)in the middle of brazing seam decreased as well as the layer of Ti2 Cu from discontinuous to continuous, and the amount of ash block Cu(s,s) decreased and eventually disappeared. When the brazing temperature was 890℃ for 10 min, the optimum shear strength was 26.4MPa. The shear strength decreased at other brazing parameters. The fracture surface analysis showed that the joints were fractured at TiSi2+TiC layer at low and middle brazing parameters and the joints were fractured at TiSi2+TiC/Ti3Cu4 interface at high brazing parameters.To release residual stress and increase mechanical properties, C_f/LAS composite and TC4 alloy were brazed with AgCu filler metal. The interface structure was C_f/LAS composites / TiSi2+TiC / Ti2Cu3 / Cu(s,s) / Ag(s,s) / TiCu / Ti2 Cu / Ti2Cu+Ti(s,s) /TC4 alloy at low brazing parameters. With the increased brazing parameter, Ti2Cu3 disappeared, and Ti3Cu4 appeared. The interface structure was C_f/LAS composites / TiSi2+TiC / Ti3Cu4 / Cu(s,s) / Ag(s,s) / TiCu / Ti2 Cu / Ti2Cu+Ti(s,s) /TC4 alloy. The thicknesses of TiSi2 + TiC / Ti2Cu3 and Ti2 Cu / Ti2 Cu + Ti(s,s) reaction layers increased, while the Ti3Cu4 / TiCu and Ag(s,s) + Cu(s,s)in the middle of brazing seam decreased as well as the layer of Ti2 Cu from discontinuous to continuous, and the amount of ash block Cu(s,s) decreased and eventually disappeared with the increased brazing temperature and holding time, meanwhile, Ti2Cu3 reaction became to Ti3Cu4 phase. When the brazing temperature was 880℃ for 10 min, the optimum shear strength was 33.5MPa. The shear strength decreased at other brazing parameters. The fracture surface analysis showed that the joints were fractured at TiSi2+TiC layer at low and middle brazing parameters and the joints were fractured at TiSi2+TiC/Ti3Cu4 interface and Ti3Cu4 layer at high brazing parameters.Making the thermodynamic analysis in each phase in the brazed joint and analyzing atomic diffusion theory in the brazing process of TC4 alloy / C_f/LAS composite, interface forming process of concrete can be divided into the following four stages:(1) interaction of TC4 alloy and molten liquid solder;(2) formation of TC4 lateral diffusion layer, Ti atom diffusion to composite side, began form reaction layer at the composite side;(3) TC4 alloy continue to dissolve into solder liquid, liquid phase composition in the weld homogenization, reaction layer formation and thickening at the composite side;(4) TC4 alloy stop to dissolving into solder liquid, the end of reaction at the composite side, intermetallic compound solidified in center of brazed joint.