Active Brazing of C/C Composite to Copper by AgCuTi Filler Metal
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ARBON-FIBER-REINFORCED carbon composites (C/C composites) have been widely used in many areas due to their superior thermal and mechanical properties at high temperatures.[1,2] However, some applications in the aerospace and nuclear industries will require joining such C/C composites to metals or alloys. For example, diverters used in the International Thermonuclear Experimental Reactor need C/C composites to be joined to the copper alloy as part of the lower vertical target.[3] There are several ways to join C/C composites to metals, including direct casting after surface modification of C/C composites and active brazing.[4,5] Brazing is one of the most popular methods for joining C/C composites to metals or other substrates, since it has many advantages such as low brazing temperature, low pressure, and low residual stress in the joints. Therefore, there has been considerable interest in brazing C/C composites to different metals or other substrates using different filler metals in recent years due KEXIANG ZHANG, Ph.D. Student, is with Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China, and also with the University of Chinese Academy of Sciences, Beijing 100049, China. LIHONG XIA, Ph.D. Student, and FUQIN ZHANG, Professor, are with the National Key Laboratory of Science and Technology on High-strength Lightweight Structural Materials, Central South University, Changsha 410083, China. LIANLONG HE, Professor, is with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences. Contact e-mail: [email protected] Kexiang Zhang, and Lihong Xia have contributed equally to this work. Manuscript submitted July 18, 2015. Article published online March 1, 2016 2162—VOLUME 47A, MAY 2016
to their technological importance.[6] There are usually two problems that deteriorate the quality of brazing of C/C composites to metals or alloys. One is the wettability of the carbon surface by liquid metals or alloys. The wettability of carbon has been partly improved by adding the active element Ti to the braze alloy, which has a strong affinity to carbon. Modification of C/C composites is also feasible in order to obtain a carbide coating wettable by metal through the slurry technique.[7] The other one is the residual thermal stress in the joint, caused mainly by the mismatch of the coefficient of thermal expansion (CTE) between C/C composites and metal/alloy substrates. For example, the CTE for C/C composites is less than 1.0 9 106 K1 in the temperature range 293 K to 523 K (20 °C to 250 °C) and 2.0 to approximately 4.0 9 106 K1 in the range 293 K to 2773 K (20 °C to 2500 °C);[8] for pure copper, the CTE is 16 to approximately 20 9 106 K1. The CTE discrepancy between them is rather large. Two methods have been used to release the residual thermal stress of the joints. One technique is inserting a soft metal foil between every two braze foils to transfer thermal stress;[9] the other process is geometry modificati
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