Joining of Advanced Aluminum‐Graphite Composite

Abstract

Advanced aluminum graphite composites have unique thermal properties due to opposite coefficients of thermal expansion of aluminum and graphite. The thermal and mechanical properties of such composites are anisotropic due to directional properties of graphite fibers and their designed orientation. A joint of components with different fiber orientations would theoretically produce an isotropic material for thermal management purpose. This paper presents research results for welding and brazing of the composite using different joining techniques. A laser beam melts the matrix and delaminates graphite fibers. The molten aluminum reacts with graphite to form aluminum carbide Al4C3. The joint strength is compromised when laser welding at optimal conditions to minimize the carbide formation. Brazing is preferred since the low melting temperature of a filler material suppresses the formation of Al4C3 while minimizing shrinkage cavities in the joint. Microstructural study and shear test confirm the success of joining this composite by laser brazing and resistance brazing with Zn-Al filler. Introduction Common metal matrix composites (MMCs) utilize ceramic as reinforcement and metal as host matrix. Although having good structural and wear resistance, these MMCs are difficult to be machined, welded, and experience high thermal stress and distortion due to thermal mismatch and high thermal expansion. A new class of advanced MMCs, aluminum graphite (Al-Gr) composites provides high machinability, lightweight, high mechanical strength, and high thermal conductivity with customized thermal expansion for applications that require superior performance and excellent thermal management. The industrial applications of Al-Gr composites include devices for wireless communication, satellite, power semiconductor, heat sink for microchips, precision electronic devices, heat exchanger for aerospace applications, design for ultra-precision and thermally stable machinery and measurement equipment, etc. The material behavior in thermal related manufacturing, such as laser micro machining and micro welding, should be understood to effectively utilize the composites in other applications. Thermal management MMCs utilize graphite that has negative thermal expansion and light aluminum matrix. This clever choice allows a new group of advanced composites with tailored thermal expansion in the range 1-10x10 /°C that matches those from other engineering materials [1]. Although extrusion is feasible, casting appears to be the most economical manufacturing process for this composite [2]. The composite can effectively remove heat while eliminate thermal stress. Its low density is ideal for portable devices or aerospace applications. The thermal and mechanical properties of Al-Gr composites, however, are directionally dependent since the graphite fibers are aligned differently in the x-y basal plane than in the normal z-direction. Table I shows the commercially available Al-Gr composites MetGraf with 50-60% difference in thermal conductivity, 350-600% difference in thermal expansion, and 40%