Bonding and Integration of Titanium to Graphitic Foams for Thermal Management Applications



Light-weight, high-conductivity graphite foams comprised of open cellular structure and dense graphitic matrix are attractive materials for thermal management applications in avionic heat sinks and heat exchangers. Integrating foam in such systems requires robust and thermally conductive joints between the foam and metals such as Ti. Graphite foams with different densities were vacuum brazed to titanium using a Ag-Cu-Ti active braze alloy, Cusil-ABA. Optical microscopy and scanning electron microscopy coupled with energy dispersive spectroscopy were used to evaluate joint integrity, interface microstructure, and elemental composition. The joints were defect-free and well-bonded, and the carbon/braze interfaces were enriched in Ti indicating chemical bonding. The low-density foams exhibited significant braze penetration with the penetration distance increasing with decreasing foam density. The room-temperature tension test on brazed foam/Ti joints revealed that the joints were stronger than the foam, so failure occurred within the foam. The thermal resistance of foam/Ti joints, estimated using 1-D, steady-state heat conduction analysis for a planar configuration, revealed a marginal effect of braze saturated foam on joint conductivity for the practical range of values of foam and metal conductivities, penetration depth, and metal-to-foam thickness ratio.