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

Abstract

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. *mrityunjay.singh-1@nasa.gov

Integration of High‐Conductivity Graphite Foam to Metallic Systems Using Ag‐Cu‐Ti and Ag‐Cu‐Pd Braze Alloys

“…As there was virtually no braze penetration in joints with Poco foam, the thermal resistance of flat‐base sandwich structures Poco/metal joints made using conductive Palcusil‐5 braze is evaluated for the purpose of illustration of the idea. Thermal projections similar to those presented below for Poco/metal joints were made for GTIH foam/Ti joints with Cusil‐ABA and foams of different densities, and the effect of braze infiltration was considered in thermal calculations …”

“…The foams were carefully sectioned into rectangular prisms to expose surfaces that were either parallel or perpendicular to the foaming direction and identified as “WR” or “AR.” The rectangular prisms had a cross‐section of 12.7 mm × 12.7 mm and thicknesses of 25.4 mm (the effect of foam thicknesses on joint strength with Ti substrates was found to be insignificant in Ref. ).…”

“…Likewise, tubular steel subelements may be used in heat exchangers for corrosion resistance and enhanced cooling when integrated to foam. Carbon‐based composites have been integrated to metals and alloys such as Ti, Hastelloy, Inconel‐625, and Cu‐clad‐Mo using active braze fillers, and Ti has been brazed to graphite foam using an Ag‐Cu‐Ti braze alloy . However, joining of foam to nickel‐base superalloys has not been demonstrated to the authors' knowledge.…”

“…In recent years, light weight and high‐conductivity carbon–carbon composites and graphite foams have attracted a great deal of interest and attention for thermal management applications . Graphite foams offer low density, large surface area, high ligament and bulk conductivities, and chemical and thermal stability.…”

“…For example, Poco foam is a lightweight (e.g., density approximately 797 kg/m 3 ), open microcellular foam with a relatively high thermal conductivity (bulk conductivity: 135–245 W/m/K, ligament conductivity: 540–630 W/m/K). Similarly, GrafTech foams (density: 150–450 kg/m 3 ) have bulk conductivity values of 31–150 W/m/K and ligament conductivity of 460–700 W/m/K . Graphite foam and bulk graphite substrates have been soldered to metals such as aluminum for low (<570 K) use temperatures .…”