Aluminum-Silicon Lightweight Thermal Management Alloys with controlled Thermal Expansion

As the power density of aircraft avionics equipment has increased, new methods and materials have been developed to manage heat load as well as interfacial stresses associated with the Coefficient of Thermal Expansion (CTE) mismatch between components. These materials include Metal Matrix Composites (MMCs) consisting of a metal matrix with ceramic reinforcements, such as Aluminum-Silicon Carbide (Al-SiC), Copper-Silicon Carbide (Cu-SiC), and Beryllium-Beryllium Oxide (Be-BeO). Metallic systems can also be applicable however, such as copper alloyed with tungsten, molybdenum, or carbon (in the form of diamond, nanotubes or fullerene), but also Aluminum-Silicon (Al-Si) alloys. Each of these materials systems provides a unique set of performance properties and manufacturing challenges.

The Al-Si system provides an attractive combination of CTE performance and high thermal conductivity whilst being a very lightweight option. While the alloy can be manufactured by several methods, this presentation will describe manufacturing of hypereutectic Al-Si alloys via a powder metallurgy route utilizing a novel high-energy mixing process. This method allows for a very fine and homogeneous distribution of silicon particles within an aluminum alloy matrix, much like that of an MMC, whilst maintaining the machining, fabrication, coating, and processing characteristics of conventional aluminum alloys. Such Al-Si alloys are of particular interest for structural heat sink applications that require high reliability under thermal cycling as well as reflective optics and instrument assemblies that require good thermal and mechanical stability. Due to the flexibility of the manufacturing route, this lightweight material system can be finely tuned to achieve specific and desirable CTE values such as 17ppm/°C (to match copper alloys) and 13ppm/°C (to match nickel alloys) and a detailed understanding of the relationship between chemistry and CTE has been developed. Critical performance relationships such as this will presented, coupled with the basic microstructure, physical and mechanical properties of these Al-SI alloys.