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Aluminum Matrix Composites-Materials of the Present or the Future?
12-20-02
Metal matrix composites with aluminum as the matrix material have been heralded as the next great development in advanced aluminum materials…for at least 20 years. MMCs have sometimes been termed "the materials of the future…and always will be." It seemed as though the promise was greater than the product. While a lot of the industry wasn't looking, however, these materials have been applied in a diversity of market areas, and are poised for further growth if current development work comes to fruition.
First, what is an aluminum matrix composite? Broadly, aluminum composites are materials in which a reinforcement, typically a ceramic-based material, is added with the purpose of improving the material's properties. Of the variety of ceramic materials that can be used as reinforcements, silicon carbide (SiC) and aluminum oxide (Al2O3) are the two that have seen the greatest use as a result of their favorable combination of density, price, and property improvement potential. Reinforcements also come in a variety of forms: continuous fibers, whiskers, and particulates. When these reinforcements are combined with an aluminum matrix, the resulting material has significant increases in elastic modulus (stiffness), wear resistance, and, in some cases, strength and fatigue resistance. In addition, the coefficient of thermal expansion of aluminum is reduced by the addition of the reinforcement, while the material retains the high thermal conductivity and low density inherent in the aluminum alloy. These types of property changes, not generally possible through conventional alloying methods, have been the source of the excitement about aluminum composites.
Now a bit of history…aluminum matrix composites were first developed to meet very high performance defense and aerospace needs. Continuous fiber reinforced aluminum was used in the Space Shuttle and Hubble Space Telescope. As material cost became a more significant consideration, the emphasis shifted toward particulate-reinforced materials, with the goal of a lower cost, high volume product that could be used in automotive and commercial aerospace applications. Many of the major aluminum companies, as well as others, had metal matrix composites development programs in the 1980's and early 1990's. Alcan, through its Duralcan subsidiary, established a 25 million pound per year production capability for particulate-reinforced aluminum composites. The Aluminum Association convened the Aluminum Metal Matrix Composites Working Group, a product of which was the ANSI H35.5 standard that established a nomenclature system for aluminum composites (available from the Aluminum Association online at www.aluminum.org). These efforts were important in moving the field forward.
Where are these materials is use today? Automotive applications include cast aluminum composite brake drums and rotors on the Prowler and EV-1, driveshafts for the Corvette and GM S/T truck, and tire studs in Scandinavia. In the aerospace field, aluminum composites are used in the fan exit guide vanes of the Pratt & Whitney 4000 series engines, which power the Boeing 777. Another exciting area of application for aluminum composites is in the fast-growing electronics packaging market, primarily for thermal management applications in which the ability to match the coefficient of thermal expansion of the electronic materials is a key attribute.
I mentioned earlier that there was significant future potential for these materials. A recent market study by Business Communications Co., Inc. has indicated that the metal matrix composites market will grow at a rate of 14.1% from 1999-2004 to a value of $179 million. A key to achieving this level, as well as future growth, is continued development of the materials, processes, and products. Key in all market areas is the further reduction of the cost of the aluminum composite materials as well as more efficient processing and increased design experience. In the automotive market area, recent efforts have focused on further reductions in raw material cost to enable more widespread application. Projects on both cast and powder metallurgy-based composites through the United States Automotive Materials Partnership (USAMP) are addressing this area. In aerospace, cost-effective expansion of the range of properties for aluminum composites is the focus, with increased elevated temperature capability and improved fracture toughness being two key objectives. Applications for aluminum composites in the electronic packaging market will continue to grow as processing methods for forming, machining, and plating continue to develop and further experience is gained.
Want to learn more about this area? Two useful web sites are:
- MMC-Assess (http://mmc-assess.tuwien.ac.at). MMC-Assess, constituted in Oct. 1998 and funded by the European Community within Brite Euram III, brings together 21 partners from industry, applied research institutions and universities - a representative cross-cut of European researchers active in metal matrix composites. The main target of this group is to increase market acceptance by collecting and evaluating information describing the outstanding potential of MMC.
- The Aluminum MMC Consortium (www.almmc.com). The Mission of the Aluminum MMC Consortium is to facilitate expanded application of reinforced aluminum and other metal matrix composite products by developing manufacturing process technology, design confidence, and increased awareness.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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