METAL MATRIX COMPOSITES
INVESTMENT CASTING
The design freedoms inherent in investment casting help keep aluminum metal matrix composites cost effective.
PAUL MIKKOLA, RETIRED, HITCHINER MANUFACTURING, MILFORD, NEW HAMPSHIRE BRUCE WILLSON, O’FALLON CASTING, O’FALLON, MISSOURI
T
he particulate silicon carbide reinforcement in alumi- num alloy metal matrix composite (MMC) enhances this lightweight material with improved mechanical property attributes for stiff ness, vibration dampening, wear resistance, high thermal conductivity and low coeffi cient of thermal expansion. With its unique set of properties, MMC has been employed in diverse applications such as moving structures in high speed equipment for manufacturing, brake rotors for ve-
hicles, heat sinks for electronics, housings and mirrors for optics. In the 1980s and 1990s, casting shapes from MMC ingot was envisioned to be a major enabling technology for manufacturing MMC metal components. T is perception prompted the development of metalcasting processes and special- ized techniques to overcome the natural tendency of the silicon carbide particles to clump or precipitate from the aluminum matrix. T e inherent abrasiveness of the silicon carbide particulates in the alloy also gave MMC a reputation, perhaps unfairly, for being diffi cult and expensive to machine. For myriad reasons, a broad market for cast and other process shapes has not developed, and so, despite its many attributes, MMC remains an underutilized material option. As with other materials that are diffi cult to machine, the near-net-shape
capability of investment casting is an eff ective counterbalance to help mitigate the cost for machining aluminum MMC. With eff ective casting designs, this capabil-
ity, combined with refi nements in the alloy and in secondary machining, makes investment cast aluminum al- loy/silicon carbide particulates a viable and aff ordable option for engineers to incorporate lightweight MMC shapes into their products.
Cast Metal Matrix Composites Although increasing development
activities have led to system solutions using metal composite materials, the use of especially innovative systems, particularly in the area of light metals, has not been realized. T e reason for this is insuffi cient process stability and reliability. Combined with production and processing problems, it results in inadequate economic effi ciency. Ap- plication areas are cost orientated and conservative. Often, the industry is not willing to pay additional costs for the
Nov/Dec 2014 | METAL CASTING DESIGN & PURCHASING | 31
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