Technical

High-Temperature Aluminum MMC

From a materials standpoint, aluminum (Al) is an attractive material for structural and thermal applications, primarily because of its availability, low cost, light weight, and high thermal conductivity. It is 66% lighter per unit volume than iron (Fe) and 39% lighter than Ti-6-4. When alloyed, Al has one of the highest strength-to-weight ratios of all the structural metals. The initial use of Al alloys for engineering applications coincided with the development of motorized vehicles and the fledgling automotive industry in the early 19th century and has increased steadily over the last 100 years. Al alloys are extensively used for electrical transmission lines, fracture-resistant parts for airframes and engines, missile bodies, fuel cells, and satellite components. Al MMCs possess significantly better specific properties than monolithic aluminum alloys while maintaining good thermal conductivity. For this reason they are of wide interest for structural and thermal applications in the aerospace and automotive industries.

Several papers and properties tables on metal matrix composites are available to download here.

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Continuously Reinforced Composites

One of the ways of classifying composite materials is on the basis of the shape and size of their reinforcements. Composites are typically referred to as either continuous fiber or particulate (discontinuously reinforced) composites. While discontinuous fiber composites have lower cost and are generally easier to process, applications for these materials are limited by their lower strength and stiffness. Continuously reinforced composites, on the other hand, are much superior in properties, especially in the direction of the fibers. Recent work on continuously reinforced Al composites has been directed towards developing lower-cost processing routes in order to drive these materials into defense-related and commercial products.

Hand Lay-up

MetPreg can be made into multidirectional, multiply parts by manually laying the tapes into a fixture. The laminate can then be consolidated in a vacuum furnace, uniaxial hot press, vacuum bagging or other method to produce the desired end item.

MetPreg layer applications include reinforced armor, selective reinforcement, crack mitigation, crack repair, or where high-strength, high-stiffness composites may be needed.

MetPreg combines beneficial attributes of light metals such as aluminum with those of high-strength, high-modulus reinforcement such as aluminum oxide continuous fiber.

Filament Winding

Touchstone has developed an innovative processing technology for creating MMC cylinders and pressure vessels. The technology uses an existing composites processing method known as filament winding. Filament winding is one of the oldest manufacturing processes employed in the composites industry. The process consists of pulling a roving or tow (a bundle of fibers or filaments) through the matrix material in a liquefied form (for example, a resin bath), impregnating or infiltrating the roving material with the matrix material, and “wrapping” the impregnated roving over a mandrel. Filament winding is considered to be a very robust, inexpensive means of creating large, high-fiber-volume composite structures. Finished parts have been produced with a fiber volume as high as 60%.

Combining a low-cost filament winding process with high-performance MMC materials can lead to great improvements in the ability to produce affordable MMC structures by driving down costs and improving manufacturing capabilities.

Potential applications include hydrogen containment, liquid hydrogen tanks, rocket motor cases, composite gun barrels, and insensitive munitions.

Joining

MetPreg structures can be assembled with adhesives, solders, and brazes, as well as spot welding and ultrasonic welding. Because of the high strength and stiffness of the aluminum matrix, metallic prepregs offer the potential of lighter weight joints than PMCs.

Metallic attachments can be readily incorporated into part design, using standard soldering or brazing techniques. An advantage of metallic attachments is their inherently high bond strength – often many times the strength of adhesives.