Castable composites target new applications.

Castable Composites Target New Applications

When two or more dissimilar materials are combined to produce a composite material, it typically incorporates some of the properties of each of its constituents. Concrete, for example, is a "particle-reinforced" composite of cement and rock. On the positive side, concrete is less expensive than cement and more difficult to fracture; on the negative side, its tensile and compressive strengths are usually less than those of cement. To increase the strength, rebar can be added, resulting in a "fiber-reinforced" composite.

Similarly, alloys, notably those of aluminum and magnesium are reinforced with fine particles, whiskers or fibers of ceramics to improve tensile properties. Almost invariably, the properties of these metal matrix composite materials (MMC) are significantly improved over the unreinforced alloy, and the density remains almost unchanged as long as the ceramic volume fraction is relatively low.

Until very recently, such improved performance without the penalty of added weight came only at very high cost. In addition, the composite materials could not be cast to near-net-shape, because remelting would degrade the composite microstructure (i.e. the distribution of the ceramic in the metal matrix) on which their mechanical properties depended.

These obstacles have been overcome in a ceramic-particle-reinforced aluminum composite. It is now available as foundry ingot DURALCAN [TM] suitable for remelting and shape casting. Standard aluminum foundry practices and equipment are used with some modifications, none of which require extensive effort or expense to implement. The castings are substantially defect-free, with consistent and improved mechanical properties. The strength, modulus (stiffness), wear resistance and thermal stability are improved when compared to unreinforced alloys.

This material has been manufactured since 1986 by Duralcan U.S.A., a division of Alcan Aluminum Corporation. The key to this patented process is to cause the molten aluminum to wet the ceramic particles (by vigorous mixing) so that the two substances are strongly bonded. This breakthrough in MMC technology cleared the way for the gravity casting of foundry ingot as well as the direct-chill (DC) casting of extrusion billet and rolling slab.

These new production methods are simpler and less costly than the previously developed methods for manufacturing MMCs, such as powder metallurgy, thermal spray, diffusion bonding and squeeze casting, all of which are energy or labor intensive. Despite the many technological advances and improved economies of scale during the last two decades, the products made by these other methods are still prohibitively expensive for most applications, even in the aerospace/defense sector.

The ceramic used in the new castable material is finely powdered (about 9-13 [mu]m) silicon carbide (SiC), in concentrations of 10, 15 or 20 volume percent; the aluminum matrix alloys are primarily A356, A357, and F332, which are suitable for casting. [For the wrought composites, powdered alumina ([A1.sub.2O.sub.3]) is used, in the same volume fractions, to reinforce alloys such as 6061 and 2014.]

Applications

The first commercial products made from this material were introduced in 1989, and more are expected to be produced in 1990. There are numerous potential applications for an inexpensive, castable aluminum composite. The most promising applications are in the automotive, aerospace/defense, industrial equipment, and sporting goods industries. Since most of the possible applications involve moving components, any weight saving from composite materials will result in increased fuel efficiency.

Furthermore, if a component fabricated from an aluminum alloy can be made lighter because of the composite's greater strength or stiffness, it may be possible to lighten other components of the same system, because they are now subject to lower inertial or vibrational loads. This can initiate a trend toward weight-saving and fuel-saving economies.

The savings can also be applied to heavier alloys such as titanium, cast iron, or steel which are replaced by aluminum composites because of the composite's high specific strength (strength-to-weight ratio) and specific stiffness (stiffness-to-weight ratio). Composite replacements include the following applications.

* Composite disk-brake rotors fabricated

by green sand casting are

currently in use in approximately 30

stock racing cars in the United States.

Several drivers have attributed improved

performance to the use of

these rotors. They are much lighter

than the cast iron rotors they replaced

and offer the added advantages of

high wear resistance (a natural consequence

of the composite's ceramic

content) and high thermal conductivity,

which facilitates heat dissipation.

Auto manufacturers are currently

evaluating cast composite rotors for

use in production vehicles.

* Belt pulleys for vehicle engines and

industrial machinery appear to be a

promising application for composites.

Composite pulleys are currently

prototyped in green sand at a U.S.

foundry, and it is expected that permanent-mold

casting and die casting 

will be the production methods.

The primary reasons for using composite

pulleys are the lower density

and higher wear resistance when

compared to conventional cast iron

belt pulleys. Improved performance is

expected from the composite, based on

abrasion and wear test resulting.

* Aircraft structural and auxiliary components

must be as lightweight as

possible to maximize fuel efficiency.

The advantage of composites in this

application is that they combine low

density with a low coefficient of thermal

expansion (CTE), high modulus

and high strength.

An aircraft camera gimbal was originally designed to be made from titanium, for its high modulus and low CTE. The manufacturer found that the gimbal could be investment cast from an aluminum composite at a reduced weight and cost. Six different composite configurations are currently in production at two North American foundries.

Vehicular components such as a composite gearbox can be fabricated easily and economically by high-pressure diecasting, using existing equipment. The high strength and high wear resistance of these composite castings offer significant advantages over unreinforced aluminum alloys such as A380. Extensive tests are under way at a leading European foundry. Current results demonstrate the composite's quality to be equal to or better than comparable high-pressure aluminum alloy die castings.

Production and Fabrication 

Until early in 1990, all castable composites were produced at a pilot-scale facility in San Diego. There are currently about 20 different composite formulations. Large-scale production will begin at a 25-million-lb/yr plant recently built in Quebec, near an existing aluminum smelter. This facility is expected to meet the projected demand for aluminum composites - both foundry and wrought - well into the 1990s.

The composite is currently manufactured in standard 26 lb notched foundry ingot, which can be remelted and cast to near-net shape. Currently, nearly 100 foundries worldwide have poured composite castings to the same standards as those that apply to existing aluminum castings. Casting methods that have been successfully demonstrated are:

* Sand casting (green sand, dry

sand, and nobake)

* Permanent mold casting (gravity

and low pressure)

* Investment casting (shell and plaster)

* Expendable pattern casting (EPC)

* Squeeze casting

* Diecasting (high-pressure)

The results demonstrate that composites can be cast in a wide variety of sizes and shapes.

Properties

The properties of the composite result by combining the strength, stiffness and wear resistance of silicon carbide with the formability and toughness of aluminum, while retaining the lightweight and corrosion resistance of aluminum. The room-temperature tensile properties show the improvements in strength and modulus relative to the unreinforced alloy.

The strength values depend somewhat on the uniformity of the particle distribution, which in turn depends on the solidification rates of the castings. Those that rapidly solidify exhibit uniform particle distributions.

In slow-cooling investment castings the particle distribution is reasonably uniform. Consequently, the typical room-temperature yield strength of an investment-cast F3A.205 composite is approximately 50% higher than the matrix alloy. In permanent-mold and sand castings, the improvements are about 65% and 75%, respectively. For elevated temperatures (below about 300F), the composite's yield strength is also sharply improved relative to the matrix alloy.

The ductility of the composite is lower than most unreinforced aluminum alloys. The low ductility is not a reflection of poor casting quality, because it is not accompanied by low fracture toughness. The fracture toughness is only slightly lower than the aluminum alloy: KIC = 15.0 ksi-in 1/2 for F3A.20S, vs 15.8 for unreinforced A356. The composite value is high enough to indicate that these materials are resistant to unstable crack growth, therefore, they are suitable for many structural applications.

The abrasion resistance of F3A.20S is about 2.5 times of unreinforced A356 aluminum (sand-abrasion test, ASTMG-65, Procedure B), and its wear resistance is about 3.7 times greater (block-on-ring wear test, ASTM G-77). The ceramic content is advantageous for many applications, especially those involving reciprocating parts. Since composites can be difficult to machine, carbide or polycrystalline diamond cutting tools are required for successful machining.

The composite's coefficient of thermal expansion (CTE) decreases linearly with increasing SiC content. Values as low as 8.2x 10-6/F have been measured in F3A.20S castings. The CTE of the composite can thus be closely matched with those of many other metals and alloys. Applications requiring the close fit of dissimilar metals that are subject to thermal cycling, such as pistons and valve components, will benefit from composite materials.

The electrical conductivity of the composite is slightly lower than the unreinforced aluminum alloy, and the thermal conductivity is slightly higher. The SiC is more dense than the aluminum, therefore the composite density is slightly higher - but approximately 3% maximum.

Recent Advancements

Research and development programs on castable composite materials continues. Certain programs involve quality control in the composite manufacturing process. For example, recent progress in the use of optical emission spectroscopy (OES) for melt and cast ingot analysis has enabled the matrix alloy elements to be determined by direct reading (except for Si). The matrix Si level can be calculated, however, by factoring out the amount of Si contained in the SiC particles.

Nondestructive testing (NDT) is the preferred detection technique for determining SiC content. Recent studies on aluminum composites demonstrate accurate and reproducible results with two such techniques, each of which should allow measurements of the SiC content within approximately [+ or -] 1 vol%.

Electrical resistivity, an NDT technique, is as applicable and accurate in the composite melt as in solid castings - a feature that makes it potentially useful for on-line process control. It can be used to monitor the distribution of ceramic particles in the aluminum matrix, another important element of quality control.

Ultrasonic Testing is another NDT technique that shows promise for measuring the SiC content (but only in the solid casting). Each casting shape must be calibrated at its own characteristic resonant frequency, therefore, the ultrasonic technique may prove useful for very large production runs, where it may be more cost-effective.

Another important new development is a method for removing hydrogen gas from the composite melt in the foundry. Although the foundry ingots themselves are free from hydrogen when shipped, contamination can easily occur during handling and remelting unless adequate prevention methods are enforced. This new degassing method effectively removes hydrogen without disturbing the SiC particles.

Finally, it is essential to create new aluminum foundry alloys with properties that lend themselves more readily to the development of composites for specific applications. Such programs are under way and have already resulted in a new diecasting composite designated F3D.xxS (xx denotes the percentage of SiC). The chemistry of the matrix alloy was tailored for high strength and stiffness while minimizing sticking of the composite to the die, without extremely impairing the ductility.

Other alloys are currently under development to create gravity cast composites that are more "foundry-friendly" in the sense of having less rigid requirements for temperature control and gating and risering. Additional composites are being designed for higher strength and modulus (at room temperature and elevated temperatures), especially in the as-cast and T5 conditions.

The advent of a castable aluminum/ceramic composite material represents a revolutionary development in MMC technology for two specific reasons:

* The engineering benefits of composite

materials can now be realized

in the form of castings.

* The new composites are inexpensive

when compared to MMC materials

manufactured by traditional

methods.

Products cast from the new materials are characterized with excellent mechanical properties. These properties, combined with low density and lower cost, will permit composite castings to compete effectively with many aluminum forgings, as well as with many products that are currently made from cast iron, steel, magnesium or titanium.

PHOTO : Disk-brake rotor (shiny piece in center) in a racing-car brake assembly, green sand-cast from an aluminum composite by Texas Metal Casting Co, Inc, for Race Car Products. The composite rotor weighs 7 lb less than the cast iron rotor it replaced.

PHOTO : Aircraft camera gimbal (31-in. diameter, 38 lb), investment-cast from F3B.20S composite by Cercast, Inc, for Ball Aerospace, Inc. It is one part of a six-part assembly made from the composite.

PHOTO : Photomicrograph (100X) of a polished, diecast specimen of F3D.20S (20 vol% SiC). The particles are uniformly distributed in the aluminum matrix and are strongly bonded.

PHOTO : Gearbox made from F3D.10S composite by high-pressure die casting, by Metallic A/S (Skive, Denmark). There is no visible porosity on the machined surfaces. This part possessed excellent wear resistant properties.

PHOTO : Tensile properties of composite sand castings made from F3A.xxS (xx vol% SiC in A356 aluminum) in the T61 condition. The test specimens were cast-to-size tensile bars.