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Foundry practice for the first castable aluminum/ceramic composite material.

Foundry Practice for the First Castable Aluminum/Ceramic Composite Material

The ceramic particle-reinforced aluminum ingot offers foundries the first economical means of realizing the engineering benefits of aluminum composite materials in cast components.

Aluminum/ceramic composite materials have been manufactured for over 20 years and have found their way into a variety of products in the automotive, aerospace and defense industries.

The attraction of these engineered materials is their exceptional tensile properties--especially their specific stiffness (stiffness-to-weight ratio), which is much higher than those of commonly used metals such as aluminum, cast iron and steel (see Fig. 1).

Thus, composite materials offer the potential for significant weight savings in many applications. Of the various products that have been fabricated from these high-performance materials, however, none were made by casting processes until very recently.

The reason is that most such metal matrix composite (MMC) materials could not be melted without ruining the microstructure on which their mechanical properties depended. For aluminum-based MMCs, these properties typically include dramatically enhanced stiffness, strength, wear resistance and thermal stability compared with the unreinforced matrix alloy.

To be castable in an economic as well as a practical sense, an aluminum composite must satisfy three basic criteria: * the ingot must be remeltable without impairment of its properties; * the melt must be amenable to standard aluminum foundry practices; * the casting must be substantially defect-free, with mechanical properties that are both consistent and superior. These criteria now have been met.

New Composite Material

An ingot-metallurgical method for producing a castable MMC material was invented in 1986 by Dural Aluminum Composites Corp (a wholly owned subsidiary of Alcan Aluminum Corp) in San Diego, CA. The material, produced by a patented process, is called Duralcan[TM] aluminum composite.

The product consists, in foundry ingot form, of foundry alloys to which 10, 15 or 20 vol% of particulate silicon carbide (SiC) had been added. Wrought alloy formulations of the material, with alumina (A [l.sub.2] [O.sub.3]) as the reinforcing agent, also are being made in the form of direct chill-cast extrusion billet.

The introduction of this new material represents a major development in the field of MMC technology for two important reasons: the engineering benefits of aluminum composite materials now can be realized in the form of castings; and the new aluminum composites are extremely inexpensive compared with MMC materials manufactured by traditional methods.

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

The composite is produced as standard 30 lb notched foundry ingot, which can be remelted and cast to near net shape in almost exactly the same fashion as unreinforced aluminum, as detailed below.

The casting methods successfully demonstrated to date have been sand, permanent mold, low pressure permanent mold, high pressure diecasting and investment casting, both shell and plaster. The results have shown convincingly that products in a wide variety of sizes and shapes can be cast without difficulty, as shown in Fig. 2.

Mechanical Properties

The mechanical properties of the composite result from the "blending" of some of the stiffness (modulus), strength and wear resistance of silicon carbide with the formability and toughness of aluminum. The material also retains the lightness and corrosion resistance of aluminum. The room temperature tensile properties shown in Table 1 clearly reveal the improvements in strength and stiffness relative to those of 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 solidify rapidly exhibit very uniform distributions, as shown in Fig. 3.

In some investment castings, however, when the solidification is very slow, the distribution is less than optimal. This is caused by the "pushing" of SiC particles by the leading edges of growing aluminum dendrites. Even then, however, the composite's yield strength (at 20% SiC) is about 50% higher than that of the matrix alloy. In both sand and permanent mold castings, the enhancement is about 55%.

It is also clear from Table 1 that the enhanced strength and stiffness are achieved at the expense of tensile ductility. One might expect that such low ductility would mean very low fracture toughness, but fortunately, this is not true. The plane-strain fracture toughness values of A356-15% SiC and 20% SiC, for example, are 16.6 ksi and 14.6 ksi . in. 1/2, respectively. These values indicate that the materials are resistant to unstable crack growth and thus are suitable for many structural applications.

Considering the material's abrasive content, its strikingly enhanced abrasion resistance--about 2.5 times that of unreinforced aluminum--is not surprising. This is clearly advantageous in many applications, especially those involving reciprocating parts. A consequence of the high abrasive content, however, is that the material must be machined with carbide or diamond tools; ordinary tool steel is inadequate. By the same token, cut-off and finishing of castings are best accomplished with heavy-duty tools.

With increasing temperature, the strength of the composite material decreases, as expected. Relative to the strength of the unreinforced alloy, however, the composite's yield strength increases dramatically, from the 66% enhancement at room temperature to well over a 200% enhancement at 600F. Thus, as a result of adding 20 vol% SiC to the aluminum alloy, the useful temperature range for a cast product can be extended by up to 200 [degrees] F.

The coefficent of thermal expansion (CTE) of the composite decreases linearly with increasing SiC content; values as low as 8.2 x [10.sup.-6]/[degrees] F have been measured in A356-20% SiC castings. The CTE of the composite, therefore, can be closely matched with those of many other metals and alloys. This is a great advantage in applications requiring the close fit of dissimilar metals that are subject to thermal cycling, such as pistons and valve components.


Some recent examples of castings made from this new composite material are shown in Fig. 2. The piston, made from F332-20% SiC, provides a good example of some of the advantages of using a composite material: its low CTE, high wear resistance and greatly improved high-temperature strength are particularly useful in this demanding application.

High stiffness, as well as wear resistance, is particularly useful in the bicycle sprocket.

Development of MMCs

Until 1986, virtually all aluminum-based MMC materials and products were produced by energy- or labor-intensive methods, such as powder metallurgy, thermal spray, diffusion bonding and high-pressure squeeze casting.

None of the composites could be remelted and shape cast and each proved to be prohibitively expensive for most applications, even in the aerospace/defense sector. Despite many technological advances and improved economies of scale, it remains unlikely that the products of these manufacturing methods can be sold at a price close to that of aluminum.

A solution to both the casting and cost problems was found in the patented process used to produce the new composite foundry ingot. Powdered ceramic is mixed into molten aluminum with total wetting of the particles by the aluminum--a feat that is much more difficult than it sounds. The melt is cast into foundry ingot and extrusion billet, which are sold to foundries and extruders for conversion to product forms.

The most attractive feature of this ingot-metallurgical process is its low cost, especially at industrial production levels. At present, the material is in pilot-scale production (four million lb/yr capacity), at a price in the $3-4/lb range. Prior to 1988, the price was over $10/lb. Another significant drop in price is expected when a 25 million lb/yr production plant, presently under construction in Quebec by Alcan Smelters and Chemicals, Ltd, goes on line in early 1990.

Casting Guidelines

The composite foundry ingot can be remelted and shape cast easily using standard aluminum foundry practices and equipment. Only minor variations from the norm are entailed and none require much effort or expense to implement.

The most significant requirement is stirring the melt to prevent settling of the SiC particles, which are denser than aluminum. Stirring is accomplished with a motor-driven graphite impeller, except in most induction furnaces, where the convection currents make mechanical stirring unnecessary. A stirring rate sufficient to create visible turbulence (but not a vortex) at the surface of the melt is about right.

Salt fluxing must be avoided because it would remove the SiC particles along with everything else. Good foundry practice is required, therefore, to keep the composite ingots and the melt itself as clean as possible.

An inert cover gas, such as dry argon, helps to prevent atmospheric contamination of the melt and ceramic filters can be used to trap any large inclusions.

Good temperature control of the melt is needed in order to suppress the following unwanted chemical reaction between the molten aluminum and the SiC particles:

4Al + 3SiC [right arrow] [Al.sub.4][C.sub.3] + 3Si.

The silicon that forms is relatively harmless, but the aluminum carbide precipitates as crystals that can substantially degrade the quality of the castings. This is prevented easily however, by keeping the melt temperature below 1450F at all times. Below this value, the reaction proceeds too slowly to be a problem.

Sound metal-handling practices are vital. Despite its good casting fluidity, the composite melt does not feed quite as readily as does ordinary aluminum. Hence, a conservative gating design, with well-gated and well-risered molds, is necessary to prevent shrinkage porosity.

The tendency for the SiC particles to settle can cause a SiC-poor layer to form at the top of slow-cooling regions of the casting. Appropriately sized and located risers can be used to allow this layer to form in the risers rather than in the casting.

Finally, the gates and risers from the composite castings can be recycled if sound foundry practices were observed to avoid hydrogen pickup during the original melting and casting operations. Any oxides and inclusions picked up by the gates and risers can be filtered out using special feeding and gating techniques.

Current R&D

Dural Aluminum Composites maintains close contact with many foundries nationwide and worldwide to pursue experimental casting programs for its composite materials, as well as with many manufacturers who are engaged in product feasibility and development programs.

In addition, a variety of studies are under way at Alcan International, Ltd's Arvida and Kingston Research and Development Centres in Canada to expand the database of mechanical and physical properties for the materials and to refine the procedures for casting and forming them.

The current effort is directed primarily at the recovery and recycling of returns. Special techniques for these purposes are expected to be introduced in foundries by the end of this year.

PHOTO : Fig. 2. Shown are a variety of Al-SiC composite castings produced from the composite

PHOTO : ingot: a sand cast automotive disk-brake rotor and upper control arm; a permanent mold

PHOTO : piston; a high pressure diecast bicycle sprocket; an investment cast aircraft hydraulic

PHOTO : manifold; and three investment cast engine cylinder inserts.

PHOTO : Fig. 3. Photomicrograph (125X) of a permanent mold test specimen of A356-20 vol% SiC shows

PHOTO : the highly uniform distribution of ceramic particles in the aluminum matrix. [Figure 1 Omitted] [Table 1 Omitted]

Donald E. Hammond, Foundry Program manager for Dural Aluminum Composites Corp, has 13 years' experience in the design, inspection and production of sand, permanent mold and investment castings. Prior to joining Dural, he was foundry superintendent and quality control manager for two large nonferrous aerospace foundries, as well as a casting design specialist for General Dynamics. He holds a B.S.I.E. from California State Polytechnic Institute-Pomona.
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Article Details
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Author:Hammond, Donald E.
Publication:Modern Casting
Date:Aug 1, 1989
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