Justifying Aluminum Metal Matrix Composites in an Era of Cost Redution.

The increase in strength, modulus and other mechanical properties provided by cast MMCs has made the material a viable alternative for specific applications.

Aluminum metal matrix composites (MMCs) have been under development as a casting material for more than 30 years and have been commercially available for the last decade. In the material's brief period of existence, however, the metalcasting community has largely ignored this material as less than 10 jobbing foundries worldwide regularly cast it. Why? Much of the problem is related to the material's perceived high cost. In addition, MMCs face many of the same dilemmas all new materials face when they are presented as a substitution for an established material like iron and aluminum.

For example, in the 1930s, Owens-Corning introduced fiberglass as a replacement for wood and metals in selected applications. The company established a "Fiberglass Policy" to guide its search for applications for its new material: either our material must do a job that no other material can do effectively; or for the same price it must perform better than competitive materials; or at a lower cost it must do as good a job as alternative materials; or its unique characteristics must enable the manufacturer using it to make corollary savings not possible with other materials.

This policy is as applicable to MMCs as it was to fiberglass. If MMCs are more expensive than existing materials, do its benefits exceed the extra costs? How can these benefits be quantified for casting customers whose purchasing, manufacturing and engineering are separate profit centers that often do not share costs or cost savings? What is the impact of MMCs on associated purchased components or manufacturing operations? Also, can a true cost be calculated based on MMC fabrication and use characteristics?

This article will discuss some of the cost and design considerations for MMCs. By analyzing some of these considerations, foundries and OEMs can discover opportunities that may be available to them with this material.

MMC Overview

Aluminum MMCs have expanded the property range of aluminum alloys. Initially, MMCs were developed with the use of fibers as continuous reinforcement in the matrix of composites for the aerospace industry. The result was a material of high strength, modulus and wear resistance, good corrosion resistance and low thermal expansion. Although this material provided the high specific strength and modulus desired to achieve weight savings for aerospace, the cost and limiting fabrication methods restricted the use of MMCs to this niche market.

Work continued on the development of MMCs and resulted in the introduction of lower-cost discontinuous reinforcements with simpler fabrication methods. This material could be handled with normal aluminum foundry equipment and required minimal changes to standard melting, recycling and casting procedures, allowing non-specialized foundries access to the material. Although the discontinuous MMCs did not provide the highest strength levels of the continuous reinforcements, its enhanced properties (when compared to aluminum), ease of fabrication and lower cost expanded the market opportunities for the material.

There are many discontinuous reinforcement materials available, but alumina ([Al.sub.2][O.sub.3]) and silicon carbide (SiC) are the most widely used. The matrix materials receiving the greatest attention due to suitable availability, strength and low density are aluminum, magnesium and titanium.

Cost Benefits

The fundamental theory behind the development and use of MMCs is that they are not meant as a replacement material for all aluminum and iron applications. MMCs are to be used as a substitute material that can provide reduced weight and increased mechanical properties for some current casting applications, but also can open new markets for casting with components currently made via other manufacturing methods with materials not suited for casting.

MMCs have increased stiffness, strength and wear resistance combined with enhanced thermal conductivity and a reduced coefficient of thermal expansion when compared to unreinforced aluminum. As a result, MMC castings can be less expensive than the product forms that they replace in a number of ways.

First, commercial particulate-reinforced aluminum alloys sell for as low as $1.50/lb. The low cost and net shape characteristics of casting combine to produce product forms at substantial savings to fabricated or wrought and machined composite components. In some applications, cast particulate-reinforced composites can replace high-cost fiber, filament and whisker-reinforced materials. Second, high-production MMC casting methods such as die, squeeze and semi-solid molding can compete effectively in terms of cost with ferrous sand casting.

Cast brake rotors have been held up as the example where MMCs make a difference. In terms of weight, MMC rotor designs provide up to a 60% reduction when compared to cast iron. In addition, aluminum MMC rotors outperform their iron counterparts in terms of their mechanical properties and practical use. Ford Motor Co. has placed the value of weight reduction in the automotive industry at between $0.35-3.50/kg depending on vehicle platform. Therefore, a 7-kg-iron rotor produced in aluminum MMC at 50% the weight could result in a savings of $ 1.23-12.25.

High volume automotive components are not the only benchmark. In freight transport, the weight savings of a component translates to additional freight that can be hauled. Component manufacturers for heavy-duty trucks value that savings from $2-16/kg depending on the equipment's operational cycle. In the commercial aircraft industry, this weight savings can reach $450/kg; and in spacecraft, it can reach $40,000/kg.

Beyond weight savings, a component also must be judged by the foundry and its customer on: purchase price; post-purchase processing; technical quality, including durability; performance; marketability, including operational efficiency, operating cost, maintenance cost and esteem value; maintainability; reliability; and environmental cost. In a comparison of an aluminum MMC brake rotor vs. an iron one, the iron component will be the higher value in terms of purchase price, post-purchase processing and maintainability, but the MMC component wins out in terms of performance, marketability and maintainability.

Casting & Applications

In general, standard furnaces and tools are used in the melting and processing of aluminum MMCs. However, the material does require some special considerations including: accurate temperature control, stirring of the melt with an impeller to avoid reinforcement settling and controlled pouring.

In terms of casting processes for MMCs, both die and squeeze casting are the preferred methods. Aluminum MMC allays are suited to diecasting because of the thixotropic nature of the MMC flow that results in good casting soundness and increased die filling efficiency (with cycles reduced up to 30%). The high pressures exerted in squeeze casting also contribute to high casting soundness, but also permit easy infiltration of ceramic preforms and the production of particulate-reinforced aluminum MMCs. Other casting processes such as permanent mold and sand casting (gravity or low pressure/vacuum counter-gravity) also are used when heavy coring is required or for short runs and prototypes.

Composite material properties offer designers opportunities for high product performance. Aluminum composites using SiC or [Al.sub.2][O.sub.3] as the reinforcement have higher tensile and yield strengths and lower elongation and fracture toughness than the matrix alloy. Some section size decrease may be possible when designing for static strength, but where fracture toughness is important, a section size increase may be required. See Tables 1 and 2 for typical aluminum MMC physical and mechanical property data.

The material's elastic modulus increases with particle loading, with 20% SiC material having the same modulus as cast gray iron (14 Msi). When designing for stiffness, section sizes can be equivalent to cast iron sections or reduced relative to aluminum sections.

Under both sliding and abrasion wear, the wear rate of MMCs decreases with an increase in volume fraction. Abrasive wear rates are reduced 55-90% from the wear rate of unreinforced aluminum alloy (depending on particle loading). Twenty volume fraction silicon carbide aluminum MMCs are reported to have better sliding wear resistance than cast iron.

While it is safe to assume that aluminum MMCs outperform aluminum over a wide range of wear mechanisms, caution must be used when comparing wear rates with iron or steel. Aluminum MMC brake rotors combined with proper pad materials have lower wear rates than cast iron rotors. In some erosive applications, however, wear in aluminum MMCs will be higher since the soft matrix alloy erodes with little protection from the ceramic particle. Depending on the wear mechanisms expected, testing using actual conditions is important for the successful application of materials.

The coefficient at thermal expansion (CTE) for MMCs changes predictably with reinforcement type and content and is significantly less than the matrix alloy. This property can be taken advantage of in systems that see temperature variation reducing stresses in assembled or bonded components. The composites often can be formulated to match a desired CTE. The thermal conductivity of 20% SiC reinforced material is 105 Btu/ft x hr x F. This compares with 27.3 for grade 30 cast iron and 80 for unreinforced 359 aluminum alloy.

According to Stratecasts, Inc., Ft. Myers, Florida, aluminum MMC usage in automobiles will increase significantly over the next decade due to the ever-tightening weight restrictions imposed on automakers. Based on conservative estimates, the following growth in usage will be shown for aluminum MMC components from production last year to production in 2008: disc brake rotor usage will increase from 360,000 units to 800,000; brake caliper usage will increase from 400,000 units to 900,000; and suspension component unit usage will rise from 1 million to 3.

Although the majority of applications for MMCs have been in the aerospace and automobile markets, the material also is being used for train brake rotors. A 50% decrease in weight, less thermo-cracking, low wear, energy savings and a lack of rotor hot spots are the properties cited for their use in this application. In addition, the thermal stability of MMC castings has led to their use in laser camera bases and housings, and the material's lightweight and wear-resistant nature has made it suitable for golf club heads.

There also are ancillary benefits to using aluminum composites in many applications. The reduced weight of the composite component often allows weight reduction elsewhere such as in supporting structures. In applications involving moving or rotating equipment, higher acceleration and deceleration rates are attainable. Reduced mass translates into less energy consumed to move, lift, rotate and stop.

Making a Change

Casting aluminum metal matrix materials can be rewarding when the properties and processing needs of the material are taken into consideration. Weight reduction, improved noise, vibration and harshness reduction, and superior wear resistance are properties that are important to many cast components. By using the material properties to maximum advantage and understanding the material's unique processing challenges, a foundry can redefine itself and find a new niche in the design of lightweight, robust and economical components.

However, it is easier to convince a customer to substitute one material for another than to design a product using the strengths of the new material. Often, the substitution is a disappointment, and a clear benefit is not derived from its use. The question that needs to be answered is: how would a product be designed to take advantage of the properties of a composite? How would a brake drum look if the thermal properties of SiC or [Al.sub.2][O.sub.3] reinforced aluminum was used? What would a transmission case or cylinder head look like if it were designed for the stiffness and high temperature strength of an aluminum composite?

The strengths of cast composites as well as their difficulties in use are well documented. But, designing for the advantages and away from the disadvantages will create opportunities for use that have yet to be explored.

         Typical Physical Properties of Two Duralcan Aluminum MMCs
                                 Aluminum MMC
Property                            F3S.20S         F3N.20S
                                 (359/SiC/20p)   (360/SiC/20p)
Density (gm/cu cm)                   2.77            2.71
Electrical conductivity              26.4            24.7
(%IACS) at 22C
Thermal conductivity (cal      0.442 (T6 temper)     0.401
 /cm x s x K) at 22C
             at 260C           0.48 (T71 temper)
Specific heat (cal/gm x K)            0.2            0.193
 at 25C
             at 200C                 0.239           0.238
             at 300C                 0.259           0.259
Average coefficient of thermal
 expansion ([10.sup.-6]/K)
            at 5O-100C               17.5            16.6
            At 50-500C               21.4            20.2
Information courtesy of Alcan Engineered Cast
Products, Farmington Hills, Michican.
     Typical Tensile and Hardness Values of Two Duralcan Aluminum MMCs
Aluminum            Ultimate     Yield   Elongation Elastic Rockwell
   MMC              Tensile     Strength    (%)     modulus  (HRB)
& Temper         Strength (MPa)  (MPa)               (GPa)
359/SiC/20p--T6       359         338       0.4       98.6     77
359/SiC/20p--T71      262         214       1.9       98.6     --
360/SiC/20p--F        303         248       0.5      108.2     73
360/SiC/20p--T5       365         338       0.3      108.2     73
Information courtesy of Alcan Engineered Cast
Products, Farmington Hills, Michigan.

Eck Industries Leads the Pack in Casting MMCs

Eck Industries, Inc., a 360-employee foundry in Manitowoc, Wisconsin, was looking for an edge over its competition. The foundry had been successful in casting difficult-to-handle aluminum alloys, such as 201 and 206, and believed there could be another niche to carve with MMCs. After researching the possibility of adding this niche to its current aluminum and magnesium green and chemically bonded sand, permanent mold (gravity and low-pressure), shell mold, and diecasting capabilities, Eck decided to move forward.

"We were interested in the composite alloys for all the potential applications that could develop and we wanted to get in on the ground floor," said David Weiss, vice president of engineering.

In 1990, Eck began casting prototypes in two composite alloys: 359/SiC/20p and A360/SiC/20p. Today, the foundry uses 500-1000-lb gas-fired crucible furnaces to melt the composites and a graphite mixer to continuously stir the melt and keep the particulate in suspension. Eck uses sand and permanent mold casting for the 359 composite alloy and diecasting for the A360 composite alloy. The foundry segregates the composite casting operations from the rest of the operation to avoid cross-contamination of the melt.

Eck ships up to 200,000 lb of composite castings/year (total shipments=8 million lb/year) to customers such as Chrysler, Boeing, Ford, GM, Eaton and the U.S. military. To grow this niche, Eck constantly markets its capabilities and actively seeks applications where a conversion to aluminum composites may increase component quality.

The greatest difficulty for Eck in casting the composites is the pouring, as the technique and gating systems used for composite castings must eliminate all turbulence from the melt. Mishandling of the alloy during pouring or in the gating system will entrap air bubbles that are difficult to remove because they are stabilized by the reinforcement phase of the alloy.

Alfred T. Spada, managing editor

Current Applications of MMCs

* Brake rotors, drums, calipers and pad backplates

* Turbocharger impellers

* Differential housings

* Cylinder liners

* Valve train components

* Suspension arms

* Steering links

* Brackets

* Stabilizer bars

* Crankcase girdles/ladders

* Clutch plates

* Structural oil pans