MMC low cost mfg

7/29/2019 MMC low cost mfg

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The economics of the MC-21 rapidmixing process and lower-cost rawmaterials show a strong potential forbeing able to produce a castable359/SiC/20p aluminum MMC

product for close to $2.20 per kg($1 per pound).

Darrell R. Herling*Glenn J. Grant*Pacific Northwest National LaboratoryRichland, Washington

Warren Hunt, Jr.**Aluminum Consultants GroupMurrysville, Pennsylvania

Aluminum metal matrix composites(MMC) are appealing because of theirlow density and high specific stiffness.In addition, the ceramic-particle re-

inforcement significantly increases wear resistance.Nevertheless, high cost relative to conventional alu-minum alloys has prevented widespread indus-trial applications.

Two primary factors account for the high cost ofmetal matrix composites. The first factor is the rawmaterial cost of both the aluminum matrix mate-rial and the ceramic reinforcement particles. The

second factor leading to higher cost is related to thefabrication process. If these two factors could becontrolled and reduced, then a wider range of ap-plications becomes possible.

Toward this end, Metal Matrix Composites forthe 21st Century (MC-21) Inc., in Carson City,Nevada, has developed a novel process in whichthe particulate is rapidly mixed into the matrixalloy. The process significantly reduces the time re-quired for mixing, and therefore can reduce thelabor and ultimately the cost. In addition, themixing system may be placed at a foundry, and the*Member of ASM International** Fellow of ASM International

molten material may be transferred directly to the

casting floor without the need for remelting of ingot.Furthermore, the selection of a lower-cost materialfor the reinforcement, rather than the expensive F-500 grade, would also reduce expense.

This article describes current MMC mixingprocesses, then discusses the MC-21 process, pro-viding details of its operation and showing how itcan reduce costs.

Conventional processesOver the years, a number of compositing and

mixing processes have been investigated and uti-lized for the production of aluminum MMC. Theseinclude both liquid and non-liquid techniques, such

as mechanical mixing of reinforcement into moltenmetal, and powder metallurgy approaches. Powdermetallurgy involves blending metal and ceramicpowders that are compacted together and then sin-tered to create a solid, dense form. This method hasproven expensive due to the high cost of metalpowders and the associated production equipment,as well as time-consuming processing steps. How-ever, powder metallurgy methods do have thebenefit of being able to produce unique MMCmaterial chemistries, with very fine particle rein-forcement size distributions that otherwise are notpossible with most molten metal techniques.

Stir-casting techniques are currently the most

Low-cost aluminum

METAL MATRIXCOMPOSITES

Fig. 1 Conventional stir-casting setup for the compositing of aluminum MMCmaterial. This is the most common commercial method.

Vacuum chamber

Heating/meltingelements

Mechanicalmixing head

Reinforcement

particle addition

Mixing crucible

ADVANCED MATERIALS & PROCESSES/JULY 2001 37


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common commercial method. This approach in-volves mechanical mixing of the reinforcement par-ticulate into a molten metal bath. A simplified com-positing apparatus is shown in Fig. 1, and typicallyis comprised of a heated crucible containing moltenaluminum metal, with a motor that drives a paddle,or mixing impeller, that is submerged into the melt.The reinforcement is poured into the crucible abovethe melt surface and at a controlled rate, to ensure asmooth and continuous feed.

As the impeller rotates at moderate speeds, itgenerates a vortex that draws the reinforcementparticles into the melt from the surface. The im-peller is designed to create a high level of shear,which helps strip adsorbed gases from the surfaceof the particles. The high shear also engulfs the par-ticles in molten aluminum, which promotes wet-ting. Proper mixing techniques and optimized im-peller design are required to produce adequate meltcirculation and homogeneous distribution of thereinforcement.

Early stir-casting processes maintained a melttemperature between the solidus and liquidus tem-

peratures. The higher viscosity of the semi-solidmelt produced a higher degree of shearing duringthe mixing step, and this assisted in particle wet-ting. However, this technique, known as com-pocasting, has technical issues and limitations. Ma-terials are typically compocast under atmosphericpressure, with ambient air or an inert cover gasabove the melt surface.

Unfortunately, the same beneficial vortex thatdraws the reinforcement into the melt and producesadequate melt shearing, also draws in gas and oxidefilms from the surface. These contaminants aredetrimental to the quality of the MMC and limitthe rate of particle wetting and mixing. Other lim-

itations of these early compositing processes in-cluded small batch sizes and a high level ofporosity. These methods were ultimately rejectedbecause they were not commercially viable on alarge manufacturing scale.

Stir-casting technology developed little after theinvention of the compocasting process, until it wasmerged with vacuum technology. Duralcan, a di-vision of Alcan Ltd., is currently the leader in stir-cast aluminum MMC production and sales world-wide. The technology development that led to theDuralcan compositing process incorporated thebasic concept of the compocasting method into avacuum environment, in which the uptake of gasin the melt could be avoided. In addition, mixingin a vacuum environment eliminated the gasboundary layer at the melt surface and eased theconditions for particle entry into the melt from thesurface.

An improvement in mixing efficiency and com-positing temperatures above the liquidus point re-sulted in the production of a superior aluminumMMC. Alcan currently produces castable alu-minum MMC in 6800 kg (15,000 lb) batch sizes,with good quality and a range of silicon carbide(SiC) particle reinforcement levels from 10 to 30vol%. After stir-casting, the aluminum-SiC com-posite is cast into 14 kg (30 lb) ingots that are sold

to foundries for remelting and casting.Currently, the cost for Duralcan F3S.20S MMCproduct, which is a stir-cast 359/SiC/20p castablealuminum MMC material, is approximately $5.50 to$6.60 per kg, depending on the quantity of mate-rial purchased. The relatively high cost comparedwith high-silicon aluminum alloys or high-strengthlightweight steel, is a direct contributor to its lim-ited application in many markets. In cost-sensitiveindustries such as automotive, the availability of alower cost, castable aluminum MMC materialwould enable widespread application to manypowertrain components, such as brake rotors orsuspension arms. To address this need, MC-21, in

conjunction with researchers at the Pacific North-west National Laboratory, has developed an evo-lutionary rapid mixing and compositing processfor the production of low-cost aluminum MMCmaterials for casting applications.

The rapid mixing processThe MC-21 MMC rapid mixing concept, as de-

scribed by U.S. Patent No. 6,106,588, avoids many ofthe technical issues associated with the Duralcanprocess by mixing the aluminum and reinforce-ment materials at or near atmospheric conditions.Ambient air eliminates the need for the complexand expensive vacuum system, and potentially

lowers capital equipment costs. A controlled inertatmosphere, such as argon or nitrogen, can preventthe melt from reacting with the oxygen in the air,effectively reducing the build-up of an oxide layer.

The mixing head is similar to that for other stir-casting methods, in that its primary purpose is tobreak up particulate agglomerations, create a highlevel of shear forces in the melt, and effectively mixand distribute the reinforcement. However, unlikeother stir-casting mixing head designs, the specificdesign does not produce a vortex when rotated inthe melt. In addition, baffles at the top of the melthelp eliminate significant surface movement andprevent pulling oxides from the surface into the melt.

Fig. 2 Rapid mixing concept and compositing chamber setup developed by MC-21.

Reinforcementparticle addition

Mixing crucible

Hollow mixingshaft for powderintroduction

Heating/meltingelements

Mechanicalmixing head

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The novel approach of this concept is based onhow the particulate reinforcement is introduced tothe melt. The powder is metered into the meltthrough a hollow mixing shaft, and is introducedinto the melt below the surface, as shown in Fig. 2.The mixing head is designed to promote thebreakup of particle agglomerations and to effec-tively distribute the reinforcement in a verticalrolling motion. Higher shear forces are achievedby processing in a semi-solid state, where the vis-cosity of the melt is higher.

Since the impeller design does not create a vortexin the melt, the rotating speed can be significantlyincreased, which tends to further enhance the sizeof the shear region and particle wetting efficiency.In the rapid-mixing process, the higher level ofshear and the larger shear region that builds in themelt improve the mixing action of the particles. Inaddition, the position of the shear zone is at an op-timum location, where the particles are first intro-duced into the matrix material as they disseminatefrom the hollow mixing shaft. This reduces the timerequired for the particles to reach the shear region,and significantly increases the fraction of particlesthat pass through the shear zone. This leads to an

increase in the rate of wetting.The addition of a stationary impeller base un-

derneath the impeller head can further boost theshearing action, if it is placed in close proximity tothe outlet of the hollow mixing shaft and has a pro-file complementary to that of the impeller. Flutingor contouring the top of the mixing base can tailorthe specific shear characteristics. This modifies themixing action in the shear zone and helps controlthe compositing process.

Through mixing trials, MC-21 has shown thatthis rapid mixing technique significantly reducesthe time required to stir-cast aluminum MMC ma-terials by at least 80%. The reduced compositing

time ultimately influences the labor costs. Manu-facturing cost estimates for the MC-21 rapid mixingprocess indicate that the process should enable eco-nomical compositing of aluminum MMC for ap-proximately $0.33 per kg.

Additional cost reductionThe key to the MC-21 rapid mixing process is the

ability to quickly and efficiently incorporate the re-inforcement particles into the matrix material, andallow for more versatile mixing conditions. How-ever, other aspects of the process besides the me-chanical mixing can make it more efficient. For ex-ample, more efficient equipment and processdesigns can be applied to provide an overall sys-tems approach that is less labor-intensive. To fur-ther promote casting efficiency of MMC compo-nents and reduce labor time, the MC-21compositing system is designed in a modular formthat can be placed on the foundry floor.

The system consists of three primary units, amelter, a mixer, and a holding furnace. The meltingunit melts the matrix alloy ingot prior to mixing,

via either resistive heating methods or a reverbra-tory heating unit. After the matrix alloy is melted,the molten material is transferred into the mixingunit, where the particulate reinforcement is addedin a controlled manner through a proprietarypowder injection system, and is then mixed intothe matrix material. After the compositing step, theMMC material is transferred to the holding furnace.The furnace has a low-speed mixing head, pro-ducing a gentle agitation that keeps the particulatefrom settling out.

This modular approach provides several bene-fits for the production of aluminum MMC mate-rials. Because it is transferred to a holding furnace

after the mixing step, the mixer is then immediatelyfree to produce another batch. The process stepsare sequenced so that production is semi-contin-uous, which can effectively increase the overalloutput quantity during every shift. In addition, bytransferring the material to a holding furnace in aliquid state, it is subsequently poured or ladled di-rectly into the casting molds without the need forremelting. Elimination of the remelting step can in-crease productivity by eliminating the time neces-sary to melt the MMC ingot prior to casting. In ad-dition, part-manufacturing costs can be reducedsince the labor, furnace energy, and other expenseassociated with remelting and shipping of MMC

ingot are removed.Because the system is modular, each unit can be

scaled according to foundry casting needs, and maysupply MMC material only when necessary. Thishelps to reduce inventory and overhead costs. Aprototype rapid mixing unit, shown in Fig. 3, wasbuilt by MC-21, and is capable of producing 600 kg(1320 lb) of aluminum MMC per cycle. The mod-erate size and flexibility of the compositing processallows for quick changeover to different MMCproducts (different matrix alloy and reinforcementcombinations), and provides for more versatility,especially for smaller foundries that specialize incasting unique MMC materials and components.

Fig. 3 Prototype 600 kg modular mixing system built by

MC-21, with the melter (furthest back), mixer (middle posi-tion), and holding furnace (front).

The keyto the

MC-21rapid mixing

process is

the ability toquickly andefficientlyincorporatethereinforcement

particlesinto thematrixmaterial.

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Cost of silicon carbideA significant portion of the cost of MMC ma-

terials is in the raw material expenses (the alu-minum, alloying elements, and reinforcement). Cur-rent castable aluminum MMC typically utilizes anF-500 grade of high-purity green SiC. The F-500designation refers to the sorting and classificationthat establishes a well-defined particle size distri-bution. Because the F-500 SiC reinforcement issorted and sized, and a surface treatment is applied,the additional processing adds cost. To help fur-ther reduce the cost beyond that made possible bythe rapid mixing and compositing method alone,an alternative, lower-cost, black SiC material hasbeen utilized. This grade of SiC has lower puritythan the green SiC, and has a wider size distribu-tion. Since the sorting and pretreatment require-ments are relaxed, the manufacturing time is lower.Despite the lower purity of the SiC and the widersize distribution, this material is adequate for rein-forcement of MMC.

Estimated cost for black SiC supplied at mod-erate to high volumes is less than $2.20 per kg. This

is approximately half of the cost of the F-500 grade.The lower cost SiC results in a cost savings of $0.48per kg for aluminum MMC with 20 vol% SiC. Thetotal estimated MMC material cost for the produc-tion of a 359/SiC/20p product, utilizing the MC-21 rapid mixing process and lower cost SiC, is ap-proximately $2.40 per kg. This, combined withfurther cost savings from the reduction in ingot

shipping charges and labor time associated withremelting, could drive costs even lower, and rep-resents an enabling step forward in more wide-spread applications of aluminum MMC. I

For more information: Darrell Herling, Senior Develop-ment Engineer, Pacific Northwest National Laboratory,902 Battelle Blvd., MS: P8-35, Richland, WA 99352; tel:509/376-3892; fax: 509/376-6034; e-mail: darrell.her-

[email protected].

This project was funded by the Department of Energy,Office of Transportation Technologies, Office of Ad-vanced Automotive Technologies. The authors wouldlike to thank David Schuster and Mike Skibo at MC-21,Inc. for their involvement in this project and providingthe materials for evaluation. The Pacific Northwest Na-tional Laboratory is operated by Battelle Memorial In-stitute for the U.S. Department of Energy under contractDE-AC06-76RLO 1830.

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MMC low cost mfg