synthesis and processing of cast metal-matrix composites and … · 2018-12-11 · cast composites,...

Synthesis and Processing of CastMetal-Matrix Composites andTheir ApplicationsPradeep K. Rohatgi, University of Wisconsin-MilwaukeeNikhil Gupta, Polytechnic Institute of New York UniversityAtef Daoud, Central Metallurgical Research and Development Institute (CMRDI)

METAL-MATRIX COMPOSITES (MMCs)are an important class of engineered materialsthat are increasingly replacing a number of con-ventional materials in the automotive, aerospace,and sports industries, driven by the demand forlightweight and higher-strength components(Ref 1–6). MMCs are engineered combinationsof two or more materials (one of which is a metalor alloy) in which tailored properties, not achiev-able in monolithic metals or alloys, are achievedby systematic combinations of different constitu-ents. MMCs, in general, consist of continuous ordiscontinuous fibers, whiskers, or particles dis-persed in a metallic alloy matrix. MMCs can besynthesized by vapor phase, liquid phase, or solidphase processes. In this article, the discussionemphasizes the liquid-phase processing wheresolid reinforcements are incorporated in themolten metal or alloy melt, which are thenallowed to solidify to form a composite.Cast MMCs are not new to the industry.

Although traditionally not called a composite,the aluminum-silicon eutectic alloy (Fig. 1a) iscomposed of silicon needles embedded in analuminum matrix. Such a microstructure maybe called an in situ composite. Another exampleof an in situ composite commonly produced byfoundries is ductile cast iron (Fig. 1b), in whichgraphite nodules are dispersed in a ferrite matrix.A limitation of composites such as the alumi-num-silicon eutectic alloy and ductile cast ironis that the volume percentages of the two phasesare restricted to narrow ranges predicted by theirphase diagrams. Additionally, the morphologyand spatial arrangement of reinforcements can-not be varied as freely as in synthetically pro-duced composites, which are synthesized byphysically mixing a reinforcement phase in ametallic melt. Therefore, in situ composites areexcluded from the discussion in this article inorder to focus on synthetic composites.

In synthetic composites, one can change,almost at will, the chemistry, shape, volumepercentage, and distribution of the second phase(reinforcement). The restrictions, if any, arerheology and flow of metal between the reinfor-cements. Given the large number of metals andalloys used in synthesizing composite materials,it is not possible to cover all types of compo-sites in this short discussion. Therefore, majorissues pertaining to processing of cast MMCsare discussed by means of examples related toaluminum-matrix composites.

Classification of MMCs

MMCs are classified into three broad cate-gories depending on the aspect ratio of the rein-forcing phase. These categories are defined as:

� Continuous fiber-reinforced composites� Discontinuous or short fiber-reinforced

composites� Particle-reinforced composites

The structures of the three types of MMCsare schematically shown in Fig. 2. A unidirec-tionally aligned continuous fiber-reinforcedcomposite is illustrated in Fig. 2(a). The fibermaterial can be carbon or a ceramic such as sil-icon carbide (SiC), and the matrix can be ametal such as titanium, copper, or aluminum.Extensive literature is available on mechanicaland thermal properties, such as modulus andthermal conductivity, of fiber-reinforcedMMCs. Generally, mechanical properties,including strength and stiffness, are higher inthe longitudinal direction than in the transversedirection to the fibers. Figure 2(b) illustrates ashort fiber-reinforced composite. The short

Fig. 1 Phase-diagram-restricted metal-matrix composites. (a) Aluminum-silicon alloy. (b) Ductile cast iron

ASM Handbook, Volume 15: Casting ASM Handbook Committee, p 1149-1164 DOI: 10.1361/asmhba0005339

Copyright © 2008 ASM International® All rights reserved. www.asminternational.org

fibers can be either aligned or randomly ori-ented in the matrix material. Figure 2(c) demo-nstrates a particle-reinforced composite in whichirregularly shaped particles of a second phaseare dispersed in a metallic matrix. The particlesor whiskers incorporated in these compositescan be made of graphite or ceramics, such asSiC, alumina silica, and the matrix can be ametal, such as aluminum, copper, titanium, ormagnesium.Fiber-reinforced composites are used in

applications where higher specific strength andspecific modulus are required. Particulate com-posites normally provide significant improve-ment in wear properties of composites butonly a marginal improvement in mechanicalproperties compared to the matrix alloy. Unlikefiber-reinforced MMCs, the particle-reinforcedMMCs are generally isotropic and more amena-ble to secondary processing practices, such asrolling, extrusion, and forging after casting.Figure 3 schematically illustrates the magnitude

and range of stiffening and strengthening ofcomposites in relation to the matrix as a func-tion of reinforcement aspect ratio (ratio oflength to diameter) and volume fraction ( f ).Particle-reinforced MMCs differ from disper-sion-hardened materials, in which the particlesize varies between 0.001 and 1 mm (0.039and 39 min.) and the volume fraction of parti-cles is below 0.10. Dispersion-hardened alloysare excluded from this discussion on syntheticcomposites.

Solidification Processing of MMCsand Possible Effects ofReinforcement on Solidification

The initial discovery of the synthesis of castMMCs was made in 1965 at the Merica Labora-tory of the International Nickel Company(Fig. 4) (Ref 7). The initial work confirmed that

nickel-coated graphite particles could be eitherintroduced to molten aluminum through aninert gas stream (Fig. 4) or stir mixed in themelt using an impeller rotating in the melt toform cast aluminum-graphite composites. Testsshowed that cast aluminum-graphite compositescould be run against aluminum alloys withoutseizing or galling, even under conditions ofboundary lubrication. The initial work alsoincluded incorporation of SiC and alumina inthe matrix of cast aluminum alloys.The initial work on small-sized engines at the

Associated Engineering Company demon-strated that aluminum-graphite liners were ableto run against aluminum pistons without seizingin the Alpha Romeo and the Ferrari automo-biles used in Formula One races. Figure 5shows a centrifugally cast aluminum-graphitecomposite cylinder block of a motorcycleengine, where the graphite particles wereconcentrated near the inner periphery of thecylinder due to the centrifugal force duringcasting. The aluminum-graphite cylinder linershown in Fig. 5 was fitted to demonstrate thatone could run aluminum cylinders againstaluminum pistons without seizing, as a resultof solid lubrication provided by the graphiteparticles incorporated in the castings.Since this initial work, considerable progress

has been made in the field of cast MMCs. Table 1shows the wide variety of combinations of metaland alloy matrices and the reinforcements thathave been used to synthesize composite materi-als. This table is by no means comprehensivebut illustrates the MMC development activities.In solidification processing of composites,

the reinforcement phase is incorporated in the

Fig. 2 Classification of metal-matrix composites. (a) Aligned continuous fiber-reinforced composite. (b) Shortfiber-reinforced composite. (c) Particulate composite

Fig. 3 Magnitude of stiffening and strengthening of composites relative to the matrix as a function of reinforcementaspect ratio and volume fraction, f

Fig. 4 Injection of nickel-coated graphite particles inmolten aluminum alloys in an initial

experiment on casting aluminum-graphite particlecomposites at Merica Laboratory, International NickelCompany, 1965

1150 / Nonferrous Alloy Castings

liquid metal, which is then solidified in a mold.The possible effects of reinforcements on solid-ification microstructure, the property motiva-tion for using MMCs, and present applicationsare discussed next. The solidification proces-sing of composites can be divided into twomain classes: stir casting and melt infiltration.

Stir Casting

In stir casting, molten metal is stirred with thehelp of either amechanical stirrer (Ref 8) or high-intensity ultrasonic treatment (Ref 9–11) whilethe particles are added to the melt. This actiondisperses the reinforcing phase to create a com-positemelt, containing a suspension of reinforce-ments. Figure 6 shows a schematic view of thestir-mixing process using an impeller submergedin the melt (Ref 1). Processing of MMCs by stirmixing and casting requires special precautions,including temperature control and design ofpouring and gating systems. The stir-mixed sus-pension of reinforcement inmelts can be cast intodesired shapes using a variety of conventionalcasting processes.The thermodynamics and kinetics of transfer

of reinforcements from the gas phase to the liq-uid phase through the oxide film and finallyfrom the liquid to the solid phase is known,

facilitating the development of processes to syn-thesize different cast MMCs (Ref 12). In severalinstances, coatings of wettable metals are depos-ited on reinforcements to facilitate their transferinto melts. For example, nickel or copper coat-ings are used on graphite particles that areincorporated in aluminum melts. In other cases,wetting agents, including magnesium and lith-ium, are added to the melts to promote wettingbetween the melts and reinforcements.Stir mixing and casting are now used for

large-scale production of particulate MMCs,and companies such as ALCAN market ingotsof select composites including Al-SiC, whichcan be remelted and cast in a variety of shapes(Ref 8). Various metals, such as aluminum,magnesium, nickel, and copper, have been usedas the matrix, and a wide variety of materials,such as graphite, SiC, SiO2, Al2O3, Si3N4, andZrSiO4, have been used as reinforcements.The study of cast composites has significantlycontributed to the understanding of the solidifi-cation of conventional monolithic castings,especially to the issues related to the effects ofinclusions on the fluidity, viscosity, nucleation,growth, particle settling, and particle pushing.Sand and Permanent Mold Casting. Metal-

lic melts containing suspended ceramic parti-cles can be cast in sand as well as permanentmolds. The slow solidification rates obtained

in insulating sand molds permit buoyancy-driven segregation of particles. Depending onthe intrinsic hardness and density of thedispersed particles, high volume fractions ofreinforcements can be concentrated at the topor bottom of a component cast by this method,to serve as selectively reinforced surfaces.Tailor-made lubricating or abrasion-resistantcontact surfaces for various tribological appli-cations have been made by this technique. Useof smaller-sized particles, thin sections of sandcast composites, and the codispersion of twotypes of ceramic particles, as in the case ofhybrid composites, can reduce the segregationof reinforcements.Centrifugal casting of composite melts con-

taining particle dispersions results in the forma-tion of two distinct zones in the solidifiedmaterial: a particle-rich zone and a particle-freezone. If the particles are less dense than the melt(for example, graphite, mica, or fly ash hollowspheres in aluminum), then the particle-rich zoneforms at the inner circumference. The outer zoneis particle rich if the particles are denser than themelt (for example, zircon or SiC in aluminum).Centrifugal casting has been used to producecylindrical bearings of aluminum- and copper-base composites. In these composites, the solidlubricants such as graphite are segregated at theinner periphery where they are needed for

Fig. 5 (a) Centrifugally cast aluminum-graphite com-posite. (b) Cast iron cylinder block of a motor-

cycle engine fitted with an aluminum-silicon graphiteliner

Table 1 Selected matrix-reinforcement combinations used to make cast metal-matrixcomposites

Matrix Dispersoid Size, mm Amount(a)

Aluminum base Graphite flake 20–60 0.9–0.815Graphite granular 15–500 1–8Carbon microballoons 40, thickness 1–2 . . .Shell char 125 15Al2O3 particles 3–200 3–30Al2O3 discontinuous 3–6 mm long, 15–25 mm diam 0–23 vol%SiC particles 9–12 3–60SiC whiskers 5–10 10, 0–0.5 vol%Mica 40–180 3–10SiO2 5–53 5Zircon 40 0–30Glass particles 100–150 8Glass beads (spherical) 100 30MgO (spherical) 40 10Sand 75–120 36 vol%TiC particles 46 15Boron nitride particle 46 8Si3N4 particle 40 10Chilled iron 75–120 36 vol%ZrO2 5–80 4TiO2 5–80 4Lead . . . 10

Copper base Graphite . . . . . .Al2O3 11 74 vol%ZrO2 5 2.12 vol%

Steel TiO2 8 . . .CeO2 10 . . .Illite clay 753 3Graphite microballoons . . . . . .

Tin-base Babbit metal Graphite particle . . . . . .Al2O3 . . . . . .

Tin base Zinc . . . . . .Magnesium base Graphite fibers . . . 40–60 vol%Zinc base Nickel-coated graphite . . . . . .

Lead . . . 7

(a) In wt% unless otherwise indicated

Synthesis and Processing of Cast Metal-Matrix Composites and Their Applications / 1151

lubrication. This technique is being used to pro-duce selectively reinforced brake rotors, wherethe hard SiC reinforcement particles segregateon the outer surface of the component, improvingthe wear resistance.Compocasting. Particles and discontinuous

fibers of silicon carbide, titanium carbide,

alumina, silicon nitride, graphite, mica, glass,slag, magnesium oxide, and boron carbide havebeen incorporated into vigorously agitated, par-tially solidified aluminum alloy slurries by thecompocasting technique. The discontinuousceramic phase is mechanically entrappedbetween the proeutectic phase present in the

alloy slurry, which is held between its liquidusand solidus temperatures. Under mechanical agi-tation, such an alloy slurry exhibits thixotropy inthat the viscosity decreases with increasing shearrate. This effect appears to be time-dependentand reversible. Increasing the particle residencetime in the slurry promotes wetting and bond for-mation due to chemical reactions at the interface.This semifusion process reduces deformationresistance and allows near-net shape fabricationby extrusion or forging.

Infiltration Processes

In the infiltration technique, liquid metal isinfiltrated through the narrow crevices betweenthe fibers or particulate reinforcements, whichare arranged in a preform to fix them in space(Ref 13, 14). Unlike the stir-mixing processwhere the reinforcements are free to float or set-tle in the melt, the reinforcements do not havefreedom to move in the infiltration processes.As the liquid metal enters between the fibersor particles during infiltration, it cools and thensolidifies, producing a composite. In general,the infiltration technique is divided into threedistinct operations: the preform preparation(the reinforcement elements are assembledtogether into a porous body), the infiltrationprocess (the liquid metal infiltrates the pre-form), and the solidification of liquid metal.In this method, a transient layer of solidifiedmetal can form as soon as the liquid metal comesin contact with the cold fiber. Melt infiltrationcan be achieved with the help of mechanicalpressure (Ref 15), inert gas pressure, or vacuum(Ref 16, 17). Techniques of pressureless infiltra-tion have also been developed (Ref 18, 19), alongwith electromagnetic and centrifugal infiltration.The fundamental phenomena involved in

infiltration processes are surface thermodynam-ics, surface chemistry, fluid dynamics, and heatand solute transport. The processing variablesgoverning the evolution of microstructures are:

� Fiber preheat temperature� Metal superheat temperature� Interfiber spacing� Infiltration pressure� Infiltration speed

If the melt or fiber temperature is too low,poorly infiltrated or porous castings will be pro-duced. If the temperatures are too high, exces-sive fiber/metal reaction will degrade castingproperties. Finally, if the plunger speed is toofast, the preform can be deformed on infiltra-tion. A threshold pressure is required to initiatemelt flow through a fibrous preform or powderbed. This pressure begins in the thermodynamicsurface energy barrier because of nonwetting.The threshold pressure increases with increas-ing infiltration rates because wetting is afunction of time.Pressure Infiltration Casting. Various pres-

sure infiltration techniques have been used tomanufacture cast composites. In pressure

Fig. 6 (a) Schematic view of stir-casting process. Courtesy of N. Chawla. Microstructures of typical particulate castmetal-matrix composites made by stir casting: (b) Al-Si/20 vol% Grp, (c) Al-Si/20 vol% spherical Al

2O

3p,

made by Comalco Australia, and (d) Al-SiCp, made by Duralcan Corporation


infiltration casting, the mold and the preformsare kept at a temperature close to the meltingpoint of the metal; this gives the metal time toflow and fill the mold without choking (Fig.7). Once the metal has filled major voids inthe mold and preform, the pressure can beincreased rapidly. After this, a uniform pressurecan be maintained through the mold, and theremaining voids can be infiltrated isostatically.This makes it possible to use relatively thin-

walled molds and to infiltrate very fragile pre-forms with little damage. Even though the moldand preform start out at a high temperature, useof thin mold walls allows the mold to be cooledrapidly to minimize matrix-reinforcement inter-facial reaction.Squeeze Casting. In the pressure infiltration

process, the liquid metal is initially underhydrodynamic pressure, which changes tohydrostatic pressure at the conclusion of pre-form infiltration but prior to solidification. Onthe other hand, during squeeze casting a pre-mixed suspension of chopped fibers, whiskers,or particles in the matrix melt is solidifiedunder a large hydrostatic pressure with virtuallyno movement of the liquid (Fig. 8) (Ref 1).Recently, squeeze casting was developed toinvolve unidirectional pressure infiltration offiber preforms or particle beds in order toproduce void-free, near-net shape castings ofcomposites. Aluminosilicate fiber-reinforcedaluminum alloy pistons for use in heavy dieselengines have been produced using squeeze cast-ing. A considerable amount of work has beendone on the squeeze casting of ceramic particle

and fiber-reinforced MMCs for industrial appli-cations. The fibers or particles exert a consider-able influence on grain size, coarseningkinetics, and microsegregation in the matrixalloy. Squeeze casting promotes a fine,equiaxed grain structure because of large under-coolings and rapid heat extraction.Vacuum infiltration is effected by creating

a negative pressure differential between thepreform and the surroundings, which drivesthe liquid through preform interstices againstthe forces of surface tension, gravity, and vis-cous drag. Vacuum infiltration is usually donein conjunction with chemical methods of wetta-bility enhancement, for example, fiber surfacemodification or addition of wetting-enhancingelements to the matrix alloy. Vacuum infiltra-tion has been used by AVCO Specialty Materi-als (Textron) to infiltrate silicon-coated SiCmonofilaments with aluminum, and by DuPontto infiltrate alumina fiber preforms with analuminum-lithium alloy.Electromagnetic Infiltration. The use of

electromagnetic forces to infiltrate liquid metalinto nonwetting preforms can dispense withthe need for application of inert gas pressureand mechanically driven rams (Ref 20). Alu-mina fiber/aluminum composites have beenfabricated using the electromagnetic infiltrationtechnique. In this technique, the metal is sub-jected to a high-frequency electromagnetic fieldthat interacts with the eddy currents producedwithin the metal. The interaction results in infil-trating the metal into the preform when the pre-form is suitably oriented with respect to thedirection of the force. In a typical experimentaldesign, a fiber preform is immersed in liquidmetal contained in a ceramic crucible. A batteryof capacitors is used to produce intense burstsof high-frequency axial magnetic field. A fluxconcentrator intensifies the magnetic fieldaround the crucible. The entire assembly is con-figured such that body forces produced withinthe metal are oriented radially inward to enablemelt ingress into the preform. Porosity, preformdeformation, and fiber breakage are absent inthe castings at the completion of infiltrationand solidification. Aluminum-matrix compo-sites reinforced with up to 25 vol% aluminafiber (Saffil, Saffil Ltd.) have been successfullyproduced using this technique.Centrifugal Infiltration. Infiltration can be

achieved using centrifugal force to fabricateMMCs. High rotational speeds are able togenerate enough centrifugal force to overcomethe capillary forces for melt penetration andviscous forces for metal flow in the preform.Aluminum-matrix composites containing flyash and carbon microballoons were success-fully fabricated by using centrifugal infiltration(Ref 21). The microballoons were packed inan alumina tube, which acts as a crucible forthe molten metal poured onto the powder bed.The infiltration and dispersion of powders werecarried out at rotational speeds of up to 4000rpm, and the rotation was continued until thesolidification was completed. During rotation,

Fig. 7 Formation of composite during nonisothermalinfiltration of a fiber preform by liquid metal

Fig. 8 Schematic representation of squeeze casting process. Courtesy of N. Chawla


the centrifugal force pressurizes the moltenmetal into the interstices of the particle bed.As the rotation is continued through the solidi-fication process, the particles are “frozen-in”at their final position in the casting.Pressureless Spontaneous Infiltration.

Spontaneous infiltration of preforms or perme-able bodies of reinforcements by metals canbe effected if the compositions of the melt andpreform, temperatures, and gas atmosphere arecontrolled such that good wetting conditionsare achieved. A cost-effective pressureless infil-tration process to fabricate void-free, net-shapeMMCs having high structural integrity has beendeveloped by Lanxide Corporation, USA. Thetechnique permits fabrication of compositecomponents containing a wide range of reinfor-cements. Near-net shape SiC-reinforced alumi-num-matrix composite components forelectronic applications such as heat sinks havebeen synthesized by using pressureless infiltra-tion of preforms. In the directed-metal oxida-tion process (a form of the Lanxide process),an ingot of aluminum alloy containing magne-sium is placed on top of a permeable body ofreinforcement or preform contained within arefractory vessel. The entire assembly is heatedto a suitable temperature in an atmosphere of afree-flowing, nitrogen-bearing gas mixture.Spontaneous infiltration of the permeablereinforcement is assisted by the presence ofmagnesium in the alloy and an oxygen-freeatmosphere. In a modified pressureless metalinfiltration process (called PRIMEX, M CubedTechnologies, Inc.), magnesium in the matrixalloy volatilizes and reacts with nitrogen inthe atmosphere to form a thin Mg3N2 coatingon the reinforcement surface that makes thereinforcement system wettable. During infiltra-tion, Mg3N2 reacts with aluminum to form alu-minum nitride (AlN) and magnesium, which isreleased in the alloy. The AlN could be formedthrough direct reaction of nitrogen in theatmosphere with liquid aluminum. The A1Nphase is present mainly as small precipitatesin the composite, although thin films of thiscompound are also found as coating on the rein-forcement surface. The spontaneous infiltrationstep can be followed by a mixing step prior tocasting.Advanced Pressure Infiltration Casting.

MMCs containing hard reinforcements such asSiC are difficult to machine. Therefore, devel-opment of near-net shape components ofMMCs is desirable. Generally, pressure infiltra-tion technology is directed to producing near-net shape components. Special preforms canbe prepared for these MMCs. These preformshave fine interstices through which the liquidmetal is infiltrated. The preforms areencapsulated in a graphite mold and placed ina furnace. Liquid metal is poured and pressureis applied, causing the melt to pass throughthe mold and infiltrate the preform. Figure 9shows alumina preforms of piston connectingrods developed by MMCC, Inc. and Fig. 10schematically shows the advanced pressure

infiltration casting (APIC, MMCC, Inc.) pro-cess that can use such preforms (Ref 5). Themain feature of this process is that the meltingof the matrix metal and the preheating of thepreform take place outside of the autoclave.This technique is now available to producenear-net shape pressure-infiltrated parts ofMMCs.

Effects of Reinforcement Present in theLiquid Alloy on Solidification

The solidification microstructure of thematrix alloy is affected by the presence of rein-forcement phase. The reinforcement can:

� Act as a solute and diffusion barrier withinthe melt

� Catalyze heterogeneous nucleation in themelt

� Influence the conductivity of the liquid

� Influence the viscosity and fluidity of themelt and impart thixotropic properties

� Restrict fluid convection due to restriction ofliquid in narrow interstices between fibers orparticles

� Settle or float due to density differences� Influence morphological instabilities, includ-

ing planer-to-dendritic solidification� Be pushed or entrapped ahead of the solidi-

fying interface� Influence dendrite arm coarsening� Influence grain size� Reduce the amount of latent heat to be

extracted out of the casting

The aforementioned effects have been dis-cussed in several studies. Particles have beenshown to settle or float in the melt and are fre-quently pushed in the last freezing interdendri-tic liquid by the growing solid-liquid interface(Ref 22–24). The effect of reinforcements onsolidification processes, more specifically, on

Fig. 9 Alumina preforms printed at MMCC, Inc.

Fig. 10 Schematic of the advanced pressure infiltration casting (APIC, MMCC, Inc.) process


dendrite formation and microsegregation, isdiscussed in some detail.Figure 11 shows the effect of alumina fiber

spacing on themicrostructure ofAl-4.5%Cualloy(Ref 25). The primary phase morphology and dis-tribution of second phases depend on the relativemagnitudes of dendrite arm spacing (DAS) andthe interfiber spacing. Figure 11 shows that whenthe DAS is less than the interfiber spacing, a nor-mal solidification microstructure is observed,and the fiber does not alter the microstructure.When the DAS is equal to the interfiber spacing,the second phase, that is, Al-CuAl2 eutectic,segregates onto the fiber surface. Finally, whenthe DAS is more than the interfiber spacing, thedendritic morphology is ill-defined, and there isless precipitation of secondary phase. In this case,the fibers act as barriers to solute precipitation,and, as a result, the concentration of solute inaluminum will be more than the equilibriumvalue. This is important because it is possible tocreate microstructures with no dendrites andreduce microsegregation, resulting in decreasinghomogenization time during heat treatment.Figure 12 shows the microstructure of SiC

platelets incorporated in the matrix of an

aluminum-copper alloy by pressure infiltration.It shows frequent enrichment of Al-CuAl2eutectic at the SiC platelet-matrix interface asa result of nucleation of the a-phase away fromthe platelets (Ref 26). Figure 12 also shows sig-nificant effects of platelet spacing onmicrosegregration.It is clear from Fig. 11 and 12 that reinforce-

ments alter the solidification microstructure ofalloys to a great extent, due to the restrictionof spaces between which the liquid solidifies.This can be exploited to create improved micro-structures in composites.

Property Motivation forUsing MMCs

In MMCs, the second phases (fibers, whis-kers, particles) are incorporated in metals oralloys to achieve properties that are not obtain-able in conventional monolithic materials.Desired improvements in mechanical, tribologi-cal, electrical, and thermal properties can beachieved by intelligently selecting the rein-forcement materials and their size, shape, and

volume fraction. Table 1 shows the selectedreinforcements and metallic matrices com-monly used for synthesis of MMCs.Figure 13 shows a plot of specific modulus

versus specific strength for a number of materi-als (Ref 27). Ideally, one would like a materialto be near the upper right-hand corner of thegraph, where one can obtain a combination ofhigh specific strength and high specific modulusto minimize the weight of a component. It isclear that MMCs provide a better combinationof specific strength and modulus compared tomonolithic alloys of aluminum, magnesium,copper, nickel, and iron. Only a few materialsare better than reinforced-aluminum compo-sites. These include graphite-epoxy composites(along the fiber direction), pure ceramics, anddiamond, although they are more expensiveand some of them are brittle. Figure 14 showsthat the specific cost of organic composites ismuch higher than MMCs for similar specificstiffness (Ref 27).In many applications, the resistance to distor-

tion from thermal or mechanical loads is impor-tant, especially in precision devices. The higherthe stiffness and thermal conductivity of thecomposite and the lower the coefficient of ther-mal expansion (CTE), the higher will be itsresistance to mechanical and thermal distortion.

Fig. 11 (a) Graph of dendrite arm spacing (DAS) versus solidification time. (b), (c), to (d) Microstructural formation inthe interfiber regions of an Al-4.5Cu-alumina fiber composite. The solidification times will vary with material.

Fig. 12 Solidification microstructure of discontinuouslyreinforced Al-SiC alloy composites showing

the influence of spacing between SiC platelets on themicrosegregation pattern in aluminum-copper alloys


Figure 15 shows the materials selection chartfor resistance to mechanical and thermal distor-tions (Ref 27). It is observed that the closer amaterial is to the top right-hand corner of thechart, the higher will be its resistance to thermaland mechanical distortion. This figure showsthat MMCs have one of the highest resistancesto mechanical and thermal distortion, with theexceptions of beryllium and diamond.MMCs are also becoming very popular for

thermal management applications (Ref 28). Forsuch applications, the CTE, thermal conductiv-ity, and density are the important properties.Figure 16 shows a plot of CTE versus specificthermal conductivity. It is desired that for a given

CTE value, the thermal conductivity should be ashigh as possible. This figure shows that for theallowable range of thermal expansion for thermalmanagement in electronic packaging, MMCshave one of the highest specific thermal conduc-tivities, making them a desirable choice. In fact,only beryllium-containing materials and dia-mond are better than MMCs. Therefore, anincreasing role ofMMCs in thermalmanagementapplications is expected.The hardness of MMCs is significantly

higher than that of the unreinforced-matrixalloys. Hardness increases with increasing vol-ume fraction of reinforcement and is affectedby processing variables. Also, the addition of

ceramic phase in metallic matrices improvesthe friction and wear behavior. Figure 17 showsthe wear resistance of A356 alloy and A356-SiC composite against 4140 steel. The wearloss of the composite is approximately 10 timesless than that of the matrix alloy. The wearresistance of aluminum-matrix composites isbetter than that of cast irons, which are muchheavier. Further improvement in the wear resis-tance of alloys can be achieved by developinghybrid composites reinforced with hard phasesand soft-phase lubrication (Ref 29). For exam-ple, the transition from mild to severe wearoccurred in the range of 225 to 230 �C (435to 445 �F) for A356 alloys (Ref 30, 31). Withthe addition of 20 vol% SiC to the A356 alloy,the transition temperature increased to 400 to450 �C (750 to 840 �F). A hybrid A356-matrixcomposite containing 20 vol% SiC and 10 vol%graphite remained in a mild wear regime evenat the test temperature of 460 �C (860 �F).

Components of MMCs Currentlyin Use

MMCs, in view of their excellent combina-tion of properties such as high levels of strengthand stiffness, improved thermal stability, lowCTE, and improved wear and seizure resis-tance, have already found several applications.It is estimated that in the year 1999, the

worldwide composite use was approximately2.5 million kg (2750 ton) and increased toapproximately 3.5 million kg (3850 ton) in theyear 2004 (Ref 32, 33). The annual growth rateof MMCs has been approximately 6% in recentyears (Ref 34). One of the reasons for limiteduse of MMCs is their cost. The higher cost ofMMCs compared to the corresponding basealloy is due to the higher cost of fiber, more rig-orous processing methods, and the difficulty inmachining. However, in recent times, decreas-ing cost of fibers, modification of conventionalmetal-processing methods for processing com-posites, and the development of net or near-net shape components are contributing to thedecrease in the cost of MMC components.

Fig. 13 Specific modulus and specific strength of various structural materials

Fig. 14 Specific cost of various structural materials inprimary product forms

Fig. 15 Materials selection chart for resistance to mechanical and thermal distortions


There has been a dramatic decrease in the costof particle-reinforced MMCs primarily due tothe low cost of stir-mixing and casting pro-cesses and the use of inexpensive particles.

MMCs in Automotive Industries

A number of automotive components, includ-ing pistons, cylinder liners, brake rotors, andconnecting rods, have been made of alumi-num-matrix composites. Table 2 shows somesignificant early developments of compositeautomotive components (Ref 35).Properties of interest to the automotive engi-

neer include increased specific stiffness, wearresistance, and improved high-cycle fatigueresistance. While weight-savings is also

important in automotive applications, the needto achieve performance improvements withmuch lower cost premiums than tolerated byaerospace applications drives attention towardlow-cost materials and processes. The rawmaterial cost of cast iron or steel is less than$0.82/kg ($0.37/lb), and discontinuously rein-forced cast aluminum ingot currently costsapproximately $4.0 to 5.0/kg ($1.8 to 2.3/lb)for large-scale production, so the raw materialcost of MMCs is higher than that of the mate-rial typically replaced. However, the MMCcomponent would typically weigh 60% lessthan steel, significantly improving the costcomparison. Examples of MMCs in successfulautomotive applications, in which the combina-tion of properties and cost satisfied particularneeds, are discussed as follows.

The first commercially made automotiveMMC component was a diesel engine pistonproduced by Toyota, Japan, in the early 1980s(Ref 6). The piston was produced by squeezecasting technology, in which liquid aluminumwas infiltrated in alumina-silica short fibers.The ceramic preform was selectively placed inthe ring portion of the piston. Later on, thistechnology was refined by blending nickel pow-der into an alumina fiber preform. In thismethod, nickel powder reacted with aluminumand formed NiAl3, which improved the adhe-sive wear resistance of the ring (Ref 6). Selec-tively reinforced aluminum composite dieselengine pistons have now become very popularthroughout the world. In fact, this has triggereda flurry of activities in making other enginecomponents from cast composites. Further,Toyota developed another MMC piston inwhich the preform was made of alumina-boriawhisker and was selectively reinforced in thelip of the combustion bowl of the piston.In 1999, Mazda began producing pistons offiber/particle-reinforced MMCs.Another major use of Al-SiCp composite is

for brake rotor applications. Testing has shownthat composite brake rotors are as effective ascast iron rotors in braking applications, whilehaving a higher capacity for heat dissipationand 50 to 60% lower weight. Figure 18 showsan Al-SiCp composite brake rotor used in theChrysler Prowler. Toyota has also used A359/20 vol% SiCp composite brake rotors in theirelectric vehicle (RAV4-EV), as shown in Fig. 19(Ref 36). The Lotus Elise was also released with

Fig. 16 Materials selection chart for thermal management applications

Fig. 17 Volume loss of aluminum alloy, aluminumcomposite, and cast iron during sliding wear

Table 2 Use of metal-matrix composites (MMCs) in automotive applications

Year Company Composite and component

1980 Toyota First commercially produced MMC diesel piston by squeeze infiltration in ceramicpreform

1990 Honda Four-cylinder 2.2 L MMC engine block selectively reinforced with mixture of Saffil,alumina, and graphite fibers

1992 Toyota Selectively reinforced low Coefficient of thermal expansion fiber engine pulley1996 Toyota Alumina-boria whisker-reinforced piston1997 Toyota Al/SiCp front brake rotors for an electric vehicle (RAV4-EV)1997 Toyota Heat-spreader plates of an electronic cooling device (Prius hybrid vehicle)1997 Porsche Porsche Boxter selectively reinforced cylinder bores1997 GM Al-SiCp MMC engine cradle1997 GM Extruded aluminum MMC propshaft on the Corvette1997 GM Al-SiCp driveshaft for extended S/T trucks1998 Honda Produced selectively reinforced bore V6 engine for Acura NSX1998 GM Used several MMC components (rear brake drum, electronic cooling package) for

GM electric vehicle (GM-EVI)1998 Chrysler Used Al-SiCp brake rotors for its Prowler model1999 Mazda Produced fiber/particle MMC piston1999 Lotus Released Al-SiCp MMC brake rotors in Elise1999 Volkswagen Low-fuel 3 L version of the Lupo model with rear-wheel Al-SiCp brake drums2000 Honda Bore-reinforced in-line four-cylinder engine for its S2000 roadster2000 Toyota In-line four-cylinder engine for high-performance Celica with MMC bores

Source: Ref 35

Fig. 18 An Al-SiCp composite brake rotor developedfor the Chrysler Prowler


Al-SiCp composite brake rotors. The Volkswagenlow-fuel version of the Lupo model also had Al-SiCp composite brake rotors in the rear wheels.

Aluminum-SiC composites were found to bevery effective as heat-spreader plates of anelectronic cooling device for the world’s first

hybrid vehicle, the Prius (Fig. 20) (Ref 36).Toyota has also developed a cylinder blockwith MMC bores for a high-performance sparkignition engine. The ceramic preform was madefrom alumina-silica short fibers filled withmullite particles. The MMC bore was made bya laminar flow die casting process. This cylin-der block, with MMC cylinder bore, was intro-duced in the 2000 model Celica sports car.Toyota developed an MMC crankshaft pulley

that has a low CTE and does not expand awayfrom the shaft during thermal cycling (Fig. 21).In this application, a ceramic preform was madeof SIALON (Si-Al-O-N) and was infiltrated withaluminum.Figure 22 shows prototypes of an automobile

brake drum and a refrax apex insert madeof Al-SiCp composites produced at the labora-tories of Council of Scientific and IndustrialResearch in Bhopal, India. There is a large mar-ket for cast composites in Asia, where theenergy costs are very high, and replacingenergy-intensive materials such as aluminum

Fig. 19 An A359/20 vol% SiCp composite brake rotor for a Toyota electric vehicle

Fig. 20 Aluminum-SiC composites as heat-spreader plates of an electronic cooling device for the world’s first hybridvehicle, the Prius

Fig. 21 Metal-matrix composite (MMC) crankshaft pulley made by infiltration of SIALON preform with aluminum

Fig. 22 Aluminum-SiC composite components. (a)Automobile brake drum. (b) Apex insert for

cyclone


with particulate reinforcements can result inconsiderable energy-savings (Ref 4).

MMCs for Space Applications

The extreme environment in space presentsboth a challenge and an opportunity for mate-rial scientists. In the near-Earth orbit, typical

spacecrafts encounter naturally occurring phe-nomena such as vacuum, thermal radiation,atomic oxygen, ionizing radiation, and plasma,along with factors such as micrometeoroidsand human-made debris. Therefore, aerospaceapplications were the original driving force forthe development of MMCs. This developmentwas due to the quest for dimensionally stablestructures and weight reduction for improvedperformance and payload capability. The

primary motivation for the insertion of MMCsinto aerospace applications is the excellent bal-ance of specific strength and stiffness offeredby MMCs relative to competing structuralmaterials.The SiC particle-reinforced MMCs are used

in the Lockheed F-16 fighter aircraft. Forinstance, the use of MMCs reduces the maxi-mum deflection of the leading-edge tip of theventral fin in an F-l6 aircraft by over 50%,reducing the aerodynamic loads and increasingthe service life from 400 to 8000 h. Aluminumalloy skins used in the honeycomb structure ofthe fin were replaced by 6092 Al-17.5 vol%SiCp composite sheet material. The service lifeof the skins was significantly increased becauseof the increased specific stiffness of the alumi-num-matrix composite material (Ref 37).Although not produced by casting, one of the

major space-based applications of continuousfiber-reinforced MMCs is structural tubes forspace shuttles. Figure 23 shows space shuttleOrbiter’s midfuselage main frame MMC tubes(Ref 38). These tubes were made of 50 vol% Bfiber-reinforced aluminum-6061 alloy compo-sites, produced by the diffusion-bonding tech-nique. There are 243 MMC tubes of 25 to 92mm (1 to 3.6 in.) diameter and 0.6 to 2.4 m(2 to 8 ft) length. The MMC tubes savedapproximately 145 kg (320 lb) over the initialdesign of aluminum tubes. Another majordevelopment is the application of MMCs in anantenna wave-guide mast on the Hubble SpaceTelescope (Fig. 24) (Ref 38). The mast is madeby infiltration technology using a preform con-taining 40% P-100 carbon fibers. Aluminumalloy 6061 was used as the matrix material.The mast is rectangular and has dimensions of43 by 86 by 2000 mm (1.7 by 3.4 by 79 in.)(Ref 38). Cast Al-SiCp and Al-SiCw compositeshave also found applications as multi-inlet trussnodes and electronic packaging (Fig. 25)(Ref 38). General Electric Company designedan advanced-composite optical system gimbal,using 6061–40 vol% SiCp composite in selectedareas, requiring low expansion and high wearresistance.

MMCs for Electronic PackagingApplications

MMCs are also finding applications in space-craft thermal management systems. Cast MMCsare very suitable for thermal managementapplications due to their high conductivity andlow CTE. Figure 26 shows a typical radio fre-quency packaging for microwave transmittersused in commercial low-Earth orbit communi-cation satellites (Ref 27). This Al-SiC compos-ite part is cast to near-net shape. Aluminum-SiCis not only lighter than the existing materials,but it is much more affordable. The PRIMEXpressureless melt infiltration process is used toproduce high-volume-fraction SiC-reinforcedaluminum composites for such applications.

Fig. 23 Boron-aluminum composite tubular strutsused as the frame and ribs in space shuttle

Orbiter. Struts are not cast but represent a category ofmetal-matrix composite applications.

Fig. 24 The P-100/6061 aluminum high-gain antennawave guides/boom for the Hubble Space

Telescope (HST). (a) Before integration in the HST. (Theruler is 12 cm, or 5 in., long.) (b) On the HST as it isdeployed in low-Earth orbit from the space shuttle Orbiter

Fig. 25 Discontinuously reinforced aluminum metal-matrix composites. (a) Multi-inlet SiCp-Al

truss node. (b) Grp-Al composite components forelectronic packaging applications

Fig. 26 Aluminum-SiC microwave radio frequencypackaging for communication satellites


MMCs consisting of carbon fibers havinghigh conductivity are also being employed inpackaging and thermal management applica-tions. There are relatively few applications todate of MMCs reinforced with continuousfibers, because they are relatively expensive.However, recent developments in fabricationmethods have resulted in significant cost reduc-tions for discontinuous-fiber MMCs, and thesematerials are now entering production lines.

Development of Select Cast MMCs

Aluminum-Graphite Composite

Cast aluminum-graphite particulate compo-sites have been used in several applications, suchas pistons, engine cylinder liners, and connectingrods, as shown in Fig. 27. These are the forerun-ners of aluminum-graphite engines beingdeveloped for antiseizing applications (Ref 12).

Lead-Free Copper Alloy/GraphiteComposites

Since lead is banned in a number of copperalloys for bearing and plumbing applications,lead-free copper-graphite composites have beendeveloped as a substitute. Graphite particleshave been shown to impart machinability andlubrication characteristics similar to lead atmuch lower costs and without the associatedtoxicity. Graphite is also much cheaper andabundantly available compared to bismuth andselenium, which are being proposed as alterna-tives to lead. Figure 28 shows a collection ofplumbing fixtures and bearings cast in copper-graphite composites at the University of Wis-consin-Milwaukee. By centrifugally castingcopper alloy/graphite composite, the graphiteparticles can be concentrated on the innerperiphery, where they selectively reinforce andprovide solid lubrication for bearing applica-tions. Figure 29 shows the microstructure fromthe cylinder graphite-rich zone near the innerperiphery, which shows dendrites of copperand graphite particles present in the interdendri-tic regions (Ref 39). A comparison of wearlosses of copper-graphite and lead-copperagainst 1040 steel reveals that the copper-graphite composite shows much less wearthan the lead-copper alloy, demonstrating itssuitability for bearing applications (Ref 39).

Hybrid Composites

Figure 30(a) shows hybrid composites con-taining both small SiC particles, which are hard,and large graphite particles, which are soft, in thematrix of a cast aluminum alloy. The productionprocess is simplified by the simultaneous pres-ence of two types of particles (the lighter graph-ite and heavier SiC). Together, they result in abuoyancy-neutral mixture where SiC is unableto settle down (due to the graphite, which is ligh-ter and is trying to float up), and the graphite par-ticles are unable to float up (due to the hinderingeffect of the SiC, which is trying to settle down).Tests at International Nickel Company and other

Fig. 27 Components made of aluminum-graphite composites. (a) Cylinder. (b) Cylinder liner. (c) Piston. (d)Connecting rods

Fig. 28 Montage of lead-free copper-graphite composite castings

Fig. 29 (a) Schematic view of centrifugally castcopper-graphite alloy. (b) Microstructure of

the graphite-rich zone


organizations have shown that the wear resis-tance of these hybrid Al-SiC-graphite compo-sites is greater than that of cast iron, and theyare much easier to machine compared to Al-SiCcomposites. Figure 30(b–d) show cylinder liners,a brake rotor, and a disc brake, all made from alu-minum-hybrid composites that are being testedas a replacement material for much heavier castiron components (Ref 40). Several other hybridcomposites, including Al-aluminum-graphite,have been developed, and their applications arebeing explored.

Fly-Ash-Filled Syntactic Foams

In recent years, techniques have been devel-oped to introduce and disperse fly ash into mol-ten metal, resulting in castings with reducedweight, cost, and CTE and higher abrasionresistance than the matrix alloy. Such compo-sites containing hollow particles in their struc-ture are called syntactic foams. Fly ash is areadily available waste by-product of coal com-bustion, and its use as a filler can result in costreduction of MMCs because it replaces themore expensive matrix alloy in the composite.Fly ash is already used as an additive to cement,but most of it is still disposed of in landfills.Syntactic foams can be made by pressure

infiltration (Ref 41). Figure 31(a) shows ceno-spheres (hollow particles of fly ash), which arepacked loosely in a tube and infiltrated by alu-minum alloy to create aluminum/fly ash syntac-tic foam. Figure 31(b) shows the microstructure

of cenospheric fly ash dispersed in a cast alumi-num matrix. The density of this cast aluminum-base foam is 1400 kg/m3 (87 lb/ft3) (approxi-mately half the density of the matrix alloy),which demonstrates the potential of reducingthe weight of aluminum by the incorporationof cenospheres. This opens up the possibilityof also producing syntactic foams with othertypes of hollow particles, not just fly ash, infoundries. These foam materials are likely tohave very high damping capacity and energy-absorption characteristics of interest to automo-tive industries.Figure 32(a) shows fly ash particles dispersed

in amatrix of aluminum-silicon alloy produced bystir mixing and casting from a 180 kg (400 lb)melt. From such melts, it has been possible tomake a variety of prototype castings, includingthe sand cast intake manifold shown in Fig. 32(b).This intake manifold is made of aluminum-10%fly ash and is lighter and cheaper than theconventional aluminum alloy manifolds. Figure33 shows other sand castings, pressure die cast-ings, and squeeze castings made from alumi-num/fly ash melts, demonstrating the castabilityof such melts using a variety of casting processes(Ref 4). Fly ash has been filled in a variety ofother matrices, including lead and magnesium,to synthesize lightweight composites.

Cast Metallic Foams

The problem of stabilization of gas bubblesby the reinforcement phase in composite meltscan be converted into an opportunity to createnew materials (Ref 42). Figure 34 shows a gas

Fig. 30 (a) Microstructure of Al-SiC-graphite hybrid composite. (b) Cylinder liners made out of hybrid composites.(c) Hybrid composite disc brake. (d) Hybrid composite brake rotor

Fig. 31 (a) Fly ash cenospheres. (b) Fly ashcenospheres in a-aluminum matrix

Fig. 32 (a) Microstructure of A356–10 vol% fly ashcomposite. (b) Intake manifold made of such

a composite


bubble stabilized by copper-coated carbonfibers in an aluminum melt (Ref 43). The cop-per-coated carbon fibers have stabilized the gasbubble in the melt, creating porosity, whichresults in making a stable foam structure. A sim-ilar phenomenon is observed in Al-SiCp melts,which helped in synthesizing composite foams(Ref 44, 45). The structure of an Al-SiC com-posite foam is shown in Fig. 35 (Ref 45). Thesetypes of foams from melts of MMCs have poten-tial uses in energy-absorbing applications inautomotive and aerospace structures.

Research Imperatives

There are still many research challenges inthe area of cast MMCs. To date, researchershave incorporated fibers or particles mainly inconventional monolithic alloy matrices, buthave not been able to obtain the best advantageof cast MMCs. It is necessary to develop spe-cial alloys to serve as matrix materials in castMMCs. These alloys can be specially tailoredto bond to the reinforcement by having favor-able thermodynamics and kinetics for desiredinterfacial reactions. There is a need to developspecial particulate reinforcements, includingsurface-treated particles, for cast MMCs. Therehas been a significant amount of work on devel-oping special fibers as reinforcements, but com-paratively very little work has been done to

develop particles specially suited for castMMCs. For the most part, SiC that was devel-oped for grinding applications is beingincorporated in aluminum matrices to makecast MMCs.There is a need to develop techniques for

rapid infiltration of preforms, including techni-ques for rapid pressureless infiltration. Currentpressureless infiltration processes take a longtime, while costly tooling is required for pres-sure infiltration. There is further need todecrease the cost of synthesis of cast MMCsto the point where their life-cycle cost, as wellas their initial cost, is lower than the existingmaterials. Attempts should be made to developthe use of low-cost reinforcements such as flyash in cast MMCs to bring the cost of castMMCs below monolithic alloys. Applicationsof such lightweight composites in the transpor-tation industry can result in the conservation ofsignificant amounts of energy.There is also a need to enhance the ductility

and fracture toughness of cast MMCs throughthe use of special matrices, reinforcements,and processing techniques. Basic research isneeded to promote nucleation of the primaryphase on reinforcements so that the particlesor fibers will be encapsulated within the grainor dendrite and can act as grain refiners.Research is needed to promote engulfment ofparticles by growing dendrites to obtain a moreuniform distribution of particles in the micro-structure of cast MMCs. This is likely toincrease ductility and fracture toughness of castMMCs. Techniques to cast thin-sectionedMMC components need to be developed forelectronics packaging.Low-cost techniques for machining cast

MMCs, especially for drilling and tapping, alsoneed to be developed. Techniques for near-net

Fig. 33 Sand casting, pressure die casting, and squeeze casting of aluminum/fly ash composite. (a) Sump pumpcover. (b) Greenlee hydraulic tool handle cast at Eck Industries. (c) Mounting bracket die cast at Eck

Industries. (d) Motor mounts cast at Thompson Aluminum

Fig. 34 Gas bubble stabilized in aluminum alloy meltby the presence of copper-coated carbon

fibers

Fig. 35 Foam material created by introducing gas inan Al-SiC melt


shape production of components are needed.Nondestructive techniques must be developedto characterize the percentage of reinforcementsand porosity in cast MMC components forfacilitating quality control and ensuring unifor-mity in successive components. Techniquesfor degassing and grain refining compositemelts as well as technologies for recycling ofcast MMCs are needed to either recover thematrix alloy or reuse the composite itself. Tech-niques to improve the distribution and bondingof reinforcements and the matrix are required.In addition, it is necessary to ensure that similardistribution and bonding is achieved in succes-sive components during bulk manufacture.Porous and cellular materials are receiving a

great deal of attention, both in automotive andspace applications, and can be produced usingMMCs. Polymer-matrix cellular composites havebeen developed as functionally graded materials,nanocomposites, smart composites, and biomedi-cal implants. Similar microstructures can also bedeveloped in MMCs, which can have additionaladvantages related to the matrix metals.

ACKNOWLEDGMENTS

The authors would like to express gratitudeto the National Science Foundation for fundingunder the grant CMMI-0726723. The help ofBenjamin F. Schultz in preparing the manu-script is gratefully acknowledged. The authorsthank David Weiss, Daniel Miracle, SurajRawal, James Cornie, Warren Hunt, NikhileshChawla, and others for providing figures.

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