Discontinuously
reinforced
aluminum
MMC
extrusions
Don
Hashiguchi
1,*
,
David
Tricker
2,
Andrew
Tarrant
2,
Jeffrey
Campbell
1and
Charles
Pokross
11
MaterionBrush,Elmore,OH,USA
2MaterionAerospaceMetalComposites,Farnborough,UK
What
do
you
do
when
you
need
to
design
energy
or
weight
saving
part
for
an
aerospace,
marine,
automotive
or
robotics
system
and
the
perfect
material
is
not
available?
Desirable
properties
not
normally
found
in
an
off
the
shelf
material.
Properties
for
example
low
density,
high
strength,
high
modulus
and
low
coefficient
of
thermal
expansion
(CTE).
One
answer
is
to
select
several
materials
that
have
the
desired
properties
and
mix
them
together
to
produce
an
engineered
material
with
the
combination
of
desired
properties.
This
is
primarily
how
composites
are
designed,
including
metal
matrix
composites
(MMCs).
One
lightweight
MMC
system
specifically
aluminum-SiC
MMCs
is
described
in
this
article
which
combine
a
low
density
moderate
strength
ductile
aluminum
matrix
with
a
low
CTE,
high
strength
silicon
carbide
reinforcement.
MMC
history
Metalmatrixcompositesusingcopperoraluminumasthemetallic
matrixwereinvestigatedinthe1950sand1960sasamethodto
reduceweightwhilemaintaininghighmechanicalstrength.
Bo-roncarbide,aluminum oxideandsiliconcarbidewerecommon
ceramic reinforcement constituents. There have been waves of
developmentandlimitedproductionoverthefollowingdecades
affectedprimarilybytheeconomicenvironment.Oneconsistent
barrier wasthe relativelyhighcostto producethe materials or
partsfromtheMMCs.
Several manufacturing methods to produce Al-MMCs were
developedinattempttoaddresscostandmanufacturability.These
include stir castingand/or squeezecasting a slushy mixture of
moltenaluminumwithSiCreinforcement.Someofthetechnical
weaknessesofthisprocessincludeareactionbetweenthemeltand
reinforcement, uniformity of the dispersion or high viscosity
which tendsto limit the amountofreinforcement loaded into
themelt.Thismethodamongafewotherscouldusefibersasthe
strengtheningcomponent.AnotherexampleofaMMCsprocessis
toinfiltrate askeleton ofhard reinforcementwithliquidmetal.
Thetechnicalweaknessesofthisprocessincludetheabilitytoform
ahandleableskeleton,uniformityofinfiltrationandachemical
reactionbetweenthemeltandreinforcement.
Powdermetallurgyopensaprocessingdimensionthatoperates
inthesolidstatethereforeminimizesachemicalreactionbetween
themetalmatrix andreinforcementceramic.Asimpleand
eco-nomicalmethodisto premix metalpowder withceramic
rein-forcement to produce the Al-MMC composition prior to
consolidation. Depending on the particle size of the powders
andmixingmethodtheweaknessesofthisprocessinclude
unifor-mity of the reinforcement material and weak adhesion of the
reinforcementwiththemetalmatrix.Insomecasesfurther
ther-momechanicalmetalworkingisnecessarytoimprovethe
disper-sionofthereinforcementmaterial.
AresurgenceofprocessdevelopmentusingMechanical
Alloy-ing(MA)asahighenergymixingprocessoccurredinthe1990s
and 2000s. This process continuously mashes the components
together and breaks the mixture apart over and over again to
produce powder particles. When performedunder ideal
condi-tionseachparticlecontainsanintimateanduniformmixtureof
thereinforcementmaterialwithinthematrix.AdvantagesofMA
includenochemicalreactionbetweentheconstituents,uniform
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TURE
*Correspondingauthor:Hashiguchi,D.(Don.Hashiguchi@materion.com)
dispersionofreinforcement,abilitytoaddarelativelyhigh vol-umepercentofreinforcementandtheabilitytoreducethesizeof
thereinforcementand/ormetalgrainsize.
Reinforcementmorphologycanbedescribedorcategorized,per
ANSIH35.5‘Nomenclature system forAluminumMetalMatrix
Composite Materials’ as continuous fibers, chopped filaments,
whiskersordiscreteparticles(Fig.1).TheAl-SiCMMCdescribed
in this article utilizes discrete equiaxed SiC particles with the
aluminummatrix.SinceeachMAMMCpowderparticlecontains
auniformdispersionofSiCthepowdercanbeconsolidatedtofull densityandusedas-isbyHotIsostaticPressing(HIP).Itcanfurther
be transformed intoother product forms by globally available
conventionalthermomechanicalmethodssuchasforging,sheet
rollingorextrusion.
Industrial
growth
Aproprietaryhighenergymechanicalalloyprocesswasdeveloped
inthe1990sbytheUKMinistryofDefense.Aprivatecompany,
Aerospace Metal Composite (AMC) located in Farnborough,
Hampshire,Englandwasformedasanoutcomeofthe
develop-mentwork.TheprocesswasexpandedtoproduceseveralAl-SiC
intomillproductforms(Fig.3).
Thereare3distinguishingadvantagesofmaterialsmadebyMA:
1. Theenergyinputduringthemechanicalalloyingstepallows
production of finer SiC particles plus a higher loading in
comparisontoMMCmaterialsthatmightbemelted,stirred
thencast.P/Misthe‘solution’thatpreventsdissolutionofthe
particulatecomponentifitweresolubleinametalmelt.
2. Particlesproducedduringmechanicalalloyingaresomewhat
analogous to prealloyed atomized powder particles in that
eachpowderparticle containsauniformdistributionofSiC
withineach‘compositepowderparticle’(Fig.4);contrastedto separateanddiscretemetalandSiCparticlesmadefroma
pre-mix prior to consolidation. This produces a uniformMMC
microstructurewithnometalworkingrequiredtohomogenize
thecomponentsandisoneattributethatleadstosuccessful
nearnetshape(NNS)HIPconsolidationprocessing.
3. Arelativelyhigherparticulateloadingwithuniform
distribu-tionand goodadhesionbetweenthe particulateand
alumi-num matrix is achievable through high energy milling in
comparisontopre-mixmanufacturingtechniques.Porosityis
not observed after consolidation of the composite powder
particles.
The ability to combine finer particulate size, high ceramic
loadingandsubsequentlyshortmeanfreepathbetweenparticles
canproducehighstrengthandhighmodulusmaterials.Thefiner
MatrixComposites’TWClyneandPJWithers,CambridgeUniversityPress, 1995.
FIGURE2
MaterionCorporationinFarnborough,Hampshire,UKandElmore,Ohio,USA.
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particlesizeisalsoakeyenablertoreducethecostofdownstream
manufacturingstepssuchasextrusionandmachining.The
sub-micronSiCparticlesallowlesshighspeedtooldamagebecausethe
cuttingedgeofthemachinetoolwillcanmoreeasilypassbetween
versusthroughlargerhardparticlestherebyreducingchippingand
dullingofthetool’scuttingedgein comparisontoMMC’s that
contain larger ceramic particles (Fig. 5). Having stated this it
should be noted that a higher volume percentage loading of
ceramicwillincreasewearofthemachinetoolinsert.
Extrusion
Beforethestructureandpropertiesarediscussedadescriptionof
two typesofextrusions,in whichapreheatedbillet ofMMCis
forcedthroughasmallercrosssectiondie(Fig.6)ismade includ-ing:(1)‘BulkExtrusions’thatcanbeusedtomakerod,bar,tube
andshapesthatareextrudedinasinglemetalstream.Extrusion
diesaretypicallymadefromconventionalH-13Alloysteelorwith ceramicinserts.(2)‘PrecisionExtrusions’madethroughaporthole orbridgedieinwhichacylindricalextrusionbilletseparatesatthe
dieentry thenfusesintoahollow shapebeforeitexitsthe die,
typicallymanufacturedfromH-13Alloysteel.
Bulk
extrusions
Materion hasbeendeveloping ‘bulk’extruded productformsof
aluminumMMCsincludinguseofaconventionalhorizontalpress
inElmore,Ohiowith3000-tonpressstemcapacity.Theextrusion
pressisnormallyusedfornon-ferrouscopperbasedandother non-ferrousalloysversusaluminumorferrousalloys.AHIPconsolidated cylindricalorhollowbilletorslugistypicallyusedasinput.Someof theextrudedproductformsincludeflatbar(Fig.7),rod(Fig.8)and seamlesssimpleshapedgeometry(Fig.9).Thesurfacefinishofthe
extrusion allows use of the product form with minimal to no
machining.Oneadvantageofusingthelargenon-ferrousextrusion
FIGURE3
ProcessflowdiagramusedtoproducewroughtproductformsofAl-SiC MMCs.Al-SiCMMCsproducedbyMAdonotrequiremetalworkingimprove thedistributionoradhesionofreinforcementparticles,extrusionorforging producesengineeredshapedproductforms.
FIGURE4
SEMimageofapolishedcrosssectionofamechanicalalloyedpowder particlecontaining25%SiCina2124Aaluminummatrix(2124A/SiC/25p). ThedistributionofSiCparticlesisuniformwithinthecompositepowder particlebeforeconsolidation.
FIGURE5
Exampleofthefinesurfacefinishofanenginepistonmachinedfrom mechanicalalloyed2124A/SiC/25paluminumMMC(SupremEXW
225XE).
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press withhigh stemcapacity isthat itopensthe possibilityof makingrelativelylargecrosssections.The significanceof under-standingstemcapacityisthatthek-factororflowstressoftheAl-SiC
MMCincreasesasvolumeofparticulateloadingincreases.
Precision
extrusions
Manyaluminumalloystructuralorengineeredshapeextrusions
are madeusing porthole or bridge dies that produce bright or
precisionfinish.Asimilartechniqueusingaportholediecanbe
usedtoproducetheAl-SiCMMCshapes(Fig.10).Thedimensional precisionofbrightextrusiontechniqueworkswellwith6xxxseries
aluminum MMCsforhighthroughputincommercialextrusion
presses.Withportholedietechnologythebilletenteringintothe
diesplitsintoseparateflowstreams.Ashapedinternalmandrel
andoutersurfaceformedbydieproducestheextrudedshape.The
materialisforcedbacktogetherfusingthestreamstogetherbefore
the hollowextrusion exitsthedie.The advantageof usingthis
processisthatasimplelowercostsolidcylindricalbillet canbe usedasinput.Inadditionproductionofhightolerancenet-shaped
hollow or complex shapes can be made requiring no further
processingotherthanheattreatmentormachiningofboltholes.
Useofthemechanically milledinputAl-SiCmaterial,with
uni-form dispersion of fine particulate allows fusion to take place
FIGURE6
Schematicofextrudingarodorshapefromabilletorslugofmaterial.
FIGURE7
3.5mm120mmflatbarextrudedfrom6061B/SiC/20paluminumMMC (SupremEXW
620XF)aluminumMMC.
FIGURE8
Closeupviewof2124A/SiC/25paluminumMMC(SupremEXW225XE)rod
extrudedthrougha25-mmrounddieshowingabrightsurfacefinish.
FIGURE9
Extruded50mmOD35mmID2124A/SiC/25paluminumMMC (SupremEXW225XE)tubesittingontheextrusionpressrun-outtablewith
smokingdielubricantshortlyafterextruding.
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whichcouldbeproblematicwithMMCscontaininglarger
diame-terparticles.Extrusionsmadeusingthismethodarecurrentlyin
theprocessofbeingevaluatedforautomotiveandother
applica-tions. The use of 6061 alloy matrix for precision extrusions
achieves goodstrength and highspecificmodulus witha high
corrosionresistantmatrixalloy.
Mechanical
property–structure
relationship
SiC’shighceramichardness,highmodulusof410GPa(60Msi)
andlowCTEof1ppm/C(1.8ppm/F)arekeycharacteristicsthat
improvethealuminummatrixproperties.Aluminumalloys
gen-erallyhaveamodulusofabout69MPa(10ksi)andCTEof23ppm/
C(13ppm/F).IncreasingSiCparticulateloadinginMMC
compo-sitionsincreasesmodulusoftheMMCmaterial(Fig.11).
SiC has is a density higher than aluminum alloys therefore
higherparticulateloadingincreasesdensityofthematerial.SiC
hasa densityof about3.2g/cc(0.12lb/cu.in.) whilealuminum
alloysaregenerallyabout2.7g/cc(0.097lb/cu.in.).Fulldensityof
theAl-SiCMMCsisinthe2.86–2.90g/ccwhenSiCcontentis15–
40%byvolume.Al-SiCcompositeshaveahighspecificmodulus
(moduluspermaterialvolumeordensity)whencomparedtoother
structuralmaterials(Fig.12)includingcommonaluminum,steel, titaniumormagnesiumalloys.Thisisakeyintrinsicrelationship
for light weight stiffness driven structural designs helping to
deliverhigherperformance, lowerenergyusageorwhere lower
momentumisfactoredintothedesign.
Typical mechanical properties and CTE of extruded Al-SiC
materialsareshowninTable1 benchmarkedtoextrude6061a
commonstructuralaluminum alloy.Higherstrengthand lower
ductilityisseenintheAl-SiCMMCscomparedto6061.Lowor
controlledCTEcanbeanimportantpropertyinapplicationsthat
have temperature excursions such as in combustion engines,
FIGURE10
Twoexamplesofprecisionshapesextrudedthroughportholediesmade from6061B/SiC/20paluminumMMC(SupremEXW
620XF).Oneshapeis 62mmH50mmW1.8mmwall.Theothershapeis50mm H44mmW1.8mmwallwitha30mmprojectedfin.
FIGURE11
ElasticmodulusasafunctionofSiCparticleloadinginanaluminummatrix.
FIGURE12
SpecificmodulusofAl-MMC’swithdifferentreinforcementloadingsversus severalcommonalloys.Thespecificmodulusorstiffnesspervolumeof materialisakeyengineeringattributeforweight.Savingsstructuresand designs.SpecificmodulusisimportantforAerospace,Automotiveand Roboticsystems.
TABLE1
TypicalpropertiesofAl-SiCMMCsinHIP’dandextrudedproductform.
Productform Extruded6061 HIP’dAMC225xe ExtrudedAMC225xe ExtrudedAMC217XG ExtrudedAMC640XA
Materialdesignation 6061 2124A/SiC/25p 2124A/SiC/25p 2124A/SiC/17p 6061B/SiC/40p
AverageSiCsize(mm) – 3 3 0.3 3
Orientation L Isotropic L L L
Temper T6 T4 T4 T6 T6
0.2%YSMPa(ksi) 275(40) 470(68) 480(70) 545(79) 490(71)
UTSMpa(ksi) 310(45) 570(83) 680(99) 670(97) 620(90)
Elongation(%) 12 1.8 5 7 2.5
ModulusGpa(Msi) 70(10) 115(16.7) 115(16.7) 98(13.9) 140(20)
CTE25C–100Cppm/C(ppm/F) 23(12.7) 16.1(8.9) 16.1(8.9) 17(9.4) 13.4(7.4)
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brakesandelectronicsubstrateswhereeitherfailurefromthermal
fatiguecannotbetoleratedor wheretimetothermallystabilize
shouldbeminimized.HigherparticleloadingcontrolsbothCTE
andincreasesmodulusaspreviouslyillustrated.Finerparticlesize
increases mechanical strength. Extrusion produces higher
strength and elongation along the major axis versus isotropic
HIPproductforms.
AHIP’dversionof2124A/SiC/25pisshowninthe3rdcolumnof
Table1whichcanbecomparedtothesameMMCcompositionbut
initsextrudedforminthe4thcolumn.ExtrusionofAl-MMCsasis
commonlyfoundinotherextrudedmaterialsresultsinatextured
microstructureandhigherstrengthsandelongationinthe
longi-tudinaldirection(Fig.13).Transversepropertyevaluationis on-goingonlargercrosssectionextrusions.
FIGURE13
Microstructureofextruded2124A/SiC/25p(SupremEXW
225XE)showingtheeffectontextureresultingfrommetalworking.Transverseisontheleftand longitudinalontheright.
FIGURE14
MicrostructureofHIPconsolidated2124A/SiC/25p(SupremEXW225XE)showingthedifferencebetweenreinforcementparticlediameter.d
50of3-mmison
theleftandd50of0.7isontheright.
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Future
Developmentworkisbeingperformedtoutilizeandprocessfiner
SiCparticles.AluminumMMCscurrentlyproducedbyMaterion
AMCcontaineither3-mmor0.7mmSiCparticles(Fig.14)
desig-natedwithXE(3mm)andXF(0.7mm)suffix.Finerparticlesand
shortermeanfreepathbetweenparticlesconcomitantwith
inti-matebondingoftheparticleswiththealuminummatrixthrough
mechanical alloying results in improved mechanical strength.
Property characterizationand developmentof aluminumMMC
materialsproducedwith0.3mmSiCiscurrentlybeingperformed
(designatedwithXGsuffix).Asanexample0.2%yieldstrength
significantlyincreaseswithafiner0.3mmSiCparticlediameteras
withhighervolumeloading(Figs15and16).
Withincreasedglobalinterestofthelightweight,highspecific
modulus Al-SiC MMCs and broader product form availability,
including the HIP’d and wrought extruded forms, statistically
analyzedmaterialpropertieswillbereviewedandsubmittedfor
industrialspecificationacceptancesuchasSAE-AMS.
FIGURE15
EffectofSiCparticlediameterand%loadingonstrength.Developmentof finersub-micronSiCparticulateandhigher%particulateloadingleadsto higher0.2%yieldstrength.
FIGURE16
Yieldstrengthasafunctionof%particulateloadingandSiCreinforcement massmediandiameter.Smallerdiameterandhighervolumeleadsto higher0.2%yieldstrength.
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