镁合金熔炼

MaterialsCharacterization58(2007)989–

996

CharacterizationofAl–MnparticlesinAZ91Dinvestmentcastings

S.LunSin,D.Dubé⁎,R.Tremblay

DepartmentofMining,MetallurgicalandMaterialsEngineering,LavalUniversity,Quebec,Quebec,CanadaG1K7P4

Received18September2006;accepted12October2006

Abstract

ManganeseiscurrentlyaddedtoMg–Alalloysinordertoimprovethecorrosionbehaviorofcastcomponents.ApartofthismanganeseisdissolvedinthemagnesiummatrixandthebalanceisfoundasfineAl(Mn,Fe)particlesdispersedwithincastings.ForAZ91Dspecimenspreparedusingtheplastermouldinvestmentcastingprocess,theseparticleswereobservedinverylargequantityatthesurfaceofcastings.Theseparticleswerecharacterizedbyscanningelectronmicroscopyandelectronprobemicroanalysis.ItwasfoundthattheyconsistofAl8Mn5phaseandthattheirmorphologyandsizedependonlocalsolidificationconditions.Theirpresenceatthesurfaceofthecastingsisrelatedtolowsolidificationratesandreducedthermalgradientsatthemould/metalinterface.

2006ElsevierInc.Allrightsreserved.

Keywords:AZ91Dmagnesiumalloy;Solidmouldinvestmentcasting;Microstructure;Al–Mnparticles

1.Introduction

CastmagnesiumalloysaremainlybasedontheMg–Alsystem,wheremanganeseisaddedtoreducetheeffectofirononcorrosionresistance.SmalladditionsofmanganesedecreasetheconcentrationofironinthemeltthroughtheformationofAlx(Fe,Mn)yparticles,someofthemsettlingatthebottomofcrucibles,theothersbeingembeddedincastingsduringsolidification[1–4].Thecompositionofintermetallicparticlesdictatestheirelectrochemicalbehavior.Al-richparticleslikeAl4MnandAl6Mnshowarelativelylowcathodiccurrentdensity,whilethosecontaininghighMnconcentrations

⁎Correspondingauthor.Tel.:+[1**********];fax:+[1**********].

E-mailaddress:[email protected](D.Dubé).1044-5803/$-seefrontmatter2006ElsevierInc.Allrightsreserved.doi:10.1016/j.matchar.2006.10.010

suchasAl8Mn5showacontinuouslyhighcathodiccurrentdensity[1–3,5–7].ExcessMnconcentrationscanbedetrimentaltothecorrosionresistanceofMg–Alalloys.

Besidestheireffectoncorrosionresistance,Alx(Fe,Mn)yparticlescanalsoactasheterogeneousnucleationsitesleadingtograinrefinement[8–10].However,thisinfluenceisstillnotclear[11–13].Itwasobservedthatthemetastablehcpε-AlMnphasecanactasanucleant[9,10]inMg–Alalloys,buttheinfluenceofAl8Mn5particlesonthegrainsizeinMg–Alcastingsisnotelucidatedyet[12,14].

IncastMg–Alalloys,particlescontainingaluminumandmanganeseappearundervarioussizesandforms,asitcanbeseeninTable1.Thesizeofparticlesusuallyrangesfrom0.1to30μm[4,12,15–20].Variousmorphologies,moreorlessregular,wereobserved[4,12,14–25].Amongtheregularshapes,needles[19,25],crosses[24],flowermorphologies[4,20]and

990S.LunSinetal./MaterialsCharacterization58(2007)989–996

Table1

Literaturereviewaboutmanganese-containingparticlesdetectedinMg–AlbasealloysProductionmethod

Alloy

ParticletypeAl–MnAl–MnAl8Mn5Al8Mn5

Al–Mn–(Fe)

Particlesize(μm)N/A0.30.1–0.2N/A0.1–1

ParticlemorphologyAngularblockymorphology(HPDC)

Irregularshapes(HPDC)AngularblockymorphologyAngularblockymorphologyFlower-likestructure

AngularblockymorphologyFlower-likestructure

AngularblockymorphologyBar-likeshapes

AngularblockymorphologySphericalparticlesIrregularshapesinLPDCsamples

AngularblockymorphologyinHPDCsamplesNotmentionedCross-shapedfeatures

ParticlelocationNotmentioned

InsidethegrainandintheboundaryregionNotmentioned.Notmentioned.

MainlyformedonthecoarseMg17Al12phase.Afewsmallparticlesisolatedonthegrainboundaries

Inornearthegrainboundaries

Ref.[21][15][16,17][22][4]

Die-castingAM50A

AZ91DAM60AZ91DAZ91DAZ91

AM60AM50AM20AM50AM50

Al–MnAl–Mn1N/A

IntheinterdendriticregionNotmentioned.

Someinthemiddleofgrains,themajority

intheareasofsupersaturatedα-Mgsolidsolution

[18][23]

AZ31

RapidAZ91solidifiedribbonsIngotsAM50

AM60AZ91

ConeladleAM60(sampling)Semi-solid

AZ91

Al8Mn5β-Mn

Al–Mn–Fe

1N/APreferentiallyatgrainboundariesandintheinteriorofthegrainsNotmentioned

[12][24]

Al–Mn7

Blockyandneedlemorphologies

Nearlysphericalordevelopingamulti-armedflower-likestructure

Notmentioned

Notmentioned[19]

Al8(Mn,Fe)510–30Notmentioned[20]

Al8(Mn,Fe)5N/AWithintheprimaryMgphase[14]

angularblockystructures[4,16,17,19,21–23]werereported.

Alx(Fe,Mn)yparticlesareusuallyfoundinthebulkofMg–Alalloyscontainingmanganese[4,12,15,19,23].Particleswerehabituallyobservedinsidethegrainsandatgrainboundariesindie-castspecimensofAM50,AM60,AM20andAZ91alloys[4,12,15,23].

Whilestudyingmould-metalreactivityofAZ91Dspecimensinvestmentcastinplastermoulds,particlescontainingbothAlandMnwereobserved[26].How-ever,themicrostructureofthesecastingsrevealedthatAl–Mnparticleswerenothomogeneouslydistributedwithinthebulkbutthattheyweremostlylocatedatthesurfaceofthecastings.Toourknowledge,suchahigh

densityofparticlesatthesurfaceofcastcomponentshasnotyetbeenreportedforothercastingprocesses.

Inordertofindanexplanationtothisphenomenon,adetailedcharacterizationoftheAl–Mnparticleswasconductedandfocusedon(1)thedistributionand

Table2

ChemicalcompositionofAZ91Dmagnesiumalloyingots(wt.%)MgBal.

Al9.0

Zn0.83

Mn0.24

Si0.005

Cub0.001

Nib0.001

Fe0.003

Be0.0008

Fig.1.Backscatteredelectronimageofthecross-sectionofa1.6mmthickspecimeninvestmentcastinplastermould.

S.LunSinetal./MaterialsCharacterization58(2007)989–996991

Fig.2.CharacteristicX-rayenergyspectrumofAl–Mn

particles.

morphologyofAl–MnparticlesinAZ91Dinvestmentcastingsand(2)thecompositionoftheseparticles.2.Experimentalprocedure

FlatspecimensofAZ91Dalloywerepreparedusingthevacuum-assistedsolidmouldinvestmentcastingprocess,whichwasdescribedwithmoredetailsina

previouspaper[27].Waxpatternswereplacedintoaperforatedflaskandaplaster-basedslurrywaspouredaroundthepattern.Themouldwasessentiallycom-posedofcalciumsulphatehemihydrate,andofsilica,asquartzandcristobalite.Aftersetting,thewaxpatternswereeliminatedandthemouldswerebakedatamaximumtemperatureofapproximately730°Candthendecreasedto350°Cpriortocasting.IngotsofAZ91Dalloy,withcompositiongiveninTable2,weremeltedunderaprotectivegascover(CO2+0.5%SF6)inastainlesssteelcrucibleandthemoltenalloywasthenpouredintothemould.Thecastingtemperaturewassetto750°C.Attheendofthesolidification,castspecimenswereremovedfromplastermouldusingpressurizedwaterandairdried.Thethicknessofcastspecimensrangesfrom0.4to3.2mm.

Thesurfaceandthepolishedcross-sectionsofcastspecimenswerebothexamined.Thelatterwerepreparedbyembeddingselectedspecimensinacrylicandpolishingaccordingtousualprocedures[28].Mechanicalwetgrindinginvolvedsuccessively

finer

Fig.3.SEMmicrographsofthesurfaceof(a)0.8mm,(b)1.0mm,(c)1.3mmand(d)1.6mmthickAZ91Dspecimensinvestmentcastinplastermould.

992S.LunSinetal./MaterialsCharacterization58(2007)989–996

Fig.4.MorphologyofAl–Mnparticlesobservedatthesurfaceof:(a)0.4mm,(b)0.6mm,(c)1.3mmthickspecimensinvestmentcastinplastermould.

gritsofsiliconcarbidepapersdownto1200grit.Finepolishingwascarriedoutwithdiamondsuspension(particlesizedownto0.1μm).AllspecimenswerecoatedwithathinlayerofAu–Pdtoenhanceresolutionofobservationwithscanningelectronmicroscope(SEM).Backscatteredelectronmodewasusedtopro-videimageswithabettercontrast.Quantitativecom-

positionalanalysisoftheparticleswasconductedusinganelectronprobeX-raymicroanalyzer(EPMA)equippedwithawavelengthdispersivespectrometer.X-raymap-pingswereobtainedfromthesurfaceandinthecross-sectionsofcastspecimenstorevealthedistributionofAl,Mn,andFeelements.3.Results

Fig.1showsatypicalSEMmicrographinthecross-sectionofa1.6mmthickflatspecimenobtainedbytheinvestmentcastingprocess.Themicrostructureofthespecimensconsistsofα-Mgsolidsolution,alongwithMg17Al12phaseandparticlescontainingmanganeseandaluminum,butnoiron,asdeterminedbyenergy-dispersivespectroscopy(Fig.2).ThebackscatteredelectronsproduceasharpcontrastbetweentheAl–Mnparticles(white)andthemagnesium-richmatrix.Itwasobservedthatmostparticlesarelocatedatthesurfaceofspecimens,whileonlysomewerefoundinthebulk.Figs.3and4showtheinfluenceofthethicknessofcastspecimensonthesizeandmorphologyofmanganese-richparticlesatthesurfaceofcastspeci-mens.Thesizeofparticlesincreaseswithspecimenthickness(Fig.3),fromapproximately1μmto10μmin0.4mmand1.3mmthickspecimens,respectively.ThesevaluesareintherangeusuallyreportedintheliteratureforAl–Mnparticles(from0.1to30μm).Thesemicrographsalsoillustratethatthedensityofparticles(numberofparticle/unitsurface)atthesurfaceofthecastspecimensdecreasesasthesectionthicknessincreases.Forthinspecimens,theparticlesexhibitcrossshapes(Fig.4a)and,asthesectionthicknessofthespecimenincreases,theygrowasfaceteddendrites(Fig.4bandc).Particlesarebigger,lessnumerousandtheirdendriticstructureismoredevelopedwhensectionthicknessincreases,andhencewhenthecoolingratedecreases(secondarydendritearmspacingsvaried

from

Table3

ChemicalcompositionofAl–Mnparticles(standarddeviationinparentheses)MgAlMnFeReference(at.%)(at.%)(at.%)(at.%)–

59.9(0.4)39.6(0.5)–Thiswork––∼62.560

(1.3)2537.1–40(1.1)01.3–15(0.4)[31][14]7.79a57.01

35.14

–[16]––

60.758.4(3.1)(7.0)LPDC39.3(3.1)LPDC–41.6(7.0)–

[23][23]

HPDC

HPDC

a

Mglikelycomesfromthematrix.

S.LunSinetal./MaterialsCharacterization58(2007)989–996993

Fig.5.ElementalEPMAmappingsofa3.2mmthickspecimenshowingthedistributionof:(a)Mn,(b)Al,and(c)Feinthebulk,and(d)Mn,(e)Al,and(f)Featthesurface.

994S.LunSinetal./MaterialsCharacterization58(2007)989–996

17to23μmwhenthesectionthicknessincreasedfrom0.8to1.6mm).

EPMAwasfurtherusedtoquantitativelydeterminethecompositionoftheAl–Mnparticles.Thetypicalcompositionofthebiggestparticles(10μm)isgiveninTable3and,forcomparisonpurposes,thecompositionofmanganese-containingparticlesreportedintheliteratureisalsogiven.ReferringtotheAl–Mnbinaryphasediagram,theMnconcentrationmeasuredinanalyzedparticles(between31and50at.%),corre-spondstothatofAl8Mn5[29].Theseparticles,whichcontainahighMncontent,couldbedetrimentaltocorrosionresistance.ItwashoweverdifficulttodeterminethecompositionofthesmallerAl–Mnparticles(1–8μm),wheremagnesiumfromthematrixwasalsodetected,distortingtheresults.Nevertheless,itispresumedthattheyarealsoAl8Mn5particles.ManystudiesmentionedthepresenceofironinAl–Mnparticles[14,30–32].Howeverinthepresentwork,noironwasdetectedintheparticles,inagreementwithotherworks[10,22,33].Corbyetal.[20]alsoobservedthepresenceofMncontainingparticles,whoseFecontentistoolowtobereliablydetected.Theysug-gestedthattheseparticlescouldbenearlyiron-free.Fig.5showstypicalelementalX-raymappingofmanganese,aluminumandironatthesurfaceandinthecrosssectionofa3.2mmthickspecimen.Thesemappingswereusedtocomparequantitativelythedistributionofparticlesinthebulkandatthesurfaceofcastings.ImageanalysisfromthesemappingsindicatesthatthesurfaceproportionofAl–Mnparticlesisapproximatelytentimeslargeratthesurfaceofcastspecimensthaninthebulk(7.3±0.1%ascomparedto0.6±0.1%).Theabsenceofironisalsoconfirmedinparticlesbothatthesurfaceandinthebulkofthespecimens.4.Discussion

Inthiswork,Al–MnparticleswereobservedinAZ91Dspecimensmoldedbyinvestmentcasting.Theseparticlesprecipitateduringcooling,sincethesolubilityofMninliquidMgdecreaseswithdecreasingtem-peratureanditisfurtherreducedbythepresenceofAlandFeinthemelt[31].InmoltenAZ91alloycontaining0.24wt.%Mn,supersaturationinMnoccursbelow646°C.Al–Mnparticlescanthennucleateandgrowinthemelt.At595°C,α-Mggrainsbegintocrystallize.Atthistemperature,thesolubilityofMninthemoltenAZ91alloyis0.122wt.%[31].Therefore,underequi-libriumconditions,approximatelyhalfofthemanga-nese,initiallypresent,wouldbeprecipitated.ItwasalsoobservedthattheAl–Mnparticlesatthesurfaceofthespecimensexhibitdifferentdimensionsandmorphologydependingonthesolidificationcondi-tions.Inthinspecimens,therelativelyhighcoolingratesresultintheformationofalargequantityofsmallAl–Mnparticles.Conversely,thereducedcoolingratesexperiencedinthickspecimenspromotethegrowthofbiggerbutfewerAl–Mnparticles.ThisrelationbetweenthesizeoftheparticlesandthesolidificationconditionsisinagreementwithpreviousobservationsmadebyByunetal.[14],BakkeandKarlsen[30]andCorbyetal.[32].

However,forspecimenscastinpermanentmoulds(diecastingandgravitycasting),mostAl–Mnparticlesareformedinthebulkduringsolidification[4,12,15,19,23],onlyfewparticlesbeingformedatthesurface.Onthecontrary,thenumberofAl–Mnparticlesatthesurfaceofspecimensinvestmentcastinplastermouldsisunexpect-edlylarge.Inthiscontext,whatparticularcastingconditionsthatprevailinplastermouldsduringsolidifi-cationcouldstimulatetheformationofthesenumerousparticlesatthesurface?

Inpermanentmoulds,heatisrapidlyconductedthroughthemould/metalinterface.Undercoolingclosetothemouldwallisrelativelyimportantandaskincomposedoffineα-Mggrainsusuallyformsatthesurfaceofcastingsshortlyaftermouldfilling.Nucle-ationofAl–Mnparticlesatthesurfaceofpermanentmouldsislikelybutwhenthemelttemperaturelocallyreaches595°C,nucleationofα-Mgstartsandrapidgrowthoccurs.Therapidgrowthofα-MggrainspreventsthedevelopmentofAl–Mnparticles,whichhaveaninherentlymorecomplexstructureandpresentafacetedmorphology[34,35].Consequently,forperma-nentmouldcastingprocesses,thetimeavailableforthenucleationandgrowthofAl–Mnparticlesatthesurfaceofcastingsisrathershort,thelocaltemperaturedecreasingrapidlyduetotheeffectiveheattransferthroughthemould.

Theoppositesituationprevailsinplastermouldswhichhavealowerthermalconductivity.Inthiscase,coolingofcastalloyswithinmouldcavityisslower.Therelativelylowerundercoolingatthemould-metalinterfacepreventstheearlynucleationandgrowthofα-Mggrains.Consequently,thereisarelativelylongerperiodoftimeduringwhichtheAl–Mnparticles,whichhavenucleatedatthemouldsurface,cangrow.

Asasummary,theroleofthethermalconditionsatthemould/metalinterfaceonthecompetitivegrowthofα-MggrainsandAl–Mnparticlesisseriouslyconsid-eredtoexplainthepresenceoftheAl–Mnparticlesatthesurfaceofthespecimenscastbyinvestmentcasting.

S.LunSinetal./MaterialsCharacterization58(2007)989–996995

5.Conclusions

Inthiswork,thedistribution,morphology,andcom-positionofMn-richparticlesformedinAZ91Dalloyflatspecimenscastinplastermouldswerecharacterized.TheseMn-richparticleswereobservedinverylargenumberatthesurfaceofcastspecimens.ItwasfoundthattheyconsistofAl8Mn5phaseandthattheycontainnoiron.Theseparticlesarebigger,fewerinnumberandmoredevelopedwhenthecoolingratedecreasesduringsolidification.ThepresenceofAl8Mn5particlesatthesurfaceofinvestmentcastingscouldbedetrimentaltothecorrosionperformancesofcastcomponentsbecauseoftheirhighcontentinMn.

Therelativelylowcoolingrateprevailingwithinplastermouldsduringsolidificationandthereducedthermalgradientsatmould/metalinterfaceallowthenucleationandgrowthofAl–Mnparticlesatthesurfaceofspecimenscastinplastermoulds.MoretimeisallowedforthenucleationandgrowthofAl–Mnparticlesontothesurfaceofplastermouldsthanwithpermanentmouldcastingprocesseswhereα-Mggrainsrapidlyformaskin.Acknowledgements

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