www.elsevier.com/locate/matlet
Microstructurecharacterizationandtensilepropertiesof
squeeze-castAlSiMgalloys
M.T.AbouEl-khair*
NonFerrousLaboratory,CentralMetallurgicalResearchandDevelopmentInstitute(CMRDI),P.O.B.87Helwan,Cairo,Egypt
Received24March2004;receivedinrevisedform20November2004;accepted27November2004
Availableonline10December2004
Abstract
Aresearchprogramwasconductedtostudytheeffectsofsqueezepressure(70,100and160MPa)andheattreatmentT6onthestructure,hardnessandtensilepropertiesofcastAl6Si0.3Mgalloys.Theinfluenceofsqueezepressureonmacro-andmicrostructuresofAl6Si0.3Mgalloyshasbeeninvestigated.Someofcastingsweresolutiontreatedat5408Cforvarioustimesandothersweresubjectedtoagingat1708Caftersolutiontreatment.Theresultsindicatedthatprecipitationoccurredwithinabout30minforbothcastandsqueezecastalloys.Thehardnessbegantoincreaseandmaximumvalueswereobservedafterabout10hforas-castalloy.Increasingofsqueezepressure(70–160MPa)acceleratedstrengthofthealloysfrom8to4h,respectively.Squeezepressuresdecreasedthepercentageofporosityandincreasedthedensity,alsoitdecreasedthegrainsizeofa-AlandmodifiedtheSieutectic.Hardnessandtensilepropertiesincreasedwithbothheattreatmentandincreasingofsqueezepressure.D2004ElsevierB.V.Allrightsreserved.
Keywords:AlSiMg;Squeezecasting;Soundness;Macro-/microstructure;Aging;Tensileproperties
1.Introduction
Theuseoflight-metalcomponentsinvariousapplicationshasincreasedduringthelastdecade,partlyasaresultoftheincreasedamountoflightmetalsbeingusedfortransportationpurposes.Oneadvantageisthatlight-metalcomponentsleadtoanoverallreducedweightand,thus,toreducedenergyconsumption.Anotheradvantage,whichmaybejustasimportantfromanenvironmentalpointofview,isthefactthataluminiumcomponentsmayberecycledwithrelativelowenergydemands.Forsuchpurposes,alloysofthetypeAlSiMgarefoundtogivegoodresultssincetheyshowexcellentcastingcharacteristicsandmechanicalproperties;theadditionofMgmakesthealloysheattreatable[1–5].
*Tel.:+2025010642;fax:+2025010639.E-mailaddress:rucmrdi@rusys.eg.net.
0167-577X/$-seefrontmatterD2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.matlet.2004.11.041
ThemechanicalpropertiesofcastAlSiMgalloysaredeterminedbychemicalcomposition,rateofsolidification,caststructureanditsintegrityandheattreatment.Generally,alloycompositioneffectsareindependentofcastingtechniqueswhereastherateofsolidificationofcaststructure,andheattreatmentproceduresareprocessdependent[6–8].
Also,themechanicalpropertiesofthecastingsareaffectedsignificantlybythemorphologyoftheeutecticSiandporosity.TheporosityisknowntoaffectadverselythemechanicalpropertiesofAlalloys[1,4,9,10].
Inthesqueezecastingprocess,thehighpressureappliedduringsolidificationcaneliminateporositycausedbybothgasandshrinkage,andtheincreasedcoolingratecausedbyimprovedthermalcontactbetweenthecastinganddieresultsintheformationoffinegrainedstructures[10–12].Con-sequently,thesealsoimprovecastingsoundnessandmechanicalproperties.Inthepresentcontribution,aspectsrelatedtothecastingtechniqueandheattreatmentofthecastingareconsidered.
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895
Table1
TheDensitymeasurementsandporositypercent
SqueezepressureAscast
70MPa100MPa160MPaTotalporeareasq-m/g2.7830.6980.6590.490BulkDensityg/ml2.58462.67352.67532.69762.6527*2.6894*2.6902*2.6917*Porosity%
2.27
0.71
0.63
0.48
*Archimedesmeasurements.
Thispaperpresentsresultsontheeffectofoptimumprocessconditionsforobtainingsoundcastingsandalsotoexplorethesqueezepressureeffectonmacro-andmicro-structure,soundness,agingcurves,eutecticmorphologyandtensileproperties.
2.Experimentalprocedure
AnAlSialloyofcompositionSi5.9,Fe0.52,Cu0.385,Mg0.333andAlbalancewasusedascastingmaterial.Aftermeltinginanelectricresistancecruciblefurnace,degassingwithpreviouslypurifiedliquidnitrogenwascarriedout.Liquidmetalwithacertainsuperheat(508C)waspouredintothepreheatedtoolsteeldiewithacylindricalcavityofaninternaldiameterof50mm,aheightof100mmandawallthicknessof20mm.
Afterpouringthemeltintothediecavity,pressurizationwasachievedusinga60Thydraulicpress.Thedelaytime,whichisnecessaryforthepressurizationofthemeltafterpouringwas120s.Thedietemperaturewas2508Candthepouringtemperaturewas7508Candthesqueezepressureswere70,100and160MPa.
Thedensitiesofas-castandsqueezespecimensweredeterminedusingArchimedes’principleandalsobyusingporesizerdevicemicromeritiesmodel9310toquantifytheporositypercent.
Thesqueezecastspecimenswerecutintotwopartsinthelongitudinaldirection,andonepartwaspreparedforobservationofmacrostructurebypolishingandetchingwithTurker’sreagent.ThenthecastingwasformedastensilespecimensofA-370standard.
Someofthespecimensweresolutiontreatedat5408Cfor32h,theotherforhardnessandtensiletestsweresubjectedtoT6condition(solutiontreatedat5408Cfor8h,quenchedinwaterat258Candagedat1708Cfordifferenttimestill36h).
Theas-castandsolution-treatedspecimenswerepol-ishedandetched,andthemicrostructureswereexaminedusingopticalmicroscopy.ThetensilepropertieswereevaluatedwiththetestspecimensfabricatedbyScHE-MATZUEDCStestingmachineunderthecross-headspeedof0.3mm/minandfracturesurfacesofthetestspecimenswereexaminedwithJEOLscanningelectronmicroscope.
3.Soundness
ThedensitymeasuredbyArchimedes’principleandalsothedensitiesmeasuredbytheporesizedevicearetabulatedinTable1.
Fromtheresultsbothmeasurementsshowthatwithincreasingthesqueezepressure,thedensityincreases.
Fig.1.Macrostructureofsqueezecastalloys(a)70(b)100and(c)160MPa.1X.
896M.T.AbouEl-khair/MaterialsLetters59(2005)894–900
Densityincreasesmarkedlyfromas-castto70MPa,andtheincreaseisgradualthereafterfrom100to160MPa.Also,theporeareaandporositypercentdecrease.Increasingofpressurefrom0to70MPadecreasestheporosityabout69%whileitis32%from70to160MPa.
Asthesqueezecastingprocessdoesnotmakeuseoffeedermaterial,thecavityresultingfrommetalshrinkagemustbecompensatedbytheapplicationofpressure.Hashemietal.[11]statedthatthedensitymeasurementsindicatedthatthereisacriticalpressurethatcanbeachieved.Allsqueezedcastspecimensatapressureabove50MPawerefullydense.
4.Macrostructure
Fig.1showsthemacrostructuralcharacterizationcon-ductedonthesqueezecastalloysat70,100and160MPa,respectively.Itrevealednoevidenceofblowholesormacropores.Themacrostructureisanequiaxisedstructureandthegrainsizesdecreasewithincreaseofsqueezepressurefrom70to160MPa.
andheattreatmentonthemechanicalpropertiesofAl6Si0.3Mgalloy.
Themicrostructureofas-castalloyisshowninFig.2awhichshowstheexistenceofaciculareutecticSisurroundedbya-Aldendrites.Therelativelycoarsea-Aldendriteofabout150AmsizetogetherwiththecoarseSiparticleswereobservedinthegravitycastmaterial.Thiswasattributedtotheslowsolidificationrateduringcasting.Ontheotherhand,inFig.2b,theapplicationofpressure70MParesultsinastructureofa-Alofabout50AmsizewithfineeutecticSi.Increasingtheappliedpressurefrom100to160MPareducesthegrainsizeoftheprimaryphasefrom30to20Am(Fig.2candd).Also,increasingtheappliedpressureleadstotheformationofaveryfineeutecticSi.However,applicationofpressuredoesnotaffecttheeutecticSimorphology.
Onthesolidificationunderpressure,thefollowingchangesinmicrostructurewereobservedFig.2:1.2.3.4.
increasinginthevolumefractionoftheAl-richa-phasewithincreaseinpressure
decreaseinthesizeoftheprimaryAl-richdendritesdecreaseinthevolumefractionoftheeutecticconsiderablerefinementoftheSioftheeutectic
5.Microstructure
Themajorpurposeofthisworkistoclarify,throughmicrostructuralstudies,theinfluenceofsqueezepressure
Themicrostructuresofas-castandsqueezecastsolutiontreatmentalloysareshowninFig.3.Themicrostructureinthesolutiontreatmentconditionconsistsofa-Alcells
Fig.2.Microstructureof(a)as-castandsqueezecastalloysat(b)70,(c)100and(d)160MPa.
M.T.AbouEl-khair/MaterialsLetters59(2005)894–900897
Fig.3.Microstructureofsolution-treatedalloys(a)as-cast(b)squeezecastat70MPa.
boundedbyirregularlyshapedSiparticles.Eutecticisacicularandrandomlydistributed.
ThetypicaldistributionofSiparticlesaftersolutiontreatmentisshowninFig.3.ForsqueezecastalloytheSiparticleshavespheroidizedandcoarsenedtosomeextent,ascomparedwithas-castalloy.Mostofthemaremoreorlessspherical.Someparticlesstillhavealongitudinalshape.Initially,Siparticlesarebrokendownintosmallerfragmentsandaregraduallyspheroidized.Prolongedsolutiontreat-mentleadstocoarseningoftheparticles.Bothspheroidiza-tionandcoarseningaresurfaceenergy-driven,i.e.,thesystemtriestoreduceexcesssurfaceareatotheminimumpossible[13,14].
6.Agingbehavior
Theagingcurvesatroomtemperaturehardnessofas-castandsqueezedcastalloysareshowninFig.4.
Thecurvesbehaveinasimilartrend.Theyrevealthattheprecipitation-strengtheningeffectbecomepro-nouncedafter30minofagingandreachesapeakvalueat10,8,6and4hforas-castandsqueezecast(70–160MPa)alloys,respectively.Littledifferenceintimetopeakhardnesswasobservedfordifferentconditions.
ThealloysarestrengthenedbytheprecipitationofanintermetalliccompoundMg2Siduringtheagingtreatment.
898M.T.AbouEl-khair/MaterialsLetters59(2005)894–900
Fig.4.Theagingcurvesofas-castandsqueezecastalloysatdifferentsqueezepressuresat1708C.
Thisenhancementofstrengthwasobtainedalsowithincreasingthesqueezepressurefrom70to160MPa.
IncreasingthepressureincreasestheSisolubilitysothestrengthincreasedwithincreasingthepressure[10].BecauseofthepresenceofexcessSiinthesolidsolution,precipitationofMg2Sioccursmuchfasterinsqueezedalloys[6].
ThesolutiontreatmentstageoftheT6heattreatmentperformsseveralimportantfunctions:dissolutionofMg2Siphase;homogenizationofthesolidsolution;andfragmen-tation,spherodizationandcoarseningoftheeutecticsilicon.DissolutionofMg2Siandhomogenizationofthematrixoccurswithin15mininA356alloyat5408C[1].
Thechangestoeutecticsiliconmorphologyaregenerallyslower,takinguptoseveralhours,anddependonparameterssuchassolutiontemperatureandoriginalparticlesize/shapewhichinturnaredeterminedbysolidificationconditions,grainsizeandeutecticmodifi-cation[9].
7.Tensileproperties
Table2showsthevaluesofultimatetensilestrength(UTS),yieldstrength(YS)andelongationpercent(El%)ofthenon-heat-treated,solution-treatedandagedalloysatdifferentsqueezepressures.FromthetableUTSandYSshowanincreasingtrendwithincreasingofsqueezepressurefrom70to160MPaforallalloys.
AgedalloyshavethelargestvalueofUTSandYS,thevaluesofsolution-treatedalloysarelesswhilethevaluesofnontreatedalloysarethelowest.
TheeutecticSiinas-castalloyispresentascoarse,acicularneedleswhichactasstressraisers,andconse-quently,thematerialiseasyfractured.TheheattreatmentmodifiesthemorphologyoftheSifromaciculartospherodizedshape,therebyimprovingthemechanicalproperties[5,9,14].
Theenhancementofstrengthpropertiesobtainedduringagingtreatmentisprimarilyowingtothemetastablephasefromthesupersaturatedsolution[8].Whenboththesolutionandagingtreatmenteffectsonthetensilepropertiesareconsidered,thepropertieswouldshowanincreaseinstrengthanddecreaseinductility.Thepresentresultsthereforeagreequitewellwiththeresultsreportedinliterature.
TheincreaseofstrengthwithincreasingsqueezepressureisduetotheincreasedsolubilityofSiandthevirtualeliminationofshrinkageand/orgasvoidscouldallcontributetowardstheobservedimprovementsinstrengthofthealloys[6,10,11].
Theelongationpercentincreaseswithincreasingsqueezepressure,thevaluesofsolution-treatedalloysarethelargestones.Theimprovementsinelongationvaluesaremostlikelytobeduetotheincreasedvolumefractionoftheprimaryaphaseandimprovedsoundnessofthealloys.TheeutecticSimorphologyplaysavitalroleindeterminingthemechanicalproperties.Particlesize,shapeandspacingarefactorsthatcharacterizeSimorphology.Undernormalcoolingconditions,Siparticlesarepresentascoarseacicularneedles.Theneedlesactascrackinitiatorsandlowermechanicalpropertiesappreciably.TheSiparticlecharacteristicscanbealteredbysubjectingthecastingtoahightemperatureheattreatmentforlongperiods.Therefore,forprolongedsolutiontreatment,theobservedchangeintensilepropertiesareattributedtochangeinSiparticlecharacteristic[4,9,10].
8.Tensilefracturesurface
Fig.5revealstheSEMmicrographsofthetypicalfracturesurfacesofnontreatedandagedtensilespecimens.Amixedmodeofbrittlecleavageandductilefracturewithdimpleswasobservedatbothheat-treatedandnontreatedalloys.Applicationofsqueezepressuresimprovethefracturesurfaces.Itindicatesamoreductilefailuremode.
Table2
Mechanicalproperties(UTS,YSandEl.%)ofinvestigatedalloys
PressureMPa
EffectofheattreatmentNontreatedSolutiontreatmentAgingUTSMPa
Ascast10314715070128173198100131185200160132190208YSMPa
Ascast64114827087136136100103139165160114140190EI.%
Ascast2.543.57041351005.5146160
6.5
15
8
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Fig.5.SEMmicrographsoftensilefractureof(a)as-castandsqueezecastalloysat(b)70,(c)100and(d)160MPa.
Thefracturebehaviorofthealloysisaffectedbythesizeofa-particlesandSimorphology[15].5.6.
9.Conclusions1.2.3.4.
Bothheattreatmentandsqueezecastingincreasethestrengthoftheinvestigatedalloys.
Increasingofsqueezepressuredecreasesthea-AlgrainsizeandmodifiedtheeutecticSi.
Increasingofsqueezepressure(70–160)MPadecreasestheporosityandimprovesthetensileproperties.
Squeezepressureaccelerateshardnesspeakfrom8to4h.
AgingenhancesUTSandYSandreducestheductility.TheSiparticlesstarttofragmentizeandspheroidizealmostimmediatelywithsolutiontreatment.Thisleadstopronounceimprovementinmechanicalpropertiesoftreatedalloys.
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