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Preparation of semi-solid AlSi7Mg alloy slurry with big capability

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Preparation of semi-solid AlSi7Mg alloy slurry with big capabilityof,OF,alloy,with,big,ALLOY

    Preparation of semi-solid AlSi7Mg alloy

    slurry with big capability

    RAREMETALS

    VoL29,No.6.Dec2olo,p.642

    DoI:10.1O07/s12598.01001851

    PreparationofsemisofidAISi7Mgalloyslurrywithbigcapability

    LIShaandMAOWeimin

    SchoolofMaterialsScienceandEngineering,UniversityofScienceandTechnologyBeijing,Beo'ing100083,China

    Received9June2009;receivedinrevisedform28April2010;accepted6Mrdy2010 TheNonferrousMetalsSocietyofChinaandSpringer-VerlagBerlinHeidelberg2010 Abstract

    Semi

    solidA1Si7Mgalloyslurrywaspreparedbylowsuperheatpouringandweaktraveling-waveelectromagneticstirring.Theeffectsof

    poutingtemperatureandstirringpoweronthemicrostructureofA1Si7Mgalloyslurrywerestudied.Theresultsshowthatthesemisolid

    A1Si7Mgalloyslurryof5kgcallbeprepared.Thisnewtechnologycallsaveenergyandmakethepouringprocessconvenient.Whenthe

    poutingtemperatureisdecreasedatastirringpowerof0.41kW.theshapeofprima~Oc

    A1grainsgraduallychangesfromdendriticliketo

    spherica1.Whenthealloymeltispouredatthetemperature(630.C)withacertainsuperheat,thepouringprocessbecomeseasier,andthe

    sphericalmicrostrucmreofAlSi7Mgalloyslurrycanbepreparedbytheweaktraveling-waveelectromagneticstirring.Whenthepouting

    temperatureis630.C,increasingthestirringpowerappropriatelycanresultinbettersphericalprimary0-A1grains,butifthestirringpower

isincreasedtoacertainvalue(1.72kW),theshapeofprimaryot

    A1grainsdoesnotobouslyimprovewhenthestirringpoweriscontinually increased.

    Kevwords."aluminumalloys;metalforming;semisolidslurry;electromagneticstirring;mi

    crostmcture

    1.Introduction

    Semisolidmeta1formingtechnologyoriginatedinthe UnitedStates[1].Becausethistechnologyhasmanyspecial advantages.muchattentionhasbeengiventoitinthede- velopedcountriesandithasbecomeaveryexcitingfieldof materialformingtechnologyresearches[2-5].Atpresent, therearemanymethodsforpreparingsemisolidmetalslur-

    ties,suchasmechanicalstirringmethod[6],electromagnetic stirringmethod[1],straininducedmeltactivationprocess[3, 7,andultrasonicvibrationmethod8].Inpracticalproduc

    tion,electromagneticstirringmethodandstrain-induced meltactivationmethodstillholdadominantposition.but theproductioncostofthesetwokindsofpreparationmeth

    odsaremuchhigher,andthewidespreadapplicationofthe semisolidmeta1thixoformingtechnologyisrestricted.To makefuUuseoftheadvantagesofsemi.solidmetalforming technology,searchingfornewandlow-costpreparation methodshasbeenanobjectformanydomesticandintema- tionalexr'ertsandmanufacturers.Inrecentyears.somenew semisolidformingtechnologieshaveappeared,suchas controlledcrysmlmethod91,controllednucleationmethod

    [10],andliquiduscastingmethod[11].Althoughanideal microstructurecanbegainedwhentheliquidalloyispoured C0rresp0ndingauthor:LIShaE?mail:hbylyl@126.corn atthenearliquidustemperature.itisverydiHiculttocontrol

    thepouringprocessduringpracticalproduction.Maoeta1. developedalowsuperheatpouringandweakelectromag

    neticstirringtechnologyforpreparingsemisolidalloyslur-

    riesorbillets[1213].Thisnewtechnologyhasmuchlower

    energyexhaustionandismoreeasilycontrolled,whichcan decreasethepreparationcostofsemisolidalloyslurriesor

    billetsandmakethepouringprocesscontrolsimple.Mean? while,anidealmicrostructureofthesemisolidalloyslurry

    canbeobtained.Moreattentionhasbeengiventosemisolid

    A1Si7Mgalloyslurrieswithsmallcapabilityduringthe previousresearchstudy[1417],andthemassofthe

    preparedslurryisabout1.5-2.6kgeverytime,which satisfiestheproductionrequirementofsmallerrheoforming

    parts.Inpracticalapplication,however,biggerrheo-forming partsareneededtosomeextent.Whetherthetechnologyof lowsuperheatpouringandweakelectromagneticstirring canmeetthepreparationofsemi-solidalloyslurrywithbig capacityandhowtherulesofpreparationchangewillalso havetobestudiedsignificantly.Inthisarticle,thesemi-solid A1Si7Mgalloyslurrywithbigcapacity(about5kg)was preparedbylowsuperheatpoutingandweaktravelling

    waveelectromagneticstirring,andthepouringtemperature andstirringpowerwerestudied.

    (f&etaL.PreparationofsemisolidAISi7Mgalloyslurrywithbigcapability 2.Experimental

    TheexperimentalmaterialwasacommercialAISi7Mg alloy.ItschemicalcompositionislistedinTable1.The theoreticalalloyliquidusandsolidustemperaturestestedby differentialthermalanalysisare615and555.C,respec

    tively.

643

    Metallographicspecimenswerecutofrfromthe

    quenchedslurries,thenroughlyground,fmelypolished,and etchedbyanaqueoussolutionof0.5vo1.%HF.Theetched specimenswerecleanedwithanalcoholsolutionanddried. Themicrostructureswereobservedandanalyzedwithan opticalmicroscopeoftheNeuphoto21mode1.

    Table1.CompositionoftheAlSi7Mgalloywt.%3.Resultsanddiscussion TheA1Si7Mgalloywasmeltedinanelectricresistance furnaceandthenrefined.Themeltedtemperaturewas 720.C.mentheliquidalloywascooledtothechosen pouringtemperature,itwaspouredintoastainlesssteelcrtl

    cibleof127mmindiameterand255rflrninheight.Atier pouring,themeltinthetrueiblewasstirredbyanelectro. magneticstirreratlowpowerfor8s.Theslurrywasthen rapidlyquenchedintocoldwatertomaintainthemicrostruc

    tureofAISi7Mgalloyslurryathightemperature.Specific preparationparameterswereasfollows:thepouringtem? peraturewas630,650,and700.C;thestirringpowerwas 0.41,1.72,and3.90kW.Tepreparationprocessschemat

    icsoflowsuperheatpouringandweaktraveling-waveelec

    tromagneticstirringisshowninFig.1.

    ×(5)Fig.1.Preparationprocessschematicsoflowsuperheat pouringandweaktraveling-waveelectromagneticstirring:(1) traveling-waveelectromagneticstirrer;(2)refractory;(3) stainlesssteelmould;(4)AlSi7Mgalloymelt;(5)melting crucible.

    3.1.Effectofpouringtemperatureonthemicrostructure Fig.2showsthemicrostructuresofAISi7Mgalloyslur

tiespreparedwithweaktravellingwaveelectromagnetic

    stirringatdifferenttemperatures.Whentheliquidmeltis pouredat700.C,themicrostructureoftheAISi7Mgalloy slurryconsistsofdendriticlikeprimary0Algrains.The

    primarydendriticanTiSaremuchcoarserandlongerthanthe secondaryarms,andthemicrostructureisnothomogeneous, asshowninFig.2(a).Whentheliquidmeltispouredat 650.C,themicrostructurehasanobviouschange,mostof

    likeandthemicrostruc theprimarycc_Algrainsarerosette

    tureismorehomogeneous,asshowninFig.2(b).Whenthe liquidmeltispouredat630.C,theprimary~x-A!grains changesfromroseReliketospherical,mostoftheprimary 0【一A1grainsarespherica1.afewofrosettelikeprimarycc-Al

    grainsappear,andthemicrostructurebecomesmorehomo

    geneous,asshowninFig.2(c).

    Theseexperimentalresultsshowthatanidealmicro

    structureoftheA1Si7Mgalloyslurrypreparedatlowsu

    perheatpoutingandweakelectromagneticstirringfora shorttimecanbeobtained,whichcansatisfytheneedfor rheoformingorthixoforming.Whenthepouringtempera

    tureisdecreased,themicrostructureoftheAISi7Mgalloy slurryisgraduallychangedandobviouslyimproved,as showninFig.2.Iftheliquidalloymeltispouredattheliq

    uidustemperature,primaryot-Algrainsappearinspherical shapeandthemicrosmactureishomogeneous.However,in practicalproduction,itisverydifficulttocontrolthepouring processatsuchalowpouringtemperature.Therefore,itis oftenhopedthatthepouringtemperaturewouldbeappro- priatelypromotedtobeinghigherthan615.C.Whenthe Fig.2.MicrostructuresofAlSi7Mgalloyslurriespouredatastirringpowerof0.41kWatdiffer

entpouringtemperatures:

    (a)700.C;(b)650.C;(c)630.C.

    pouringtemperatureisincreasedsomewhat,itiseasierto controlthepouringprocess.Butthesolidifiedmicrostruc

    turewillbecomeworseandprimary0【一A1grainsgradually

    changefromsphericaltorosettelike.Itisreportedthatthe

    meltflowmotionattheinitialsolidificationhasanimportant effectontheshapeofprimary0【一AIgrains.whichhelpsthe

    formationofsphericalprimary0cAlgrains[18].Whenthe

    alloymeltisstirredbyaweakelectromagneticfieldfora shortperiodoftime.therewillbeacertainextentofflow motioninthemelt.Theflowmotionmakesthetemperature fieldofthemelthomogenousandtheheatdiflusionandheat exchangewiththemouldwillincrease.Thesolidification timeoftheslurryisreduced.thegrowthofprimaryAl

    grainsisrestrainedandprimary0.Algrainsarerefinedin

    theend.Moreover,whenthepouringtemperatureislower, suchas630.C.thealloymelthasalowersupercoolingde. greeduringthesolidification,andmuchfinergrainswillbe formed.Whenthepouringtemperatureisappropriatelyde- creased.thepouringprocessbecomeseasierandideal RAREMETALS,VaL29,No.6.Dec2010

    sphericalprimary0cAIgrainsoftheA1Si7Mgalloyslurry

    canalsobeobtained.Inthisexperiment,theoptimized pouringtemperatureparameteris630.C.

    3.2.Efleetofstirringpoweronthemierostructure Fig.3showsthemicrostructuresofAISi7Mgalloyslur

    riespreparedatthepoutingtemperatureof630.C,stirring frequencyof30Hz,andstirringtimeof8swithdifferent

stirringpowers.Itshowsthatthestirringpowerhasanob

    viouseffectonthemicrostructure.Whenthestirringpower is0.41kW.mostoftheprimary0cAIgrainsappearin

    sphericalshapeandafewprimarytl-A1grainsarero

    serelike,asshowninFig.3(a).Whenthestirringpoweris increasedto1.72kW,themicrostructureisobviouslyim

    proved.Mostoftheprimary0cAIgrainsaresphericaland

    finer,andthemorehomogeneousmicrostructureisobtained asshowninFig.3(b).Whenthestirringpowerisincreased tO3.90kW.themicrostructureisnotfurtherimproved comparedwiththatofFig.3(b),asshowninFig.3(c). Fig.3.MierostructuresofAlSi7Mgalloyslurriespreparedatapouringtemperatureof630.C

    withdifferentelectromag-

    neficstirringpowers:(a)0.41kW;(b)1.72kW;(c)3.90kW. Whenthestirringpoweris0.4lkW.thealloymeltisnot easilysti~ed.Thereisnoobviousliquidconcaveonthetop surfaceofthemeltinthepreparingcrucibleandthereisonly aweakflowmotionontheliquidsurface.Therefore,mostof theprimary0-AIgrainsappearinsphericalshapeandafew primaryGt.Algrainsarerosette.1ike.,enthest1'rring powerisincreasedto1.72kW.amoreobviousliquidcon. caveisgeneratedonthetopsurfaceofthemelt,andthemelt flowmotionispromoted.Therefore.themicrostructureof theAISi7Mgalloyslurryisobviouslyimproved.Mostofthe primary0.Algrainsaresphericalandfiner,andthemicro. structureismorehomogeneous.Whenthestirringpoweris furtherincreasedto3.90kW.thereisadeeperIiquidcon. caveonthetopsurfaceofthemeltandtheflowmotionin themeltisfurtherpromoted.Hence.themicrostructureof theAISi7Mgalloyslurryisobviouslyimprovedandmostof

    theprimary0cA1grainsarespherica1.too.Thisintenseflow motioninducedbystirringmakesthetemperaturefieldof themeltmorehomogeneous,andincreasestheregionarea inwhichmanyprimary0cAlgrainsseparateatthesame

    timeandsoprimary0c.Algrainsaregreatlyfined.Mcan. while,theflowmotionacceleratesthemasstransferprocess andmakesthesolutefieldnearthesolid-liquidinterface homogenousanddecreasestheconstitutionalsupercooling. The1owerconstitutionalsupercoolingmakesPrimaryden

    driteseparationdifficultandmostoftheprimary0c.Al grainsaDpearrosettelike.Thisflowmotionalsomakesthe

    separatedfinerosettelikeprimary0cAlgrainsmoveeve.

    rywhere,andastrongmanmadetemperaturefluctuationon therosettelikeprimary5-A1grainsexists.Thisphenome

    nonmakesthesecondaryarmrootsofrosettelikeprimary

    0c.Algrainsfusetogetherandtherosettelikeprimary仅一Al

    grainsgraduallychangetosphericalgrains[19-20].1]heex

    perimentalresultsshowthattheabove.mentionedphe. nomenonseemstobetrueandanappropriatestirringpower, suchas1.72kW.isneededforthepreparationofidealslur- des.Yurkoeta1.studiedthesemi.solidalloyslurryprepared bylowsuperheatpouringandweakmechanicalstirring technology[21].Theirstudyindicatesthatasemisolidalloy

    slurrywithaperfectshapeoftheprimaryphasecanbepre- paredifthemechanicalstirringrateexceeds60r/minandit isnotnecessarytousehighermechanicalstirringrates.The Lis.etaL.Preparationofsemi?solidAISi7Mgalloyslurrywithbigcapability

    experimentalresultsalsoshowthefollowingview.When thepouringtemperatureis630.C,appropriatelymc~asing

thestirringpowercangainbettersphericalprimary0_Al

    grains,butifthestirringpowerisincreasedtoacertain value.theshapeofprimaryO【一Algrainsdoesnotimprove

    furtherwhenthestirringpoweriscontinuallyincreased.In thisexperiment,theoptimizedstirringpowerparameteris 172kW

    4.Conclusions

    (1)Semi?solidAISi7Mgalloyslurryof5kgcanbepre

    paredbylowsuperheatpoutingandweaktravelingwave

    electromagneticstirringtechnologyandthisnewtechnology cansaveenergyandmakethepouringprocessconvenient. (2)Whenthepouringtemperatureisdecreasedatastir- ringpowerof0.41kW.theshapeofprimaryot-Algrains graduallychangesfromdendriticliketospherica1.Whenthe

    alloymeltispouredatacertainsuperheat,thepouringproc

    essbecomeseasier,andthesphericalmicrostructureofthe AISi7Mgalloyslurrypreparedbyweaktraveling?wave electromagneticstirringisformed.Inthisexperiment,the optimizedpoutingtemperatureparameteris630.C. (3)Whenthepouringtemperatureis630.C,increasing thestirringpowerappropriatelycanresultinbetterspherical primary0【一A1grains.butifthestirringpowerisincreasedto acertainvalue(1.72kW),theshapeofpfimarya-Algrains doesnotimprovefurtherwhenthestirringpoweriscontinu

    allyincreased.Inthisexperiment,theoptimizedstirring powerparameteris1.72kW.

    Acknowledgements

    TheworkwasfinanciallysupportedbytheNational HighTechResearchandDevelopmentProgramofChina

(No.2006AA03Z115),theNationalBasicResearchPro

    gramofChina(No.2006CB6052O3),andtheNational NaturalScienceFoundationofChina(No.50374012). References

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