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Rheological_0Rheolo

    Rheological

580J.Mater.Sei.Techno1..Vo1.20No.5.2004

    ;RheologicalBehaviorofSemiSolidAZ91DAlloy

    ;IVeimin.\L40t,ZishengZHEN,ShidianE4NandXueyouZHONG ;SchoolofMaterialsScienceandEngineering,UniversityofScienceandTechnologyBeijing,Beijing10

    0083,China

    ;Manuscriptrecei~edJune23,2003,inrevisedformSeptember12,2003] ;TherheologicaIbehaviorandmicrostructureofsemisolidAZ91DwasstudiedusingaCouettetypeviscometer.The

    ;resultsshowthattheapparentviscosityofsemisolidAZ91Dincreaseswiththecoolingrateincreasingwhenitis

    ;continuouslycooledandmechanicaIIYstirredandtheempiricrelationoftheapparentviscositywiththec

    ooIingrate

    ;andsolidfractionatshearingrate937slisexpressedasr/=0.660.63~3_o1)1e

    ,s/o_l41,andwiththeshearing

    ;rateandsolidfractionatcoolingrate4.C/rainisshownasr/=0.55e(,s/o?u’/87.59).asweIIasthecooIingrateand

    ;shearingratehaveanimportanteffectonthemicrostructureofthesemisolidAZ91DaIloysIurry,anddecreasing

    ;thecoolingrateandincreasingtheshearingratearefavorabletothenondendriticevoIutionoftheDrimary

    graias

    ;KEYWORDS:Rheologicalbehavior,Magnesiumalloys,Semisolid

    ;1.Introduction

    ;Magnesiumalloyshavebeenappliedtotheaeronautic

    ;astronautic,mobile,motorcycle,electronic,andinformation ;fieldsonalargescalebecausethesealloyshavesmallden

    ;sity,highspecificstrengthandspecificrigidity,goodshock ;resistanceandmachinability.Duringmakingmachineparts ;withmagnesiumalloys,thesemisolidfornlingtechnologies

    ;arebeingusedmoreandmoret.Inordertocontrolthe

    ;semisolidformingprocessandmanllfacturebetterpartswith ;magnesiumalloys.itisnecessarytorealizethenatureofthe ;semisolidmagnesiumalloy.Tberheologiealbehaviorandmi

    ;crostructureofthesemisolidmagnesiumallo)risoneofthe

    ;basicnaturesandthere.however.islackof’therheologicalbe—

    ;haviorandnlicrostructureuptodate.Therefo,’e.thispaper

    ;hasselectedtheAZ91Dmagnesiumallo~rastheresearched ;materialandstudiedtheapparentviscosityandmierostruc

    ;tureofthesemisolidslurryofAZ91Dbymechanicalstirring

    ;duringc.tinu.usc..ling.

;Fig.1

    ;2.Experimental

    ;2.1Materiaj

    ;ThematerialusedinthisstudywasAZ91Dalloyprovided ;byaprofessionalmanufacturer.Itschemicalcompositioncon

    ;tains9.22wtpctA1,O.70wtpctZn0.19wtpctMn,0.O1 ;wtpetCa,andbalancedMg.

    ;2.2Rheologicaltesting

    ;2.2ExperimentalapparatusACouettetypeviscometer ;wasfabricatedfortherheologicaltesting.Theschematicdi

    ;agralnisshoxxninFig.1.Themagnesiumalloywasmelted ;inthecruciblebytheresistantfurnaceoutside.Thestirring ;headwithsmoothsurfaceisassembledcoaxiallvwiththestir

    ;ringcrucible.shapingacirquewith2.5nlnlinwidth.The ;stirringCrlxcibleandstirringheadconnectedwithamotor ;andatorquetransducer,respectively.Bvanaccuratespeed ;controlunit,motordrivesthecrucibletorotateatacertain ;speedandatorqueproducedonthestirringheadbecauseof ;theviscosityoftheslurry.Thetorquetransducertransforms ;thetorqueintoelectricsignals.Acomputerwithadataac

    ;quisitioncardthencollectedthetorqueelectricsignals.The ;temperatureoftheslurrywascontrolledbythreethermocou

    ;piesplacedonthestirringheadsymmetrical1)rfronltopto ;bottom.andthedifierencebetweenthem,vaslessthan2.C ;duringtheexperiments.

    ;2.2.2ExperimentalprocessSincethespacebetween ;thestirringcrucibleandbeadisverythin.whichisabout ;tProf.,PhD.,towhomcorrespondenceshoIfldbeaddressed ;E.mail:weiminmao@263.net.

    ;oF0Uedata

    ;co1ectio1

    ;TomotoF

    ;cOntFO

    ;Schematicdiagramofexperimentalapparatus1. ;heightadjuster,2.torquetransducer,3.coolingwa

    ;ter4.thermocouple.5.heatingfurnace6.stir

    ;ringhead.7.semi.solidslurry.8.crucible.9.heat ;insulationlayer.10.coolingwater.11.motor.12. ;protectiongas

    ;2.5min.theforceontheliquidduringthetestingCallbe ;consideredaspureshear.Accordingtothecorrelatixeknowl? ;edge.thetorquecanbetrimmergeddepth

    ;ofthestirringheadintothemetallicslurry;istherotat

    ;ingangularspeedofthemotor;Ristheinnerradiusofthe

    ;crucibleandristheradiusofthestirringhead.Theaverage ;shearrateproducedontheslurrycanbecalculatedbvthe ;followingfornlulat~J.

    ;.

    ;2Rr

    ;1=

    ;Themagnesiumalloywasmeltedinthecrucible.thenput ;thestirringheadintotheliquid.Prom600.C.maketheliq- ;uidcooledata4.C/minratetodifierenttemperatureinthe ;semi.solidrmlge,whichmeansdifierentsolidfraction,and ;thenholditisothermal13r.Thec0rrespondingsolidfraction ;()oftheslurryunderacertaintemperaturecanbegiven ;12345678901

    ;

    ;J.Mater.Sci.Techno1..,1.20No,5.2004

    ;Fig.2PlotofapparentviscosityofsemisolidAZ91Dv8

    ;solidfractionunderthesameshearingratebutdif- ;ferentcontinuouslycoolingrate

    ;bythefollowingScheilequation[.

    ;,s=1TMTL

    ;?一

    ;(3)

    ;where,T,(isthemeltingpointofpuresoh,entelement,;TLis ;theliquidustemperatureoftilealloy;Tistheactualtemper

    ;atureoftheslurry;kisthecoefficientofequilibriumsolute ;distribution.

    ;Duringthisprocess,startthemotoratacertainspeed ;aadthedataacquisitionprogramatthesametime.When ;thetorquereachesastead),value.stopthetest.Repeattile ;testwithdifferentshearingratesanddifferenttemperature ;(,s)undereachshearingrate.

    ;3.ResultsandDiscussion

    ;3,1Influenceofthecoolingrateo12theapparentviscos#,3? ;Theapparentviscosities8thesolidfractionsofthesemi

    ;solidmagnesiumallovsolidifiedat(1ifierentcontinuouscool

    ;ingrateareshowninFig,2.ItcanbeseenfromFig.2thatthe ;apparentviscosityincreaseswiththeincreasingsolidfraction ;ordecreasingtemperatureoftileslurryandalsoriseswhen ;thecoolingrateispromoted.Sotindertheconditionofthe ;samefractionsolidtheapparentviscosityoftheslurrysolid

    ;ifledatlargecontinuouscoolingrateishigherthanthatat ;smallcontinuouscoolingrateandthedifferencebetweenthe ;viscositiesenlargesifthesolidfractionisincreased. ;Alongwiththemeltcooling.thesolidfractioniscontin

    ;uouslyrisingandtheshearedfluidflowresistancefrolnthe ;meltviscosityincreasessothattheapparentviscosityofthe ;slurryisraised.Inadditiontothesolidfraction,theshapeof ;theprimarysolidgrainsalsoaffectstheapparentviscosityre

    ;markablv.Theroundertheprimarysolidgrainsthesmaller ;theshearflowresistancebetweentheinnerfluidlayersandthe ;lowertheapparentviscosity.Itwillbeknownfromthelater ;3.3sectionthattheprimarysolidgrainsseparatedoutatslow ;coolingratearerounderthanthatatfastcoolingrate,Asa ;result,theslurryapparentviscosityundertheslowcooling ;conditioniscomparativelylov~.

    ;Accordingtotheexponentiallawq=Aexp(x/B).theex

    ;perimentaldataareregressedandanempiricformulaisob

    ;tainedasthefollowing,

    ;=[0.66—0.63e’..e’,s.(4)

    ;whereistheapparent,’iscosiD,ofsemi—solidAZ91Dmag

    ;nesiumalloyanditsunitisPa?s.Eistilecooling1.ateandits ;unitis.C/min,,sistheslurrysolidfraction.Empiricfor

    ;mula(4)describestherelationoftheapparentviscositywith ;thecoolingrateandsolidfractionatshearingrate93.7s_.. ;581

    ;Fig.3Plotofapparentviscosit)’v8solidfractionunderthe

    ;samecontinuouslycoolingratebutdifferentshear

    ;ingrate(a)coolingrate:0.25.C/min:shearingrate: ;46.9s-.,937s_.and187.4s_.(b)coolingrate:

    ;4.C/nfin;shearingrate:46.9s.93.7s_..187.4s

    ;and281.2s

    ;3.2InfluenceoftheshearingrateOIltheapparentviscosit~? ;Theshearingrateisanotherimportantparaaneterinthe ;semisolidslurrystirringprocessandhasanobviousetfecton ;theapparent,iscosity(1uringcontinuouslycooling.asshown ;inFig,3.Ifotherconditionsarenotchanged.theapparent,is

    ;cositvofAZ91Dalloyslurrydecreaseswiththeriseofshearing ;rate.

    ;AftertheexperimentaldatainFig.3(a,arefurtherana

    ;13rzed,itisfoundthatifthecoolingrateis0,25.Cmin,the ;shearingrateis46.9sandthesolidfractionsare0.2and

    ;0.5,theapparentviscositiesofAZ91Dalloyslurryare0.47 ;and4.27Pa?srespectivelyandtheirdiIrerenceis3.8Pa,s.But ;iftheshearingrateisincreasedto187.4sandthesolidfrac

    ;tionsarethesameasabove,theapparentviscositiesofAZ91D ;alloyslurryare0.19and0.48Patsrespectivel3randtheirdip ;ferenceisonly0.29Pars,Thistendencyismoreobviousat ;highercoolingratesandhighersolidfractions.InFig.3(b),if

    ;thecoolingrateis4.C/mintheshearingrateis46.9sand ;tilesolidfractionsare0.2and0.4.theapparentviscositiesof ;AZ91Dallovslurryare1.22and11,16Pa,srespectivelyand ;theirdifferenceis9.94Pa,s.However,iftheshearingratein

    ;creasesto2812sandthesolidfractionsarethesamea.s

    ;aboxre,theapparentviscositiesofAZ91Dallovslurryare0.21 ;and0,92Pa.srespectivelyandtheirdifferenceisonly0.71 ;Pars.Whenthedifferenceofviscositybecomessosmal1.it ;hasnmchsmalleffectOilthesemisolidforminginfactands0

    ;tileshearingratehasveryimportanteffectonthesemi..solid ;forming.

    ;Accordingtotheexponentiallawq=Aexp(x/B).theex

    ;perimentaldatainFig.3(b)areregressedandanempiricfor

    ;mulaisobtainedasthefollowing.

    ;q=0.55e(f./.-i/.)(5)

    ;whereistheapparentviscosityofsemisolidAZ91Dmag

    ;nesiumalloyanditsunitisPa?s’istheshearingrateand

    ;?m}ls8?一三墨Jedd

    ;

    ;582J.Mater.Sci.nchno1.,Vo1.20No.5,2004

    ;Fig.4MicrostructureofcontinuouslxrstirredsentisolidAZ91D(580.C),(a)coolingrate:1.C/min:shearrate

    ;93.7s,(b)coolingrate:5.C/lnin:shearrate:469s.(c)coolingrate:5.C/min:shearrate:93.7s

    ;(d)coolingrate:5.C/lnin:shearrate:1874s.C

    ;itsunitiss.istheslurrysolidfraction.Empiricfornmla ;(5)describestherelationoftheapparentviscositywiththe ;shearingrateandsolidfractionatcoolingrate4.C/lnin. ;3.3Alicrostructuralcharactensticofthesemisolidslurr3

    ;TheselnisolidslurryofAZ91Dmagnesiumallovat588.C ;solidifiedfrOll1.above1iquidustempelattireunderthecontinu

    ;OUScoolingandmechanica1stirringconditionswasquenched ;inwateranditsmicrostructureisshownThelargerand ;lighterareasaretheprimarysolidgrainsandtheblackregions ;aremicroholesmainl~formedwhenthe1netallographicsam. ;pleswerequenchedfrolnthesemi.solidstate,butthemicro ;holescannotinfluencethenlicrostructura1analysis.Figure ;4(a)and(f)showthemicrostructuressolidifiedatthesame ;shearingratebutnotthesamecoolingrate.Theprimar3 ;solidgrainssolidifiedat1owcoolingratearelargerthanthat ;athighcoolingrateandtileshape.however.fornledat1ow ;coolingrateisrounderthanthatathighcoolingrate.Thisis ;becausetheprimarysolidgrainsseparatedoutgraduallycon

    ;tinuetochangetheirshapefromthedendritestonondendrites ;duringstirringandcooling.Theexolutionincludesthegrains

    ;fracture,bending,andagglomeratingandtilenondendritic ;extentisproportionaltothestirringtime.\Vhentilecool

    ;ingrateishigh.tileprimar)’dendriteshaveshortertimefor

    ;themtobeshearedandthenondendriticextentisnotenough ;comparedwiththeslow”coolingcondition,andtheprimary

    ;solidgrainsarenotrounderthanthatsolidifiedatslowcool

    ;ingrate.Figures4(b),(c)and(d)arethemicrostructures ;solidifiedatthesamecoolingratebutnotthesameshearing ;rate.Itcanbeseenthattheshapeoftheprimarysolidgrains ;isrounderandrounderwithincreasingshearingrate.This ;ismainlxrforthereasonthatthestirringforceOlltheslurry ;increaseswiththehighshearingratethatisfavorabletothe ;nondendriticevolutionoftheprimarygrains.

    ;4.Conclusions

    ;(1)TheapparentviscosityofsemisolidAZ91Dincreases

    ;withthecoolingrateincreasingwhenitiscontinuouslycooled ;andmechanicallystirred;theempiricrelationoftheapparent ;viscosit5withthecoolingrateandsolidfractionatshearing ;rate937sisshownasrJ=0.660.63e(/.0(/04.

    ;f21TheapparentviscosityofsemisolidAZ91Dincreases

    ;withtheshearingrateincreasingwhencontinuouslycooled ;andlnechanicallvstirred;theempiricrelationoftheapparent ;viscositywiththeshearingrateandsolidfractionatcooling ;rate4.C/nil11isshownas”=0.55e(fs/0/89.

    ;(3)Thecoolingrateandshearingratehaveallimpor. ;tanteffectonthemicrostructureofthesemi.solidAZ91Dal

    ;lo3slurry,anddecreasingthecoolingrateandincreasingthe ;shearingratearefavorabletothenondendriticexolutionof ;theprimarygrains

    ;.4cknou’ledgement

    ;TheresearchwassupportedbytheNational’’973’’Project

    ;FoundationofChina(No.G2000067202)andtheNational”863”

    ;PlanofChina(No.G2002AA336080).

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