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Microstructures of electromagnetic casting and direct chill casting LY12 aluminum alloys

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Microstructures of electromagnetic casting and direct chill casting LY12 aluminum alloys

    Microstructures of electromagnetic casting

    and direct chill casting LY12 aluminum

    alloys

Vo1.13No.2Trans.NonferrousMet.Soc.ChinaApr.2003

    ;ArticleID:10036326(2003)02034805

    ;Microstructuresofelectromagneticcastingand

    ;directchillcastingLY12aluminumalloys?

    ;CAOZhiqiang(曹志强),JINJun-ze(金俊泽),HAOHai(郝海),JIAFei(贾非)

    ;(ResearchCenterofFoundryEngineering,DalianUniversityofTechnology,Dalian116024,China) ;Abstract:LYI2aluminumalloysmadebyelectromagneticcasting(EMC)anddirectchillcasting(DCC),werean

    ;alyzedbyopticalmicroscope,differentialscanningcalorimetry,transmissionelectronmicroscopeandX-raydiffrac

    ;tion.Itisfoundthatthesurfaceandsubsudacequalityoftheingotisimprovedlargelyduetotheabsenceofaningot

    ;mold,whichisimpossibletoachievewithconventionalDCC.Itisalsofoundthattheintenseforcedconvectioncan

    ;promotethefastsuperheatevacuationandbreakthedendritealms,leadingtothegrainmultiplicationandtheap-

    ;pearanceofafineequiaxedgrainsoverthewholecrosssection.Asaresult,thehardnessofEMCspecimensincrea

    ;sesonetimethanthatfromDCCintheas-caststate.Eventhoughafterthesolidsolutiontreatmentandtheartificial

    ;aging,theDCCingotstillcannotgetthesamehardnessasEMCones.

    ;Keywords:electromagneticcasting;continuouscasting;aluminumalloy;microstructUre;hardness ;CLCnumber:TG249.7DocumentaIe:A

    ;Electromagneticcasting(EMC)isatechnolo-

    ;gydevelopedbythecombinationofmagnetohydro

    ;dynamics(MHD)andcastingengineering[卜引.The

    ;EMCtechnologydependsontheelectromagnetic

    ;forcetopreventthemetalfromtouchingthemold.

    ;Dependingontheinteractionofeddycurrentsin

    ;ducedinthemetalandthemagneticfieldofthein

    ;ductor,theliquidmetalcolumniskeptstable[.

    ;ThecontactlesscastingofEMCmayeliminatethe

    ;liquidationbuild-upsandfeathercrystals,relieve

    ;thescalpingoperationbeforethehotrollprocess,

;whichisinevitablefordirectchillcasting[‘.The

    ;moreimportantmeritisthefineandhomogeneous ;grainsizecausedbytheelectromagneticstirringin ;themelt.Asaresult,EMChasbeenpaidgreatat

    ;tentioninChinarecently[‘.Earlyresearches

    ;concentratemoreontheinvestigationoftheelec- ;tromagneticformabilityandmagnetohrodynamic ;phenomenaofEMCtechnology[.?.thispaper

    ;aimsmainlyatthemicrostructuresbeforeandafter ;heattreatmentofEMCspecimens.

    ;2EXPERIMENTAL

    ;TheLY12aluminumalloywascastbyDCC

    ;andEMCtechniquestocontrasttheirmicrostruc

    ;tureandaginghardness;itsnominalcomposition ;wasA14.3Cu1.6Mg0.7Mn[1.Asakindof

    ;widelyappliedhardeningalloy,itisoftenusedto ;fabricatetheplanewrappingsandaeroenginevanes ;workingathightemperatures..

    ;Theexperimentalprocedurewascarriedout ;onapilotscalecaster,inwhichtriaand-error

    ;shapingexperimentswereperformedandtheopti- ;mizedtechnicalparametershavebeenobtained. ;ThebasicapparatusofEMCconsistsofade

    ;liverysystem,acastingcontrolsystem,asha.ping ;andacoolingsystem,ameltingfurnaceandapow

    ;ersupplyetc,asshowninFig.1.Themagnetic ;fieldgeneratedbyamediumfrequencyalternating

    ;currentwasusedtopinchuponthemoltenalumi

    ;RumL’.andflheavyeddycurrentwasinducedon

    ;thesurfaceofthemoltenaluminum,whichwasin ;Fig.1SchematicillustrationofEMCapparatus ;?Foendationitem:Projects(59901001,59995442)supportedbytheNationalNaturalScienceFoundationo

    fChina;project(G1998061500)

    ;supportedbythestatekeyFundamentalResearchandDevelopmentProgramofChina

    ;Receivedd-te:20020416;Acsetotheimposedcurrentoftheelec

    ;tromagneticcoil.Bythewayoftheinteractionof ;alternatingmagneticfieldandinducedcurrent,a ;bodyforce,directedtowardsthecenterofingots, ;wasgeneratedtopreventthemetalfromtouching ;themold.

    ;Thetechniqueprocedureswereasfollows: ;firstly,themetalwasmelted;secondly,theposi

    ;tionofinductor,screen,wateriacketandbottom

    ;blockwereadjiusted;thirdly,coolingsystemand ;powersupplywereturned~on;atlastthecasting ;beganinawithdrawalspeeduntilthecastingend. ;Themanufacturingconditionswere:inductorcur- ;rent4800A,heightofliquidcolumn40mm;solidi

    ;ficationfront10mmuptobottomofinductorI ;pouringtemperature710730?lflowrateof

    ;coolingwater3m?h_.lcastingspeed0.31.5

    ;mm’s_..

    ;Besidestheas-caststate.someLY12speci

    ;mensweresolidsolutiontreatedat495?insalt

    ;bathfurnacefor1handthenquenchedinicewa- ;ter.Afterthis,theywereimmediatelyagedinthe ;190?siliconoilbathforvariousholdingperiodto ;obtainthehardeningeffect.

    ;ThemetaUographicspecimenswereprepared ;bycutting,grinding,polishingandetching.The ;etchantwasKeller’8solution.Thespecimensfrom

    ;DCCandEMCingotswereexaminedandcontras

    ;tedbyopticalmicrostructure(0M)andX-raydif

    ;fraction(XRD).

    ;Hardnesstestswereperformedbeforeandaf

    ;terheat-treatmentbytheloadof60kgwiththe ;RockwellhardnesstesterofFtype.Takeanaver

    ;ageof7timesafteromittingthehighestandthe ;lowestvalues.

    ;Differentialscanningcalorimetry(DSC)and ;transmissionelectronmicroscopy(TEM)were ;usedtoinvestigatetheprecipitationkineticsof ;agedspecimens.TheDSCspecimenswereheated ;to530?from30?attherateof10??rain_..

    ;TheTEMspecimenswerepreparedbytwin-jet ;electro-polishingin30HN037OCH3OH

    ;(volumefraction)solutionat30?.

    ;3RESULTSANDDIsCUSSIoN

    ;Fig.2showsthetransversesectionalmicro

    ;structuresoftheLY12DCCandEMCspecimensin ;as-caststate.ItisclearthatthegrainsofEMCare ;muchfinerthanDCC’s,nomatteratedge,

    ;midthicknessorcenterofingots.AndinDCCcon

    ;dition,thegrainsizebecomescoarsefromthesur

    ;facetothecenterofingots.Incontrast,thegrain ;sizedecreasesfromthesurfacetothecenterofin

    ;gotsduetothedecreaseofthecoolingratecaused

    ;bytheJouleheatingatthesubsurfacezoneofthe ;ingot.

    ;However,fortheEMC,thegrainsarestill ;homogeneousandfinethantheDcC’sovertheca-

    ;tirecrosssectionduetothestrongelectromagnetic ;stirringintheliquidpoolE’.Thisintenseforced

    ;convectionpromotesafastsuperheatevacuation ;andbreaksthedendritearms,whichleadstothe ;grainmultiplication.Thesuspendednucleilocal

    ;izedinthevicinityoftheliquid-solidinterfaceare ;carriedawayanddispersedinaslightlysuper-heat

    ;edmelt.Asaresult,thecrystallizationtakesplace ;simultaneouslyinmostofthesumpandcausesthe ;appearanceofafineanduniforrnequiaxedstructure ;overtheentirecross-section~17-19]. ;XRDpatterninFig.3showsthatthemain

    ;phasesinEMCLY12alloyare,0-CuAl2andS-

    ;CuMgAI2.Theprincipalstrengtheningphasesare ;S’-CuMgAI2,and0’-CuAI2[12].Underthecondi

    ;tionofnon-equilibriumcoolingduringcasting,ele

    ;mentscopperandmagnesiumareretainedinsolid ;solutions,then,thesuper-saturatedsolidsolutions ;(SSSS)aredecomposedinthefollowingsequence ;withincreasingagingtemperature:

    ;SG.P.[I]G.P.[?]S

    ;S(CuMgAI2)

    ;SSSSG.P.[I]G.P.[1I]

    ;(CuAl2)

    ;Theoretically,astheG.P.[I](solute-rich ;clusters)andG.P.[1I](vacancy-richcluster)e- ;merge,thestrengthofaluminumalloysincreases. ;WhenthetransitionprecipitationphaseS() ;comesup,thestrengthofaluminumalloysreaches ;thepeaksataround190?.Atlasttheequilibrium

    ;phaseS(turnup,andthemeehanicalproperties ;begindropE?.

    ;Fig.4showstheaging-hardeningcurvesof ;LY12alloys.ItisfoundthatthehardnessofEMC ;specimens(HRF31.7)istwotimeshigherthan ;thatofDCCones(HRF16.0)duetothegrain ;multiplicationinas-caststate.Thestrengthin

    ;creaseswiththestrengtheningphaseprecipitation ;afterthesolutiontreatment.TheEMCspecimens ;reachesapeak(HRF48.5)ataround12h.while

    ;theDCConesgetthepeak(HRF44.4)atabout ;36hafterartificialaging.ThismeanstheEMCin

    ;gotshavemoreobviousresponsetosolutionand ;agingtreatment,comparedtoDCCones.

    ;Theprecipitationprocessofintermediatepha

    ;sesisshowninFig.5.Theexothermicpeaksata

    ;round280?and495?arerespectivelycausedby

    ;G.P.[1I]andS’()precipitates.Noendother—

    ;micbumpcorrespondingwiththeG.P.[I]zones ;emergesduringthedecomposingofalloys.The ;EMCspecimenshavehigherexothermicprecipitati

    ;onpeaksbutnarrowpeakbroadnessthanDCC ;ones.

    ;Theoretically,theG.P.[I]zonesare

    ;formedatarelativelylowtemperatureandeasier ;

    ;?350?Trans.NonferrousMet.Soc.ChinaApr.2003 ;Fig?2MicrostructuresofLY12alloyingots(d174ram)atascaststate

    ;(a)’(c)’(e)DCC,fromsurfacetocenter;(b),(d),(f)--EMC,fromsurfacetocenter

    ;Agingtime/h

    ;Fjg?3xRDpatternonspecimensof

    ;Fig.4Age-hardenincurvesofLY12alhEMCLY12alloysrxge-gotLalloys’n-dIUenlIl ;(_置一??od_p朋王

    ;

    ;

    ;

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