Grain-refinement-mechanism-of-Al-5C-master-alloy-i

Trans.NonferrousMet.Soc.China23(2013)31673172GrainrefinementmechanismofAl5CmasteralloyinAZ31magnesiumalloyAi-minZHANG,HaiHAO,Xing-guoZHANGSchoolofMaterialsScienceandEngineering,DalianUniversityofTechnology,Dalian116023,ChinaReceived22October2012;accepted8January2013Abstract:Al5%Cmasteralloywaspreparedbypowderinsitusynthesisprocess,anditseffectsongrainrefinementofAZ31alloyandrefiningmechanismwereinvestigated.TheresultsindicatethattheAl5Cmasteralloyconsistsof(Al)andAl4C3phases,andthesizedistributionofAl4C3particlesiscontrolledbysinteringtime.TheAl5CmasteralloycanremarkablyreducethegrainsizeofAZ31alloy,whichdecreaseswiththeincreasingadditionamountofAl5Cmasteralloywhentheadditionamountisbelow2%.TherefiningmechanismisattributedtotheformationofnewcompoundsofAlCMnparticlesbyAl4C3andMn,whichmightactasnucleatingsubstratesfor-Mggrain.Keywords:AZ31magnesiumalloy;grainrefinement;Al5Cmasteralloy;AlCMncompounds1IntroductionAsthelightestmetallicstructurematerials,magnesiumalloyshavebeenwidelyusedinautomotiveandaerospaceindustriesforsavingmass.ThecommercialMgAlalloys,suchasAZ31andAZ91alloys,arethemostcommonlyusedmagnesiumalloysinindustries.Theyaresuitableforstructuralcomponentsinairplanesandcars,suchascylinderheads,airbaghousings,steeringwheelsandseatflames[1].InordertobroadentheapplicationfieldsofMgAlalloys,itisessentialtoimprovetheirlowstrengthandpoorformability.Itiswellknownthatthegrainrefinementisanimportanttechniqueforimprovingthemechanicalpropertiesofstrengthandductility.ZristhemostpromisinggrainrefinerforMgalloyswhichdonotcontainAl;however,itcannotbeusedinMgAlalloysduetoimmediatereactionwithAlformingAl3Zrphase[2].ItissignificanttofindgrainrefinerswhichcanbeusedforMgAlalloys.ManypotentialmethodshavebeendevelopedforrefiningMgAlgrainincludingsuperheating,carboninoculation,theagitationmethods,addingmeltagents(Al4C3,AlN,SiC,TiB2)andmicro-alloyingwithsoluteelements(Sr,Ca,Ce,Y)[310].Amongthesemethods,theadditionofcarbon-containingagents(C2Cl6)isknowntobethemosteffectivemethodandofferspracticaladvantagesbecauseofthelowoperatingtemperatureandthelessfadingwithalong-timeholding[11].However,theproblemofthismethodisthereleasingofharmfulchloridegasduringrefining.Asotherpowderrefineragents(suchasAl4C3andSiC),MgCO3[12]andMnCO3[13]alsohavetheproblemofun-wettingwithmagnesiumalloysandreleasingCO2gas.Therefore,areliable,efficientandconvenientcommercialgrainrefinerhastobedeveloped.Accordingtothesuccessfulexperiencesofgrainrefinementinaluminumalloy,addingmasteralloyisanimportantmethodforitsconvenientoperationandlesspollution.TherearefewinvestigationsonthepreparationofAlCmasteralloyduetothebadwettabilitybetweengraphiteandaluminummelt.PANetal[14]preparedtheAl1Cmasteralloythroughputtingacertainamountofcarboncontainingpreformsintothealuminummelt,buttheparticlesizeoftheresultantAl4C3isfarawayfromtheoptimalsizeproposedbyQUESTEDandGREER[15].HANetal[16]fabricatedtheAl2.5Cmasteralloybypowdermetallurgy,butitseemsthatthereactionofAlandCisnotsufficientandthelamellarresultantwithalargesizeislocatedatthegrainboundariesof(Al).ItisalsohardlytoseethemorphologyoftheformationparticlesofAl4C3clearly.Therefore,itiscriticaltocontroltheparticlesizedistributionoftheresultantFoundationitem:Project(2011921065)supportedbyLiaoningBaiQianWanTalentsProgram,China;Project(DUT11ZD115)supportedbytheFundamentalResearchFundsfortheCentralUniversities,ChinaCorrespondingauthor:HaiHAO;Tel/Fax:[phone];E-mail:[e-mail]:10.1016/S1003-6326(13)62848-4Ai-minZHANG,etal/Trans.NonferrousMet.Soc.China23(2013)316731723168Al4C3.Besides,thegrainrefinementmechanismsofAl4C3arestillunclear.Inaddition,itisstillunclearwhataretheidealvolumefraction,particlesizedistributionforagivenalloycomposition.Hence,inthepresentworkAl5CmasteralloywaspreparedbypowderinsitusynthesisprocessanditseffectsongrainrefinementofAZ31magnesiumalloywereinvestigated,andtherefiningmechanismswerealsodiscussed.2ExperimentalForpreparationoftheAl5Crefiner,themixtureofaluminumpowdersandgraphitepowders(massratio19:1)wasmilledinaplanetaryballmillfor5h,andthenthemixedpowderswerepressedintoacylindricalpreformwithadiameterof69mmat400°Cfor30minunderthepressureof100MPa.Subsequently,thecylindricalpreformwassinteredat1000°Cforagivendurationinvacuumconditionandcooleddowntoroomtemperatureinthefurnace.TheXRDandSEManalyseswereemployedfortheanalysisofphaseidentificationandmicrostructuresoftheAl5Cmasteralloys.Forgrainrefinementtests,commerciallypureMg(99.9%),Al(99.9%),Zn(99.9%)andMg4.5MnmasteralloywereusedforpreparingAZ31alloyinamagnesiacruciblewith1%protectingflux.Theflux(RJ2)consistsof35%45%KCl,5%8%CaF2,40%50%MgCl2,and5%8%(NaCl+CaCl2).Al5Cmasteralloywasaddedintothemeltat770780°C,theamountofAlintheAl5CmasteralloywascarefullytakenintoconsiderationinordertoexactlycontrolAlcontentintheAZ31alloymelt.Themeltwasheldfor30minandthenpouredintoapreheatedgraphitemoldwithdimensionsofd55mm×70mm×120mm.ThepouringprocessingwasunderprotectionofCO2andSF6mixedgas.Allthegrainrefinementtestswererepeated