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(1.AutomationResearchInstitute,BohaiUniversity,Jinzhou121013,China；2.SchoolofMaterialsScienceandEngineering,DalianUniversityofTechnology,Dalian116024,China;3.SchoolofCirilEnginering,HebeiInstituteofArchitectureCirilEnginering,Zhangjiakou075000,China)



EffectofultrasonicandelectromagneticfieldsonsolidificationstructureofhypoeutecticAl-Sialloy

FUYing1,LIUChang1,ZHANGYu-bo2,JIEJin-chuan2,LITing-jui2,XIEJun*,3

(1.AutomationResearchInstitute,BohaiUniversity,Jinzhou121013,China；2.SchoolofMaterialsScienceandEngineering,DalianUniversityofTechnology,Dalian116024,China;3.SchoolofCirilEnginering,HebeiInstituteofArchitectureCirilEnginering,Zhangjiakou075000,China)

Abstract：Themorphologyofprimaryα-Alandeutecticsiliconin4004aluminumalloyunderdiversephysicalfieldswasinvestigated.Thediversephysicalfieldsareultrasonic(UST)orelectromagnetic(EMS),andtheircompound(UE).TheresultsshowedthatthesolidificationstructuresofingotscouldberefinedbyusingUSTorEMSsolely.Inparticular,thehigh-intensityultrasonicvibrationhadasignificanteffectonrefiningeutecticsiliconphase.Whenthecompoundfieldwasimposed,boththeprimaryaluminumphaseandtheeutecticsiliconphasewererefined,therefinementeffectisnearlyuniformwiththesamplestreatedbypowerultrasonicsolely,however,therefinementzoneincompoundfieldwouldbelargerthansoleUSTorEMS.

Keywords：powerultrasonic;electromagneticstirring;hypoeutecticAl-Sialloy

IIntroduction

Al-Sialloyshavewidespreadusageinthefieldsofautomotive,aerospaceandmilitaryduetotheirexcellentcastability,highspecificstrengthandgoodwearproperties〔1〕.Inordertoimprovebothcastabilityandmechanicalpropertyofthem,thestandardpracticeisthemodificationofsolidificationmicrostructure.Inrecentyears,manymethodstomodifythesolidifiedstructureofAl-Sialloyhavebeendeveloped.Theseinvolve,seeding〔2〕,mechanicalorelectromagneticstirring〔3〕,ultrasonictreatment〔4〕,electromagneticvibration〔5〕,andsoon.Ithasbeenreportedthatultrasonichasasignificanteffectonrefiningboththeprimaryaluminumphaseandtheeutecticsiliconphase〔6〕.ThecombinationofultrasonicandrotatingelectromagneticfieldwasusedtomodifythemorphologyofprimaryAlandeutecticSiinA356alloy〔7〕.Inthepresentstudy,theeffectofdiversephysicalfields(ultrasonic,electromagneticstirring,andthecombinationoftwofields)onthemorphologyofprimaryα-AlphaseandeutecticsiliconinhypoeutecticAl-Sialloywasinvestigated.

ⅡExperimental method

Inthisstudy, 4004aluminumalloywasused.Theexperimentalsetupsconsistofultrasonicgenerator200Wand22.5KHzfrequency,temperaturemeasurementsystem,andelectromagneticstirringequipment.TheschematicdiagramispresentedinFig.1.The4004alloywasmeltedinanelectricresistancefurnace,andheatedto1023K.Afterholdingforonehour,themoltenmeltwastakenoutfromthefurnaceanddegassedbyusinghexachloroethane(1wt.%ofmoltenmelt),whenthetemperaturedecreasedto967K,itwaspouredintothepre-heatedgraphitemold.Whenthetemperatureofthemeltwas957K,itwastreatedbydiversephysicalfieldi.e.onlyUSTorEMS,thecompoundfieldandwithoutanyphysicalfieldtreatment.Finally,theingot(40mmindiameter, 60mminheight)wascutverticallyfromthecenterandusedformetallographicexaminationandSEManalysis.ThedetailedexperimentalparametersandconditionsareshowninTable1.

ⅢResults and discussion

Themicrostructuresof4004alloyingotsunderdiversephysicalfieldareshowninFig.2.Fig.2ashowsthetypicaldendritestructureofthenormalhypoeutecticAl-Sialloywithoutanytreatment,andthecoarseaciculareutecticsilicondispersesamongthesedendrites.InFig.2b,undertheeffectofelectromagneticstirring,theprimaryα-Alchangesfromdendritetoirregularfinegrain,butthethickandthefibrouseutecticsiliconalsodistributeamongirregularprimaryaluminumphases.Whenthe4004alloymeltistreatedbypowerultrasonic,thesolidificationstructurechangesfromdendriticcrystaltoglobulargrain,andtheaveragegrainsizereducesto137.1μmfrommilimeterrangewithoutanytreatment,asshowninFig.2c.Fig.2ddisplaysthesolidificationstructureofthe4004alloyunderthecompoundphysicalfieldofUSTandEMS.Itcanbeseenthatthereareglobulargrainsandirregularprimaryaluminumgrains,meanwhilethefineeutecticsiliconintersperseamongtheseprimaryα-Alphases.ThissolidificationstructureofcompoundfieldissimilartothatofUST.Owingtotheheateffectofpowerultrasonicandelectromagneticfield,forthesmallingot,thesolidificationrateincompoundfield(UE)isslowerthantheothersolephysicalfields,therefore,therefinementeffectisnotthesuperpositionofUSTandEMS.

Fig.3Averagegrainsizeofprimaryα-Alin4004alloyunderdiversephysicalfield

Fig.4EutecticSimorphologyingots

(a)without, (b)EMS, (c)UST, (d)UE

Thequantitativemetallographicanalysisofα-AlgrainsareshowninFig.3.Withoutanytreatment,theaveragegrainsizeofα-Alisabout480μm,whilewithEMS,thesizedecreasesto240μm.WhentheUSTisapplied,theaveragegrainsizedecreasesto137μm.However,withthecompoundfieldofEMSandUST,theaveragegrainsizeisonly140μm,therefinementeffectisnearlyidenticalwiththeUSTsolely.

Thegrainrefinementmechanismthatoccursunderconditionsofelectromagneticstirringcanbeexplainedasthefragmentationofthedendritescreatedbyturbulenceoftheliquid,there-meltofdendriterootsbytemperaturefluctuationsandflowofliquidarounddendritearms.Relatively,theforcedconvectioncausedbyultrasonicacousticstreamingalsocanleadtothefragmentationofthedendrites〔3〕.Theultrasonicgrainrefinementmechanismshavebeenproposedbasedoncavitation,i.e.cavitation-inducednucleation.Whenthepowerultrasonicisinjectedintothemelt,thenonlineareffectssuchascavitation,acousticstreamingwillgenerate.Ontheeffectofcavitation,therearelotsofcavitationbubblesinthebulkmelt.Lotsofnucleicanbeproducedbycavitation-inducednucleation,resultinginthegrainrefinement.Naturally,thecombinationofultrasonicandelectromagneticfieldwillincreasetheforcedflowofthemelt,therefinementeffectshouldbebetterthanthesolelyultrasonicorelectromagneticstirring.Zhang〔6〕appliedthecompoundfieldofultrasonicandrotatingelectromagneticfieldtotheA356alloyaboveliquidus(about887K)at923Kandwhenthetemperaturedecreasedto883K,thecompoundfieldwasstopped,atthesametimethetreatedmeltwaspouredintoanothergraphitecrucible.Theyshowedthatboththeprimaryaluminumandeutecticsiliconweresignificantlyrefined.However,inthisexperiment,thetreatedmeltwascoolinginthesameonegraphitemold.Whenthecompoundfieldwasappliedintothemelt,theturbulenceofthemeltincreased,inaddition,theheateffectalsoincreased,therefore,forthesmallingot(40mmindiameter, 60mminheight),therefinementeffectofcompoundfieldwasweakened.Therefore,theaveragegrainsizeoftheingottreatedbycompoundfieldisonly140μm,ratherthanthesuperpositionrefinementeffectofUSTandEMS.TheSimorphologyofthe4004alloywasobservedbySEM,asshowninFig.4.

Ingotswithoutanytreatmentexhibitcoarseeutecticsilicondispersedamongtheprimaryα-Al,aspresentedinFig.4a.Fig.4bshowsthattheingottreatedbyEMS,theinter-phasespacingofSiphaseincreasescomparingwiththeingotwithoutanytreatment.Further,theingotswithUSTdisplayfineeutecticsilicon,thesizeismuchsmallerandthemorphologychangesintoshortstickfromacoarseacicularplate-like,asshowninFig.4c.Fig.4dpresentstheeutecticsiliconmorphologyofingotstreatedbycombinationofpowerultrasonicandelectromagneticfield,thesizeandmorphologyarenearlysimilartotheingotwithUSTsolely.

Fig.5EutecticSimorphologicalanalysesof4004alloyunderdiversephysicalfields

Fig.6Three-dimensionaleutecticSimorphology

(a)without, (b)EMS, (c)UST, (d)UE

FromtheFig.5,itcanbeseenthattheaveragelengthofeutecticsiliconwithoutanytreatmentisabout40μmandtheaveragewidth8.5μm.Whilewiththeelectromagneticstirringtreatment,theeutecticsiliconisabout42μminlength,and8μminwidth.However,withtheultrasonictreatment,themorphologyofSigrainisgreatlymodified,theaveragelengthofeutecticsilicondecreasesto17.5μmandtheaveragewidth2.5μm,whilewiththecompoundfield,theaveragesizeofeutecticsiliconisabout15μminlength,and2μminwidth.

Fig.6presentsthethree-dimensionalmorphologyofeutecticsiliconusingSEMonthedeep-etchedsamples.Theeutecticstructureofthealloyundertheascastconditionisrelativelycoarsesiliconflakes,asshowninFig.6a.Fig.6bshowsthattheinter-particlespacingofSigrainsincreaseundertheeffectofEMS.Fig.6cdisplaysthesiliconmorphologyoftheingotwithUST.Thesiliconphasechangestotheshortplatefromthecoarseplate-likeform.Thesimilarresultsalsopresentinthesampletreatedbycompoundfield,asshowninFig. 6d.Themovementofliquidaroundgrowingsiliconflakes,resultingfromtheforcedflowinducedbendingstresscausesfractureinthesilicon,whichhasaverylittlestrength〔7〕.Thebrokensiliconflakes“swim”tootherpartsoftheliquid,duringthistravel,becomegraduallythickenedandbroadened,Fig. 4bandFig.6b.Andtheinter-phasespacingofSiphaseincreases.ThereasonisthattheforcedconvectioncausedbyEMSenablesthediffusiondistanceofsoluteatomstoincrease,thusthecrosswisetransferabilityincreasesresultingintherealizationoftwophasecooperationgrowthbylong-distancediffusion〔8,9〕.

ⅣConclusions

Diversephysicalfieldswereintroducedintothe4004aluminiumalloy.Theresultsshowedthat:

(1)When4004alloymeltwastreatedbyelectromagneticstirringorpowerultrasonicsolely,thesolidificationstructurechangedfromdendriticgrainstonon-dendriticgrains,fortheultrasonictreatedingot,theaveragegrainsizereducedto137.1μm.Whilefortheingotwithcompoundfield,theaveragegrainsizewasabout140μm,therefinementwasnearlyuniformthanwithpowerultrasonicsolely.Itwasconsideredthat,forthesmallingot(40mmindiameter, 60mminheight),duetotheheateffectofultrasonicandelectromagneticfield,thefurtherrefinementeffectwasweakened.However,therefinementzoneincompoundfieldwouldbelargerthansoleUSTorEMS.

(2)Themorphologyofeutecticsiliconof4004alloywasmodifiedfromacoarseacicularplate-likeformwithoutanyphysicalfieldtoafineshortstickformwithultrasonictreatment.Withthetreatmentofcompoundfield,thefinersiliconphasealsoobtained,itwasthemaincontributionofpowerultrasonic,ratherthantheelectromagneticstirring.

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超声及电磁场场对亚共晶铝硅合金凝固组织的影响

付莹1，刘畅1，张宇博2，接金川2，李廷举2，谢军3

(1. 渤海大学 自动化研究院，辽宁 锦州 121013； 2. 大连理工大学 材料学院，辽宁 大连 116024；3. 河北建筑工程学院 土木工程学院，河北 张家口 075000)

摘要：本文研究了4004铝合金在不同物理场作用下初生α-Al和共晶硅的形貌特征．不同物理场主要包括超声场、电磁场、超声和电磁复合场．实验结果表明，单独应用超声或电磁场情况下，铸锭的凝固组织都达到了细化效果．特别是功率超声场，可以细化铝合金中的共晶硅相．当施加超声和电磁复合场时，初生α-Al相和共晶硅相都得到细化，细化程度与同等条件下施加功率超声场的细化效果几乎等同，但是，复合场处理的铸锭细化区域明显得到扩大．

关键词：功率超声；电磁搅拌；亚共晶铝硅合金

收稿日期：2015-02-05.

基金项目：国家自然科学基金项目(No:51074031); 渤海大学博士启动基金项目(No:bsqd201442).

作者简介：付莹(1983-)，男，博士，讲师，主要从事真空冶金技术方面的研究.

通讯作者：5117832@163.com.

中图分类号：O242；P423

文献标志码：A

文章编号：1673-0569(2016)01-0070-05