JournalofUniversityofScienceandTechnologyBeqingVolume15,Number7,February2008,Page48ELSEVIERFillingabilityofsemi-solidA356aluminumalloyslurryinrheo-diecastingYuelongBail.,WeiminMao,Son&Gao”,GuoxingTang”,andJunXu1)SchoolofMaterialsScienceandEngineering.UniversityofScienceandTechnologyBeijing,Beijing100083.China2)NationalEngineeringResearchCenterforNan-ferrousMetalsComposite.GeneralResearchInstituteforNon-ferrousMetals,Beijing100088,China(Received2007-02-28)Abstracts:Theeffectsofslurrytemperature,injectionpressure,andpistonvelocityontherheo-fillingabilityofsemisolidA356al-loyswerestudiedbythereho-diecastingmethods.Theresultsshowthattheslunytemperatureofthesemi-solidA356aluminumal-loyhasanimportanteffectonthefillingability；thehighertheslurrytemperature,thebetteristhefillingability,andtheappropriateslurrytemperatureforrheo-fillingisintherangeof585-595C.Theinjectionpressurealsohasagreateffectonthefillingability,anditisappropriatetotherheo-finingwhentheinjectionpressureisintherangeof15-25MPa.ThePiStORvelocityalsohasagreateffectonthefillingability,anditisappropriatetotherheo-fillingwhenthepistonvelocityisintherangeof0.072-0.12ds.Thefillingabilityoftheslurrypreparedbylowsuperheatpouringwithweakelectromagneticstirringisverygoodandthemicrostructuraldistributionintherheo-formeddiecastingsishomogeneous,whichisadvantageoustothehighqualitydiecasting.02008UniversityofScienceandTechnologyBeijing.Allrightsreserved.Keywords:A356aluminurnalloy；rheo-diecasting；fillingability；semi-solidTheworkwasfinanciallysupportedbytheNationalNatureScienceFoundationofChinu(No.50774007),theNationalHighTech-nologyResearchandDevelopmen1Programofchina(N0.2006AA03Z115).andtheNationalKeyBasicResearchProgramofChina(N0.2006CB605203).I1.IntroductionEversincesemi-solidmetalformingwasdevelopedbyFlemingsatMITinthe1970s.ithasattractedcon-siderableattentioninoverseasanddomestic.Severalexpertshaveparticipatedinthebasicandappliedre-searchofthesetechniques.Afterdevelopingforoverthirtyyears,thesehavesuccessfullybeenappliedtomanufacturingvariouskindsofcarandaircraftparts1-31.However,atpresent,onlythethixo-formingtechniqueshavebeenusedforcommercialproductionandthecommercialproductionscaleissmallyet.Thisismainlybecausethepreparingcostofsemi-solidslunyandtheenergyconsumptionofbilletreheatingarehigher,andtherecyclingofbleedingdrops,bis-cuits,gatingsystem,andrejectedcastingsaredifficult.Whencomparedwiththixo-formingtechnologies,reho-formingtechniquesofsemi-solidmetalshaveseveraladvantages,suchasconsiderablyshorterproc-ess,easyrecyclingofthegatingsystemandwasteCorrespondingauthor:YuelongBai.E-mail:02IH)8UniversityofScienceandlechnolopyBeijing.Allrightsreserved.castings,lowproductioncost,etc.Thus,reho-formingtechniquesarenowbecomingoneofthemostimpor-tantstudysubjects4-91.Lowsuperheatpouringwithweakelectromagneticstirringforashorttime(LSPWES)isanewmethodforthepreparationofsemi-solidalloysslurries10-111,whichrequireslowenergyconsumptionascomparedwiththepreparationofsemi-solidalloysbilletsusingonlyelectromagneticstirringatahighpower12-131；andtheprocesscon-trolisconsiderablyconvenient141andtheslurrymi-crostructureisbetterthanthatpreparedbytheliquiduscastingtechnology.Therefore,LSPWEScanbeap-pliedasthekeytechnologyofanewsemi-solidreho-formingprocess15.Inthisstudy,thefillingabilityofsemi-solidA356alloyslurrypreparedbytheLSPWESmethodwasinvestigatedandtheexperi-mentalresultscanbeusefulforthedevelopmentofanadvancedreho-formingtechnology.2.ExperimentalTherawmaterialsusedintheexperimentswereAY.L.Baiefal.,Fillingabilityofsemi-solidA356aluminumalloyslurryinrheo-diecasting49commercialA356aluminumalloy.ItscompositionwasSi,7.49wt%；Mg,0.46wt%；Fe,0.16wt%；andbalanceAl.Theliquidstemperatureandthesolidstemperatureareabout615and555OC,respectively.Intheexperiments,theslurryofA356alloywasfirstpreparedbylowsuperheatpouringwithweakelectromagneticstimng,andthentheslurrywassoakedinaninductionheater.Afterthesoakingproc-ess,theslurrywasfinallydeliveredquicklyintotheinjectionchamberofthediecastingmachineandformed.Toinvestigatetherheo-diecastingfillingabilityoftheabove-preparedslurry,thediewitha1300mmx10mmx3mmspiralcavitywasused.Theschematicdia-gramandcrosssectionofthedieisshowninFig.l.Therheo-formedspiralsamplelengthmaybeusedasthestandardtoevaluatetheeffectsofslurrytempera-ture,injectionpressure,andpistonvelocityonthefillingability.Theslurrytemperaturesusedintheseexperimentswereintherangeof585-595C,thein-jectionpressureswereintherangeof5-25MPa,andthepistonvelocitieswerebetween0.024and0.072ds.Thepreheateddieandtheinjectionchambertemperaturewere120and300OC,respectively.Fig.1.Schematicdiagramofthedie(a)anditscrosssection(b)(unit:mm).Themetallographicspecimenswerecutfromthedifferentsitesofthespiralsamplesandroughlyground,finelypolished,andetchedbyanaqueoussolutionof0.501%HEThemicrostructurewasob-servedandanalyzedwithanopticalmicroscopeforanalyzingthemicrostructuraldistribution.3.Resultanddiscussion3.1.Effectofslurrytemperatureonthefillingabil-ityTheslurrytemperaturehereisthetemperatureatwhichtheslurrywasdeliveredintotheinjectionchamber.Theexperimentsindicatethattheslurrytemperaturehasagreateffectonthefillingabilityofthesemi-solidA356aluminumalloyslurry.Thefillinglengthsofrheo-formedspiralsamplesatdifferenttemperaturesareshowninFig.2,inwhichthepistonvelocityis0.072m/s.280E240E,220J=200-2180-.-2160-m-10MPa-0-15MPa-100584586588590592594596Formingtemperature/CFig.2.Effectofslurrytemperatureonthefillinglengthofthesemi-solidA356alloyslurry.ItcanbeseenfromFig.2thatthevariationtrendoffillinglengthwithincreasingslurrytemperatureissimilarwhethertheinjectionpressureis10,15,or20MPa,anditisobviousthatthefillinglengthincreaseswithincreasingslurrytemperature.Namely,thefillingabilityofthesemi-solidA356aluminumalloyslurrybecomesgraduallybetterwiththeincreasingslurrytemperature.Thisisbecausethesolidfractionandap-parentviscositydecreasewiththeincreasingslurrytemperature,andtherefore,thefillingresistanceislowerwhentheslurryflowsinthecavityandthefill-inglengthincreases.Ontheotherhand,thefreezingtimeoftheslurryresultingfromthehighertempera-turewillbelonger,andthefillingtimemaybelonger.Thus,thehighertheslurrytemperature,thelongeristhefreezingtime,andthebetteristhefillingability.However,iftheslurrytemperatureisconsiderablyhigher,theturbulentflowandthegasentrainmentmaybecaused.Therefore,theappropriateslurrytempera-turesthatensureacompletelycompactedfillingmustbeadopted.ItcanalsobeseenfromFig.2thatthefillingabilityoftheslurrypreparedbylowsuperheatpouringwithweakelectromagneticstirringisverygood.Evenwhentheslurrytemperatureandthereheateddietem-peratureare585and120C,respectively,thefillinglengthstillreachesupto100mm,whichindicatesagoodfillingabilityforthecastingswith3mmthick-nessand10mmwidth.Iftheslurrytemperaturecon-tinuouslyincreasesto595C,thefillinglength50J.Univ.Sci.TechnoLBeijing,VoLlS,No.1,Feb2008reachesuptomorethan180nunandisinarangeof180-270mm,namely,thefillinglengthwillincreaseto270nunwhentheformingtemperatureandthein-jectionpressureare595Cand20MPa,respectively.Thus,thecompletefillingforanycomplexdiecavitycanbeensuredbyappropriatelyadjustingtheslunytemperatureandtheinjectionpressure.3.2.EffectofinjectionpressureonthefillingabilityTheinjectionpressureisthestaticpressureexertedonthesemi-solidA356aluminumalloyslurryintheinjectionchamber.Itisalsoanimportantparameterintherheo-diecastingprocess,andtheeffectofinjectionpressureonthefillinglengthofthespiraldiecastingsundervariousinjectionpressureconditionsisshowninFig.3,inwhichtheslurrytemperatureis585C.5I0152025ln,jrctionpressure!MPaFig.3.Effectofinjectionpressureonthefillinglength.FromFig.3,itcanbeseenthatthefillinglengthisshorterwhentheinjectionpressureislow；however,thefillinglengthwillincreasewhentheinjectionpres-sureincreases.Forexample,whentheinjectionpres-sureis5MPaandthepistonvelocityis0.048ds.thefillinglengthisonly71mm,whichindicatesapoorfillingability.However,whentheinjectionpressureincreasesto15MPa,thefillinglengthwillreachupto110mm,wluchindicatesagoodfillingability.Moreover,iftheinjectionpressureincreasestoabove15MPa,thefillinglengthwillincreaseconsiderablyandwillreachupto120-190mm.Thevaryingtrendofthefillinglengthwiththeincreasinginjectionpres-suremaybeexplainedasfollows:whentheinjectionpressureislow,thefillingpowerisnotenoughtoovercometheflowresistanceoftheslurry,therefore,thefillinglengthisshort；iftheinjectionpressurein-creases.theslurrywillgetlargerpowertoovercometheflowresistanceoftheslurry.However,ifthein-jectionpressureisveryhighandreachesacriticalvalue,theairinthecavitycannotescapeimmediately,andthefillinglengthwilldecreasebecauseofthehigheradversepressure,andthegasentrapmentmayalsobecaused1,31；thus,ahigherpressuremustbeadoptedifgasentrapmentmustbeprevented.ItcanalsobeseenfromFig.3thatwhiletheinjec-tionpressureincreases,thefillinglengthwillincreaseconsiderablyifthepistonvelocityincreasesatthesametime.Forexample,whenthepistonvelocityis0.096ds,iftheinjectionpressureincreasesto25MPa,thefillinglengthwillreachupto240mm,thatistosay,thefillingabilityisconsiderablybetter.Themainreasonmaybethatthehigherfillingvelocityde-creasestheheatemissionoftheslurryandisfavorablefortheslurrytoflowforalongertimebeforeitfreezes,andtherefore,thefillingabilitywillbecomeconsiderablybetter.TheresultsinFig.3indicatethatiftheexhaustionofthedieisgoodandgasentrainmentcannotappear,ahigherinjectionpressureisfavorabletothefillingability,andthefillingabilityoftheslurrypreparedbylowsuperheatpouringwithweakelectromagneticstir-ringisalsoverygood.3.3.EffectofpistonvelocityonthefillingabilityThepistonvelocityisthevelocityatwhichthepistonmovesforwardwhentheslurryisfilled.Itisalsoanimportantparameterintherheo-diecastingprocess.Fig.4showstheeffectofpistonvelocityonthefillinglength,inwhichtheslurrytemperatureis585C.-mIOMPa-0-I5MPa-A-20MPaPistonvelociv/(ms)Fig.4.Effectofpistonvelocityonthefillinglength.FromFig.4,itcanbeseenthatthefillinglengthin-creaseswithincreasingpistonvelocity.Whenthein-jectionpressureis10MPaandthepistonvelocityis0.024ds,thefillinglengthisonly70mm,whichin-dicatesapoorfillingabilityunderthiscondition.Ifthepistonvelocityincreasesto0.072dsandtheinjec-tionpressureisalso10MPa,thefillinglengthin-creasesandreachesupto100mm.However,ifthepistonvelocityincreasesto0.12ds,thefillinglengthwillincreaseandwillreachupto140mm,whichindi-Y.LBuietuZ.,Fillingabilityofsemi-solidA356aluminumalloyslurryinrheo-diecasting51catesabetterfillingabilityatthistime.Withincreas-ingpistonvelocity,theslurryfillingspeedwillin-creaseandthefillingtimeinthesamediecavitywillbeshortened；theheatemissionalsodecreasessothattheslurrywillflowatahighertemperature.Asaresult,theapparentviscosityoftheslurrywillbelowerandtheflowabilitywillalsobegood；thus,thefillinglengthgraduallybecomeslongerintheend.ItcanalsobeseenfromFig.4thatiftheinjectionpressurein-creases,thefillinglengthwillconsiderablyincreasewhilethepistonvelocitywillincreasesimultaneously.Forexample,whentheinjectionpressureis20MPaandthepistonvelocityis0.024m/s,thefillinglengthisonly105mm,butifthepistonvelocityincreasesto0.12m/s,thefillinglengthwillincreaseto180mm,whichindicatesthatthisconditionhasagoodfillingability.Thus,itisfavorableforgoodfillingabilitywhentheinjectionpressureisabove15MPaandthepistonvelocityisabove0.096m/s.Thevaryingtrendofthefillinglengthwithin-creasingpistonvelocityalsoshowsthatthefillingabilityoftheslurrypreparedbylowsuperheatpouringandweakelectromagneticstirringisverygood.3.4.MicrostructuraldistributionTogettheisotropicandgoodmechanicalproperties,itisrequiredinth