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Fig.1.Majorcastngqualityissuesinaluminum,grayiron,ductileiron,andsteelcastings,basedon121responsestoquestionnaire.

Castingisasolidificationprocess,whichmeansthesolidificationphenomenoncontrolsmostofthepropertiesofthecasting.Moreover,mostofthecastingdefectsoccurduringsolidification,suchasgasporosityandsolidificationshrinkage.^4

Solidificationoccursintwosteps:nucleationandcrystalgrowth.Inthenucleationstagesolidparticlesformwithintheliquid.Whentheseparticlesformtheirinternalenergyislowerthanthesurroundedliquid,whichcreatesanenergyinterfacebetweenthetwo.Theformationofthesurfaceatthisinterfacerequiresenergy,soasnucleationoccursthematerialactuallyundercools,thatisitcoolsbelowitsfreezingtemperature,becauseoftheextraenergyrequiredtoformtheinterfacesurfaces.Itthenrecalescences,orheatsbackuptoitsfreezingtemperature,forthecrystalgrowthstage.Notethatnucleationoccursonapre-existingsolidsurface,becausenotasmuchenergyisrequiredforapartialinterfacesurface,asisforacompletesphericalinterfacesurface.Thiscanbeadvantageousbecausefine-grainedcastingspossessbetterpropertiesthancoarse-grainedcastings.Afinegrainstructurecanbeinducedbygrainrefinementorinoculation,whichistheprocessofaddingimpuritiestoinducenucleation.島

Allofthenucleationsrepresentacrystal,whichgrowsastheheatoffusionisextractedfromtheliquiduntilthereisnoliquidleft.Thedirection,rate,andtypeofgrowthcanbecontrolledtomaximizethepropertiesofthecasting.Directionalsolidificationiswhenthematerialsolidifiesatoneendandproceedstosolidifytotheotherend;thisisthemostidealtypeofgraingrowthbecauseitallowsliquidmaterialtocompensateforshrinkage.^

CoreandMoldGasEvolution

Accordingtoasurveyof121productengineers,designengineers,metallurgists,laboratorypersonnel,andothersassociatedwiththedesign,production,anduseofcastings,themetalcastingindustry'sgreatestneedfoioVenpientliesineliminatingporosityincastings.Byconsensus,thisappliestoaluminum,grayiron,andductileiron,andtoalltypesofsteelcastings.ThedistributionofqualityissuesbasedonthisbroadsurveyisshowninFigure1.Concernsoverporosityratedtwiceashighasanyotherissueinthesurvey.

OneresearcheraddressingthisconcernisLeonardWinardi,aPh.D.candidateattheUniversityofAlabama-Birmingham.Hehasdevelopedthreetechniquesfordetermininggaspressureinmolds:insertingaprobeattachedtoapressuretransducerincores;immersingsamplesinmoltenmetaltovariousdepths,dependingonthecalculatedgaspressureinthemoldorcore;andobservingwhetherornotbubblesareejected,andobservinggasevolutionfromcoresduringpouringinareal-timeX-rayunit.Anyorallofthesethreetechniquesmaybeusedtodeterminepeakpressures,atleastinsimplecores.

Winardimeasuredthevolumeandrateofgasevolutionfromacoreproducedinacommercialfoundry,andcalculatedthepressureinsidethecoreasafunctionoftime(seeFig.2).Thebinderbeingusedproducedtwogas-evolutionpeakswhichresultedintwogas-pressurepeaks.Thepeakcalculatedpressurewasalmostidenticaltothepeakmeasuredpressure.Furtherimprovementsintheshapeofthepressurecurvesmaybeobtainedwhenmoreaccurateinformationonthecompositionofthegasesproducedisobtained.

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Nucleationistheextremelylocalizedbuddingofadistinctthermodynamicphase.Someexamplesofphasesthatmayformvianucleationinliquidsaregaseousbubbles,crystals,orglassyregions.Creationofliquiddropletsinsaturatedvaporisalsocharacterizedbynucleation(seeCloudcondensationnuclei).Nucleationofcrystalline,amorphous,andevenvacancyclustersinsolidmaterialsisalsoimportant,forexampletothesemiconductorindustry.Mostnucleationprocessesarephysical,ratherthanchemical,butfewexceptionsdoexist(e.g.electrochemicalnucleation).AgoodexamplewouldbethefamousDietCokeandMentoseruption.Nucleationnormallyoccursatnucleationsitesonsurfacescontactingtheliquidorvapor.Suspendedparticlesorminutebubblesalsoprovidenucleationsites.

Thisiscalledheterogeneousnucleation.Nucleationwithoutpreferentialnucleationsitesishomogeneousnucleation.Homogeneousnucleationoccursspontaneouslyandrandomly,butitrequiressuperheatingorsupercoolingofthemedium.Nucleationisinvolvedinsuchprocessesascloudseedingandininstrumentssuchasthebubblechamberandthecloudchamber.

Homogeneousnucleation

Nucleationgenerallyoccurswithmuchmoredifficultyintheinteriorofauniformsubstance,byaprocesscalledhomogeneousnucleati.oTnhecreationofanucleusimpliestheformationofaninterfaceattheboundariesofanewphase.

Heterogeneousnucleation

Heterogeneousnucleationoccursmuchmoreoftenthanhomogeneousnucleation.Itformsatpreferentialsitessuchasphaseboundariesorimpuritieslikedustandrequireslessenergytothanhomogeneousnucleation.Atsuchpreferentialsites,theeffectivesurfaceenergyislower,thusdiminishedthefreeenergybarrierandfacilitatingnucleation.

Segregationincastings

Whencastinganalloy,segregationoccurs,wherebytheconcentrationofsoluteisnotconstantthroughoutthecasting.Thiscanbecausedbyavarietyofprocesses,whichcanbeclassifiedintotwotypes:

Microsegregation;whichoccursoverdistancescomparabletothesizeofthedendritearmspacing.Thisoccursasaresultofthefirstsolidformedbeingofalowerconcentrationthanthefinalequilibriumconcentration,resultinginpartitioningoftheexcesssoluteintotheliquid,sothatsolidformedlaterhasahigherconcentration.MoreaboutmicrosegregationcanbefoundintheTLPonSolidificationofalloys.

Die-castingissimilartopermanentmoldcastingexceptthatthemetalisinjectedintothemoldunderhighpressureof10-210Mpa(1,450-30,500)psi.Thisresultsinamoreuniformpart,generallygoodsurfacefinishandgooddimensionalaccuracy,asgoodas0.2%ofcastingdimension.Formanyparts,post-machiningcanbetotallyeliminated,orverylightmachiningmayberequiredtobringdimensionstosize.

Die-castingcanbedoneusingacoldchamberorhotchamberprocess.

Aluminum,ZincandCopperalloysarethematerialspredominantlyusedindiecasting.Ontheotherhand,pureAluminumisrarelycastduetohighshrinkage,andsusceptibilitytohotcracking.ItisalloyedwithSilicon,whichincreasesmeltfluidity,reducesmachinability.Copperisanotheralloyingelement,whichincreaseshardness,reducesductility,andreducescorrosionresistance.

Aluminumiscastatatemperatureof650°C(1200°F).ItisalloyedwithSilicon9%andCopperabout3.5%toformtheAluminumAssociation380alloy(UNSA03800).Siliconincreasesthemeltfluidity,reducesmachinability,Copperincreaseshardnessandreducestheductility.BygreatlyreducingtheamountofCopper(lessthan0.6%)thechemicalresistanceisimproved;thus,AA360(UNSA03600)isformulatedforuseinmarineenvironments.Ahighsiliconalloyisusedinautomotiveenginesforcylindercastings,AA390(UNSA03900)with17%Siliconforhighwearresistance.Commonaluminumalloysfordiecastingaresummarizedasfollows:

Material

Silicon

Copper

TensileStrengthMPa(ksi)

Properties

AA380

8.5%

3.5%

324

Faireasytofill

(UNSA03800)

(47)

AA384

11%

4%

331

Easytofill

(UNSA03840)

(48)

AA386

9.5%

0.6%

317

Goodcorrosionresistanee

(UNSA03860)

(46)

AA390

17%

4.5%

283

Goodwearresistanee

(UNSA03900)

(41)

ZinccanbemadetoclosetolerancesandwiththinnerwallsthanAluminum,duetoitshighmeltfluidity.ZincisalloyedwithAluminum(4%),whichaddsstrengthandhardness.Thecastingisdoneatafairlylowtemperatureof425°C(800°F)sothepartdoesnothavetocoolmuchbeforeitcanbeejectedfromthedie.This,incombinationwiththefactthatZinccanberunusingahotchamberprocessallowsforafastfill,fastcooling(andejection)andashortcycletime.Zincalloysareusedinmakingprecisionpartssuchassprockets,gears,andconnectorhousings.

Copperalloysareusedinplumbing,electricalandmarineapplicationswherecorrosionandwearresistanceisimportant.

Minimumwallthicknessesan