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B.KOSECFAILURESOFDIESFORDIECASTINGOFALUMINIUMALLOYSReceived–Prispjelo20061223Accepted–Prihva}eno20070420PreliminaryNote–Prethodnopriop}enjeINTRODUCTIONDiecastingisthemostcostefficientandtechnicaleasymethodofcastingsophisticatedandaccuratealuminiumalloyspartsingreatscaleseriesc911c93.Aluminiumalloysdiecastingsrequirelittlemachiningpriorthefinalinstallation.Approximatelyhalfofallcastingsworldwidemadeofaluminiumalloysaremanufacturedinthiswayareusedforawiderangeofautomotivepartsandotherconsumergoodsc912c93.Thecomparisonofnineparametersofthediecastingversusstamping,forging,sandcasting,permanentmoldcastingandplasticmoldingc913c93ispresentedinTable1.Aluminiumalloysdiecastingdiesfailbecauseofanumberofdifferentandsimultaneouslyoperatingstresses.Thestressesareoftwobasickindsc914c93stressescreatedbythemanufacturingofthedieandstressesformedbytheexploitationprocess.Alongdieworkinglifeisofessentialimportancefortheeconomicalproductionofaluminiumalloysdiecastingsc914,5c93.Thereplacementofadieisexpensiveinbothmoneyandproductiontime.Themostfrequentfailuresofaluminiumalloysdiecastingdiescangenerallybedividedintofourbasicgroupsc911c93heatchecking,cornercracking,sharpradiiorsharpedgescracking,andcrackingduetowearorerosion.Itisgenerallyagreedthatoneoftheprincipalcausesofterminationofdielifeisheatchecking,whichoccursthroughaprocessofcrackinitiationandpropagationinducedbythethermalstressfatiguingofadiesurfacec9168c93.Someofthefactorsthataffectdiefailuresmaybecontrolledtosomeextentbythediecastingexpertsdesigners,manufacturersandoperatorsc919c93.Thesefactorsincludec9110c93design,materialsselection,heattreatment,finishingoperations,andhandlinganduse.TESTINGOFDIECASTINGDIESIntheframeofourinvestigationworkacomplexanalysisofatypicaldiesfordiecastingofaluminiumalloyshasbeencarriedoutc9111c93.ThewholediecastingmachineisshowninFigure1,andthefixedhalfofthetestingdiecastingdieisinFigure2.METALURGIJA4720081,515551Diecastingdiesforcastingofaluminumalloysfailbecauseofagreatnumberofdifferentandsimultaneouslyoperatingfactors.Materialselection,diedesign,andthermalstressfatiguegeneratedbythecyclicworkingprocessheatchecking,aswellastolowandinhomogeneousinitialdietemperaturecontributetothefailuresandcracksformationon/indiesfordiecastingofaluminiumalloys.Intheframeofthepresentedinvestigationworktheintensityandhomogeneityofthetemperaturefieldsontheworkingsurfaceofthetestingdiewerecheckedthroughthermographicmeasurements,andfailuresandcracksontheworkingsurfaceofthediewereanalysedwithnondestructivemetallographicexaminationmethods.Keywordsdiecasting,workingsurface,aluminiumalloy,temperaturefield,failureanalysisO{te}enjekalupazalijevanjealuminijskihlegura.Doo{te}enjakalupazalijevanjealuminijskihleguradolazizbogistodobnogutjecajabrojnihrazliitihradnihimbenika.Kodkalupazalijevanjealuminijskihleguraizbormaterijala,konstrukcijakalupa,zamornotermalnonaprezanjezbogciklikogradnogprocesateniskainehomogenapolaznatemperaturekalupadoprinoseo{te}enjuinastajanjupukotina.Uokviruovogistraivanjakontroliranjeintezitetihomogenosttemperaturnapoljanaradnojpovr{iniispitanogkalupapomo}utermografskihmjerenja,ao{te}enjaipukotinenaradnojpovr{inianaliziranisunedestruktivnimmetalografskimmetodama.Kljunerijeilijevanjeukalup,radnapovr{ina,aluminijskalegura,temperaturnopolje,analizao{te}enjaISSN05435846METABK47151552008UDC–UDK621.74.043669.71111B.Kosec,FacultyofNaturalSciencesandEngineering,UniversityofLjubljana,Ljubljana,SloveniaThehotworkdiesteelmusthaveexcellentpropertiesc9112c93.RequestedpropertiesanddamagemechanismsofthediematerialareshowninTable2c9113c93.ThetestingdiewasmanufacturedfromtheBOEHLERW300ISODISCc9114c93hotworktoolsteel,whichiswidelyusedforallkindsofhotworkingtoolsanddies.ThethermalandmechanicalpropertiesofBOEHLERW300ISODISCsteelaregivenbytheproducer.TheliquidustemperatureofcastedaluminiumalloyAlSi9Cu3isapproximatelyof593°C,andcastingtemperatureisapproximately50°Chigher,thereforethepropertiesinthetemperatureintervalfromtheambienttemperatureuptoapproximately700°Careimportantfortheanalysisofthediscussedcase.ThedensityofBOEHLERW300ISODISCsteelatambienttemperature20°Cisapproximatelyof7800kg/m3,anditdecreaseswithhighertemperature.Uptothetemperatureof700°Citdropsforabout200kg/m3.Thissteelhasarelativelylowandnearlylinearincreasingtemperaturedependentheatconductivityfrom19,2to26,3W/mK,andproportionallyconstantthermaldiffusivitythewholetimeitisapproximatelyof5c215106m2/s.Specificheatisincreasedwithhighertemperatureanditisfrom456to587J/kgK,respectively,fortheboundaryvaluesofthechosentemperaturerange.Thelinearcoefficientofelongationslowlyincreasesfrom10,7c215106/Kat20°Cto13,2c215106/Kat700°C,whilethemodulusofelasticity,withboundaryvaluesof211and168GPa,decreaseswiththeincreaseoftemperature.TEMPERATUREMEASUREMENTSANDANALYSISWhenthemeltswetsthedieactiveworkingsurfacethedieexpandsandthencontractsasthesurfacetemperatureisloweredbythediffusionofheatintothesteelbe52METALURGIJA4720081,5155B.KOSECFAILURESOFDIESFORDIECASTINGOFALUMINIUMALLOYSTable1.Comparisonofnineparametersofthediecastingandotherprocessesc913c93ComparedwithNinepointsofcomparisonStampingsForgingsSandcastingsPermanentmoldcastingsPlasticmolding1CostLowermachiningLowerfinalLowerproductionandmachiningLowerlabor,productionandmachiningGenerallyhigher2DesignflexibilityMorecomplexshapesMorecomplexshapesThinnerwallsectionspossibleThinnerwallsectionspossible,lessdraftrequiredMuchgreater3FunctionalversatilityBetterdesignspossibleMoreversatilewithlessmachiningMoreversatilewithlessmachiningMoreversatilewithlessmachiningManymoreuses4TolerancesCloserCloserCloserCloserCloser5WallthicknessGreatervariationsThinnersectionsThinnersectionsThinnersectionsThinnersectionsforthesamestrength6SurfacefinishWidervarietySmootherSmootherSmootherWidervariety7MaterialwasteLessLessLessLessLess8StrengthDependsondesignLowertensileGreaterwithsamealloyGreaterwithsamealloyMuchgreater9WeightDependsondesignLighterLighterLessLessFigure1.DiecastingmachineFigure2.Fixedhalfofthetestingdiecastingdielowthesurfaceofthediec9115c93.Thegreaterdifferencebetweenthetemperatureofthedieandthatofthehotaluminiumalloyshotintothedie,thegreaterwillbetheexpansionandcontractionofthediesurface,andsoonerthediesurfacewillshowtheeffectofheatcheckingc9116c93.Sincethestressesproducedonthediesurfaceareinverselyproportionaltothedietemperature,itisgoodpracticetokeepthediesashotasitiseconomical.Aluminiumalloysdiecastingdiesshouldbepreheatedtoapproximately240to300°C.Experiencehasshownthatbyincreasingthedieoperatingtemperaturefrom205to315°C,dieproductionmaybedoubledc9117c93.Therequiredintensityandhomogeneityoftheinitialtemperaturefieldontheworkingsurfaceofthefixeddiehalfwasexaminedwiththermographicmeasurementsc9118,19c93.Thetestingthermographicmeasurementswerecarriedoutonadieofrelativelysimplegeometryandsimplethermographsheatimageswereobtained.Incomparisonwithopticalpyrometers,whichapplicationislimitedtotheverysmallsurface,investigatedobjectisenabledbythermographiccameraFigure3.Camerafieldvisionisofabout30°horizontallyandof20°vertically.Withinthatfieldofvisionthetemperatureimageofabout30.000informationpointsontemperaturewereobtainedwiththecamera.Thegeometricresolvingpowerofsingledetailsdependsonthedistanceofcameratoobject.Ontheworkingsurfaceofthefixeddiehalfthermographicmeasurementshavebeencarriedoutinthediepreheatingperiodtotheinitialoperatingtemperatureapproximately240°Candhomogeneousthroughthewholeworkingsurfaceofthedie.CheckingtemperaturemeasurementsonthediesurfaceandcalibrationofthethermographiccamerahavebeencarriedoutusingacontactNiNiCrthermocoupleandthetemperatureof61,2°Cwasmeasuredattimeof42minTable3inthemarkedpointonthesurfaceofthefixeddiehalf.Afewsecondslaternotcalibratedthermographiccamerawiththevirtualvalueofemissivityequal1,0wascenteredtothesamepointwiththevirtualtemperatureof67,1°C.Theratiobetweenbothmeasuredtemperaturesrepresentsthevalueofemissivityofc1010,91.Theemissivityhastobedeterminedexperimentallybeforeeachmeasurement.Thermographs,showninFigure4,arejustpartsoflongercontinuousprints.Thetemperaturedistributiononworkingsurfaceofthediecastingdieisshownbythecolouronthethermographs.Blackandwhitethermographshavebeencolouredwithsixteendistinctcolours.Distincttransitionsbetweencoloursshowthedifferenceintemperature,whilethegeometricdetailsarelessclear.Foreachthermograph,thetimeofformationofimageprintisveryimportantTable3.Thefirstthermographontheleftispresentedwithextendedcolourscaletobedirectlycomparabletothesecondwhichwasdonelater,whenthesurfacetemperaturesofthepreheateddiewassignificantlyhigher.Onlythesametemperaturerangecolouredthermographscanbedirectlycompared.ThermographsleftinFigure4arepresentedforthetemperaturerangeof90to161°C,withblackuncolouredregionsbelow90°C.Rightthermographisthesameastheleftthermograph1,butitispresentedinthelowertemperaturerangebetween90and124°C.METALURGIJA4720081,515553B.KOSECFAILURESOFDIESFORDIECASTINGOFALUMINIUMALLOYSTable2.DamagemechanismsandrequestedpropertiesofdiematerialDamagemechanismRequestedpropertyHighmechanicalloadingHighhardnessSuitablefracturetoughnessHighmechanicalloadingatelevatedtemperaturesHighhothardnessHighthermalstabilityofthemicrostructureRepeatedmechanicalloadingfatigueHighhardnessHighfatigueresistanceFinemicrostructureLowcontentandsmallsizeofinternaldefectsWearAbrasionHighhardnessHighvolumefraction,optimumsizeanddistributionofhardwearresistantparticlesAdhesionHighhardnessOxidelayeratthesurfaceLowchemicalreactivitybetweentoolandworkmaterialSurfacefatigueHighhardnessHighfatigueresistanceHightemperatureHighthermalstabilityofthemicrostructureHighoxidationresistanceThermalcyclingHighthermalstabilityofthemicrostructureHighhardnessatelevatedtemperaturesHighcreepresistanceHighresistanceagainstplasticcyclingLowthermalexpansionHighoxidationresistanceFigure3.PositionofthethermographiccameraInthefoundrypraxisthepreheatingtimeisforsimilardiesmuchshorterthanitwasbyourtestsmaximallyuptotwohours.Furthermore,theflowofheatingoilwiththetemperatureapproximately250°Cwasincreasedduringourtestmeasurementafterapproximately1hourfrom30l/mininthefoundrypraxisusuallyappliedto60l/minfor100.FAILUREANALYSISThecracksappearedontheworkingsurfaceofthefixeddiehalfafterlessthanthousandshotswererevealedandidentifiedwithpenetrants.Someofthemwerealsoclearlyseenbytheuseofmagnifyingglassorevenbyvisualobservation.IntheframeofourexperimentalworkalsonondestructivemetallographicexaminationbyopticalmicroscopyOMandbyscanningelectronmicroscopySEMofpolymerreplicaswasappliedc9120c93.Readilyaccessibleconvexpartsofthefixedhalfofthetestingdiewerepolishedwithfinegradehigherthan500emerypaperanddiamondpasteandexaminedinopticalmicroscope.Polymericfoilswereusedtotakeimprintsfromthesurfaceofthepreparedspotsc9121c93.Thereplicasobtainedweresosharpthatevensmalldetailsofthesurfacee.g.microstructureconstituentscouldeasilybeobservedwithanopticalmicroscopeaswellasascanningelectronmicroscope.Highdepthoffieldcharacteristicsofscanningelectronmicroscopyresultedinasharpthreedimensionalimageoftheobservedobjectc9122c93.Naturally,concavepartsofthediesurface,wherethefirstlongcracksinitiated,werenotaccessibleformachinepolishingandmicroscopeobservation.Thecontourlinesoflettersandnumbersofanidentificationmarksarewellrounded.However,manycracksstartedfromthesesignsandtheirlengthsarewithin2054METALURGIJA4720081,5155B.KOSECFAILURESOFDIESFORDIECASTINGOFALUMINIUMALLOYSTable3.TestingcasecronologicalflowofthepreheatingprocessOperationTime/minMaximalsurfacetemperature/°CStartofpreheating0–Startofmeasurements4091Calibrationofthermographiccamera42–Openingofthedie160125Increasingofheatingoilflow90–Openingofthedie190150Openingofthedie2Endofmeasurements250161Figure4.WorkingsurfaceofthefixedpartoftestingdiecastingdieFigure1andTable3.Preheatingprocess.Thermographs.Atthebeginning1,andattheend2–initialtemperaturefieldofthediepreheatingprocessFigure5.Workingsurfaceofthetestingdiecastingdie.Surfacepitsandcracksatidentificationmarks.OM
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