B.KOSECFAILURESOFDIESFORDIE-CASTINGOFALUMINIUMALLOYSReceivedPrispjelo:2006-12-23AcceptedPrihvaeno:2007-04-20PreliminaryNotePrethodnopriopenjeINTRODUCTIONDie-castingisthemostcostefficientandtechnicaleasymethodofcastingsophisticatedandaccuratealu-miniumalloyspartsingreat-scaleseriesc911c93.Aluminiumalloysdie-castingsrequirelittlemachiningpriorthefi-nalinstallation.Approximatelyhalfofallcastingsworldwidemadeofaluminiumalloysaremanufacturedinthiswayareusedforawiderangeofautomotivepartsandothercon-sumergoodsc912c93.Thecomparisonofnineparametersofthedie-castingversusstamping,forging,sandcasting,permanentmoldcastingandplasticmoldingc913c93ispre-sentedinTable1.Aluminiumalloysdie-castingdiesfailbecauseofanumberofdifferentandsimultaneouslyoperatingstresses.Thestressesareoftwobasickindsc914c93:stressescreatedbythemanufacturingofthedieandstressesformedbytheexploitationprocess.Alongdieworkinglifeisofessentialimportancefortheeconomicalproductionofaluminiumalloysdie-castingsc914,5c93.Thereplacementofadieisexpensiveinbothmoneyandproductiontime.Themostfrequentfailuresofaluminiumalloysdie-castingdiescangenerallybedividedintofourbasicgroupsc911c93:heatchecking,cornercracking,sharpradiiorsharpedgescracking,andcrackingduetowearorerosion.Itisgenerallyagreedthatoneoftheprincipalcausesofterminationofdielifeisheatchecking,whichoccursthroughaprocessofcrackinitiationandpropa-gationinducedbythethermalstressfatiguingofadiesurfacec916-8c93.Someofthefactorsthataffectdiefailuresmaybecontrolledtosomeextentbythedie-castingexperts(de-signers,manufacturersandoperators)c919c93.Thesefactorsincludec9110c93:-design,-materialsselection,-heattreatment,-finishingoperations,and-handlinganduse.TESTINGOFDIE-CASTINGDIESIntheframeofourinvestigationworkacomplexanalysisofatypicaldiesfordie-castingofaluminiumalloyshasbeencarriedoutc9111c93.Thewholedie-castingmachineisshowninFigure1,andthefixedhalfofthetestingdie-castingdieisinFigure2.METALURGIJA47(2008)1,51-5551Die-castingdiesforcastingofaluminumalloysfailbecauseofagreatnumberofdifferentandsimultaneouslyoperatingfactors.Materialselection,diedesign,andthermalstressfatiguegeneratedbythecyclicworkingprocess(heatchecking),aswellastolowandinhomogeneousinitialdietemperaturecontributetothefailuresandcracksformationon/indiesfordie-castingofaluminiumalloys.Intheframeofthepresentedinvestigationworktheintensityandhomogeneityofthetemperaturefieldsontheworkingsurfaceofthetestingdiewerecheckedthroughthermographicmeasurements,andfailuresandcracksontheworkingsurfaceofthediewereanalysedwithnon-destructivemetallographicexaminationmethods.Keywords:die-casting,workingsurface,aluminiumalloy,temperaturefield,failureanalysisOteenjekalupazalijevanjealuminijskihlegura.Dooteenjakalupazalijevanjealuminijskihleguradolazizbogistodobnogutjecajabrojnihrazliitihradnihimbenika.Kodkalupazalijevanjealuminijskihleguraizbormaterijala,konstrukcijakalupa,zamornotermalnonaprezanjezbogciklikogradnogprocesateniskainehomogenapolaznatemperaturekalupadoprinoseoteenjuinastajanjupukotina.Uokviruovogistraivan-jakontroliranjeintezitetihomogenosttemperaturnapoljanaradnojpovriniispitanogkalupapomoutermo-grafskihmjerenja,aoteenjaipukotinenaradnojpovrinianaliziranisunedestruktivnimmetalografskimmetodama.Kljunerijei:lijevanjeukalup,radnapovrina,aluminijskalegura,temperaturnopolje,analizaoteenjaISSN0543-5846METABK47(1)51-55(2008)UDCUDK621.74.043:669.71=111B.Kosec,FacultyofNaturalSciencesandEngineering,UniversityofLjubljana,Ljubljana,SloveniaThehotworkdiesteelmusthaveexcellentpropertiesc9112c93.RequestedpropertiesanddamagemechanismsofthediematerialareshowninTable2c9113c93.ThetestingdiewasmanufacturedfromtheBOEHLERW300ISODISCc9114c93hotworktoolsteel,whichiswidelyusedforallkindsofhotworkingtoolsanddies.ThethermalandmechanicalpropertiesofBOEHLERW300ISODISCsteelaregivenbythepro-ducer.Theliquidustemperatureofcastedaluminiumal-loyAlSi9Cu3isapproximatelyof593C,andcastingtemperatureisapproximately50Chigher,thereforethepropertiesinthetemperatureintervalfromtheambi-enttemperatureuptoapproximately700Careimpor-tantfortheanalysisofthediscussedcase.ThedensityofBOEHLERW300ISODISCsteelatambienttemperature(20C)isapproximatelyof7800kg/m3,anditdecreaseswithhighertemperature.Uptothetemperatureof700Citdropsforabout200kg/m3.Thissteelhasarelativelylowandnearlylinearincreasingtem-peraturedependentheatconductivity(from19,2to26,3W/mK),andproportionallyconstantthermaldiffusivity(thewholetimeitisapproximatelyof5c21510-6m2/s).Spe-cificheatisincreasedwithhighertemperatureanditisfrom456to587J/kgK,respectively,fortheboundaryvaluesofthechosentemperaturerange.Thelinearcoeffi-cientofelongationslowlyincreasesfrom10,7c21510-6/K(at20C)to13,2c21510-6/K(at700C),whilethemodulusofelasticity,withboundaryvaluesof211and168GPa,de-creaseswiththeincreaseoftemperature.TEMPERATUREMEASUREMENTSANDANALYSISWhenthemeltswetsthedieactiveworkingsurfacethedieexpandsandthencontractsasthesurfacetempera-tureisloweredbythediffusionofheatintothesteelbe-52METALURGIJA47(2008)1,51-55B.KOSEC:FAILURESOFDIESFORDIE-CASTINGOFALUMINIUMALLOYSTable1.Comparisonofnineparametersofthedie-castingandotherprocessesc913c93ComparedwithNinepointsofcomparisonStampingsForgingsSandcastingsPermanentmoldca-stingsPlasticmolding1CostLowermachiningLowerfinalLowerproductionandmachiningLowerlabor,produc-tionandmachiningGenerallyhigher2DesignflexibilityMorecomplexsha-pesMorecomplexshapesThinnerwallsec-tionspossibleThinnerwallsectionspossible,lessdraftre-quiredMuchgreater3FunctionalversatilityBetterdesignspos-sibleMoreversatilewithlessmachiningMoreversatilewithlessmachiningMoreversatilewithlessmachiningManymoreuses4TolerancesCloserCloserCloserCloserCloser5WallthicknessGreatervariationsThinnersectionsThinnersectionsThinnersectionsThinnersectionsforthesamestrength6SurfacefinishWidervarietySmootherSmootherSmootherWidervariety7MaterialwasteLessLessLessLessLess8StrengthDependsondesignLowertensileGreaterwithsamealloyGreaterwithsameal-loyMuchgreater9WeightDependsondesignLighterLighterLessLessFigure1.Die-castingmachineFigure2.Fixedhalfofthetestingdie-castingdielowthesurfaceofthediec9115c93.Thegreaterdifferencebe-tweenthetemperatureofthedieandthatofthehotalu-miniumalloyshotintothedie,thegreaterwillbetheex-pansionandcontractionofthediesurface,andsoonerthediesurfacewillshowtheeffectofheatcheckingc9116c93.Sincethestressesproducedonthediesurfacearein-verselyproportionaltothedietemperature,itisgoodpracticetokeepthediesashotasitiseconomical.Alu-miniumalloysdie-castingdiesshouldbepreheatedtoapproximately240to300C.Experiencehasshownthatbyincreasingthedieoperatingtemperaturefrom205to315C,dieproductionmaybedoubledc9117c93.Therequiredintensityandhomogeneityoftheinitialtemperaturefieldontheworkingsurfaceofthefixeddiehalfwasexaminedwiththermographicmeasurementsc9118,19c93.Thetestingthermographicmeasurementswerecarriedoutonadieofrelativelysimplegeometryandsimplethermographs(heatimages)wereobtained.Incomparisonwithopticalpyrometers,whichappli-cationislimitedtotheverysmallsurface,investigatedobjectisenabledbythermographiccamera(Figure3).Camerafieldvisionisofabout30horizontallyandof20vertically.Withinthatfieldofvisionthetempera-tureimageofabout30.000informationpointsontem-peraturewereobtainedwiththecamera.Thegeometricresolvingpowerofsingledetailsdependsonthedis-tanceofcameratoobject.Ontheworkingsurfaceofthefixeddiehalfthermographicmeasurementshavebeencarriedoutinthediepreheatingperiodtotheinitialoperatingtemper-ature(approximately240Candhomogeneousthroughthewholeworkingsurfaceofthedie).Checkingtemperaturemeasurementsonthediesur-faceandcalibrationofthethermographiccamerahavebeencarriedoutusingacontactNi-NiCrthermocoupleandthetemperatureof61,2Cwasmeasuredattimeof42min(Table3)inthemarkedpointonthesurfaceofthefixeddiehalf.Afewsecondslaternotcalibratedthermographiccamera(withthevirtualvalueofemissivityequal1,0)wascenteredtothesamepointwiththevirtualtemperatureof67,1C.Theratiobe-tweenbothmeasuredtemperaturesrepresentsthevalueofemissivityofc101=0,91.Theemissivityhastobedeter-minedexperimentallybeforeeachmeasurement.Thermographs,showninFigure4,arejustpartsoflongercontinuousprints.Thetemperaturedistributiononworkingsurfaceofthedie-castingdieisshownbythecolouronthethermographs.Blackandwhitethermo-graphshavebeencolouredwithsixteendistinctcolours.Distincttransitionsbetweencoloursshowthedifferenceintemperature,whilethegeometricdetailsarelessclear.Foreachthermograph,thetimeofformationofim-ageprintisveryimportant(Table3).Thefirstthermographontheleftispresentedwithextendedcol-ourscaletobedirectlycomparabletothesecondwhichwasdonelater,whenthesurfacetemperaturesofthepreheateddiewassignificantlyhigher.Onlythesametemperaturerangecolouredthermographscanbedi-rectlycompared.Thermographs(left)inFigure4arepresentedforthetemperaturerangeof90to161C,withblack(uncol-oured)regionsbelow90C.Rightthermographisthesameastheleftthermograph(1),butitispresentedinthelowertemperaturerangebetween90and124C.METALURGIJA47(2008)1,51-5553B.KOSEC:FAILURESOFDIESFORDIE-CASTINGOFALUMINIUMALLOYSTable2.DamagemechanismsandrequestedpropertiesofdiematerialDamagemechanismRequestedpropertyHighmechanicalloadingHighhardnessSuitablefracturetoughnessHighmechanicalloadingatelevatedtemperaturesHighhothardnessHighthermalstabilityofthemicrostruc-tureRepeatedmechani-calloading(fatigue)HighhardnessHighfatigueresistanceFinemicrostructureLowcontentandsmallsizeofinternaldefectsWearAbrasionHighhardnessHighvolumefraction,optimumsizeanddistributionofhardwearresistantparti-clesAdhesionHighhardnessOxidelayeratthesurfaceLowchemicalreactivitybetweentoolandworkmaterialSurfacefatigueHighhardnessHighfatigueresistanceHightemperatureHighthermalstabilityofthemicrostruc-tureHighoxidationresistanceThermalcyclingHighthermalstabilityofthemicrostruc-tureHighhardnessatelevatedtemperaturesHighcreepresistanceHighresistanceagainstplasticcyclingLowthermalexpansionHighoxidationresistanceFigure3.PositionofthethermographiccameraInthefoundrypraxisthepreheatingtimeisforsimi-lardiesmuchshorterthanitwasbyourtests(maximallyuptotwohours).Furthermore,theflowofheatingoil(withthetemperatureapproximately250C)wasin-creasedduringourtestmeasurementafterapproxi-mately1hourfrom30l/min(inthefoundrypraxisusu-allyapplied)to60l/min(for100%).FAILUREANALYSISThecracksappearedontheworkingsurfaceofthefixeddiehalfafterlessthanthousandshotswerere-vealedandidentifiedwithpenetrants.Someofthemwerealsoclearlyseenbytheuseofmagnifyingglassorevenbyvisualobservation.Intheframeofourexperi-mentalworkalsonon-destructivemetallographicexam-inationbyopticalmicroscopy(OM)andbyscanningelectronmicroscopy(SEM)ofpolymerreplicaswasap-pliedc9120c93.Readilyaccessibleconvexpartsofthefixedhalfofthetestingdiewerepolishedwithfinegrade(higherthan500)emerypaperanddiamondpasteandexaminedinopticalmicroscope.Polymericfoilswereusedtotakeimprintsfromthesurfaceofthepreparedspotsc9121c93.Thereplicasobtainedwere