Ceramic dies selection for electrical resistance sintering of metallic materials
Abstract
Processing metallic powders by electrical resistance sintering requires the use of insulating ceramics dies. Selecting the appropriate ceramic material according to the electrical, thermal and mechanical properties is a need. Dies produced with several ceramic materials have been tested during the production of cemented carbide in order to check their behaviour in the process and final product properties. Tialite/mullite, zircon/mullite, zirconium phosphate based ceramic, yttria-stabilized zirconia and sialon, in most cases with modified compositions and shaping processes in order to achieve a high density, have been tested. Dry powder processing by cold isostatic pressing and furnace sintering resulted to be the better process for dies production. The effect of die properties on the produced cemented carbide, and the behaviour and life of the die during the production have been analysed. Very smooth die surface increases the number of cycles withstood during metallic parts production, because of lower extraction stresses, as checked for sialon dies. Zirconium phosphate based dies, with low thermal conductivity, show the most densified hard metal parts surface.
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1 Ceramic dies selection for electrical resistance sintering of metallic materials F.G.Cuevas1,D.Andreouli2,J.M.Gallardo3,V.Oikonomou2,J.Cintas3,Y.Torres3,J.M. Montes3 1DepartmentofChemicalEngineering,PhysicalChemistryandMaterials Science, Escuela Técnica SuperiordeIngeniería,UniversidaddeHuelva,CampusElCarmen,Avda.TresdeMarzos/n,21071Huelva, Spain 2MIRTEC(MaterialsIndustrialResearch&TechnologyCenter),72ndkmofAthens‐LamiaNationalRoad, P.O.Box18646,34100,Chalkida,Greece 3MetallurgyandMaterialsEngineeringGroup,EscuelaTécnicaSuperiordeIngeniería,Universidadde Sevilla,CaminodelosDescubrimientos,s/n,41092Sevilla,Spain Correspondingauthor:F.G.Cuevas,e‐mail:[email protected]Tel.:+34959217448 ABSTRACT.Processingmetallicpowdersbyelectricalresistancesinteringrequiresthe useofinsulatingceramicsdies.Selectingtheappropriateceramicmaterialaccordingto theelectrical,thermalandmechanicalpropertiesisaneed.Diesproducedwithseveral ceramicmaterialshavebeentestedduringtheproductionofcementedcarbideinorder to check their behaviour in the process and final product properties. Tialite/mullite, zircon/mullite,zirconiumphosphatebasedceramic,yttria‐stabilizedzirconiaandsialon, inmostcaseswithmodifiedcompositionsandshapingprocessesinordertoachievea highdensity,havebeentested.Drypowderprocessingbycoldisostaticpressingand furnacesinteringresultedtobethebetterprocessfordiesproduction.Theeffectofdie propertiesontheproducedcementedcarbide,andthebehaviourandlifeofthedie during the production have been analysed. Very smooth die surfaceincreasesthe number of cycles withstood during metallic parts production, because of lower extractionstresses,ascheckedforsialondies.Zirconiumphosphatebaseddies,withlow thermalconductivity,showthemostdensifiedhardmetalpartssurface. KEYWORDS:ceramicmaterials;insulatingdies;electricalresistancesintering;cemented carbide 1.INTRODUCTION This is an Accepted Manuscript of an article published by Elsevier in Ceramics International Volume 45, on August 2019, available at: https://doi.org/10.1016/j.ceramint.2019.04.172 Copyright 2019 Elsevier. Licencia Creative Commons CC BY-NC-ND
2 Appropriatematerialsselectionisakeyaspectforanyengineeringapplication[1‐3]. Improvements in the properties of the selected material usually lead to significant economicsavingsduringtheuseoftheproducedparts.Thismaterialselectiondoesnot onlyapplytothefinalproducts,butalsotothedifferentmaterialsinvolvedintheir production. Thus, when the equipment necessary to obtain the final product is susceptibleofbeingaproblemssource,itsadequateselectioncanbeascriticalasthe properties of the final product itself. One such case is the fabrication of cemented carbides. Hardmetalorcementedcarbideisthenamegiventoamixtureoftungstencarbide(WC) andabindermetalascobalt(Co)[4‐5],althoughothercompositionsarealsopossible. Theresultingcomposite,withcompositionusuallytailoredtothedesiredapplication,is averyhardmaterialusedfortheproductionofcuttingtools,asdrills,sawsorinserts, neededforprocessingmetallicmaterials,orgeo‐engineering[6].Theresultingproduct shows,ingeneral,betterglobalbehaviourthanotheroptionsashigh‐speedsteels. The production of these cemented carbides is usually carried outbyacompaction processintoaspecificform,agreenpart,thatisthenfurnacesinteredtoobtainthefinal part.Duringsintering,thebinderpartiallymelts,surroundingthecarbidesthatremain insolidstate atthesinteringtemperatures.The obtainedmetalmatrix compositeis additionallygrindedandpolishedtothefinalshape[7]. A new technique for the production of cemented carbides, based on sintering the pressedpowdersmixturebyleadinganelectricalcurrentthroughit(Jouleeffect),has beenrecentlystudied[8].Thistechnique,knownaselectricalresistancesintering(ERS), isoneofthemodalitiesoftheknownasFieldAssistedSinteringTechniques[9],having thepossibilityofrestraininggraingrowthandthereforeimprovingproperties.During ERS,powdersareplacedintoanisolatingdie,andtwoconductiveelectrodes/punches acttoexertthedesiredpressuretocompactthepowders,atthe time that a high intensitycurrent,ofuptoapproximately15kA,withalowvoltageofupto10V,passes throughthepowdermass.Thesefeaturesareeasilysatisfiedbyadaptingaspotwelding machinetocarryouttheERSprocess[10,11].
3 Averyimportantfactorthataffectstheelectricalconsolidationistheselectionofthe diematerial.Thediematerialhastobegood electricalandthermalinsulator,being resistant to thermalshocks, chemically inert, andhaving good mechanicaland wear properties.Thiswillmaketheelectriccurrentpassingthroughthemetallicpowders, avoidingheatleaksandamoreefficientheatingandsinteringofthecementedcarbides. Inaddition,havingtheadequateroughnessforde‐mouldingwilldecreasewear,anda goodmechanicalresistancewillhelpresistingstresses.Thismakestheselectionofthe appropriatediematerialakeypointinthedevelopmentofthisnovelcementedcarbides processing. Requirements for the ceramic dies were set based on bibliography and previousexperienceasfollows:flexuralstrengthhigherthan50MPa,thermalexpansion lower than 4∙10‐6 1/°C, electrical resistivity higher than 107Ω∙mandrefractoriness (temperatureshigherthan1000°C). Accordingtotheliterature[12‐31],thefollowingrefractoryoxide materials with attractivepropertiescanbeconsidered:aluminiumtitanateortialite(Al2TiO5),sodium zirconium phosphate NZP based ceramics, mullite (3Al2O3∙2SiO2), aluminum oxide (Al2O3),zirconiumoxideorzirconia(ZrO2),zirconiumsilicateorzircon(ZrSiO4),cordierite (2MgO∙2Al2O3∙5SiO2),siliconnitride(Si3N4),boronnitride‐zirconiumoxidecomposites (BN‐ZrO2) and zircon‐mullite (ZrSiO4‐3Al2O3∙2SiO2). These advanced ceramics exhibit excellentpropertiesandareusedinhightemperatureandverydemandingapplications: metallurgicalindustry,glassindustry,electronics,thermalbarriercoatingsandthermal shockresistant,hightemperaturefuelcells,oxygendetectors,catalystssupports,wall flowfiltersfordieselparticulateemissioncontrolinautomotiveindustry,etc.However, alumina does not withstand thermal shocks, and cordierite has amaximumservice temperature close to 1000 °C, therefore not being considered for the current application. The materials with properties fulfilling most of the high demanding requirementsset above are mainly:modified aluminum titanate‐mullite composites, NZPceramics,mullite,zirconia,siliconnitrideandzircon‐mullitecomposites. Inthispaper, severalceramic materials havebeentestedin order to establish their benefitsanddrawbacksintheERSprocess.Someoftheseceramics have been specificallypreparedforthisapplication,whereasotheronesarecommercialceramics. Among the many possible available compositions and production techniques, an
4 importantgoalwasalwaysconsideredduringdiesproduction.Takingintoaccountthat thedurabilityoftheceramicdiesislimited,relativelycheapmaterialsandproduction techniqueswereselected.Therefore,consideringsomebasicpropertiesofcandidate ceramics,severaldifferentceramicsandproductiontechniqueshavebeentested. 2.EXPERIMENTALPROCEDUREandMATERIALS Severaldifferentceramicdies,producedfromcustomorcommercialpowders,aswell as net‐shape acquired dies, have been tested in this work. Criteria called for in the selectionofthosecandidatematerialsarelowelectricalandthermal conductivity, thermalshockresistance,highhardnessandtoughness,highdensitygivingwaytolowroughness surface finish, etc. Three of the tested powdered materials have been synthesizedinthiswork,i.e.,modifiedtialite/mullite(Al1.6Fe0.4TiO5‐3Al2O3∙2SiO2)AT/M, modifiedzircon/mullite(ZrO2‐3Al2O3∙2SiO2‐MgO)Z/M,andmodifiedsodiumzirconium phosphatebasedceramics(Ba1.22Zr4P5.56Si0.44O24‐ZnO)NZP.Commercialpowderused wasyttria‐stabilizedzirconia(ZrO2‐8mol%Y2O3)8YSZ.Inallthesecases,theuseoffine powderfractionsandofvarious sinteringagentsweretestedinordertoobtainlow porositysamples. Regardingdieshapingmethodssuitablefortheproductionoflargeandcomplicated ceramicparts,variousmulti‐stepsolidstatereactionprocessesweretestedwiththe newdevelopedandcommercialpowders,i.e.,slipcasting,extrusion,uniaxialpressing (UP)andcoldisostaticpressing(CIP). Theoptimummanufacturingprocessforeachceramicwasselectedandoptimizationof themanufacturing parameters (e.g. pressurizingload,viscosity,solidsload, sintering temperature)wascarriedout.Dieswerecharacterizedregardingtothefollowingkey properties: porosity, coefficient of thermal expansion (CTE), electrical resistivity and thermalconductivity. Finally,asbenchmarking,readytouseβ‐SiAlON(Si3N4‐Al2O3‐Y2O3)dieswereacquired andtested.
5 TheoptimumERScycle(temperatureandtime)wasusedinordertoobtainsamples withtheminimumpossibleporosityandthecombinationofthepropertiesdescribed above. 2.1.DIESPRODUCTION.Takingintoaccounttheaforementionedsetofrequiredvalues, ceramicmaterialssynthesisprocessanddieproductionprocesswerebothoptimizedfor eachmaterial.Optimizedfinalceramicswereafterwardstestedregardingtheireffect onERS.Thefollowingfourmaterialsweresynthesizedandshaped. Modifiedaluminumtitanate‐mullitecomposites:Tialitepowder(AT,aluminumtitanate) wasin‐houseproducedbysolid‐statereaction,startingfromceramic oxides, while mullite(M)wasacommercialpowder(SymuloxM72,Nabaltec,Germany).Afterwet mixing,grindinganddrying,theresultingmodifiedtialite/mullite(AT/M)isacomposite powderwithD90=29.6μmthatcanbeusedfordiesproduction. DieswereproducedbyusingCIPforshapingtheAT/Mpowders,at2000barfor1min. Aftersinteringthegreendiesat1450°Cfor3hours,theyexhibitedaporosityof6%, measuredbyArchimedesmethod.Sinteringathighertemperature,closertomaterials meltingpoint,orlongersinteringtimesleadtomicrocracking,makingthematerialmore brittle.Therefore,theabovesinteringconditionswereselectedastheoptimumforthis composite.Theadditionofsinteringagents,e.g.1wt%ZrO2or0.5%wtSiO2[23‐24],was examinedtofurtherdeceasetheporosity,althoughnoimprovementwasachieved. OptimizationofthesurfacequalityoftheproducedAT/Mceramicdieswastriedusing afinerfractionofthecompositepowder.Itwasprovedthatshapingafractionwith approximatelyhalfthevalueofD90comparedtotheabove(D90=13.05μm)resultsto smothersurfacesoftheproduceddies. ExtrusionwasalsousedforshapingAT/Mpowders.Severalcylindricalsampleswere produced,andpreliminarytestsshowedthatextrusionwassatisfactoryfortheAT/M ceramic composite. In general, extrusion of ceramic powders requires additions of binders,plasticizersandwater,inordertoproduceaplasticextrudablemass.Inthe present work, additions should be at the lowest percentage in the mix, in order to achieveverydenseparts,sinceorganicadditives(binders,plasticizers)promoteporosity
6 aftertheirremovalinthesinteringstep.Finally,itwasdecidedtofocusmainlyondry powdershapingmethods,forthedifferentmaterials. Modifiedzircon/mullitecomposites:Commercialzircon/mulliteZ/Mpowder(Symulox Z72,Nabaltec,Germany),composedofmicrocrystallineZrO2and3Al2O3∙2SiO2,hasa particle size D90 = 24.4 μm, outstanding high temperature strength, thermal shock resistanceandhighchemicalandmechanicalresistance. ThesepowderswereshapedbyCIPat3000barfor1minandsinteredat1550°Cfor2 h, attaining 80% density. Customized powders by adding various oxides in different amounts[25‐27],i.e.,zircon,aluminumoxide,zirconandaluminumoxide,mulliteand magnesiumoxide,wereproduced.Bestresultswereobtainedfor1wt%MgOaddition and1600°C,8hsintering,leadingtoaporosityreductiondownto1.5%. Modified NZP ceramics: A modified sodium zirconium phosphate NZP ceramic with similarstructuretosodiumzirconiumphosphate(NaZr2P3O12)wasalsosynthetizedina multi‐stepprocess[28].ThesenewNZPpowderswereshapedbyUPat2000barand sinteredat1400°Cfor16h.Producedpartsexhibitveryhighporositycloseto50%. Therefore,theadditionofZnOassinteringagentwasdecided[29].Oxygenvacancies appearintheNZPcrystalbecauseofZrpositionssubstitutedforZnionswithalower oxidation state. This, in turn, increases the diffusion promoting densification. The modifiedpowders,producedafterappropriatemixing,millinganddrying,resultedina sizeD90=4.1μm.Additionsof1and2wt%ZnOtotheNZPpowderwereexamined. Measuredporosityvaluesoftheproducedsamplesaftersinteringwere3.1%and2.4% respectively,revealingtheeffectiveroleofZnOforpartsdensification. FurtherdecreaseofporositywasachievedusingthemoreeffectiveCIPtechnique(2000 barfor1min),reachingaporosityof1.7%forNZPsampleswith2wt%ZnO,sinteredat 1400°Cfor16h. Stabilized zirconia powders: Regarding commercially available powders, two fully stabilizedzirconiapowders8YSZwereused:arawpowderconsistingofagglomerates (D90=19.5μm),andaspraydriedreadytopresspowderwithD90=2.9μmformore effective powder compaction and better quality of parts. Both powders were from
7 ImerysFusedMinerals,Germany.Betterresultsregardingthequalityoftheproduced dieswereobtainedwiththereadytopresspowder. The8YSZreadytopresspowderwasisostaticallypressedat2000barfor1min.After sinteringat1550°Cfor8hsamplesexhibitedporositiesof2.1‐2.3%.Furtherdecrease ofporosity,downto1.7%wasachievedaftersinteringat1600°C.Thesurfacequalityof theseceramicdieswasquitegood,withaverysmoothfinishing.Slipcastingwasalso effectiveforYSZpowder.However,wallthicknessanddimensionalaccuracyachieved withthistechniquewerenotgoodenough. Cylindrical ceramic dies of all the previous powders were producedbyCIPusinga polymericmouldandametallicmandrel,inthedesireddimensions.DieshadDin12mm, Dout17mmandvariouslengthsupto60mm(Figure1). (a)(b) (c) Figure1.DiesproducedbyCIPandsinteringfrom(a)AT/M,(b)Z/Mand(c) NZPpowders. Powdercharacteristicsandconditionsappliedduringdieproduction,aswellasthefinal porosityachieved,aresummarizedinTable1.
8 Table1.Selectedconditionsfordiesproductionfromceramicpowdersandfinal porosityobtained. Powder ParticleSize (μm) Shapingparameters Sintering Porosity (%) AT/M D90=13.05 CIP,2000bar,1min 1450°C,3h 6.0 Z/M D90=24.4 CIP,3000bar,1min 1600°C,8h 1.5 NZP D90=4.1 CIP,2000bar,1min 1400°C,16h 1.7 8YSZ D90=2.9 CIP,2000bar,1min 1600°C,8h 1.7 Finally,commercialsialontubeswiththefinaldimensionsof12mminnerdiameterwere acquired(Syalon050,InternationalSyalons,Newcastle,UnitedKingdom)tobetested andcomparedtodiesdevelopedatthelab.Thesematerialswereacquiredwithahigh quality surface finish to check the difference between machinedandnotmachined surfaces. 2.2.DIESMATERIALSPROPERTIES.BeforeERStests,thethermalshockbehaviorofthe dies, being a key property, was studied. Cylinders were tested by two methods: (1) pouringmoltenmetalatapproximately1400°Cdirectlyinthediesand,(2)quenching thediesfromahightemperature(1200°C)tocoldwater(20°C).Frombothtests,itwas revealedthat8YSZdiesdonotappropriatelywithstandseveretemperaturechanges.On theotherhand,AT/McompositesandNZPceramicsprovedtobethebesttoresistthe tests. BulksampleswerealsopreparedfrommodifiedAT/M,NZPandcommercial8YSZfor additional characterization. The results gathered in Table 2 showed that noncommercialceramicsfulfillmostofthehighdemandingrequirementsfortheceramic dies.Theyareexcellentthermalandelectricinsulators,andexhibitverylowthermal expansiononheating.From commerciallyavailablematerials,sialonseemsto bean additionalsuitablecandidate. Table2.Characterizationresultsofdieceramicmaterials.(*Basedonliterature [30‐31]) Property AT/M NZP 8YSZ Sialon Elect. Resist. (Ω∙cm) 1.4∙1010 (25°C) 6.11∙1014 (25°C) 1.9∙1013 (25°C) 1.0∙1010 (25°C)
9 Thermal Conductivity (W/mK) 2.1(25°C) 2.0(300°C) 1.1(25°C) 1.1(300°C) 1.9(25°C) 2.0(300°C) 25* (25°C) CTE(ppm/K) 2.2(25‐100°C) 3.4(25‐520°C) 1.6(25‐90°C) 1.8(25‐510°C) 8.3(25‐110°C) 9.1(25‐510°C) 3.0* (25‐1200°C) 2.3.ERSTESTINGCONDITIONS.AlltheceramicdiesweretestedinERSexperienceswith thefollowing conditions: (i) thematerialto beERSedwassubmicron WC‐6wt.% Co cementedcarbide,(ii)apressureof100MPawasappliedalloverthesinteringcycle,(iii) currentintensityof7kAand(iv)dwellingtimeof700mswereused.Meanproperties offinal12mmindiameter,approximately4mmheightcementedcarbidepartswere: density11.86g/cm3,hardness1980HV30andKIC9.7MPa∙m1/2.Thosearevaluesinthe typicalrangeforthesecementedmaterials. Themechanicalrequirementsofthedies,mainlyduetotheappliedpressure,adviseto wrapthemwithametallichoop.However,acertainlackofdimensionalprecisionmakes thisaquitecomplicatedtask.AsshowninFigure2,wrappinginasteeltubeconsisted ininsertingaleadfoilinbetweentheceramicandanexternal25mminnerdiameter steeltube,thenheatingslowlyupto450ºCandcoolingslowly, and repeating the processuptofillingthecavity.Portlandcementwasusedtoclosethetubeslowerend. Consideringdimensions,freezingshrinkageandthermalexpansioncoefficients,slight compressionstresses are producedonthe ceramic tubes.Unfortunately,sometimes cement flows up between the die and the steel tube. In such cases, lead did not completelycoverdiesmouth,leadingtostressesincreaseattheceramictubeendwhen extractingthefinishedpart.
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