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EDM 时间:2021-01-27 阅读:()
EDM|ARTICLEEDMEffectonSurfaceIntegrityAuthor:JerryMercerINTRODUCTION—ProtectingthesurfaceintegrityofthecavityisoneofthemostcriticalfacetsofEDMingtoday.
TheintegrityofthesurfacefinishinthecavityisdeterminedbytheformationofthethermalalteredlayerscreatedbytheEDMprocess.
TheEDMprocessinvolvesthetransferenceofacontrolledelectricaldischargebetweenanelectrodeandtheworkpiece.
Thecurrentappliedtotheworkpieceduringthisdischargemeltsandvaporizesthemetal,thereforecreatingthethermalalteredlayersofthecavity.
BeforewecanunderstandhowtheEDMprocessaffectstheintegrityofthemoldsurface,wemustfirstunderstandthevariouslayersofthecavitythatarethermallyalteredbythisprocess.
TheEDMprocesschangesnotonlythesurfaceoftheworkmetal,butthesubsurfacelayersaswell.
THERMALLYALTEREDLAYERS—ThevariouslayersaffectedbytheEDMprocesshavebeenreferredtobymanynames.
Theculminationoftheselayerswillbereferredtoasthealteredmetalzone.
AsyoucanseefromFigure1,thealteredmetalzoneiscomprisedoftwothermallyaffectedsub-layersofmaterialknowastherecastorwhitelayerandtheheat-affectedannealedlayer.
Thewhitelayeristhelayerthathasbeenheatedtothepointofamoltenstate,butnotquitehotenoughtobeejectedintothegapandflushedaway.
TheEDMprocesshasactuallyalteredthemetallurgicalstructureandcharacteristicsinthislayerasitisformedbytheunexpelledmoltenmetalbeingrapidlycooledbythedielectricfluidduringtheflushingprocessandresolidifyinginthecavity.
Thislayerdoesincludesomeexpelledparticlesthathavesolidifiedandhavebeenre-depositedonthesurfacepriortobeingflushedoutofthegap.
Thewhitelayerisdenselyinfiltratedwithcarbontothepointthatitsstructureisdistinctlydifferentthanthatofthebasematerial.
ThiscarbonenrichmentoccurswhenthehydrocarbonsoftheelectrodeanddielectricfluidbreakdownduringtheEDMprocessandimpenetrateintothewhitelayerwhilethematerialisessentiallyinitsmoltenstate.
Figure2showsabreakdownoftheelementalanalysisofabasematerialpriortobeingEDMedandthewhitelayerafterEDM.
ThecarboncontentaftertheEDMprocessismuchgreaterthanthebasematerialpriortoEDM.
Beneaththewhitelayeristheheat-affectedzone.
Thislayerisminimallyaffectedbythecarbonenrichmentofthewhitelayerandhasonlybeenheated,butnottoapointtoreachmeltingtemperature.
Atthispoint,theheat-affectedzoneretainsthemetallurgicalstructureoftheparentmaterialasthetemperatureabsorbedisnottotheleveltochangethestructure.
Belowtheheat-affectedzoneistheparentmaterialandthisareaisunaf-fectedbytheEDMprocess.
CarbonEnrichmentofWhiteLayerCSiMnCrMoFeOtherBasematerial1.
55.
0550.
30.
03.
02RemainderAfterEDM19.
0912.
252.
14Remainder0.
43Figure2RedepositedlayerWhitelayerAnnealedlayerUnaectedworkingmaterialFigure12MICROCRACKING—Amajorconcernformoldmakersistheamountofmicrocrackspresentinthemold.
AsseeninFigure3,microcrackingisextremelyprominentinthewhitelayer.
IfthislayeristoothickorisnotremovedbyfinerEDMfinishesorpolishing,theeffectsofthismicrocrackingcancauseprematurefailureofthepartinsomeapplications.
Furthermore,ithasbeenknownthattheexistenceofthesemicrocrackslowersthecorrosionandfatigueresitstanceofthematerial.
Therefore,surfaceintegrityshouldbetheprimaryconsiderationwhenevaluatingtheperformanceoftheEDMtechniqueandtheprimeobjectiveofEDMmachiningmustbetoestablishtheconditionwhichsuppressesthisformation.
ThemicrocracksproducedbytheEDMprocessaretheresultofthermalstressescreatedduringtheon-timephaseoftheEDMcycle.
ThedepthofthemicrocrackingispartiallycontrolledbytheEDMprogramandasthesparkintensityincreasessodoesthedepthofthewhitelayer.
Thisalsofacilitatesanincreaseinthenumberandsizeofmicrocrackspresentinthecavity.
ThesurfaceintegrityaffectedbytheEDMprocesscanbecontrolledbythetechnologiesoftoday'sEDMpowersupply.
Thespecificparametersthataffectthesurfaceintegrityarevoltage,amperage,on-time,anddutycycle.
Theseparameterscanbemanipulatedtooptimizeefficienciesintheroughing,semi-finishing,andfinishingstages,andcontrolthesurfaceintegrityaccordingly.
SincetheEDMdischargeproducesthewhitelayerandmicrocracking,thedepthwillbeasthick(orasthin)astheintensityofthesparkenergy.
Asthesparkenergyisreduced,suchasweseeinchangingfromaroughingconditiontoafinishingcondition,thedepthofthewhitelayer,andthecrackingwillalsobereduced.
WORKPIECECHARACTERISTICS—Thesparkintensityisnottheonlydeterminingfactoraffectingthesurfaceintegrity.
Thisisalsodependentonthethermalconductivityoftheworkmetal.
Highthermallyconductivemetalswillusuallyhaveasmallerwhitelayerandlessmicrocrackingthanalowerconductivematerial.
Thisisduetotheenergydissipationthroughoutthesurfaceofthehigherconductivematerials.
Inthiscase,wecanexpectforacopperalloymaterialtohaveathinneraffectedlayerwithlesscrackingduetoitshighthermalconductivityandductilenature.
Contrarytothis,amaterialwithalowconductivityvalue,suchastoolsteel,canbeexpectedtohaveathickeraffectedlayerwithmorecrackingbecausethesparkintensityremainsinthesparkarealongerbeforethematerialcandissipatetheenergytothesurroundingareas.
BurningcarbidecreatesanotherconcernfortheEDMerasthismaterialisverybrittleandthereforeexhibitshigherlevelsofthermalcrackingthanothermaterials.
Someconsiderthismaterialtobehighlyconductive,however,carbideiscomprisedoftungstencarbideorsiliconcarbideparticlesheldtogetherwithacobaltbinder.
ItisthisbinderthathasthehighconductivityvalueandistheareathatisaffectedbytheEDMprocessinsteadofthecarbideitself.
Thesparkenergydisintegratesthebinderandreleasesthecarbideparticlesintothegap.
Figure3.
Crosssectionillustratingmicrocracksinthewhitelayer.
3SURFACEFINISHANDINTEGRITY—Surfacefinishandintegrityaretwodifferentfacetsofthecavityquality,butbothplayanimportantpartinthecharacteristicsofthemold.
Muchasthemachineparametersaffecttheintegrityofthesub-layersofthecavity,theyalsoaffectthesurfacefinish.
Figure4showshowtheamperageandon-timeaffectthesurfacefinishduringtheEDMprocess.
Anotherimportantfactorincontrollingthesurfacefinishthatisoftenoverlookedisthetypeofelectrodematerialbeingusedintheapplication.
Intermsofthesub-surfaceintegrityofthecavity,thetypeofelec-trodehaslittleeffect.
Wheretheelectrodematerialdoesplayanimportantroleisthefinishonthecavity'ssurfaceitself.
Thesurfacefinishofthecavityisanareawherethereisopportunitytoreducemanufacturingcostsanddeliverytimewhilestillbeingabletoprovideaqualitymold.
AmoldwithafineEDMfinishcanbeusedrightoutofthetankwhileonethatneedspolishingoretchingrequiresaddedcostsastheseprocessesarecarriedout.
AfineEDMfinishisachievedbyusingahighfrequencyEDMprogramwithlowamperagesandshorton-times.
Thequalityoftheelectrodematerialgoeshand-in-handwiththeEDMprogramtoachievethedesiredresults.
TakingtheEDMprogramoutoftheequation,thecavitysurfacewillreflectthequalityoftheelectrodematerialusedtofinishthecavity.
Thematerialparticlesizeandcorrespondingporesizeplayasignificantroleinthematerial'sabilitytoproduceafinefinish.
Ifthestructureoftheelectrodematerialisnotcapableofproducingthespecifiedsurfacefinish,theEDMmachinewillcontinuetorunwithouteverreachingthedesiredsurfacefinish.
Figure5showsthesurfacefinishoftwomaterialsatthesamemachineparameters.
Thematerialwiththefinermaterialstructureprovidesthesmoothersurfacefinishandthereforewillrequirelesspolishingtoachievethedesiredfinishresults.
Anelectrodematerialwithapoormicrostructure—suchaslargeorirregularparticleshapes—willwearunevenlycausingthecavitytoalsohaveunevensurfaceareas.
Thissituationisespeciallycriticalonmulti-cavitymoldswherethesurfacefinishisrequiredtobeconsistentonallcavities.
Whenusingagraphiteelectrodematerial,strictcautionshouldbetakentoensureconsistentqualityforallelectrodes.
Becausegraphitegradesfromdifferentmanufacturerswillweardifferently,itispossiblethattheEDMmachinemaynotprovidethespecificprogrammedsurfacefinish.
ThisismostoftenseenwhenmoldsareproducedusingdifferenttypesofelectrodematerialsormakesofEDMmachines.
Figure4212502007006005004003002001000MicrosecondsOn-TimeEDMEectonSurfaceFinishToolSteel;PositivePolaritySurfaceFinishRa-inch55Amps35Amps25Amps15AmpsFigure521225501002007006005004003002001000MicrosecondsOn-TimeElectrodeMaterialEectonSurfaceFinishToolSteel;PositivePolarity@35AmpsSurfaceFinishRamicroinchSuperneUltrane4KNOWTHELIMITATIONS—Thesurfacefinishandsurfaceintegrityaretwofacetsthataffectthequalityofthemold;itisimperativethattheEDMerknowthelimitationsoftheseareas.
AlthoughmostcurrentEDMmachinesproduceveryfinefinishesusingstandardtechnologiesintegratedwithinthemachine,acommonpracticeistooverridethesetechnologiesinordertogainoptimizationoftheprocess.
Withoutknowinghowtocalculatethedepthofthewhitelayer,orhowtheelectrodematerialaffectstheEDMprocess,theattempttooptimizetheprocessmayactuallyresultindecliningperformanceorfailuretoproduceaqualitypart.
ELECTRODEMATERIAL—Toimprovethepotentialforsuccessinqualitymoldproduction,moldmakersneedtounderstandsomespecificcharacteristicsoftheelectrodematerialaswellascertainfacetsoftheEDMprocessthataffectsurfacefinishandintegrity.
Anunderstandingofthesecomponentswillsignificantlyimprovethepotentialofsuccessinproducingaqualitymold.
TherehavebeenmanyarticleswrittenaboutthevarioustypesofelectrodematerialavailableforEDM.
Themajorityofthesearticlesfocusonthemicro-structureofthematerialandhowthisstructure—alongwiththeparticlesizeofthematerial—playsapartinthefinalEDMsurfacefinish.
Whiletheimpor-tanceoftheparticlesizeandmicrostructureoftheelectrodematerialshouldnotbediscounted,thereisanothercharacteristicthatshouldbeconsidered.
ThisistheElectricalResistivity(ER)valueoftheelectrode.
Theelectricalresistivityofamaterialisthepropertythatdeterminestheresistancetotheflowofanelectricalcurrentasitpassesthroughtheelectrode.
MaterialswithlowerERvaluesarebetterconductorsandallowmoreenergytobeappliedintotheEDMcut.
Conversely,materialswithhigherERvaluesseemtoretainagreateramountoftheenergywithintheelectrodeandincreasethepotentialofoverheatingandincreasingtheoverburn.
Whilegraphiteisinherentlyresistanttothecurrentbeingappliedtotheelectrode,thevaryingporositywithinthemicrostructureplaysalargeroleintheamountofelectricalresistivityofaspecificmaterial.
Theporosityofthematerialgreatlyaffectstheelectricalresistivity,astheseporesarepocketsoftrappedairwhichhaveahighinsulatingvalue.
Forinstance,Figure6illustratesthephotomicro-graphsoftwomaterialsbeingmarketedwithinthesamegraphitematerialclassification.
Theelectricalresistivityofthematerialsample"A"is871μOhms/inchwhiletheresistivityvalueforthematerialsample"B"is605μOhms/inch.
Aspreviouslyexplained,thehigherERvaluescausethematerialtoretainmoreoftheenergyintheEDMprocess,andtherefore,couldresultinlargeroverburnatthesameparametersofthelowerERmaterial.
Materialsample"A"–871Ohms/inchMaterialsample"B"–605Ohms/inchFigure65UsingcopperastheelectrodematerialsignificantlyreducestheERvalue.
Thisisduetothefactthatthecopperisformedinamoltenstateandhaslittletonoporositytoaccountfor.
Inadditiontothis,copperisoneofthebestconductorsavailabletodaysovirtually100percentofthecurrentappliedtotheelectrodepassesthroughwithlittlebeingretained.
Whilethismaybringaslightadvantagetotheuseofcopperastheelectrodematerial,thereareotherfacetsoftheEDMprocessthatlimitthematerialsperformance.
AnotherelectrodematerialavailabletotheEDMerisacombinationofgraphiteandcopper.
Thisiscopper-impregnatedgraphite,wheremoltencopperisforcedintotheporosityofagraphitematerial.
Theadvantagetothisisnotonlyanincreaseinthestrengthofthematerial,butalowerelectricalresistivityasthecopperintheporositychangesaninsulatingporeintoaconductiveone.
Thesematerialsareveryusefulwhenworkingwithexoticmetalssuchascarbide,titanium,orcopperalloysastheworkpiece.
Figure7illustratesaphotomicrographofacopperimpregnatedgraphite.
Theelectricalresistivityofthismaterialis177μOhms/inch,muchlessthantheERvaluesofthematerialsfeaturedinFigure6.
ThelowerelectricalresistivityoftheelectrodeallowsforgreateramountsofenergytobeappliedintheEDMcutandreducestheamountretainedwithintheelectrode.
Theadvantageofthisisoftenhighermetalremovalrateswithreducedoverburn.
SPARKENERGY—Thesparkenergyisaculminationofthreefactors:voltage,amperage,andon-time.
Eachofthesethreefactorsaffectsthesurfaceintegritydifferentlyandcanbeadjustedtoalterthiseffect.
However,itmustbenotedthatalteringanyofthesefactorswillnotonlychangethesurfacefinishandintegrity,butalsocouldaffecttheoverallEDMperformance,intermsofmetalremovalandelectrodewear.
VoltageBeforethecurrentcanflowbetweentheelectrodeandtheworkpiece,theopen-gapvoltageincreasesuntilithascreatedanionizationpaththroughthedielectricfluid.
Oncethecurrentstartstoflow,thevoltagedropsuntilitstabilizesattheworkinggaplevel.
Theworkinggapvoltageisthedistancebetweentheleadingedgeoftheelectrodeandtheworkpiece,andispresetbyeithertheEDMoperatororthetechnologywithintheEDMmachine.
Ahighworkinggapvoltagesettingwillincreasethegapandallowforbetterflushingconditionswhilestabilizingthecut.
Alowerworkinggapvoltagesettingwillsqueezethegapandoftenresultsinanincreaseofmetalremoval.
Withthissetting,thesparkenergyiscompressedintoareduceddistancebetweentheelectrodeandtheworkpiece.
Whilethisdoesallowforimprovedspeeds,thiscompressionofthesparkalsoincreasesthedepthofthethermallyaffectedlayersandincreasesthealteredmetalzone.
AmperageWhendiscussingamperage,thefocusisgenerallyontheaverageampsappliedtotheelectrodeduringacompletesparkcycle.
Thereasonforthisistopreventoverpoweringanddamagingtheelectrode.
Intermsofsurfaceintegrity,theamperageconsideredisthepeakcurrent.
Thepeakcurrentisthemaximumcurrentappliedfromthepowersupplyduringeachinitiationpulse.
Forinstance,iftheprogramofanEDMcutcalledfor40ampsinaroughingconditionandranat50percentdutycycle,theaverageampsappliedthroughoutthecyclewouldberoughly20amps.
However,whenconsideringpeakcurrent,thedutycycleisnotafactorandtheamperageconsideredFigure7-Materialsample"B"-177Ohms/inch6wouldbe40ampsregardlessofthedutycycle.
Astheamperageincreases,thewhitelayerofthecavitybecomesthickerandadeeperlayerofcarbonispresentinthecavity.
Figure8illustratesthevarianceoftwoEDMcutswithdifferentamperages:5ampsappliedtotheEDMcutinthetopphotoand25ampsappliedtotheEDMcutinthebottomphoto.
On-timeTheon-timesegmentoftheEDMcycleisthelengthoftimethatthepeakcurrentisappliedtotheEDMcut.
Theon-timehasadirecteffectonthesurfacefinishofthecavity(Figure9)aslongeron-timesresultinaroughercavityfinishthanshorteron-times.
Itisprimarilytheon-timeportionoftheEDMcyclethatremovesmaterialandaffectstheintegrityofthecavity.
Amperageandvoltageinthiscasearesecond-aryandaffecttheEDMprocessthroughtheon-time.
Forthesamereason,longeron-timesresultinroughersurfacefinishesinthecavity;usinganexcessivelylongon-timealsowillresultinathickeralteredmetalzone.
Astheheatfromthesparkenergytransfersintothecavity,theunaffectedmaterialsbegintoheat,eventuallybecomemolten,andareultimatelyejectedintotheEDMgap.
Astheon-timeisincreased,thisenergyiscarriedfurtherintothematerialandchangesthepropertiesofthesub-surfacelayersunderneaththeEDMcut(seeFigure9).
DETERMININGTHEALTEREDMETALZONE—MostmanufacturersofEDMsinkerstodayhavetechnologiesdevelopedwithinthegeneratortocalculateandcontrolthedepthofthealteredmetalzone.
Thesetechnologiesshouldbeutilizedwheneverpossible,astheyaredevelopedspecificallyforthatparticularmanufacturer.
However,thetechnologieswithintheEDMsinkergenerallyallowforasafetyfactorandwillcalculateforasmallamountofstocktoremainwithinthecavitytoensuretheabilitytoachievethefinaldesiredsurfacefinish.
InordertomaximizetheefficienciesoftheEDMcut,EDMoperatorshavebeenknowntocalculatethealteredmetalzoneusingaformulatodeterminetheamountofenergyappliedtothecut.
Figure9.
Theamountofon-timedeterminesthedepthofthesparkcraters.
Longeron-timecreatesadeeperheat-affectedlayer.
50μS12μSFigure8Asmentionedearlier,energyisderivedbyacombina-tionofvoltage,amperage,andon-time,andisoftenexpressedinunitsofJoules.
Theformulaforcalculat-ingenergyisasfollows:E=V*A*μS=Joules;whereE=energy;V=voltage;A=amperageandμS=microsecondsofon-time.
IfanEDMcutweretohaveprogrammed80voltswith20ampsandanon-timeof200microseconds,thecalculationwouldbe80*20*0.
0002=0.
32J.
Inthisscenario,theenergyappliedtothecavitywouldbe0.
32Joules.
7ByfollowingthechartinFigure10,itcanbedeter-minedthatthisexamplewouldresultinanalteredmetalzonedepthofapproximately30μ(0.
0012")ifa5μelectrodematerialwasused.
Iftheelectrodematerialwascopper,thealteredmetaldepthwouldbeapproximatelytwicetheamountofthegraphiteelectrodeat60μ(0.
0023").
WhyisthisRemember,thecopperelectrodeisessentiallyapureconductorandappliesmoreenergyintheEDMcutduetoaverylowelectricalresistivityvalue.
Themoreenergythatisappliedinthecut,thedeeperthealteredmetalzonewillbe.
CONTROLLINGSPARKENERGY—Asalreadymentioned,theenergyofthesparkisdeterminedbyvoltage,amperage,andtheon-timeusedintheEDMprocess.
So,whathappensifwewanttochangeEDMparameterstooptimizetheapplicationbutwishtomaintainaconstantsparkCanwehaveanycontroloverthesparkenergyanditsimpactonthesurfaceintegrityTheansweris"yes,wecan.
"Inregardtocontrollingsparkenergy,wemustequallyconsiderthefactorsinfluencingthisandhowtheseinteracttogether.
Forexample,anEDMsinkerwillgenerateamachineprogrambasedontheinputprovidedbytheoperator.
Generally,theseprogramsprovidesufficientperformancetocompletetheEDMprocesswithintheparametersofthejob.
However,duetothemanyvariousEDMelectrodematerialsavailableonthemarket,theprogramgeneratedmayormaynotbeoptimumformaximumperfor-manceandthereforeresultinaproductionloss.
Let'sconsideranEDMprogramthatgeneratedaninitialmachinesettingof80Volts,40Amps,and100Son-time.
Usingtheformulatocalculateenergy,thisamountsto0.
32Joulesofenergyappliedtothecut.
DidyouknowthereareseveralothermachineparametersthatcanalsoapplythissameamountofenergytothecutForexample,Figure11illustratesdifferentEDMparametersthatwillgeneratethesameamountofenergyintheEDMcut.
Withthis,providedthesameelectrodematerialwasused,itisfeasiblethateachoftheseparameterswillprovidethesameEDMperformanceinregardtometalremoval,overburn,recast,andsurfacefinish.
However,theseresultscoulddifferwithelectrodematerialsofvaryingelectricalresistivity.
Usingthisinformation,machineparameterscanbeoptimizedspecifictothetypeofelectrodematerialbeingused.
Thisallowsforenhancedperformancewithouttheconcernsofjeopardizingsurfaceintegrity.
Voltage(V)808080606060Amperage(A)2040802754108On-Time(μS)2001005020010050FormulaV*A*μS=E80*20*0.
000280*40*0.
000180*80*0.
0000560*27*0.
000260*54*0.
000160*108*0.
00005TotalEnergy(J)0.
320.
320.
320.
320.
320.
32Figure11Figure1000.
050.
10.
20.
150.
250.
30.
350.
480706050403020100JoulesAlteredMetalZoneDmax(Microns)Copper10Graphite5GraphiteFORMOREINFORMATIONPleasecallyourlocaldistributortolearnwhatPOCOcandoforyou.
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comorasanappforyouriOSorAndroiddevice.
iOSDeviceAndroidDeviceCONCLUSION—Maintainingtheintegrityofthecavityisessentialtoproducingaqualitymold.
Iftheintegritywerejeopardized,thelifeofthemoldwouldbesignificantlyreduced—thereforeincreasingmanufacturingcosts.
Withanunderstandingofthefacetsaffectingtheintegrityofthecavity,theEDMerwillbeabletoopti-mizetheEDMprocesswhilemitigatingtheprobabilityofmoldfailureduetoexistingmicrocracksorflawswithinthecavity.
AstheEDMprocessisoptimized,thepotentialexistsforincreasedthroughputandreduceddeliverytimes.
TheintegrityofthesurfacefinishinthecavityisdeterminedbytheformationofthethermalalteredlayerscreatedbytheEDMprocess.
TheEDMprocessinvolvesthetransferenceofacontrolledelectricaldischargebetweenanelectrodeandtheworkpiece.
Thecurrentappliedtotheworkpieceduringthisdischargemeltsandvaporizesthemetal,thereforecreatingthethermalalteredlayersofthecavity.
BeforewecanunderstandhowtheEDMprocessaffectstheintegrityofthemoldsurface,wemustfirstunderstandthevariouslayersofthecavitythatarethermallyalteredbythisprocess.
TheEDMprocesschangesnotonlythesurfaceoftheworkmetal,butthesubsurfacelayersaswell.
THERMALLYALTEREDLAYERS—ThevariouslayersaffectedbytheEDMprocesshavebeenreferredtobymanynames.
Theculminationoftheselayerswillbereferredtoasthealteredmetalzone.
AsyoucanseefromFigure1,thealteredmetalzoneiscomprisedoftwothermallyaffectedsub-layersofmaterialknowastherecastorwhitelayerandtheheat-affectedannealedlayer.
Thewhitelayeristhelayerthathasbeenheatedtothepointofamoltenstate,butnotquitehotenoughtobeejectedintothegapandflushedaway.
TheEDMprocesshasactuallyalteredthemetallurgicalstructureandcharacteristicsinthislayerasitisformedbytheunexpelledmoltenmetalbeingrapidlycooledbythedielectricfluidduringtheflushingprocessandresolidifyinginthecavity.
Thislayerdoesincludesomeexpelledparticlesthathavesolidifiedandhavebeenre-depositedonthesurfacepriortobeingflushedoutofthegap.
Thewhitelayerisdenselyinfiltratedwithcarbontothepointthatitsstructureisdistinctlydifferentthanthatofthebasematerial.
ThiscarbonenrichmentoccurswhenthehydrocarbonsoftheelectrodeanddielectricfluidbreakdownduringtheEDMprocessandimpenetrateintothewhitelayerwhilethematerialisessentiallyinitsmoltenstate.
Figure2showsabreakdownoftheelementalanalysisofabasematerialpriortobeingEDMedandthewhitelayerafterEDM.
ThecarboncontentaftertheEDMprocessismuchgreaterthanthebasematerialpriortoEDM.
Beneaththewhitelayeristheheat-affectedzone.
Thislayerisminimallyaffectedbythecarbonenrichmentofthewhitelayerandhasonlybeenheated,butnottoapointtoreachmeltingtemperature.
Atthispoint,theheat-affectedzoneretainsthemetallurgicalstructureoftheparentmaterialasthetemperatureabsorbedisnottotheleveltochangethestructure.
Belowtheheat-affectedzoneistheparentmaterialandthisareaisunaf-fectedbytheEDMprocess.
CarbonEnrichmentofWhiteLayerCSiMnCrMoFeOtherBasematerial1.
55.
0550.
30.
03.
02RemainderAfterEDM19.
0912.
252.
14Remainder0.
43Figure2RedepositedlayerWhitelayerAnnealedlayerUnaectedworkingmaterialFigure12MICROCRACKING—Amajorconcernformoldmakersistheamountofmicrocrackspresentinthemold.
AsseeninFigure3,microcrackingisextremelyprominentinthewhitelayer.
IfthislayeristoothickorisnotremovedbyfinerEDMfinishesorpolishing,theeffectsofthismicrocrackingcancauseprematurefailureofthepartinsomeapplications.
Furthermore,ithasbeenknownthattheexistenceofthesemicrocrackslowersthecorrosionandfatigueresitstanceofthematerial.
Therefore,surfaceintegrityshouldbetheprimaryconsiderationwhenevaluatingtheperformanceoftheEDMtechniqueandtheprimeobjectiveofEDMmachiningmustbetoestablishtheconditionwhichsuppressesthisformation.
ThemicrocracksproducedbytheEDMprocessaretheresultofthermalstressescreatedduringtheon-timephaseoftheEDMcycle.
ThedepthofthemicrocrackingispartiallycontrolledbytheEDMprogramandasthesparkintensityincreasessodoesthedepthofthewhitelayer.
Thisalsofacilitatesanincreaseinthenumberandsizeofmicrocrackspresentinthecavity.
ThesurfaceintegrityaffectedbytheEDMprocesscanbecontrolledbythetechnologiesoftoday'sEDMpowersupply.
Thespecificparametersthataffectthesurfaceintegrityarevoltage,amperage,on-time,anddutycycle.
Theseparameterscanbemanipulatedtooptimizeefficienciesintheroughing,semi-finishing,andfinishingstages,andcontrolthesurfaceintegrityaccordingly.
SincetheEDMdischargeproducesthewhitelayerandmicrocracking,thedepthwillbeasthick(orasthin)astheintensityofthesparkenergy.
Asthesparkenergyisreduced,suchasweseeinchangingfromaroughingconditiontoafinishingcondition,thedepthofthewhitelayer,andthecrackingwillalsobereduced.
WORKPIECECHARACTERISTICS—Thesparkintensityisnottheonlydeterminingfactoraffectingthesurfaceintegrity.
Thisisalsodependentonthethermalconductivityoftheworkmetal.
Highthermallyconductivemetalswillusuallyhaveasmallerwhitelayerandlessmicrocrackingthanalowerconductivematerial.
Thisisduetotheenergydissipationthroughoutthesurfaceofthehigherconductivematerials.
Inthiscase,wecanexpectforacopperalloymaterialtohaveathinneraffectedlayerwithlesscrackingduetoitshighthermalconductivityandductilenature.
Contrarytothis,amaterialwithalowconductivityvalue,suchastoolsteel,canbeexpectedtohaveathickeraffectedlayerwithmorecrackingbecausethesparkintensityremainsinthesparkarealongerbeforethematerialcandissipatetheenergytothesurroundingareas.
BurningcarbidecreatesanotherconcernfortheEDMerasthismaterialisverybrittleandthereforeexhibitshigherlevelsofthermalcrackingthanothermaterials.
Someconsiderthismaterialtobehighlyconductive,however,carbideiscomprisedoftungstencarbideorsiliconcarbideparticlesheldtogetherwithacobaltbinder.
ItisthisbinderthathasthehighconductivityvalueandistheareathatisaffectedbytheEDMprocessinsteadofthecarbideitself.
Thesparkenergydisintegratesthebinderandreleasesthecarbideparticlesintothegap.
Figure3.
Crosssectionillustratingmicrocracksinthewhitelayer.
3SURFACEFINISHANDINTEGRITY—Surfacefinishandintegrityaretwodifferentfacetsofthecavityquality,butbothplayanimportantpartinthecharacteristicsofthemold.
Muchasthemachineparametersaffecttheintegrityofthesub-layersofthecavity,theyalsoaffectthesurfacefinish.
Figure4showshowtheamperageandon-timeaffectthesurfacefinishduringtheEDMprocess.
Anotherimportantfactorincontrollingthesurfacefinishthatisoftenoverlookedisthetypeofelectrodematerialbeingusedintheapplication.
Intermsofthesub-surfaceintegrityofthecavity,thetypeofelec-trodehaslittleeffect.
Wheretheelectrodematerialdoesplayanimportantroleisthefinishonthecavity'ssurfaceitself.
Thesurfacefinishofthecavityisanareawherethereisopportunitytoreducemanufacturingcostsanddeliverytimewhilestillbeingabletoprovideaqualitymold.
AmoldwithafineEDMfinishcanbeusedrightoutofthetankwhileonethatneedspolishingoretchingrequiresaddedcostsastheseprocessesarecarriedout.
AfineEDMfinishisachievedbyusingahighfrequencyEDMprogramwithlowamperagesandshorton-times.
Thequalityoftheelectrodematerialgoeshand-in-handwiththeEDMprogramtoachievethedesiredresults.
TakingtheEDMprogramoutoftheequation,thecavitysurfacewillreflectthequalityoftheelectrodematerialusedtofinishthecavity.
Thematerialparticlesizeandcorrespondingporesizeplayasignificantroleinthematerial'sabilitytoproduceafinefinish.
Ifthestructureoftheelectrodematerialisnotcapableofproducingthespecifiedsurfacefinish,theEDMmachinewillcontinuetorunwithouteverreachingthedesiredsurfacefinish.
Figure5showsthesurfacefinishoftwomaterialsatthesamemachineparameters.
Thematerialwiththefinermaterialstructureprovidesthesmoothersurfacefinishandthereforewillrequirelesspolishingtoachievethedesiredfinishresults.
Anelectrodematerialwithapoormicrostructure—suchaslargeorirregularparticleshapes—willwearunevenlycausingthecavitytoalsohaveunevensurfaceareas.
Thissituationisespeciallycriticalonmulti-cavitymoldswherethesurfacefinishisrequiredtobeconsistentonallcavities.
Whenusingagraphiteelectrodematerial,strictcautionshouldbetakentoensureconsistentqualityforallelectrodes.
Becausegraphitegradesfromdifferentmanufacturerswillweardifferently,itispossiblethattheEDMmachinemaynotprovidethespecificprogrammedsurfacefinish.
ThisismostoftenseenwhenmoldsareproducedusingdifferenttypesofelectrodematerialsormakesofEDMmachines.
Figure4212502007006005004003002001000MicrosecondsOn-TimeEDMEectonSurfaceFinishToolSteel;PositivePolaritySurfaceFinishRa-inch55Amps35Amps25Amps15AmpsFigure521225501002007006005004003002001000MicrosecondsOn-TimeElectrodeMaterialEectonSurfaceFinishToolSteel;PositivePolarity@35AmpsSurfaceFinishRamicroinchSuperneUltrane4KNOWTHELIMITATIONS—Thesurfacefinishandsurfaceintegrityaretwofacetsthataffectthequalityofthemold;itisimperativethattheEDMerknowthelimitationsoftheseareas.
AlthoughmostcurrentEDMmachinesproduceveryfinefinishesusingstandardtechnologiesintegratedwithinthemachine,acommonpracticeistooverridethesetechnologiesinordertogainoptimizationoftheprocess.
Withoutknowinghowtocalculatethedepthofthewhitelayer,orhowtheelectrodematerialaffectstheEDMprocess,theattempttooptimizetheprocessmayactuallyresultindecliningperformanceorfailuretoproduceaqualitypart.
ELECTRODEMATERIAL—Toimprovethepotentialforsuccessinqualitymoldproduction,moldmakersneedtounderstandsomespecificcharacteristicsoftheelectrodematerialaswellascertainfacetsoftheEDMprocessthataffectsurfacefinishandintegrity.
Anunderstandingofthesecomponentswillsignificantlyimprovethepotentialofsuccessinproducingaqualitymold.
TherehavebeenmanyarticleswrittenaboutthevarioustypesofelectrodematerialavailableforEDM.
Themajorityofthesearticlesfocusonthemicro-structureofthematerialandhowthisstructure—alongwiththeparticlesizeofthematerial—playsapartinthefinalEDMsurfacefinish.
Whiletheimpor-tanceoftheparticlesizeandmicrostructureoftheelectrodematerialshouldnotbediscounted,thereisanothercharacteristicthatshouldbeconsidered.
ThisistheElectricalResistivity(ER)valueoftheelectrode.
Theelectricalresistivityofamaterialisthepropertythatdeterminestheresistancetotheflowofanelectricalcurrentasitpassesthroughtheelectrode.
MaterialswithlowerERvaluesarebetterconductorsandallowmoreenergytobeappliedintotheEDMcut.
Conversely,materialswithhigherERvaluesseemtoretainagreateramountoftheenergywithintheelectrodeandincreasethepotentialofoverheatingandincreasingtheoverburn.
Whilegraphiteisinherentlyresistanttothecurrentbeingappliedtotheelectrode,thevaryingporositywithinthemicrostructureplaysalargeroleintheamountofelectricalresistivityofaspecificmaterial.
Theporosityofthematerialgreatlyaffectstheelectricalresistivity,astheseporesarepocketsoftrappedairwhichhaveahighinsulatingvalue.
Forinstance,Figure6illustratesthephotomicro-graphsoftwomaterialsbeingmarketedwithinthesamegraphitematerialclassification.
Theelectricalresistivityofthematerialsample"A"is871μOhms/inchwhiletheresistivityvalueforthematerialsample"B"is605μOhms/inch.
Aspreviouslyexplained,thehigherERvaluescausethematerialtoretainmoreoftheenergyintheEDMprocess,andtherefore,couldresultinlargeroverburnatthesameparametersofthelowerERmaterial.
Materialsample"A"–871Ohms/inchMaterialsample"B"–605Ohms/inchFigure65UsingcopperastheelectrodematerialsignificantlyreducestheERvalue.
Thisisduetothefactthatthecopperisformedinamoltenstateandhaslittletonoporositytoaccountfor.
Inadditiontothis,copperisoneofthebestconductorsavailabletodaysovirtually100percentofthecurrentappliedtotheelectrodepassesthroughwithlittlebeingretained.
Whilethismaybringaslightadvantagetotheuseofcopperastheelectrodematerial,thereareotherfacetsoftheEDMprocessthatlimitthematerialsperformance.
AnotherelectrodematerialavailabletotheEDMerisacombinationofgraphiteandcopper.
Thisiscopper-impregnatedgraphite,wheremoltencopperisforcedintotheporosityofagraphitematerial.
Theadvantagetothisisnotonlyanincreaseinthestrengthofthematerial,butalowerelectricalresistivityasthecopperintheporositychangesaninsulatingporeintoaconductiveone.
Thesematerialsareveryusefulwhenworkingwithexoticmetalssuchascarbide,titanium,orcopperalloysastheworkpiece.
Figure7illustratesaphotomicrographofacopperimpregnatedgraphite.
Theelectricalresistivityofthismaterialis177μOhms/inch,muchlessthantheERvaluesofthematerialsfeaturedinFigure6.
ThelowerelectricalresistivityoftheelectrodeallowsforgreateramountsofenergytobeappliedintheEDMcutandreducestheamountretainedwithintheelectrode.
Theadvantageofthisisoftenhighermetalremovalrateswithreducedoverburn.
SPARKENERGY—Thesparkenergyisaculminationofthreefactors:voltage,amperage,andon-time.
Eachofthesethreefactorsaffectsthesurfaceintegritydifferentlyandcanbeadjustedtoalterthiseffect.
However,itmustbenotedthatalteringanyofthesefactorswillnotonlychangethesurfacefinishandintegrity,butalsocouldaffecttheoverallEDMperformance,intermsofmetalremovalandelectrodewear.
VoltageBeforethecurrentcanflowbetweentheelectrodeandtheworkpiece,theopen-gapvoltageincreasesuntilithascreatedanionizationpaththroughthedielectricfluid.
Oncethecurrentstartstoflow,thevoltagedropsuntilitstabilizesattheworkinggaplevel.
Theworkinggapvoltageisthedistancebetweentheleadingedgeoftheelectrodeandtheworkpiece,andispresetbyeithertheEDMoperatororthetechnologywithintheEDMmachine.
Ahighworkinggapvoltagesettingwillincreasethegapandallowforbetterflushingconditionswhilestabilizingthecut.
Alowerworkinggapvoltagesettingwillsqueezethegapandoftenresultsinanincreaseofmetalremoval.
Withthissetting,thesparkenergyiscompressedintoareduceddistancebetweentheelectrodeandtheworkpiece.
Whilethisdoesallowforimprovedspeeds,thiscompressionofthesparkalsoincreasesthedepthofthethermallyaffectedlayersandincreasesthealteredmetalzone.
AmperageWhendiscussingamperage,thefocusisgenerallyontheaverageampsappliedtotheelectrodeduringacompletesparkcycle.
Thereasonforthisistopreventoverpoweringanddamagingtheelectrode.
Intermsofsurfaceintegrity,theamperageconsideredisthepeakcurrent.
Thepeakcurrentisthemaximumcurrentappliedfromthepowersupplyduringeachinitiationpulse.
Forinstance,iftheprogramofanEDMcutcalledfor40ampsinaroughingconditionandranat50percentdutycycle,theaverageampsappliedthroughoutthecyclewouldberoughly20amps.
However,whenconsideringpeakcurrent,thedutycycleisnotafactorandtheamperageconsideredFigure7-Materialsample"B"-177Ohms/inch6wouldbe40ampsregardlessofthedutycycle.
Astheamperageincreases,thewhitelayerofthecavitybecomesthickerandadeeperlayerofcarbonispresentinthecavity.
Figure8illustratesthevarianceoftwoEDMcutswithdifferentamperages:5ampsappliedtotheEDMcutinthetopphotoand25ampsappliedtotheEDMcutinthebottomphoto.
On-timeTheon-timesegmentoftheEDMcycleisthelengthoftimethatthepeakcurrentisappliedtotheEDMcut.
Theon-timehasadirecteffectonthesurfacefinishofthecavity(Figure9)aslongeron-timesresultinaroughercavityfinishthanshorteron-times.
Itisprimarilytheon-timeportionoftheEDMcyclethatremovesmaterialandaffectstheintegrityofthecavity.
Amperageandvoltageinthiscasearesecond-aryandaffecttheEDMprocessthroughtheon-time.
Forthesamereason,longeron-timesresultinroughersurfacefinishesinthecavity;usinganexcessivelylongon-timealsowillresultinathickeralteredmetalzone.
Astheheatfromthesparkenergytransfersintothecavity,theunaffectedmaterialsbegintoheat,eventuallybecomemolten,andareultimatelyejectedintotheEDMgap.
Astheon-timeisincreased,thisenergyiscarriedfurtherintothematerialandchangesthepropertiesofthesub-surfacelayersunderneaththeEDMcut(seeFigure9).
DETERMININGTHEALTEREDMETALZONE—MostmanufacturersofEDMsinkerstodayhavetechnologiesdevelopedwithinthegeneratortocalculateandcontrolthedepthofthealteredmetalzone.
Thesetechnologiesshouldbeutilizedwheneverpossible,astheyaredevelopedspecificallyforthatparticularmanufacturer.
However,thetechnologieswithintheEDMsinkergenerallyallowforasafetyfactorandwillcalculateforasmallamountofstocktoremainwithinthecavitytoensuretheabilitytoachievethefinaldesiredsurfacefinish.
InordertomaximizetheefficienciesoftheEDMcut,EDMoperatorshavebeenknowntocalculatethealteredmetalzoneusingaformulatodeterminetheamountofenergyappliedtothecut.
Figure9.
Theamountofon-timedeterminesthedepthofthesparkcraters.
Longeron-timecreatesadeeperheat-affectedlayer.
50μS12μSFigure8Asmentionedearlier,energyisderivedbyacombina-tionofvoltage,amperage,andon-time,andisoftenexpressedinunitsofJoules.
Theformulaforcalculat-ingenergyisasfollows:E=V*A*μS=Joules;whereE=energy;V=voltage;A=amperageandμS=microsecondsofon-time.
IfanEDMcutweretohaveprogrammed80voltswith20ampsandanon-timeof200microseconds,thecalculationwouldbe80*20*0.
0002=0.
32J.
Inthisscenario,theenergyappliedtothecavitywouldbe0.
32Joules.
7ByfollowingthechartinFigure10,itcanbedeter-minedthatthisexamplewouldresultinanalteredmetalzonedepthofapproximately30μ(0.
0012")ifa5μelectrodematerialwasused.
Iftheelectrodematerialwascopper,thealteredmetaldepthwouldbeapproximatelytwicetheamountofthegraphiteelectrodeat60μ(0.
0023").
WhyisthisRemember,thecopperelectrodeisessentiallyapureconductorandappliesmoreenergyintheEDMcutduetoaverylowelectricalresistivityvalue.
Themoreenergythatisappliedinthecut,thedeeperthealteredmetalzonewillbe.
CONTROLLINGSPARKENERGY—Asalreadymentioned,theenergyofthesparkisdeterminedbyvoltage,amperage,andtheon-timeusedintheEDMprocess.
So,whathappensifwewanttochangeEDMparameterstooptimizetheapplicationbutwishtomaintainaconstantsparkCanwehaveanycontroloverthesparkenergyanditsimpactonthesurfaceintegrityTheansweris"yes,wecan.
"Inregardtocontrollingsparkenergy,wemustequallyconsiderthefactorsinfluencingthisandhowtheseinteracttogether.
Forexample,anEDMsinkerwillgenerateamachineprogrambasedontheinputprovidedbytheoperator.
Generally,theseprogramsprovidesufficientperformancetocompletetheEDMprocesswithintheparametersofthejob.
However,duetothemanyvariousEDMelectrodematerialsavailableonthemarket,theprogramgeneratedmayormaynotbeoptimumformaximumperfor-manceandthereforeresultinaproductionloss.
Let'sconsideranEDMprogramthatgeneratedaninitialmachinesettingof80Volts,40Amps,and100Son-time.
Usingtheformulatocalculateenergy,thisamountsto0.
32Joulesofenergyappliedtothecut.
DidyouknowthereareseveralothermachineparametersthatcanalsoapplythissameamountofenergytothecutForexample,Figure11illustratesdifferentEDMparametersthatwillgeneratethesameamountofenergyintheEDMcut.
Withthis,providedthesameelectrodematerialwasused,itisfeasiblethateachoftheseparameterswillprovidethesameEDMperformanceinregardtometalremoval,overburn,recast,andsurfacefinish.
However,theseresultscoulddifferwithelectrodematerialsofvaryingelectricalresistivity.
Usingthisinformation,machineparameterscanbeoptimizedspecifictothetypeofelectrodematerialbeingused.
Thisallowsforenhancedperformancewithouttheconcernsofjeopardizingsurfaceintegrity.
Voltage(V)808080606060Amperage(A)2040802754108On-Time(μS)2001005020010050FormulaV*A*μS=E80*20*0.
000280*40*0.
000180*80*0.
0000560*27*0.
000260*54*0.
000160*108*0.
00005TotalEnergy(J)0.
320.
320.
320.
320.
320.
32Figure11Figure1000.
050.
10.
20.
150.
250.
30.
350.
480706050403020100JoulesAlteredMetalZoneDmax(Microns)Copper10Graphite5GraphiteFORMOREINFORMATIONPleasecallyourlocaldistributortolearnwhatPOCOcandoforyou.
Visitpoco.
comandselecttheEDMDistributorslinkforthelocationnearestyou.
TERMSANDCONDITIONSOFSALEAllpurchasesaresubjecttoPocoGraphite'sTermsandConditionsofSale.
Toviewandprintthisinformation,visitpoco.
comandselecttheTerms&Conditionslink.
POCOandotherproductnamesaretrademarksofPocoGraphite,Inc.
aslistedonentegris.
com/trademarks.
Allthird-partyproductnames,logos,andcompanynamesaretrademarksorregisteredtrademarksoftheirrespectiveowners.
Useofthemdoesnotimplyanyaffiliation,sponsorship,orendorsementbythetrademarkowner.
2010-2018PocoGraphite,Inc.
|Allrightsreserved.
|PrintedintheUSA|6207-10139ENT-1218EDMTECHNICALMANUAL—ThePOCOEDMTechnicalManualisnowavailableonlineatwww.
EDMTechMan.
comorasanappforyouriOSorAndroiddevice.
iOSDeviceAndroidDeviceCONCLUSION—Maintainingtheintegrityofthecavityisessentialtoproducingaqualitymold.
Iftheintegritywerejeopardized,thelifeofthemoldwouldbesignificantlyreduced—thereforeincreasingmanufacturingcosts.
Withanunderstandingofthefacetsaffectingtheintegrityofthecavity,theEDMerwillbeabletoopti-mizetheEDMprocesswhilemitigatingtheprobabilityofmoldfailureduetoexistingmicrocracksorflawswithinthecavity.
AstheEDMprocessisoptimized,thepotentialexistsforincreasedthroughputandreduceddeliverytimes.
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