10外文资料原文.docx
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10外文资料原文.docx
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10外文资料原文
Experimentalinvestigationonsurfacemicroscopiccharacteristicsinelectrochemicalmechanicalfinishing
LiZhi-yongNiuZong-wei
SchoolofMechanicalEngineering,ShanDongUniversityofTechnology,ZiBo,China
Keyword:
electrochemicalfinishing;surfacequality;microscopiccharacteristic
Abstract:
Themechanismofelectrochemicalmechanicalfinishing(ECMF)processwasinvestigated.Theexperimentalsystemwasdevelopedandelectrochemicalprocessparameterssuchasoperatingvoltage,electrolytecomponentandconcentration,electrolytetemperature,electrodegapandcurrentdensitywereevaluated.Theimportantsurfacemicroscopiccharacteristics,suchasheightandspacingcharacteristicsofsurfaceroughness,surfacewavinessandmicroscopicstructurebeforeandafterECMFwerecomparedandanalyzedindetail.TheexperimentresultsindicatethatECMFisaneffectivemachiningmethodtoreducesurfaceroughnessandpromotesurfacequalityandshapeaccuracy.
1.Introduction
Withtherapiddevelopmentofmodernindustries,thesurfacequalityofproductsbecomesmoreandmoreconcernful.Poorsurfacequalitycanincreasesurfaceirregularroughnessandcausereducedthermalandmechanicalfatiguelifeandreductioninimpactstrength,leadingtoprematurediefailure.Thesurfacequalityrelatesnearlytothefinishingmethodusedafterinitialmachining.Finishingingeneralisatediousandtime-consumingprocess.Themanualoperationsofgrindingandpolishingarelaborintensiveandpromptforhumanerror.Theothermechanicalfinishingmethodssuchasshotpeeningandsandblastingareroughandimprecise.Incomparison,electrochemicalmechanicalfinishing(ECMF)technologyisacompoundfinishingtechnologybycombiningthemechanicalactionofaconventionalgrindingprocessandtheelectrochemicalreactionofanelectrochemicalmachining(ECM)process[1,2].ThemainadvantagesofECMFare:
(1)ECMFcanimprovesurfacequalityandreducesurfaceroughnessobviously,whichcontributestotheincreaseinproductslife.
(2)ECMFisinsensitivetothestrength,hardnessandtoughnessofmetalmaterials,suchasnickel-basedsuperalloys,titaniumandmolybdenumalloys.(3)Thereisnotoollossinnormalmachiningconditions.(4)TheapplicationofpassiveelectrolyteinECMFprocessaccordswiththetendencyof“greenmanufacturing”[3~5].Therefore,ECMFhasbecomeamainmachiningmethodforproductsfinishingandbeenwidelyappliedinvariousindustries(suchasaerospace,mouldanddie,defenseandautomotiveindustries).
InthisstudythemechanismofECMFprocessandtheinfluencesofsomekeyfactorssuchaselectrodegapdistribution,passivefilmandmicroscopicelectricfielddistributionalongworkpiecesurfaceprofileontheECMFprocedurewereinvestigatedanddiscussed.Theexperimentalsystemandcontrolunitweredevelopedandelectrochemicalmachiningparameterswereevaluated.Furthermore,thecharacteristicsofsurfaceroughness,wavinesspropertyandmicroscopicappearanceofworkpiecesurfacebeforeandafterECMFalsowerecomparedandanalyzed.
2.MechanismofECMFprocess
Themicroscopicappearanceofworkpicecsurfaceconsistsofagreatdealofmicroscopiclocalpeaksandvalleysafterinitialmachining,asshowninFig.1.Thepurposeoffinishingistoeliminatetheselocalpeaksandvalleysandobtainasmoothsurface.Hence,theremovedratesoflocalpeaksshouldbegreaterthanthoseoflocalvalleys.SupposethegapofpeakApointonanodesurfaceisΔa,andthegapofvalleyBpointisΔb,andΔa<Δb,whichmeansthegapdistributionalongtheworkpiecesurfaceisnotuniform.Sotheelectrolyteelectricalresistancedistributionisquitedifferentalongtheworkpicecsurface.SupposetheelectrolyteelectricalresistancevaluesofAandBareRA1andR2respectively.
Furthermore,
thesodiumnitrate(NaNO3)aqueoussolutionisoftenchosenasthepassiveelectrolyteinECMFprocess,becauseNaNO3isagenericelectrolyteforawiderangeofmetals,readilyavailableandinexpensive.However,thepassivefilmcanformandcoveronthemachinedsurfaceoftheworkpicecinthepresenceofNaNO3solution.Thispassivefilmiscomposedofmetallichydroxideandoxideandhashigherelectricalresistance.Itcanpreventtheworkpiecefromadirectcontactwiththeelectrolyteandnormalanodicdissolutioncannotproceed.Hence,thepassivefilmcanprotecttheworkpiecesurface.ThepassivefilmofpeakAisquitethinandtransportsintoelectrolyteeasily.Whereas,thepassivefilmofvalleyBisthickeranddifficulttotransfer,whichresultsinthepassivefilmdistributionalongworkpicecsurfaceisnotuniform.SupposethepassivefilmthicknessvaluesofAandBareδAandδBrespectively,andδA<δB,sotheelectricalresistancevaluesofAandBarecalculatedas
Wherekmistheelectricalresistancecoefficientofpassivefilm.Considertheelectrolyteelectricalresistance,thetotalelectricalresistancevaluesofAandBarecalculatedas
BecauseofδA<δBandΔa<Δb,thusRA
•From(3)and(4),themetalremovedratesofAandBcanbereformulatedas
Whereηiscurrentefficiency,ωthevolumetricelectrochemicalequivalentofanodemetal,URtheoperatingvoltage,κtheelectrolyteconductivity,Δtheelectrodegap,νAthemetalremovedrateofAonanodeandνBthemetalremovedrateofBonanode.SinceiA>iB,soνA>νB.Furthermore,thesmallertheelectrodegap,thelargerthemetalremovedratewillbe.Becauseofthedifferencesintheremovedrate,thoselocalpeakscanbeleveledoffgraduallywiththeECMFproceeding.
3Experimentwork
3.1Experimentsystem.TheelectrochemicalsubsystemofECMFsystemissimilartothatoftheECMprocessexceptthattheoperationalvoltageandcurrentdensityismuchlowerthanthoseoftheECMprocess.TheECMFprocessusualoperatesintherangeofseveralvoltagesandthecurrentdensityislessthan10A/cm2.TheschematicdiagramofexperimentsystemforECMFisshowninFig.2.
Theworkpiece,7,astainlesssteelrod(d=30mm)withtitaniumalloycomponents,isanodeandthefinishinghead,1,iscathode.Twopiecesofsoftgrindingbars,werepressedtightlyontheworkpiecesurfacewithsprings.NaNO3aqueoussolutionwasusedaselectrolyte.Thecontrolsubsystemis
composedofpositionsensor,numericalcontrolunitandotherassistantapparatus.Itsfunctionistodetectandadjustthepressureofgrindingbarsinordertoretainenoughpressureagainsttheworkpiecesurface.ApumpwasusedtocirculatetheNaNO3electrolytetothemachiningdomainandafilterwasusedbeforetheelectrolytewasrecirculatedtotheholdingtank.WhenthegapbetweentheelectrodesisfilledwithNaNO3electrolyte,theelectrochemicalreactiontakesplaceundertheappliedvoltage,whichmeansthepassivefilmbeginningtoformatthesametime.Thegrindingbarswereusedtobreakdownthepassivefilmandmadethefreshsurfaceofthesubstrateexposetotheelectrolyteforthattheelectrochemicalreactioncancontinue.Hence,aproperbalancebetweentheelectrochemicalreactionandthemechanicalgrindingisvitalinordertoobtaintheoptimalresult.Thisbalancecanberealizedthroughadjustingworkpicecrotaterateandthepressurebetweenworkpieceandgrindingbars.
3.2Optimizationofexperimentalparameters.TheaboveanalysisofmechanismofECMFprocessdemonstratesthemaininfluencefactorsonfinishingprocessaretheoperatingvoltage,electrolytecomponentsandconcentration,electrodegap,temperatureandcurrentdensity,inwhichtheoperatingvoltageandelectrolyteconcentrationarethemostimportantparameters.Soitisnecessarytoevaluatetheseparametersandfindouttheoptimalmatch.Atwo-variable,theoperatingvoltageandNaNO3solutionconcentration,andthree-levelfullfactorialdesignexperiment,totalofninetests,wasdeveloped,inwhichthevoltagesare7,10and12Vandtheconcentrationlevelsare10,15and22%.TheoptimalparametersmatchafterexperimentaltestsareshowninTable1.
Table1Optimalparametersmatchresultingfrom
ECMFexperimentaltests
Parameter
Value
Operatingvoltage[V]
10.0
Electrolytecomponent
NaNO3
Electrolyteconcentration[wt%]
15%
Electrolytetemperature[ºC]Electrolytetemperature
25.0
Electrodegap[mm]
0.6
Currentdensity[A/cm2]
7.5
4Surfacemicroscopiccharacteristicsanalysis
4.1Heightcharacteristicsofsurfaceroughness.Theinitialworkpiecesurfaceisgrindedsurface.ECMFexperimentswereconductedwiththeparameterslistedinTable1.AfterECMF,aSurftestprofilometer,ModelTALS-212,wasusedtomeasurethesespecimenssurfaceroughness.Table2summarizesthechangesinheightparametersofsurfaceroughnessbeforeandafterECMF.Thesedataareallaveragesofthreeexperimentalspecimensinordertoeliminatetherandommeasureerrors.
Table2Changesintheheightparametersofsurfaceroughness
Roughnessheightparameter
Initialsurface
Finishingsurface
RoughnessAverageRa[μm]
0.750
0.095
RootMeanSquareRoughnessRq[μm]
0.975
0.114
MaximumHeightoftheProfileRy[μm]
1.50
0.62
AverageMaximumHeightoftheProfileRz[μm]
1.375
0.210
MaximumProfilePeakHeightRp[μm]
0.86
0.24
Table2showstheroughnessaverageRa,themaximumheightoftheprofileRyandtheaveragemaximumheightoftheprofileRzoftheinitialsurfacereducerespectivelyby87.4%,58.6%and84.7%afterECMF.Therootmeans
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