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Analysis_1

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Analysis_1Analys

    Analysis

BaosteelTechnicalResearch

    ;Volume3,Number2,June2009,Page24

    ;Analysisofpointdefectsinautomobilesheetcoating ;xuFengandZHULan

    ;Welding&SurfaceTechnologyDivision,BaosteelResearchInstitute,BaoshanIron&SteelC

    o.,Ltd.,Shanghai201900,China

    ;Abstract:Theanalysisofthepointdefectsisanimportantmethodtosolvetheproblem.Typicalpointdefe

    cts

    ;distinguishedaspinholesorbumpsinautomobilesheetcoatingwereinvestigatedinthepresentstudy.Cr

    osssectional

    ;imagesofpinholesinprimecoatingortopcoatingshowthattheywerecausedinthesanqewaybyfractures

    inbasesteel

    ;orazincalloyedlayer.Furthermore.theformationofthedefectsisinferred.alongwiththespecificpaintin

    gprocess.The

    ;analysisalsoshowsthatbumpsfrequentlyoccurredintheecoatinglayerweremainlycausedbycontamination.

    ;Keywords:pinhole;bump;crosssectionalanalysis

    ;1Introduction

    ;Thereisastrictrequirementforthequalityofthe ;surfaceofautomobilesheets,especiallyoutersheets. ;Withthedevelopmentofcoatingtechnologies,some ;advancedcoatingsandsomewithahighcoveringabmtV ;havebeengraduallyapplied.Inthisway,theinfluenceof ;steelsurfacequalityonfinalcoatingappearancehasbeen ;reduced.However,ontheotherhand,automobile ;companieshavebeengraduallyadoptingsomenew ;measuresincludingthatofreducingpaintfilmthickness ;andthatofthe3C1Bpaintingprocess.insteadofthe ;traditional3C2Bprocess,becauseofthecostpressure. ;111esenleasureshave.tosomeextent.weakenedcoating’s

    ;capacitytocoversteelsheets.

    ;Generallyspeaking,thequalityofsteelsurface ;affectscoatingperformancesinthreeaspects. ;(1)Thechemicalreactivityofsteelsurfaceisa ;significantfactorindeterminingthepropertiesof ;chemicalconversionlayers.whicharecloselyrelated ;tOthecoatingfilm.sheetadhesionandproduct

    ;-

    ;I

;corrosi0nresistance[.

    ;(2)Theprofileofstee1surface.namelyroughness ;andwaveness,caninfluencedistinctnessofimage ;(DOI)oftopcoatingJ.

    ;(3)Thedefectsofsteelsurfacecanbetransferred ;tOthesurfaceoftopcoating.whichwillnotonly ;damagetheappearance,butalsoformweakpointsof ;corrosionresistance.thusgreatlyreducingproduct ;durability[.

    ;Coatingdefectsarearecurringprobleminfl ;paintingworkshopandcausedbyavarietyofreasons ;duetothecomplexityofthecoatingprocess.whichis ;illustratedinFig.1.Inmostcases.itisdifficultto ;determinethespecificcauseofdefectsmerelyby ;observation.Effectiveanalysisofdefectscanoffer ;greatbenefitsandisoftenashortcuttotrouble ;shooting.Thepresentstudyinvestigatesintothetwo ;typesofpointdefectsthataremostcommonly ;mentionedbycustomers.Basedontheinformation ;obtainedfromthedefectanalysisandthespecific ;paintingprocess,possiblecauseswereexamined. ;————1

    ;厂一!!_11:!:!!!..

    ;2Experimentalmethods

    ;Fig.1Schematicdiagramofthecoatingprocess ;Eachspecimencontainingapointdefectwascutintoa ;10mill×15mlnpiece.Thesuperficialmorphologyof ;thepointdefectwasrecordedwithanopticalmicroscope ;equipped,Vim3Dimagecapturefunction.Afterthat.the ;specimenwassealedverticallywithresinwhichwas ;Correspondingauthor:XUFeng;Email:xu_

    ;feng@baostee1.tom

    ;curedatroomtemperatureuntilitturnedhighly ;transparent.Thespecimenwasgroundwithcoated ;abrasivesfromcoarsetofineabrasivesinturn.The ;wholeprocesswasaccomplishedwithgreatcareand ;patience.Thefinalcrosssectionwaspolishedslightly. ;Athinlayerofcarbonwasdepositedonthesurface ;ofthesampleinaJEE——420vacuumevaporatorto

    ;rendersuperficialconductive.Thecrosssectionwas ;XUFeng,eta1.Analysisofpointdefectsinautomobilesheetcoating

    ;observedbyusinganOxford$4200scanning

    ;electronicmicroscope(SEM)whilethecomposition ;wasdeterminedbytheenergydispersivespectrum

;(EDS)accessory.

    ;3Resultsanddiscussion

    ;nisratherchallengingtoobtaintherealcrosssection ;ofapointdefectofasizeoftensofmicrometers.This ;kindofspecimensusuallyconsistsofmultilayers.

    ;includingbasestee1.azincalloyed1ayer,aphosphate ;layerandorganiccoatinglayers.Greatdifferencesboth ;inchemicalandphysicalpropertiesexistamongthe ;layers.Sofar.therehasn’tbeenanyadvancedtechrfique

    ;tocarvethesemultilayerspecimensthroughthecenter ;withoutdestroyingtheinterfaces.However,thetraditional ;grindingandpolishingmethod,widelyutilizedin ;metallography,isstillthemostcompetentmethodin ;crosssectionpreparation.althoughitrequiresgreat ;fa1

    ;skills,aswellascarefulnessandpatience

    ;3.1Pinholes

    ;25

    ;Withregardtopinholesorcratersine-coatingafter ;curing,MorishitaH,eta1.kaJbelievesthattheyare ;causedbyelectricaldischargeinthehydrogengas ;bubblesgeneratedbyelectrolysis,whichcausesgelation ;offilmge1inthevicinityofthedischargezoneand ;preventscoatingflattingduringtheprocessofheat ;curing.Thisexplanmionhasbeenprovedbythe ;occurrenceofcratersduringelectrophoreticdepositionof ;galvannealed(GA)steelsheetathighvoltage. ;However,pinholesdonotalwaysappearinane

    ;coating1ayer.Fig.2showsapinholeinaprime ;coating1ayeranditscrosssectionalimage.Theair ;bubbleinthesealingresinwasintroducedduring ;sealingprocessing.Unliketheaboveelectrical ;dischargingmechanism,thisdefectwascausedbya ;fractureofthezincalloyed1ayer.

    ;fb1

    ;Fig.2Apinholeinprimecoatinganditscr.0sssectionimage ;Itisinterestingtonotethattheshapeofbubblesin ;thee-coating1ayerisnearlyroundandtheinterface ;betweentwoorganiccoating1ayersisflat.Ina ;modemcarpaintingworkshop,e-coatingandprime ;coatingarebakedseparately.Itissuggestedthatthe ;bubbleinthee.coating1ayershouldnotburstwhen ;thee-coatingiscuredbyheatandthusthesurfaceof ;e-coatingkeepflatandflawless.

    ;Similarly,Fig.3showsanotherpinholeintop ;coatinganalyzedbyPittsburghP1ateGlassCompany ;(PPG).Thefractureofsteelandthezinc1ayercaused ;theformationofabubbleandthisbubblepenetrated ;theprimecoatingandexpandedinthebasecoating ;andtheclearcoatinglayer.

    ;Infact.thepinholedefectcausedbyafracturein ;metalcoatingorsteelsubstrateisacommonissue. ;Therefore,thecoatingblistertestisrequiredinthe ;generalautomobilequalificationtestprogram. ;3.2Bumps

    ;Itisnoticedthatinmanycases,bumpsine-coating ;areinducedbycontaminationsintroducedduring ;electrophoresis.Onetypicalcontaminationisscrap ;iron.Fig.4showsthecrosssectionalmorphologyofa

    ;bumpinanecoatinglayer.Thecomponentofthe

    ;particleinecoatingwasidentifiedasiron

    ;(CitedfromPPGTechnicalRepo~)

    ;Fig.3Crosssectionimageofapinholeintopcoating ;Asgrindingsteelsheetsurfacewithcoatedabrasive ;isawidelyacceptedpracticeinmanycarfactories,a ;greatdealofscrapironisproducedanditisquitea ;challengeforcleaningfacilitiestocompletelygetrid ;ofscrapiron,especiallVthatofasizeofdozensof ;micrometers.Onceelectrocoatingiscontaminatedby

    ;26

    ;scrapirontosomeextent,bumpswillinevitablybe ;formed.

    ;BaosteelTechnicalResearch,Vo1.3,No.2,Jun.2009 ;basecoatingsurface.Bumpsareusuallycausedby ;contaminationsine-coatingandscrapironisthe ;majorcause.

    ;References

    ;[1]

    ;[2]

    ;[3]

    ;Fig?4Crosssectionimageofabumpcausedbyscrapiron[41 ;4C!onclusion

    ;Thegrindingandpolishingmethodisprovedtobea ;goodmeasureinpreparingcrosssectionofpoint ;defectsofasizeof300llxn.Inthepreviouscasestudies, ;ithasbeenfoundthatpinholedefectshavetwo ;differentcauses.Itmaybeinducedbyelectrical ;dischargeofhydrogengasgeneratedduring

    ;electrophoresis.AfractureinbasemetalalSOmay ;causetheformationofpinholesinorganiccoatings.In ;thiscase.pinholesmayappearinprimecoatingor ;NarayananTSNS.Surfacepretreatmentbyphosphate ;conversioncoatingslJ1.RevAdvMaterSci,2005,9: ;130177.

    ;ScheersJ,VermeulenM,DeMareC,eta1.Assessment ;ofstee1surfaceroughnessandwavenessinrelationwith ;paintappearance1J.IntJMachToolsManufact,1998. ;38:647656.

    ;ChoiYMandChungJH.Topographyassessmentof ;coatedsteelsheetsurfaceimperfectionsinrelationto ;appearanceafterpainting[c]//SAETechnicalPaper. ;Detroit:SAE,2003:237242.

    ;MorishitaHandKanekoK.Developmentofleadfree ;cationice-coacingforautomobilebodyinwhiteinSAE ;Inmrn~ionalCl//SAETechnicalPaper.Detroit:SAE. ;2003:716-723.

    ;XUFengZHULan

    ;

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