chalmers university of technologyse - 412 96 Gothenburg, SwedenTelephone: + 46 - (0)31 772 10 00www.chalmers.se

Experimental Study on Cold Lap Formation in Tandem Gas Metal Arc Welding

peigang li

Department of Materials and Manufacturing Technologychalmers university of technologyGothenburg, Sweden 2011

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ExperimentalStudyonColdLapFormationinTandemGasMetalArcWeldingPeigangLiPeigangLi,2011.TechnicalReportno69/2011.ISSN:16528891DepartmentofMaterialsandManufacturingTechnologyChalmersUniversityofTechnologySE41296Gothenburg,SwedenTel:+46(0)317721000.PrintedbyChalmersReproserviceGothenburg,Sweden2011

To Xiaolin and my coming baby

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AbstractTandemgasmetalarcwelding(GMAW)isahighproductivityweldingprocesswhichisappliedinmanyindustries.However,atypeofimperfection,knownascoldlap,wasrevealedthatmostlyaccompaniesthetandemGMAWprocess.Coldlapisasmalllackoffusioninsizeattheweldtoewhichcanhaveanegativeinfluenceonthefatiguelifeoftheweld[1,2].Itgenerallyrunsparalleltothesurfaceoftheparentplate.Duetoitssmallsize,ithasnotbeensuccessfultodetectcoldlapbyanynondestructivetest(NDT)method.Withalotofwork,ithasbeennoticedthatitishardtoproduceaweldwithoutcoldlaps.Evenforsomeposttreatmentmethods,e.g.TIGdressing,coldlapcannotbeeliminatedcompletely[2].Therefore,abetterunderstandingoftheformationmechanismisrequiredtobeabletoavoidtheirformation.Themainobjectivesofthisthesisaretoclassifycoldlaps,tocharacterisecoldlapinterfaces,andtoinvestigatethemaininfluencingfactorsoncoldlapformation.Forthispurpose,severalseriestandemGMAWexperimentswereperformed.Domex355MCwasusedasthebasemetalandOKAutrod12.511.2wasusedastheconsumablematerial.Differentshieldinggases(pureargonandpurecarbondioxide)andsurfaceconditions(blastedsurfaceandmilledsurface)ofthebasemetalwereapplied.Asealedchamberwasusedintheexperimenttoensureanonoxidisingoroxidisingweldingenvironment.Crosssectionsofthecoldlapswerepreparedbyaconventionalmetallographicmethod,e.g.cutting,mounting,polishing,andetching(ifnecessary).Themetallographicsampleswereevaluatedwiththehelpofbothalightopticalmicroscopeandascanningelectronmicroscope(SEM)withanattachedenergydispersivespectroscopy(EDS).Also,theinterfacebetweenspatterandbasemetalwasinvestigatedusingthesamemethodasanassistantstudyofcoldlaps.Theresultsshowedthreetypesofcoldlaps,i.e.spattercoldlap,overlapcoldlap,andspatteroverlapcoldlap.Thecoldlapismostlycomposedofvoidsandoxides.Forthematerials(weldingconsumablesandparentmaterials)usedintheexperiments,theoxideswereshowntobemanganesesiliconoxides(MnSioxides).ItwasalsofoundthatMnSioxideshaveasignificantinfluenceontheoccurrenceofoverlapcoldlap.Theblastedsurfaceconditioncanhaveaminoreffectonenhancingcoldlaps.Bystudyingthespatter/basemetalinterface,itisbelievedthattemperature/energyaretheotherimportantfactorsfortheformationofcoldlaps.Keywords:tandemGMAW,coldlap,imperfection,lackoffusion,spatter,overlap,overflow,manganese,silicon,oxides,temperature/energy.

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Acknowledgements

InmystudiesIhavehadalotofhelpfrommanypeople.Firstofall,IwouldliketoexpressmydeepgratitudetomymainsupervisorProfessorLarsErikSvenssonforhispatience,discussions,suggestions,andhelp.IalsowouldliketogivemysincerethankstomyexmainsupervisorProfessorPerNylnforhisvaluableencouragement.MysincerethanksareduetomyassistantsupervisorNicolaieMarkocsanforhispositiveadviceandfrankdiscussion.SpecialthankstomyexaminerProfessorUtaKlementforhereffortstopushmyPhDstudyforward.MyheartfeltthanksgotoProfessorBillLucas,ProfessorYoshinoriHirata,andDr.StephanEgerlandfortheirvaluablecomments,helpfuldiscussionsandfaithfuladviceduringtheIIWconferenceandevents.Manypeoplehavegivenmesupportanddeservemythanks:Dr.NiklasJrvstrt,AssociateProfessorHkanWirdlius,Mr.ToreRonnhult,ProfessorNilsStenbacka,Dr.YimingYao,Mr.HasseOlsson,AssociateProfessorKennethHamberg,Mr.PeterNerman,Mr.JoakimHedegrd,Mr.KjellArnePersson,Mr.LarsHammar,andMr.NicolasCurry.Ithankallmycolleagueswhohelpedtocreateaninspiringandmoreinterestingworkingenvironment.WithoutthefinancialsupportfromtheLOSTandWIQprojects,thisworkwouldnothavebeenpossible.Also,IwouldliketothankVolvoConstructionEquipment(VCE)forimportantsupport.Finally,Iwouldliketodedicateallofmyloveandthisworktomywife,XiaolinSu,andmycomingbaby.Youmeantheworldtome!Trollhttan,June2011PeigangLi

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AbbreviationsandnotationsAbbreviations/notations Description

Al AluminiumAr ArgonASM AmericanSocietyforMetalsBSE BackScatterElectronC CarbonCo CobaltCO CarbonmonoxideCO2 CarbondioxideCr ChromiumCTWD ContactTipWorkingDistanceEDS EnergyDispersiveSpectroscopyFe IronGMAW GasMetalArcWeldingLarc ArcLengthMAG MetalActiveGasMg MagnesiumMIG MetalInertGasMn ManganeseMnO ManganeseoxideMnS ManganesesulphurMo MolybdenumN NitrogenNb NiobiumNDT NonDestructiveTestNi NickelRLoF Ratiobetweenthelengthoflackoffusionattheinterfaceandtheentirelengthofthe

spatter/basemetalinterfaceSE SecondaryElectronSEM ScanningElectronMicroscopySi SiliconSiO SiliconmonoxideSiO2 SilcondioxideSMAW ShieldedMetalArcWeldingSTEDV StandardDeviationTA TrochAngleTi TitaniumTIG TungstenInertGasTS TravelSpeedV VanadiumVCE VolvoConstructionEquipmentWFS WireFeedSpeed

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WireC WireCombinationWS WeldingSpeedVT VisualTestZr Zirconium

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Contents

Abstract.....vAcknowledgements..viiAbbreviations.ixContents.xi1 Introduction.....................................................................................................................................11.1 Background..............................................................................................................................11.2 Objective..................................................................................................................................11.3 Limitations.............................................................................................................................1

2 GMAWprocess................................................................................................................................33 Coldlaps..........................................................................................................................................73.1 Definitionofcoldlaps..............................................................................................................73.2 Detectionofcoldlaps............................................................................................................103.3 Previousclassificationofcoldlaps........................................................................................113.4 Influencefactorsforcoldlaps...............................................................................................15

4 OxidesinGMAWprocess..............................................................................................................235 Experimentaltechniques...............................................................................................................275.1 Steelandconsumablematerials...........................................................................................275.2 Shieldinggases......................................................................................................................275.3 Sealedchamber.....................................................................................................................285.4 SEMandEDS..........................................................................................................................28

6 Summaryofappendedpapers......................................................................................................316.1 PaperI....................................................................................................................................316.2 PaperII...................................................................................................................................326.3 PaperIII..................................................................................................................................32

7 Conclusionsandfuturework.........................................................................................................358 References.....................................................................................................................................379 Appendedpapers..........................................................................................................................39

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1

1 Introduction

1.1 BackgroundThisthesiscontributestowardsthefundamentalunderstandingoftheformationofcoldlap,whichisatypeofweldingdefect,tobeexplainedlater.Weldingisveryimportantinmodernmanufacturingindustry.GasMetalArcWelding(GMAW)isespeciallyutilizedinoffshoreindustryandvehiclemanufactureduetoitshighweldingproductivity.ItiswellknownthatsomeimperfectionsgeneratedintheGMAWprocess,suchaspores,inclusions,undercuts,etc,areinevitableincivilindustrialmanufacture.Theseimperfectionscaninfluencethepropertiesoftheweldedstructures,especiallythefatigueproperties.Fatigueisalocalphenomenoninwhichthelocalstressleveland/orlocaldefectscaninfluencetheservicelifeofthematerialsseverely.Thefatiguepropertiesoftheweldedstructuresalwaysneedtobeimprovedforthedevelopmentofnewproducts,whichmustusuallyfeatureincreasedloadcapacity,highertravelspeedsandlongerlife.Sincethelastcentury,manyfatiguetestshavebeenperformedandevaluatedonweldedsamplesusingdifferentweldingmethods,fillermaterials,parentmaterials,weldingparameters,shieldinggases,jointtypes,andfatigueloadings[16].Besidesweldgeometry,coldlapwasrevealedasanotherimportantfactorwhichcanhaveadetrimentaleffectonthefatiguelifeoftheweldedstructures.Recently,anewcorporatestandardwasdevelopedwithinVolvoConstructionEquipment(VCE),inwhichcoldlapwasfirstlygivenandrelatedtodifferentqualityclasses[7].However,verylimitedknowledgeaboutcoldlapisavailableinliterature.Eventhedetectionofcoldlapisanobstacleforstudyingit.PreviousinvestigationshaveshownthatitisverydifficulttoproduceaweldwithoutcoldlapwhenusingGMAW.Therefore,itisimportanttounderstandcoldlapfundamentally,e.g.classification,characterisation,mechanismofformation,etc.

1.2 ObjectiveTheobjectiveofthisongoingworkistostudycoldlapsinordertobetterunderstandthephenomenabehindtheirformationandthemechanismsinvolved.First,aclassificationofcoldlapsisgiven.Physicalcharacterisationofthecoldlapinterfaceisasecondimportantpartofthisthesis.Thirdly,thefactorsthataresupposedtohaveasignificantinfluenceoncoldlapformationarepresented.Regardingtheweldingprocess,differentinfluencefactorsareillustratedtoexplainthecoldlapformation.

1.3 LimitationsInthisstudy,thedetectionofcoldlapshascontinuedtobeanobstacle.Sincecoldlapsaresmallinsize,allstandardNDT(NonDestructiveTest)methodshavefailedindetectingthem,andonlyvisualtests(asapreliminarytest)anddestructivetestswerefoundtobeefficientandreliablemethods.Blastedsurfacesofthebasematerialshavebeenbroadlyintroducedinindustry.Therefore,thisstudyonlyfocusesonblastedorburrgrindingsurfaceconditions.Accordingtopreviousstudies,thesolidwirecangivethehighestfrequencyofcoldlapintheweldingprocess.Onetypeofbothweldingfillermaterial(solidwire)andbasemetalwereusedinthisstudy.

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Onlytwotypesofjointwereusedinthisstudy,i.e.buttjointandbeadonplate,toreducetheuncertaintiesoftheweldingprocess.TandemGMAWwasselectedastheweldingprocessinthisstudysinceitwasmostlyappliedintheworkshopofVCE.Itisalsoimportanttopointoutthatthecoldlapsdiscussedinthisthesisonlyrefertolocalcoldlaps,whicharecomparedwithatypeofcoldlapduetoalackoffusionalongtheentireweldtoeinthelongitudinaldirection.Usually,thecoldlapsalongtheentireweldtoeareduetoincorrectweldingsetuporaverythickoxidelayeronthebasemetalsurface.Therefore,itisbelievedthatthecoldlapalongtheentireweldcanbeeasilyavoidedinthemodernGMAWprocess.

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2 GMAWprocessGasMetalArcWelding(GMAW)referstoanarcweldingprocessusinganarcbetweenacontinuouslyfillingelectrodeandtheweldingpoolwithashieldinggas.GMAWwasintroducedinthe1920s,butitwasnotuntil1948thatitwasmadecommerciallyavailable.Whenusingdifferentshieldinggases,GMAWisalsocalledMIG(MetalInertGas)/MAG(MetalActiveGas)iftheshieldinggasisaninertoractivegas.InEuropetheprocessisalsosimplycalledMIGwelding.TheuseofGMAWcanbringmanyadvantages.Itcanbeusedforalmostallcommercialmetalsandalloys;itcanbedoneinallpositions;continuouswirefeedingispossible;depositionratesaresignificantlyhigh;itiseasytoautomatewelding;andminimalpostweldcleaningisrequired.Therefore,GMAWisutilizedbroadlyinindustry.ThetypicalequipmentsforGMAWincludepowersource,wiresupplyreel,wirefeedingdriverollers,flexibleconduit,hosepackage,weldinggun,contacttube,gasnozzle,andshieldinggas,etc[8].

Figure1TypicalGMAWequipment.(1:Arc;2:Metalwireelectrode;3:Wiresupplyreel;4:Wirefeedingdriverollers;5:Flexibleconduit;6:Hosepackage;7:Weldinggun;8:Powersource;9:Contacttube;10:Shieldinggas;11:Gasnozzle;12:Weldpool.)(Adaptedfrom[8])TheGMAWweldingprocessisdependentonanumberofweldingparameters,themostcommonvariablesbeing[8]:

Electrodediameter Voltage Wirefeedspeedandcurrent Weldingspeed Electrodestickout Choiceofshieldinggasandgasflowrate Torchandjointposition

Toensuregoodweldingperformance,mostoftheseparametersmustbeoptimizedandmatchedtoeachother.Theworkingpointmustbewithintheworkingrangeortoleranceboxfortheparticularweldingsituation.Generally,thesizeofelectrodeischosenaccordingtotheweldingcurrent.Whenahigherweldingcurrentisused,anelectrodewithahigherdiametershouldbechosen.Regardingvoltage,increasedvoltageincreasesthearclengthandgivesa

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widerweldbead.Buttoohighavoltagewillbringtheriskofundercut.Ifshortarcweldingisused,ahighervoltagereducestheshortcircuitfrequency,whichwillgivelargerdropsandmorespatter.Ontheotherhand,toolowavoltagewillincreasetheriskofstubbingandpoorstartperformance.Normallythevoltageshouldbesetforastabilityarc.ForGMAW,thecurrentissetindirectlybythewirefeedspeedanddiameter.Currentisthemainparameterforweldingtoachievesufficientweldingpenetration.However,itisalsoimportanttostrikeabalancewithweldingspeed,voltagewithrespecttothearcstability,andweldquality.Whensettingweldingspeed,thisalsohasaconsiderableeffectontheshapeandpenetrationoftheweld.Therefore,ahigherweldingspeedalwaysaccompanieshighercurrentandvoltageandmayresultinpoorstability.Electrodeextension,alsocalledstickout,referstothedistancefromthecontacttiptothemeltedelectrodetip.Practically,itiseasytousethedistancefromthecontacttiptotheworkpiece(CTWD)toexpresselectrodeextension.Therelationshipbetweenstickout,CTWDandarclengthisshowninFigure2.Toosmallastickoutincreasestheriskofburnback,wherethearcwillweldtheelectrodetogetherwiththecontacttip.Toolongadistancewillincreasetheriskofstubbing,especiallyatthestart.Thecontacttiptoworkdistancealsohasaninfluenceonthecurrentandpenetrationprofile.Iftheelectrodeextensionisincreased,thecurrentandheatinputdecreasewhiletheamountofdepositedmetalremainsthesame.Thisreducesthepenetration,andevenifitwasunintentionalariskofalackoffusionarises.Therefore,thestickoutisusuallykeptconstantduringtheweldingoperation.Torchanglesrelativetothejointarealsoanimportantweldingparameter.Ifthetorch/wireisdirectedawayfromthefinishedpartoftheweld(forehandtechnique),thismakesthepenetrationprofilemoreshallowandthewidthoftheseamwider.Ontheotherhand,ifitisdirectedtowardsthefinishedpartoftheweld(backhandtechnique),thepenetrationwillbedeeperandtheseamwidthwillbenarrower[810].

Figure2Relationshipbetweenstickout,CTWDandarclengthinGMAWSolidwiresandcoredwiresarethetwotypesofwiresusedinGMAW.Thelattertypeconsistsofametallicoutersheath,filledwithfluxormetalpowder.Thefluxcoredwirescanhaveeitherarutileorbasicfilling.Theycanalsobeselfshieldedforusewithoutshieldinggas.Thecostper

unitsupcoregiviGMwel

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Tanwelareareindicomwelmag

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tofcoredwiperiortosolidedwires.Basngatougha

MAWissmalleding.ThewiGMAW,thearc.OnecanterialtranspondemGMAWdingspeedcmeltedintousedfortheividually.Homplicatedanddingparamegneticarcblo

ure3Tandemreover,dueametersneetancebetwee

iresisconsiddwires.Ahigsicfluxcoredandcrackresercomparedireisusuallyestabilityoftndistinguishort:thespra

Wisregardedcanbeobtainonecommoewires.Therwever,becaddifficulttoetersettingwoweffect.

GMAWwithtoanadditioedtobesetfenthewires

derablyhigheghdepositiodwireshavesistantweldmdwiththatus0.9mmto1thearcdepeessentiallybayarcandthasatypeofnbyusingthonweldpoolrefore,thewusethetwosettheweldwillincrease

twoindividuonalwirebeiforthetande,andtherat

erthanthatnrateandgesimilarperfmetal[8].Gesedforsubm1.6mmindiaendslargelyobetweentwoeshortarc(fhighefficienhedoublewiusingtwoa

weldingparamarcsareverdingcurrenttheriskofin

ualwiresandaingintroduceemGMAW,etioofcurrent

ofsolidwireoodsidewaformancetoenerally,themergedarcaameter[9].onhowthemodifferenttyshortcircuitncyGMAWwre.Intandemrcs,asshowmetersforeayclosetoeaandvoltagenterference

arcsedintothewe.g.theangletandvoltage

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3 ColdlapsColdlapisarelativelynewwordtodescribeanimperfectionattheweldtoe.Theimperfectionwasfirstdiscoveredinthe1990sduringexperimentalworktoimprovethefatiguepropertiesofwelds.Itwasalsoobservedatthattimethatcoldlapscanhaveanimportantinfluenceonthefatiguelifeoftheweldedstructures.However,intermsofavailableliterature,therearealimitednumberofpublicationspresentingstudiesonweldprocessinfluenceoncoldlapformationorphenomenawhichpromotecoldlaps.Inthischapter,thefewavailablearticlesaresummarizedinordertogiveabriefoverviewofcoldlaps.

3.1 DefinitionofcoldlapsInthisthesis,coldlapreferstoacracklikeimperfectionattheweldtoewhichhasanegativeinfluenceonthefatiguepropertiesoftheweld.ThecoldlapwasfoundinfatiguetestscarriedoutbyLopezMartinezandKorsgren[1]inthe1990s.Intheirfatigueinvestigation,twoweldingmethodswereevaluated,i.e.ShieldedMetalArcWelding(SMAW)andGasMetalArcWelding(GMAW).Theweldingrunswereperformedinthebestweldingposition(1G).Fourdifferenttypesofweldingconsumablematerialswereinvolved:OK48.00,abasiccoatedelectrode;PZ6130,abasicfluxcoredwire;PZ6111,arutilefluxcoredwire;andOK12.51,asolidwire.ForGMAW,twotypesofshieldinggaseswereappliedintheweldingexperiments.Differentweldingparameters,i.e.weldingcurrent,weldingvoltage,andinterpasstemperature,wereusedduetothedifferentmethodsanddifferentdiametersoftheelectrodes.ThedetailsareshowninTable1.Twoparentmaterialswereused,i.e.Domex350XPandWeldox900.

Table1ExperimentdetailsofLopezMartinezandKorsgrensinvestigation[1]Weldmethod MMA GMAFillermetal OK48.00 PZ6130 PZ6111 OK12.51

Gas Mison25,15l/minFogon20,14l/min

Mison25,8l/min

Weldingposition 1G 1G 1G 1GThroatthickness 5mmNominal 5mmNominal

5.5mmMeasured

34mmNominal

ElectrodeSize(mm) 4 1.6 1.6 1.0

Current(A) 185 185 257 140Voltage(V) 24 23 25 20Heatinput(kJ/mm) 2.0 1.6 1.3 0.6Interpass

Temperature()25,110,125,

13025,125,135,

80 20,20,20,20 20,20,20,20Investigation

points 712 720 5824 2896Numberofspotswithdefects(Coldlaps)

38 83 112 475Percentage 5.3 12 1.9 35

Twotestingmethodswereinvolvedintheinvestigation:onewasfatiguetestingandtheotherwastheseparatetest.

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Inthefatiguetest,thefracturesurfaceswereinvestigated.Atypeofmicrodefectwasidentifiedattheweldtoeasthefatigueinitialspotinalmost70%oftheexaminedfracturesurfaces.Thesesocalledcoldlapsareparalleltoandatthesamelevelastheoriginalparentplatesurface.Coldlapsappearedasplanardefectsattheweldtoeanddifferedinshape.Thedefectscanbeverydifferentindepth(transversetoweldlongitudinaldirection)andlength(inweldlongitudinaldirection).Inthestudy[1],twocaseswereidentifiedregardingcoldlaps:oneisthecoldlapscreatedbyshrinkagecrackintheweldingprocessandanotheristhecoldlapswithoutanyinfluencefromshrinkagecrackintheweldingprocess;seeFigure4.Forthedetectedcoldlaps,twodimensionsweredefinedasdepthandlength,asmentionedabove.Theresultsshowedthatthecoldlapcanvarybetween0.1mmand3.5mminlengthand0.05mmand0.8mmindepth[1]seeTable2.

Figure4TwocasesofcoldlapsidentifiedinLopezMartinezandKorsgrensinvestigation:a)Coldlapscreatedbyshrinkagecrack;andb)Coldlapswithoutinfluencefromshrinkagecrack.(Adaptedfrom[1])

Table2DimensionsofthecoldlapsfoundinLopezMartinezandKorsgrensstudy[1].

Dimensionsketchofacoldlap(a:dimensioninlongitudinaldirection;b:

dimensionindepthdirection/transversedirection)

Samplenumber a(mm) b(mm) Remarks11 0.8 0.15 13 1.0 0.5 14 3.5 0.5 Nodefects.Shapeofearlycrack16 1.0 0.5 18 1.5 0.4 21 2.0 0.8 22 0.3 0.3 Twocoldlapswerefoundforsample2222 1.5 0.4 Twocoldlapswerefoundforsample2233 0.25 0.125 34 N.A.

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Intheseparatetest,theweldwascutandthecrosssectionsweremilled,polished(downto3m),andetched(3%Nital).Newcrosssectionswerepreparedusingthesameprocedure2mmawayfromtheformercrosssection.Amicroscopewasusedtoexaminethedefectsonthecrosssectionattwomagnifications(50xand200xmagnification).Thdepth(d)andorientation()ofdefectswereevaluated,asshowninFigure5[1].

d

d

= 0

Figure5Depth(d)andorientation()ofthetestedsample(Adaptedfrom[1])Thenumberofcrosssections(investigationpoints)fordifferentweldingspecimensislistedinTable3.InLopezMartinezandKorsgrenspaper[1],itisstatedthatthedefectsfoundinseparatetestscanberegardedascoldlapsbasedonthediscoveryinthefatiguetest.However,thecoldlapsfoundinthefatiguetestwerestatedtobemainlyparalleltotheparentmetalsurface.Thedefectsof90oorientationcanbeanexceptionofcoldlaps,ortheymaybeduetheshrinkagecrack[1].Also,theirformationcouldbedifferent.Therefore,thedefectswith90oarenotconsideredinthisthesis.Toillustratethefrequencyofdefectsfordifferentweldingmethodsandmaterials,thepercentageofinvestigatedpointswithdefectswasevaluated.Themeanvalueofthedefectlengthofthecrosssectionwasrecordandcalculated.TheresultsareshowninTable3.

Table3Separatetestresultfordefectsonthepreparedcrosssections(Adaptedfrom[1])Weldmethod MMA GMAFillermetal OK48.00 PZ6130 PZ6111 OK12.51

Investigationpoints 712 720 5824 2896Numberofspotswithdefects(coldlaps) 38 83 112 475

Percentage(%) 5,3 12 1,9 35Meanvalueind(mm) 0.270 0.250 0.047 0.130

STEDV 0.300 0.250 0.050 0.150FromtheresultsshowninTable3,onlyasmalldifferencebetweenOK48.00(basicelectrode)andPZ6130(basicfluxcoredwire)intermsofcoldlapmeandepthwasfound.ThePZ6111(rutilefluxcoredwire)gavethesmallestdefectsindepth.TheOK12.51(solidwire)gavethehighestfrequencyofcoldlaps.Hence,thetypeofelectrodehasthemostsignificantinfluenceoncoldlaps.Basicfillermaterialstendtogiveadeepercoldlap.Solidwirestendtogiveahigherfrequencyofcoldlaps.Incomparison,asshowninTable1,theeffectsofcurrent,voltage,andheatinputoncoldlapsareweak.AccordingtoSamuelsson[2],coldlapsarealocallackoffusion.Thisdefinitionoflackoffusionmakesiteasiertounderstandcoldlapcomprehensively.Furthermore,theimportanceofthe

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lackoffusiondefinitionistoimplycoldlapformationastheconventionallackoffusion,e.g.lackofenergy/heat,improperpreweldcleaning,andincorrectweldingtechniques/operation,etc[11].

3.2 DetectionofcoldlapsAsmentionedabove,coldlapsareverysmallinsize(0.1mm3.5mminlengthand0.05mm0.8mmindepth).ItisveryhardtodetectcoldlapsbyanyNDTmethod.Theconventionalmethodfordetectingcoldlapsiscutting,polishingandinvestigatingthecrosssectionbymicroscopy.However,thismethodcannotgiveinformationaboutthelengthofthecoldlapsalongtheweldbead.Also,itishardtogetanoverviewofthenumberofcoldlapsintheweld.Therefore,anewdestructivemethodwasdevelopedbyHolstetal.[5]whichcansuccessfullydetectcoldlaps.Topreparethespecimen,thefirststepistogrindagapontheoppositesideoftheweld.Thelocationofthegapistheplatethicknessplushalfthethroatthicknessfromtheedgeofthebasemetal,andthedepthishalftheplatethickness.ThepositionanddepthcanbeseenschematicallyinFigure6[5].Ifthegapistoodeeportoofarfromtheedge,thebendingwillnotoccurattherightplaceandnothingcanbegathered[5].

Figure6Thecorrectpositionanddepthofthegap(Adaptedfrom[5]).Thelocationofthegapistheplatethicknessplushalfthethroatthicknessfromtheedgeofthebasemetalandthedepthishalftheplatethickness.Tobendthespecimen,thespecimenneedstobeplacedonplateswhichcanslideapartasthepressgoesdownuntiltheweldistorncompletely.Aspacercanbeplacedonthetoptospreadtheforceoverthespecimen.Theforceneededvarieswiththedepthofthegap,lengthofthespecimen,thicknessofplate,design,throatthickness,andgeometry.Inthefirststage,theweldtearsupattheweldtoe.Inthenextstage,thecrackpropagatesthroughthematerialandfinallythespecimensplitsintotwopieces,asshowninFigure7.Itisimportanttoprotecttheedgeofthefracturedsurface.

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Figure7Fracturedweldtoe.(Adaptedfrom[5])Thedetectedweldtoeistornupwiththepressureandtowslideblocks.Finally,thespecimensplitsintotwopieces.Toexaminethespecimen,astereomicroscopewithmagnificationof46timesissuggested.Foramoreexactdefectsizedetermination,acameraconnectedtoacomputercanbeused.ColdlapsandspatterswereidentifiedonthefracturesurfaceoftheweldtoeintheworkofHolstetal.[5].Therefore,usingthemethodabove,itispossibletoperformadestructivetestofaspecimentoindicatecoldlaps,spatters,lackoffusionandporesforsolidelectrodeMAGwelds.Boththelengthandthedepthofthecoldlapscanbedetermined.

3.3 PreviousclassificationofcoldlapsColdlapswerecharacterizedandcategorizedbyFarajianSohiandJrvstrt[12].Intheexperiment,tandemGMAWweldingwasused.Structuralsteel(EN10025S275JR)plateswereusedasthebasemetalandwereplasmacutintosmallercouponswiththedimensionsof12*50*300mm.Weldingwasperformedontheplate(beadonplate).AsmilledsteelplatesandweldpositionPAwerechosenfortheexperiment.Thirtyeightweldedspecimenswithdifferentweldingparameterswereproduced.Whenselectingtheweldingparameters,spraymodewasensured,evenfordifferentarclengthsengaged.Also,theratioofwirefeedspeed(WFS)andtravelspeed(TS)waskeptfixedtokeepthesamedepositionrateinwhichdifferentweldprofileswereproduced.Theinfluenceofelectrodestickoutcanbestudiedbyvaryingcontacttubetoworkpiecedistance(CTWD).Theinfluenceoftheweldingtorchangle(forwardandbackward)wasalsostudied.TheWFSoftheleadingwirewaskeptalwaysequaltothatofthetrailingwire.Twocombinationsofelectrodes,i.e.solidsolidandsolidcoredwire,wereusedintheexperiment.Adevelopedrapiddestructivemethodwasusedtodetectcoldlapsinwhichthespecimenswerehitbyapendulumtipinanimpacttestmachine.Afterbreakingthespecimens,thefracturesurfaceofthebrokenpartswasstudiedusinglightopticalandscanningelectronmicroscope.Thetestswithdifferentloadingspeedsshowedthatafasterloadingspeedandalowertemperaturecouldgiveabetterpictureoftheimperfections.Inordertofracturethepartsinabrittlemanner,thespecimenswerefirstcooleddowninacontainerfilledwithliquidnitrogenfor10minutesbeforetheimpacttest.Thetestcouponswerepreparedwiththedimension11*12*50mm.Thependulumhitsthespecimensfrombehindandbreaksthembypropagatingacrackstartingattheweldtoes.InFigure8,thespecimenpreparationandimpacttestingaredescribedschematically[12].

Figint[1pe

ByanprUskn

InshfraFighilothso

gure8Schemtosubspecim2](Originallyermission.)ystudyingthndbasematereviousfatigusingtheseexnownasoveFigure9,ovharpcontrastacturesurfacgure9.Thedghermagnifngand0.45hebasemateolidinclusion

maticaldescripmens.c)Hittinpublishedin

hefracturesuerialcouldbeuetestswerxperiments,erlapandspverlapwasobtcanbeobseceoftheovedetailsofthefication.Asemmdeep.Terial.Thesubnscanbeobs

ptionofthetengwithapendSteelResearc

urfaces,theeobserved.ecomparedFarajianSohpatter.bservedassoervedbetweerlaps(seeFeencircledimmiellipticalTheobservatbsurfaceoftserved[12].

est:a)Beadodulum.d)LigchInternation

initialimperThefracture.ColdlapscahiandJrvstr

olidifiedoveeenthebaseigure9a).Thmperfectioncracklikeimionoftheimthecoldlaps

nplateweldhtorelectronnal,77(2006)

fectionswhiesurfacesfroanbeobservrt[12]obse

rflowofthematerialfra

hreeoverlapsinFigure9a

mperfectionwmperfectionssappearedp

specimen.b)nmicroscopy),No.12,page

chexistbetwomthisimpavedclearlyinervedtwotyp

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Slicingthespofthefractures889895.W

weentheweacttestandtnbothmethopesofcoldla

nthebasemceandthesuwnatthewelnFigure9baedwhichis1.metallicbindelargepores

12

pecimenresurfaceWith

eldtoetheods.aps,

metal.Aubdtoeinata.5mmingwiths,some

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gure10Surfaternational,7

apfoundin[1ainingoverlaplishedinStee

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esentontheigationusingnsweredomonandmangowninTableetweenthewweldingand

apinvestigate.12,pages88

ngelectronmaps.b)Magnifternational,7

observedcogthescanninminant,asshganesewere4.Thissuggweldandthesolidification

edusingEDS[89895.Withp

icroscopyoftficationofthe77(2006),No

oldlapsurfacngelectronmowninFigurpresentont

geststhattheebasemetanprocess.

[12](Originalpermission.)

thefracturesecoldlapscir.12,pages88

ce,energydimicroscopewre10.Therethesurfaceiesiliconandl,formingth

lypublishedi

surfaceofthercledina)[1289895.With

spersivewasperformeesultsfromtnadditiontomanganese

henonmetal

inSteelResea

13

welded]

edattheheEDSoiron,

llic

arch

14

Table4ChemicalcompositionobtainedbyEDSinvestigationofonecoldlapsurface,asshowninFigure10.(Modifiedfrom[12])

Element Weight(%) Atomic(%)C 2.07 7.31O 9.11 24.15Si 0.91 1.37Mn 1.50 1.16Fe 85.75 65.13

AnothertypeofcoldlapfoundbyFarajianSohiandJrvstrt[12]isnamedspatter.Thelengthofasinglespatterismuchshorterthanthatoftheoverlaptypecoldlaps,andingeneraltheaspectratio(a/ca:crackdepth,2c:cracklength)ofspatterlikecoldlapsislargerthanthatofoverlaplikecoldlaps.InFigure11,asinglespattertrappedwithintheweldmetalattheweldtoeisshown.Theobservedspatterhasadiameterofaround200mandwasbelievedtobecoveredbytheweldweaves.Someoxideswerealsofoundsurroundingthespatter,whichisthesamematerialastheinclusionsintheporesontheoverlapsurface.Aclusterofspatterwasalsoobservedattheedgeofthefracturesurface,asshowninFigure12.Betweenthestickingspatters,oxidesandporeswerestatedtobefound.Thelengthofthisspatterclusteris1.8mmanditsdepthis0.6mm.

Figure11Spatterfoundin[12].a)Singlespatteronthetopofthefracturesurface.b)Magnifiedpictureofthespatterintheleftimage.[12](OriginallypublishedinSteelResearchInternational,77(2006),No.12,pages889895.Withpermission.)

15

Figure12a)Clusterofspatteronthetopofthefracturesurface.b)Magnifiedpictureofthepartcircledina)[12](OriginallypublishedinSteelResearchInternational,77(2006),No.12,pages889895.Withpermission.)Inthediscussionofthepaper[9],theauthorsstatedthatseveralforcefieldsintheweldingprocess,e.g.arcpressure,electromagneticfieldsandtheforcefromtheshieldinggasflow,resultinthefluctuationoftheweldpool.Thefluctuationsarethesourceoftherippleoftheweld.Iftheforcefieldswhichcontrolthefluctuationsoftheweldpoolweredisturbed,largerrippleswouldbeproduced.Whenthelargerrippleswithhigherkineticenergythannormalripplesreachtheweldtoe,theyoverlaponthebasemetalandformtheoverlaptypeofcoldlaptogetherwiththecoldbasemetalsurface.Thesolidinclusionscontainingsiliconandmanganesewereregardedtobetrappedparticleswhichareproducedduringwelding.Thesiliconandmanganeseinclusionswereassumedtobeproductsofdeoxidizationoftheelectrodesandcleanersforbasemetalsurfacescontainingoxides/millscale.Itwasstatedin[12]thatgasescanbereleasedwhiletheinclusionissolidifying.

3.4 InfluencefactorsforcoldlapsForadeeperunderstandingofcoldlaps,itisimportanttounderstandtheinfluencefactorswhicharesupposedtobeabletosignificantlycontroltheoccurrenceanddimensionsofcoldlaps.RoboticRapidArcGMAW(highspeedwelding)withsolidwirewasusedbyHedegrd[13]toproduceweldingspecimens.Filletweldswereproducedintheexperiment.Adestructivemethod(crosssectionsoftheweld)wasusedtodetectandevaluatecoldlaps.Table5Sevenvariedimportantweldingparametersinexperiment.(Adaptedfrom[13])

Examinedweldingparameter ValueoftwolevelsElectrodestickout 22/29(mm)

Heatinput 0.8/1.1(kJ/mm)Weldinggunangles A/B(notspecifiedinpaper)Weldinggunposition e.g.badposition:1/0(mm)offWeldingposition 1F/2F

Surfacequalityoftheplate Blasted/asdeliveredWeldsize(throatsize) 4/6(mm)throatsize

16

Intheexperiment,multifactorialtestswereperformedandeighttestserieswerewelded,consistingofthreespecimenseach.Sevenimportantweldingparameterswerevariedinastandardisedandreducedtwolevelmultifactorialtest.Theweldingparametersexaminedareshownbelow.Intotal,twentyfiveindependentcutsweremadeforeachseries.Byevaluatingthecrosssections,threesignificantparameterswererevealedforcoldlapoccurrence,i.e.weldingposition,surfacecondition,andweldinggunposition[13].ThemostimportantparametersforGMAW,asconsideredbyFarajianSohiandJrvstrt[14]intheirexperiment,arelistedinTable6.

Table6Nominalweldparameters.(Controlledparametersaredenotedby*,thoseheldfixedaredenotedbyo,andthosefullydeterminedbycontrolledparametersaredenotedby.)(Adaptedfrom[14])

Weldingparameter Basematerial

FillermaterialsandShieldinggas

Totalwirefeedspeed*(21m/s)

Alloy o(Steel,EN10025S275JR) Wiretype*(solid:OKAutrod12.51,metalcored:OKTubrod14.12)

Weldtorchangle*(0,vertical)

Platethickness o(12mm)

Wirediameter o(1.2mm,leadingandtrailing)

Arclength*(4mm) Surfacequalityo

(Asrolled) Distancebetweenthetwocontacttubeso(20mm)Contacttubetoworkdistance(16mm)* geometry

o(50300mm,beadonplate) Gastype*:92%Arand8%Co2

Electrodestickout Weldingpositiono

(Horizontal,PA) Gasflowrateo(28l/min)Current Voltage

Weldingspeed AfractionalfactorialtwolevelmodelwithonereplicationwaschosentodesigntheexperimentusingthesoftwareMODDE.SixvariablesoftheweldingparametersfortandemGMAWwithtwolevelswereinvolvedinexperimentaldesign,asshowninTable7[14].Atotalnumberof38weldspecimenswithdifferentandcontrolledweldingparameterswereproducedaccordingtotheworksheet,asshowninTable8[14].Strangely,thetwolevelofCTWDusedare13mmand15mminsteadofthelevelsshowninTable7.

17

Table7Weldingvariablesandtheirrangesinthepaper.(Adaptedfrom[14])Tandemarcweldingparameters Notation Range

WireFeedSpeed WFS 18,21,24(m/s)*ArcLength(Leading) Larc 2,4,6(mm)

ContactTubetoWorkDistance(Leading) CTWD 15,16,17(mm)*TorchAngle TA 10(Pull),0,10(Push)degrees

Wirecombinations WireC Solidsolid:0Solidcored:1

WeldSpeed(SettoWFS/15) WS 1.2,1.4,1.6(m/s)**Thethreevaluesrefertothelow,midandhighlevelofthevariables.Withthemiddlevaluetherepetitionsweregiven.Table8Worksheetofweldingparameters.(Adaptedfrom[14])

No. WFS(m/min)

WS(m/min)

Larc(mm)

CTWD

(mm) TA WiretypeVoltageLeading

(V)1.20 18 1.2 2 13 10 Cored 27.52.21 24 1.6 2 13 10 Solid 29.03.22 18 1.3 6 13 10 Solid 29.84.23 24 1.6 6 13 10 Cored 34.85.24 18 1.2 2 15 10 Solid 26.06.25 24 1.6 2 15 10 Cored 29.57.26 18 1.2 6 15 10 Cored 32.08.27 24 1.6 6 15 10 Solid 34.89.28 18 1.2 2 13 10 Solid 26.010.29 24 1.6 2 13 10 Cored 29.511.30 18 1.2 6 13 10 Cored 31.512.31 24 1.6 6 13 10 Solid 34.813,32 18 1.2 2 15 10 Cored 27.514.33 24 1.6 2 15 10 Solid 29.515.34 18 1.2 6 15 10 Solid 29.516.35 24 1.6 6 15 10 Cored 34.817 21 1.4 4 14 0 Cored 31.018 21 1.4 4 14 0 Cored 31.019 21 1.4 4 14 0 Cored 31.036 21 1.4 4 14 0 Solid 29.637 21 1.4 4 14 0 Solid 29.638 21 1.4 4 14 0 Solid 29.6

18

Toevaluatethespecimens,boththeconventionalcrosssectionmethodandthedevelopedimpacteddestructivemethod(asmentionedintheabovechapter)wereengaged.Lightopticalandscanningelectronmicroscopewasusedfortheinvestigations.Twotypesofcoldlapwereobservedinthisstudy[14],i.e.overlapandspatters,andMnandSiinclusionwasalsofoundonthesurfaceoftheflaw[14].Thedepthofthetwotypesofcoldlapsforallthefracturedspecimenswasmeasured.Themaximumdefectdepthwasregisteredoveradistanceofabout11cmforeachsetofweldparameters.InFigure13,histogramsshowingdefectdepthrangesmeasuredfromallthedetectedtoeflawsaregiven.

Figure13Histogramsofthedepthofa)overlapandb)spatter[14](ReprintedwithpermissionofASMInternational.Allrightsreserved.www.asminternational.org)Thehistogramsshowthattheoverlapdepthhasanormaldistributionwithanaveragevalueof0.25mmandthatspattersarerandomlydistributedwithanaveragedepthof0.34mm.TheeffectofthedifferentparameterswasstatisticallyevaluatedusingthesoftwareMODDE.Foroverlaps,FarajianSohiandJrvstrt[14]statedthatthefactorswiththemostpronouncedeffectontheoverlapdepthwerethetotalwirefeedspeed(WFS)andthetorchangle(TA),asshowninFigure14.Thecontacttubetoworkpiecedistance(CTWD)hadaminoreffectbutstillstatisticallysignificant.

Figure14Scaledandcanteredcoefficientplotofparametersaffectingmaximumoverlapdepthin110mmofwelds.[14](ReprintedwithpermissionofASMInternational.Allrightsreserved.www.asminternational.org)Thestatisticallyderivedempiricalmodelswerealsogiven,asshowninequation1and2.Forsolidsolidwire:

19

4 0.22 0.006 0.01 0.06 0.001 0.004 0.5 0.21 0.03 Eq.(1)Forsolidcoredwire: 3.45 0.23 0.006 0.01 0.04 0.001 0.004 0.56 0.23 0.03 Eq.(2)Forthespatters,usingthesamemethod,theauthorsstatedthatthetotalwirefeedspeed(WFS)andthetorchangle(TA)weredominatingfactors,asshowninFigure15.

Figure15Scaledandcentredcoefficientplotofparametersaffectingmaximumspatterdepthin110mmofwelds.[14](ReprintedwithpermissionofASMInternational.Allrightsreserved.www.asminternational.org)

Regardingtheoverlap,iftheequations(Eq.1andEq.2)arereliableandabletobesimplified,neglectingtheinteracteditems,thenewequationscanbewrittenas:Forsolidsolidwire: 4 0.22 0.06 0.5 0.21 Eq.(3)Forsolidcoredwire: 3.45 0.23 0.04 0.56 0.23 Eq.(4)Consideringthechangerangeofthevalueforeachparameter,themaximumeffectofeachparametercanbecalculated,asfollows:Forsolidsolidwire: 0.22 0.22 24 18 1.32Eq.(5) 0.06 0.06 10 10 1.2Eq.(6) 0.5 0.5 6 2 2Eq.(7) 0.21 0.21 17 15 0.42Eq.(8)Forsolidcoredwire: 0.23 0.23 24 18 1.38Eq.(9) 0.06 0.04 10 10 0.8Eq.(10) 0.5 0.56 6 2 2.24Eq.(11)

Franovthhaals

Mto17cocoexauquthco

Figfoww

romEq.5toEndCTWD.Hoverlapcoldlahevoltageinasthemostssoberaised

Moreover,theoasolidcore7.However,ontradictoryoredwireinaxplanationfouthorsstatedualitywasnohecontradictonditionhas

gure16Statisrsolidcoredww.asminter

Eq.12,someowever,itisapforbothstheweldprosignificantef.

eauthors[14edwirecombfromthepreconclusionwasinglewireorthecontradthattheexotoptimal.Ttoryresultswadominant

sticalmodelrwire.[14](Renational.org)

0.23

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udedthatswultedindeepofLopezMahatthesolidcweldingprltswasgiven

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idsolidwire.national.Allri

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dwirecombFigure16anegrdetal.[apscompareand2.5).Onrvstrt[14].heetsandtheffectinprotthesurface

b)Depthofoightsreserved

20

Eq.(12)

FS,TApthofhmeansvoltagescan

binationndFigure[13],aedwitheTheesurfaceoducinge

overlapd.

Figsoww

Fuimleexfasig

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urthermore,mportantforngthandconxperimentalctorwiththegnificanceinopezMartineaterial.Befopecimeninthsinganoptictigue.Bythiwerdepth.Teldtoe,but

sticalmodelre.[14](Reprinational.org)

itiswellknotheexperimntacttipwordesign.Asdiefixedsticktheexperimezetal.[15]orethefatiguhefatiguetealmicroscopsobservatioTheresultsscanonlyred

epresentingantedwithper

ownthatthementaldesignrkdistance(Ciscussedincout.Therefomentaldesignproducedwuetest,TIGdstwasprepapeandascann,coldlapswhowedthatTducethedep

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findependeninthestudyhefactorsintearclengthependencecer.Gusingblastesperformedthefractureonmicroscopinthespecimgcannotelimdlaps.

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edWeldox70ontheweldsurfacescoupetoidentifymensafterTIminatetheco

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statisticallythorsselectenalfactorialarenotindepvepoorstatis

00asthebastoes.ThefauldbeanalysytheinitialpIGdressingboldlapsfrom

21

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22

23

4 OxidesinGMAWprocessWeldingcanberegardedasametallurgyprocesswithquickmelting,coolingandsolidification.Fromthisrapidprocess,asoundweldisproducedwithhighstrengthandtoughnesscomparedwiththeparentmaterial.Thealloyelementsintheweldmetalarethekeytoensuringtherequiredmechanicalproperties.FortheGMAWweldingprocess,bothdropletandweldpoolwillexperienceanextremelyhightemperature,andoxidizationisinevitable.Thisoxidisationwillconsumeacertainamountofthebeneficialalloyelements.Fordifferentparentmaterials,thebeneficialelementscanbedifferent.Ontheotherhand,thealloyelementsareimportantfortheweldabilityintermsofbeingfreefromweldingflaws,e.g.properviscosityofthemoltenmetal,alighteroxidewhichcaneasilyflowuptotheweldpoolsurface,etc.Forthesepurposes,therearealargenumberofalloyelements,e.g.C,Si,Mn,Cr,Ni,V,Ti,Co,Mg,N,Mo,Zr,NbandAl,etc[8,9,11].However,onlythebehaviourofmanganeseandsiliconisfocusedoninthisstudy,sinceMnSioxideswerefoundtobethemajoroxidesincoldlaps.Manganeseisagreyishwhitemetallicelementwithanatomicweightof54.9andameltingpointof1245oC.Acertainquantityofmanganesecancommonlybefoundinmoststeels,asitisahelpfuladditiveinironalloys.Manganesehasastrongeraffinitytooxygen,sulphurandcarbonthaniron.Whenaddedtomolteniron,manganesereactswithoxygentoformmanganeseoxide(MnO).Hence,manganeseisadeoxidizerforiron,butitisstilllessimportantthanaluminiumandsilicon.Inthemelt,Manganesecanalsoreactpreferentiallywithsulphurtoformmanganesesulphur(MnS).Therefore,theMnSinclusioniscommoninhotrolledsteelasathinlayerinclusion.Manganeseiscommonlyfoundasanalloyingadditioninalltypesofcarbonandlowalloysteelbasemetalsandfillermetalsforwelding.Thepurposeofmanganesecangenerallybesummarizedasbeingthreefold[16]:

Combineswithoxygeninthemoltensteelasadeoxidizer; Tiesupanysulphurthatmaybepresenttoavoidhotcracking; Promotesgreaterstrengthbyincreasingthehardenabilityofthesteel(toughnessis

usuallyimprovedasabonuseffect)Siliconhasanatomicweightof28andameltingpointof1427oC.Siliconisusedmainlyinsteelsasadeoxidizingagent.Insteelmaking,theamountofsilicontobeaddedisslightlyinexcessofthequantityneededtocombinewiththeoxygen.Siliconandoxygenreactvigorouslytoliberatealargeamountofheat,andsilicondioxide(SiO2)isformed.Thesilicondioxidecaneitherescapetothemoltenmetalsurfaceorforminclusionsinthesolidifiedsteel.Withthepresenceofmanganeseinthemoltensteel,thesilicondioxideinclusionswilloftenoccurascomplexironmanganesesilicatecompounds.Ifmanganeseandsiliconareaddedtogetherandactsynergistically,themanganeseandsiliconcanachieveanonmetalliccompositionthatwillremainliquidatthefreezingtemperatureofthesteel[16].Thesiliconcanalsobeusedasaferritestrengthener,anditisstrongerinthisrespectthanmostothercommonlyusedalloyingelements.Also,siliconisastronghardenabilitypromoter.Therefore,heattreatablealloysteelshaveahighproportionofsilicon.Finally,siliconcanpromotethefluidityofthemoltensteel.Itissometimesusefulinpouringcastingsandincertainfusionweldingprocesses.

24

Inthemoltensteel,thedeoxidationofsiliconandmanganesecangivecomplexproducts,evenfortheequilibriumcondition.AccordingtoTurkdogan[17],theproductswillbedeterminedbyallthreefactors,i.e.thecontentofsiliconandmanganese,thecontentratiobetweensiliconandmanganese,andthetemperature,asshowninFigure18.

Figure18Productsofoxidationofsiliconandmanganeseinequilibriummoltensteel.(Adaptedfrom[17])However,theoxidationproductsofsiliconandmanganesecouldbemorecomplexbecausetheweldingprocessgenerallyfeaturesarapidcoolingprocess,makingithardtoachieveequilibriumconditionsinthemoltenmaterial.Also,itishardtomeasurethetemperatureintheweldingprocess.GrongandChristensen[18]performedanexperimentinanenclosedchamber.Differentshieldinggaseswereusedintheexperiment.Basically,twotypesofspecimenswereprepared:oneisachilledweldmetalandtheotheroneisamultilayerweldment.Theelectrodetipafterweldingwasalsoanalysed.Forthechilledweldmetalsamples,theweldwasmadeonawatercoolingcopperwheelwhichwascoveredbyaTeflonscrapertoeasetheweldmetalseparation.Thechilledmetalwascollectedintheformofintermittentstrips(1to2mmwide,thickness0.05to0.1mm,length20to150mm).Forthemultilayerweldmetal,theweldingtrialswereperformedonmildsteelcoupons(135mmx95mmx15mm).Sixlayersweredepositedforeachsample.Theweldingspeedwas3mm/s.OnlythetopbeadswereinvestigatedwithrespecttoC,O,SiandMncontent.Afillerwireof0.8mmdiameterwasused.Therateofwirefeed(about125mm/s)wasadjustedtotheoperationalconditions(90to100Aand24to26V,respectively).ThechemicalcompositionofthefillerwireisshowninTable1.Table9Chemicalcompositionofthefillerwire.(Adaptedfrom[18])

Elements C O Si MnContent(wt %) 0.097 0.020 0.83 1.52

25

Bothelectronprobeanalysisandchemicalanalysiswereperformedforthesamples.Acomparisonbetweentheresultsofthechilledweldmetalandthemultilayerweldmetalrevealedthattheoxidizingreactionofcarbondoesnotnormallytakeplaceintheweldpool,butoccurseitherattheelectrodetiporinthearcplasma,ormaybeboth.GrongandChristensen[18]alsodiscussedthatitisreasonabletoassumethattheoxidationmainlyoccursontheelectrodetip,ratherthaninthearcplasma.Thelargemetal/atmosphereinterfaceisthemainreasonforCOnucleationduringdropletformation.Twopartsofthedropletweredistinguishedinthediscussionofcarbonoxidation.Oneisthehotlayer,whichreferstothepartfacingthearc.Thispartismostlikelythelocationofcarbonoxidation.Theotherpartisthepartonthesideoftheelectrodewire(thelowertemperaturepartcomparedwiththepartfacingthearc)inwhichSiandMnareexpectedtoprotectcarbonfromoxidation.WhenreachingthecriticalCOgaspressure,thecarbonreactionisblocked.Thus,siliconandmanganesewilltakeoverandcontroltheoxygenlevelinthemetal.OnereasonforthelackofcarbonoxidationinthearcplasmaisaninadequatesupplyofoxygenduetoMnandFevapourarisingfrommetalsuperheating.Inthehotpartoftheweldpool(thepartsimultaneouslybeneathandfollowingthearc),theplasmajetblowstheMnandFevapourawayandprovidesasteadysupplyofoxygen.However,themetal/atmosphereinterfacemaybetoosmallinvolumecomparedwiththedropletstolimitthecarbonoxidationinthisstage.Inthecoolerpartoftheweldpool(thepartnexttothearcintheweldpool),SiandMnarenormallyexpectedtopreventanycarbonreaction.Regardingthelossesofsilicon,GrongandChristensen[18]concludedthatlossesofsiliconmainlyoccurintheweldpoolandtheamountofSilostontheelectrodetipandinthearcplasmaismuchsmaller.Sincenoslagisformedonthesurfaceofthechilledmetalsample,theauthorsbelievethatthesiliconmustescapeintheformofagaseousproduct.Ontheotherhand,evaporationlossescanbeexcludedinthiscaseduetoaverylowSivapourpressureattheprevailingtemperature.Therefore,itisreasonabletoassumethatSiOisformedonthetipdropletsurface.Thesiliconlossesofchilledmetalwithincreasingoxygenpotentialofshieldinggasarebelievedtobecausedbythereaction:

SiO2(inslag)+CO(g)=SiO(g)+CO2(g)Whencomparingthemanganesecontentbetweenthechilledmetalsamplesandthemultilayerweldmetalsamples,GrongandChristensen[18]believedthatthelossesofMnaremainlyduetovaporizationduringextremelyhightemperaturearcplasma(2400oC).Intheirconclusion,theauthorssummarizedtheresultsbydifferentlocationintheweldingprocess,i.e.electrodetip,arcplasma,hotpartoftheweldpool,andthecoolingpartoftheweldpool.Fortheelectrodetip,thetemperaturewasstatedtobeabout1600oCandcarbonmonoxideandmanganesesilicateslagwereformed.Carbonwaslostmainlyinthisstageaswell.AcertainamountofsiliconwaslostasaresultofSiOformationinthefumes.Forthearcplasma,withatemperatureofabout2400oC,theprecipitatedmanganeseslagintheformerstageredissolvesinthemetaluponheating.FurtherabsorptionofoxygenwillbepreventedbyFeandMnvapours.Fumesareformedbymetalvapourreactingwithoxygen.

26

Forthehotpartoftheweldpool,thetemperaturerangeis2000oCto1800oC.Oxygenis,toalargeextent,dissolvedintheweldpoolatthearcrootresultingfromtheinteractionwiththeplasmagas.Manganesesilicateslagformationtakesplacesimultaneously,preventingtheoxygenconcentrationfromexceedingacertainlimit.Aseparationofmicroslagproceedscontinuouslyundertheexistingturbulentconditions.ForCO2welding,aCOgasbarrierabovethemetalsurface,fromthedecompositionofCO2,preventstheabsorptionofoxygen.Inthecoolingpartoftheweldpool,thetemperatureisbelow1800oC.Siliconandmanganesereactfurtherwithdissolvedoxygenuponcooling.Themicroslagseparationwillbehighlyunfavourableduetoanearlystagnantbath.Forthisreason,theanalyticaloxygencontentishighlyunpredictable,butisneverthelesscloselylinkedtothesiliconandmanganeseconcentrationsintheweldmetal.

27

5 Experimentaltechniques5.1 Steelandconsumablematerials

Intheexperimentalworkofthisthesis,Domex355wasusedastheparentmaterialandESABOKAutrod12.51wasusedastheweldingwire.Domex355isahotrolledcoldformingsteelwhichmeetssteelS355MCinEN101492.Itcanbeusedinapplicationssuchastruckchassis,cranesandearthmovingmachines.ThechemicalcompositionofDomex355isshowninTable10anditsmechanicalpropertiesinTable11[19].Table10ChemicalcompositionofDomex355MC(Adaptedfrom[19])

C Si Mn P S Al Nb V TiBasemetal:Domex355MC 0.10 0.03 1.50 0.025 0.010 0.015 0.09 0.200.15

Table11MechanicalpropertiesofDomex355MC(Adaptedfrom[19])

YieldstrengthReff(Nmm)

minTensilestrengthRm(Nmm)minmax

Elongationonfailure

5.3 SeInexwguththanthAlcoch

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60

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tiontoolthatgyscanningeghvoltagefiearthesamptronsandchdusinganelectron(SE)imdesinSEM.

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28

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29

Figure20SignalsproducedinSEMThesecondaryelectronshavelowenergyduetotheinelasticinteractionwiththeatomsinthesample.Duetotheircharacteristics,thesecondaryelectronscomefromanareaveryneartothesamplesurface.Also,theedgesoftheobjectstendtogivemoresecondaryelectrons.Therefore,SEimagescanusuallygiveveryhighresolutionsurfaceimagesandcanalsobeusedtoproduceathreedimensionalimageofthesamplesurface.BSEsarebeamelectronsthatarereflectedfromthesamplebyelasticscattering.TheBSEshavehighenergyandcangeneratealargeinteractionvolumebeneaththespecimensurface.Sincetheheavyelementscangivemorebackscatteredelectrons,BSEsareusedtodetectcontrastbetweenareaswithdifferentchemicalcompositions.ThespatialresolutionoftheSEMdependsonthesizeoftheelectronspotandthesizeoftheinteractionvolume,asshowninFigure21.ThelargerelectronspotandgreaterinteractionvolumegivealowerresolutionoftheSEM.Sincetheinteractionvolumeisdependentontheenergyofthebeam,lowerbeamenergyissuggestedwhenperformingimageanalysisatthespecimensurface.

Figure21ElectroninteractionvolumeTheSEMusedinthisstudywasaJSM6490LVfromJOEL,whichhasaresolutionof3.0nm.Thelowvacuummodewasusedtoinvestigatetheinfluenceofmountingmaterialinthefirst

30

paper.Thehighvacuummodewasusedintheworkpresentedinthesecondandthirdpapersduetotheutilizationofconductivemountingmaterials.Standardautomatedfeatureswerealsousedintheabovementionedexperiments,includingAutoFocus,AutoGun(saturation,biasandalignment),andautomaticcontrastandbrightness.Energydispersivespectroscopy(EDS)isananalyticaltechniqueusedfortheelementalanalysisorchemicalcharacterizationofamaterial.Asmentionedabove,acharacteristicXraycanbeemittedwhenthehighenergyelectronbeamhitsthesample.ThenumberandenergyoftheXraysemittedfromaspecimencanbemeasuredusinganenergydispersivespectrometer.ByanalysingtheuniqueXray,EDSallowstheidentificationofdifferentelementsinthesample.TheaccuracyoftheEDSspectrumcanbeaffectedbymanyfactors.WindowsinfrontofthedetectorcanabsorbthelowenergyXray(i.e.EDSdetectorscannotdetectelementswithanatomicnumberoflessthan4,i.e.H,He,andLi).Also,manyelementswillhaveoverlappingpeaks(e.g.TiKandVK,MnKandFeK).Theaccuracyofthespectrumcanalsobeaffectedbythenatureofthesample.Xrayscanbegeneratedbyanyatominthesamplethatissufficientlyexcitedbytheincomingbeam.TheseXraysareemittedinanydirection,andsotheymaynotallescapethesample.ThelikelihoodofanXrayescapingthespecimen,andthusbeingdetectedandmeasured,dependsontheenergyoftheXrayandtheamountanddensityofthematerialithastopassthrough.Thiscanresultinreducedaccuracyinthecaseofinhomogeneousandroughsamples.TwotypesofSEMandEDSsystemwereusedinthisstudy.IntheexperimentofpaperI,theSEMandEDSsystemisahighresolutionfieldemissiontypeLEO1550GeminiequippedwithanOxfordEDSsystem.IntheexperimentofpaperIIandpaperIII,thesystemisaBrukerQuantax800system.ThedetectorisanXflash4010witharesolutionof127eVfortheMnKpeak.

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6 SummaryofappendedpapersThreepapershavebeenorwillbepublishedregardingtheobjectivesofthisthesis: Classificationofcoldlaps, Characterizationofcoldlapinterface, Investigationoftheinfluencingfactorsoncoldlapformation.Thischapterpresentsabriefsummaryofthepaperswhichhavebeenreformattedforuniformityandincreasedreadability.

6.1 PaperIThemainpurposeofthispaperistoclassifycoldlaps.Secondly,theinterfaceofthespatter/basemetalissupposedtobecharacterized.Atandemweldingexperimentwasperformedusing92Ar8CO2shieldinggas,Domex355MCbasemetal,andESABOKAutrod12.51wire.Avisualtest(VT)wasfirstperformedontheweldedspecimens.Thespecimenswerecutandpolishedtopreparefortheconventionalmetallographicsamples.Thesampleswereevaluatedbylightopticalmicroscopyandscanningelectronmicroscope(SEM).Threetypesofcoldlapswereobserved,i.e.spattercoldlap,overlapcoldlapandspatteroverlapcoldlap.Spattercoldlap,namedastypeI,isformedwhenaspatterissituatedclosetotheweldtoesothatthespatterisfusedandpartiallyincludedintheweld.Itisworthpointingoutherethatthecoldlapsfromspattersstatedin[12]canalsoberegardedasaspecificsituationinwhichthespatterisfullyembeddedintheweldandisinvisible[12].Inthiscase,itcanbecharacterizedbyL

32

findcoldlapsalongtheentireweldtoe(inthelongitudinaldirectionoftheweld),whichareusuallycausedbythepoorsurfaceconditionofthebasemetalandimproperweldingparameters.TypeIIIisreferredtoasspatteroverlapcoldlap,andcanberegardedasacombinationoftypeIandtypeII.Thisusuallyappearsinawormlikeshapeandcanbeseveralmillimetresawayfromtheweldtoe.Microscopyanalysisofthespatter/basemetalinterfacehasrevealedalackoffusionconsistingofbothvoidsandoxides.ThemainoxidesdetectedwereMnSioxides,whichcanbesymbolisedwiththegeneralformula(MnSi)Onwherenvariesfrom0.3to0.5.

6.2 PaperIIThemainpurposeofthispaperistoinvestigatetheinfluenceoftheMnSioxidesobservedinpaperIoncoldlaps.Secondly,anyotherinfluencingfactorsoncoldlapswerestudied.AseriesoftandemarcweldingexperimentswasperformedinasealedchamberfilledupeitherwithpureargonorwithpureCO2withthepurposeofcreatingoxidizingornonoxidizingweldingenvironments,respectively,andstudyingtheinfluenceoftheshieldinggasontheformationofcoldlaps.Twosurfaceconditions,i.e.anironsandblastedsurfaceandaburrgrindingsurface,werepreparedforthebasemetal.Theweldedspecimenswerevisuallyexaminedusingthenakedeye.Spatterswerealsoevaluatedintermsofsizeanddistributionontheweldedspecimens.InthepureArweldingprocess,smallerspattersweregeneratedandconcentratedinasmallareaclosetotheweldtoe,whereasthespattersinpureCO2weldingwerelargerinsizeandmoreevenlydistributed.Allthreetypesofcoldlapswereidentifiedandanalysed.Theinterfaceofdifferenttypesofcoldlapswasexaminedusingalightopticalmicroscope,SEMandEDS.VoidsandMnSioxideswerefoundinthecoldlapinterface.TheresultsrevealedthattheMnSioxideshavethemostsignificantinfluenceonoverlapcoldlapoccurrence.Itwasalsoobservedthattheblastedsurfacecanenhancethelackoffusionbyeasingtheairentrapmentintheinterface.ByevaluatingthespattersurfaceandbasemetalsurfaceafterweldingregardingMnSioxides,itcouldbeconcludedthattheMnSioxidesincoldlapscamemainlyfromtheoxidizationofdropletsbeforetheyreachedtheweldpool.

6.3 PaperIIIThemainpurposeofthispaperistorevealthefactor(s)behindthecoldlapsfoundinthepureArweldinginpaperII,bystudyingthespatter/basemetalinterface.Thespatter/basemetalinterfacewasinvestigatedfortwodifferentweldingconditions,i.e.inpureArandinpureCO2.ThecontributionofMnSioxideswasconfirmedforthelackoffusionformationatthespatter/basemetalinterface.Theratiobetweenthelengthoflackoffusionattheinterfaceandtheentirelengthofthespatter/basemetalinterface(RLoF)wasintroducedinthisinvestigation.StudyingtheRLoFagainstspatterdistanceandspatterdiameterrevealedthattheRLoFincreasedabruptlyuntilthespatterdiameterreachedacriticalvalue(approx.1mm)asthespatterdiameterdecreasedfromitshighestvalue.Forlowerspatterdiametervaluesthanthecriticalvalue(i.e.under1mm),theRLoFremainedconstant(100%)withthecontinuousdecrementofspatterdiameter.AdirectproportionwasrevealedbetweenspatterdistanceandRLoF.However,theheat/temperatureofthespatterisinfluencedbyboththe

33

spatterdiameterandthespatterdistanceregardingthefusionbetweenspatterandbasemetal.Therefore,theheat/temperaturecanberegardedastheotherimportantinfluencefactorforlackoffusionformation.

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7 ConclusionsandfutureworkThroughtheexperimentalstudyofcoldlaps,threetypesofcoldlapswereidentified,i.e.overlapoldlap,spattercoldlap,andspatteroverlapcoldlap.Foralltypesofcoldlaps,theinterfacewascharacterizedbyvoidsandoxides.Theoxidesdependedontheweldingconsumables.Forthematerialsappliedinthisstudy,manganesesiliconoxideswereshowntohavesignificantinfluenceoncoldlapoccurrence.Theinfluenceoftemperatureoncoldlapswasshownbytheindicatorsofspatterdiameteranddistancefromspattertotheweld.Inthefuturework,thetemperatureinfluenceoftheweldingworkpieceoncoldlapsshouldbeinvestigated.Also,thebehaviouroftheweldingpoolcanbestudiedwithhelpofhighspeedimagingtechnology.Regardingtheinfluenceofoxidesoncoldlaps,newconsumablesofGMAW,whichcanavoidoxidesinweldingprocess,areofinteresttostudy.

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8 References1. L.Lopez.Martinez,P.Korsgren.CharacterizationofInitialDefectDistributionandWeld

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4. M.Lundin,L.Lopez.Martinez,J.Hedegrd,K.Weman.HighproductiveMAGweldingFatiguepropertiesofweldments.In,WeldedHighStrengthSteelStructures.Stockholm,Sweden:EMAS;1997

5. L.Holst,M.Lundin,J.Hedegrd,K.Weman.HighproductiveMAGWeldingWeldtoedefectsAnewdetectionmethod.In,WeldedHighStrengthSteelStructures.Stockholm,Sweden:EMAS;1997

6. K.Weman,M.Lundin,J.Hedegrd.HighproductiveMAGweldingofvehiclecomponents.In,WeldedHighStrengthSteelStructures.Stockholm,Sweden:EMAS;1997

7. Volvoweldqualitystandard.In,STD1810004.Sweden;20088. K.Weman.WeldingProcessHandbook:CRCPrssInc(5Aug2003);20039. R.L.O'Brien.WELDINGPROCESS.Eighthed.MIAMI:AMERICANWELDINGSOCIETY;199510. PritchardD.Soldering,Brazing&WeldingAManualOfTechniques:TheCrowoodPress;200111. L.P.Connor.WELDINGTECHNOLOGY.Eighthed.Miami:AMERICANWELDINGSOCIETY;198712. M.FarajianSohi,N.Jrvstrt.AFractographicalInvestigationofWeldToeImperfectionsin

TandemGasMetalArcWelding.Steelresearchinternational2006;77(2006)No.12:88989513. J.Hedegrd,L.Lopez.Martinez,N.Moradashkafti,H.Trogen.Theinfluenceofwelding

parametersonthesizeanddistributionofwelddefectsforvariousweldmethods.In,VTTSymposium156:FatigueDesign1995.Helsinki:VTTOffsetprintEspoo1995;1995

14. M.FarajianSohi,N.Jrvstrt,M.Thuvander.EffectofWeldingParametersonFormationofToeImperfectionsinTandemGasMetalArcWeldingIn,7thInternationalConferenceonTrendsinWeldingResearch.Geogia,USA:ASM;2005

15. L.Lopez.Martinez,A.F.Blom,J.Samuelsson.WelddefectsbeforeandafterpostweldtreatmentforMAGandhighproductivityMAGwelding.In,WeldedHighStrengthSteelStructures.Stockholm,Sweden:EMAS;1997

16. G.E.Linnert.WeldingMetallurgy.HiltonHeadIsland,SouthCarolina:AWS;199417. E.T.Turkdogan.Deoxidationofsteel.In,ChemMetallofIronandSteel.Sheffield,U.K.:The

IronandSteelInstitution;1973:15317018. O.Grong,N.Christensen.FactorsControllingMIGWeldMetalChemistry.Scandinavian

JournalofMetallurgy1983;12:15516519. SSAB.Domex355MCDatablad.In.Brolnge:SSAB;2011

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9 Appendedpapers

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