Electrtricarcwelding(part2)

（電弧焊）Lesson14Carbonarcwelding（碳弧焊）Theearliestofthearcweldingprocesses.(electrodeismadeofeithercarbonorgraphite).Incontrasttographiteelectrodes,thecarbonelectrodesaresoftandmorecontrollable,cannottakeupveryhighcurrentdensities.Thoughcarbonorgraphiteelectrodesarenotexpectedtomeltasdoestheconsumableelectrodes,theydogetheatedtoaredhottemperatureandtheelectrodeisslowlyconsumed.Thelifeofthegraphiteelectrodeishigherthanthatofthecarbonelectrode.Carbonarcwelding（碳弧焊）Inthecarbonarcweldingpractice,thearccanbeobtainedbetweenthecarbonelectrodeandtheworkpieceorbetweentwocarbonelectrodes,asshowninfollowingFig.Carbonarcwelding（碳弧焊）Inthecaseofcarbonarcwelding,bettercontrolofheatinputispossiblebecauseoftheseparationoftheheatsourcefromthefillermetal.Itispossibletoweldthethickerplateusingtwincarbonelectrodeweldingbyprovidingadditionalheattothethickplatebeforeprovidingthefillermetalandheatingthejoint.Carbonarcweldingisnotsuitabletooverheadorverticalweldingpositions,butintheflatposition,veryhighmechanisedweldingspeedscouldbeobtained.InertgasshieldedarcweldingThewelderisalwaystriedtoobtainajointwhichisasstrongasthebasemetalandatthesametimethejointisashomogeneousaspossible.Thecompleteexclusionofoxygenandothergaseswhichinterferewiththeweldpooltothedetrimentoftheweldqualityisveryessential.(ahighpressureinertgasflowingaroundtheelectrodewoulddisplacealltheatmosphericgasesaroundtheweldmetaltofullyprotectit)

Theshieldinggasesmostcommonlyusedareargon（氬氣）,helium（氦氣）,mixturesofthemandcarbondioxide.Thedevelopmentofinertgasshieldedarcwelding

InertgasshieldedarcweldingArgon（氬氣）isnormallypreferredoverheliumbecauseofanumberofspecificadvantages.Itrequiresalowerarcvoltage,allowsforeaserarcstartingandprovidesasmootharcaction.Longerarccanbemaintainedwithargon,sincearcvoltagedoesvaryappreciablywitharclength.Itistheheaviestoftheshieldinggasesused,assuchitgenerallyrequiresalowerflowrateforgoodshieldingaction.Thedevelopmentofinertgasshieldedarcwelding

Argonisusefulforweldingthinsheetsandforflatpositionwelding.InertgasshieldedarcweldingThemainadvantageofheliumisthatitcanwithstandthehigherarcvoltages.Itisusedintheweldingwherehigherheatinputisrequired,suchasforthicksheetsorforhigherthermalconductivematerialssuchascopperoraluminium.Thedevelopmentofinertgasshieldedarcwelding

Carbondioxideisthemosteconomicalofalltheshieldinggases.Underthearc,thecarbondioxidedecomposestocarbon

monoxide(CO)andoxygenwhichsubsequentlycombinesbacktoformcarbondioxidewhentheycool.Itrequiresslightlyhighercurrentwhichcausemoreagitationintheweldpuddleresultinginthetrappedgasestoriseandthustoreducetheweldporosity(焊縫氣孔）.

InnergasshieldedarcweldingTungstenInertGasWelding(TIG)

Tungsteninertgas(TIG)weldingorgastungstenarcwelding(GTAW)isaninertgasshieldedarcweldingprocessusingnon-consumableelectrode.Atypicaltungsteninertgasweldingsetup

Thepowersourcesusedarealwaystheconstantcurrenttype.Bothdirectcurrent(DC)andalternatingcurrent(AC)powersuppliescanbeusedforTIGwelding.InnergasshieldedarcweldingTungstenInertGasWelding(TIG)

TheelectrodesofTungsteninertgas(TIG)weldingmayalsocontain1to2%thoria

（氧化釷）or0.15to0.40%Zirconia(氧化鋯).TheTIGweldingprocesscanbeusedforthejoiningofanumberofmaterials:Thepowersourcesusedarealwaystheconstantcurrenttype.BothDCandACpowersuppliescanbeused.WhenDCisused,theelectrodecanbenegative(DCEN)(直流反極性)orpositive(DCEP)(直流正極性).Thecurrentcarryingcapacity（載流容量）ofaDCENelectrodeisabout10timesashighasthatofaDCEPelectrod.Inertgasshieldedarcwelding

ThepowersourcesofTIGDCEP（直流反接）isnormallyusedforweldingthinmetalswhereasfordeeppenetrationweldsDCEN

(直流正接)isused.InertgasshieldedarcweldingCarbonarcwelding（碳弧焊）GasMetalArcWelding(GMAW)

Metalinertgasarcwelding(MIG)ormoreappropriatelycalledasgasmetalarcwelding(GMAW)utilizesaconsumableelectrode.Thoughgastungstenarcwelding(GTAWorTIG)canbeusedtoweldalltypesofmetals,itismoresuitableforthinsheets.Whenthickersheetsaretobewelded,thefillermetalrequirementmakesTIGdifficulttouse.Inthissituation,TheMIGcomeshandy.Metaltransfer短路過渡溶滴過渡噴射過渡脈沖噴射過渡旋轉射流過渡InertgasshieldedarcweldingIntheGMAWprocess,thefillermetalistransferredfromtheelectrodetothejoint.deShortcircuit（短路過渡）Advantageofusingtheshortcircuitmetaltransferisthelowerpenetrationwhichcanbeeffectivelyusedforweldingthinsheets.Inertgasshieldedarcwelding

Globularordroptransfer（溶滴過渡）Globularordroptransferofmetaloccursatslightlyhighercurrentsthanthoseusedinshortcircuittransfer.Themetalistransferredbygravity,thismethodcannotbeusedinoutofpositionweldingandcanonlybe

usedforflat(downhand)

weldingposition.

InertgasshieldedarcweldingSpraytransferofmetaloccurswhenthecurrentisincreasedbeyondthatoftheglobulartransfercurrent.Gravityhasnoeffectonthemetaltransfer.Spraytransferweldingcanbedoneinanyposition,horizontal,verticaloroverhead.

Spraytransfer（射流過渡）InertgasshieldedarcweldingSubmergedArcWelding（埋弧焊）Insubmergedarcwelding(SAW)，itispossibletouselargerweldingelectrodes(12mm)aswellasveryhighcurrents(4000A)sothatveryhighmetaldepositionratesoftheorderof20kg/hormorecanbeachieved.ThefeatureofthesubmergedarcweldingAlso,veryhighweldingspeeds(5m/min)arepossibleinSAW.Somesubmergedarcweldingmachinesareabletoweldplatesofthicknessashighas75mminbuttjointsinasinglepass.

Thoughsubmergedarcweldingcanbeusedevenforverysmallthicknesses,oftheorderofImm,itismoreeconomicalforlargerweldsonly.Theschematicofatypicalsubmergedarcweldingprocess:SubmergedArcWelding（埋弧焊）ThearcisproducedwhiletheconsumableelectrodewirewhichiscontinuouslyfedintotheweldzoneasinGMAW.RepresentationofSAWTheweldingzoneiscompletelycoveredbymeansofalargeamountofgranulatedfluxwhichisdeliveredaheadoftheweldingelectrodebymeansofaweldingfluxfeedtube.Thearcoccurringbetweentheelectrodeandtheworkpieceiscompletelysubmergedunderthefluxandnotvisiblefromoutside.Apartofthefluxmetalandformstheslagwhichcoverstheweldmetal.

SubmergedArcWelding（埋弧焊）Thepowersource（電源）

inthesubmergedarcweldingEitherACorDC.ThecurrentratingsoftheSAWmachinesaretwotothreetimeshigherthanthatofthesimilarGMAWmachines.WithasinglewireSAW,arcblowisnoencounteredwithACsupply.Buttwowiresmaybeusedtodepositlargeramountsofmetal.Thesetwoelectrodesbeingconnectedtotwoseparatepowersources,thearcblowislikelytooccur.Thewaytoreducethisproblem.Thetwopowersuppliesaresoadjustedthatwhenoneofthesuppliesisinpeak,theotherwouldbesettozerocurrentsothatnoopposingmagneticfieldswouldbeset.Oneoftheelectrodeiscalledleading（主導）andtheothertrailing（從動）.Thetwoelectrodesarephysicallyseparated,inthejoint,onepositionedslightlyaheadoftheother.

SubmergedArcWelding（埋弧焊）Sincethearciscompletelysubmergedintheflux,thereisnospatter（沒有飛濺）ofthemoltenmetal.Sincethisprocessusesloosegranulatedfluxtocoverthejoint.（flatordownhandposition）ParametersinthesubmergedarcweldingTheweldmetalbacking

（襯墊），Themostcommonlyemployedarethecopperplateswhichcanbecooledwithinternalrunningwater（活水）.Theelectrodewiresnormallyusedareofsizes1.6，2,2.5，3.15，4,5,6.3and8mm.Thewiresshouldbesmoothwithnosurfaceimperfectionsorcontaminants（污染）.Thewirefeedrate

（送絲速度）isnormallyveryhigh,itisnotpossibletomanuallyfeed（手工送絲）thewireintothejoint.SubmergedArcWelding（埋弧焊）Thetypeofedgepreparationandjointdesign

SubmergedArcWelding（埋弧焊）

Theamountofcurrentsetting（電流設定）

Itdeterminesthewireburnoffratefordifferentelectrodes.SubmergedArcWelding（埋弧焊）SubmergedArcWelding（埋弧焊）Theexpectedmetald