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Electrtricarcwelding(part2)
(电弧焊)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(氩气),helium(氦气),mixturesofthemandcarbondioxide.Thedevelopmentofinertgasshieldedarcwelding
InertgasshieldedarcweldingArgon(氩气)isnormallypreferredoverheliumbecauseofanumberofspecificadvantages.Itrequiresalowerarcvoltage,allowsforeaserarcstartingandprovidesasmootharcaction.Longerarccanbemaintainedwithargon,sincearcvoltagedoesvaryappreciablywitharclength.Itistheheaviestoftheshieldinggasesused,assuchitgenerallyrequiresalowerflowrateforgoodshieldingaction.Thedevelopmentofinertgasshieldedarcwelding
Argonisusefulforweldingthinsheetsandforflatpositionwelding.InertgasshieldedarcweldingThemainadvantageofheliumisthatitcanwithstandthehigherarcvoltages.Itisusedintheweldingwherehigherheatinputisrequired,suchasforthicksheetsorforhigherthermalconductivematerialssuchascopperoraluminium.Thedevelopmentofinertgasshieldedarcwelding
Carbondioxideisthemosteconomicalofalltheshieldinggases.Underthearc,thecarbondioxidedecomposestocarbon
monoxide(CO)andoxygenwhichsubsequentlycombinesbacktoformcarbondioxidewhentheycool.Itrequiresslightlyhighercurrentwhichcausemoreagitationintheweldpuddleresultinginthetrappedgasestoriseandthustoreducetheweldporosity(焊缝气孔).
InnergasshieldedarcweldingTungstenInertGasWelding(TIG)
Tungsteninertgas(TIG)weldingorgastungstenarcwelding(GTAW)isaninertgasshieldedarcweldingprocessusingnon-consumableelectrode.Atypicaltungsteninertgasweldingsetup
Thepowersourcesusedarealwaystheconstantcurrenttype.Bothdirectcurrent(DC)andalternatingcurrent(AC)powersuppliescanbeusedforTIGwelding.InnergasshieldedarcweldingTungstenInertGasWelding(TIG)
TheelectrodesofTungsteninertgas(TIG)weldingmayalsocontain1to2%thoria
(氧化钍)or0.15to0.40%Zirconia(氧化锆).TheTIGweldingprocesscanbeusedforthejoiningofanumberofmaterials:Thepowersourcesusedarealwaystheconstantcurrenttype.BothDCandACpowersuppliescanbeused.WhenDCisused,theelectrodecanbenegative(DCEN)(直流反极性)orpositive(DCEP)(直流正极性).Thecurrentcarryingcapacity(载流容量)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短路过渡溶滴过渡喷射过渡脉冲喷射过渡旋转射流过渡InertgasshieldedarcweldingIntheGMAWprocess,thefillermetalistransferredfromtheelectrodetothejoint.deShortcircuit(短路过渡)Advantageofusingtheshortcircuitmetaltransferisthelowerpenetrationwhichcanbeeffectivelyusedforweldingthinsheets.Inertgasshieldedarcwelding
Globularordroptransfer(溶滴过渡)Globularordroptransferofmetaloccursatslightlyhighercurrentsthanthoseusedinshortcircuittransfer.Themetalistransferredbygravity,thismethodcannotbeusedinoutofpositionweldingandcanonlybe
usedforflat(downhand)
weldingposition.
InertgasshieldedarcweldingSpraytransferofmetaloccurswhenthecurrentisincreasedbeyondthatoftheglobulartransfercurrent.Gravityhasnoeffectonthemetaltransfer.Spraytransferweldingcanbedoneinanyposition,horizontal,verticaloroverhead.
Spraytransfer(射流过渡)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(电源)
inthesubmergedarcweldingEitherACorDC.ThecurrentratingsoftheSAWmachinesaretwotothreetimeshigherthanthatofthesimilarGMAWmachines.WithasinglewireSAW,arcblowisnoencounteredwithACsupply.Buttwowiresmaybeusedtodepositlargeramountsofmetal.Thesetwoelectrodesbeingconnectedtotwoseparatepowersources,thearcblowislikelytooccur.Thewaytoreducethisproblem.Thetwopowersuppliesaresoadjustedthatwhenoneofthesuppliesisinpeak,theotherwouldbesettozerocurrentsothatnoopposingmagneticfieldswouldbeset.Oneoftheelectrodeiscalledleading(主导)andtheothertrailing(从动).Thetwoelectrodesarephysicallyseparated,inthejoint,onepositionedslightlyaheadoftheother.
SubmergedArcWelding(埋弧焊)Sincethearciscompletelysubmergedintheflux,thereisnospatter(没有飞溅)ofthemoltenmetal.Sincethisprocessusesloosegranulatedfluxtocoverthejoint.(flatordownhandposition)ParametersinthesubmergedarcweldingTheweldmetalbacking
(衬垫),Themostcommonlyemployedarethecopperplateswhichcanbecooledwithinternalrunningwater(活水).Theelectrodewiresnormallyusedareofsizes1.6,2,2.5,3.15,4,5,6.3and8mm.Thewiresshouldbesmoothwithnosurfaceimperfectionsorcontaminants(污染).Thewirefeedrate
(送丝速度)isnormallyveryhigh,itisnotpossibletomanuallyfeed(手工送丝)thewireintothejoint.SubmergedArcWelding(埋弧焊)Thetypeofedgepreparationandjointdesign
SubmergedArcWelding(埋弧焊)
Theamountofcurrentsetting(电流设定)
Itdeterminesthewireburnoffratefordifferentelectrodes.SubmergedArcWelding(埋弧焊)SubmergedArcWelding(埋弧焊)Theexpectedmetald
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