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1焊接专业本科生用1WHATISWELDING?Weldingtechniqueshavebecomesoversatilethatitisdifficultnowadaystodefine“welding”.Formerlyweldingwas“thejoiningofmetalsbyfusion”,thatis,bymelting,butthisdefinitionwillnolongerdo.Eventhoughfusionmethodsarestillthemostcommon,theyarenotalwaysused.Weldingwasnextdefinedasthe“joiningofmetalsbyheat”,butthisisnolongeraproperdefinitioneither.Notonlymetalscanbewelded,socanmanyoftheplastics.Furthermoreseveralweldingmethodsdonotrequireheat.Everymachinistisfamiliarwithheatlesswelding.Whenthechipslidesoverthecuttingtoolonalathe,itoftenleavesabuilt-upedgeofchipmaterialweldedtothetopofthecutterbit.Thisiscoldpressurewelding,whichunderothercircumstancesisproperproductionweldingmethod.Besidespressurewelding,wecanweldwithsoundandevenwithlightfromthefamouslaser.Facedwithadiversityofweldingmethodsthatincreaseyearbyyear,wemusthereadoptthefollowingdefinitionofwelding.“weldingisthejoiningofmetalsandplasticsbymethodsthatdonotemployfasteningdevices”.Thejoiningofmetalsbymethodsthatdonotemployfasteningdevicesisanartasoldasblacksmithing.Neverthelessweldingasamanufacturingprocessmustbeconsideredadevelopmentofthetwentiethcentury.Inthe1880’scarbonarclampswereusedforstreetlighting.Atthattimeitwasnotedthatthecarbonarclamp,likealllamps,producedmoreheatthanlight,andthefirstattemptsweremadetousethisheatforweldingmetals.Thusthefirstweldingmethodofthosestillinusewascarbonarcwelding,perfectedabouttheturnofthecentury.Stickelectrodesandoxyacetyleneweldingalsoappearedabout1900.weldingwasgenerallyusedonlyforrepairandmaintenanceuntilthe1920’s.X-rayexaminationofweldscamesoonafterwardanddidmuchtodevelopconfidenceinweldedjoints.Themiddleofthe20thcenturysawgreatdevelopmentinweldingaswellasinothertechniques.Italsogaveus,amongotherthings,inertgaswelding.Sincethenweldinghasprogressedatatremendousrate.Ithelpsdevelopscienceandtechnology,includingtheelectronictechnique.Atthesametime,thetechnologyofweldingembracesawidearea.Manyelectroniccircuitsarerequiredtocontrolthemoreintricateweldingmachines.Evenradiofrequencieshavetheirapplicationsinwelding,ininductionbrazingandtheultrasonictestingofwelds.PhotographyisalsodrawnintothescopeofweldingforX-rayandγ-rayphotographyisusedintheexaminationofweldedjoints.Weldinghasmadeimportantcontributionstothemodernizationofournationaldefenceindustry,suchasthemakingofnuclearreactorsandman-madeearthsatellites,forbothareweldments.Ontheothersideofthecoin,thoseproductsofthenuclearreactors,radioisotopesarefamiliararoundweldingshopsintheirfunctionofweldingtesting.Alltheseandotherdevelopmentsshowthatweldingisnolongerwhatitwasintimepast,thesimplematterofrunningaheadwithagasflameorastickelectrode.Theoperatorofthemodernautomaticwelderwearsneitherweldinghelmetnorgoggleswhenweldingtheheavy-walledpressurevessel.Butdepositingweldmetalisaboutfifteentimesfasterthanbymanualmethods.Hisweldingrodsarethetwolargecoilsofwireatthetopoftheweldingmachine,andtheseareautomaticallyfedintotheweld.Thearcisnotvisibleinthismethodofwelding,andsonohelmetisrequired.Theweldingcontrolsaregroupedoperator.ThisisUnionmeltsubmergedarcwelding,acommonlyusedweldingmethodthatcaninanhourdepositweldmetalequalinweighttothatoftheoperatorhimself.Attheoppositeextremeis2microwelding,inwhichtheoperatormayneedamicroscopetoseewhathehaswelded.Betweenthesetwoextremesinsizeandweldingcapacityareperhapsfivedozenotherweldingmethods.2ClassificationofWeldingProcessesMostweldingprocessesrequiretheapplicationofenergy(heatorpressure)toproduceasuitablebond.Thereforeweldingprocessesmaybeclassifiedandnamedaccordingtobythetypeofenergysourceemployed.Ifincludingbrazing,theclassificationofweldingprocessesislistedbelow(1)Fusionwelding①Oxyacetylenewelding(OAW)②ArcWeldingShieldedmetalarcwelding(SMAW)orManualmetalarcwelding(MMAW)Gasmetalarcwelding(GMAW),includes:metalinertgasesarcwelding(MIG)andmetalactivegasesarcwelding(MAG)Gastungstenarcwelding(GTAW)ortungsteninertgasarcweldng(TIG)Fluxcoredarcwelding(FCAW)Electrogaswelding(GMAW-EG;FCAW-EG)Plasmaarcwelding(PAW)Submergedarcwelding(SAW)③Electroslagwelding(ESW)④Thermitwelding(TW)⑤Laserbeamwelding(LBW)⑥Elecrtonbeamwelding(EBW)(2)Pressurewelding(resistancewelding)①Spotwelding②Seamwelding③Buttwelding④Coldpressurewelding⑤Frictionwelding⑥Ultrasonicwelding⑦Explosionweldinginvacuum⑧Diffusionwelding⑨Highfrequencywelding(3)BrazingweldingThiswayofclassifyingweldingprocessesiscalledfamily-treemethod.Infact,thereisnouniformmethodofnamingweldingprocesses.Manyprocessesarenamedaccordingtotheheatsourceorshieldingmethod,butcertainspecializedprocessesarenamedafterthetypeofjointproduced.Examplesarestud,spotandbuttwelding.Anoverallclassificationcannottakeaccountofthisbecausethesametypeofjointmaybeproducedbyavarietyofprocesses.Studweldingmaybedonebyarcorprojectionweldingandspotweldingbyelectricresistance,arcorelectron-beamprocesses.Buttweldingmaybyresistance,flashoranyofanumberofothermethods.Althoughincommonusagemanyprocesseshaveabbreviatednames,thefullnamesoftenfollowthepattern:first,astatementofthetypeofshielding;secondly,thetypeofheatorenergysource;thirdly