1、Welding TechniquesWelding Parameters and Techniques Their Effects On The Weld Afterhavingselectedthewireandgasforaweld,operatingconditionsmustbechosen.Thefourimportant parametersaretheweldingcurrent,wireelectrodeextension,weldingvoltageandarctravelspeed.These parameterswillaffecttheweldcharacteristi
2、cstoagreatextent.Becausethesefactorscanbevariedovera largerange,theyareconsideredtheprimaryadjustmentsinanyweldingoperation.Theirvaluesshouldbe recordedforeverydifferenttypeofweldtopermitreproducibility.WELDING CURRENT Theweldingcurrentistheelectricalamperageinthepowersystemastheweldisbeingmade.Itis
3、usually readfromthepowersourcemeter,butaseparateammeterisoftenused. Inthemigprocess,weldingcurrentisdirectlyrelatedtowire-feedspeed(ifthewireextensionbeyondthe guidetipisconstant).Asthewire-feedspeedisvaried,theweldingcurrentwillvaryinthesamedirection.In otherwords,anincrease(ordecrease)inthewire-fe
4、edspeedwillcauseanincrease(ordecrease)ofthe current.Figure7-1showsthetypicalwire-feedspeedvs.weldingcurrentrelationshipforvariousdiameter E70S-3wires.Thisrelationshipiscommonlycalledthe”burn-offcharacteristic.Thegraphalsoshowsthat whenthediameterofthewireelectrodeisincreased(ordecreased)atanywire-fe
5、edspeed,thewelding currentishigher(orlower).Eachtypeofwire(steel,aluminum,etc.)hasadifferentburn-offcharacteristic. Oneimportantfactthatshouldbenoticedin Figure 7-1istheshapeofeachburn-offcurve.Inthelower currentrangeforeachwiresize,thecurveisnearlylinear.Inotherwords,foreveryadditiontothecurrent, t
6、hereisaproportional(andconstant)increaseinthemeltoff.However,athigherweldingcurrents, particularlywithsmalldiameterwires,theburn-offcurvebecomesnon-linear.Inthisregion,higherwelding currentscauselargerincreasesintheburn-off.Thisisduetoresistanceheatingofthewireextension beyondtheguidetube.Thisresist
7、anceheatingisknownatPRheatwhereI=weldingcurrentandR= resistance.Thegreatertheweldingcurrent,thegreaterthePRheating.WIRE ELECTRODE EXTENSION Wire extension or ”stick-out” is the distance between the last point of electrical contact, usually the end of the contact tip, and the end of the wire electrod
8、e. Figure 7-2 schematically shows wire extension. It is in this area that PR preheating effect occurs.Thecontacttip-to-workdistance,becauseofitseffectonthewireextension,affectstheweldingcurrent requiredtomeltthewireatagivenfeedspeed.Fig.7-3showstheextenttowhichtheweldingcurrent requirementcanvarywit
9、htip-to-workdistance.Basically,asthetip-to-workdistanceisincreased,the amountofI2Rheatingincreasesandtheweldingcurrentrequiredtomeltthewireisdecreased.The converseisalsotrue.Controllingtip-to-workdistanceisimportant.Longextensionsresultinexcessweldmetalbeingdeposited withlowarcheat.Thiscancausepoorb
10、eadshapeandlowpenetration.Inaddition,asthetip-to-work distanceincreases,thearcbecomeslessstable.Forshortarcwelding3/8in.(9.6mm)tip-to-workdistance isrecommended.Itisveryimportantthatthewireextensionbekeptasconstantaspossibleduringthe weldingoperation.Inviewofthesubstantialeffectontheweldingoperation
11、,itisalwayswisetorecordnot onlycurrentandvoltage,butalsothewire-feedspeed.WELDING VOLTAGEAlthoughdiscussedinChapter3,itshouldbere-emphasizedthatthevoltagesettingdirectlycontrolsthearc length.Inaddition,acertainrangeisrequiredtomaintainarcstabilityatanygivenweldingcurrentlevel.ARC TRAVEL SPEED Thearc
12、travelspeedisthelinearratethatthearcmovesalongtheworkpiece.Thisparameterisusually expressedasinchesormetersperminute.Threegeneralstatementscanbemaderegardingthearctravel speed: 1)Asthematerialthicknessincreases,thetravelspeedmustbelowered. 2)Foragivenmaterialthicknessandjointdesign,astheweldingcurre
13、ntisincreased,soisthearc travelspeed.Theconverseisalsotrue. 3)Higherweldingspeedsareattainablebyusingtheforehandweldingtechnique.WELDING TECHNIQUES Thefirstgeneralweldingtechniquethataffectsweldcharacteristicsistorchposition.Thisreferstothe mannerinwhichthetorchisheldwithrespecttotheweldjoint.Thepos
14、itionisusuallydescribedfromtwo directionstheanglerelativetothelengthoftheweldandtheanglerelativetotheplatesasillustratedin7-4 and 7-5respectively.Bothbackhandandforehandweldingtechniquesareshownin7-4.Thebackhand methodmeansthetorchispositionedsothatthewireisfeedingoppositetothedirectionofarctravel.F
15、iller metalisbeingfedintotheweldmetalpreviouslydeposited.Fortheforehandmethod,thetorchisangledso thattheelectrodewireisfedinthesamedirectionasarctravel.Nowthefillermetalisbeingdeposited,for themostpart,directlyontheworkpiece.Itshouldbenotedthatachangeinweldingdirectionisnotrequired tofacilitateforeh
16、andorbackhandwelding,onlyareversalinthelongitudinaltorchpositioning.Generally, operatorsfindthatthebackhandtechniqueyieldsamorestablearcandlessspatterontheworkpiece.The angle relative to the plate for the fillet weld shown in Figure 7-5 is usually 45 deg. However, for a beveled butt joint, this angl
17、e may only be a few degrees from the vertical to allow for proper wetting of the weld metal to the side wall.Thesecondgeneralweldingtechniquethatshouldbeconsideredisthatofarctraveldirectionwhenthe weldingmustbeperformedintheverticalposition.AsFigure 7-6illustrates,therearetwomethodswith whichthiswel
18、dingcanbedoneverticalupandverticaldown. Herethetorchpositioningisextremelyimportantandweldingshouldbeperformed onlyasshown.Ineither case,thearcmustbekeptonthepuddlesleadingedgesoastoinsurecompleteweldpenetration. Thiscompletesadefinitionofthefactorswhichmakeupthecontrollableweldingparametersand tech
19、niques.Weshallnowturnourattentiontothemannerinwhicheachoftheseaffectcertainweld characteristics.PENETRATION Weldpenetrationisthedistancethatthefusionlineextendsbelowthesurfaceofthematerialbeingwelded. Weldingcurrentisofprimaryimportancetopenetration.AsFigure7-7illustrates,weldpenetrationisdirectly r
20、elatedtoweldingcurrent.Anincreaseordecreaseinthecurrentwillincreaseordecreasetheweld penetrationrespectively. Weld Bead CharacteristicsHowever,wehaveseenthatweldingcurrentcanbevariedwithoutchangingthewirefeedspeed;namely, throughthevariationofthetip-to-workdistance.Theeffectoftip-to-workdistanceonwe
21、ldpenetrationis oppositeinnaturetothatofweldingcurrent.Anincreaseinthetip-to-workdistancewill decreasewelding currentandpenetration.Ofcourse,theconverseisalsotrue.Insomeapplications,manyoperatorshave foundithelpfultousethispropertytocontrolpenetration.Changingthetip-to-workdistancewhilewelding preve
22、ntsburnthroughwhentherearediscontinuitiesinmaterialthicknessesorjointgap.Theremainingfactorshavecomparativelylittleeffectonpenetrationanddonotprovideagoodmeansof control.Figure7-8illustratestheeffectofweldingvoltage.Inthisexample,penetrationisgreatestat24volts anddecreasesasthevoltageiseitherincreas
23、edordecreased.Twenty-fourvoltsistheoptimumvoltagefor theamperageusedandyieldsthemoststablearc.Arcinstabilitydecreasespenetration.Effectsofarctravelspeedaresimilartothatofweldingvoltagepenetrationisamaximumatacertainvalue anddecreasesasthearctravelspeedisvaried.Figure 7-9showsthatat12inchesperminute(
24、30.5cm/min) travelspeed,penetrationisatamaximum.Ateither7ipm(17.8cm/min)or17ipm(43.2cm/min)itis decreased.Withthelowerspeeds,toomuchmetalisdepositedinanareaandthemoltenweldtendstoroll infrontofthearcand”cushions”thebaseplate.Thispreventsfurtherpenetration.Athighspeeds,theheat generatedbythearchasnts
25、ufficienttimetosubstantiallymelttheareaofbasematerial.Torchpositionhasaslightlygreatereffectthandoesweldingvoltageorarctravelspeed.Theeffectof changingthelongitudinaltorchangle,orswitchingfromaforehandtobackhandweldingtechniqueisshown inFigure 7-10.Itcanbeseenthatgenerallytheforehandweldingtechnique
26、yieldsshallowerpenetrationthan doesthebackhandtechnique.Maximumweldpenetrationisachievedwithatorchangleof25deg.andthe backhandweldingtechnique.However,beyondthisdegreeoftorchangle,arcinstabilityandspatterwill increase.Forverythinmaterialsorwherelowpenetrationisrequired,aforehandtechniqueisgenerallyu
27、sed.DEPOSITION RATEThedepositionratedescribeshowmuchusableweldmetalwillbedepositedinonehourofactualarc-ontime.Becausethemigprocessisveryefficient,onlyaverysmallamountofweldmetalislostasspatter.Thedepositionrateforanywireiscalculatedbytheequation:Table7-1givestheinchesofwireperpound(m/kg)forvariouswi
28、reelectrodesinavarietyofsizes. Depositionrateissynonymouswithwirefeedspeed.Figure7-11givesdepositionrateversuswirefeed speed.Thecurrenttoachieveagivendepositionratecanalsobevariedbychangingthetip-to-work distance.AsFigure 7-12shows,thewirefeedspeedcanbeincreasedwithincreasingtip-to-workdistance toma
29、intainaconstantweldingcurrent.Thisresultsinahigherdepositionratethanusuallyassociatedwitha givencurrentlevel. Longtip-to-workdistancesandhighwire-feedspeedsareusedforhighspeedweldingofthinmaterials,as theweldingcurrentcanbekeptlow.Usuallytheforehandweldingtechniqueisemployed.Increasingthe deposition
30、rateinthismannerwillalsohaveaneffectonweldpenetration.Becausemoremetalisbeing depositedatagivenweldingcurrent,thepenetrationwillbereduced.Thisresultsfroma”cushioning”ofthe arcforcebytheextraweldmetaldeposited.Weld Bead Appearance Twocharacteristicsoftheweldbeadarethebeadheightandwidth,asshowninFigur
31、e7-13.These characteristicsareimportanttoassurethattheweldjointisproperlyfilled,withaminimumofdefects, particularlyinmulti-passweldments.Inthiscase,ifthebeadheightistoogreat,itbecomesverydifficultto makesubsequentweldpassesthatwillhavegoodfusion.Themorepeakedandnarrowtheweldbead,the greaterthechance
32、thatpoorfusionmayoccur.Theweldbeadcharacteristicsmaybealteredviabothsize andshape.Inordertochangeweldbeadsize,thelbs.(kg)ofweldmetaldepositedperlinearfoot(m)oftheweldment mustbechanged.Weldingcurrentandtravelspeedaretheweldingparametersprimarilyusedtocontrol weldbeadsize.Forinstance,whenthecurrentis
33、decreased,theweldbeadwillbecomesmaller.The converseisalsotrue.ThisrelationshipcanbeseenbyreferringtoFigure7-7. Weldbeadsizecanalsobechangedbyvaryingthearctravelspeed.AsseeninFigure7-9,beadsizeand travelspeedareinverselyrelated.Adecreaseintravelspeedwillresultinanincreaseintheweldbead heightandwidth.
34、Anincreaseintravelspeedwillresultinadecreaseintheweldbeadheightandwidth. Again,thepounds(kgs)offillermetaldepositedinalinearfoot(m)ofweldareincreased(ordecreased).Both welding current and travel speed have little effect on weld bead shape. The bead width and height increase or decrease together.Arc
35、voltage is used to control the shape of the weld bead. As can be seen in Figure 7-8, as the arc voltage (arc length) increases, the bead height decreases and bead width increases. Here the overall size of the weld bead remains constant. Only the shape or contour of the bead is changed. By increasing
36、 the bead width, the bead height becomes flatter and the weld metal is said to ”wet” the base materials more efficiently. Fusion to the base plate is improved.Wire extension and the welding technique employed (backhand or forehand welding) also affects these characteristics, but only to a limited ex
37、tent. When long extensions are used to increase deposition rates, bead height will increase to a greater extent than bead width. Although larger, the weld bead becomes more peaked as shown in Figure 7-14. A backhand welding technique will also produce a high, narrow weld bead. Decreasing the lagging
38、 torch angle will decrease the bead height and increase the width. The forehand technique yields the flattest, widest weld bead. Nodiscussionofweldingtechniqueswouldbecompletewithoutsomereferencetothemethodsoftorch manipulation.Therecommendationswhichfollowareonlytoserveasaguidetobeusedduringwelder
39、training.AstheindividualweldersbecomemoreproficientwiththeMigprocess,theywilladapttheirtorch manipulationstobestsuitthejobathand.FLAT POSITION Recommendedweavingpatterns,torchpositionsandbeadsequenceareshowninFigure 7-15.Forthe single-pass,buttedjoint,aslightback-steppingmotionisused.Gappedrootpasse
40、saremadewithasmall, back-and-forthweavepattern.Forfillandcoverpasses,thesameweave,withanadjustmentforthedesired width,isused,withcaretakentopauseatthesidewallstoobtainadquatefillintheseareas.Torch ManipulationsHORIZONTAL POSITION Recommendedweavingpatterns,torchpositionsandbeadsequencesareshowninFig
41、ure7-16.Forfillet welds,acircularmotionisrecommended.Forbuttweldrootpassesandfillpasses,anin-line,back-and- forthmotionisusedwithwidthadjustmentsasrequired.Aslightpauseisusedatthetie-intotheprevious bead.VERTICAL POSITION Recommendedweavingpatternsandtorchpositionsforverticalupandverticaldownareshow
42、ninFigure 7- 17.Withverticalup,forasquareedgepreparationanin-line,back-and-forthweaveisused.Forabevelled, multipassjointa”U”patternisusedfortheroot.Thefillandcoverpassesaremadeusingaside-to-side weavewithabackstepatthewalls.Thelengthofthebackstepisontheorderofawirediameter.Fora verticalupfilleta”Chr
43、istmasTree”patternisusedwithpausesatthesidewalls. Forverticaldownaninverted”U”patternisused,pausingatthesidewallsfortheroot,fill,andcover passes.Alwaystakecareinverticaldownweldingtokeepthearcontheleadingedgeofthepuddle. Preventingthemoltenmetalfromrunningaheadofthearcwillimproveweldsoundness.OVERHEAD POSITION RecommendedweavingpatternsandtorchpositionsfortheoverheadpositionareshowninFigure7-18. Again,aback-and-forthweaveisusedwithpausesattheplatesidewalls.Thisappliestoroot,fill,andcover passes.
