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LASERINSIGHTSNO.1Fundamentals of Laser WeldingOverthe past30 years,laserweldinghasbeenusedinan increasing numberofmanufacturingapplicationsinthe medical,automotive,electronics,aerospace,solarandmanyotherindustries.Inthe 1970'slaserweldingwasperformedwithcarbondioxidegaslasers. Theselasersemittedatawavelengthof10.6microns. Thislasersourcehadseveraldrawbacksforweldingapplications. Becausemostmetalsdo notreadilyabsorbthe10.6micronwavelength,considerableheatwasintroduced intothe part. Thisexcessheatoftencauseddistortionorotherthermaldamage-particularly inthinnermaterials. Furthermore,thiswavelengthcanonlybe deliveredwithfixedopticsandcannotbe deliveredwithaflexible fiberopticcable.Nd:YAG: ThesolidstateNd:YAG laseremergedduringthe 1980'sas the preferredweldinglaserformetalsup to1mmthick. Theshorterwavelength(1.064microns)ofthe YAGlaserismorereadilyabsorbedby metalsandintroduceslessheatorthermaldamage. Additionally, the YAG

laser

beam

may

be

delivered

to

the

weld

zone

with

either

fixed

optics

or

the

energy

may

be

coupledintoaflexible fiberopticbeamdeliverycable. Thisisasignificantadvantageoverthe carbondioxidelasersincethe Nd:YAGlaserpowersupplycouldbe locatedseveralmetersfromthe laserweldsiteorworkstation. PulsedYAGlasersusedforspotandseamweldingapplicationsare availablefrom 10-500wattsaveragepower. Thehighpeakpowerofthe pulsedYAGisthe preferredapproachforachievingdeeppenetrationwithminimalheataffectedzone.Thislaserisalsosuitedforweldingmorereflective metalssuchas aluminum,goldorsilver.(SeeFigure1:Electro-magneticspectrum).

Excimer Tripled Doubled YAG CO2Lasers YAG YAG Fiber Laser

.355 .532 1 10.6

FiberLasers: Therecentintroduction offiberlasertechnologyhasprovidedmanufacturerswithanotherlaserweldingoption. FiberlasersofferasmallerspotdiametercomparedtoNd:YAG.Thisresultsinhigherpowerandenergydensitiesthanthoseproduced by Nd:YAGlasers.Additionally, the "wallplugefficiency"ofthe fiberlaserisupwardsof30%comparedto2-3%forNd:YAG. Fiberlaseroutputmayalsobe deliveredtotheworkpiecevia aflexible fiberopticcable. Fiberlasersofferexcellentbeam qualitiesandare scalabletomulti-kWpowerlevels.Finally,unlike pulsedNd:YAGlasersthatrequirethe flashlamptobe changedafterseveralmillionshots,the fiberlaserdiodearraymaylast50,000hoursormorebeforeneedingreplacement. Thereare somedisadvantageshowever. Fiberlasersoperateinthe continuouswaveorCW modeandthereforedo notprovidehighpeakpoweroutput. Thisdisadvantageismostnoticeableforspotweldingapplicationsandforweldingaluminum orotherhighlyreflective metals. As aresult,bothfiberlasersandpulsedNd:YAG lasershouldbe consideredandevaluated.

Gamma X-Rays UV Visible Infrared Microwave TV Radio

Fig.1.

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Howdoesalaserwork? Theword"LASER"isan acronym forLightAmplification by StimulatedEmissionofRadiation. Creation oflaserlightrequires severalkeyelements. Thereare three(3) majorelementsinatypicalweldinglasersystem.

/ Powersupply / Resonator

oLasing

medium

o Pumpsource

/ BeamdeliverysystemPowerSupply: Thepowersupplygeneratesthe highvoltageandpowertodrivethe resonatorassemblyandachievethe averagepowerandpeakpowerneededforlasermaterialprocessingapplications. Lasercontrols providethe operatormeanstoestablishinputvoltage(V),pulsewidth(milli-seconds),pulserepetitionrate(pulsespersecond),pulseenergy(Joules),averagepower(watts),peakpower(kilo-watts)andotherlaseroutputfunctions. Thepowersupplyisconnectedtothe Resonatororlaserhead,via an electrical umbilical.Resonator: Theresonator(alsocalleda"laserhead")isan opticalengine. Itconsistsofapairofopposingreflective mirrors. TheREAR mirroristypically 98-100% reflective. TheFRONTmirrorispartially reflective andpartially transmissive. Thisallowsthe laserenergytoreflectoffthe rearmirroranddeliveredtothe workpiecevia the partially transmissivefrontmirror. Theresonatorcontainsapumpchamberthatcontainsthe lasingmediumandpumpsource. Coolingwateriscirculated insidethe pumpchambertokeepthe lasingmedium andflashlampunderstabletemperatureconditions. Low averagepowerlasersusuallyhaveaclosedloopwater-to-aircoolingsystems. Higherpowerlasersrequirewater-to-watercooling.

TheLasingMedium: Thelasingmedium containsasmallamountofalasingmedium (alsocalledthe "activeelement"). Inthe caseofNd:YAGlasers,Neodymium (Nd)isthe activeelement. TheYttrium,Aluminum,Garnet(YAG)components form ahostcrystalthatisprecisionmachinedintoalaserrod.YAGlaserrodstypicallycontainonly1-3%Neodymium. Thelasingmedium inafiberlaserisYtterbium.PumpSource: Thepumpsource"stimulates"the lasingmediumuntilthe lasingthresh holdisreachedandenergyisemittedtoconductusefulwork. ThepumpsourceforpulsedYAGlasersisaflashlamp. Thepumpsourceforafiberlaserisalaserdiodearray.

LaserCharacteristics: Lasersphotonsexhibitthreefundamentalcharacteristics. Coherent-samewavelength(frequencyorcolor) Outputinphase Photonstravelinthe samedirection

LaserWelding:Mostspotandseamweldingapplications are performedwithpulsedNd:YAG orFiberlasers.Becauseofthe smallHAZ,YAGtheselasersare ideallysuitedforweldingverythinmetalssuchas

small

diameter

wires

or

heat

sensitive

components.

These

performance

characteristics

are

particularly criticalinthe medicaldeviceindustry. Pacemakers, nervestimulatorsanddrugdispensersusuallycontainheatsensitive electronics, IC chipsetsororganicmaterials.Fiberlasersprovideverysmallspotdiameters. Thisresultsinveryhighpowerdensityrequiredforseam weldingapplicationsinthickermetals. Fiberlaserpowerisalsoscalablefromlow averagepower(10 watts)to10 kilo-watts(kW)ormore.

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LaserBeamAccess&FocalLength:WhilelaserweldingisaNONCONTACTprocess-beamaccessisrequiredtogetthe focusedlaserspotoverthe weldzone. Ifanyportionofthe beamisblocked,the weldwillbe incompleteandunacceptable. Keyfactorstoconsiderare focusedspotdiameterandfocallength. Longerfocallengthsproduceanarrowerbeamconeangle,largerspotdiameterandlongerdepthoffield. Longerfocallengthsofferamoreacutebeam anglemakingitmoresuitablefortightgeometries. Ashorterfocallengthresultsinasmallerspotdiameterbutlimitthe workingdepthoffield.(Figure3).FocalLength:120mm100mm80mm60mm

BeamAccess BeamAccessBlockedAs canbe seenfromthe aboveFigure3,the beamwiththe longerfocallengthwillnot be blockedandwillreachthe weldtargetthroughasmallopening. On the otherhand,the shorterfocallengthbeamwillbe blockedby the component(ortooling)featuresandcannotdeliversufficientenergytocompletethe weld.MeltingvsVaporizing:Thelaserpowerandenergydensitymustbe sufficienttomeltthe metalsandformaweldnuggettothe desireddepthofpenetration. Toolittlepower/energydensityandthe metalwillnotmeltorthe weldpenetrationwillbe insufficienttojointhe materials. Toomuchandthe materialmaybegintovaporizeandblowout. (SeeFigure4)

0.025"depth304LStainless

The weldsshowninthe greenareacontainsufficientpowerandenergydensitytoproducegoodweldswithno evidenceofweldsplatter. Power/energydensitywasmaintainedbelowthe vaporizationpoint.

Fig.4.

GoodWelds Weldblowout

Fig.3

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BeamProfile&PowerDensity: Onefactortoconsiderwhencalculating powerdensity/energydensityisthebeamprofile. Atop hatbeamprofile,forexample,ismoreforgivingsincethepowerandenergyisevenlydistributed acrossthe weldzone. Tophatenergydistributionisachievedwhenthe laserbeamisdeliveredtothe workpiecevia afiberopticcable. Increasingthe pulseenergyincreasesthe welddepth. Vaporizationdoesnotoccurifthe power/energydensityremainsbelowthe vaporizationpoint. However,excessivepower/energydensityispossible evenwithatop hatbeamprofileifthe peakpowerreachesthe vaporization point. (SeeFigures5&6)

Whenfixedopticsare employed todeliverthe laserbeamtothe workpiece,the marginoferrorandpointoffocusaremuchmorecriticaltomaintain. Thebeamprofile from fixedopticsbeamdelivery systemismoreGaussianinprofile. Controllingpeakpowerismoredifficultbecausethe centerofthe beam mayproducevaporizationtemperatureswhilethe outerportionofthe beamonlymeltsthe metal. Weldsplatterandmetalvaporizationismorelikelyto occurusingfixedoptics. (SeeFigures6&7).

VaporizationPointMeltingPoint

MetallurgicalConsiderations: Somemetalscannotbe laserweldedduetotheirmetallurgicalcomposition. Brass,forexamplecannotbe laserwelded. Why? Brassisan alloyofcopperandzinc. Theproblemwithbrassisthatthe meltingtemperatureofthe copper(1084C)exceedsthe boiling(vaporization)pointofthe zinc(907C). Zincmeltsat419C. By the timethe coppermeltsthe zinchasvaporized. Whenthe zincvaporizesitexplodesandsplattersmetaloveralargearea. Thesameistruewithbronzeandphosphorbronze.Summary: Laserweldingisamaturematerialjoiningprocess. Laserengineershaveanumberoflaserandbeamdelivery andprocesscontroloptionsavailabletooptimizethe weldprocessfortheirparticularapplication. However,becausethereare so manyoptions,achievingreliable,production worthyresultswillrequireagoodunderstandingoflaserfundamentals,processcontrols,laser,beamdeliveryandmaterialchoices.

Fig.5TopHat Fig.6.CrossSection-Top HatWelds

Fig.6Gaussian Profile Fig.7Blowout

MeltingPoint

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