Engineering5(2019)702–720Contents lists available at ScienceDirectEngineeringResearch
AdditiveManufacturing—Review
DevelopmentofMicroSelectiveLaserMelting:TheStateoftheArtandFuturePerspectives
BalasubramanianNagarajan,ZhihengHu,XuSong,WeiZhai,JunWei?SingaporeInstituteofManufacturingTechnology(SIMTech),AgencyforScience,TechnologyandResearch(A*STAR),Singapore637662,Singaporearticleinfoabstract
Additivemanufacturing(AM)isgainingtractioninthemanufacturingindustryforthefabricationofcom-ponentswithcomplexgeometriesusingavarietyofmaterials.Selectivelasermelting(SLM)isacommonAMtechniquethatisbasedonpowder-bedfusion(PBF)toprocessmetals;however,itiscurrentlyfocusedonlyonthefabricationofmacroscaleandmesoscalecomponents.ThispaperreviewsthestateoftheartoftheSLMofmetallicmaterialsatthemicroscalelevel.IncomparisonwiththedirectwritingtechniquesthatarecommonlyusedformicroAM,microSLMisattractiveduetoanumberoffactors,includingafastercycletime,processsimplicity,andmaterialversatility.AcomprehensiveevaluationofvariousresearchworksandcommercialsystemsforthefabricationofmicroscalepartsusingSLMandselectivelasersintering(SLS)isconducted.InadditiontoidentifyingexistingissueswithSLMatthemicroscale,whichincludepowderrecoating,laseroptics,andpowderparticlesize,thispaperdetailspotentialfuturedirections.Adetailedreviewofexistingrecoatingmethodsinpowder-bedtechniquesisconducted,alongwithadescriptionofemergingeffortstoimplementdrypowderdispensingmethodsintheAMdomain.Anumberofsecondary?nishingtechniquesforAMcomponentsarereviewed,withafocusonimplementationformicroscalefeaturesandintegrationwithmicroSLMsystems.ó2019THEAUTHORS.PublishedbyElsevierLTDonbehalfofChineseAcademyofEngineeringandHigherEducationPressLimitedCompany.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).Articlehistory:Received27July2018Revised31January2019Accepted14March2019Availableonline3July2019Keywords:AdditivemanufacturingSelectivelasermeltingMicrofabricationHybridprocessingPowder-bedrecoating1.IntroductionInrecenttimes,therehasbeenanever-increasingdemandformicrofabricationtechnologiestocatertothedrivetowardminiaturizationthatisoccurringinseveralsectors,includingtheelectronics,medical,automotive,biotechnology,energy,communi-cations,andoptics[1].Numerousproductsandcomponents,includingmicro-actuators,micro-mechanicaldevices,sensorsandprobes,micro?uidiccomponents,medicalimplants,micro-switches,opticaldevices,memorychips,micro-motors,magneticharddriveheads,computerprocessors,inkjetprintingheads,leadframes,electricalconnectors,microfuelcellsand,mostimpor-tantly,micro-electromechanicalsystems(MEMS)devices,aremadebymeansofmicrofabricationtechniques.Microscalemanu-facturingprocessesaregenerallyclassi?edintoMEMS-based(orlithography-based)andnon-MEMS-based(ornon-lithography-based)processes.Theutilizationofmetallicmaterialsinmicrocomponentshasgainedmomentum,largelyduetothe?Correspondingauthor.E-mailaddress:jwei@SIMTech.a-star.edu.sg(J.Wei).applicabilityresultingfromtheirmechanicalandelectricalproper-ties(i.e.,strength,ductility,electricalconductivity,etc.)[2].Theprocessingofmetalsinmicrofabricationiscommonlyachievedthroughnon-lithography-basedtechniquessuchasmachining,forming,andjoining[3].Traditionalmicromanufacturingmethodshaveoneormoreofthefollowinglimitations:dif?cultyinfabricat-ingcomplexshapes,materiallimitations,tooling-relatedissues,inabilitytoperformrealthree-dimensional(3D)fabrication,andsoforth.Thedevelopmentofadditivemanufacturing(AM)technologyoverthepasttwodecadeshasopenedupnewhorizonsinmetalfabrication,giventheabilityofAMtorealizeanycomplexgeome-try[4,5].AMconsolidatespowderorwirefeedstockintoa?nalproductinalayer-by-layermanner.AMprocessesstartwith3Dmodelingofthedesiredcomponent,whichisthenslicedintodifferenttwo-dimensional(2D)layers.Thefeedstockisthendeposited,followedbytheselectiveadditionofeverylayerusinganenergysource[6].AMtechniquesarecommonlyclassi?edintosevencategories:materialextrusion,vatphotopolymerization,materialjetting,binderjetting,sheetlamination,directedenergydeposition(DED),andpowder-bedfusion(PBF)[7].Materialextru-sion,vatphotopolymerization,andmaterialjettingaregenerallyB.Nagarajanetal./Engineering5(2019)702–720703usedfornon-metallicmaterials.Sheetlaminationiscapableofprocessingmetals,basedontheprecisionslicingofmetalsheetswithsubsequentstackingusingbonding,welding,orultrasonicconsolidation[8].However,binderjetting,DED,andPBFhavebeenidenti?edasthemostsuitableprocessestoprocessmetals[6,7,9].Binderjettingworksbydepositingbinderadhesiveonmetalpow-der,followedbycuringtoforma‘‘green”part[10].The?nalpartisachievedbysinteringthegreenpartwithanoptionalin?ltrationofanothermaterialorofnanoparticlesofthesamemetal.Mandatoryheattreatmentandhighporosityarethecommonlimitationsofthebinderjetprocess,astheyhinderitsabilitytobeapplicableatthemicroscale[11].DED—whichisalsoknownaslasercladding,lasermetaldeposition(LMD),andlaser-engineerednetspacing(LENS)—isanothersigni?cantAMprocessusedtofabricatemetalcomponents[12].InDED,thefeedstockisdirectlydepositedintothemeltpool,whichiscreatedbyafocusedenergysource.Thefeedstockcaneitherbepowderorwire,wherepowder-fedDEDtypicallyhasbetterresolutionthanwire-fedDED[7].SinceDEDpro-ducesonlynear-netshapes,furtherpost-processingisnecessary.PBFistypicallypreferredformanufacturingsmallcomponentsthatrequireagoodsurface?nish,asPBFdemonstratesbetterresolutionthanDED[4].PBFgenerallyhasasmallermeltpoolandlayerthick-ness,resultinginbetterresolutionandsurface?nish.PBFprocessesinvolvetheselectivemeltingorsinteringofalayerofpowderusinganenergysource.ElectronbeamandlaserbeamarethetwomainenergysourcesusedinPBFprocesses—thatis,inelectronbeammelting(EBM)andinselectivelasermelting(SLM)/selectivelasersintering(SLS),respectively.Inaddition,SLMiscapabletoproducecomponentswithmechanicalpropertiesthatarecomparabletothatofthetraditionalmanufacturingprocesses[13].EventhoughmetallicAMhasalreadybeencommercializedforvariousapplicationsinthebiomedicalandaerospacesectors,includingtheproductionandrepairofaerospacecomponents[5],theapplicationofAMhasbeenlimitedtomacroscaleandmesoscalefabrication.AMtechniquesformicroscalefabricationareonlyrecentlybeingdevelopedfortheproductionof3Dmicro-featuresonavarietyofmaterialsincludingceramics,polymers,andmetals[14].ThefollowingsectionfocusesonpastAMapproachesforfabricatingmetallicmicrocomponents.2.MicrometallicAMAMatthemicroscaleandnanoscalehasattractedattentioninrecentyears,asisevidentfromtheemergenceofreviewpapersofcorrespondingtechniques[14–16].Engstrometal.[15]pub-lishedareviewofadditivenanomanufacturing(ANM)techniquesthatproduce?nalpartswitharesolutionofsub-100nmusingvariousmaterialsincludingmetals,polymers,andorganicmolecules.ThereviewbyHirtetal.[16]focusedexclusivelyonmicroAMtechniquesformetals,whichareclassi?edintometaltransferandinsitusynthesistechniques.Byde?nition,thebench-markfeaturesizeformicroAMtechniquesisdescribedas10lm.Vaezietal.[14]classi?ed3DmicroAMtechniquesintotwomaincategories—namely,3DdirectwritingandscalableAM—asillus-tratedinFig.1.3Ddirectwritingiscomprisedofink-basednozzledispensingandaerosoljettechniques,lasertransfertechniques,andbeamdepositionmethodssuchaslaserchemicalvapordepo-sition(LCVD),focusedionbeam(FIB)writing,andelectronbeam(EB)writing.Althoughthedirectwritingprocesstypicallyhasahighresolutionthatissuitablefornanoscalefabrication,thepro-cessinghasbeenhighlycomplexandslow[15,16].InthecategoryofscalableAMtechniques,micro-stereolithography(MSL)hasbeenthemostsuccessfulmicroAMtechniqueduetoitshighres-olutionandrepeatability,althoughitislimitedbythechoiceofmaterials[17].Fuseddepositionmodeling(FDM)andlaminatedobjectmanufacturing(LOM)techniqueshavedif?cultiesinprocessingmetals,besidestheirlimitationtoachievehighfeatureresolution.Whilemetalinkshavebeenusedininkjetprinting[18],thismethodisstillstronglyrestrictedtonon-metals.3Dprinting(3DP)/binderjetprinting(BJP)showspromiseintermsofmulti-materialprintingandcoldprocessing,buttheprintedpartstypicallyhavehighporosity[19].Forprocessingmetalswithoutanyresins(asinMSL)orbinders(asin3DPorBJP),SLMandSLS—thatis,powder-bed-basedlayer-by-layermeltingorsinteringusinglasers—havedemonstratedpotentialduetotheirabilitytofabricatetrue3Dmicropartswithhighresolution[14,20].ThevastamountofavailableknowledgeontheuseofSLMandSLSinmacroscaleprocessingcouldbeusedtoscaledownthetechniquetothemicroscale.ThisreviewfocusesexclusivelyonSLMandSLSforthefabricationofmicroscalefea-tures.ThedifferencebetweenSLMandSLSliesinthedegreeofmelting[6].SLMachievescompletemeltingofthepowder,whereasSLSonlysinters—orpartiallymelts—thepowder.Withtheexceptionofthefullorpartialmeltingofpowderparticles,thereisnodifferencebetweenSLMandSLSintermsofprocesssetupandmechanisms.Therefore,inthispaper,SLMandSLSareconsideredtobeidenticalforthepurposeofcomparingprocesscomponentsandparameters.Thediscussiononthepowder-recoatingsystemandhybridprocessinginthelatersectionscanalsobeappliedtotheminiaturizationofotherPBFtechniques.3.SelectivelasermeltingFig.2showsaschematicoftheSLMprocesssetup.InSLMandSLS,alayerofpowderis?rstspreadonthebuildsubstrate.Thelaserbeammeltsorsintersthepowderaccordingtotherequiredgeometry.Therecoaterthenappliesthenextlayerofpowderoverthesolidi?edpart,followedbyfurtherlasermelting/sintering.TheheatingandcoolingratesareveryhighduringtheSLMprocessduetotheshortinteractiontimebetweenthelasersourceandthepowder.Sincetheresultantmeltpoolgeometrysigni?cantlyin?u-encesthemicrostructurefeatures,themechanicalpropertiesofthefabricatedpartdifferfromthoseofconventionalprocesses[13].DetailedreviewsoftheprocessmechanismsduringSLMcanbeFig.1.Majorclassi?cationofAMtechniquesformicroscalefabrication.MSL:micro-stereolithography;FDM:fuseddepositionmodeling;LOM:laminatedobjectmanufacturing.ReproducedfromRef.[14]withpermissionofSpringer-VerlagLondon,ó2012.704B.Nagarajanetal./Engineering5(2019)702–720ThecharacteristicsofAMcomponentsmadeusingSLMaretypicallyevaluatedthroughanumberofprocessoutcomes,dependingontheapplication.Fig.4summarizessomeoftheimportantfeaturesofSLM-fabricatedparts.Asinanyconventionalprocess,thefeatureresolution,surface?nish,mechanicalproper-ties,andmicrostructurearecharacterizedinordertoevaluatethequalityofthe?nalbuiltpartandtherebytheSLMprocess.Fig.5illustratesthedifferentpossibledefectsthatmayoccurinSLM.Theformationofdefectsisessentiallydependentonthepro-cessvariables,whichneedtobeoptimizedinordertofabricatedefect-freecomponents.AdetailedreviewofthedefectsinAMprocessesisavailableelsewhere[7].4.MicroselectivelasermeltingCommercialSLMsystemsgenerallyemploypowderparticlesizesof20–50lmandalayerthicknessrangingfrom20to100lm.TheefforttoscaledownconventionalSLMinordertoincreasethefeatureresolutioninvolvesthreemainfactors:laserbeamdiameter,layerthickness,andparticlesize,asillustratedinFig.6.Fischeretal.[31]de?nedthescaleofmicroSLMtobethefollowing:alaserbeamdiameter<40lm,alayerthickness<10lm,andaparticlesize<10lm.Fig.2.SchematicoftheSLMprocess.foundelsewhere,inRefs.[6,7,21].The?nalqualityoftheSLMpartsisin?uencedbyalargenumberofprocessparametersduetothecomplexsystemandmechanismsinvolved[22–29].TheSLMprocessparameterscanbeclassi?edroughlyintopowder-related,laser-related,andpowder-bed-relatedvariablesaccordingtotheproperties,asillustratedinFig.3.Mostofthepowder-relatedprocessparameters,suchasthechemicalcomposi-tion,sizeandshapeoftheparticles,andsurfacemorphology,areinvariantsinanactualproductionenvironment[7].Theparame-tersrelatedtolasersystemsthatin?uencetheSLMprocessincludethelasertype(i.e.,continuouswave(CW)orpulsed),laserpower,andspotsize.Thescanningparameters—suchasscanningstrategy,hatchspacing,andscanningspeed—signi?cantlyaffecttheSLMbuiltpartcharacteristics[30].Thethirdclassi?cationofSLMpro-cessparametersispowder-bedcharacteristics.Inmostpowder-bedprocesses,thepowderisappliedontothebuildingplatformbymeansofarakingmechanism,whichisalsoknownasrecoating.Theef?ciencyofthepowderdeliverysystemisin?uencedbyanumberofparameters,includingtherecoatertype,numberofrecoatingpasses,amountofretrievedpowderduringeachpassand—mostimportantly—powderproperties.Thethicknessoftherecoatinglayerisoneofthesigni?cantprocessparametersthatcontrolthepartproperties.Layerthickness,particlesizedistribu-tion(PSD),andlaserparametersin?uencethelaser–materialinter-actionandhencethemeltpoolcharacteristics.4.1.CurrentstateoftheartThe?rstmicroSLSsystem—knownaslasermicrosintering—wasdevelopedmorethanadecadeagoattheLaserinstitutMittel-sachsene.V.[32]usingaQ-switchedneodymium-dopedyttriumaluminumgarnet(Nd:YAG)laser(0.5to2kW).Thissysteminvolvesaspecialrakingprocedurethatappliesathicklayerofpowder?rst,whichissuccessivelyshearedofffromoppositedirec-tionstoproduceathinlayer.Thedrivesforthepowderdispenserandbuildingplatformhavearesolutionof0.1lminordertocon-trolthelayerthicknesswithsub-micrometeraccuracy.Withthis?rstapproach,themicropartsthatwerefabricatedhadastructuralresolutionoflessthan30lmandanaspectratiogreaterthan10,withasurfaceroughnessof5lm.Variousmetalsincludingtung-sten(W),aluminum(Al),copper(Cu),andsilver(Ag),withanaver-agepowderparticlesizerangingfrom0.3to10lm,weretestedforthisstudy,asshowninFig.7[20,33,34].Fig.7(a)[33]showsoneoftheinitialfeaturesbuiltbythissetupusing300nmtungstenpow-der.Althoughtheexposureofthepowdertoavacuumof10à3Paproducedbetterraking,thepowder-beddensity(PBD)afterrakingwasstillaround15%.Amaximumpartdensityof90ˉtersinter-ingwasobservedwithaWandCupowdermixture.Fig.3.SummaryofSLMprocessparameters.B.Nagarajanetal./Engineering5(2019)702–720705Thesameresearchgrouphasdevelopedanimprovedsysteminvolvingtworakeswithacircularcross-sectiontospreadthepowder[20,35].Figs.7(b)–(d)[20,33,34]showthedifferentfeatureshapesthathavebeenfabricatedwiththismodi?edsetup.Thedif-ferenceliesinthepowder-recoatingmechanism,astherakestra-verseinacircularmotionbetweenthepowderreservoirandthebuildingplatform.Metalcylinderswithasharpenededgeareusedastherakeblade.Thisdesignwithtwoormorerakesfacilitatesbuiltpartswithmultiplematerialsoragrainsizegradientalongthepartthickness,asshowninFig.7(d).Inadditiontoraking,therecoatingsystemcanbeusedtomanuallycompactthepowderbypressure.Withthisuniquesetup,micropartsofvariousmetalsincludingtungsten,aluminum,copper,silver,316L,molybdenum(Mo),titanium(Ti),and80Ni20Crcanbeproducedusinglasermicrosintering.Aftercontinuousimprovementoftheprocesscharacteristics,thelasermicrosinteringofmetalshasyieldedaFig.4.SummaryofSLMprocessoutputcharacteristics.Fig.5.TypicalSLMprocessdefects.Fig.6.RequirementsfortheSLMofmicroscalefeatures.minimumresolutionof15lmandasurfaceroughnessof1.5lm.Amaximumpartdensityof98%and95%wasreportedforoxideceramicsandalloys,respectively[36].Giesekeetal.[37,38]developedamicroSLMsystemin2013toproduceAmericanIronandSteelInstitute(AISI)316Lhollowmicroneedleswithaminimumwallthicknessof50lm.Thelaserspotdiameterwasscaleddownto19.4lminordertoachieve?nefeatures.Aparticlesizerangingfrom5to25lmwasusedtobuildneedleswithaninnerdiameterof160lmusingalayerthicknessof20lm.Despitethecombinationof?nespotsizeand?nerpow-ders,thesurfaceroughnessofthebuiltpartswaspoor(Ra%8lm).Agglomerationof?nepowdercouldhaveresultedinnon-uniformpowderspreading,whichwouldexplainthepoor?nish.Asigni?-cantpowderadherenceoccurredalongthewallduetothehighenergyinput.Morecomplexhelixshapeswithaminimumstrutdiameterof60lmwerealsoproduced,albeitwiththeoccurrenceofpartialstrutfailure[38].Thesameresearchgroup[39]fabri-catedpartsusingshapememoryalloys(Ni–Ti),asshowninFig.8(a),witharesolutionof50lmatalowerlaserpowerandhigherscanningspeed.YadroitsevandBertrand[40]usedacom-mercialsystem,PM100,tofabricatemicro?uidicsystemsmadeofstainlesssteel(SS)904L,asshowninFig.8(b).ThespotsizeFig.7.Fabricationofmicrofeaturesbylasermicrosintering.(a)Sinteredteststructuresmadeoftungstenpowder(300nmsize);(b)threenestedhollowspheres;(c)concentricrings;(d)lasersinteringofmulti-materials(CuandAg).(a)and(d)arereproducedfromRef.[33]withpermissionofEmeraldGroupPublishingLimited,ó2007;(b)isreproducedfromRef.[20]withpermissionofWILEY-VCHVerlagGmbH&Co.KGaA,ó2007;(c)isreproducedfromRef.[34]withpermissionofEmeraldGroupPublishingLimited,ó2005.Fig.8.PartsfabricatedusingmicroSLM.(a)Ni–Timicroactuators;(b)topviewofSS904Lmicro?uidicsystems,theinsertimageisitsinternalstructure.(a)isreproducedfromRef.[39]withpermissionofElsevierB.V.,ó2010;(b)isreproducedfromRef.[40]withpermissionofDAAAMInternational,ó2010.706B.Nagarajanetal./Engineering5(2019)702–720andlayerthicknesswere70and5lm,respectively.Fullyfunc-tionalpartsof100–500lmwithstructuralelementsof20lmwereproduced.Itisworthnotingthatthespotsizewasstilllargeandthesurfaceroughnesswaspoor.In2014,Fischeretal.[31]investigatedtheprocessparametersofmicroSLMusinganEOSINTl60system.Theminimumrough-nessandthemaximumfeatureresolutionachievedwere7.3and57lm,respectively.Amaximumrelativedensityof99.32%wasattainedfortheSLMofcuboidalstructures.Despitearelatively?nerpowderof3.5lm,theachievedresolutionisnotsuf?cientforthedimensionalspeci?cationsofmicrocomponents.AbeleandKniepkamp[41]furtherimprovedthesurfacequalityofthepartsfabricatedbymicroSLMbyusingthecontour-scanningstrat-egy.Aminimumsurfaceroughnessof1.69lmwasachievedalongthewallsparalleltothebuildingdirection.Kniepkampetal.[42]alsoreportedonthefabricationofmicroSLMpartswithatopsur-faceroughnessoflessthan1lm,usingparametricoptimization.Veryrecently,RobertsandTien[43]reportedonthefabricationofSSmicroelectrodearraysusingmicroSLSwithaverticalandlat-eralresolutionof5and30lm,respectively.ThelatesteffortinmicroAMisfromtheUniversityofTexasatAustin[44,45],whereamicroSLSsystemcomposedofanultrafastlaser,amicro-mirror-basedopticalsystem,substrateheating,andapreciserecoatingsystemhasbeendevelopedtoachieveafeatureresolutionof1lm.Threesigni?cantmodi?cationstotypicalSLSsystemshavebeenperformed:??Anewspreaderdesignhasbeenincorporated,whichincludesacombinationofaprecisionbladeandaprecisionroller.Therollerisattachedwithalinearvoicecoilactuatortoprovideverylowamplitudehigh-frequencyvibration.Withthenewsetup,vibrationcompactionisincludedtoachieve?nelayersofafewmicrometers.??ThegalvanometricmirrorscommonlyusedinSLMmachineshavebeenreplacedwithdigitalmicro-mirrordevices(DMDs)inthissystemtoincreasethesystemthroughput.??Additionalfocusingopticshavebeenaddedtoachieveaspotsizeof1lm.Inaddition,alinearactuatingsystemfordisplac-ingthepowderbedwitharesolutionofafewtensofnanome-tershasbeenimplemented.Despitetheinclusionofavibratingrollerasthepowdersprea-derintheSLSsystem[45],agglomerationofthepowderparticleswasstillobserved.Twomodi?cationstothemicroSLSsystemhavebeenimplemented:①replacingdrypowderwithnanoparticleinks,and②changingtheparticle-dispensingmechanismfromtra-Table1
LiteraturereviewofSLM/SLStechniquesformicroscalefabrication.Speci?cationsStructuralresolution(lm)AspectratioLayerthickness(lm)Surfaceroughness(lm)Laserspeci?cationsRegenfussetal.[32]<30>10NS<3.5Nd:YAGlaser(CW)Power:0.1–10WFreq:0.5–50kHz25W,Al,Cu,Ag0.3–10Vacuum(10à3Pa)orreducedshieldgaspressures(104–105Pa)CustomizedStreeketal.[35]15NS1–101.5–3.5Nd:YAGlaser(pulsed)ditionalblade/rollerstoslotdie-orspin-coatingtechniques.Intheimprovedsetup,themicroSLSdesignwaschangedtoincludeaslotdie-coatingmechanism,duetoits?exibility.Slotdiecoatingiscapableofdepositingawiderangeofthicknessesrangingfrom20nmto150lmthroughprecisemeteringandcontrolleddis-pensing[44].Inaddition,aprecisenanopositioningstageusingvoicecoilactuatorswasusedtoachieve?neprecision.However,thissystemwasonlyapplicableforslurriesorinks,duetonanoparticleagglomerationofthe?nedrypowdercausedbyvanderWaalsforces[46].Table1[31,32,35,37,38,42,43]summarizestheresearchworksthathavebeencarriedoutontheuseofmicroSLM/SLStoprocessmetallicmaterials.ItisworthnotingthatbothCWandpulsedlasersarebeingusedinmicroSLMsystems,whichisdifferentfromtheprominentuseofCWlasersinconventionalSLMsystems.Regenfussetal.[33]initiallyusedaQ-switchedpulsedlaserforalasermicrosinteringsetup.TheQ-switchedlaserwasshowntobeeffectiveforthefollowingreasons:①anincreaseinpartreso-lution;②areductioninresidualstress;③areductioninoxidationeffect,possiblyduetogasorplasmaexpansion,whichprovidedashieldingeffect;④theeliminationofissuessuchaspoorsub-strate–partadherenceandmaterialsublimationatlowpressure,whichtypicallyoccurwiththesinteringofsub-micrometerpow-dersusingaCWlaser;and⑤suitabilitytoprocessdielectrics.Thepulsedlasersproducednarrowanddeepcut-ins,frozenjets,and?attenedcratersduetoahigherlaserintensityincomparisonwiththeCWlasers.However,thepulsedlaserresultedinapoorsurface?nish,irregulartracks,andballing,duetoanunstablemeltpool.Keetal.[47]comparedCWandpulsedlasermodesinthelasermicrosinteringof?nenickel(Ni)powderwithameanparti-clesizeof4lm.ItwasreportedthattheCWlaserresultedinamorepronouncedballingphenomenonthanthepulsedlaser;useofthelatterreducedtheballingduetothe?atteningeffectbytheplasmaandtherapidcoolingrate.Moreover,thepulsedlaserwasobservedtoresultinabetterwettability.However,thesingletracksproducedbythepulsedlaserhadcorrugation,trenchforma-tion,andpoorsurface?nish.Similarly,Kniepkampetal.[42]reportedonthepoorsurface?nishanddiscontinuoustracksthatoccurredwhenusingthepulsedmodeofa50W?berlaser.Fischeretal.[31]observedthatthepulsedlasercouldnotproducehomogenoussingletrackswithoutdefects,despitetestingawiderangeoflaserpowersandpulserepetitionrates.Inadditiontoitsusewithmetals,apulsedwavelaserinmicroSLShasbeentestedwithceramicsandwasfoundtobeeffective[48].Giesekeetal.[37,38]<5030:1208FiberlaserPower:25W/50W19.4SS316L5–25O2<300ppmFischeretal.[31,42]<402627>7.29PulsedlaserPower:30WFreq:1kHz–1MHz30SS316L3.5Argon(O2&H2O<10ppm)EOSINTl60RobertsandTien[43]30NS55NSSpotsize(lm)MaterialPowderparticlesize(lm)Environment25W,Al,Cu,Ag,316L,Mo,Ti,80Ni20Cr1–10Vacuum(10à3Pa)30316Land17-4PHD90:6Argon(O2&H2O<1ppm)DMP50GPMachineCustomizedNSNS:notspeci?ed;Freq:frequency;D90:thediameteroftheparticlethat90%oftheparticledistributionisbelowthisvalue.
微观选择性激光熔化技术发展的现状及未来展望 - 图文
