B.Nagarajanetal./Engineering5(2019)702–720707TheQ-switchedpulsedlaserproducedbetterresolutionthantheCWlaserforceramicmaterials,duetothenon-accumulationofheatwiththepulsedlaser.Despitethesuccessfulsinteringofcer-tainmetallicandceramicmaterialsusingalasermicrosinteringsetupwithQ-switchedpulsedlasers,itcanbeperceivedthatpulsedlasersinmicroSLMstillpossesslimitationsintermsofsur-face?nish,meltpoolstability,anddefects.Theselimitations—alongwiththewideapplicationofCWlasersinconventionalSLM—couldbethereasonwhymostrecentresearchworksinthis?eldhavebeencarriedoutusingCWlasers.ItshouldbenotedthatresearcheffortsonmicroSLMhavebeenquitelimited,andthatthisisdisproportionatetothegeneralenthusiasmthatexistsforthe?eldofconventionalmacroscaleSLM.ForconventionalSLM,theeffectsofvariousprocessparame-ters(asillustratedinFig.3)ontheprocesscharacteristicshavebeenwidelyreportedintheliterature[13,23,25,27,49,50].AlthoughmicroSLMprocessparametersareexpectedtoexhibitasigni?cantin?uenceontheprocessoutcomes,includingfeatureresolution,defects,surface?nish,andmicrostructure,therearelimitedparametricstudiesonmicroSLMavailableintheliterature.Kniepkampetal.[42]reportedontheincreaseinthedimensionalaccuracyofcertainpartfeatureswithareductioninlaserpowerduringthemicroSLMof316Lpowder.Fischeretal.[31]studiedtheformationofsingletracksandbulkfeaturesusingthemicroSLMof316Lpowderoverarangeofscanningspeedsandlaserpowers,andtheprocesswindowforhomogeneoustracksanddensecuboidswasidenti?ed.AbeleandKniepkamp[41]investi-gatedtheeffectsofthecontour-scanningstrategy,laserpower,andscanningspeedonthesurfaceroughnessandmorphologyoftheverticalwallsduringthemicroSLMof316Lpowders.Contourscanningreducedtheverticalsurfaceroughnessofthepartsattheoptimizedexposureparameters.Despitetheseefforts,noneofthepreviousresearchworksonmicroSLM/SLShavereportedonthemechanicalproperties,microstructure,orresidualstresspro?leofthefabricatedfeatures.Sincethefocusofthoseworkswaspri-marilyonachieving?nedensefeatureswithasmoothsurface,onlycharacteristicssuchasfeatureresolution,partdensity,andsurface?nishwerereported.MostofthecomponentsfabricatedbymeansofconventionalSLMhavestructuralapplicationsinwhichthemechanicalpropertiesandmicrostructuralfactorssuchasgrainmorphologyandcrystallographictexturearesigni?cant.SincethepartsfabricatedbymeansofmicroSLMmightalsohaverequirementsforthemechanicalproperties,residualstress,andmicrostructure,itisessentialtounderstandtheunderlyingbehav-ioroftheprocess.MicrostructureformationinSLMisin?uencedbyanumberofmechanismsincludingheattransfer,thermophysicalpropertiesofthematerials,andphasetransformations[51].Themodeofsolidi?cationandtheresultantmicrostructurearecontrolledbythetemperaturegradient(G)andtheliquid–solidinterfacevelocity(i.e.,solidi?cationrate,R)ofthemeltpool,whicharerepresentedthroughsolidi?cationmaps(Gvs.Rmaps)[21].Thesolidi?cationmodesareequiaxeddendritic,columnardendritic,cellular,andplanar.ThecommonlyobservedmicrostructureinSLMhasbeenfoundtobecolumnardendritic,asAMprocessestypicallyundergorapidheating,solidi?cation,andreheatingduringthemeltingofadjacentlayers[7,11,21,51].ThepredominantformationofcolumnardendritesinSLMcanbeattributedtothelargetemper-aturegradientalongthebuildingdirection[11].TheresultingmicrostructureinSLMismainlycontrolledbyprocessvariablessuchaslaserpower,scanningspeed,andscanningstrategy,althoughanumberofotherfactorsincludingelementalcomposi-tion,buildingdirection,andpartgeometryalsoplayarole[51].DespiteavastquantityofliteraturebeingavailableontheresultantmicrostructureinconventionalSLM,therehavebeennosimilarstudiesreportedformicroSLM.Inrecenttimes,attemptshavebeenmadetoinvestigatetheeffectoflaserspotsize(seeSection4.2)bydefocusingthebeaminPBFprocessessuchasEBMandSLM.Al-Bermani[52]reportedthatdefocusingtheelectronbeambychang-ingthefocusoffsetsigni?cantlyin?uencedthemeltpoolmorphol-ogyduringtheEBMofSS.AsimilarapproachbyPhanetal.[53]usinganarrowlyfocusedbeamintheEBMofacobalt(Co)-basedalloyresultedinhorizontaldendritesrestrictingthegrowthoftyp-icalcolumnardendrites.McLouthetal.[54]studiedthelaserbeamfocusshiftintheSLMofIN718,andobservedthatasmallerspotsizeproduced?nerandequiaxedmicrostructuresduetohigherpowerdensity.Inourrecentstudyonthesingle-trackformationof316LpowderinmicroSLM,theobservedmolten-poolmorphol-ogyofa‘‘double-crest”surfacewasquitedifferentfromthatofthesingletracksformedduringmacroscaleSLM,duetothe?nelaserspotsizeinourresearch[55].Theabove-mentionedresearchondefocusingeffectsindicatesthatthelaserspotsizemayplayasig-ni?cantroleintheprocesscharacteristicsofmicroSLM.Duetothe?nerspotsize,smallerlayerthickness,and?nerpowdersinmicroSLM,themicrostructureformationisexpectedtodifferfromthatofconventionalSLM.Furthermore,asmicroSLMinvolvesa?nespotsize,thetemperaturegradientandthesolidi?cationrateareexpectedtobehigher,whichmayleadtofastercoolingratesandhenceto?nerdendrites.Nevertheless,itisdif?culttopredictthemicrostructureofmicroSLM,asitdependsuponanumberoffac-torsinvolvingcomplexmechanisms.ThemechanicalbehaviorofpartsfabricatedbymeansofconventionalSLM,includingthehard-ness,tensile,andfatiguepropertiesofvariousmaterials,hasbeenwidelyreported[11,25,50,56,57].However,themechanicalproper-tiesofmicroSLMpartshavebarelybeeninvestigatedinthelitera-ture.Themechanicalpropertiesaretypicallyin?uencedbydefects,microstructure,residualstress,andpost-heattreatment[7].AccordingtopublishedreviewsrelatedtoSLMorPBFingen-eral,thefollowingpost-processingheattreatmentsarecommonlyused:stressrelieving,aging,solutiontreating,andhotisostaticpressing(HIP)[7].Themotivationforheattreatmentistoreduceoreliminatedefects,controlthemicrostructure,improvetheprop-erties,andrelieveresidualstresses[21,56,58].HIPistypicallyusedtocloseinternalporesandcracks,whereasrecrystallizationre?nesthemicrostructuretoequiaxed?negrainsandagingcontrolspre-cipitateformation[7,21].SinceSLMproducesmicrostructuresthataredifferentfromthoseformedbytraditionalprocesses,theheattreatmentstrategyisdifferentaswell[59–62].Asdiscussedearlier,microSLMmayresultinmicrostructuresthatdifferfromthoseformedbyconventionalSLMduetotheextremely?nespotsize.Throughsuitableheattreatment,themicrostructureisexpectedtobecontrolledwhileimprovingthemechanicalproperties.Asthepost-heattreatmentforSLMcomponentsdependsonanumberoffactors,includingtheinitialmicrostructure,defects,residualstress,elementalcomposition,anddesiredoutputcharacteristics,itischallengingtopredictasuitableheattreatmentformicroSLM.Thus,futurestudiesontheheattreatmentofmicroSLMwillbeveryvaluable,astheywillbringsigni?cantopportunitiestobroadenrelevantapplications.First,however,itisnecessarytounderstandthemicrostructurecharacteristics,suchasgrainmor-phologyandphaseformation,thatarecreatedbythemicroSLMofvariousmaterialsinordertoidentifyoptimizedpost-processingheattreatments.Table2[63–68]comparesvariouscharacteristicsofcommer-ciallyavailableAMsystemsformicromanufacturingintermsofthebuildvolume,achievablelayerthickness,laserspeci?cations,laserspotsize,recoatingsystem,processingmaterials,andsoon.The?rstcommercialsystemformicroSLSwasbuiltoverapatent[69]basedonlasermicrosintering[20,33].ThemicroSLSprocesswascommercializedas‘‘EOSINTl60”by3DMicroPrintGmbH,acompanyfoundedby3D-MicromacAGandEOSGmbHexclusivelytodevelopmicroSLSsystemsformetallicmicrofabrication.Itcan708B.Nagarajanetal./Engineering5(2019)702–720Table2
BenchmarkingofcommerciallyavailableAMsystemsformicromanufacturing.
DMP64/EOSINTl60[63]REALizerSLM50/SLM100[64]RealizerGmbH/70?H4020–50Fiberlaser;20–120W$20BladeCoCr,SS316L,Ag,Au,Pd,TialloysArgonJewelry,precisionengineeringPRECIOUSM080[65]EOSGmbH/80?H9530Yb-?berlaser;100W<30BladeAg,Au,Pd,PtalloysMYSINT100[66]TruPrint1000[67]ProXDMP100[68]ManufacturerBuildvolume(mm)Layerthickness(lm)Laserspeci?cations3DMicroPrintGmbHL60?W60?H301–5Fiberlaser;50WSismaSpA/100?H10020–40Fiberlaser;200WTRUMPF/100?H10010–50Fiberlaser;200W3DSystems,Inc.L100?W100?H100NSFiberlaser;50WLaserspotsize(lm)RecoatingsystemMaterials<30BladeSS,Ti,Mo,Al55/30BladePreciousmetals,bronze,CoCr,SS,maragingsteel,NialloysNitrogen,argonPreciousmetal,jewelry55BladeSS,toolsteel,CoCr,Al,Nialloys,Ti,preciousmetals,bronzeNitrogen,argonMedicine,dental,aerospace,energy,automotiveNSRollerCoCr,SS17-4PHControlenvironmentIndustryArgonMedical,jewelry,mechatronics,moldmaking,automotiveNSWatches,jewelryNitrogen,argonPrecisionengineering,researchanddevelopmentbeseeninTable2thattheexistingcommercialsystemshavealaserspotsizegreaterthanorequalto20lm.Itshouldbenotedthatthislaserspotsizemustbereducedsigni?cantlyinordertobuildpreciseparts.AstheSLM/SLSprocessbuildspartsinalayer-by-layerfashion,itisnecessarytoachieveassmallalayerthicknessaspossibleinordertoreducethefeatureresolution.WiththeexceptionofEOSINTl60,theotherexistingmicroSLSsystemstypicallyproducealayerthicknessbetween10and50lm,whichcannotbeusedtoachievemicrofeatureswithsub-micrometerdimensions.Despitedifferenteffortstousevarioustypesofrecoatingsystems,thecommercialsystemsuseeitherabladeorrollersystem,whichissimilartomacroscaleSLMsystems.Theabilitytoreducethelayerthicknessiscorrelatedtothesizeofthepowderused.ConventionalSLM/SLStypicallyusespowdersof20–50lmdiameter,whereasmicroSLSprocessesrequireparticleswithadiametermuchsmallerthan10lm.Recently,theauthors(i.e.,SingaporeInstituteofManufacturingTechnology,SIMTech)developedanin-housemicroSLMsystem(Fig.9(a))witha?nelaserspotsizeandanovelpowder-recoatingsystemwiththeabilitytohandle?nepowders.InitialexperimentalresultsusingSS316Lpowders(D50lm,inwhichD50isthediameteroftheparticlethat50%oftheparticledistributionisbelowthisvalue)demonstratethatthedevelopedmicroSLMsystemiscapableofproducingmicrofeatureswitha?nesurface?nish.Varioustrialswereconductedtovalidatethesystembyvaryingthelaserpower,scanningstrategy,scanningspeed,andhatchingdensity.Fig.9(b)showsvariousfeaturesthatwerefabricatedusingthemicroSLMsystemwiththefollowingprocessparameters:alayerthicknessof10lm,spotsizeof15lm,laserpowerof50W,scanningspeedof800–1400mmásà1,andhatchspacingof10lm.Atpresent,aminimumfeaturesizeof60lmandaminimumsurfaceroughness(Ra)of1.3lmcanbeachieved.However,thesystemiscapableofhandlingsub-micrometerandnanoscalepowderstoproducealayerthicknessof1lm.Withafurtherreductioninthelayerthicknessandpow-derparticlesize,amuch?nerfeatureresolution(<15lm)andasurfaceroughnessoflessthan1lmcouldbeachievedusingthedevelopedsystem.ScalingdownfromconventionaltomicroSLMeffortsnecessi-tatescertainconsiderations,whichcanbeclassi?edinto①equipment-related,②process-related,and③post-treatmentfactors.Mostoftheprocessmechanismsandtheeffectsofprocessparameterscanbereadacrossthescales.A?nespotsizeandpar-ticlesizewillnaturallyreducethelayerthicknessandhatchFig.9.(a)MicroSLMsystemdevelopedbySIMTech;(b)variousfabricatedfeaturesusingmicroSLM;(c)scanningelectronmicroscope(SEM)imagesoffeaturetopsurfaces.B.Nagarajanetal./Engineering5(2019)702–720709spacing,leadingtoanincreasedprocesscycletime.Streeketal.[35]reporteda12-foldincreaseintheprocessingtimeoflasermicrosinteringtoprintthesamecomponentwhenthelayerthick-nessandparticlesizewerereducedbyanorderofmagnitude.Withtheapplicationof?nespotsatmicroscales,thepowerdensitywillbemuchhigher.Therefore,theprocessthroughputmightbeincreasedbyusingreducedlaserpowerand/orfasterscanning.SupportstructuredesignisanotherconcernwithmicroSLM,asremovingthestructuresisdif?cultandmightaffectthepartdimensions.Similarly,preheatingcouldbeanissueinthecaseofhigh-aspect-ratiothinwalls,especiallywhenbuildingsupportstructureshasbeenadif?culty.Equipment-relatedscalingfactorsincludethebuildingplat-form,opticalsystem,powderrecoating,powderhandling,andpowderrecycling.ThesizeofthebuildingplatformandhencetheentireequipmentfootprintissmallerformicroSLMsystems.Inordertosatisfyoneofthemajorrequirementsofachievinga?nespotsize,theopticalunitsmustbemodi?ed,whichwillbedescribedinSection4.2.AnotherimportantrequirementformicroSLMisachievingasmallerlayerthickness,whichcanberealizedbyprecisiondrivesforthepowderdispensingandbuildingplat-form.Themajorequipment-relatedissueswiththescalingdownhavebeentheneedtouse?nepowdersofthesub-micrometerscaleorevennanoscale.Sincetheexposureof?nenanopowderstotheenvironmentcarriessafetyandhealthhazards,itisadvis-abletominimizethemanualhandlingofsuchpowders.Itisoftheutmostnecessitytoprovideatightenclosuretothebuildingchamber,asforanySLMmachines.Theeffectofthepowderparti-clesizeandtherecoatingsystemwillbediscussedinSections4.3and4.4,respectively.Post-treatmentdifferencesincludethesur-face?nishingandheattreatmentperformedontheAMparts.Heattreatmentofmicropartswiththinfeaturescouldresultinpartdis-tortion.Powderadhesiontothewallshasbeenacommonoccur-renceinSLM,whichnecessitatesfurther?nishingafterprinting.Inmicroscales,thereisapossibilitythatthemachiningofthinwallswillnotbepossible.Anon-contact?nishingsuchaselec-tropolishingmightbeineffectiveaswell,asobservedbyNoelkeetal.[38].Thus,itisnecessarytofabricatepartswithagoodsur-face?nishbothonthesurfaceandalongthewalls,ratherthanrelyingonsecondarysubtractiveprocessing.Thesurface-?nishingeffectisdiscussedindetailinSection5.4.2.LaserspotLaserbeamdiameterisoneofthemostsigni?cantparametersin?uencingthefeatureresolution[31].Theminimumspotsize,whichoccursatthelaserfocalpoint,istypicallyusedforAMpro-cesses,asthepowerdensityismaximizedatthefocus.PBFpro-cessesusealaserbeamdiameterintherangeof50–100lm,whereasDEDprocessesusemillimeter-sizedspots[21].Maetal.[70]studiedthedifferenceinthemetallurgicalbehaviorsofSS316Lfabricatedbymeansoflasercladdingdeposition(LCD)andSLM,withthespotsizeoftheLCD(>1mm)beingmuchlargerthanthatoftheSLM(0.12–0.15mm).SLMresultedinahigherdepth-to-widthratioofthemoltenpool,highercoolingrate,smallerprimarycellulararmspacing,lowergrainaspectratio,highermicrohard-ness,andgreaterstrength.Althoughitisdif?culttoattributetheSLMbehaviortothebeamdiameterthroughthisstudy,thisworkprovidessomeindicationoftheconsequencesintermsofvaryingenergyinputs,solidi?cationrates,meltpools,andmicrostructurethatresultfromachangeinspotsize.Liuetal.[71]investigatedtheeffectofthelaserbeamdiameterinSLMusingSS316Lpow-ders.Forareductioninbeamdiameterfrom48to26lm,improve-mentsinthepartdensity,surface?nish,andmechanicalpropertieswerereported.Makoanaetal.[72]usedtwodifferentsystemswithdifferentbeamdiameters(80and240lm)toinvestigatetheeffectofspotsizeupscalingduringlaser-basedPBF.Thepowerdensitywaskeptconstantinordertostudythebeamdiametereffect.Itwasfoundthatasmallerbeamdiameterandsmallerlaserpowerresultedinanarrowerandshallowermoltenpool,leadingtosmal-lerhatchspacingandlayerthickness.Helmeretal.[73]studiedtheeffectofspotsizeinEBMbychangingthelaserfocusshift.Theresultsindicatedasigni?cantdifferenceinthemeltpoolgeometryandmicrostructurefordiffer-entspotsizescorrespondingtothefocused(400lm)anddefo-cusedbeam(500lm).ArecentpaperbyMcLouthetal.[54]extendedtheanalysisofalaserfocusshifttoSLM.IN718samplesfabricatedatthefocalpointhada?nermicrostructureincompari-sonwiththesamplesfabricatedusingthedefocusedbeam.Thisbehaviorwasattributedtoahigherpowerdensityresultingfromthesmallerspotsize.Aconcurrentstudy[74]ontheeffectoflaserfocusshiftonporosity,surfaceroughness,andtensilestrengthreportedasigni?cantchangeinthebuiltpartpropertieswiththefocusshift.Varyingmeltbehaviorsrangingfromalackoffusionatthenegativeshift(à2mm)tokeyholeformationduetoexces-siveenergyatthepositiveshift(+3mm)wereobserved.Thechangeinenergyinput,alongwiththefocusshiftandspotsize,correspondstothedivergenceoftheGaussiandistributionofthebeam.However,itwasnotedthattheoptimumfocusshift,andhencethespotsize,iscorrelatedtothescanningspeedandlaserpower.Studiesonasimilarprocess—namely,laserwelding—high-lighttheeffectofsmallerspotsinimprovingweldingbehaviorbyachievingeitherafasterweldingspeedordeeperpenetration,duetotheincreaseinpowerdensity[75].DespiteextensiveresearcheffortsinSLMgenerally,itisnotedthatstudiesontheeffectofspotsizeontheprocessbehavior—especiallyonthefeatureresolution—isquitescarce.ItcanbeseeninTable1thatthespotsizeofmicroSLMsystemsrangesfrom20to30lm,whilethecorrespondingminimumfeatureresolutionissimilartoorslightlylargerthanthespotsize.Similarly,commer-cialmicroSLMsystemshavealaserspotsizegreaterthan20lm(Table2).Inordertorealize?nemicrofeatures,itisnecessarytoachieveeven?nerbeamspotsizes.DebRoyetal.[21]emphasizedthatsmallspotsizesandlowpowerarerequiredtoachieve?nerpartresolution.Thespotsizeistypicallyafunctionofthe?bercorediameter,focusinglens,andcollimatorlens.Reducingthelaserspotsizeisquitestraightforwardwithanappropriateopticaldesign.TheopticalsysteminSLMtypicallyconsistsofacollimator,beamshaper,scanner,andobjectiveF–hlens.ThescanningsysteminconventionalandmicroSLMmachinestypicallyusesagal-vanometer,whichconsistsoftwomirrors,toguidethelaserbeaminatleasttwoaxes.Inoneofthe?rstSLSsystems,developedbyRegenfussetal.[32],aSCANLABbeamscannerwithascan?eldof25mm?25mmwasusedalongwithaQ-switchedNd:YAGlaserwith0.1–10WpowerinTEM00mode.Theopticaldesigncanalsoconsistofothermechanisms,suchasadigitalmirrordevice,toachieve?nespotsizes[44].However,adetailedreviewofopticalsystemsisbeyondthescopeofthisstudy.4.3.PowdersSeveralpowdercharacteristics(Fig.3)in?uencetheSLMpro-cessperformanceand,hence,thefabricatedpartquality.Powdershape,size,andsurfaceroughnessarethemostsigni?cantparam-etersthatin?uencethepowder?owabilityand,consequently,thepowder-bedproperties,meltpoolbehavior,andpartcharacteris-tics[76–78].Olakanmi[79]studiedtheeffectofpowderpropertiesontheSLM/SLSofpureAlandAlalloys.Theresultsindicatethattheshapeofthepowderparticlehasasigni?canteffectontheprocessingmapsanddensi?cationprocess.Thepowderparticleswithirregu-larshapesinthepowderswerefoundtoexacerbatetheformation710B.Nagarajanetal./Engineering5(2019)702–720ofagglomeratesandporosity.AnanalysisofrawTi–TiBpowdershapesinSLMshowedthatirregularlyshapedpowderparticlesaredetrimentaltodensi?cationand,hence,tothetensilestrength[80].AstudyonpowdercharacteristicsbyCordovaetal.[78]usingdifferentmetalpowdersreportedanoccurrenceofthemaximumpowderpackingdensitywiththemosthomogeneousmorphology(i.e.,mostspherical).Liuetal.[71]observedthatwater-atomized11lmpowderhasalowerPBDcomparedwiththeapparentandtappeddensities,duetotheirregularangularmorphologyandthe?neparticlesize.Thesestudiesdemonstratecommonagree-mentregardingtheneedtoemploypowderparticleswithaspher-icalshapeforSLMandAMprocessesingeneral[7,76,77].Thein?uenceofparticlesizeinSLMhasbeenwidelyinvesti-gated,ashasbeenreviewedbySuttonetal.[76].Asmallerparticlesizetypicallyresultsinbetterpowderpacking(increaseinappar-entdensity)andpoor?owability[81].Incontrast,apoorerappar-entdensity,tapdensity,andPBDwerereportedwith?nerIN718powders[71].Finerpowdersresultinbettersurfaceroughnessofthe?nalpartafterSLM[82,83],butanincreaseinporosity[84].Simchi[85]reportedbetterpartdensi?cationduringSLMwitha?nerpowderparticlesizeorgreatersurfacearea,intheabsenceofagglomeration.Anoptimalpowderparticlesizeisdependentonotherprocessvariables,astheuseofapowderparticlesizethatislargerthanthelaserspotsizeandlayerthicknesstypicallyresultsinnon-uniformenergydistributions,whichfurtheraffectthemeltpoolbehavior[86].Inadditiontotheparticlesize,thePSDsigni?cantlyin?uencestheSLMprocess[76,77].Liuetal.[71]revealedthatawiderPSDachievedbettersurfaceroughnessandpartdensity,whereasbetterhardnessandtensilestrengthoccurredwithanarrowerPSD.Iden-tifyinganoptimumpowderparticlesizeandPSDischallenging,as?nepowderswithanarrowPSDresultinagglomeration,whereascoarsepowderswithawiderPSDleadtosegregation[85].Further-more,anumberofstudies[87–89]haveemphasizedthatabimo-dalormultimodalpowderdistributionincreasesthepowderpackingdensityandpartdensity.Basedonthisadvantage,Vaezietal.[14]proposedabimodalapproachforthemicroscalebinderjettingprocesstoimprovethepartsurfacequality.ConventionalSLM/SLStypicallyusespowderswithaparticlediameterof25–50lm,whereasmicroSLSprocessesrequireparti-cleswithadiametermuchsmallerthan10lm.Microscaleandsub-micrometer-scalepowdershavebeentestedinmicroSLSsys-tems,butexhibitlimitationsintermsofpartquality[20,31].Regenfussetal.[33]usedpowdersas?neas0.3lmforlasermicrosinteringprocesstoproducethefeaturesshowninFig.7.Fischeretal.[31]usedpowderswithasizeof3.5lm,butthe?nestfeatureresolutionwasabout57lm.Tofabricatesub-micrometerfeatures,nanopowdersarenecessary.However,nanopowdersresultinexcessiveagglomerationandoxidationduetothehighsurface-area-to-volumeratio[44].Fig.10[33,90]showstheagglomerationofbothirregularlyshapedand?nesphericalpowderparticles.VanderWaalsforcesbecomedominantovergravityatthenanoscale[90].Agglomerationincreasestheinterparticlefrictionandreducesthepowder?owability,leadingtoinhomogeneouspowderlayer-ing[76].Furthereffectsincludetheballingeffectandanincreaseinporosity.Inadditiontoagglomeration,?nepowderparticlesresultinanumberofotherissuesthatneedtoberesolvedinthecaseofmicroSLMsystemdevelopment:??There?ectivityof?nepowdersishigher,whichreducestheabsorptivityofthelaserirradiationduringSLM.??Nguyenetal.[82]observedthat?nerpowderswithaparticlesizelessthanafewmicrometerswerecarriedawaybytheinertgas?owduringtheSLMofIN718.??Finepowdersmightvaporizeatveryhighenergydensities,leadingtoareductioninpartdensity,aswasobservedwithSLM[71].??Anotherdrawbackisthereactivityofthe?nepowder,whichnecessitatesadditionalsafetymeasuresduringhandlingandtransportation.4.4.Powder-recoatingsystemThemajorissuethathasbeenreportedformicroSLM/SLSformetalshasbeentheinabilityofthetraditionalrecoatingsystemstoeffectivelydepositthepowderonthepowderbed.Thereisacommonconsensusontheneedforinnovativepowder-recoatingmechanismsthatcanhomogenouslyspreadpowdersatthesub-micrometerscaleornanoscale.However,asmentionedearlier,nanopowdersarepronetoexcessiveagglomerationduetothehighsurface-area-to-volumeratioandresultinghighsurfaceenergy.Atthenanoscale,vanderWaalsforcesbecomedominantovergravity,leadingtonon-uniformpowderlayersduringtherecoatingstepoftheAMprocess.Inordertoachieveef?cientlayeringwithagoodpowderpackingdensity,oneormoreofthefollowingapproachesareneededformicroSLM:??Aneffectivepowderdistributionstrategytoavoidpowderclogging;??Mechanicalseparationofagglomeratedpowders;??Thermalenergytoincreasethepackingdensity(preheating/pre-sintering);??Useofanadditionalbindingagentforeffectivedistribution(slurry-based).Fig.10.Agglomerationof(a)sub-micrometergrainedWpowder;(b)Cunanoparticles(averageparticlesizeof100nm)withanirregularshape;(c)sphericalCunanoparticleswithasizeof40nm.(a)isreproducedfromRef.[33]withpermissionofEmeraldGroupPublishingLimited,ó2007;(b)and(c)arereproducedfromRef.[90]withpermissionofElsevierB.V.,ó2018.B.Nagarajanetal./Engineering5(2019)702–720711InordertodevisenewpowderdistributionstrategiesthatarenotlimitedtomicroSLM,itisnecessarytounderstandtheexistingtechniquesthatarecurrentlyusedinconventionalSLM.4.4.1.CurrentrakingmethodsPowder-bedrecoatingdependsonthe?owabilityofthepow-der,whichisin?uencedbyacombinationofthepowderandequipmentcharacteristics[91].The?owabilitymustbeincreased?rstforbetterpowderdistribution,whereasthepowderneedstobeintactafterthespreading.MostcommercialSLM/SLSsystemsuseeitherabladeorrollerforrecoatingthepowderlayers(Fig.11)[20,45,92,93],asdescribedinTable2.Themostcommonspreadingmechanismisrakingwithadoctorblade,asillustratedinFig.11(a).Adoctorbladeissimplyathinpieceofmetalorceramicthatisusedtoscrapethepowderacrossthesurfaceofapowderbed.Sincethepowderisnot?uidizedwiththebladespreader,highshearforcesareappliedtothepreviouslydepositedlayer[94].Applyingultrasonicvibrationtothebladeisexpectedtoreducetheseshearstresses.Rollersarethesecondmostcommondeviceforpowderraking.Translationoftherolleracrossthepowderbed,orclockwiserota-tion,producesaforwardrotatingmotion,whichiscalledaforward-rotatingroller(FR),asshowninFig.11(b).Thismethodtendstoimpartcompactionofthepowder,asthereismorepow-derinfrontoftherollerduringitstranslation[91].However,alumpofpowderstickstotherollerduringtheforwardmotionandcreatescratersinthepowderbed.Rollerrotationintheoppo-sitedirection,calledacounter-rotatingroller(CR),hasbetter?owability,asitforcesthepowderupwhile?uidizingthepowder(Fig.11(c)).However,thereisnocompactionofthepowderwiththeCRmethod.NiinoandSato[92]proposedacombinedsetupofFRandCR,asshowninFig.11(d).TheCR?rstscrapsofftheexcesspowderfromthebed,whichfacilitatesbettercompactionbytheFR.BuddingandVaneker[91]replacedtheCRwithadoctorblade,inordertoimpartthesamescrapingeffectwhilereducingtheprocesstime.However,theirmethodstillproducedcratersanddragonthepowderbed.RoyandCullinan[45]usedadoctorbladeandaCRinstead,inordertoproviderakingandcompaction,respectively.InthesetupshowninFig.11(e),vibrationoftheCRwasaddedtoinducecompactionofthepowderthatwas?rstspreadbythedoctorblade.Haferkampetal.[93]usedacombina-tionofthreerollerstoprovideboththeforward-andcounter-rotatingrollingaction(Fig.11(f)),wherethelayerthicknesswascontrolledbythedistancebetweentherollers.Regenfussetal.[20]usedacompactioncylinderinadditiontotherakingblades,inordertodisperseandcompactthe?nepowderusedformicro-scalepowder-bedprocesses.Theschematicofthepowder-rakingsystemisillustratedinFig.11(g).Inthissetup,thebuildsubstrate,fusedpart,andremainingpowderbelowthefreshpowderlayerareliftedupwardtowardamanualcap,inordertoprovidepowdercompaction.Table3[20,37,45,91–93]comparesthedifferentpowder-rakingsystemsdescribedintheliterature.TheexistingrakingsystemsareeffectivefortheconventionalSLMprocess,inwhichminorinaccuraciesinthepowderspreadingcanbeconsiderednegligible.Atthemicroscale,however,similarissuescouldleadtoalargedeviationinthefabricatedpartdimen-sions.Theeffectswouldbeexacerbated,as?nepowdersareusedinmicroSLM.Despiteconsistenteffortstoimprovetherakingmethods,theylacktherequiredprecisionformicroSLM.Theinabilityofexistingrecoatingmethodstoachieveahomogeneous,denselayerof?nepowderonthepowderbedhasconsistentlybeenreported[33,38,45].Theinteractionbetween?nepowderparticlesandtherakingcomponentsgreatlyin?uencetheef?-ciencyofpowderspreading.Theliteraturereviewrevealsthatrakingsystemsareexpectednotonlytodispersethepowderontothepowderbed,butalsotoprovidebettervolumetricpackingdensityoftheappliedlayer.Therefore,aneffectivepowder-recoatingsystemisrequiredtocon-trolthelayerthicknesstosub-micrometer-scaleornanoscalepre-cisionwhileresultinginahomogeneouspowderdistributionalongthepowderbed.4.4.2.DrypowderdispensingInordertoovercometheissueswiththecurrentpowderdistri-butionsystems,Vaezietal.[14]suggesteddrypowder-dispensingtechniques,especiallyformicroscalePBFprocesses.Themechani-calmethodsofdrypowderdispensingincludethepneumatic,vol-umetric,andscrew/augermethods,whichhavelimitationssuchasaslowfeedrateandaninabilitytohandle?nepowders[95].Thespatialresolutionofthesemethodsisatleasttwoordersofmagni-tudelowerthanwhatisnecessaryformicroSLM.Vibrationmethodshaveattractedincreasingattentioninthe?eldof?nepowderfeeding.Thesemethodsusevibrationalbehav-iortocauseanincreaseinfreevolume,whichimprovestheparticledisplacement[95].Thebreakingofparticleagglomeratescanalsobeachievedthroughvibration.Matsusakaetal.[96]?rstusedthevibrationofaverticalcapillarytube(asshowninFig.12(a))tocontrolthe?owof?nealuminapowderwithaparticlesizeof20lmandanirregularshape.Duetoadhesiveness,the?nepow-dercouldnot?owthroughthecapillarytubeentirelybygravity.Whenvibrationwasinducedonthecapillarytubethroughavari-abledirectcurrent(DC)motor,itpropagatedintothepowder,causingareductioninfrictionalstressbetweenthetubewallandthepowder.Boththeamplitudeandthefrequencyofvibrationarecriticalparametersaffectingthe?owrate.Thepowder?owrateisproportionaltothefrequency,butinverselyproportionaltotheamplitude.Thesameresearchgroupusedanultrasonictransducertoinducevibrationofthecapillarytube[97].AsimilarsetupwasdevelopedbyYangandEvans[98](asshowninFig.12(b))toprintpolygonal-shapedtungstencarbide(WC)pow-derparticleswithasizeof12lmonasubstrate.Lietal.[99]usedultrasonicvibrationgeneratedbyapiezoelectrictransducertofeed3lmcopperandSSpowders.Becauseofthemicro-vibrationsintheultrasonicfrequency,thethinpowderlayerneartheinnerwallbehavedasalubricant.Thebene?tsofultrasonicpowderfeedinglieinitsabilitytopreventpowderagglomerationandachievecon-tinuousanduniformpowderfeeding,duetothetravelingoftheultrasonicwavealongthecapillarytube.YangandEvans[95]developedasystem,asshowninFig.12(c),tomixanddepositmul-tiplematerialsusingindividualpowderhoppersandamixinghop-per,wherethe?owrateiscontrolledbyacousticvibration.Theseresearchworkshavedemonstratedthecapabilityoftheultrasonic-basedmicro-feedingdeviceswhichcanbeintegratedwithlasersandusedinatypicalAMsystem.Anotherpromisingpowder-feedingmechanismforAMiselectrostatic-baseddispensing.Electrostaticcoatingorsprayinghasbeenwidelyusedforindustrialcoatingsandinconstruction[100].Inrecenttimes,ithasfoundanapplicationinthepharmaceu-ticaldrycoatingoftablets,asdetailedbyYangetal.[101]inarecentreview.Thismethodworksontheprincipleofelectrostaticattractionbetweenoppositecharges.AsillustratedinFig.13(a)[101],thepowderparticlesarechargedwhilebeingexposedtoastrongelectrical?eld.Thenegativelychargedparticlesareattractedtothesubstrate,whichiseitherpositivelychargedorgrounded.Inelectrostaticspraying,thechargingofpowderoccurswhenpowderparticlespassthroughthespraygun,andarethendepositedonthegroundsubstrate.Incomparisonwithotherdry-coatingmethods,electrostaticcoatinggreatlyimprovesthecoatingef?ciencyandadhesionduetotheelectricalattraction.Electrophotographyisanothercommonapplicationusingtheelectrostaticmethod,inwhichthephotographicpapersareprintedwithtonerparticles[102].Inelectrophotography,alight-sensitive
微观选择性激光熔化技术发展的现状及未来展望 - 图文
