712B.Nagarajanetal./Engineering5(2019)702–720Fig.11.Schematicoftheexistingrakingsystemsinpowder-bedAM.(a)Doctorblade;(b)forward-rotatingroller(FR);(c)counter-rotatingroller(CR);(d)combinedFR–CR;(e)combineddoctorbladeandvibratingCR;(f)three-rollersystem;(g)cylindricalrakingsystemwithcompaction.(d)isreproducedfromRef.[92]withpermissionofUniversityofTexasatAustin,ó2009;(e)isreproducedfromRef.[45]withpermissionofUniversityofTexasatAustin,ó2015;(f)isreproducedfromRef.[93]withpermissionofElsevierB.V.,ó2004;(g)isreproducedfromRef.[20]withpermissionofWiley-VCHVerlagGmbH&Co.KGaA,ó2007.B.Nagarajanetal./Engineering5(2019)702–720Table3
Comparisonofpowder-rakingmethods[20,37,45,91–93].
Doctorblade(DB)IntroductionNorealcompactionoccurs;onlyaltersthelayerheightSimpleandeasilycontrollableCRMostlyforpowderspreading,aslesscompactionoccursStimulatesthepowder?owabilityinfrontofthepowderNosigni?cantcompactionofthepowderLinearandrotationalspeedoftheroller,rollerradius,angle,frictionFRMorecompaction,asmorepowderisundertherollerProvidescompactionCRfollowedbyFR[92]LimitstheamountofpowderinfrontofFRbyinitialcompactionusingCRBothspreadingandcompactionLongprocesscycletime713DBfollowedbyFRReplacementofCRbyDBforaquickeroperationHighthroughputAdvantagesLimitationsUnevennessofthebladeaffectsthedepositedlayer;also,thepowderisnot?uidizedLayerheight,bladevelocityAcompressedlumpofthepowderstickstotherollersurfaceandcausescratersLinearandrotationalspeedoftheroller,rollerradius,angle,frictionDistortionsofthepowdersurfaceatalowdensityBladeandrollertranslationvelocities,rollerdiameterProcessparametersLinearandrotationalspeedoftherollers,rollersize,frictionFig.12.Schematicofvibrationdrypowder-dispensingsystems.(a)Vibrationusingadirectcurrentmotor;(b)vibrationusinganultrasonicsource;(c)multi-powder-dispensingsystemusingacousticsforpowder-bedAM.(a)isreproducedfromRef.[96]withpermissionofSocietyofPowderTechnologyJapan,ó1996;(b)isreproducedfromRef.[98]withpermissionofElsevierB.V.,ó2004;(c)isreproducedfromRef.[95]withpermissionofSocietyofPowderTechnologyJapan,ó2007.photoconductoris?rstchargedbyahighvoltagecorona.Thelatentimageisthencreatedonthephotoconductorbyselectivelydischargingitssurfacethroughalightsource.Thechargedtonerparticlesaredepositedonthephotoconductor,whichisthentrans-ferredtothepaper.Basedontheelectrophotographytechnique,Liewetal.[103]developedasecondarypowder-depositionsystemtobeusedformulti-materialfabricationusingSLS.Inthesimpleexperimentalsetup,aTe?onscraperwasusedtodetachthenega-tivelychargedtoner,whichwasthendepositedoverapaperwithapositivecharge.KumarandZhang[104]developedelectrophotography-basedpowderdepositionforpowder-bed-basedtechniquessuchasSLM/SLS,whichcanalsobeappliedforbinderjetting[105].Aschematicoftheirsetupissimilartothatoftheelectrophotographyprocess,whichisshowninFig.13(b)[104].Polystyrenepowderswithaparticlesizeof5lmweredepositedonanaluminumbuildingplatformandfusedtogetherbyaheatrollertoachievepartswithathicknessof1mm.Inthistechnique,layerthicknesswascontrolledbyparameterssuchasthespeedofthephotoconductivebelt,chargeperunitmassofpowder,anddevelopingrollerspeed.Thomasetal.[106]alsodevelopedanelectrophotography-basedpowder-depositionmethodfortheSLMprocess.Theirsetupdemonstratedagoodtransferofpolymerpowdersfromthechargingplatetothesub-strate.Bothresearchworksproposedmulti-materialpowderdepo-sitionusingelectrophotography[104,106].Thedepositionef?ciencywasfoundtobein?uencedbytheelectricalpotentialandbythedistancebetweenthechargingplateandsubstrate.Despitetheinitialformationofauniformmonolayerofpowdersonthesubstrate,itwasdif?culttocontrolthestackingoffurtherlayers,asisnecessaryforSLM,inelectrophotography-baseddepo-sition.TwoapproachesareproposedinordertoachievepowderdepositioninatypicalSLMprocess,whichrequiresaconstantpotentialbetweenthephotoconductorandthesubstrateorsolidi-?edpartsurface:①removalofresidualchargefromthefusedlay-ers;and②additionalchargingbyacoronadeviceinordertoincreasethechargedensity.714B.Nagarajanetal./Engineering5(2019)702–720Fig.13.Schematicofelectrostaticdrypowder-dispensingsystems.(a)Electrostaticspraying;(b)electrophotography-basedpowderdispensingforSLM;(c)electrostaticpowdercompaction;(d)electrostaticpowderdispensingforpowder-bedAM.(a)isreproducedfromRef.[101]withpermissionofChineseSocietyofParticuologyandInstituteofProcessEngineering,CAS,ó2016;(b)isreproducedfromRef.[104]withpermissionofLaboratoryforFreeformFabricationandUniversityofTexasatAustin,ó2018;(c)isreproducedfromRef.[109];(d)isreproducedfromRef.[110].AsievefeedsystemwasdesignedbyMelvinandBeaman[107]tobeusedinSLS.Unlikeelectrography,thesievefeedsystemworksbytheremovalofstaticelectriccharges.Inthesievefeedsystem,thepowderisforcedontothepowderbedthroughachargedorgroundsieve,whereaslevelingisperformedbyasqueegeeorroller.Anincreaseinthebuiltpartstrengthof3to4timesandinthepartdensityof10%–15%wasachievedafterthesinteringofpolycarbon-atepowderusingthesievefeedsystem,incomparisonwithrollerfeeding.TheobservedbehaviorwasattributedtothecorrespondingincreaseinthePBDduetotheremovalofelectrostaticchargefromthepowderpassingthroughthesieve.However,thissystemhasdif?cultyachievingpreciselayeringandauniformcoatingthick-ness.Thesameresearchersdevelopedanelectrostatic-coating-basedpowder-recoatingmethodforSLS[108].Althoughtheelec-trostaticpowderlayeringproducedbetterdispersionthantheroller,thesinteredpartstillhadasigni?cantamountofporosity.ArecentpatentbyAppliedMaterialsInc.[109]useselectro-staticchargingtocompactthespreadpowderlayerwiththebaseplateorthepre-sinteredpart,asillustratedinFig.13(c).Electrostaticcompactionisappliedwhenthepotentialdropatthegapbetweentheelectrodeandthelayeroffreshfeedpowderislargerthanthepotentialdropacrossthelayerofsinteredandfreshfeedmaterial.Plasma,whichisgeneratedthroughthegas?ow,canalsobeusedtoincreasethecompactionforce.Inthiscase,mostofthepotentialdropoccursacrossanypreviouslydepositedlayersandthelayeroffreshfeedmaterial.Paascheetal.[110]con-ceptualizedapowder-bedsystemforAMusingelectrostaticpow-derdeposition,asillustratedinFig.13(d).Intheirsetup,thepositivelychargedsubstratecollectspowderfromthenegativelychargedpowdercontainerwiththeapplicationofvoltage.Oncethepowderisdeposited,thesubstratetraversestowardthelaserbeamforsubsequentmelting.Theprocessrepeatsuntiltheentirepartisfabricated.Onceimplemented,thissystemcouldhavethefollowingissues:①Positioningthesubstrateatthefocalspotforlaserirradiationandatthespeci?clocationforpowderdepositionforeverylayeristime-consumingandcouldleadtoerrors;②traversingthesubstratebetweeneachlayermightleadtoposi-tioninginaccuraciesandpartshifting;and③disposalofthetrappedpowdersbeforetheshiftingmaybedif?cult.Inaddition,theabilityofthissystemtoachievefurtherlayeringmaystillbelacking.Despitethedemonstratedfeasibilityofvibratoryandelectro-staticpowderdispensingforpreciseandselectivelayeringinpowder-bedprocesses,thesetechniqueshavecertainlimitations:(1)Powderdispensingthroughnozzle-basedsystemsisstronglyin?uencedbytheprocessenvironment,andnozzleclog-gingwillhamperreliablepowderdelivery.(2)Drypowder-dispensingsystemshaveamuchhigherdeposi-tiontimethanconventionalpowder-recoatingmethods.Thiswillincreasetheprocesscycletimeofthepowder-bedprocesses,whenAMisalreadytacklingtheissueofamuchhighercycletimeincomparisonwithconventionalmanufacturingprocesses.4.5.Powder-bedcharacterizationForSLMatthemicroscale,theapplicationofthinpowderlayersisacrucialstepthatcangreatlyaffectthepartresolution,surface?nish,porosity,microstructure,andmechanicalproperties.Liuetal.[71]reportedthatthePBDsigni?cantlyin?uencestheB.Nagarajanetal./Engineering5(2019)702–720715fabricatedpartdensityinSLS.Itisnotablethatnoprocessvariableisavailabletocomparethedifferentpowder-dispensingtech-niques.Anycomparison—ifitexists—ismadethroughthesinteredormeltedpartdensity.AsSLMiscomprisedofanumberofprocessparameters,itisdif?culttosegregatetheeffectofthepowder-bedcharacteristicswhilecomparingthe?nalparts.Thissectionpro-videsdetailsonPBD,asitisanin?uentialfactorinmicroscalepowder-bedsystems.Thepackingofthepowderduringthepowder-bedprocessingin?uencesthepartdensity.However,thereisnostandardproce-duretocharacterizethedensityofthepowderbedexperimentally[111].Elliottetal.[112]devisedamethodtocharacterizetheden-sityofthepowderbedusedforbinderjetprinting.First,aCRwasusedtodepositthepowderonthepowderbed.Next,abinderjetwasappliedalongthecontoursofacup,leavingloosepowderinthecavity.Afterprinting,thecupswereremovedandtheweightoftheloosepowderwasevaluated.ThePBDcouldbecalculated,astheweightandvolumeofthecupwereknown.AsimilarmethodwasusedbyLiuetal.[71]forSLM,inwhichthePBDwasmeasuredbymeltingthewallsofasquarecontainer.Inbothstudies,thePBDwasfoundtofallbetweentheapparentdensityandtappeddensityofthepowder.Guetal.[81]devisedamethodtocalculatethePBDwithoutbindersorsinteringalongadisc.AnSSdiskwithadiameterof60mmwasplacedonthebuildingplat-formofthesinteringmachine.Threelayersof0.03mmthickpow-derwerespreadoveritforeachmeasurement,creatingatotalheightof0.09mm.Thevolumeofthepowder,therefore,couldbedetermined.Thediskwasthenremovedfromtheplatterandweighedbothwithandwithoutpowder;thedifferencewasthemassofthethreelayersofpowder.ThePBDwascalculatedusingthemassandvolume.Nocorrelationbetweenthepowder?owa-bility(angleofrepose)andthePBDwasobservedfromtheresults.InanexperimentbyZoccaetal.[113],thedensityofthepowderbedwasdeterminedbyweighingthepowderafteradepositionof50layers(each100lmthick)intheprinter’sbuildingplatformanddividingthemassbythegeometricalvolumeobtained.5.Surface?nishingandhybridprocessingSLM-fabricatedcomponentsgenerallyhaveasurfaceroughnessgreaterthan10lm,whichmandatespost-processing[114].Despitethedrivetoachievesmoothsurfaceswitharoughnessoflessthan1lm,itmightbeinevitablethatasecondary?nishingisrequiredformicroAMparts.Thissection?rstfocusesontypicalsurface-?nishingtechniquesthatareusedforAMcomponents,andTable4
Comparisonofsurface-?nishingtechniquesforAM-fabricatedparts.ProcessCNCmachiningCapabilities(lm)Ra%0.4[116]Advantagesonthecapabilitiesofsuchtechniques.Next,itbrie?ydiscussesthesuitabilityofthesemethodsforapplicationtomicroSLMparts,whetherasseparatepost-processingorthroughintegrationwiththemicroSLMtoformahybridsystem.Table4[115–127]comparessomecommonsurface-?nishingtechniquesthatareusedforAMcomponents.Traditionalsubtrac-tivemachiningistypicallyusedtoimprovethesurface?nishofthenear-netshapedcomponentsproducedbyAM[7].Simplemechan-icalgrindingand/orpolishingmaybeadequateforsomeapplica-tions,althoughtheydonotusuallymeetthestandardsrequiredforhigh-qualityparts[115].Chemicalandelectrochemicalpolishing(ECP)haveanadvan-tageoverconventionalmachiningintermsoftheabilitytobeusedforcomplexfeatures.Pykaetal.[118]usedchemicaletching(CHE)andECPfortitaniumalloy-basedopenporousstructures;itwasfoundthatCHEmainlyremovedtheattachedpowdergrains,whileECPreducedtheroughnessfurther.Alrbaeyetal.[117]usedECPtoreducetheroughnessofSLM-madeSS316Lfrom10–17.5to0.5lm.Yangetal.[128]electropolishedTi6Al4Vsamplesfabri-catedbymeansofEBM,whichresultedinareductionofthesur-faceroughnessfrom23to6lm.Shapeaccuracylossandinconsistentpolishingacrossdifferentregionsandtimeswereobserved.ECPislimitedbyitstendencytoerodethematerial,whichresultsindimensionalinaccuracies,inadditiontotheasso-ciatedenvironmentalconcerns[115].Laserpolishingorlaserre-meltinghasemergedasapotentialcost-effectivesurface-?nishingprocessforSLMsurfacesthatcanusethesamelasersourceasAM[115,121,129].Yasaetal.[129]achieveda?nalsurfaceroughnessof1.5lmafterthelaserre-meltingofSLM-madeSS316Lwithaninitialroughnessof12lm.ThelaserpolishingofadditivelymanufacturedSSAISI420in?ltratedwithbronzereducedthesurfaceroughness(Ra)from7.5–7.8lmtovaluesbelow1.49lm,withnocracksorporesintheheat-affectedzone[120].Maetal.[121]observedareduc-tioninsurfaceroughnessfrom5lmtobelow1lmonTi-basedalloys.Marimuthuetal.[115]achievedaroughnessreductionfrom10.2to2.4lmonSLM-manufacturedTi6Al4V,withnoformationofthealphacaseorthermalcracking.DespitethefeasibilityoflaserpolishingforAMcomponents,thismethodislimitedto?atsurfacesandexternalfeatures.Inaddition,surfacere-meltingcanaffectthesurfacechemistryandthermalresidualstress.Abrasiveblasting—commonlyknownassandblasting—iswidelyusedinindustryforcleaningsurfaces,engraving,anddeburring[130].Sand,abrasives,andnutshellsareusedastheblastingmedia,whichispropelledbypressurizedairor?uid.DeWildLimitationsDif?culttomachinecomplexstructuresMicro-toolingistime-consumingDimensionalinaccuraciesduetomaterialerosionEnvironmentalconcernsDif?cultforinternalfeaturesandinclinedsurfacesRe-meltingcouldintroducethermalresidualstressesandchangesinsurfacechemistryPoorprocessrepeatabilityLimited?nishduetotheabrasive?owdirectionNolocalized?nishingHighprocesscycletimeEligibilityformicroSLMYesAbilityforhybridsystemYesEffectiveforsimplegeometriesCanachieveamirror?nishEasytoprocessinternalchannelsanddif?cult-to-accessareasSamelasersourcecanbeusedReductionin?oorspaceCHE/ECPRa%0.5[117,118]YesNoLaserpolishingRa<3[119]Ra%1.5[120]Ra%0.4[121]Ra%2.4[115]Ra<1[122–124]Ra<1[125]Ra<1[126,127]YesYesAbrasiveblastingAbrasive?owmachiningMass?nishingSimple,?exibletechniqueCanbeusedforinternalanddif?cult-to-accessfeaturesNotoolingrequirementsBatchprocessingYesYesNoNoNoNoCNC:computernumericallycontrolled;CHE:chemicaletching;ECP:electrochemicalpolishing.716B.Nagarajanetal./Engineering5(2019)702–720etal.[122]usedsandblastingto?nishporousorthopedicTiimplantsfabricatedbymeansofSLM.Thesurfaceroughness(Sa)oftheimplantwasreducedfrom3.33to0.94lmaftersandblast-ingwithcorundum.Strickstrocketal.[131]usedyttriatetragonalzirconiapolycrystal(Y-TZP)particlestosandblastY-TZPsurfacestoproducearoughnessof1.7lm.Klotzetal.[132]usedsandblastingwithcorundumsandandglassbeadstopolishSLM-fabricatedyellow-goldalloysfromaninitialroughnessof12.9to4.2lm.SandblastingwasalsousedtoimprovetheaestheticappearanceofSLM-mademaragingsteel[133].Quetal.[123]reportedthatthesurfaceroughnessofelectricaldischargemachining(EDM)rough-cutWC–Copartswereimprovedsigni?cantlybyabrasiveblasting,withtheaveragesurfaceroughness(Ra)fallingfrom1.3to0.7lm.Table5[122–124,131,132,134]summarizestheeffectofdifferentabrasiveblastingtreatmentsonthe?nalsurfacequal-ityofvariousmaterials.Itcanbededucedthatabrasiveblastingcaneffectivelyreducethesurfaceroughnessby50%–70%withaminimumRaoflessthan1lm.Despitethelimitationofprocessrepeatability,abrasiveblastingiscommonlyusedformicrocompo-nents,asitisadvantageousintermsofprocesssimplicity,?exibil-ity,cycletime,andcost.Anumberofnewanddifferenttechniqueshavebeenimple-mentedforcomplexAMcomponentsinordertoaddressthechal-lengingsurface-?nishingrequirements.TanandYeo[135]developedanewtechnique—ultrasoniccavitationabrasive?nish-ing—forAMcomponents.Inthismethod,cavitationbubblesformedbyultrasonicpressurewaveswithinaliquidmediumwereobservedtoremovethepartiallymeltedpowders.Thecollapseofcavitationbubblesinducesshockwaves,whichpropagatetheabrasiveparticlestowardthesamplesurface,resultinginmaterialremoval.Thesurfaceroughnessofas-receivedIN625wasreducedfrom6.5–7.5to3.7lm.Wangetal.[125]usedabrasive?owmachining(AFM),awell-known?nishingtechniquethatforcessemisolidabrasivemediaacrossthesurface,forSLMcomponents.Asigni?cantimprovementinthesurface?nishofSLM-madealu-minumalloywasachievedafterAFM,withareductioninsurfaceroughnessfrom14to0.94lm.Magneticabrasive?nishing(MAF),whichcreatesabrasionfrommagneticforcesactingonmagneticabrasives,wasdemonstratedtoreducethesurfaceroughnessofSS316Linternalchannelsfrom0.6to0.01lm[130].Amodi?edversionofMAF—vibration-assistedmagneticabrasivepolishing(VAMAP)—wasexploredbyGuoetal.[136]to?nishmicrochannelsandgrooves.Areductioninthesurface?nishfrom2.2to0.3lmwasachievedalongthemicro-groovesusingthisprocess.Mass?nishingtechniquessuchasvibratory?nishing[126,137]andbarrel?nishing[127],whicharebasedontheprin-cipleofslidingbetweenthecomponentsurfaceandtheabrasiveparticles,havebeenusedforAMparts.Vibratory?nishingofSLM-fabricatedTi6Al4Vwithanaverageroughnessof17.9lmledtoa?nalroughnessof0.9lm[126].However,vibratory?nish-ingresultedinalargenumberofroughnessvalleysonthesurface.Boschettoetal.[127]usedbarrel?nishing—aprocessinwhichmaterialremovaloccursthroughtumblingactionduetoarotatingTable5
Comparisonoftheeffectofvariousabrasiveblastingconditionsonthesurface?nish.SubstratematerialInitialconditionbarrel—to?nishSLM-manufacturedTi6Al4V.Alargereductioninsurfaceroughness(from13.3to0.2lmwitha48hprocesstime)ofSLMcouponswasachievedusingthistechnique.Despiteitsgoodsurface-?nishingperformanceandprocesssimplicity,itisatime-consumingprocess.Inordertoidentifyasuitablesurface-?nishingprocessformicroSLMcomponentsfromthepoolofavailabletechniquesdis-cussedearlier,anumberoffactorsmustbeconsidered,includinginitialroughnessofthefabricatedfeatures,partsize,geometry,minimumfeaturesizeresolution,processcomplexity,cycletime,andsoforth.ThesizeofmicroSLMcomponentsistypicallyonthemillimeterscale,whereastheminimumfeatureresolutionisintherangeofafewmicrometers(Table1).Theeligibilityoftech-niquestobeusedformicroSLMcomponentsislistedinTable4.Despiteachievingagoodsurface?nish,mass?nishingtechniquesmightdamagemicroscalefeaturesduringtheprocess.Computernumericallycontrolled(CNC)machiningofmicroSLMpartsisfea-sible,butmicro-toolingandtoolpathcontrolforcomplexgeome-trypresentadif?culty.Inparticular,themicromachiningofthinwallsandofinternalandhigh-aspect-ratiofeaturesisdif?cultandtime-consuming.CHEandECPtypicallyrequire?atsurfacesandcausematerialerosionalongtheedges,whichmightinducelargedimensionalinaccuraciesinmicroparts.Abrasiveblastingcouldbeanidealchoice,asitiscommonlyusedto?nishthemicro-partsthatarefabricatedinvariousindustriessuchasdentistryandjewelry.Micro-abrasiveblastingisoneofthemostfrequentlyusedsurfacetreatmentsforarangeofmedicalapplications,suchasobtainingthedesiredsurface?nishofdentalimplantstosupportosseointegration[122,131,138–140].Kennedyetal.[124]usedmicroshotblastingwithceramicbeadsonhigh-speedsteel(HSS)andcoatedcarbides,whichresultedina60%reductionofsurfaceroughness,withthe?nestsurfacehavinganRaof0.4lm.Laserpolishingisanothersuitablecandidate,althoughthethermalstres-sescausedbyre-meltingcouldresultinpartdistortion,especiallyalongthinfeatures,duetoresidualstresses.HybridmanufacturingsystemsintegrateAMwitheithersub-tractiveorotherassistivesystemstoimprovetheproductivityandcustomizationcapabilityofthemachinesystems[141–143].HybridsystemsinAMtypicallyinvolvetheintegrationoflasersys-temswithCNCmillingmachinesbymountingthelasercladdinghead(incaseofLMD)tothez-axisofthemillingmachine[143].Overall,thesystemdesignshouldimprovethebuildcapability,accuracy,andsurface?nishofthestructures,withminimalpost-processing.Inthecaseofpowder-bedfusionadditivemanufactur-ing(PBF-AM),hybridsystemsarerarelyavailable,withtheexcep-tionofSodickOPM250EandMatsuuraLUMEXAvance-25[144],althoughthesurfacequalityofthecomponentsafterPBF-AMhasalwaysbeenanissue[128].Despitethemanyeffortsthathavebeenmadetowardmicrofabricationinpowder-bedAMprocessing,nohybridsystemsthatincludeadditiveandsubtractivemachininghavebeendevelopedtofabricatemetallicmaterialsatthemicro-scale.Incomparisonwiththe?nishingprocesseslistedinTable4,laserre-melting,orlaserpolishing,seemstobethemostfeasibleAbrasiveblastingmediaRoughness,RaorSa(lm)InitialFinal0.94.20.40.70.091.7Reduction(%)Ref.TiimplantsYellow-goldalloysHSS,coatedcarbidesWC–CoTiN/Al2O3/TiCNcoatingsY-TZPSLMSLMMilling/turning/drillingEDMCVDMillingCorundumCorundumsand,glassbeadsCeramicbeadsSiCCorundumY-TZPparticles3.312.911.30.18NS7267604650NA[122][132][124][123][134][131]HSS:high-speedsteel;CVD:chemicalvapordeposition;NA:notavailable.
微观选择性激光熔化技术发展的现状及未来展望 - 图文
