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3D printing of multiple metallic materials via modifiedselective laser meltingDOI:10.1016/j.cirp.2018.04.096

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Citation for published version (APA):Wei, C., Li, L., Zhang, X., & Chueh, Y. H. (2018). 3D printing of multiple metallic materials via modified selectivelaser melting. CIRP Annals, 67(1), 245-248. https://doi.org/10.1016/j.cirp.2018.04.096

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2018-E-04R1

3DPrintingofMultipleMetallicMaterialsviaModifiedSelectiveLaserMeltingChaoWei,LinLi(1)*,XiaojiZhang,Yuan-HuiChuehLaserProcessingResearchCentre,SchoolofMechanical,AerospaceandCivilEngineering,TheUniversityofManchester,Manchester,M139PL,UK

SelectiveLaserMelting(SLM)isapowderbedlayer-by-layerfusiontechniquemainlyappliedforadditivemanufacturingof3Dmetalliccompo-nentsofcomplexgeometry.However,thetechnologyiscurrentlylimitedtoprintingasinglematerialacrosseachlayer.Inmanyapplicationssuch as themanufacture of certain aero engine components, conformably cooled dies,medical implants and functional gradient structures,printingofmultiplematerialsaredesirable.Thispaperreportsan investigation into the3Dprintingofmultiplemetallicmaterials including316Lstainlesssteel,In718nickelalloyandCu10Sncopperalloywithinasinglebuild-upprocessusingaspeciallydesignedmultiplematerialSLMsystemcombiningpowder-bedwithpointbypointpowderdispensingandselectivematerialremoval,forthefirsttime.Materialdeliverysystemdesign,multiplematerialinteractions,andcomponentcharacteristicsaredescribedandtheassociatedmechanismsarediscussed.3Dprinting,selectivelasermelting,multiplematerials

1.Introduction

SelectiveLaserMelting(SLM)usesahighpowerlaserbeamtofullymelt powderedmaterial spread on a flat surface, layer-by-layer,tobuildthreedimensionalsolidmodelswithahighdensityandwell bonded structures based on a CAD file [1–3]. SLM canproduce components of variety of materials including metals,ceramics and polymers, while Laser Metal Deposition (LMD),Wire and Arc Additive Manufacturing (WAAM), and ElectronicBeam Melting (EBM) are only suitable for printing metallicmaterials [4]. Existing SLM processes using flat bed powderspreading techniques are only suitable for printing a singlematerial across each layer, thusunsuitable forprintingmultiplematerial components, while theremay be engineering needs toprintmultiplematerials forspecificapplicationswheredifferentmaterial properties are required at different locations, such asaeroenginecomponents,medicalimplants,anddies/moulds.AcriticalrequirementinmultiplematerialSLMistodepositat

least two discrete powdermaterials within one layer. A doublepowder spreading system driven by piezoelectric transducerswasappliedinaSLMsystemtofabricateaFe/Al-12Sidualmate-rial structure [5]. Investigators from Singapore demonstratedSLM processed 316L SS/C18400 copper alloy andAlSi10Mg/C18400 copper alloy samples on a commercial SLMsystem[6,7].Adoubleringbladeassistedpowderspreadingsys-temwasalsoappliedtosinteringpartsmadeofsilver/copper[8].However, noneof thedepositionmethodsmentionedabove canproducemultiplematerialsoverthesamelayer.Dissimilarmaterialsmustbedispensedlocallyonthesamelay-

erandacrossdifferent layersattherequiredlocationtoachievereal3Dmaterialgradientstructuresandmutiplematerials.Lappoetalappliedamanualvacuumcleanertoremovepowdersinre-quiredlocationsandspreadthesecondpowderbyarollertofillthesamelocation[9].Theirexperimentshowedthatsuchanap-proach caused serious cross-contamination outside the desiredsecond powder deposition region and made multiple materiallayers shifting. A new ‘powder recoating-vacuum cleaning-sieving’ approach was described for multiple material SLM inwhichtheclassicrollermechanismwasusedtospreadthemulti-

plematerials[10].Suchasystemisdifficulttoberealizedinprac-ticedue tounavoidablematerial cross-contaminationduring re-peatedpowderspreadingandvacuumcleaningprocedures.Ultrasonic vibration assisted dry powder dispensing has been

investigatedwidelyfordifferentapplications[11–15]. Thestud-ies demonstrated that dry powder flow rates can be effectivelyandaccuratelyregulatedbycontrollingtheelectricalpulsestothepiezoelectric transducer. Selective area deposition of differentdrypowdermaterialswasachievablebyemployingprogrammedultrasonic vibration without sophisticated material pre-mixingpreparation. The first demonstration of amultiple nozzle ultra-sonic powder deposition method without the use of traditionalflat powder bed spreading, for Cu/H13 powder selective lasermeltingwasreportedbyresearchersfromTheUniversityofMan-chester in 2008 [16]. The laser printed samples producedwere2Dstructures.Until now, there have been no scientific publications showing

3D printing using SLM with multiple materials within a singlelayerbasedondrypowderdelivery.AsuitablediscretemultiplepowderdeliverysystemforSLMshouldbeahybridsystemcom-bining the traditional powder bed delivery mechanism and apoint-by-point powder deposition mechanism. Such a combina-tion is not only required to dispense multiple materials on thesameprocessinglayer,butisalsoneededtogeneratestablesup-portstructuresrequiredforcomplex3Dcomponentprinting.Thispaperdemonstratesanewapproachformultiplematerial

SLMby combining powder-bed spreading, point-by-pointmulti-ple nozzles ultrasonic dry powder delivery, and point-by-pointsingle layer powder removal to realizemultiplematerial fusionwithin the same layer and across different layers. In this work,multiplemetallicmaterial components3Dprinting via SLMwasdemonstrated. It would also have the potential to printmetal-ceramic-polymercomponents.

2.Experimentalmaterialsandprocedure

2.1.MaterialsGasatomizedspherical316Lstainlesssteelpowder(LPW-718-

AACF, 10-45 µm, LPW Technology Ltd., UK), In718 nickel alloypowder (LPW-316-AAHH, 10-45µmLPWTechnology Ltd., UK),

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CIRP Annals Manufacturing Technology

Journal homepage: www.elsevier.com/locate/cirp

and Cu10Sn copper-alloy spherical powder of 10-45 µmdiameters (Makin Metal Powders Ltd. UK) were used in thisinvestigation. The substrate plates used were ground finished304steelsheetsof120mmdiameterand12mminthickness.

2.2.ExperimentalsystemdescriptionAmultiplematerialSLMsystemwasdesignedandmanufacturedinthisstudy(seeFig.1).Anx-y-zgalvoscanner(Nutfield,3XB3-axis) was used to scan the laser beam with an 80 µm focusedbeamspot sizegenerated froma500WYtterbiumsingle-mode,continuouswave (CW) fibre laser (IPGPhotonics, YLR-500-WC)ofa1070nmwavelengthoverthetargetpowderbed.Amultiplepowder delivery system was comprised of a traditional rollerassisted powder bed delivery mechanism spreading the mainbuilding powder material (316L in this study), and a point-by-point micro-vacuum selective material removing system forselective, precision single layer powder removal at specificlocations, and several ultrasonic dry powder dispensersdepositingIn718andCu10Snpowdersrespectivelyaccordingtothe designed pattern. The ultrasonic powder dispensers weremountedonanx-ylinearstagealongwiththemicro-vacuumse-lectivepowder remover. Theprocessoperationwas inan inertgasenvironmentfilledwithnitrogenorargongashavinganoxy-gengas level less than0.3%monitoredwith a real-timeoxygensensor.Beforefillingintheinertgas,theoperationchamberwasvacuumeddownto40Pa withavacuumpump.Afumeexactionsystem was built into the system to remove fumes generated.Although the systemhad a built-in pre-heating facility, thiswasnotusedinthepresentinvestigation.AschematicdiagramoftheexperimentalsetupisshowninFigure1.

GalvoScanner Nd:YAGFiberLaser

Roller

VacuumSuckerPowderDispenser

Pressure Oxygen

BuildingChamber SupplyChamber

SparePowder

Heater

Fig.1.SchematicdiagramofthemultiplematerialSLMsystem.

2.3.MultiplematerialcomponentprintingprocessandprocedureFig.2adescribesthemultiplematerialSLMprocessimplement-

ed in this investigation. Firstly, the main powder material, i.e.316Lwasspreadforone layerof50µmthicknessoverthesub-stratewithamotorizedrollerandpowderlevelingblades. Thenthe laserbeammelted thedesiredareas.A selectivepowder re-moval process then took place to remove powders of a singlelayerthicknessindefinedareas,usingthemicro-vacuumsystem.The second/thirdmaterial powders (In718/Cu10Sn) were thendispensed into the vacuum sucked areas using the ultrasonicpowderdispensersandthenmeltedbythelaserbeamandbond-edwiththealreadymeltedmaterial.Finally,thebuildingplatformmoveddownadistanceequaltothelayerthickness.Allabovesixstepswererepeateduntilthewhole3Dmodelwasfabricated.Fig.2bshowsaselectivesingle layermaterialremovalpattern

using themicro-vacuumsystem.. Fig. 2cdemonstrates an exam-ple of multiple material deposition combining powder bedspreading (SiC), selectivepowder removalandselectivepowderdeposition(316L)beforelaserfusion.Thereweresomemarginsclosetotheedgesasindicatedbytheredarrows,duetothewidth

oftheexpandingzoneofthevacuumsuckingnozzlebeinglargerthan the toolpathoffsetvalue [17]. Suchaproblemwassolvedbyvacuumsuckingtoolpathoptimizationinthefollowingexper-iments.

(a) (b) (c)

(f)(e)(d)

a)

10mm

c)

SiC

316Lb)

10mm

Fig.2.a)TheprocessflowchartofmultiplematerialsSLM,b)Avacuumcleanedpattern,c)a316LboxandhalfYingyangpatternproducedbyselectivepowderdepositiononaSiCpowderlayerbeforelaserfusing

Since there have been no software tools formultiplematerial

SLM,anewdatapreparationprocedureandtoolwasdeveloped.As illustrated in Fig.3, a multiple-material component was con-sideredasanassembly,comprisedofasetofsinglematerialparts.Allthesepartsweredesignedwithspecialfeaturesonthemateri-alinterfacetoenhancethebond.Theywerethenassembledintoa single component. At the SLMprocess data preparation stage,the individualmaterialgeometrywasconvertedintoanSTLfor-mat.AglobalsupportstructurewasthencreatedafteralltheSTLfilesforeachmaterialwereassembled.Subsequently,slicingandhatching tookplace for eachmaterial separately and the resultswereexported into the laser control system.The toolpathsandCNCG-codesfortheselectivepowdervacuumremovalandultra-sonicpowderdeliverywerepreparedbyaproprietaryCNCCAMsoftwaretool.

STLfilesgeneration

G-codegeneration

3DCAD

SLMCAM

STLfilesrepairandassembly

Lasercontroller

CNCcontroller

Slicingseparately

Supportstructuregeneration

3Dmodelsdesignandassembly

Hatchingseparately

CNCCA

M

Fig. 3. Illustration of the data preparing procedure for multiple material SLM.

Thelaserprocessparametersforfusingthreematerialsusedinthis investigation are presented in Table 1. Thesewere derivedfrompreliminaryexperimentstoachieveoptimummeltingquali-tyandprocessingefficiency.

Table1:OptimumlaserprocessparametersformultiplematerialSLM

Material 316L In718 Cu10SnLaserpower(W) 170 180 125Scanspeed(mm/s) 800 857 150Hatchdistance(µm) 45 45 140Relativehatchangle(°) 90 90 90Layerthickness(μm) 50 50 50

2.4.MaterialcharacterizationCross-sections of the SLM parts were prepared by cutting,

mounting,grindingwith400#,800#,1000#,and1200#gridemery

papers, and finally polished using 1.0 μm diamond polishingpaste.Thepolishedsampleswereelectro-etchedin10vol.%oxal-icacidsolution.OpticalmicroscopicimagesofmaterialinterfaceswereacquiredusingaLeicaDM2700-Mmicroscope.ADurascan-80 hardness tester was used to measure the Vickersmicrohardnessonthesampleswitha0.3kgfappliedloadontheCu10Snpartanda0.5kgfonthe316L/In718part.Theinterfacesbetween 316L/In718 and 316L/Cu10Snwere examined using ascanning electron microscope (SEM, Zeiss Sigma VP FEG SEM)equippedwith energy dispersive spectroscopy (EDS, Oxford In-strumentsX-maxN150)forelementalmapping.

3.Resultanddiscussion

Multiplelayersamplesof20mm×20mmhavinga4mmwidth“fingercross”jointzonewereprintedtoinvestigatemultiplema-terial interfaces as shown in Fig. 4a for 316L/In718 and316L/Cu10Sndualmaterialsamples(Fig.4bandc).

Jointa) 316L In718 316L Cu10Sn

Joint

Joint

b) c)

5mm 5mm

Fig.4.Experimentalsamplestostudythemultiplematerialinterfaces.a)schematicofthe“figure-cross”dualmaterialinterfaces,b)andc)arethetopviewoftheprocessed20×20mm316L/In718and316L/Cu10Sn

samplesrespectivelyThe optical microscopic images of the cross-sectional view of

themultiplematerial interfacesareshown inFig.5wheresomepores were found in the ultrasonic deposited powder area asshown in Fig. 5a. The ultrasonic deposited powder surfacewasunevenduetotheridgesbetweeneachdispensingtracks.Hencethepackingdensityinsuchanareawasrelativelylower,leadingtohigherporosity.Fig.5bpresentsagoodbondbetween304SSsubstrate,316LSS layerand theCu10Sn layer.SomepartofCu,migrated into the previously molten 316L layer (see positionspointedbyarrowsinFig.5b).AstheliquidphasecontactangleofCutothemoltenFewasquitesmall,itwashelpfultoincreasethethermodynamicdrivingforcefortheinfiltration[18].

Fig. 6apresentsanSEMimageoftheregiondescribedinFig.5a,

inwhichsomecrackswereobserved.TheEDSmappingresults(

Fig. 6btod)showedthatmostofsuchdefectsweredistributed

intheIn718alloypowderregionasindicatedbytheredarrows,depositedbytheultrasonicnozzle. Intherollerassistedpowderspread process, the packing density of the powder bedwas en-hanced by the roller compressing, thus cracks/porosity wereminimised[19].However,asthepowderparticlesfromtheultra-sonicfeedingnozzlewasnotinacompactconditionduetolackofanexternalpressure,cracksandporosityappearedduringmate-rialphasechangesinthelaserfusingprocess.Some316L/In718intermixed regionswere also found as shown in the elliptic re-gionsin

Fig. 6b. In previous studies, investigators usually applied pre-

mixed powders for the graded zones, while our study demon-strated thatmaterials could bemixed in the desired regions toachieve required transition properties. SEM evaluation of the316L/Cu10Sn specimen (Fig. 7a), revealed sound metallurgicalbonding at the interface of 316L/Cu10Sn and the interface be-tween 316L and the 304 substrate. No apparent defects werepresent in the fused316L regionwith layersadded through thepowder-spread roller. On the other hand, some porosity andcrackswereobservedintheultrasonicdispensedCu10Snpowderregion indicated by the arrows. It may be caused by the samereason described above. Besides, some isolated light zones, asmarkedbyarectangleinFig.7a,werepresentinthe316Lpow-der area. Further EDS inspection (Fig. 7f) showed the chemicalcompositionof suchzonesasCu, indicating thatCuelementdif-fused into theFearea.Adualpowdermixingzonewas found inthetoprightofthescannedareabycomparingFig.7candf.Thiscouldbeduetoinsufficient316Lpowdervacuumremoval.Someresiduals of 316L powder were blended with the depositedCu10Snpowderandfusedbythesubsequentlaserfusingprocess.

a)

SubstrateCu

b)

Fig.5.Opticalmicroscopicimagesofmultiplematerialinterfacesa)

316L/In718interfacesfarfromthesubstrate,b)316L/Cu10Sninterfaceclosetothesubstrate.

Fig.6.AnSEMimageandEDSmappingsof316L-In718interfaces.a)AnSEMimageof316L-In718interfaces,b)AEDSmappingofthe316-In718interfaces.c)andd)theFeandNimappingofthe316L-In718interfaces.

Fig.7.AnSEMimageandEDSmappingsof316L-Cu10Sninterfaces.a)theSEMimageof316L-Cu10Sninterfaces,b)AnEDSmappingofthe316L-Cu10Sninterfaces.c)andf)theFe,Sn,NiandCumappingofthe316L-

Cu10Sninterfaces.

TheVickershardnessvaluesalongthehorizontaldirectionoftheSLMsamplesmadeofdualmaterialsareshowninFig.8.Itcanbeseenthatthehardnessvaluesof316Lpartrangedfrom237±6HVto251±4HVand thoseof In718part ranged from301±4HV to310±6 HV with the transition zone having hardness values be-tween those of the twomaterials. The hardness values of 316Land Cu10Sn on the 316L/Cu10Sn sample were 227±7 HV to247±8 HV and 149±8 HV to 160±6 HV respectively while thehardness values in the transition zone rangedbetween those ofthetwomaterials.Duetothespecial“fingercross”jointstructuredesign,materialelementaldiffusionandbondingwereachieved.Itisnotablethatthevaluesofmicrohardnessstandarddeviationweremuchhigherinthetransitionzonescomparingwiththoseinthesinglematerialregionasshowninbothcurves,asthemateri-alcompositionvariedinthetransitionzones..

0 2 4 6 8 10 12 14 16 18 20120

160

200

240

280

320In718/Cu10SnJoint Zone

Har

dnes

s(H

V)

Distance (mm)

Hardness(316L-Cu10Sn) Hardness(316L-In718)

316L

Fig. 8. Vickers hardness values along the horizontal direction of the SLM

316L/In718 sample and the SLM 316L/Cu10Sn sampleAsetof3Dcomplexshapesweremanufacturedusingthepro-

prietary system to demonstrate 3D multiple material printingusingtheSLM. AsshowninFig.9a,thedoorstepandthechim-ney of a simple houseweremade of Cu10Sn and In718 respec-

tively,whiletherestofthehousewasmadeof316Lmaterial.InFig.9bandc,goldenandsilvercolorsrepresenttheCu10Snand316Lmaterialseparately.ItisnotablethatthesnakeheadwearoftheSphinx(Fig.9b)wasmadeof316L/Cu10Snmaterialmatrixusingthelocalpowdermixingstrategy,whilethefacewasmadeofCu10Snandtherestwasmadeof316Lstainlesssteel.Thethinwall structures anddotdiameter as shown inFig. 9 cwere150µminthicknessand1mmindiameterrespectively.

a)

c) 1mm

150um

b)316L/Cu10Sn

Cu10Sn

316L

30mm 10mm

10mm

Fig.9.a)aminihousecomprisedthreematerials,b)Amultiplecolor,multi-materialstatueofSphinx,c)adualcolorgridpattern.

Thismultiplematerial3Dprintingmethodmayhavepromisingapplications for the prouduction of functionally gradedcomponents where the matetrial properties can be taylored atdifferent locations. Industrial sectors that may need such atechnologywouldincludeaerospace(e.g.jetenginecomponents),nuclear (e.g. compopnents that require both high thermalresistance and corrosion resistance), customized jewellery (e.g.combining several types of precious metals), and the medicalimplants (e.g. artificial teethwithmetal coreandceramic shell).Theoretically speaking, all geometry, capable of being producedbytraditionalsinglematerialSLM,shouldbeabletobeproducedby this multiple material SLM process. The challenges are thesupportstructuredesignforcomplexgeometrycomponentsandmaterialrecycling.

4.Conclusion

This paper demonstrated a multiple material SLM technologybycombiningconventionalpowder-bedspreadingwithpoint-by-pointmultiplematerial selective powder removal and point-by-pointdrypowderdelivery,forthefirsttime.Aproprietaryexper-imental SLM equipment and special CAD data preparationprocedure for SLM were developed and employed to produce316L/In718 and 316L/Cu10Sn samples successfully. Thefeasibility to deposit multiple materials on the same buildinglayer and across different layers was confirmed by theexperiment results. A clear distinct sandwich layer distributionandagoodmetallurgicalbondingwereobtainedat thematerialinterfacesforthematerialcombinationsstudied.Theresultalsoindicatedthatthespecialmaterialinterfacedesignwashelpfultoenhancematerialelementaldiffusion,whichleadtobetterbond-ing. On the other hand, some defects including porosity andcrackswerefoundintheultrasonicallydepositedpowderregiondue to un-compressed powders. Future work will include theimprovement of the ultrasonic powder dispensing quality andincorporation of ceramics andpolymermaterials in 3DprintingusingthespecialSLMsystem.

Acknowledgement

TheauthorsweregratefulforTheUniversityofManchesterPh.D.scholarshipsawardedtoMr.ChaoWeiandMr.Yuan-HuiChueh.

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