ADDITIVE MANUFACTURING DESIGN CONSIDERATIONS FOR PRODUCTION IN AEROSPACE CONSIDERATIONS FOR THE FULL VALUE STREAM CAITLIN OSWALD Additive Manufacturing Specialist LAI International
ADDITIVEMANUFACTURINGDESIGNCONSIDERATIONSFORPRODUCTION
INAEROSPACECONSIDERATIONSFORTHEFULLVALUESTREAM
CAITLINOSWALDAdditiveManufacturingSpecialist
LAIInternational
ADDITIVEMANUFACTURINGDESIGN:CONSIDERATIONSFORPRODUCTION
INTRO
Asadditivemanufacturingcontinuestomatureandbeginstotransferfromafadtoarobustproductionprocess,
complementarymethodsofdesigningforadditivemanufacturingneedtodevelopinparallel.Todaythemajorityof
designengineersweretrainedunderconventionaleducationmethodsofsubtractivemanufacturing.Switchingthat
paradigmwilltakemorethanadjustingafewrules,butanevolutionineducation,practiceandmindset.When
designersareurgedtobecreativeandinnovative,thedesignspaceboxwhichmeetsfit,form,andfunction
requirementsexpandsexponentiallywhenusingadditive.Thisnewideaofstartingfromnothinginsteadofstarting
fromablockofmaterial,whileoverwhelmingforsome,becomeslimitlessforstudentsandyoungdesignengineers
withoutdecadesofstrictrulesandguidelineshanginginthemidst.However,thefastpaceofadditivemanufacturing
adoptionrequiresthosewhopreviouslytrainedforconventionalmanufacturingtorapidlyretrainandrethinktheway
theydesigntomeetthevastnewoptionsaffordedbyadditive.It’simportanttorealizethatwhile“complexityisfree”
therearestillguidelinesandinstructionthatcanmakeyouradditivedesignasefficientaspossiblewhenconsidering
theparametersofcost,time,andquality.ThefollowingpaperfollowsPartAofatwo-partseriesfocusedonAdditive
ManufacturingDesign.Theserieswillcondensesomeoftheselimitlesspossibilities,andcapturetheadvancing
strategiesofadditivemanufacturingdesign.
THEOPTIMALADDITIVEDESIGNENGINEER
Fordecades,thethreeidealsofdesign,manufacturing,andcosthavelivedinsilos.Whileit’scleareveninthe
conventionaldesigncyclehowimportantitisforthesethreeskillsetstocollaborateandunderstandeachother,itis
DesignEngineer
ManufacturingEngineerCostAnalyst
magnifiedinanadditivemanufacturingenvironment.Whereconventionaldesignstakeweeksmaybemonthstobe
realizedandmanufactured,theadditivecycleisquick,sometimesovernight,anddesignfaultscanbeidentified
immediately.Withthesedesign,manufacturing,andcostiterationshappeningsoquickly,it’sreasonabletoseehow
thesethreeskillsetscanstarttolearnfromeachotherandmeldintooneoptimizedapproach.Byintegratingthe
impactonpostprocessingoperationsandcostconsiderationsupfront,combinedwiththedesignopportunitiesthat
additivemanufacturingprovideswithregardstocomplexity,theidealdesigncanbefoundrapidly,significantly
reducingthedevelopmenttimelineforproducts.
THEADDITIVEDESIGNPROCESS
Today,thedesignprocessforadditivemanufacturingoftenstartswithanexistingdesign,especiallywhenthegoalisto
removecostorweightfromacurrentproduct.Thepartselectionprocess,materialdecision,andadditivetechnology
(forexampleEBMvslaser)selectionscouldrepresentanentirediscussionandarticleonitsown,butwewillfast
forwardtowhathappensafterthosedecisionshavebeenmadeandit’stimetoopenaCADpackageandcreatethe
additivedesign.
Thedesigncycleisnotlinear.Thedecisionsmadeduringeachpieceofthedesignwillaffectandcontributetothe
others.Therearemanypossibleoutcomestothecycledependingonwhatparameterneedstobeoptimized.For
example,aone-offdevelopmentpiece,whereleadtimeisthemostimportantfactor,mayleadtoaflatandlow
orientationpartwhichreducesbuildheightanddoesnotconsiderthequantityofpartswhichcouldnesttogether.This
wouldbeinefficientinaproductionenvironmentbecausethenumberofbuildsneededtomeetproductionquantities
wouldequallyincreasewithlaborcostsforset-upandteardown,build-to-buildconsumablecosts,andmachinedown-
timebetweenbuilds.Thedesignthatyouusefortheone-offdevelopmentpiecewillbecompletelydifferentthanone
thatyoudesigntooptimizeforaproductionvolumewhichmaximizespartsperbuild,nestsmultiplelayersofparts,and
reducespostprocessingstepswithuniquefeatures.Investigatingeachofthesepuzzlepiecesseparatelyisdifficult
sincetheyarerelatedbycauseandeffect,howevertherearesomebasicguidelinesandconsiderationswhichshouldbe
drawnuponwheninthedesigncycle.
MATERIALPROPERTIES
Itisessentialtounderstandthatthematerialpropertiesofadditivemanufacturedmetalarenotequivalenttowrought
orcast.Additivelymanufacturedmetalshavetheirowndesignsystem,andshouldbetreatedassuch.Duetothe
temperaturegradientsduringmetaladditivemanufacturingprocesses,materialpropertiesandmicrostructurecanbe
anisotropicinnaturebeforeperformingpostthermaltreatment.Thermalprocessingcanalterthemicrostructureto
becomemoreisotropic,buttherestillmaybesomeorientationandgeometriceffectswhichremain.Theeffectsmay
beseenonmonotonicordynamicproperties.It’simportanttohaveanunderstandingofhowsignificanttheeffectsare
withinthematerial/processcombinationchosen.Iftherearedesignfeaturesthatwouldbelimitingifnotbuiltina
certaindirection,it’simportantforthedesignengineertointegratethatdesignruleintothebeginningstageoftheir
orientationdecisions.
POSTPROCESSING
Intheadditivemanufacturingprocess,therearetwotypesofCADmodelswhichexist.Oneisthefinalgeometrywhich
iswhatthefinalproducedpartwillbeinspectedto.Thedatumsareidentified,machinedtoleranceswillbecalledout,
surfacefinishrequirementswillbezoned,assemblynotes,etc.
ThesecondCADmodelwhichisequallyimportantistheas-builtmodel,orthemodelwhichisusedastheinputintothe
additivemachine,thiswilllookdifferentthanthefinishedmodels.Holesmaybefilledin,supportstructurewillbe
added,machiningstockwillbeadded,andmore.Tounderstandwhatconsiderationsshouldbeputintothe“as-built”
model,agenericprocessmapisprovided.
Therequiredpostprocessingstepswilllookslightlydifferentdependingonthematerialandtechnologyutilized.For
example,ElectronBeamMelting(EBM)doesnotrequirestressrelieformechanicalremovalofthepartsfromthebuild
plate,butdoesrequirepowderremovalofthepartiallysinteredpowderthatsurroundstheparts.LaserPowderBed
Fusion(LPBF)requiresstressreliefandeitherwireEDMorabandsawproceduretoremovethepartsfromthebuild
Build Buildremoval
ThermalProcessing
SurfaceFinishMethod
Machining Inspection&Testing
plate.ThermalprocessingforallmetaladditivemanufacturingoftenincludesaHotIsostaticPressing(HIP)processto
reducevoidsandporositywithinthematerialwithparametersbasedonmaterialchoice,andadditionallysome
materialsmayrequireaheattreattoaltertothedesiredmicrostructure.
BUILDANDPOWDERREMOVAL:
Asanexpansiontotheexampleabove,differentadditivetechnologiesofferadvantagesanddisadvantageswhenit
comestobuildandpowderremovalconsiderationswhendesigningapart.
ELECTRONBEAMMELTING(EBM)
TheconsiderationsforEBMwithregardstobuildplateremovalsurroundtwoaspects.First,itisidentifiedasabest
practicetonotbuilddirectlyontheplate,especiallyifthepartshavestrictchemistryrequirements.Therecouldbe
someinfluenceofthestainlesssteelplateforthefirstfewlayersofthebuild,soitisadvisedtostartpartsatleast3-
5mmfromthetopofthebuildplate.Those3-5mmcanbefilledbyeithersupportstructure,orsolidmaterialtobelater
cut-off,whicheverismoreefficientforthepart.OneadvantagefortheEBMprocessisthatduetolowresidualstress
fromtheentirebuildareaatelevatedtemperatures,lesssupportsarerequiredandthepartsusually“pop”offofthe
plateeasilywithslightpressureorwiththeuseofarubbermallet.
ThebiggestconsiderationduringpartremovalofanEBMbuildispowderremoval.Ononehandthepre-heatstepin
theEBMprocessprovidesbetterpartintegrity,butforpowderremovalitprovidesachallengethatmustalsobe
consideredwhendesigningyourpart.Duringthepre-heatstepintheEBMprocess,thepowdersurroundingthesolid
materialispartiallysinteredmakingitdifficultsometimestoremovefromlongcavities.Thispartiallysinteredpowder
doesnotflowfreely,however,itiscrucialthepowderisremovedbeforesendingthepartsthroughanythermal
processinglikehotisostaticpressing(HIP)orheattreatment.Designingsweepingradiiwhilemaximizinglineofsite
cavitiesintoapartcanmakepowderremovalmoresuccessful.
Figure1(a) Figure1(b) Figure1(c)
Figure1showslineofsightpowderremovalexamples:A)simpleradiusB)elongatedradiusC)straightcenterwithradii
onexitswheretheblacklinedepictshowfarthecompressedairinthepowderrecoverysystemwouldbeabletoreach
intothecavitywithlineofsight.Itisnotalwaysintuitiveastowhichgeometrywouldbebest.
Figure2showstheadvantageoflargeradiiinsteadofcornerpoints.Itismucheasiertoremovepowderfromasmooth
surfacethanonewithmanydiscontinuities.
LASERPOWDERBEDFUSION
Considerationsinlaserpowderbedwithregardstobuildremovalareslightlydifferent,butnolesscomplex.Inthelaser
systems,thereisabenefittothe“cool”buildprocessasthepowderdoesnotbecomesinteredandthusflowseasily.
Whenbuildingintricateinteriorcavitiesisrequired,theremovalprocessinlaserprovidesanadvantage.However,even
thoughthepowderflowsmoreeasilyoutofcavities,itcanstillbecomplexduetotherequirementofremovingthe
powderbeforestressreliefandbeforethepartscanberemovedfromtheplate.Therefore,iftheinternalcavitiesare
buriedwithinthesupportstructureorcannotbereachedbythevacuum,powderremovaloptionsmustbedesigned
intotheparts.Ifanorientationcannotbeconstructedtoallowforfullpowderremoval,itiscommontoaddholestothe
as-builtmodeltoeasethepowderremovalprocessofcavities,asseeninFigure3below.Theseholescanthenbe
pluggedlateronbyaweld.
Figure2(a) Figure2(b)
Inlasersystems,thepartsandtheplateareessentiallyweldedtogethermakingitabitmorecomplextoseparate,
addingtimetotheoverallprocess.PartsmustberemovedbymechanicalmethodssuchasWireEDMorabandsaw.
WireEDMisamoreaccurateprocess,sotheadditionalstockneededunderneaththepartsmayonlybe(3mm),
whereaswiththebandsaw,itisrecommendedtoaddatleast(5mm)ofsolidmaterialorsupportstructureunderneath.
THERMALPROCESSING
Thermalprocessinginmostcasesdoesnotdefineorchangemanydesigns,unlessthereissignificantdistortionduring
thethermalprocessduetodrasticallyvaryingthicknessescomparedtothepartlengthorheight.Thisisanotherarea
wherechoosingtherightadditivetechnologymayberequired.Largepartscanbedifficulttoproduceonlasersystems
duetothe“cool”processthatmaystrugglewithresidualstress.Stressreliefisrequiredonmostlaserbuilds,buteven
moresoonpartsthatspanthebuildarea.EBMmaybeabetterchoiceingeneralforlargepartsinboththehorizontal
andverticalorientationssincethestressreliefstepisnotrequired.Ifgeometricdistortionisstillanissueforapart,
thereareafewoptionswhichcanhelpthepartsholdtheirshapethroughthethermalprocessing.Forinstance,adding
asacrificialgussetorframearoundthinwalledcomponentstolaterbemachinedoffcanprovidestabilityduringthe
thermalprocessingsteps.
SURFACEFINISHMETHOD
Thesurfacefinishofmetalpowderbedfusiontechnologiesisrougherthanacasting,andalsodependsonthe
orientationofthesurface.Topsurfacescanbefairlysmooth.Verticalsurfacesnormallyhavearoughbutconsistent
surfacefinish,andsurfaceswhichareorientatedatangleslessthan90degreesfromthebuildplatecanbetheroughest.
Ingeneral,thelaserpowderbedsystemshavepowderandlaserparametersthatareoptimizedforasmoothersurface
finish.EBMhasfocusedonoptimizingforcostviabuildspeed,andthereforepartsarerougherwithafasterbuildtime
trade-off.Itisoptimalforcosttokeepthesesurfacefinishesas-built,butmanytimesstructuralandflowrequirements
orpartaestheticscandrivetheneedforvarioussurfacefinishmethodstobringdownthesurfacefinishmeasurements.
Thesurfacefinishmethodchoseninfluencestheimpactondesignchanges,buttheultimategoalistounderstandthe
materialremovalrateandcompensatefortheremovalintheas-builtmodel.Forexample,ifatumblingprocess
Figure3
removes0.005”fromthesurface,itwouldberecommendedtoadda0.005”enveloptotheas-builtCADfilesoafter
postprocessingthefeatureswillcomeintoconformingthicknesses.
Whenattemptingtocompensateforsurfacefinishmethods,designersmustadditionallyconsiderthephysicsofthe
processitself.Forexample,cornerswillhaveamoreaggressivematerialremovalrateinavibratorybowlthanaflat
surface,soprofiletolerancesmayhavetoopeninthoselocations,asseenbelowinFigure4.Foranycriticalfeaturesor
tolerances,differentmaskingtechniquesduringthesurfacefinishprocessmayaidinmaintainingtheas-builtsurfaceso
thematerialdoesnotdegrade,orcanbelatermachined.
Inadditiontoexternalsurfacefinishmethods,internalcavitiesmanytimesrequiresurfacefinishmethodstosmooth
outchannelsforflowrequirements.Similarly,thedesignersmustconsiderthematerialremovalratesofthosemethods
andcompensatefortheminthemodel.
MACHINING
Asdiscussedpreviously,mostpartsneedtobemachinedatsomelevelbeforebecomingafinalproduct.Critical
featuresandtighttoleranceswhichcan’tbeachievedbytheprintingprocessusuallymustbebroughtintoconformance
byconventionalCNCprocesses.ThereareafewdesignconsiderationsupfrontwhichmakestheCNCprocessmore
effective.
First,thedesignengineerandthemanufacturingengineershouldcollaboratetounderstandhowthepartwillbeheld
duringtheCNCprocessset-ups.UtilizingtheadvantagesoftheAdditiveManufacturingprocess,datumfeaturescanbe
addedtotheas-builtmodeltoreducecustomtooling,oratminimum,provideconsistenttoolingacrossapartfamilyof
similargeometries.Tabs,pins,holes,slots,orevenatemporaryhandlecanallbeaddedtotheprintedparttohelpalign
anyfixturesandtooling,andcanbelatercut-off.Havingthoseconversationsupfrontwillreducedevelopment
iterationsandspeedupthesetupandactualdevelopmenttime.
As-BuiltSurfaceTumbledSurface
DimensionalProfileAllowance
Figure4
Anotherdesignconsiderationwithregardstomachiningiswrapstock.Justaswithsurfacefinishmethods,therewillbe
somematerialremovalinordertobringcriticaltolerancesintoconformance.Addingwrapstocktothosefeatureswill
ensurethereisenoughmaterialtherefortheCNCprocesstoremove.Theamountofwrapstockaddedshouldbe
enoughtoreduceriskoftherenotbeingenoughmaterial,butlimitedsoastonotaddtoomuchtimeandcosttothe
CNCprocess.Thiswillvarydependingonthematerial,thegeometryoftheparts,aswellasthetechnologybeingused.
Figure5belowshowsthedifferencebetweenthe(a)finalpartgeometryand(b)theas-builtgeometrywiththeholes
filledin,andbottomstockaddedforpostmachining.
Insummary,theoptimaladditivemanufacturingdesignermustwearmultiplehats.Theyneedtohavean
understandingofthefullmanufacturingvaluestream,costanalysis,andevenstructuralandmaterialsengineering.
Onestepbetteristohaveexpertsavailabletoprovideinsightintothedesignprocessasit’sproceedingina
collaborativeatmosphere.Thedesigncyclewasdiscussed,wherethepiecesareallinterconnectedandincludecause
andeffectcorrelationsbetweenthem.Focusedfirstonmaterialpropertiesandpostprocessingtherearemanydesign
considerationswhicharedependentonmaterial,technology,andrequiredoperations.Inthenextseries,some
producibilityrulesandguidelineswillbereviewed,alongwithtopologyoptimization,andotherdesignforfunction
opportunitiesaswell.
FormoreinformationonDesignforAdditiveManufacturingandotherAdditiveManufacturingtrainingopportunities,
Figure5(a) Figure5(b)