Method Article
Standard method for microCT-based additive manufacturing quality control 2: Density measurement
Anton du Plessis
a,*, Philip Sperling
b, Andre Beerlink
b, Lerato Tshabalala
c, Shaik Hoosain
c, Ntombi Mathe
c, Stephan G. le Roux
aaCTScannerFacility,StellenboschUniversity,Stellenbosch,SouthAfrica
bYXLONInternationalGmbH,Hamburg,Germany
cNationalLaserCentre,CouncilforScientificandIndustrialResearch,SouthAfrica
ABSTRACT
MicroCTisbestknownforitsabilitytodetectandquantifyporosityordefects,andtovisualizeits3Ddistribution.
However, itis alsopossibleto obtainaccuratevolumetricmeasurements fromparts– thiscanbeused in combinationwiththepartmasstoprovideagoodmeasureofitsaveragedensity.Theadvantageofthisdensity- measurementmethodistheabilitytocombinethedensitymeasurementwithvisualizationandothermicroCT analysesofthesamesample.Theseotheranalysesmayincludedetailedporosityorvoidanalysis(sizeand distribution)androughnessassessment,obtainablewiththesamescandata.Simpleimagingoftheinteriorofthe sampleallowsthedetectionofunconsolidatedpowder,openporositytothesurfaceorthepresenceofinclusions.
TheCTdensitymethodpresentedheremakesuseofa10mmcubesampleandasimpledataanalysisworkflow, facilitatingstandardizationofthemethod.AlaboratorymicroCTscannerisrequiredat15mmvoxelsize,suitable softwaretoallowsub-voxelpreciseedgedeterminationofthescannedsampleandhenceanaccuratetotal volumemeasurement,andascalewithaccuracyto3digits.
MicroCT-basedmeandensitymeasurementmethod.
Accuratevolumemeasurementandscalemass.
10mmcubesampleallowsstandardizationandautomationofworkflow.
©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://
creativecommons.org/licenses/by/4.0/).
ARTICLE INFO
Methodname:StandardmethodformicroCT-basedadditivemanufacturingqualitycontrol2:densitymeasurement Keywords:Additivemanufacturing,MicroCT,X-rayTomography,Non-destructivetesting,Standardization,Density Articlehistory:Received7May2018;Accepted11September2018;Availableonline26September2018
*Correspondingauthor.
E-mailaddress:[email protected](A.duPlessis).
https://doi.org/10.1016/j.mex.2018.09.006
2215-0161/©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://
creativecommons.org/licenses/by/4.0/).
ContentslistsavailableatScienceDirect
MethodsX
journal homepage:www.elsevier.com/locate/mex
SpecificationsTable
Subjectarea Engineering
Morespecificsubjectarea Additivemanufacturing
Methodname StandardmethodformicroCT-basedadditivemanufacturingqualitycontrol2:density measurement
Nameandreferenceof originalmethod
Thisisanewmethod,anditismentionedinmyrecentreviewpaper:duPlessis,Anton,Igor Yadroitsev,InaYadroitsava,andStephanG.LeRoux."X-RayMicrocomputedTomographyin AdditiveManufacturing:AReviewoftheCurrentTechnologyandApplications."3DPrintingand AdditiveManufacturing(2018).Itisalsousedinarecentlypublishedroundrobintest:duPlessis, Anton,andStephanG.leRoux."StandardizedX-raytomographytestingofadditively manufacturedparts:aroundrobintest."AdditiveManufacturing(2018).
Resourceavailability ThisisallinthemethodsXpaper,includingsupplementaryvideo
Methoddetails
Additivemanufacturingisafastgrowingmanufacturingmethodespeciallyusefulformedicaland aerospaceapplications,duetothecomplexityofdesignthatispossible.Variouseffortsatqualification ofsystemsandmaterialsfordifferentapplicationsareinprocess,suchasdemonstratedformedical applicationsin[1].ThereisaneedforstandardizationinqualityinspectionsespeciallyformicroCTbut moregenerallyforallNDTmethods[2].
PartdensityisusuallymeasuredbytheArchimedesmethod[3],whichisastandardizedandvery accuratemethod.However,thismethoddoeshavesomedrawbacks:itrequiresanassumptionofthe parentmaterialdensity,whichcanpotentiallybeincorrectforalloysdependingonthecontentof differentatomicelements.Theseconddrawbackisthepossibilityforsurfaceroughnesstocapture bubblesandhencemeasureahighervolumethanexpected,whilesmallchannelsconnectedtothe surfacemayleadintolargercavities,wherethewater(orgas)willpenetratethesampleandhencea smallervolumerecorded,andthis maynotbevisibletothehumaneye.Unconsolidatedpowder trappedinsidecavitiesorporeswillalsoresultinlowerdensitybyArchimedesbutnotsignificantlyso (duetothemassofthepowder),andthepresenceofalargecavitywithunconsolidatedpowdermay bemissedornotrealized.
WhiletheuseofmicroCTforanalysisofporosityiswellknown(seeforexample[4]),itisnotso wellknownthat accurate materialvolumescanbe determined,which canbeused indirectlyto calculateameandensityforsamples.Thisisanentirelydifferentapproachthanmeasuringporosity, andcanbeusefulwhensomeporesmaybesmallerthanthevoxelsize,orwhenthealloydensity mightbeincorrect.
Inthisworkwedemonstrateasimplemethodologywherenosamplepreparationisrequired–a simple10mmcouponsample(cube)isrequired.Thescanmethodandanalysisworkflowmakesuseof commercialhardwareandsoftware,andthestepsdonotinvolveanyformofhumanjudgement.Such simplifiedunbiasedmethodsareimportanttotheproperuseofthetechnologytosupporttheadditive manufacturingcommunity,andisoneofanumberofstandardizedmethodsdevelopedinourgroup andmentionedinarecentreviewofthetechnologyappliedtoAM[5].
Themethod
Thesampleswerebuiltonacustombuiltselectivelasermeltingplatformwithinacommercial LENSenclosure.ThelaserusedwasanIPGYLS5000ytterbium5kWfibrelaser,wavelengthof1076nm withadeliveryfibrecorediameterof50
m
m.ThescannerusedwasanIntelliweld30FCVsystem.MaterialsusedwereTi6Al4VprovidedbyTLSTechnikGmbH,gasatomizedwithparticlesize20– 60
m
m.ThebaseplatematerialisTi6Al4V150mmindiameterapproximately40mmthick.Thehatch parametersusedwereapowerof3kW,speedof3m/s,anda240m
mspotsize.Thecontourscansusedalaserpowerof1kWwherethedistancebetweenthehatchandcontourandthespeedwasvariedto investigateitseffectondensity(porosity)andsurfacefinish.
A standard coupon sample of 101010mm is suggested for this test. This size allows a reasonablyhighscan resolution(15
m
m)whileallowing a largeenoughsample sizefor practicalpurposes. All scanning and image analysis steps are described and thereby standardized, and importantly,noneofthesedependonhumanselectionandallbiasisthereforeremoved.Thismethod canbecosteffectiveconsideringtheadditionalinformationobtainedvisuallyregardingtherootcause ofdensityvariationssuchasporosityorunconsolidatedpowder.
Thesampleisloadedinfoamatanangletoensurenoedgeartifactsarepresent,asshowninthe firstmethodpaperofthisseries[6].Themethoddescribedherethereforedoesnotrequireanewscan ifporosityanalysiswasalready performedasintheabovementionedmethod,makingthemean densitymeasurementsimpleand fast.Theporosityanalysismethodusesasegmentationprocess whichisalmostfreeofhumanbias.InthepresentmicroCT-densitymethod,theedgeofthecubeis segmentedwhichisentirelyfreeofhumanbiasandcanbefullyautomated.MicroCTisperformed using a standard laboratory microCT system [7], with parameters optimized according to the guidelinespresentedin[8].MicroCTscansettingsof200kV,70uA,with0.5mmbeamfilterareused, withimageacquisitionof500msperimage,2400steppositionsinafull360rotation.Ateachstep position,thefirstimageisdiscardedandtwosubsequentimagesaveraged.Thetotalscantimeisjust under 1h. When sample setup,machine warmup, background correction and reconstruction is includedthisshouldbepossibleinalmostanysystemin2htotal.Thereconstructionisdoneusinga strongbeamhardeningcorrectionfactorwithoutanyimagede-noising.
Theaccuratesurfacedeterminationisidenticaltothefirstpartoftheworkflowin[6]suchthatthe totalobjectvolumeincludesallinternalpores.Theworkflowdescriptionisrepeatedhereforthisstep.
ThedataisanalysedinVolumeGraphicsVGStudioMax3.1.Theimageprocessingstepsaredescribed hereforremovingtheexteriorairfromthedataset,butincludingallmaterialandair(closedpores,not open to surface). Despite the complex description, the video associated with this process demonstrates the simplicityof the process. This segmentation is doneby first applying a basic
“automatic”surfacedetermination,followedbycreatingaregionofinterest(ROI)fromthissurface.
Thisregionisthenmodifiedbyandopening/closingfunctionwithavalueof+3,whichclosesupsmall surfacepores.Aregiongrowingtoolisthenusedwithhightolerance(noeffectofgreyvalues)onthe airoutsidethepart,whiletheoptionisselectedfor“avoidothervisibleROIs”.Thisselectsallexterior airuptotheedgeofthepartasdesignatedbythesurfacedeterminationandsurfaceclosingfunction.
Ifsmallnoiseparticles(loosepowderforexample)arepresentoutsidethepart,anopening/closing function(+3)canbeappliedtothisregion,toremovethesefromtheselection.Invertingthisexterior-
Fig.1.MicroCTimageofsamplecubewithasurfaceview.
airselectionallowstoselecttheentirepartincludingitsinternalvoids.A newadvancedsurface determinationfunctionisthenapplied,usingthisROIselectionasastartingcontour.Inthiswaythe localoptimizationisperformedontheexteriorsurface,allowingthebestsubvoxelprecisiononthe surfacelocation.
Theaccuratetotalvolumeofthepart,includinganyinternaldefectsisthusfoundfromthisobject underthepropertiestab.Theaboveprocesscanbeautomatedandrequiresnomanualinput,therefore removingallbiasfromtheresults.Theresultingvisualizationofthesurfaceofthesampleisshownin Fig.1.AcloseupofoneregionofthesurfaceisshowninasliceimageinFig.2,wherethewhiteline indicatesthelocationofthesub-voxelprecisesurfacerelativetothevoxels.
Itmustbenotedthatlargeopenporestothesurfacewillbeseenasexterior,thesameasinan Archimedestest.Itispossibletomakeadifferentsegmentationtoincludetheseopenpores,butitis notpossibletoautomatethismethod,asopenporescanhavesignificantlyvaryingnecksizesatthe surface and thedepth of openporesand varyingsurface roughness will stronglyaffect results.
Thereforethis standard methodmakes useof onlythe automatedsteps above.Fig.3 showsan exampleof anothersample undernon-idealprocessparameterswhich containsopenandclosed porosity:theCTdensityisonlycalculatedfromthematerialwithclosedporosityinthisstandard method.
Fig.2. Accuratesurfacewithsub-voxelinterpolationshowninclose-upviewoftopsurface.
Fig.3.Openandclosedporosityinasample,CTdensityiscalculatedfromthevolumeincludingclosedporosity.Seenhereisa partwithlargeareasofunconsolidatedpowderandvoids.
TheCTdensityisaccompaniedbyimages,whichcanbeusedtoassessthebuiltpartqualitatively.In Fig.4,thepresenceof denserinclusions(whitespots)indicatesimpurity presentin thepowder feedstock.Theporosityisobservedtobemainlylocatedneartheedge,andcanberelatedtothe contouringusedinthiscase.
Aseriesof9couponsampleswereanalysedforCTdensityandtraditionalArchimedestests[9]
werealsodoneonthesamesamples:theresultscomparewell,asshowninFig.5.Clearlymostofthe sampleswerenearlyfullydense,exceptforoneoutlier.ThisoutlierisshowninaCTimageinFig.6, indicatingtheunderlyingcauseofitslowerdensity:largeporositynearthesurface.TheArchimedes methoddoesnotworkaswellasthemicroCTdensitymethodforthisoutlier,mostlikelydueto connectionsfromsurfacetothenear-surfacepores,allowinginfiltrationofthewaterintothesepores duringtheArchimedestest.Inthisseriesof9samplestheporositywasnotintentionallyproduced,but
Fig.4. CTsliceimageshowspresenceofporosity(blackareas)andinclusions(whitespots).
Fig.5.ComparisonofCT-densitytoArchimedes,foraseriesof9couponsamples,relativetotraditionalmicroCTporosity measurement.TheoutlierwithhighporosityshowsthatinthiscasethenewCTdensitymethodisimprovedcomparedtothe Archimedesmethod.
canbeattributedtoincreasingscanspeed combinedwithimpropercontourscanningtracks,not overlappingthefillingtrackssufficiently.Whenthescanspeedincreases,lackoffusionismorelikely tooccurandthisbecomesexcessiveattheinterfacebetweencontourandfillingtracks.
Conclusions
TheaboveresultsshowthatmicroCTcanbeusedtocalculateanaccurateaveragedensityfromCT- derivedvolumeandscalemass,whichisdifferentfromtraditionallywell-knownporositymeasurement (whichmaymisssmallpores,especiallysmallerthanthevoxelsize).Theaccuratedeterminationofthe sampleedgeiscrucialtotheaccuratevolumemeasurementandanerrormarginisexpecteddepending onthesurfaceroughness.Besidessurfaceroughness,dimensionalaccuracyofmicroCT systemsdepends ontheirregularcalibrationandsomesystemsareinherentlymoreaccuratethanothers.Usingtypical laboratorymicroCTsystems,weestimatetheerrormarginformeandensitymeasurementofTi6Al4V partsof 10mm diameter to be 0.02g/cm3.In most cases in AM there is more likely to be a larger error in the Archimedesmeasurementduetoroughsurfacestrappinggas,openporesallowingwatertoinfiltratethe sample, andunconsolidated powder in thesample which can mask thepresence of porosity in the density measurement.Thismethodislikelytoplayacriticalroleinoptimizingprocessparametersinthefuture.
Thestandardmethodologydescribedhereallowshigherthroughputandhencelowercosts,whenusing microCTservicelabs.Sincenohumaninputisrequired,automationinimageanalysisispossibleandcan furtherenhancethethroughputforlargenumbersofidenticalsamples.
Acknowledgements
The Collaborative Program in Additive Manufacturing (CPAM), funded by the South African DepartmentofScienceandTechnology,isacknowledgedforfinancialsupport.YXLONInternationalis alsoacknowledgedforfinancialsupportofthiswork.
Fig.6. CTimageofsamplewithlowCTdensity,duetolargeamountofporositynearsurface(leftinimage).
AppendixA.Supplementarydata
Supplementarymaterialrelatedtothisarticlecanbefound,intheonlineversion,atdoi:https://doi.
org/10.1016/j.mex.2018.09.006.
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