Page - 121 - in 3D Printing of Metals

Metals 2017,7, 64 Figure1. “Island”scanningstrategy, shapedlikecheckerboardsemployedin theSLMprocessof316L SSsamples. Vickersmicrohardness (HV)measurementswere takenalong thecross-sectionsusingFM-300 MicrohardnessTester (Future-TechCorp,Kanagawa, Japan). Thedistancebetweeneach indentation was1mmandtheapplied loadwas100gfwithdwell timeof10s. Themeasurementsateachposition wererepeatedthreetimesandtheaverageHVvalueswerecalculated. Inaddition,HVvalueswerealso obtainedfromthecube-shaped316LSSsamplemanufacturedbyWMusingsimilar testingconditions. Theaverageporosity in theSLM-built sampleswascalculatedusing thewell-knownArchimedes method. On the other hand, the pore size distributionwasdeterminedusing opticalmicroscopy (GenICam software, Basler AG, Ahrensburg, Germany) from 15 micrographs for each sample. Inaddition,oneAMsample (approximately10mm×10mm×15mm)wassubjectedtoadvanced X-ray computed tomography (XCT) scan to obtain information onporosity as this sample had a relativelycomplexgeometrycomparedto theothersamples. This samplewasfirst scannedunder low resolutionusingNikonBenchtopCT160Xi (NikonMetrology,Herts,UK) toobtain thegeneralporosity distribution. Then,asmall regionwith thehighestporositycontentwaschosentoundergodetailed scanningusing160kVZeissXRadia510Versa (CarlZeissMicroscopyGmbH,Jena,Germany)with highresolutionof3.2μmfor20h.After reconstructionandhandlingofrawimages fromtheXCTscan results,VGStudioMaxsoftware (VolumeGraphicsGmbH,Heidelberg,Germany)wasusedasa3D visualisationtool toobtaindetailedporositydistribution in thesmall regionof interest. Theporosity inXCT-scannedsampleswasdefinedusingtheOtsumethoddescribed inRef. [24]. 3.ResultsandDiscussion 3.1.Microstructure Figure2a shows the cross-sectionalviewson thex–y (scandirection)plane,whileFigure2b,c showsthecross-sectionalviewsonthex–zandy–zplanes (builddirection), respectively. Thesemelted scantracksarerepresentativeof thesolidifiedmeltpool foreach layeronthepowderbed. Thecurved “fish-scale”—like geometries observed in the x–z and y–z (build direction) planes are due to the semi-circularshapeof themeltpoolandthepartial re-meltingofsuccessivelydeposited layerswhich havebeensolidified[23]. Theseoverlappinggeometriesalsodemonstratesuccessful fusionofpowder particlesandbondingwithineach layer, similar to theworkcarriedoutbyCherryetal. [1]andYasa andKruth[20]. 121