Seite - 125 - in 3D Printing of Metals

Metals 2017,7, 64 Figure8.PoresizedistributionforSLMsampleshowninFigure7b,obtainedviaXCTscan.Redcurve represents thecumulativeporedistribution. Figure 9. (a)Majority of the pores in theCT-scanned samplewere observed close to the surface (innerwall)of theregion instudy; (b)Exampleofporeshapeandsizenear the innerwall region. Nevertheless, the XCT scan could be a better method to study the porosity distribution in AM-fabricatedsamples. This isbecause it isanon-destructive techniquewhichenablesdetailed3D visualisationof internalpores in thesamples (Figure9)withoutphysicallyandchemicallydisrupting the sample, as compared to the conventionalmetallographic preparation for opticalmicroscopy observation.However,onlyasmallpercentageofsmallporesizes (<5μmmeandiameter) couldbe capturedwiththe3.2μmresolutionof theXCTscanusedinthisstudycomparedto thoseobtained usingopticalmicroscopy. In addition, amuchhigher cost and longer time are required to obtain ahigherresolutiontodetect smallerporesizes,which is thecurrent limitationof this technology. Porosity is a commondefect observed in theAMofmetal parts, and it can be controlled by adjusting various processing parameters, e.g., the scan speed, laser power and layer thickness. In general, there are two types of porosities includinggas-inducedporosity andprocess-induced porosity [34]. Spherical-shaped gas pores could arise during the gas atomisation of the 316L SS feedstockmaterial prior to SLMprocessing and continue to be present in the final parts. On the other hand, pores resulting fromprocess-induced porosity are typically non-spherical. They are formedwheneither: (a) theenergyapplied is insufficient tocompletelymelt thepowder feedstock, causing lackof fusionbetweeneachadjacentscanandbetweensuccessive layers [35];or (b)excessive 125