Page - 120 - in 3D Printing of Metals

Metals 2017,7, 64 316L SS, in particular, is highly attractive for biomedical andmarine applications due to its excellent corrosionresistanceandrelativelysuperiorductilitycompared toothermaterials [14–16]. Current researchonAMof316LSS isnotonly limitedtosingle-materialprocessing,butalsoextends to composites. For example, AlMangour et al. [17] studied the SLMof TiC-reinforced 316L SS matrixnanocompositesandfoundthat theadditionoffineTiCparticles remarkably improved the microhardness andwear performance of the fabricated parts. This is because of the increase in the densification level and the homogeneousmicrostructure distribution as a result of enhanced reinforcement/matrixwettability. Inaddition, studiesontheSLMofTiB2/316LSSnanocomposites were also carriedoutwithvarying results. For example, superior compressionyield strengthand ductilitywereobtainedwhenprocessing thisnanocompositewithoutahot isostaticpressing(HIP) post-processingdueto the formationofhomogenouslydispersedTiB2particles formingnanoscaled structures [18].However,HIPtreatmentwas foundtoreduce thehardnessandwearresistancedueto thehigh-temperatureannealingeffect [19].Nevertheless, theflexibilityofAMprocesses to fabricate suchcompositesprovidesapromisingfuture,especially forparts requiringcomplexgeometries. Although SLM is able tomanufacture almost fully dense parts (~98%–99%), the presence of residualporosity inSLM-fabricatedpartshindershigh-strengthandfatigueresistanceapplications[20]. Similar toconventionallymanufacturedparts, themechanicalpropertiesofcomponentsbuiltbySLM are influenced by the resultingmicrostructure andporosity profiles (size andmorphology) [1,21]. Hence, it is important to understand the microstructure and porosity formation and how their behaviour influences themechanical properties of the completed parts. Thus, this study aims to investigate themicrostructure,porositydistributionandmicrohardnessof316LSSparts fabricatedby SLM, inparticularbyusingtheadvancedX-raycomputedtomography(XCT) technique. 2.MaterialsandMethods Gas-atomised316LSSpowders (ConceptLaserGmbH,Lichtenfels,Germany)withdiameters ranging from15 to 40μmwere used in this study. The as-suppliedmaterial composition of this alloy is shown inTable 1. The lowP,CandS contents in 316LSS reduce the susceptibility of this material tosensitisation(grainboundarycarbideprecipitation), inwhichsensitisationcouldreduce themechanicalpropertiesof the fabricatedparts. Table1.Chemicalcomposition(wt.%)of316LSSpowdersusedin this study. Component Fe Cr Ni Mo Mn Si P C S wt.% Bal. 16.5–18.5 10.0–13.0 2.0–2.5 <2.0 <1.0 <0.045 <0.030 <0.030 AllAM316LSSsampleswere fabricatedbyusingConceptLaserM2LaserCusingSLMmachine (ConceptLaserGmbH,Lichtenfels,Germany) inan inertgasenvironment. Theprocessingparameters used in this studywere as follow: (i) laser power: 200W; (ii) scan speed: 1600mm/s; and layer thickness: 50μm.Thesampleswerebuiltusing the“island”scanstrategytoreduce theresidual stress in thecompletedparts (Figure1) [22,23]. In this study, three sampleswere fabricatedbySLMandone samplewasmadebyusing the conventionalwroughtmanufacturing(WM)technique. TheSLMsampleswerebuiltalongthez–axis (vertically). Foropticalmicroscopy, cube-shapedAMsamples (originally 8mm× 8mm× 8mm) were cut into 4mm× 4mmsquare cross-sections along the x–y, y–z and x–zplanesusing awire electricaldischargemachine. Theyare thenmountedonconductivebakelite,groundusing120,800, and1200gritsabrasivepapersandpolishedusing6μmand1μmdiamondpaste toobtainmirror-like surfacefinish. Inorder toreveal themicrostructures, thepolishedsampleswereetchedusingKalling’s No. 2reagent (50mLHCl,50mLethanol,2gcopperchloride for100mLofetchant) forapproximately 30s.OlympusBX41M-LEDopticalmicroscope (Tokyo, Japan)wasusedtoobserve themicrostructure onthemetallographicspecimens. 120