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metals Article InvestigationonPorosityandMicrohardnessof316L StainlessSteelFabricatedbySelectiveLaserMelting ShahirMohdYusuf1,YifeiChen1,RichardBoardman2,ShoufengYang1 andNongGao1,* 1 MaterialsResearchGroup,FacultyofEngineeringandtheEnvironment,UniversityofSouthampton, SouthamptonSO171BJ,UK;symy1g12@soton.ac.uk(S.M.Y.); chenyifeiuk@sina.com(Y.C.); S.Yang@soton.ac.uk(S.Y.) 2 μ-VISX-RayImagingCentre,FacultyofEngineeringandTheEnvironment,UniversityofSouthampton, SouthamptonSO171BJ,UK;rpb@soton.ac.uk * Correspondence: n.gao@soton.ac.uk;Tel.:+44-23-8059-3396 AcademicEditor:ManojGupta Received: 5 January2017;Accepted: 15February2017;Published: 20February2017 Abstract: This study investigates theporosity andmicrohardness of 316L stainless steel samples fabricatedbyselective lasermelting(SLM).TheporositycontentwasmeasuredusingtheArchimedes method and the advanced X-ray computed tomography (XCT) scan. High densification level (≥99%)witha lowaverageporositycontent (~0.82%)wereobtainedfromtheArchimedesmethod. Thehighestporositycontent in theXCT-scannedsamplewas~0.61.However, thepores in theSLM samples forbothcases (opticalmicroscopyandXCT)werenotuniformlydistributed. Thehigher average microhardness values in the SLM samples compared to the wrought manufactured counterpart are attributed to the fine microstructures from the localised melting and rapid solidificationrateof theSLMprocess. Keywords: porosity; microhardness; Selective LaserMelting (SLM); advancedX-ray computed tomography(XCT) 1. Introduction Additivemanufacturing(AM)isanadvancedmanufacturingprocesswhich involves layer-wise materialadditionto fabricate three-dimensional (3D)objectsbasedonpre-definedComputerAided Design(CAD)data. This technologypossesses theadvantageofdesignflexibility,whichenables the fabricationofpartswithcomplexgeometriesandintricate featurescomparedto traditionalprocesses suchas castingand forging. Inaddition, theadditivenatureof thisprocess allowscomponents to bemanufacturedwithmuch less rawmaterialwastagewhichcould reducematerial costs and the environmental footprint [1,2]. TheAMofmetal componentshasevolvedfromrapidprototyping(RP) to fabricationof functionalmetallic components forendusesuchas in theautomotive,biomedicaland aerospace industries [3,4]. VariousAMprocesses formetalshavebeenwelldescribedandreviewed[5,6]. Selective laser melting(SLM) isoneof themajorAMtechnologies thathasbeenusedtoprocessanumberofmetals andalloys, e.g.,Ti6Al4V[7],β-typeTi–24Nb–4Zr–8Sn[8,9],Nisuperalloy [10]and316Lstainlesssteel (316LSS) [11], forawiderangeofapplications, includingbone implants [12], turbineblades [13]and automotivepistons [3]. InSLMtechnology, the laserbeamisusedtocompletelymeltmetalpowder layersspreadonapowderbedto formnear-net-shapedcomponents. In thisprocess,3Dmodelsare firstslicedinto2Dcross-sectionswithaset thicknessvalue. Thelaserbeamthenscansthecross-section of thedesignedpart layerby layerbeforeselectively fusingthemontopofeachother,whichenables thefinal3Dpart tobe formeddirectly. Metals 2017,7, 64 119 www.mdpi.com/journal/metals