Page - 39 - in 3D Printing of Metals

metals Article MicrostructureandWearPropertiesofElectronBeam MeltedTi-6Al-4VParts:AComparisonStudyagainst As-CastForm WeiQuanToh1,PanWang2,XipengTan1,*,MuiLingSharonNai2,ErjiaLiu1,*and ShuBengTor1 1 SingaporeCentre for3DPrinting,SchoolofMechanicalandAerospaceEngineering, NanyangTechnologicalUniversity,50NanyangAvenue,Singapore639798,Singapore; tohw0023@e.ntu.edu.sg (W.Q.T.);msbtor@ntu.edu.sg (S.B.T.) 2 Singapore InstituteofManufacturingTechnology,73NanyangDrive,Singapore637662,Singapore; wangp@simtech.a-star.edu.sg (P.W.);mlnai@simtech.a-star.edu.sg (M.L.S.N.) * Correspondence: xptan1985@gmail.com(X.T.);mejliu@ntu.edu.sg (E.L.);Tel.:+65-6790-5504 (X.T.&E.L.) AcademicEditor:ManojGupta Received: 29October2016;Accepted: 15November2016;Published: 18November2016 Abstract:Ti-6Al-4V(Ti64)partsofvaryingthicknesseswereadditivelymanufactured(AM)bythe powder-bed-basedelectronbeammelting(EBM)technique.Microstructureandwearpropertiesof theseEBM-builtTi-6Al-4Vpartshavebeen investigated incomparisonwithconventionallycastTi64 samples. Slidingwear testswereconductedusingaball-on-discmicro-tribometerunderambient conditions. Experimental results reveal thatEBM-builtTi64samplesexhibitedhighermicrohardness andanoverall largercoefficientof frictionascomparedto theas-cast counterpart.Of interest is that thecorrespondingspecificwearvolumeswere lower forEBM-builtTi64samples,while theas-cast Ti64showedthepoorestwearresistancedespite its lowercoefficientof friction.Wearmechanisms wereprovided in termsofquantitativemicrostructural characterizationanddetailedanalysis on coefficientof friction(COF)curves. Keywords: additive manufacturing; 3D printing; electron beam melting; titanium alloys; microstructure;wearproperties 1. Introduction Metaladditivemanufacturing(AM),popularlyknownasmetal three-dimensional (3D)printing, ischangingthewayhowmetalsoralloysaremanufactured. Powder-bedfusionis the latestdeveloped metalAMtechnology,whichopensupnewopportunities tocreatecomplexmetallic componentswith relativelyhighresolutionandgooddimensionalaccuracycontrol [1–3]. Ti-6Al-4V(Ti64) is themost commonlyinvestigatedmetallicmaterial inAMbecauseofitshighspecificstrength,excellentcorrosion resistance, and good biocompatibility, which are highly demanded in aerospace and biomedical industries [4,5]. Electron beammelting (EBM®) is a representative powder-bed fusionmetalAM techniquethat isbeingincreasinglyemployedtoprocessTi64. Itutilizesanelectronbeamtoselectively meltametallicpowderbedgivenaninputofacomputer-aideddesign(CAD)model [6]. Thedistinct advantages associatedwith EBMare its ability to fabricatemetallic partsmore rapidly andwith greaterenergy-efficiencyascomparedto itscomparative techniqueofselective lasermelting(SLM). Moreover,EBM-builtpartswereshowntohave less residual stress thantheircounterparts fabricated by laser-basedsystems [6–9]. Hence,post-heat treatmentmaynotberequired forEBM-builtmetal parts,givinganaddedadvantage that leads togreatersavingof resources. Many studies have reportedon theprocessing,microstructure, andmechanical properties of powder-bedfusionAMTi64parts [5–7,9–13]. Ithasbeenfoundthatsomeof themechanicalproperties, Metals 2016,6, 284 39 www.mdpi.com/journal/metals