Seite - 43 - in 3D Printing of Metals

Metals 2016,6, 284 of 1kgandadwell timeof 15 s. Slidingwear testswere carriedoutonpolishedsurfaces, usinga ball-on-discmicro-tribometer (CSMmodel,AntonPaarGmbh,Graz,Austria)asshowninFigure3b at roomtemperatureof 23 ◦Cand in theambient environment. A100Cr6 steelball ofΦ6mmwas usedascounter-facewithanapplied loadof1NontherotatingTi64samples inacircularpathof3 mmindiameterata linearslidingvelocityof2cm/sfor50,000 laps. Forallof theEBM-built samples, theX–Zplanewaswear tested. Basedonthemeasuredwearwidthandweardepth,wearvolume was calculatedvia a simplegeometrical equation. Specificwear rates [22]were thenevaluatedby normalizingthewearvolumewith the loadapplied(N)andslidingdistance (m). 3.ResultsandDiscussion 3.1.Microstructure Figure4showsthemicrostructureof theEBM-built andas-castTi64samples. It canbeclearly seenthatboth the0.5mmand1mmsampleshavesimilaralternateα/βmicrostructuresmixedwith acicularα′martensite,whichisdifferent fromtherestof theEBM-built samplesandtheas-castsample, whichhaveα/βmicrostructures.Ofparticulardifference is thatcoarseβwasobservedin theas-cast sample. Additionally, theresultsobtainedfromtheXRDpatternsshowninFigure5revealedpeak shiftingphenomenonintheEBM-built1mmsampleascomparedto the20mmandtheas-castTi64 sample. Thispeakshiftingphenomenonis in linewith thestudiesbyZengetal. [23] that suggest the presenceofα′martensite. Theappearanceofα′phase in0.5mmand1mmwasalsoconfirmedby TEMobservation[5,18]. In thecaseof theEBM-built samples twotypesof typical transformedα/β structurewasobserved, namely, the colonyand thebasket-weave (alsoknownasWidmanstätten) morphologies.Asaresultof thedifference inmicrostructure, theyhavedifferentmicrohardnessvalues whichwillbegiven in the followingsections. Fromtheresults, it isknownthat theacicularα′phase causesahigherhardnessvalueascomparedtotheα/βmicrostructureseenintherestof theEBM-built samples. Inaddition, themicrohardnessofEBM-builtTi64 samplesdecreaseswith the increase in samplethickness. This isduetothefastcoolingratecoupledwiththeEBMbuildtemperature, favoring the formationofα′martensite in the thinsamples [10,18]. Theas-cast samplehas the lowesthardness valuedue to its coarseα/βmicrostructure,which is largely causedby themoderate cooling rate duringthecastingprocess [14,24]. Figure4. (a–f)SEMmicrographsofEBM-builtTi64sampleswiththicknessesof0.5,1,5,10,and20mm andtheas-castTi64samples, respectively.Microstructural featuresare indicatedbyarrows. 43