Page - 64 - in 3D Printing of Metals

Metals 2017,7, 91 The values determined for hardness correlate to values found in the literature [38,46,47]. BySongetal. [13] was shown that mechanical properties depend on the processing parameters. The determined value for hardness for a perfect SLM-manufacturedpartwithout heat treatment was450HV.Murretal. [48]haveshownthedifferencesbetweentheselective lasermeltingandthe electronbeammeltingprocessandthe influenceonthemechanicalproperties, especiallyhardness. Hardnessvalues forEBM-manufacturedpartswere foundin therangeof410 to427HV.Wuetal. [49] investigated thepositive effect ofhot isostaticpressingon the improvementof theanisotropiesof bendingandimpactproperties. Theyfoundthathot isostaticpressingandbuildingdirectionaffect mechanicalproperties.While theas-builtpartshaveahardnessof41and42HRC, theHIPedparts haveahardnessof34HRC. IntheworkofWuetal. the influenceof theHIPprocess isgreater thanthe orientation in thebuildingprocess. Thephase transformationof theα′-martensite toα+βstructures isanessential reasonfor thedifferences inmechanicalproperties. Theseresults, showincomparisonwitheachother, thatahardness typical forTi6Al4Vmaterial andadditivelymanufacturedpartswasachieved. Inaddition, thiselucidates that, asacomplementary measure, hardness measurement is excellently suited as a qualitative manufacturing control. Significantdifferencesarecausedmainlyby thedifferences in thestructure. Theheat-treatedparts consistpredominantlyofamicrostructurewithanαphaseanda lamellarβphase (brighter thanthe αphase). Theprimaryαphasewasdistributedalong thepriorβgrainboundary. Thestructure is uniformlybuiltandalignedparallel to thebuildingdirection(Figure14) in the0◦orientation. Figure14. Overviewabout themicrostructureof allmanufacturedparts in relation to thebuilding orientation(0◦ and45◦)aswellas topost-treatmentconditions (heat treatment—HIP—hot isostatic pressingorNoHIP).Thearrowshowsthebuildingdirection. 64