Page - 63 - in 3D Printing of Metals

Metals 2017,7, 91 45◦orientedpartsbecauseparticlesadhereonthestrut side facingthepowderbed.OnEBMparts, thesamephenomenonleads toa lowerroughnessof the45◦orientedparts. Theparticles thatadhere onthesurfacedecrease theroughnessbecausesomeunevennessgetsfilled. Foxetal. [44] foundin theirworkashiftbetweensurfacesdominatedbypartiallymeltedpowder particlesandsurfacesdominatedbymaterial fromthere-solidifiedmelt track.Withadecreaseof the surfaceangle, theRa value increased.Adecreasedsurfaceangle isequivalent to the45◦ in thiswork. The0◦ orientation in thisworkmeansahigher surfaceangle. These results are consistentwith the previous resultsofTriantaphyllouetal. [45]whofounddifferencesbetweenupskinanddownskin surfacesaswellas theorientationangles. Thedifferences inaverageroughnessRa foundbetweenthe EBMandtheSLMmethodarealsoconfirmedhere. There isasignificantdifferencebetweentheSLMparts (HIPandNoHIP) in the0◦orientation (p<0.001). Inproductionthisdifference,however, is likelytobewithintheaccuracyfluctuationsrange. TheroughnessRa ofSLM-manufactured(HIPandNoHIP)partsweresignificantly (p<0.001) lower for the0◦orientation thanfor theEBMparts. In the45◦orientation, theroughnesswasonlydifferent betweenSLMHIPandEBM(SLMHIP<EBM;p<0.05). These relationships are also seen for the roughnessRz. ThevaluesobtainedforSLM-andEBM-manufacturedTi6Al4Vpartscorrespondtothosefromthe literature.Accordingly, theSLM-partsshowafar lowerdeviationfromatheoretical smoothsurface thatcanbeconsideredanindicator forastableandaccuratemanufacturingprocess. Besidespowder sizedistribution, the layer thickness in thepowder feedandthepowderbedsystem, the laserbeam diameter itself hasan influenceonsurface roughness [13]. Higher roughnessvaluesofEBMparts are typicallydue to theprocessparameters. Asmoother surface and therefore lower roughness is thereforepossible throughtheoptimizationofprocessparameters. 3.3.VickersHardness Thehardnessofpartsmanufacturedinthe45◦orientationweresignificantly(SLM-NoHIPp<0.05; EBM-NoHIPp<0.001) lower than in thosemanufactured in the0◦orientation. Thiscorrelation isnot seen in theheat-treatedSLM-manufacturedparts.Heat treatmentof theSLM-manufacturedparts led toasignificantly lowerhardness (inbothorientationsp<0.001).WhenSLM-andEBM-manufactured partswithnoheat treatmentwerecompared, theSLMpartshadinbothorientationsasignificantly (p<0.001) lowerhardness thantheEBMparts. Theresults forhardnessareshowninFigure13. Figure13.BoxplotsofthemeasuredhardnessHV10. Boxplotsgivethemedianvalue, theinterquartile range(IQR: intervalbetweenthe25thand75thpercentile,bluerectangle)andtheextremumvalues (n=40); statistical significance levels (*p<0.05; ***p<0.001).Nosignificantdifferences fromsimilar otherpossiblemanufacturingsystemswererevealed. 63