Page - 69 - in 3D Printing of Metals

Metals 2017,7, 91 Figure 19. Correlationbetweencompressive strengthandslenderness ratioofEBM-manufactured parts. Resultsareshownasmeanvalueswith thecorrespondingstandarddeviations. For the linear regressions, thecoefficientofdetermination isshown. Compressive strength increasedwith a decreasing slenderness ratio for all tested variations. Theslopesshow, that thepossible change incompressivestrengthbyvariations in thegeometrical parameters (diameter or height) leads at least to the same result in the case of theHIPed parts. Theresults fromtheNoHIPedpartsdifferclearly fromtheHIPedparts.Here is seenamoredifferent behavior.Nevertheless, therelationbetweentheobtainedmechanicalpropertiesandthegeometrical properties iswelldescribedwith the linearrelationships found. Theseresults showthat theorientationof thepartduringthebuildingprocessplaysan important role inviewofmechanicalproperties. The45◦orientedpartsshowbyvariationof thediameteradirect influenceonthemechanicalproperties. The influenceofamodification in thediameter isgreater than achange in theheightwhile retaining thesameslenderness ratio. Agoodindicator for thisare the differentslopes in thecorrelations found.The influenceoforientationonthemicrostructureandon themechanicalproperties isknown[19,25]. This influencealsobecomesclearhere.With increases in thediameter, theproportionof thestructure-orientedstructuralareasalsogrows. Theseobliquely orientedstructural regionscanabsorb increasingly less load. For the0◦ orientedsamples, it is less importantwhether thediameter is increasedor theheight isvaried. ThedifferencebetweentheSLMparts (HIPandNoHIP) isclearlycausedbythemicrostructural difference. TheHIPedpartshaveauniformα+βstructurewitha lowerhardness thantheNoHIPed structurewithamartensitic fraction. This relationship isalsoseenforcompressionstrength. In theEBMparts, thecompressivestrengthalso increasedwithadecreasingslenderness ratio for all testedvariations.Here isnoteworthythat the45◦orientedsamplesbehavesimilar. Regardlessof whether thediameteror theheightvaries, theresult is thesame.However,differencesareseen in the 0◦orientedsamples.Here,achange in thediameterhasagreatereffectonchanges in themechanical properties thanachange inheight. TheEBMsamplesdeviatemoreandmorefromthedefault (dCad) in theirmeasuredouterdiameter (Figure10). SinceallEBMsampleswereproducedwithoutheat treatment,adirect comparisonwith theSLMsamples (0◦ and45◦) ispossiblehere. TheEBMsamples, thoughachievinghighervalues in thehardnessmeasurements,have incomparisonto theSLMparts (bothwithoutheat treatment) the lowercompressivestrength. Thereasonlies,asalreadyexplained, onthesignificantlyhigherroughnessvaluesRZandRa of theEBMparts. Thecrack initiationof the rough“as-built”surface leads toa lowercompressionstrengthofEBMparts. 69