Page - 65 - in 3D Printing of Metals

Metals 2017,7, 91 In the45◦orientation(x–zplane)aswell in the0◦ and45◦orientation(x–yplane), thisalignment is not seen. It seems that allmicrostructural anisotropywas eliminated. The parts that did not receiveanyheat treatmentaftermanufacture, showedaprimarilyacicularmartensiticalpha(α′)and fine lamellar beta (β)microstructure. Theβgrains appear tobebrighter and theα′ grainsdarker. Thismicrostructureandits inhomogeneitywastheresultof fastheatingandcoolingduringtheprocess. Theprevailingα′ grains grewalmostparallel to thebuildingdirection. This is visible inboth the 0◦ and45◦ buildingdirections. Partswithα′ grains in comparison topartswithoutα′ grainshave ahigherhardness. Thehigherhardnessof theα′phaseasagainst theα+βstructures is typical for this structure. Therepresentativecross-sectionmicrostructures inFigure14confirmtheserelationships. TheHIPedSLMpartsshowauniformα+βstructurewithaclearlyvisiblebuildingorientationonly in0◦orientation. Theas-builtparts,however, showaclearlyorientedstructure (inx–zplaneandin x–yplane)withacicularmartensite. Thehigherhardnessof the0◦orientedparts is theresultof the betteranduniformorientationof theα′grains (martensite). In the45◦orientedparts, thestructure ismore irregular. This leads to a smaller resistance to an applied compressive force, because the 0◦ oriented struts are vertical to the acting force. The force required toovercome the resistance is thengreater. 3.4. CompressiveStrengthasaFunctionofGeometricalParameter All fabricatedpartswere testeduntil themaximumpossible loadwasexceeded. Thesamples showedtwotypesof failurebehavior independent fromtheoutputquantity. Figure 15 shows representative examples for failure behavior. In Table 5 are listed all tested samples thatwereconsideredfor theevaluationof results. The compression tests performed in thiswork on these specimens lead to the deformations as shown inFigure 15. As specifiedby theMaxwell criterion (M<0), theoccurringdeformations are typical for flexure-dominated structures [36]. According to the literature, the stability failure of the rod-likeelements that arepresent in the specimensunder investigationcanbeexplainedby differingbucklingbehaviors (load cases). Load cases showing this are known. Thedeformations that have occurred here are clearly assignable to such known load cases.While Case 1 showing the typical deformation for symmetrical buckling, Case 2 is an example for antimetrical buckling. AsdescribedbySattler [50], thepathof thedeformation is thecurveofaparabola. Thesamebuckling lengthcoefficient that isof interest fordeterminingtheslenderness ratioapplies inbothcases. For the deformationpattern,similarresultswerefoundbyUshijimaetal. [33]intheirtheoreticalconsiderations ofcompressiveproperties. Thestability failure inboththenaturalmodesshownischaracterizedby anabruptevent. Case1: (longcurve) Here, thestraingraduallyandsteadilyincreases,reachesashortplateauuntil itabruptlydecreases. Thiscanbecausedbyimperfections (duetoqualitydeficienciesduringmanufacturing) that leadto areduction in thestiffness. Case2: (steepabruptdecrease) Thestrain increasesand,afterexceedingthestability limit, abruptlydrops. InCase2stability failureoccursatahigherstiffness level than inCase1 (strain increase is steeper than inCase 1). The imperfections inCase 2have a lower influenceon stiffness (i.e., the stiffness basedonthematerial-dependentelasticmodulusandthemanufacturingdependentproperty)until thestability limit isabruptlyexceeded. 65