Seite - 90 - in 3D Printing of Metals

Metals 2016,6, 280 Figure2.Dimensionsof the tensile test sample. The experimentswere performed in the universalmaterial testingmachine INSTRON1346 (Instron,Norwood,MA,USA)at roomtemperaturewitha loadingspeedof0.008mm·s−1. Thestrain rate isapproximately1×10−3 s−1. Themaximumloadof thismachine is2000KNwitha loading accuracyof±5%.Thesampleswereclampedonthemachineandthe loadwasgradually increased until thesampleswere fracturedtoget their tensilecurve. Thetensilestrengthandotherproperties canbecalculatedbyEquation(1)andthe tensilecurve: σ= P S (1) whereσ is thesectionstress,PandSare themaximumloadandsectionarea, respectively. ε= L−L0 L0 (2) Theelongationpercentageεcanbecalculatedbyinitial lengthL0 andfracture lengthL. 2.3. CompressionTest ofLatticeStructure Compressiontest candidatesweredesignedaccordingto theChineseGB/T1452-2005standard. The compression samples have the sandwich constructionwith 16 cylinder units in themiddle. Theheight of a cylinderunit is 15mmand the thickness of thepanels is 1mmwithaparallelism toleranceof0.1mm. Becauseof the thermal stress brought by the temperaturedifferenceduring theSLMprocess, thewarpingdeformationofthin-walledpartssuchasthepanelwillalwaysoccurasshowninFigure3a. Thedeformationwillhavean impacton thedimensionalaccuracyandstructuralpropertiesof the parts. So thesampleswereproducedbytheSLMprocesswithadditional skinontheverticaldirection toavoidlargedeformationandgetahigherparallelismtoleranceasshowninFigure3b. Theadditional skinwascutbywire-electrodecuttingasshowninFigure3c. 90