Page - 88 - in 3D Printing of Metals

Metals 2016,6, 280 Thecellular structuredesignmethodreplaces the interior structuresof thepartswithcellular structureswhichhaverelatively lessmass thansolidmaterial. Cellular structurescanbeclassified into twocategories: stochasticporousstructuresandperiodiccellular latticestructuresaccordingto the distributionof the internalmicro-structure. Stochasticporousstructuresarefilledwithopenorclosed voids thatdistributerandomly,whereasperiodiccellular latticestructuresconsistof repeatingunit cells leadingtoauniformstructure. Becauseof theunevendistributionofmaterials,periodic lattice structures showfinermechanical characteristics thanstochasticporous structureswhen theyhave thesamevolumefraction [4,5]. Afewpreviousworkshavebeendone tostudythemanufacturing of lightweight cellular structureswith the use of SLM.Campanelli et al. [6] investigated a lattice structurecomprisedof fourvertical strutcolumnsandfourpairsof struts. This structurewasmadeby SLMusingtitaniumalloyTi6Al4V.Acampaignwasplannedtocompare thebehaviorof the lattice structurewithvariable cells, truss sizes andvertical bars as reinforcements. TheyusedTaguchi’s method tominimize thenumberof experiments. Compression testswereperformed to study the mechanicalbehaviorof the lattice structures. Theydemonstrated thatwhenthecell size is smaller, andthesizeof thestrutedgeandtherelativedensityare thehighest, this structureshowsthehighest strength.Wangetal. [7] investigatedthedesignrulesandthekeypoints to fabricateporousstructures. Thinwalls, cylinderswithdifferentgeometricaldimensionsandoverhangingstructureswithdifferent inclinedanglesweredesignedandfabricatedtoobtain theSLMfabricatingresolutionandthecritical inclinedangle fordesigningtheporousstructures. Theexperiments for fabricatingporousstructures withdifferent sizeswerealsoconducted. Theresults showedthat theporousstructurecanbewell fabricatedbySLM. Hernandez et al. [8] researched the micro-structure and dimensional accuracy of the as-manufacturedlatticeswith thesamemorphologywith theexceptionof thestrutdiameter,which was varied systematically. They found that the variation in the compressive yield stresswith the strutdiameter is ingoodaccordancewithsimplemodelsbasedoncompressivedeformationrather than shearing or buckling. Besides, struts normal to the build direction showedmore significant defects. Toexaminetheeffectofcellular latticestructuresonthestrengthofparts,Mahshidetal. [9] analyzedthecollapsestrengthof latticestructuresbyfiniteelement (FE)andmathematicalmodels. Compression testsof thesolid,hollow,non-rotated-closed-cellularandrotated-closed-cellular samples wereperformed. Theresults showedthat thecellular structuresheavily influence thecompression strengthandshowedonlyan increaseof11.4%at thehighestpointwithrespect tohollowspecimens. Singetal. [10] investigatedthedimensionalaccuracyandcompressivebehaviorofcellular lattice structureswithdifferentunit cell types,namely thesquarepyramid, truncatedcubeandoctahedron unit cells.Analysisofvariance (ANOVA)wasalsocarriedout todetermine thesignificanceofvarious processanddesignparametersonthedimensionalaccuracyandcompressivestrengthof the lattice structures. They found that theprocessingparameters, such as laser power and laser scan speed, havenosignificanteffectontheelasticconstantbuthaveasignificanteffectonthepowderadhesion onthestruts, affecting thedimensionalaccuracy.However,geometricaldesignparameterssuchas theunit cell type and strutdiameterhave significant effects on the elastic constant butnot on the dimensionalaccuracyof the latticestructures. Thesepreviousworksshowedthat themanufacturabilityofcellular latticestructureswasoften affectedbythe latticeunit size. Besides, themechanical characteristicsof latticemicro-structuresare alwaysaffectedbymanufacturingdefects, especiallywhentheunit size isverysmall. Another commonmethod of lightweighting is the use of structure topological optimization. Thisdesignmethodgeneratesanoptimizedmaterialdistributionforparticularpurposesbyminimizing asuitableobjectivewithin the limitedspace. Somepreviousworkshave investigatedthismethodto takeadvantageof theSLMprocess.Brandtetal. [11]studiedthedesign,manufactureandexamination ofhigh-valueaerospacecomponentsand investigated thegeometricoptimizationof theaerospace bracket fromTi-6Al-4Valloy. Xiao et al. [12] examined thedesign and fabricationof bio-material scaffolds using topological optimizationwith the SLMprocess. An optimization procedurewas 88