Page - 104 - in 3D Printing of Metals

Metals 2016,6, 280 compression. Thesamplesof2.0mmand2.5mmwerenot fracturedbecauseof the limitationof the testmachine. The compression curveswere recorded inFigure 27. At thebeginningof the experiment the deformationofsampleswaselastic. Somesupportcylindersof the0.5mmsampleyieldedwhenthe loadincreasedto180MPabecauseof the localdefectscausedbytheSLMprocess.At thesametime, themodulusofelasticityof thepartsdecreasedandtheelasticdeformationcontinuedtooccur.All the supportcylindersyieldedwhenthe loadincreasedto338MPa. ThedefectscreatedbytheSLMprocess mayweakentheperformanceofparts, especiallywhenthesizeof thestructure is rathersmall. Figure26.Resultsofcompressiontests. Elasticdeformationalsooccurred in the compression test of the1.0mmand1.5mmsamples. When the displacement exceeded 0.75mm, these parts yielded. Because of the limitation of the equipment,onlyaportionof thecompressioncurvesof the2.0mmand2.5mmsampleswererecorded. Theequivalent elasticmodulus andyield stress canbe calculatedby the compression curves. The calculation results are shown in Table 5. It can be seen fromTable 5 that the performance degradationof thepartsbecomessignificantwhenthesizeof thestructure issmall.Themechanical propertiesof thesmall-size latticestructuremightbepoorduetothehighaspectratioof therods.Higher aspectratios increasethetendencyof thestructure tobuckle, leadingtopoormechanicalproperties. Table5.Calculationresultsofcompressiontests. Number EquivalentDiameter/mm YieldStress/MPa EquivalentStiffness/GPa 0.5mm 2 180(First)338 (second) 49.03 1.0mm 4 620.2 63.85 1.5mm 6 694 57.33 2.0mm 8 35.60 2.5mm 10 30.45 Figure27.Compressioncurvesofdifferentsizedsamples. 104