Seite - 149 - in 3D Printing of Metals

Metals 2016,6, 313 3.2. Influenceof theDistancebetween theNozzle and theTopSurfaceof thePreviousLayer In this section, only the layer thickness is discussed in order to maintain other parameter consistency.Onegoalof thisexperimentwas toachievemultilayerdeposition.Afterone layerwas deposited, theplatformwasmoveddownwardforadistance in thez-direction,andanother layerwas depositedover thepreviousone. The formingprocessof theMLSPismorecomplicated thantheSLSP due to thecomplexmorphologiesof thepre-deposited layer. In this section, the formingprocess is basedontheoptimizedprocesswindowsof theSLSP.Eachthin-walledspecimenhas twelve layers thatare100mminlength. If the formingdefectoccurs in thecurrent layer, the formingprocessmust be terminated. TheprocessparametersofCategoryBinFigure2wereused. In theexperiment, thedeposition velocitywas18mm/s, theflowratewas50mm3/s,andthe layer thicknesswasset to0.9mm,1.2mm, 1.5mm, 1.8mm, and 2.1mm, respectively, the platformwasmoveddownward for a distance in thez-direction. Figure4mainlyshowstheexperimental results. AsshowninFigure4a, thenozzle contactedcompletelywith theworkpiecewhenthethicknesswas0.9mm,resulting inwider layers, whichwas beneficial for the formation of a new layer, as good bonding quality can be achieved between layers. AsshowninFigure4b,when the thicknesswas1.2mm, the thin-wallwasnot too widewithheatcapillaryflowpressureandheatcapillaryflow,resulting in themostcomplex forming state.AsshowninFigure4c, themoltenmetalgeneratedadraganglewhenthe thicknesswas1.5mm, whichwillbediscussed indetail in thenextsection.AsshowninFigure4d,whenthe thicknesswas 1.8mm,therandomjetfromthemoltenmetalaffectedtheformingmorphology,whichledtothefailure of thesubsequentdepositionprocess.AsshowninFigure4e, thenozzleseparatedcompletely from theworkpiecewhenthe thicknesswas2.1mm. Inaddition, thehigh-speedmelthadastrongimpact onthe freesurfaceof themelt. 149