Seite - 151 - in 3D Printing of Metals

Metals 2016,6, 313 To conclude, layer thickness is one of themajor factors influencing the formingmorphology. Different layer thicknesseswillproduceacompressionora jet.Whenthe layer thickness is larger than 1.5mm, themoltenmetal jetwillnegativelyaffect the freesurfaceof themelt,usuallycausingdamage to theprevious layers ormassive overflowing. When the layer thickness is smaller than 1.2mm, thewidth of thin-wall specimens increases due to the nozzle extrusion pressure and, as a result, usuallyreduces the formingefficiency. Theflat formingmorphologycanbeachievedwhenthe layer thickness isbetween1.2mmand1.5mm. The forming morphology of the workpieces in Figure 4b,c are better than the others. Consequently, the process parameters of these twoworkpieceswere applied to forma thinwall, asshowninFigure5. InFigure5a, thepreviousseveral layersweredepositedwithagoodoverlapping rate.However, thecumulativeerrormadethebondingqualitydecrease,andtheporosity increased duringthesubsequentdepositionswhenthe thicknesswas1.5mm. InFigure5b, the layer thickness matcheswellwith theheightof theactual specimens. The idealworkpiecewasobtainedwithoutany defects, andthemorphologyofeach layerwasuniformwhenthe thicknesswas1.2mm. Figure5.Thedepositedspecimens indifferent layer thickness. (a)δ=1.5mm. (b)δ=1.2mm. 3.3.WavinessandCross-SectionMorphologyAnalysis ofThin-WalledSpecimens The layer thickness and surface quality remain themost important issues to be taken into considerationforthethin-walledexperiments.Thewavinesswasappliedtoevaluatethesurfacequality, includingthreemajorindexes(Wa,WqandWz).Theindexes,Wa,Wq,andWz,denotethearithmetical meandeviationof theassessedprofile, therootmeansquaredeviationof theassessedprofile,andthe maximumheightof theassessedprofile, respectively. Inaddition, itwasfoundthat thewavinessof the formedspecimens isgreatlyaffectedbythe layer thickness.As illustrated inFigure6a, the thin-walled specimenwasformedbasedonthefollowingprocessparameters: adepositionvelocityof18mm/s, aflowrateof50mm3/s,anda layer thicknessof1.2mm.Thecrosssectionof thespecimenisshown inFigure6b,andthecorrespondingmeasureddatawasshowninFigure6c. Thewaviness indexes wereanalyzed,andtheresultsareas follows:Wa=0.9448,Wq=0.963,andWz=0.303. 151