Seite - 14 - in 3D Printing of Metals

Metals 2017,7, 2 study. Flatter surfaceswithoutanysurfacedefectswereobtainedata lower layer thicknessvalues of150–250μmasseen inFigure5.Also,noheataffectedzonewasobservedat lower layer thickness values. Since theamountofmaterial tobemelted issignificantly less,heatconducted inthemolten poolhassufficientenergytocompletelymelt theneighbouringparticlesasopposedtobeingpartially melted.However, loweringthe layer thicknessbelowminimalpermissible level forapowdersystem cancreateproblems inspreadinghomogenous layersof freshpowder [84]. Ingeneral,densification is improvedwith the increase in laserpoweranddecrease in thescanspeed, layer thicknessandhatch spacing. Similar trends in thedensificationbehaviour isalso foundtobe true forothermaterials such asAl-12Si [84],Ni-alloy-CuandFe-Cu[85]. Figure5.Effectofvariationinlayerthicknessonthemicrostructureof lasermeltedmagnesiumpowder at laserpowerof17.7W,scanratesof1mm/s: (a) 150μm;(b) 200μm;(c) 250μmand(d) 300μm[58]. 3.3. Effects ofPowderProperties on theSLMDensificationMechanism Thecharacteristicsandqualityof thepowderhaveastrong influenceontheprocessstabilityand indetermining theresultingmaterialsproperties. Thequalityof thepowder isdeterminedbysize, shape, surfacemorphology, composition,andamountof internalporosity. Thequalityofpowderalso determinesphysicalvariables, suchasflowability (howwellapowderflows)andapparentdensity (howwell apowderpacks) [86]. Powdermorphology isone important factor inSLMaffecting the processingconditionssuchasflowability,packingofthepowders,andnatureofheat transferaswellas beingacrucial factor indefiningthelayerthicknessandsurfaceroughnessduringtheSLMprocess[79]. HighbuildratesandpartaccuracycanbeachievedinSLMbyemployingsphericalparticlesratherthan irregularparticlesas theycontribute towards improvedflowabilityof thepowders [87]. Thepowder particlesusedshouldbe freeofdefects suchas satellites and inter-agglomeratepores, because the presence of suchdefects can cause lack of fusionbetween theparticles affecting thedensification behaviour. Thedistributionof thepowderparticlesizesalsoaffects theflowabilityof thepowdersas finepowderswithanarrowparticlesizedistributiontendtoagglomerateandcoarseparticleswith abroaderparticlesizedistributiontendtosegregate leadingtoprocess instabilities [88]. The fraction ofsmallerparticles shouldbeassmallaspossible toavoid inhomogeneousdistributionof thepowder overthebuildplatformasfrictionamongtheparticlesincreaseswithdecreaseintheparticlesize[29,59]. Awiderparticlesizedistribution(morefineparticles) inSLMofstainlesssteel316Lwasobservedto result inhighdensity (>99%)acrossawider rangeofprocessparameters (beamdiameter,beamspeed) thanpowderwithasmallerparticle sizedistribution [89]. It shouldbealsonoted that,powdersof differentsizebehavedifferentlyduringlaserprocessing.Highersizepowdersmeltslowerthansmaller size. Therefore, incaseofwidersizedistribution,moreattention is requiredforoptimizing theprocess parameters to avoidpartialmeltingof themetal powder surface leading to inter particle residual porosities [90].Although,higherdensitypartswithbettersurfacefinishcanbebuiltwith lower laser energydensity for thefinerparticles thanthecoarserparticles, theycanbeeasilyblownawaybythe gasflowintheshieldingchamberandadhereonthescanningsurfaceimpedingthedepositionofathin layerofpowdercritical totheSLMprocess[59].Also,owingtothelowdensityofmagnesiumparticles, verystrong interparticlecohesionmaytakeplaceduetoVanderWaals forcesaffecting theflowability of thepowders. Ithasbeenshowninsomestudies thatpowderswithanarrowrangeofparticlesize 14