Page - 12 - in 3D Printing of Metals

Metals 2017,7, 2 densification levelobtainedwasrestrictedat82%andanarrowprocessingwindowwasobtaineddue to limitations intheSLMsystemasoperatingata laserpowerbeyond20W,causedsevereevaporation andsubsequentoxidationofmagnesiumowingto its lowboilingpointandalowevaporationheat (5.272kJ/kg)atambientpressure[64].Thelowerdensificationlevelsachievedinthisstudymayalsobe attributedto the irregularshapeofmagnesiumparticlesusedas itaffects theflowabilityof thepowder layers.AswaspostulatedbyAttaretal. [79] inSLMprocessingofTi-TiBcompositepowders,usageof non-spherical shapedpowderparticlesnegatively influences theuniformdepositionofpowders,as powderswith irregularshapemaynotfloweasilyandtendto interlockmechanicallyandentangle with eachother, causinganobstruction inpowderflow, consequently leading to the formationof porosities. It is alsoworthnoting that, thoughSLMisnormallyprocessedbasedon the complete meltingmechanism, application of lower laser energydensities can lead topartialmelting of the metalpowdersurface [80]. Thesesurface-meltedpowders join togetherdueto thepresenceof liquid metalat theparticle interfaces, leavingsomeinterparticle residualporosity. Thus, thepartialmelting mechanism,whereincompletemeltingof thepowder isavoided,canbeeffectivelyusedtoproduce complex shapedporous structures [80]. Partialmelting of the powder surface could also lead to formationofpartiallymoltenzoneswithineachparticleasaresultofdifferentmeltingtemperatures of thephasespresent inmagnesiumalloys, affecting themicrostructureandmechanicalproperties of SLMprocessed samples. Comparisons canbedrawn to selective laser surfacemelting (SLSM) process,whereinapplicationof lower laserenergy input leads tomeltingof just thesecondaryphases withoutaltering thepropertiesof theMgα-matrix, causingchanges in thephasemorphologyand distribution[81]. Weietal. [61] investigatedtheroleofSLMlaserprocessingparametersonthe formabilityand densificationbehaviourofAZ91Dalloytooptimize theprocessingwindowtoobtainpartswithhigher densityandlowerporosity. Theresultsareextractedfromthepublishedgraphs to thebestpossible accuracyandarereported inTable4,whichshowstheeffectofvariation inscanningspeedandhatch spacingontherelativedensitiesof theAZ91Dparts formed. Itwasobservedthat therelativedensities of thepartsdecreasedwith increase inboth the scanning speedandhatch spacings. Ataconstant laserpower (p=200W),decreasing the scanningspeedcauseda longerdwelling timeof the laser beamonthesurfaceof themoltenpool, therebyboosting the laser energydensitydelivered to the powderbedresulting inbetterdensification[82].Hatchspacingwhich isalsocalled“scanspacing” isanother importantparameter thatdetermines thedegreeofoverlapof the laserspotwhenanew laser linescansover thepreviouslyscannedline. Thehatchspacing isusuallychosen insuchaway that it varies between the halfwidth and the fullwidth ofmelt pool to ensure good bonding of the adjacent tracks [28]. Decreasing thehatch spacing increased thepart’s density asflowingand spreadingof the liquidwas increasedwhenthescan lineswerebroughtcloser tooneanother.Also, whenthehatchspacingmore thanthespotsizeof the laserbeam(100μm)waschosen,overlapping was found to reduceasadjacentmelt linesdidnot fuse together completely resulting in increased porosity.Maximumdensificationof99.52%wasachievedundera lowerscanningspeedof0.33m/s andahatchspacingof90μm.Energydensityof166.7 J/mm3wassufficient tobreakupanysurface oxide layers formed toproducealmost fullydense (>99.5%)AZ91Dparts. Near full densification levelsachievedin this studycanalsobeattributedto thestrict controlofconcentrationsofbothH2O andO2below50ppminthebuildchamber,asnopeakscorrespondingtoMgOandAl2O3were found intheXRDpatternsof theSLMedsamplebuiltatdifferentenergyinputs (Figure4). Similar results wereobservedforZK60alloys [62],whentheeffectofscanningspeedonpartdensitywas investigated with laserpower, layer thickness,hatchspacingandlaserspotsizeheldatconstantvaluesof200W, 20μm,80μm,and150μm,respectively.Whenthescanningspeedwas increasedfrom100mm/sto 900mm/s, itwasobservedthatrelativedensityof thepartpeakedwithavalueof94.05%at300mm/s. At100mm/s,severevaporizationandburningoutof themetalpowderswasobserved, leavingan ablatedpitonthesubstratesurface, resulting in terminationof theprocess.Atscanningspeedshigher 12