Page - 26 - in 3D Printing of Metals

Metals 2017,7, 2 in surfaceenergyof the liquidat short lengthscales, small sized liquiddroplets splashed fromthe liquidsurface. Further,ballingobstructedthe formationofcontinuousmelt lines, formingroughand bead-shapedsurfaces. Inmoreseverecases,ballingmayaggravate insubsequent layersand jamthe powdercoatingmechanismwith largemetallicbeads thatextendabovethepowderbed. Theballing effectcanbeavoidedif thestabilityof themeltpool is improvedbyreducingthe length/widthratioof meltpooland/or increasingcontactwidth,whichcanbeachievedbyincreasingthe laserpoweror decreasing thescanningspeed[111].AccordingtoLietal. [112], lowering theoxygen levelduring the processand introducingrepeatexposureof laser tobreakuptheoxidefilmsareways tominimize theoccurrenceof balling, as ballingbehaviourduring the SLMprocess is related to theoxidation ofmetals [112]. In their study regardingSLMprocessingofNiandstainless steelpowders, itwas observed that balling couldbe reduced significantly bykeeping the oxygen level at 0.1%, andby applyingacombinationhighlaserpowerandlowscanningspeedorapplyingre-scanningoflaser[112]. However,eventhoughtheoxygencontent inthechamberwaskeptbelow0.2%,Huetal. [59]observed theoccurrenceofballingduringSLMprocessingofmagnesium,asmagnesiumbeingveryactive is susceptible tooxidation. Figure 14. SEMimage showing cluster of balls (balling effect) in SLMprocessedMg-9%Aldue to non-optimizedlaserprocessingparameters [60]. 6.MechanicalProperties Properties observed fordifferent SLMedmagnesiumalloypowders are compiledandshown inTable6. Figure15presents thecomparisonofhardnessandYoung’smodulusvaluesreportedfor lasermeltedpartswith conventionally cast andwroughtmagnesiumalloys in literature. SLMof magnesiumalloysresults inhighsurfacehardnessthanthatofas-castalloys,asaresultofsolidsolution strengtheningeffects (becauseofsolutesupersaturationwithrapidcoolingorsoluteenrichmentwith selectiveevaporationofelements). Themicrohardnessisfoundtograduallyfluctuatefromthecentreof themoltenpool tothemarginzoneduetothevariations inthemicrostructurebetweenthetworegions. However, themicrohardness fordepositedsamplesshoweddirectional independence [61].According toMercelis andKruth [113], residual stresses are not always disadvantageous in SLM fabricated partsbecause their retentionatareasonable levelenhances thepart’shardnessprovidedsufficiently high densificationwithout formation of cracks or pores could be achieved. Moreover, the rapid solidificationeffect impartedontheas-processedpartsbytheSLMwhichresults ingrainrefinement of themicrostructure is another reason that couldbe attributed to increasedhardness of the SLM fabricatedparts. Figure16showsthevariationofhardnessofSLMprocessedmagnesiumtrackswith different laserenergy inputsasmeasuredbyNgetal. [57]. It is evident that theresultinghardness values can effectively be controlled by adjusting the laser energydensities in the formation zone. Thehardnessvalueswerefoundtoincreasewithdecrease inthe laserenergyinput. Thehighercooling rate inducedbylowlaserenergydensityresulted insmallergrainsizes, so that thehardnessvalues weremainlyrelatedto thegrainsize in themeltedzone.Also, thereportedYoung’smodulusvalues 26