Page - 25 - in 3D Printing of Metals

Metals 2017,7, 2 lossescanbemoreproblematic incaseof lowzinczirconiumalloyswhereinevaporationanddecline in thecontentofalloyingelementscanbedetrimental to themechanicalperformanceandcanleadto degradationof functionalpropertiesofSLMedsample. Evaporation of alloying elements taking place at higher laser energy densities during laser meltingwill affect the stability of themoltenpool alongwith causing a variation in composition andmicrostructure of the deposited layers [106]. Metal vaporization leads to formation of recoil pressure in themoltenpool,which tends topush the liquidaway fromthemelt zone, resulting in formationofakeyholedefect [108]. Formationofkeyholedefects results in inferiorsurfacequalityof thespecimenshowingvaryingdepthsasobservedduringSLMofZK60alloys [62].Thedepletionof alloyingelements leads toahigherdegreeofporosity in the laserprocessedparts limiting the levelof maximumdensificationthatcanbeachieved.Composition inhomogeneities resultingfromselective evaporationofelements isa functionof thevaporizationrateandthevolumeof themoltenpool [94]. Thus, careful controlandmanipulationofSLMprocessparameters is required toensure reduction in theoccurrence of elemental redistribution andporosity. Loss of highvapourpressure alloying elementscanbeeffectivelyminimizedbyregulatingthemoltenpool temperature,whichinturncanbe achievedbycontrollingthebeampowerdensitydistributionincaseofcontinuouswaveirradiationand adjusting thepulsingparameters incaseofpulsedwave irradiation[52].Although,high laserenergy densities lead togreater evaporative losses, compositional inhomogeneities aremost pronounced at lowenergydensity levelsdue to small size, and thehighsurface-to-volumeratio, of themolten pool [94]. Inorder toavoidvaporizationofelementsduringSLMprocessingofmagnesiumalloys, it is recommendedtoemploy laserprocessingparameterscombiningmediumorhighpowerwithhigh scanningspeedsandalso it isnecessary to furtherunderstandthemechanism, to investigate themain influencingfactorsandtobuildupthequantitativerelationshipbetweenthevaporization lossesand SLMprocessparameters. 5.3. Balling Balling phenomena are regarded as the typical microstructure occurring on surfaces of SLM-processedparts fromabedof loose powder [39]. Balling is defined as an agglomeration of theparticles,occurringwheretheliquidphasebreaksupintoarowofspherestoreducesurfaceenergy. Asdiscussedpreviously,occurrenceof“balling”regionischaracterisedbytheagglomerationofaseries ofball likeparticles to formlargesizemeltpoolsdueto insufficient input laserenergydensitycaused byacombinationof lowlaserpower,highscanningspeed,andlarge layer thickness [41]. Themain factor leadingtoballing is theGibbs-Marangonieffect,which is themass transferalonganinterface betweentwofluidsdueto thesurface tensiongradient [109]. In termsof temperatureassociation, this phenomenonisalsocalled thermo-capillaryconvection.DuringSLM, laserscanning isperformedline by lineandthe laserenergycausesmeltingalongarowofpowderparticles, formingacontinuous liquid scan track in a cylindrical shape. Thediminishing in the surface energyof the liquid track continuesuntil thefinalequilibriumstate throughthebreakingupof thecylinder intoseveralmetallic agglomerates in spherical shape [39]. Three kindsof ballingmechanismshavebeen identifiedby Guetal. [73,110]duringSLSprocessingofCu-30CuSn-10CuPandstainlesssteelpowder. “First line scanballing”isobservedwheninitial tracksonacoldpowderbedarescanned,causedbyhighthermal gradients imposedonthemelt. “Shrinkage inducedballing” iscausedbythecapillary instabilityof themeltpoolwhenhigherscanningspeedsareused. “Splash-inducedballing”with the formationof a largeamountofmicrometre-scaleballsoccursatahigh laserpowercombinedwitha lowscanspeed, becauseof theconsiderably lowviscosityandlonglifetimeof the liquid. Figure 14 shows the occurrence of balling phenomenonobservedduring SLMprocessing of Mg-9%Alpowders[60],whereinalargeamountofmicrometer-scaled(diameterof10–20μm)spherical splashes, formedaroundthesinteredsurface, canbeseen.Whenrelativelyhigherscanningspeeds were applied, laser energydensity of the laser input decreased causing a significant shrinkage in interparticleareas, leadingtocapillary instabilityof themoltenpool. Therefore,dueto thereduction 25