Page - 10 - in 3D Printing of Metals

Metals 2017,7, 2 Theaboveequations ((1)and(2))present theenergydensity that isavailable toheatupandmelt thepowders for fabricatingsingle (J/mm2)andmultiple layers (J/mm3), respectively. Theoccurrence ofvariousregions in theprocessingwindowaswellas theresultingmicrostructure isdependenton the laserenergydensity, rather than laserpowerorscanningrate in isolation. Basedonthe formation qualityof thespecimensduringtheSLMprocess, fourregionsofbehaviourcouldbedefinedover the consideredrangeof the laserpowerandthescanningspeedfordifferentmagnesiumandmagnesium alloypowders: I Highenergy inputzone: In this zone the input energydensity is toohighunder ahigh laser power forall speedsused.Asaresult,hightemperatureswitnessed in themoltenpoolscauses evaporationandionisationof thepowdersdueto the lowboilingpointofmagnesium(1093 ◦C). Theevaporatedpowdersexpandrapidly inducingastrongrecoil effectonthemoltenpooland blowingtheliquidandpowderawayresultinginnotrackformation.Vaporizationofmagnesium powdersoftenresults incondensationofvolatilizedmaterialsonthe laserwindow,disrupting thedeliveryof the laserpower[50].Moreover,effectivetemperature inthemeltpool increasedby thehigh-energyinputaffects thedynamicviscosityof theoverheatedliquidmagnesiumresulting in the instabilityof themeltpool.Moltenmagnesiumpossessesamuchlowerdynamicviscosity (1.5Pa-s) thaniron(6.93Pa-s)andtitanium(2.2Pa-s)alloyswhicharewellestablishedintheSLM process. Thehighenergy inputduring laserprocessing induceshighthermalstresses resulting in reducedviscosityof themeltpoolwhichmayleadtodeformationofparts [64]. II Lowenergy inputzone: Thepresenceof thiszone is influencedbythe lowestenergydensity for all thescanningspeedsusedcombinedwithrelatively lowlaserpower.Usinga lowlaserpower anda relatively high scanning speedmaynot allow themelting temperature ofmagnesium (650 ◦C)tobereachedwhichresults inpartialmeltingof thepowders. The laserenergydensity is insufficient togenerate adequate liquidphaseandconsequently leads toapoorbondneck between the particulates. Even though therewas some fusion between theparticles, fragile sampleswithoutmechanical strengthareobtainedandnumerousunmoltenmetal fragments existedon thesurface. Samples fabricatedusing the laserparameters in thiszonehavemany defectssuchasdelamination,brittle fractureandhighporosity [67]. Furthermore,aprominent heat-affectedzone(HAZ)canbedevelopedduringthemeltingofthetracks.HAZisdevelopedas aresultofpartialmeltingofparticlesdue toradialheatconduction fromthecentreof themolten pool to theneighbouringpowders [8]. If thescanningspeedwas toofast,morepowders,due to the lowdensityandchemicalactivityofmagnesium,wereblownupandthenoxidizedto form ablack fogMgOcontaminating theprotect-gaschamber [59]. It canbeobservedfromTable3 that thereexistsaminimumcritical laserenergydensityabovewhich theSLMofmagnesium powdersoccurred. III Formationzone: In this zone, acceptable melting of magnesium powders can be achieved in a relatively stablemelt poolwhichyields trackswith goodmetallurgical bonding.As the formationofa fullymoltenpowderbed isessential inSLMprocessing tosuccessfullybuildnear full-densityparts, sufficient laserenergydensity isapplied topowdermaterials. Fairlydense structures (i.e., 75%–99.5%)without obviousmacro-defects canbebuilt in aproper range of energydensitieswhicharederivable fromtherangesof laserpowerandscanratesasshownin Table3 fordifferentmagnesiumalloypowders. It isbelievedthat therangeofenergydensities in the formationzone increase thepowderbedtemperature,while reducingtheviscosityof theof themeltpool suchthat themelt canbespreadproperlyonthe formerlyprocessedpowder layer, therebyfacilitatingmoreefficientdensificationwithsolidpowderedparticlesaselucidatedby Attaretal. inSLMprocessingofcommerciallypureTi [68]andTi-TiB2 composites [69]. IV Ballingregion:Occurrenceof“balling”region ischaracterisedbytheagglomerationofaseries ofball likeparticles to formlargesizemeltpoolsdueto insufficient input laserenergydensity causedbyacombinationof lowlaserpower,highscanningspeed,andlarge layer thickness [41]. Ballingeffect is causeddueto lackofwettingof themoltenpoolwith thepreceding layerwhich 10