Page - 23 - in 3D Printing of Metals

Metals 2017,7, 2 Figure 12. Typicalmicrostructural imagesof laser-meltedmagnesiumunder, (a) continuouswave irradiationat1.27×109 J/m2and(b)pulsedwave irradiationat1.13×1012 J/m2 [57]. 5.MetallurgicalDefects 5.1.Oxide Inclusions Thepresenceofsurfaceoxidefilmonthepreceding layer impedes interlayerbondingwhich in turn inhibits thedensificationmechanismandleads toballingas liquidmetalsgenerallydonotwet oxidefilms in theabsenceofachemical reaction. Incombinationwith thermalstresses,poor interlayer bondingcanalsocausedelaminationofSLMprocessedparts. Similar to thecastingprocessaspointed outbyCampbell [103], oxidemayget into themeltpooleitherviaalloyingaddition into themetal powderduringprimaryprocessingor air/gas entrapmentvia surface turbulentflowduringSLM processing.Also,as thevaporizationofalloyingelements in thescantracks isnotuniformwhile the scantrackpositionsvarywith time, rapidlyfluctuatingscantracks tendtoentraptheshieldinggasor evenairwhichmaycontain tracesofoxideparticles.Consequently, thesurfaceof the liquidmetal in themeltpool thenbecomeoxidizedto formoxidefilmasaresultof theentrapmentofairorshielding gas into themeltpool. TheoxidationofametalMmayberepresentedby M+O2→MO2 (3) Thefreeenergyof formationδGof theoxide isgivenby δG=−RT lnK (4) whereR is thegasconstant,T is the temperature inKelvinandK is theequilibriumconstantgivenby K=(PO2) −1 (5) wherePO2 is thepartialpressureofoxygenwhenreaction (3) is at equilibrium. Formagnesiumat 700◦C,aPO2of10−54 atmorhigherwillgiveoxidationofmagnesium[104]. Thus, thermodynamically, it shouldnotbepossible topreventoxidationof themagnesium. Since the formationofoxidefilms cannot be avoided completely, the SLMprocessmust break up these oxides if fully dense parts are tobe formed, and this iswhySLMfabricationofhighdensityparts requireshigh laserpower. According toLouvis et al. [105], theoxidefilmon theupper surfaceof thepool evaporatesunder a laserbeam.Marangoni forces that stir thepoolare themost likelymechanismbywhichtheseoxide filmsaredisrupted,allowingfusionto theunderlying layer.However, theoxidesat thesidesof the pool remain intact and, thus, create regions ofweakness andporosity, as thepool fails towet the surroundingmaterial. Therefore, further researchon theSLMofmagnesiumshouldbeprimarily 23