Page - 24 - in 3D Printing of Metals

Metals 2017,7, 2 orientated towards newmethods of controlling the oxidationprocess anddisrupting oxide films formedwithin thecomponents. Ithasbeenshownthatsignificantamountsofoxidearepresentwithin theselective lasermelted magnesiumandtendedtoformbetweenscantracksrather thanbetweenlayers,asshowninFigure13. It isalsoexpectedthat theoxidewouldformontopof themeltpool,whichmayretard its spreading andalter thewettingcharacteristicsandresult inaporousstructurewithweakmechanicalproperties. However,providedthat sufficientenergy is imparted, this surfaceoxidewillbreakupandbecome encapsulatedwithin themelt pool. However, itmay then act as amicro crack in the systemand, therefore, bedetrimental to thefinalmechanical properties of theproduct. Just as oxidefilmsare damaging tomagnesiumalloy castings, so they are to SLMprocessedpartsmade inmagnesium alloys.However, thepotential roleofoxidefilmsaffecting thedensification,mechanicalproperties, andcorrosionbehaviourofSLMprocessedmagnesiumalloypartshasnotbeenelucidateddue to abundance of somedefects such asporosity and loss of alloying elements. It is expected that the potential roleofoxidefilmswillpossiblybehighlightedbySLMprocessingofmagnesiumalloys in the future,once thesedefectsareeliminated. Figure13.SLMedmagnesiumtrackfabricatedata laserenergydensityof7.89×103 J/mm2 [57]. 5.2. LossofAlloyingElements Highervapourpressureandlowboilingpointsofelements likemagnesiumandzinccan leadto preferentialelementvaporizationduringthe interactionbetweenlaserandmagnesiumalloys [94]. It is alsowellknownthatthetemperaturesreachedwithinmeltpoolsduringtheSLMprocessarefargreater thantheboilingtemperaturesofmagnesium,aluminium,orzinc. Thus, thepreferentialevaporative lossesduringSLMwill consistprimarilyofzincandmagnesium. Intrinsicvaporizationofelementsat themeltpool surfaceduring laserprocessing takesplace initiallybythe transportationofvaporization elements fromthebulk to the surfaceof themoltenpool, followedbyvaporizationof elements at the liquid/vapour interface; then the vaporized species are transported into the surroundinggas phases [106]. Sometimes, because of lowdiffusivity, the vaporized species are re-condensed and depositedback immediatelyaroundthe laser irradiatedzonesandareremeltedduringsubsequent scanningtocompensate for the initial losses [62].However, condensationofvolatilealloyconstituents on the surfaceof themoltenpool toagreater extent can lead to formationof ablack coating [107]. During SLMofAZ91Dalloys,Wei et al. [61] observed that selective vaporization ofmagnesium causedanincrease in theconcentrationofaluminiuminthemoltenpool. Increasingthe laserenergy density from83.3 J/mm3 to166.7 J/mm3causedanincrease in thecontentofAl inα-Mgsolidsolution from8.31wt%to10.98wt%signifyingahigherburning lossrateofMgatoms. Further,dueto the interactionof“solutecapture”andelementevaporation, thecontentof thehard-brittleβ-phaseand thesolubilityofsoluteelements in theα-Mgmatrix increasedsimultaneouslywith increasing laser energy inputs. Similar resultswereobserved incaseofZK60alloys [62],whereina larger reduction in thecontentofbothMgandZnwasreportedatslowerscanningspeedwithsubsequentenrichment ofZrafterprocessing. Also, ahigherMg:Znratio in thefinalpartswasobservedwhencompared to rawmaterial. ThoughevaporationduringSLMisknowntobeaproblemforaluminiumalloys, itdidnotappear tobesodetrimental in theprocessingofMg-9%Alalloys [60].However, evaporative 24