Page - 18 - in 3D Printing of Metals

Metals 2017,7, 2 withinSLMparts: columnar (elongatedgrainmorphology), columnar-plus-equiaxed,andequiaxed (isotropicgrainmorphology). Ithasbeenfoundthatahighersolidificationratepromotes thetransition fromcolumnar to equiaxedgrainmorphologies [101] and that increasing the cooling rate,G×R, leads toafinermicrostructure. The tendencyto formacolumnarstructure increasesby increasingthe ratioG/R,whiledecreasingG/R is favourable forequiaxedstructures [99]. Microstructural featuresalongwithresultingpropertiesobservedfordifferentSLMedmagnesium alloypowders are compiledand tabulated for easeof comparison inTable 6. Conventionally cast magnesiumalloysaregenerallycharacterizedbyacoarsemicrostructureconsistingofprimaryα-Mg andlamellareutectic(α-Mg+intermetallic)phaseswithanaveragegrainsizeintherangeof50–150μm. However, rapidcoolingratesassociatedalongwithepitaxial solidification in theSLMprocess, results inahighlyrefinedmicrostructure inmagnesiumalloyswithgrainsizesofα-Mgmatrix in therangeof 1–20μmandoftenfavour the formationofpartiallyor fullydivorcedeutectic (separationofeutectic phases)homogenouslydistributedalongthegrainboundariesofdendritic/columnarprimaryα-Mg (Figure8) [49,61]. SLM,beinganon-equilibriumprocess, canextendthesolubilityofalloyingelements inMgandobtain single-phaseMgalloyswithwider composition range [61]. SLMalso results in compositionalandmicrostructuralchangescausedbythecombinedeffectsofselectiveevaporation ofelements likeMgandZnhavingveryhighvapourpressures, andconsequentenrichment in the relative contentof elements likeAl andZrat the surfaceby“solute capture”phenomenon. In the processof laser rapidmelting,veryhightemperaturegradientsgenerated in themeltpoolcontribute to theformationofastrongMarangoniconvectionandresult in improvedhomogenousdispersion ofalloyingelements in themeltpool [62]. Thenasubsequenthighrategrowthof the solid/liquid interface contributes towards “solute capture” phenomenon in α-Mg matrix, resulting in larger amountsof solute atoms tobe captured, extending the solid solution limit of alloyingelements in α-Mgandretardingthenucleationβ-phases [49,61]. Suchcompositional changescan influence the microstructure,mechanicalproperties,andcorrosionbehaviourof laser-meltedmagnesiumalloys. Figure8.HomogenousdistributionoftheprecipitatesformedaroundthegrainsduringSLMprocessing of (a)AZ91Dalloyat166.7 J/mm3 [61]and(b)ZK60alloyat600 J/mm3 [49]. 18