Page - 17 - in 3D Printing of Metals

Metals 2017,7, 2 itsmeltingpoint to51KPaat itsboilingpoint led to fasterevaporationof thepowdersandthesurface of themoltenpoolwascoveredbythemeltvapour. Further, themagnesiumvapourssolidifiedand depositedonthesurfaceof themetal causingformationofcauliflower likegrains. Figure7.Cauliflower likegrainsobservedduringSLMprocessingofMg-9%Alpowders [60]. As higher energy input is required to obtain parts with higher density, development of asuitableprocessingwindowathigher laserenergydensitiesbecomeschallengingduetominimal differencebetweenmeltingandvaporization temperaturesofmagnesium.However, thisproblemwas overcomebyworking inanoverpressureprocesschamberhavinganabsolutepressureof0.3MPa, which increased theboilingpoint ofmagnesiumby127 ◦Cto1220 ◦Callowing for awider range of laser power and scanning speed tobe applied [64]. In contrast to the effect of laser processing parameters, littleworkhasbeenperformedtostudythe influenceofpowderpropertiesofmagnesium on formation anddensificationof SLMprocessedparts. Muchmorework, therefore is needed to systematicallyinvestigatethecharacteristicsofdifferentmagnesiumalloypowdersonthedevelopment ofaprocessingwindowtobuild fullydenseparts. 4.Microstructure TransientcoolingpatternsemployedinSLMdictate themicrostructures formedinadeposited layer,due to therapidanddirectional solidificationresulting infinermicrostructures. SLMpossesses the capability to control grain sizes and shapes, phase percentages, and phase compositions by manipulationofprocessparametersasper thedesignrequirements to fabricatepartswith tailored mechanicalproperties. Themicrostructural characteristicsof theconsolidatedmaterials fabricated by SLM are strongly sensitive to their thermal history during the build, which may include high heating/cooling rates, significant temperature gradients, bulk temperature rises, andmore. Theresultingmicrostructures,whichareveryfineandfar fromequilibriumareaconsequenceofvery highsolidificationratesobservedin theSLMprocess, rangingbetween106 and1011 ◦C/s[96]. Since manyprocessvariables/parameters impact the thermalhistory,predictingthemicrostructural features ofSLMparts, andthedegreeof theirdependenceontheprocessparameters, is still amajorchallenge. However, overcoming this challenge is vital for establishing the effective controlmechanisms for fabricatingSLMpartswith superiormechanicalproperties. Variousauthorshave investigated the effectsofcertainparametersonthemicrostructural characteristicsandmaterialpropertiesofSLMed magnesiumparts [58–65].However, it is stillunclearhowtoapply thesefindings to fabricatecomplex partswith various shapes since theirmicrostructureswill have a uniquedependence on thermal history. Thesolidifiedmicrostructureobtainedwhen theSLMprocessingparameters arevaried is dependent on: local solidification rateswithin themelt pool, the ratio of cooling rate to thermal gradient,R, andthe temperaturegradientat thesolid-liquid interface,G.Twocritical solidification parametersare the ratio,G/R,whichaffects thesolid-liquid interfaceshapecontrolling the typeof microstructure, andthecoolingrate,G×R,whichaffects theundercoolingcontrolling thescaleof microstructure [99,100].DifferentGandRvaluesmayresult in threemajorstructuremorphologies 17