Page - 7 - in 3D Printing of Metals

Metals 2017,7, 2 Figure1.Scanningelectronmicroscopy(SEM)microstructureofselectivelasermelting(SLM)produced ZK60sample, showing(a)distributionofporosityandunmeltedareaand(b) formationofhotcracks within theZK60matrix [49]. Figure2.SchematicofSLMprocessingparameters [41]. 3. EffectsofProcessingParametersandPowderPropertiesonSLMProcessingandDensification ofPowders SLMofmetals requires careful selection of processing parameters to address the difficulties associatedwith theprocesssuchasoxidation,balling, thermalfluctuation, lossofalloyingelements, anddross formation inthemeltpoolwhich leadtopoor interlayerbonding, lowdensity, instability of the tracks, crack formation, and delamination of the SLM processed part [50–52]. In order to gain a holistic understanding of the factors that influence the process of tracks formation, densificationmechanism, part characteristics, and quality consistency of the fabricated parts by SLM, several processing andmaterial parameters have been identified and are listed in Table 1. Inanattempt toovercomethese inherentdifficultiesalongwithensuringgoodconsolidationof the depositedmaterial layers inSLMofmetals,variousapproacheshavebeenemployedbyresearchers suchascombinationofpreheating, selectionof theright scanningstrategyandprocessparameters toensureminimalballing,utilisationofoverpressureprocesschamber tocontrol thevaporisationof powders, andmelting ina shieldedgasatmosphereof argonorheliumtomoderate theoxidation process [29,53]. Accordingly, this section focuseson thepossible effects of thevariousparameters related to laser, scan, andpowderduring the buildingprocess on the formability,microstructure, andresultingpropertiesofcomponentsproducedfrommagnesiumandmagnesiumalloypowders. 7