Page - 8 - in 3D Printing of Metals

Metals 2017,7, 2 Table1.Variables influencingtheprocessinganddensificationmechanismofselective lasermelting (SLM)processedparts [54]. SLMProcessingParameters MaterialProperties Laser type Viscosity Laserpower Surface tension Mechanical layeringofpowder Thermalconductivity Atmosphericcontrol Specificheat Gasflow Absorptivity/reflectivity Heaters (bedtemperature) Emissivity Scanradius Particlesizedistribution Scanvector length Particleshape Scanspacing MeltingTemperature Scanrates BoilingTemperature Scanningtimeinterval Chemical composition Thicknessof layers Oxidationtendency 3.1. SLMProcessingWindowsofMagnesiumandMagnesiumAlloyPowders Fromtheirpreliminaryexperiments,Ngetal. [55]wereable tosuccessfullyachieve fullmelting of single magnesium tracks using a miniature SLM system in an inert argon gas atmosphere and demonstrated the potential to employ magnesium powders in the fabrication of objects using SLM. Further, the interactions between laser sources andmagnesiumpowder tracks under different processing conditions, including laser powers, scanning speeds, and irradiationmodes (i.e., continuous wave and pulsed mode) were investigated and the processing window for the single track formationwasestablished[56,57].Also, severalother investigations [58–65]have focused ondevelopingprocessingwindowsbasedon the formabilityofmagnesiumandmagnesiumalloy powders such asMg-9%Al, AZ91D, ZK60 andWE43 for fabricating single layer andmulti-layer threedimensionalparts. Thedetailsof theparametersused in these studies canbeseen inTable2. Figure3presentsanexampleofaprocessmapdevelopedforSLMMg-9%Alpowders inwhichseveral distinctzonesareobserved. Figure3.ProcessmapforMg-9%Al, resultsas functionof therangeof laserpowersandscanspeeds (modifiedfromreference [60]). During theprocessofSLM, themeltingof thepowder layer stronglydependson the inputof laserenergysuppliedto thematerial. In thisarticle, inorder tohaveasingleparameter tocompare the processing conditions for single layer andmultiple layer parts, specific laser energy density, E, isdefinedas: E= Laserpower (P) Scanningvelocity (v)×Hatchspacing (s) (1) E= Laserpower (P) Scanningvelocity (v)×Hatchspacing (s)×Layer thickness (t) (2) 8