Page - 5 - in 3D Printing of Metals

Metals 2017,7, 2 thehigh-temperaturemetallicpowder layerssuccessivelydepositedoneover theotherasultrathin two-dimensional cross-sections [26]. Generally, thegeometrically complex componentshavebeen fabricated by SLM with a high dimensional precision and good surface integrity without other subsequentprocess requirements,which theconventional techniques (e.g., castingandmachining) cannotkeeppacewitheasily [27].SLMisanefficientapproachthatcansignificantlyshortenthe lead timeandthecosts involvedinthemanufacturingofhigh-valuecomponentsandoffersadvantages of design freedom with a wide choice of materials, reduced material wastage, and elimination of expensive tooling. Over the years, the process of SLM has become very well established for titaniumalloys [28], steelandironalloys [29],nickelbasesuperalloys [30,31], cobaltbasealloys [32], and aluminium alloys [33] and is providing an ideal platform to fabricate complex products, novel shapes, hollow and functionally graded structures to exact dimensions for the aerospace, medical, andmilitary industries. Despite thecontinuingadvancement, therearevery fewexamples thathave fullyutilized the potential of SLM to produce fully dense near-net shape components ofmagnesium and to gain anunderstandingof laserprocessability.Accordingly, this reviewaimsto identify theadvancements todate inSLMprocessingofmagnesiumandmagnesiumalloypowdersandoutlines the trendfor futureresearchtoexpandtheapplicationrangeofmagnesiumbasedmaterials further. 2. SelectiveLaserMelting(SLM)Technology SLMisapowderbedfusionprocess,whereinselectiveregionsof thepre-spreadpowderparticles aremeltedand fusedbyahigh intensity laser energysource ina layerby layermanneraccording tocomputeraideddesign (CAD)data. The term“laser” implies thata laser isused forprocessing, “melting” refers to theparticular situation inwhichpowders are completelymelted and the term “selective”specifies thatonlypartialpowder isprocessed[28]. TheSLMsystemgenerally iscomprised ofaprocessinglaser,anautomaticpowderdeliverysystem,abuildingplatform,acontrollingcomputer system, andmain accessorial parts (e.g., inert gas systemprotection, roller/scrapper for powder deposition, andanoverflowcontainer) [34]. The focusandthemovementof the laserbeamonthe build table iscontrolledbyusingabeamdeflectionsystemconsistingofgalvano-mirrorsandaflat field-focusing lens. Theentireprocess including thepowder feeding,depositionsystem, scanning, temperature,atmosphere,andbuildarecontrolledbyamanufacturingsoftware. Theoverallprocessing stagesofSLMinclude(i)designinga3Dobjectmodelof thecomponent tobefabricatedusingCAD software and then slicing thedesignedpart intomany layerswith every layer definedby a layer thickness (typically20–100μm); (ii) asubstrate isfixedandlevelledforpart fabricationonthebuild platform;(iii)aprotectiveatmosphere(e.g.,argonornitrogen)isfedintothebuildchambertominimize possible surfaceoxidationandhydrogenpickup; (iv) spreadingofa thin layerofmetalpowderby powderre-coateronasubstrateplatewithathicknessas theaforementionedslicedlayer thickness; (v) scanning/processing thepowderbedinapredefinedpattern inorder toproduce layerwiseshape basedon thegeometrydefinedby theCADmodel; and (vi) loweringof thebuildingplatformby apredetermineddistance and repeating the last two stages for successive layers of powder until the requiredcomponents are completelybuilt. InSLM,each layer is fabricatedbyfirst generating anoutlineof thekeycomponent featureswhich is referred toas contouringandsubsequently the powderwithin thecontour ismeltedusinganappropriatescanningstrategy.Once the laserscanning process is completed, specimens fromthesubstrateplateareseparatedeithermanuallyorbyelectrical dischargemachining(EDM). SLM is a depositionwelding processwherein the laser beammelts the powder particles in welding beads and showsphenomenausually observed forweldingprocesses [26]. Thephysical behaviourof theSLMprocess includesabsorption, reflection, radiation, andheat transfer,melting andcoalescenceofpowderparticles,phase transformations,amoving interfacebetweensolidphase andliquidphase,fluidflowcausedbythesurface tensiongradientandmass transportationwithin themolten pool followed by solidification, and chemical reactions [26,35]. Because of the use of 5