Page - 6 - in 3D Printing of Metals

Metals 2017,7, 2 high-intensity laser, the powderparticles are heated at a faster rate as the energy is absorbedvia bothbulk-couplingandpowder-couplingmechanisms [36]. Theenergy is converted intoheatand eventually thepowderparticlesmelt, coalesce,andformanagitatedmeltpool forsomemilliseconds (typically0.5 to25ms). Themoltenpool formed,acquires theshapeofacircularorsegmental cylinder under theeffectof surface tension[37]. Extremelyshort interactiontimebetweenthe laserbeamand thepowderbedresults in the formationofa transient temperaturefieldwithahightemperatureup to105 ◦Candasignificantrapidquenchingeffectwithveryhighcoolingratesupto106–8 ◦C/s[38]. Rapidsolidificationmaycausedevelopmentofnon-equilibriummetallurgicalphenomenasuchas microstructuralrefinement,solidsolutionhardeningandtheformationofmetastablephases,whichcan haveasubstantial effect in improvingtheresultantmechanicalpropertiesandcorrosionresistanceof the laserprocessedmaterials [39,40]. LiteraturehasshownthatcomponentsproducedbySLMarecompletelydenseandhomogeneous withoutmicroscopic pores or voids and do not require any post-processing (such as infiltration withothermaterialsorheat treatment)usuallyneededto improvetheSLS(SelectiveLaserSintered) components,other thantheremovalofpartsandsupports fromthesubstrateplate.Anothermajor advantageof SLMlies in its high feasibility inprocessingnon-ferrouspuremetals likeTi,Al, Cu, Mg,etc.,whichtodatecannotbewellprocessedusingSLS[39]. Somecommonmaterialsthathavebeen investigatedforSLMinclude: ferrousalloys, titanium,cobalt-chrome,nickel, aluminium,magnesium, copper, zinc, tungsten, andgold [41]. SLMalsohas thepotential to produce components of very complexgeometrieswithagradientporositywhich in turnallowsthechoiceofpropertydistribution toachieverequiredfunctions [42]. SLMisunique in that it canbeusedfor theadditivemanufacturing (AM)of functionallygradedandpure-metalparts,aswellas for lasercladding/repair.Additiverepair ofdamagedturbineenginehot-sectioncomponents [43,44]madefromnickelbasesuperalloys isone example of such repairs. SLM is also capable ofmulti-material processing, i.e., utilizingdifferent feedstockmaterials simultaneously to produce various alloys and functionally gradedmaterials (FGMs) [45,46]. Themaingoal inSLMis toproducemetallicpartswith100%density.Obtainingthisobjective is noteasybecause there isnomechanicalpressureduringSLM,andfluiddynamics inSLMismainly drivenbygravityandcapillary forces alongwith thermal effects. Also, theabsenceofmechanical pressureduringtheprocessingmayleadtoreducedsolubilityofsomeelements in themeltduring solidificationcausingdiscontinuousmeltingofthetracksandformationofporesresultinginanuneven surface [47,48]asshowninFigure1a,wheredistributionofporosityandunmeltedareawithinZK60 sample produced by SLM is revealed. Thematerials also experience varying degrees of thermal fluctuationduring the SLMprocesswhichmay induce residual stresses in the lasermelted layer undergoingrapidsolidification[41]. Thiscanleadtoformationofhotcracksanddelaminationofparts asshowninFigure1b.Highheating/coolingratesduringSLMcanalsoleadtotheformationofnarrow heataffectedzone(HAZ)aroundthemeltpool. PresenceofHAZcanalter thecompositionand/or microstructureofmaterial influencingthequalityandpropertiesof theSLM-processedsample [28]. The transient thermalbehaviourduring theSLMprocesscanbecontrolledconsiderablybyprocessing parameters, suchas laserpower, scan speed, hatch spacing, layer thickness, andscanningpattern. Figure2providesan illustrationof theseprocessparameterscommonlystudied inSLM.Theseprocess parameters are adjusted such that a singlemelt vector can fuse completelywith theneighbouring meltvectorsandthepreceding layer.Applicationof inappropriateprocessingconditionscan leadto variousundesirableeffectssuchas irregularities inthesurfacemorphology, thermalcracks,andballing effects. Therefore, it is important toestablish therelationshipsbetweentheprincipalSLMparameters andsurfacemorphologyandtooptimize theSLMprocessingparameters toproducemetallicparts with100%densitywithoutcracksandfusiondefects. 6