Page - 22 - in 3D Printing of Metals

Metals 2017,7, 2 Figure11.Optical imagesshowingcharacteristic crystallinestructuresof lasermeltedZK60prepared atdifferent laserenergydensities: (a) 420 J/mm3; (b) 500 J/mm3; (c) 600 J/mm3; (d) 750 J/mm3 [49]. Reportsalso indicate that the typeandmodeof the laserbeamusedcanaffect themicrostructures formedinSLMprocessedmagnesiumas theresultantconsolidationmechanismofmetallicpowders isa functionofenergydensitydelivered[39].Ngetal. [57] comparedmicrostructuresof the tracks formed inSLMprocessingofmagnesiumpowders, processedunderboth continuous andpulsed modeof irradiation.Undercontinuouswave irradiation, lasermelting ledto the formationof fully recrystallizedgrains in themeltedzoneswithgrain sizes in the rangeof2.3–4.87μm(laser energy densitiesvariedfrom1.27×109 J/m2 to7.84×109 J/m2). Theα-Mgsinglephasesolidifiedintheform ofequi-axedcrystalsasseen inFigure12a.However, in thecaseof tracksmeltedunderpulsedmode of irradiation, incompletegrowthof theα-Mgphasewasobserved(Figure12b). Fullgrowthofα-Mg was inhibitedas the solidification rates achievedunderpulsedmodewashigher than continuous irradiation. Besides,due to theshorter interaction timeincaseofpulsedmode, there is insufficient time for the crystals to arrange themselves such that thermodynamic equilibriumprevails at the solid/liquid interface. Theaverage sizeof grainsobtainedunderpulsedmodewere smaller than thoseobtainedin thecontinuousmodelasermeltedtracks.Also, smaller laserspotsize (50–180μm) and layer thickness (typically 20–50μm)used in the SLMprocesswhen compared to other laser processingtechnologiessuchasdirect laserdeposition (DLD), laser rapidforming(LRF)andlasernet shapemanufacturing(LNSM)ledto the formationofasmallermeltingpool, therebyresulting in the formationofarefinedmicrostructure [29].Normally, layer thickness,alone,has little influenceonthe microstructure,butits influenceisdependentonotherparameters,suchaslaserpower,scanningspeed, specificenergydensity,andpowdermassflowrate. Forexample,as thespecific laserenergydensity is lowered, thinner layer thicknesswillbe required,as theenergyperunitarea tomelt thepowder is reduced.However, itwasobservedbySavalanietal. [58] thatdifferent layer thicknessesdirectlyaffect theoxygencontent in thematrixmaterial thereby resulting inphase andmicrostructural changes. OxidationoccurringduringSLMprocessingofmagnesiumatdifferent layer thicknesses rangedfrom approximately9.1 to11.7at%.The levelofoxidationwasfoundtobe inverselyproportional to the layer thickness,as itdecreasedfrom11.7%to9.1%withthe increase in layer thickness from150μm to300μm. 22