Seite - 19 - in 3D Printing of Metals

Metals 2017,7, 2 Table6.Summaryofmicrostructural featuresandpropertiesachievedforvariousSLMedmagnesium alloypowders. Materials System GeneralMicrostructure/IntermetallicPhase Hardness (HV) Young’s Modulus (GPa) Surface Roughness (μm) Mg[56,57] * Equiaxedα-Mggrains,precipitatesofMgOalonggrainboundaries 60–97 (fromGPa) 20.8–38.2 - Mg[58] * - 66–74 (fromGPa) 29.9–33.1 19–33 Mg[59] - 44.7–52.4 - 38.6–51.8 Mg-9Al [60] Equiaxedα-Mggrains/Mg17Al12,MgO,Al2O3 66–85 - - AZ91D[61] Equiaxedα-Mggrains/β-Mg17Al12,Al8Mn5 85–100 - - ZK60[62] Orienteddendrites/MgZn,Mg7Zn3 78 - - ZK60[49] Dendritic/columnarα-Mg;Mg7Zn3 70.1–89.2 - - *Only single trackswereused (Details regardingprocessparametersused in these studies canbe found in Table2). Microstructures obtained inmagnesiumalloyswith SLMprocessing are comparable to that achievedbyother laserprocessingtechniquessuchas lasersurfacemelting(LSM)andselective laser surfacemelting(SLSM).Figure9showsthedifferent typesofmicrostructure thatcanbeachievedby laserprocessingofAZ91Dalloy. ThecommercialdiecastAZ91alloy (Figure9a) is composedofα-Mg solidsolutionandβ-Mg17Al12 lamellareutecticphasedistributedalongthegrainboundariesandits microstructurepresentslargegrainsduetotheslowcoolingrateofthecastingprocessused. Figure9b,c present the typical appearanceof the surfaceofAZ91alloy, aftermodificationbyLSMandSLSM, respectively. SurfacemodificationbyLSM(withalaserpowerof600Wandscanningspeedof45mm/s) revealedthat themicrostructurewascharacterizedbyverysmall isolatedβ-Mg17Al12 phaseparticles immersedinacontinuoussupersaturatedsolutionofAl inα-Mgmatrix,producingahomogeneous andcontinuousmodifiedlayer. SLSM,which isamodificationof theLSMprocess, canbeachieved by the application of lower laser energy input (375Wand90mm/s) causingmodification of the β-Mg17Al12 phaseonly,without anychange in theα-Mgmatrix. SLSMalso resulting inaeutectic microstructurebasedonfineplates ofβ-Mg17Al12 andα-Mgphase. Modificationβ-phase and its surrounding led toa reduction in itshardnessand itsdispersion, resulting inamorehomogenous andcontinuousmaterial. Figure9dpresents themicrostructureof SLMedAZ91 (at a laser energy inputof166.7 J/mm3),whichalsopresentsa featureofequiaxedα-Mgwithfullydivorcedeutectic β-Mg17Al12 distributed reticularly along thegrainboundaries. As themorphologyof the eutectic in thehypoeutecticMg-Alalloysdependsonthecoolingrate [102], change in the formofβphase betweenLSM,SLSM,SLM,anddie-castAZ91Dis inducedbythehighcoolingrates inherent to laser meltingprocesses. 19