Page - 110 - in 3D Printing of Metals

Metals 2016,6, 286 cannotkeeppacewitheasily[10].Moreover,duringSLM,thetemperatureofthepowder-bedwherethe appliedhighenergylaserbeamis irradiatedreachesamaximumtemperatureof105K,andis followed byarapidcoolingatarateupto106–7K/sas the laserbeammovesaway,which involvesarapidly meltedandsolidifiedpowdermetallurgicalmechanismandaffects the forgingofnon-equilibrium phaseswithfinegrainedmicrostructureswithsuperiormetallurgicalproperties [11,12]. In thiscase, due to the extremely large cooling rate, a considerably fine microstructure is obtained, and the resultantperformancesareefficientlyenhanced. TheapplicationofSLMinthe fabricationofAMCsis expected to createnewlymechanical and technological opportunities, becauseof thepotential for developingnovel insitucompositeswithuniquemechanicalproperties. Previous researchhasmainlybeen focusedon the fabricationof high-performanceAMCsby SLMusingceramicparticles, suchasTiC[13],AlN[14], andB4C[15].Althoughtheabove-mentioned AMCsfabricatedbySLMenhancethemechanicalproperties,apoorbondingstrengthbetweenthe reinforcements and the aluminumwas also generated. To date, there are very few studies on in situoxidedispersion-strengthened(ODS)AMCsfabricatedbySLM,whicharebelieved topossess unmatchableadvantagescomparedwiththeconventionalcomponents.AlSi10Mgisnormallyused intheautomotive industry for itshighspecificstrength, inapplicationswhere fatigueperformance is also critical. Therefore, according to the excellentproperties ofAl2Si4O10, thispaper focuseson the in situ synthesizedAl2Si4O10 ODSAl-basedcomposites and theattendantgrowthmechanism. TheAl2O3/AlSi10Mgcompositepowderswerepreparedbytheprocessingofhigh-energyballmilling; subsequently, the Al2Si4O10/Al composites were produced by SLM to obtaining desirable parts withnovelmicrostructures.Theattendantchemical composition, themicrostructurecharacterization, andthe formationmechanismof thecompositeswerestudied indetail. 2.MaterialsandMethods 2.1. PreparationofSamples The gas atomized pre-alloy powders ofAlSi10Mgpowder (a purity of 99.7%)were used as the startingmaterialwith a near spherical shape and amean particle size of 30μm. TheAl2O3 powders (apurityof 99.5%)withapolygonalmorphologyandameanparticle sizeof 9μmwere usedas reinforcements. TheAl2O3/AlSi10Mgcompositepowdersweremechanicallymilledwith aweight ratioof80:20 inaFritschPulverisette4planetarymill (Idar-Oberstein,Germany),usinga ball-to-powderweight ratioof10:1,a rotationspeedof200rpm,andamilling timeof8h.Asdepicted in Figure 1a, the small-size reinforcements of Al2O3 were homogeneously dispersed around the AlSi10Mgcompositepowdersaftermilling. TheSLMexperimental systemwasdevelopedbyNanjing UniversityofAeronautics andAstronauticsandconsistedofanYLR-500-SMYtterbiumfiber laser (Burbach,Germany)withapowerof~500Wandaspotsizeof70μm,anautomaticpowder layering apparatus (Nanjing,China), an inertargongasprotectionsystem,andacomputersystemforprocess control. The schematic of SLMprocessing is shown in Figure 1b. The detailed processing of the concerned SLM procedures is described in [16]. The samples were fabricated with dimensions of8×5×5mm3.Moreover, thecorrespondingprocessingparametersofSLMweresetas follows: thepowerof the laserbeamwassettledat100W,150W,and200W,withascanspeedof400mm/s, aspotsizeof70μm,ahatchspacingof50μm,andapowder-bedthicknessof50μm. 110