Page - 111 - in 3D Printing of Metals

Metals 2016,6, 286 Figure1.Morphologiesof thehomogeneouslymixedAl2O3/AlSi10Mgcompositepowdersprocessed byballmilling(a); theschematicofSLMprocessing(b). 2.2. CharacterizationofComposition The SLM-processed specimens for metallographic examinations were cut, ground, and polished according to standard procedure, and then etched with a solution consisting of HF (1.0mL),HCL (1.5mL),HNO3 (2.5mL), anddistilledwater (95mL) for about 10 s.AnOlympus PMG3 optical microscope (OM, Tokyo, Japan) was used to observe surface morphology of the SLM-processed specimens. X-ray diffraction (XRD, Karlsruhe, German) was employed to identify the phase. Microstructures were obtained with a field emission scanning electronic microscope (FE-SEM,Tokyo, Japan), which was equipped with EDX for the determination of chemical compositions. 2.3.NumericalSimulation Tofurther investigate the thermodynamicbehaviorwithin themoltenpoolduringtheselective lasermeltingAl2O3/AlSi10Mgcomposite, aphysicalmodelwasestablishedusingafinitevolume method(FVM)andthecorrespondingphysicalpropertiesofmaterialswasproperlysettledaccording to [17]. The heat and mass transfer, the surface tension induced by the temperature gradient, andthemovementof laserbeampowerwithaGaussianenergydistributionwastakenintoaccount in thenumericalmodel.Moreover, the insitureactionofAl2O3 andAlSi10Mgwasconsidered,andthe resultantphase transition fromAl2O3 andAlSi10Mgto the insituAl2Si4O10 reinforcementswas taken intoconsideration, includingtheheat transitionduringSLM.Meanwhile, someassumptionswerealso addressed inourphysicalmodel: (1) Theevaporationofmolten liquidmaterialwas ignoredwhenthemaximumtemperatureof the moltenmaterialdidnotexceedtheboiling temperature. (2) The reflectivitywas regardedas a constant byvarying the laser absorptionof the aluminum alloypowder. (3) Theconductivityandmeltviscositywereconsideredtobe time-dependent. The effect of the applied laser power on the temperature distribution andvelocity fieldwas investigated. Thetemperaturecounterandvelocityfieldof theY-Zcross-section(X=150μm)were obtainedto investigate the influenceof the laserpoweronthe thermodynamicbehaviorwithin the moltenpoolduringSLM. 111