Seite - 114 - in 3D Printing of Metals

Metals 2016,6, 286 3.3. ThermodynamicBehaviorofMoltenPool The calculated temperature counter and velocity fields within the molten pool in the cross-sectional viewat variable laserpower are shown inFigure 4. According to the temperature contourplots, itwas clear that thepeak temperaturepresentedat the centerunderneath the laser beamwithGaussiandistributionanddecreasedradiallyoutward. Thepeaktemperaturewaselevated rangingfrom1350Kto1760Kbyincreasingthe laserpower.Meanwhile, the temperaturegradients were observed to be varied with variable laser power, resulting in the formation of Marangoni convectionwithin themoltenmaterial shownontherightsideofeachpicture inFigure4. It isevident that thevelocityof themoltenmaterial isenhancedby increasing the laserpower. This isattributedto thedecrease in thedynamicviscosity (μ)of themoltenmaterialandthesurface tension(γ) [17–20]. Thepeak temperatureof the simulationonSLM-processed compositeswas far below themelting pointof reinforcements, leadingto thereinforcements remainingsolid in themoltenpool. Thus, itwas reasonable to consider that the reinforcements within the molten pool tended to migrate under Marangoniconvection.Accordingly, the intensityof thevelocityof themoltenmaterial significantly affects thedispersionstatesof thereinforcements.AlowlaserpoweralsoweakenedtheMarangoni convection and the attendant thermal capillary forces, thereby slowing down the molten liquid flowandmigrationof reinforcements.Consequently, thenano-scaledsolidreinforcements tendedto aggregatewithin themoltenpoolunder theactionofeachparticle, resulting inasevereagglomeration of thereinforcingparticulates. By increasingtheapplied laserpower to150W,therecirculationwas intensified,andthedistributionstateof reinforcementcanthusbeenhanced.Asa large laserpower of200Wwasapplied, ahighpeak temperaturewasobtainedandresultantMarangoni convection wasgeneratedwitha lowviscosityandahighmotioningvelocity, favoring thesufficientdispersionof thereinforcementswithin themoltenpool.Ontheotherhand, repulsive forcesprefer tostrengthen betweenAl2Si4O10 reinforcementswhentheamountofAlmelt is sufficient in themoltenpool [21]. In thecombinedeffectofMarangoniconvectionandrepulsionforces, thedispersivenetwork-structure of theAl2Si4O10-reinforcedAlmatrix is accordingly obtained at the appropriate SLMconditions. Thus,areasonable laserpower isbeneficial forobtainingahomogeneousdistributionofreinforcement in thealuminummatrix. 114