Seite - 113 - in 3D Printing of Metals

Metals 2016,6, 286 3.2.MicrostructureCharacterzation Thetypicalmicrostructuresonthecross-sectionofSLM-processedcompositesatvariable laser powersareshowninFigure3.Asarelatively lowlaserpowerof100Wwasused, thecharacteristic microstructurepresentedinanetworkstructureandappearedtobeirregular.Moreover,aconsiderable number of irregularAl2Si4O10 reinforcementswere evidently observedwith a nearly nano-scale diameter.Theywerehighly inclinedtoconnectwitheachothercoherentlyandaggregatedintoclusters ina fairly largeregionof theSLM-processedcomposites (markedbywhitedash lines inFigure3a). SLMiswidelyknowntobecharacterizedbyanextremely largecoolingrateashighas106–8K/sin, a lowlaserenergydensity,andasmall lifespanof themoltenpool. In thissituation, reinforcements would spread insufficiently and tended toaggregate inmost of the regionsof the composites [12]. As the laserpowerwas increasedto150W, themicrostructurewasobservedwithanellipseshape, andtheAl2Si4O10 reinforcementsweredistributedalong theboundaryof theAlmatrix. However, a slightnumberofAl2Si4O10 reinforcementswereaggregated inacertain regionof thecomposites (markedbywhitedash lines in Figure 3b). As the laser powerwas elevated to 200W, the in situ Al2Si4O10 reinforcementswere exhibited along the boundary ofAlmatrix in a nearly sphere-like forming.Meanwhile, thenetworkmicrostructureofSLM-processedcompositeperformedregularly (Figure3c). ThisphenomenonwasattributedtoMarangoniconvection[2,17].Asa large laserpower wasused,aconsiderable intensityofMarangoniconvectionwasobtained,whichevidently favored thesufficientspreadingofreinforcements. Thus, thereinforcementscouldbedistributeduniformly underareasonable laserpower. TheAlSi10Mgmolten liquidwasformeddueto theactionsof laser beamsource irradiated thepowder-bedsystem,andunimpededmotionofmoltenaluminuminto the interconnectednetworkof channels and crevices occurs betweenAl2O3 particles via capillary action. Under this situation, theAl2Si4O10 reinforcements existing around theAlSi10Mgmatrix obviouslydisappear. Figure3.FE-SEMimagesof themicrostructuresofSLM-processedcompositesat100W,400mm/s(a); 150W,400mm/s(b); 200W,400mm/s(c). 113