Page - 116 - in 3D Printing of Metals

Metals 2016,6, 286 Duringtheinsitureaction,theAl2O3particleactedastheoxygenresource,providingsufficient[O] atomsfor the formationof thenewphaseAl2Si4O10.Hence, thealuminumsilicatephaseprecipitated attachment to theremainingAl2O3particles. Figure 5. Schematic of the formation mechanism of the in situ Al2Si4O10 phase (a) and particle distribution(b)duringSLMprocessing. Based on the above experiment results, it has been concluded that laser energy input can significantlyinfluencethedistributionofex-situAl2O3andinsituAl2Si4O10particleswithinthemolten pool. Theunderlyingfactorcontributingto thedispersionstateof reinforcement is the inputenergy andresultantoperative temperature [23–25]. Foraclearerunderstandingof thedistributionevolution ofreinforcementparticles, thecorrespondingschematicsareshowninFigure5b.Atarelatively low laserpower, the temperaturewithin themoltenpool is limited, consequently increasingtheviscosity of themelt andweakeningMarangoni convection. Moreover, thenucleationsitesof theAl2Si4O10 phase are confined to the top region of themoltenpool. Under theweakMarangoni convection, Al2O3–Al2Si4O10multi-particlesdrive toughly towardtheboundaryof themoltenpoolandfinally agglomerate in thesolidifiedfront.As theapplied laserpower increases, theMarangoniconvection is remarkablyenhanceddueto theelevatedtemperaturewithin themoltenpool, thusacceleratingthe migrationof reinforcementparticles towardthebottompartof themoltenpool. Taking intoaccount the limited temperature rise, agglomerationstill occurswhenreinforcementparticlesaremigrated to themiddleregionof themoltenpool.As laserpowerfurther increases, thenucleationsitesof the Al2Si4O10 phase significantly increase;meanwhile, theMarangoni convection is strongenough to guarantee thehomogenousdistributionof reinforcementparticles. 4.Conclusions (1) The in situ Al2Si4O10/Al composites were fabricated via SLM. The new silica-rich phase (aluminum silicate, i.e., Al2Si4O10) was identified via XRD spectrum and EDX analysis. Thecomposites also presented a network microstructure, i.e., the Al2Si4O10 reinforcements aggregatedaroundthealuminummatrix. 116