Seite - 115 - in 3D Printing of Metals

Metals 2016,6, 286 Figure 4. Simulated temperature counter and velocity fields within the melt pool in the cross-sectionalviewatdifferentprocessingparameters: (a) 100W,400mm/s; (b) 150W,400mm/s; (c) 200W,400mm/s. 3.4. FormationMechanismofSLM-ProcessedComposites Zawrah et al. [6] designed andpreparedAl2O3–SiC–mullite reinforcedAl-based composites byreactionsintering;meanwhile,newsilicaphaseAl2Si4O10was foundbydecreasing theamount of mullite. Zawrah considered that the formation of the Al2Si4O10 phase might be attributed to the reaction between Al2O3 and SiO2. Similarly, for the SLM-processed Al-based composites in this study, Al2O3 also acted as a raw material. To explicitly illustrate the formation mechanism of in situ Al2Si4O10 during SLM processing, the corresponding schematic diagram is shown in Figure 5a. As the laser beamfleetlymoves over the powder bed, powder particlesmelt rapidly, andthe temperaturewithin the formedtinymoltenpoolshowsaheterogeneousdistributiondueto theusageof aGaussianheat source.Consequently, a surface tensiongradient emergeddue to the existenceofa temperaturegradient, resulting in the formationofMarangoniconvection. Basedonan investigationbyGuetal. [22],Al2O3particlesarecontinuouslypushedandgatherunderthecombined effectsofMarangoniconvectionandtorquearoundtheparticle, formingaring-likestructureobserved inFigure3.According to theGibbs–Kelvin formula,fineembossmentexistingonthesurfaceofan Al2O3particlecangiverise toaremarkable increase insaturatedvaporpressure, thuscontributing to thepartialmeltingbehavior of theAl2O3 particle. As a result, part of thedissociative [Al] and [O]atomswerereleasedinto themoltenpoolandinteractedwithdissociative [Si]atomsinthemelt. Thereactioncouldbedescribedas 2[Al]+4[Si]+10[O]→Al2Si4O10. 115