Seite - 109 - in 3D Printing of Metals

Metals 2016,6, 286 toprepare ceramicparticle reinforcedaluminummatrix composites, coatings, or both to improve the corresponding properties of aluminum [2]. Normally, in order to enhance the performance of aluminum, the ceramic particles are employed as reinforcements to be directly added to the molten aluminum. For instance, the additive of Al2O3 and B4C ceramic particles were added to the aluminumcomposites to strengthen themechanical properties [3].However, owing to the considerablypoorwettabilitybetweenceramicsparticleandaluminum,apoor interfacialbonding betweenthereinforcementsandthematrix isgenerated,whichconsiderably influences themechanical performances of the AMCs. In the literature, the in situ synthesis of the reinforcements of the particle-reinforcedmetalmatrixcomposites isanewtechnique,which isachievedbyaddingelement and compoundpowder or performs into themolten aluminum. In the case of in situ synthesis, thedesiredreinforcedparticlescanbesynthesizeddirectlyusingthechemical reactionbetweenthe reactants in themelt [4]. Insitureactionprocesses involvingparticle-reinforcedcomposite systems eliminate interfacial compounds infavorofnucleationandgrowthfromtheparentmatrixphase to formmore thermodynamically stable reinforced compounds. Meanwhile, the compositespossess contaminant-free reinforcement/matrix interfaces, and the in situAl2O3 particles arefineandcan enhance the strength and ductility of the composite. Compared with the previous techniques, advantagesareas follows: (i) thedistributionof the insitureinforcementsaremorehomogeneous in thewholemicrostructuresandmore thermodynamicallystable; (ii) the insitureinforcementshave ametallurgicalbondingwith thematrix,whichresults inastrong interfacialbondingbetween the reinforcementsandthematrix. Recently, somemetaloxides (e.g.,Fe2O3,MnO2,andCuO)wereadded tomolten aluminum toproduceAl2O3 particle reinforcementwith ahighperformance, lowcost, andgoodwettability,which can take the external loadand thegood interfacial cohesionbetween Al2O3 and thematrix [4]. Inan insituAl/CuOcomposite system,oxygenatomsdiffusealong the powder surfaces and the lattice andgrain boundaries during synthesis, and anoxidefilmon the surfaceof ametalpowderhasa significant effecton thebondingpropertiesof thepowder. In this case, theoxidefilmcanaccelerate thebondingbetweenparticleswhenthefilmthickness is less than critical. The insituSLMreactionofAl/Fe2O3powdermixturesreleasedextraheatandcollaborated with laser energy, which can modify the visual surface and microstructural appearance or alter material characteristics,andthehardnessofcompositewas increased[5]. Further takingtheAl2O3 as the rawmaterial, thehigh-performanceAl2Si4O10 reinforcedaluminumwas insitusynthesized bypowdermetallurgytechniqueandtheresultantmechanicalpropertiesofAMCswasconsiderably enhanced[6].Althoughthepresenceofreinforcements insitusynthesizedinthealuminumcomposites canstrengthentheperformanceof thecorrespondingaluminummatrixefficiently, theconventional AMCsreinforcedbyinsitusynthesizedreinforcementsusingmicro-scaleparticlescommonlyhavea remarkablycoarsemicrostructurewithattendantpoormechanicalperformanceresulting fromaslow coolingrate in traditionalmanufacturingprocesses [7]. Additivemanufacturing(AM)/3Dprinting(3DP)refers to theprocessof fabricatingthree-dimensional near-net-shapedcomponentsdirectly frompowderparticles ina layer-by-layermanner. It enables anumberof technical andeconomicadvantages, e.g., improving the cost-competitiveness for low volume production, reducing the environmental impact of manufacture, and increasing design complexity [8]. Selective lasermelting(SLM),anewlydevelopedAM/3DPprocess, isbasedonthe principle ofmaterial incrementalmanufacturing and considered a promisingAM technology for metallicparts,due to itsflexibility in feedstockandshapes.SLMprovidespossibilities to fabricate the geometricallycomplexcomponentsbyuser-definedcomputer-aideddesign(CAD)fileswithout tools ormolds thatwouldbedifficult todevelopbyconventionalmanufacturingmethods. It enables the quick fabricationof thegeometry-complexcomponentsdirectly frompowders.Under the irradiation of thehigh-energy laserbeam,SLMfabricatesparts ina layer-wise fashionbyselectively fusingand consolidatingthethinlayersof theloosepowder[9].Generally, thegeometricallycomplexcomponents are fabricatedbySLMwithahighdimensionalprecisionandgoodsurface integritywithoutother subsequentprocess requirements,which theconventional techniques (e.g., castingandmachining) 109