W Matrix Nanocomposites Prepared By Mechanical Alloying And Selective Laser Melting Fabrication

In this work, the micrometer-scaled W and TiC powder mixture and W, Ni and C powdermixture were fabricated by high-energy mechanical alloying (MA) combined with a selective lasermelting (SLM) process. The evolutions of constitution phases, microstructural features, powdermorphology and particle size evaluation, and chemical compositions of the milled powders at variousmilling times were studied. On the other hand, the influences of the laser scan speeds (ν) and resultantlinear energy densities (η) on the surface morphology, densification behavior, phase andmicrostructure development, microhardness and wear performance of SLM-processed parts. The mainconclusions obtained in the paper were drawn as follows:High-energy MA process was applied to prepare TiC/W nanocomposite powders andhomogeneous nanocrystalline W-Ni heavy alloy reinforced by the in-situ formed WC, starting fromthe micrometer-scaled W and TiC powder mixture and W, Ni and C powder mixture. The powderparticles experienced a continuous refinement during0-35h milling, due to the operation of afracturing mechanism, and the average crystalline size of W decreased. For a milling time larger than35h, the particles had a tendency to coarsen, as a cold welding mechanism predominated, and theaverage crystalline size of W became coarsened. The milling time of35h can be regarded as a criticalpoint corresponding to microstructure and mechanism transitions, and the milled products weretypically nanocomposite powders featured by the nanocrystalline matrix reinforced with the uniformlydispersed ceramic nanoparticles.The SLM of TiC/W nanocomposite powders was performed to prepare solid solution W matrixcomposites. At a relative low η, the insufficient densification and non-continuous scan tracks on thesurface were ascribed to the formation of micropores. At a relative high η, the formation ofdiscontinuous borders and thermal microcracks were determined by the elevated thermal stress of theliquid. Interestingly, at a reasonable η=2.1kJ/m, relatively uniform and continuous tracks with soundintertrack bonding were generally formed on the surface of SLM-processed parts, the obtainabledensification rate generally approached94.7%. In this case, the microstructure of SLM-processedparts exhibited distinctly columnar crystalline morphology, and the average diameter of the columnarcrystals was0.73μm.The SLM of W-Ni-graphite powder mixture was performed to prepare in situ WC/Ni2W4C (M6C)cemented carbide based hardmetals. The WC phase was developed via a multilaminated growth mechanism and it experienced block→shaped→triangle→elliptical morphological change ondecreasing the linear energy density. The amount of the formed M6C increased and its structurechanged from a granular form to a ring shape at a lower laser energy input. At a lower ν=0.8m/s andattendant high η=187.5J/m, SLM-processed cemented carbide based hardmetals possessed a highdensification level of96.3%, a maximum microhardness of1870.9HV0.1and an obviousimprovement of the wear performance.