To obtain an optimal powder bed condition for laser powder bed fusion (L-PBF), it is essential to clarify powder flow behaviors during the recoating process. In this work, the flow behaviors of Ti-6Al-4V powders were experimentally analyzed using a simple slope model by Particle image velocimetry (PIV) and were subsequently compared with the behaviors simulated using the similar model by discrete element method (DEM). The velocity of particles and the distribution were able to be obtained experimentally using PIV. In the DEM simulation, when the coefficient of restitution was increased, the angle between restituted particles and the slope and the average particle velocity were increased. Comparing those obtained by the DEM simulation, it was found that the velocity and distribution obtained by PIV analyses were close to those obtained by the DEM simulation at the coefficient of restitution of 0.3. These results suggest that the powder flow velocity analysis on the slope using PIV analyses is an effective method to validate the parameter of powder properties in DEM simulations.
The La-Co cosubstituted M-type strontium ferrite attracts attention as a base material for high-performance ferrite magnets. It is known that the uniaxial magnetic anisotropy of the material is enhanced by increasing the amount of Co by heat treatment under high oxygen pressure, but there is a problem in obtaining a pure sample. The present study investigated the conditions to obtain a single phase with increased La-Co substitution by heat treatments under several oxygen pressures. A single phase of M-type ferrite was obtained up to x = 0.35 at pO2 = 1 atm, and up to x = 0.65 at pO2 = 10 atm with the composition formula of Sr1-xLaxFe12-xCoxO19. The magnetic anisotropy is enhanced according to the Co concentration in these samples.
The fracture of WC-Co cemented carbides with different WC particle sizes was investigated experimentally, and then a discrete element method (DEM) simulation was performed considering such experiments. The bending strength of ultra-fine grained cemented carbide with a WC particle size of 0.4 μm was extremely high. The ultra-fine grained cemented carbide sample after bending test had numerous fracture pieces and the short length of the specimens without fracture. Initial structures consisting of matrix and defect (d) particles were used in the DEM simulations. The DEM simulation results for a sample of 2% d particle, which was considered ultra-fine grained cemented carbide by comparison with experimental results, were as follows: The sample with the highest load at fracture, the most d particle bond-breaking, and the shortest length of remaining specimen without particle bond breaking was the 2% d sample. The DEM simulation was considered to be very useful in understanding the ultra-fine grained cemented carbide’s strength and fracture behavior.
Joined plates of WC-Co cemented carbides with different Co content, WC grain size and carbon content were fabricated by sintering, grinding into rectangular plates, and then reheating (joining) the two plates together. Distribution of Co, microstructure and shape of such joined plates were investigated in detail. It was confirmed that Co migrated from higher Co content to lower one, from coarser WC grains to finer ones and from higher carbon content to lower one as shown in previous studies by Lisovsky and Fang. Shapes of the joined plates were distorted caused by volume change of each plate resulting from migration of liquid Co and solute WC through the interface of the joined plates. In the joined plates with different Co content, Co distributed discontinuously at the interface. This result can be explained by the fact that Co content at the both sides of the interface had been almost fixed just after two faces were joined. This study was done under the special condition using joined plates, but the results obtained suggest that the shape of industrial-made WC-Co cemented carbides should be distorted when distribution of Co becomes non uniform.
The uniformity of shrinkages and the distribution of grain sizes were investigated for flash-sintered 3 mol%Y2O3-ZrO2 compacts in direct current and alternating current (AC) electric fields using rectangular-shaped and circular truncated cone-shaped green compacts. The use of a high-frequency AC electric field enough to suppress electrode overvoltage was confirmed to be advantageous for uniform shrinkages, regardless of the shapes of the green compacts. However, the grain size distribution became larger in cone-shaped green compacts, the cross-sectional areas of which varied along the compact. The grain sizes in flashed cone-shaped green compacts were found to exhibit a near linear relation to current density, which provides a means for determining the grain size distribution upon the flash sintering of green compacts with different cross-sectional areas along an electric field.