Abstract
Powders used in additive manufacturing (AM) are spread onto a compact layer of particles for sintering and this process is repeated layer by layer to form the final products. Spreading of rod-shaped particles in realistic AM settings is simulated using the discrete element method (DEM) to investigate the effects of particle shape and operating conditions on the bed quality, characterised by its surface roughness and solid volume fraction. It is discovered that larger particle aspect ratios, Ar, or higher spreader translational velocities result in a lower bed quality, i.e. a larger surface roughness and a smaller volume fraction. The surface roughness increases monotonically with Ar. However, the volume fraction exhibits a maximum at Ar=1.5 for randomly packed powder beds that are formed by the roller type spreaders moving at low translational velocities. It is also found that a roller outperforms a blade spreader in terms of the quality of the prepared bed at the same operating conditions. The micro-structural analysis of the beds also shows particle alignment in response to the induced flow, which is qualitatively confirmed by a set of purposely-designed experiments. In addition, a shape segregation is documented for powders with mixed aspect ratios (Ar) such that particles with larger Ar tend to accumulate on the upper layers of the bed.
Highlights
Additive manufacturing has recently been exploited by various industries, such as automotive, aerospace and medical, as a novel production technology
Accumulation on a diagonal line still characterises elongated particles and the aspect ratio histogram shows a wide log-normal like distribution with mean value Ar ≈ 2.5 and a tail extending to Ar = 5. The results of these analyses show that elongated particles with a major axis and round edges are good approximations to the milled PEK/Poly-Ether Ether Ketone (PEEK) particles
First note that for all roller simulations a maximum volume fraction is observed at Ar = 1.5 which is independent of droller and is more pronounced at lower VrToller
Summary
Additive manufacturing has recently been exploited by various industries, such as automotive, aerospace and medical, as a novel production technology. Powder bed laser sintering (LS) or laser fusion is one of such promising AM techniques. It uses polymeric or metallic particles, heated to just below their melting temperature and spread on a fabrication piston to form a thin particle bed using a counter-rotating roller or a blade. A laser beam is focused onto the bed and scans a raster pattern of a single layer of the final part. After sintering the fabrication piston lowers the part slightly and a new layer of powder is applied. The process is repeated until the product is successfully fabricated [1]
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