Discrete element simulation and experimental study of powder spreading process in additive manufacturing

Abstract

Powders used in additive manufacturing (AM) are spread into 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

Jamming during particle spreading in additive manufacturing

“…l < 0) have positive velocity, indicating they are "dragged" forward by the blade. This dragging mechanism has also been reported by Haeri et al [22], who used it to guide the optimisation of blade geometry [25]. For l/D < −3 there is not much difference in particle velocity for different gap heights except for the largest one, where the dragging effect is smallest.…”

“…1 Particle shape has a significant impact on particle dynamics in the spreading process [22]. Therefore, accurate representation of particle shape is indispensable for reliable simulation of particle flow in additive manufacturing.…”

“…Haeri et al [22] investigated the effects of aspect ratio of rod-like particles and operation conditions on the bed quality. They found large particle aspect ratio and high spreader translational velocity could lead to low bed quality.…”

Powder Bed Surface Quality and Particle Size Distribution for Metal Additive Manufacturing and Comparison with Discrete Element Model

“…The total force was the sum of the forces on all spheres that make up the composite particle and the moment of inertia was computed by removing the contribution of the overlapped volume. Haeri et al [30] also decided to look at non-spherical shapes and focused on rod-shaped particles with different aspect ratios to simulate milled PEK/PEEK. They discovered that larger particle aspect ratios or faster spreader translational velocity resulted in worse surface roughness and less density, which would mean poor mechanical performance.…”

“…The vtk.py file used in LPP had to be modified to round extremely small numbers (~10 30 ) to zero so that Paraview did not encounter errors. Paraview also takes accompanying dump STL files that can help visualize the geometries at each timestep.…”