  Spreading of fine (D50 <=20um) powders into thin layers typically requires a
mechanism such as a roller to overcome the cohesive forces between particles.
Roller-based spreading requires careful optimization and can result in low
density and/or inconsistent layers depending on the characteristics of the
powder feedstock. Here, we explore improved, roller-based spreading strategies
for highly cohesive powders using an integrated discrete element-finite element
(DEM-FEM) framework. Powder characteristics are emulated using a
self-similarity approach based on experimental calibration for a Ti-6Al-4V
0-20um powder. We find that optimal roller-based spreading relies on a
combination of surface friction of the roller and roller kinematics that impart
sufficient kinetic energy to break cohesive bonds between powder particles.
However, excess rotation can impart excessive kinetic energy, causing ejection
of particles and a non-uniform layer. Interestingly, the identified optimal
surface velocities for counter-rotation as well as rotational oscillation are
very similar, suggesting this quantity as the critical kinematic parameter.
When these conditions are chosen appropriately, layers with packing fractions
beyond 50% are predicted for layer thicknesses as small as ~2 times D90 of the
exemplary powder, and the layer quality is robust with respect to substrate
adhesion over a 10-fold range. The latter is an important consideration given
the spatially varying substrate conditions in AM due to the combination of
fused/bound and bare powder regions. As compared to counter-rotation, the
proposed rotational oscillation is particularly attractive because it can
overcome practical issues with mechanical runout of roller mechanisms. In
particular, the application to rubber-coated rollers, which promises to reduce
the risk of tool damage and particle streaking, is recommended for future
investigation.
