  A variety of tools can be used for spreading metal, ceramic, and polymer
feedstocks in powder bed additive manufacturing methods. Rollers are often
employed when spreading powders with limited flowability, as arises in powders
comprising fine particle sizes or high surface energy materials. Here, we study
roller-based powder spreading for powder bed AM using the unique combination of
a purpose-built powder spreading testbed with a proven method for X-ray mapping
of powder layer depth. We focus on the density and uniformity of nominally 100
micrometer thick layers of roller-spread Ti-6Al-4V and Al-10Si-Mg powders. Our
results indicate that when rotation is too rapid, roller-applied shear force
impedes the creation of dense and uniform layers from powders of high innate
flowability, or where inertial forces driven by particle density dominate
cohesive forces. Roller counter-rotation augments the uniformity of cohesive
powder layers, primarily though reducing the influence of particle clusters in
the flowing powder, which are otherwise shown to cause deep, trench-like
streaks. Companion discrete element method (DEM) simulations further
contextualize the experiments through isolation of the effects of cohesion on
layer attributes. Results suggest that roller motion parameters could apply a
strategic level of additional shear force to the flowing powder, thereby
mitigating the clumping behavior characteristic of highly cohesive feedstocks
while maintaining high layer uniformity.
