The effect of recoater geometry and speed on granular convection and size segregation in powder bed fusion

In metal additive manufacturing (AM), powder bed fusion technologies such as selective laser melting and binder jetting rely on the spreading of fine metal powder to build up the layers of a part. This work studies the complex interaction between metal powder particles and the recoater. We apply the Discrete Element Method (DEM) to a calibrated cohesive Ti-6Al-4V powder model, matching the particle properties to experimental measurements of size, shape and inter-particle forces. We simulate powder bed fusion recoating at varying speeds and layer thicknesses with two different recoater geometries: a toothed rake and a solid blade. Our results demonstrate that the recoating mechanism induces significant granular convection, with the recoater velocity and geometry strongly influencing the degree of particle circulation and size segregation. Notably a toothed rake recoater improved particle circulation – while the solid blade recoater increases size segregation within the heap. Furthermore the recoater was found to filter fine and coarse particles, allowing smaller particles to flow underneath the recoater, and larger particles through its teeth. Our results demonstrate the key mechanisms which drive granular convection during recoating and how the fine details of recoater geometry impacts surface layer roughness and final part quality.