  The quality of powder layers, specifically their packing density and surface
uniformity, is a critical factor influencing the quality of components produced
by powder bed metal additive manufacturing (AM) processes, including selective
laser melting, electron beam melting and binder jetting. The present work
employs a computational model to study the critical influence of powder
cohesiveness on the powder recoating process in AM. The model is based on the
discrete element method (DEM) with particle-to-particle and particle-to-wall
interactions involving frictional contact, rolling resistance and cohesive
forces. Quantitative metrics, namely the spatial mean values and standard
deviations of the packing fraction and surface profile field, are defined in
order to evaluate powder layer quality. Based on these metrics, the
size-dependent behavior of exemplary plasma-atomized Ti-6Al-4V powders during
the recoating process is studied. It is found that decreased particle size /
increased cohesiveness leads to considerably decreased powder layer quality in
terms of low, strongly varying packing fractions and highly non-uniform surface
profiles. For relatively fine-grained powders (mean particle diameter $17 \mu
m$), it is shown that cohesive forces dominate gravity forces by two orders of
magnitude leading to low quality powder layers not suitable for subsequent
laser melting without additional layer / surface finishing steps. Besides
particle-to-particle adhesion, this contribution quantifies the influence of
mechanical bulk powder material parameters, nominal layer thickness, blade
velocity as well as particle-to-wall adhesion. Finally, the implications of the
resulting powder layer characteristics on the subsequent melting process are
discussed and practical recommendations are given for the choice of powder
recoating process parameters.
