  The cohesive interactions between fine metal powder particles crucially
influence their flow behavior, which is in turn important to many powder-based
manufacturing processes including emerging methods for powder-based metal
additive manufacturing (AM). The present work proposes a novel modeling and
characterization approach for micron-scale metal powders, with a special focus
on characteristics of importance to powder-bed AM. The model is based on the
discrete element method (DEM), and the considered particle-to-particle and
particle-to-wall interactions involve frictional contact, rolling resistance
and cohesive forces. Special emphasis lies on the modeling of cohesion. The
proposed adhesion force law is defined by the pull-off force resulting from the
surface energy of powder particles in combination with a van-der-Waals force
curve regularization. The model is applied to predict the angle of repose (AOR)
of exemplary spherical Ti-6Al-4V powders, and the surface energy value
underlying the adhesion force law is calibrated by fitting the corresponding
angle of repose values from numerical and experimental funnel tests. To the
best of the authors' knowledge, this is the first work providing an
experimental estimate for the effective surface energy of the considered class
of metal powders. By this approach, an effective surface energy of $0.1mJ/m^2$
is found for the investigated Ti-6Al-4V powder. This value is considerably
lower than typical experimental values for flat metal contact surfaces in the
range of $30-50 mJ/m^2$, indicating the crucial influence of factors such as
surface roughness and chemical surface contamination on fine metal powders.
More importantly, the present study demonstrates that a neglect of the related
cohesive forces leads to a drastical underestimation of the AOR and,
consequently, to an insufficient representation of the bulk powder behavior.
