Abstract:
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Experimental and numerical studies of powder
flow during the die filling stage in powder metallurgy cold
compaction processes are presented. An experimental setting
consisting of a horizontal pneumatically activated shoe,
a vertical die and high-speed video system has been designed.
The experiments show the existence of three flow regimes:
continuous, transitory and discrete, which are identified in
terms of the particle size, the morphology and the speed of
the shoe. In the continuous regime the powder flows in a progressive
manner but in the discrete one some perturbations
appear as a consequence of a shear band formation that forms
discrete avalanches. A numerical model, based on a ratedependent
constitutive model, via a flow formulation, and in
the framework of the particle finite element method (PFEM)
is also proposed. For the purpose of this study, the use of the
PFEM assumes that the powder can be modelled as a continuous
medium. The model, provided with the corresponding
characterisation of the parameters, is able to capture the two
fundamental phenomena observed during the filling process:
(1) the irreversibility of most of the deformation experienced
by the material and (2) the quick dissipation of the potential
gravitatory energy of the granular system through the inter-particle friction processes, modelled by the plastic dissipation
associated with the material model. Experimental and
numerical results have been compared in order to study the
viability of the proposed model. |