Micromechanical Modeling of the Material Impact in the Feedstock Filament Properties for Indirect Additive Manufacturing (MEX) ^†

ASTM (52900:2015) classifies material extrusion (MEX) as an additive manufacturing (AM) technology that relies on the extrusion of a feedstock, which could be constituted by powder material and binder/additives. The material extrusion shaping process is based on a pressure-driven extrusion flow generated by applying a force directly to the feedstock, so the maximum system pressure (in the case of filament extrusion) becomes a function of the feedstock material stiffness and geometry. If a material does not meet the conditions to be forced through a nozzle at a defined rate and pressure, it will not achieve the required volumetric flow or stop due to print failure. Depending on the characteristics of the feedstock, failure can occur predominantly due to abrasive tears caused on the gears, total beak or buckling of the filament and clogging.

Highly filled feedstock (metal) materials used in SDS (shaping, debinding and sintering) processes are very complex systems with multimaterial formulations since they have to accomplish specific requirements for each process stage. This complexity makes the design of binder systems with targeted properties extremely challenging, requiring large experimental series to identify the effect of each material on the final properties. Some studies adjust the maximum metallic powder volume content (vol.%) in the feedstock by critical powder volume concentration (CPVC) and establish a relationship between the mixing torque value and filament extrudability. In contrast, others present volume content (vol.%) and shear rate results. Although it is necessary to create a feedstock with optimal debinding and sintering conditions, the ability to print the filament through a gear system without failure cannot be neglected.

The present study discusses micromechanical modeling of highly filled metallic powder-based feedstock (i.e., ferrous alloy SS316L (austenitic), H13 (ferritic/martesitic) and non ferrous as Copper with a polymer-based matrix (binder and additive)). This numerical determination of the elastic modulus and elastoplastic behavior of feedstock with different metallic powder volume contents (50, 55 and 60 vol.%) and different concentrations in the polymer matrix could reduce the process of large experimental series to identify the effect of each constituent in the feedstock printability. Hence, the micromechanic-based analytical model “Mori-Tanaka” of the Digimat-MF (mean-field homogenization) and the numerical finite-element modeling of the Digimat-FE (3D and 2D plain strain) using the concept of representative volume element (RVE) are applied in this study, and the results were validated by comparison with the elastic properties assessed by the impulse excitation technique.