Modeling of Electric Current Assisted Sintering: An extended fluid-like approach for the description of powders rheological behavior

A general theoretical framework for investigating the rheological behavior of powder undergoing electric current assisted consolidation is proposed in this work. The most relevant phenomena occurring during ECAS process are taken into account. Consolidating powders are assumed to behave as a viscous fluid. Specifically, powder viscosity is expressed by the Bird–Carreau–Yasuda's model, which includes the power-law creep description typically adopted in the ECAS modeling literature. The proposed stress-strain constitutive law allows one avoiding a priori assumptions regarding the powder rheological behavior. The resulting system of differential equations cannot be solved analytically, so that the finite-element method (FEM) numerical technique is adopted. A numerical investigation of the effect of some representative model parameters on powder consolidation is performed and the possible different rheological regimes correspondingly taking place are identified. Influence of the applied mechanical load and zero-shear rate viscosity is studied and a suitable approach to map out the powder rheological behavior is presented. It is found that powder rheological response is not only determined by the intrinsic material properties but also depends upon the process operating conditions. As an example, high values of mechanical load along with low values of zero-shear rate viscosity induce a nonlinear viscous flow while Newtonian (linear) flow takes place for low values of mechanical load associated to high values of zero-shear rate viscosity. The proposed model should be considered as a first attempt to extend the rheological theory of sintering recently proposed in the field of electric current assisted consolidation to a broader range of shear rates, which, in turn, extends the application of this theory to a wider variety of materials and operating conditions.