| The efficiency of Spark Plasma Sintering (SPS) is optimized by means of a temperature-based procedure. The correlations between temperature, porous material structure and tooling geometry are explored. The issue of agglomeration, cause of the formation of hierarchical porous structures in the powder compact, is addressed with an analytical and numerical approach. Densification kinetics, sintering stress, bulk and shear moduli of agglomerated compacts are modeled as functions of the hierarchy characterizing the porosity and the nonlinear viscous rheology of the material. The material nonlinearity is expressed by the strain-rate sensitivity parameter, which in turn is dependent on temperature. An empirically-based explicit formulation of strain-rate sensitivity as a function of temperature is provided. A subsequent tuning of the strain-rate sensitivity allows the optimization of the thermal regime in order to obtain in situ de-agglomeration of the SPS specimen or the production of tailored material structures. In order to guarantee that the optimized thermal conditions are experienced by the entire specimen, we study the geometry of the SPS tooling setup. The tooling influence on temperature distributions is assessed in both the radial and the axial directions by means of a combined experimental and finite-element study. The phenomena underlying the presence of thermal gradients are individuated and controlled by tailoring the design of the tooling setup. Novel geometries are proposed for the components of the tooling, capable to annihilate the temperature disparities and thus complete the optimization of the SPS procedure. A comprehensive finite-element framework, in which both agglomerated powder compacts and tooling setup can be reconstructed and improved, is therefore developed. |
