On the Modelling of Solid Phase Sintering of Hardmetal Based on Viscoplasticity

Abstract: This thesis is primarily concerned with issues related to the modelling of sintering of hardmetal, which is composed of WC-grains that are embedded in a Co-binder. The aims are to develop a constitutive model framework and computational tools by which it is possible, in a unified manner, to simulate the two subsequent steps of cold compaction and (free) sintering. Only solid phase sintering is considered, i.e. it is tacitly assumed that the sintering process takes place below the melting point of the binder material.

Free sintering is carried out without any external load. The "driving force" of the process (denoted the "sintering stress") arises from the change of surface energy between constituents and pores. From a continuum point of view, the sintering stress (that brings about the densification) acts as a hydrostatic pressure. An explicit expression for the sintering stress is derived from thermodynamics and from a simplified microstructure. This stress is applied in two ways: (1) As an external load on an "Representative Volume Element" (RVE), in a mesomechanical approach. (2) As a part of the effective stress that is employed in the constitutive equations in a continuum model.

An RVE-generator, that uses the initial relative density and the weight portions of Co and WC as input, is developed. The creep mechanisms of the constituents (WC,Co) are modelled using an incompressible elasto-viscoplasticity model with isotropic hardening and temperature dependent parameters. The RVE is calibrated using experimental data from free sintering and uniaxially loaded specimens. A thermo-hyperelastic-viscoplastic model with isotropic hardening and temperature dependent parameters is used as a continuum model. The particular model chosen is based on quasi-static and dynamic yield surfaces that are elliptic in the meridian planes of the stress space. This model is, after calibration and validation using experimental data, implemented into a commercial FE-code. Numerical investigations of the compaction and subsequent sintering of specimens are carried out.