Simulation of residual stresses in cemented carbides

This paper outlines how the residual stresses in WC20Co hardmetal after cooling down from sintering temperature to ambient temperature are predicted by a numerical model. A mesoscopic, viscoplastic FEM approach is pursued with an artificial 2.5D representative volume element (RVE). The model has a hardmetal microstructure with a cobalt content of 20 wt.% and an average carbide grain size of 2 μm. It is assumed that the Co binder is subjected to viscoplastic deformation at elevated temperatures, whereas the WC-phase is assumed to exhibit linear elastic isotropic material behaviour over the whole temperature regime. The temperature-dependent material data for this binder phase is obtained experimentally from a coarse-grained model cobalt binder alloy, manufactured in a vacuum melting process followed by pressure assisted cooling to avoid porosity. The mismatch in the coefficients of thermal expansion is assumed to be the sole mechanism which induces a residual stress state. In order to obtain an eigenstrain state on which no artificial strains are superimposed, free expansion boundary conditions were applied to the unit cell in order to ensure periodic or linear displacement boundary conditions. Despite an incomplete description of the constitutive behaviour of the phases, the model provides acceptable results in comparison with experimental literature data obtained via neutron diffraction experiments that allow for a correlation of the stress evolution over temperature. A partially viscoplastic model enables the dependence of residual and internal stresses on cooling rate to be determined. The results are made more representative by averaging stress values over more than one geometrical model.