Numerical investigation of the WC re-precipitation impact on the residual stress state in WC20 wt.-%Co hardmetal

In WC-Co hardmetals severe micro stresses form between the carbide and binder phases during the cooling stage of the liquid phase sintering process. The main cause of these micro residual stresses is the difference in the coefficients of thermal expansion in each of the constituent phases. There have also been discussions as to whether or not the solution and re-precipitation of elemental tungsten may be an additional driver for the temperature-dependent formation of residual stresses [1, 2]. To further investigate the influence of the re-precipitation effect on the residual stress state in hardmetals, a numerical approach combining the diffusion modelling capabilities of thermocalc/DICTRA [3] is applied in conjunction with temperature-dependent nonlinear mechanical analysis utilizing the finite element method. This combination allows for a qualitative estimate of the re-precipitation effect on the final residual stress state in hardmetals. Additionally, the temperature dependence of the residual stress state in hardmetals is investigated, with and without consideration of the solution and re-precipitation of W and C in the binder. A detailed 2.5D representative volume element based on EBSD images of a WC20wt.-%Co hardmetal has been derived for use in mechanical FE simulation. In the mechanical analysis, the cobalt binder is modelled as temperature-dependent elastic-viscoplastic isotropic material with isotropic hardening. Carbides are modelled as purely orthotropic elastic materials with temperature-dependent description of the elastic constants. The DICTRA results are applied as additional internal loads to the mechanical model via two equivalent coefficients of thermal expansion. A comparison of model results obtained with and without re-precipitation effect permits the impact of the re-precipitation effect on the residual stress state to be examined qualitatively. Finally, the model results are compared to experimental measurements of the residual stress state in WC20wt.-%Co hardmetal specimens, obtained via neutron diffraction.