Modelling of compaction of metal powder and damage accum ulation due to fatigue in powder materials

Sammanfattning: This thesis addresses powder manufactured (P/M) components, with special attention to the compaction process and to the sensitivity to damage accumulation due to fatigue within powder compacted specimens. Three dimensional simulations of the compaction process have been performed, using an explicit dynamic dilatant finite strain finite element code supplemented with the effects from contact friction within the die, modelled by Coulomb friction. The final porosity distribution as well as friction force characteristics obtained from simulations of compaction of specimens with circular and quadratic cross section were found to correlate with experimental findings. Two different porous material models where implemented using a viscoplastic formulation, the Shima Oyane and the combined Fleck – Kuhn – McMeeking (FKM) and Gurson material models. Parameters in the constitutive models calibrated from experiments were found to be adjustable so as to capture the overall final porosity distributions. The possibility to determine the final porosity distribution within complex geometries at different compaction speeds was demonstrated for gear wheels using the combined FKM and Gurson model. The damage accumulation due to fatigue loading has been modelled in two dimensions using the Gurson material model, supplemented to account for isotropic as well as kinematic hardening, rate effects and adiabatic heat accumulation. Gear root damage at the center plane and at the surface of a gear wheel have been studied, with initial porosity distributions transferred from three dimensional simulations. The damage accumulation process was found to be highly localized, which points to the importance of correctly determining the porosity at critical regions of a component, thus diminishing the importance of an accurate overall porosity distribution description. A life estimate model, relating the number of cycles to failure to the applied fatigue load and initial porosity was also presented.