Modelling and simulation of powder pressing with consideration of friction

Abstract: Finite element modelling and simulation of the powder pressing process can be used in the rational development of powder metallurgy (PM) components. Many of the nonlinear finite element (FE) codes available today can be used for powder pressing simulations. The critical features of such simulations include almost every aspect of nonlinear FE analysis. This may be material behaviour, large deformation and contact boundary conditions, as well as the temperature and density dependence of the process. It is thus important to use appropriate models which capture the event of pressing and make the analysis worthwhile. The objective of the research presented in the present thesis has been to tighten the connection between models of powder pressing and pressing reality. There are mainly two reasons for this action: to increase the predictive capability of the pressing simulations and to reach further understanding of the PM forming process. The sliding friction contact between the powder and the tool is known to prevent homogeneous densification in the die pressing of metal powders. It is also known that the coefficient of friction in the contact varies considerably during pressing. A friction model with a nonconstant friction coefficient has been developed in this research. The model has been fitted to data from friction experiments. The final density distribution and press forces have been shown to be depentend on the choisce of friction model. The influence of a continuously varying initial density distribution on the numericalsimulations has also been investigated. The final density distribution shows a significant dependence on the starting conditions. A modification has been made to the DiMaggio-Sandler cap model which takes into account density dependence in describing the cohesive strength of powder materials. As friction models have been included in the analyses, it has also been of interest to effectively calibrate the models. Here, the friction coefficient has been estimated by combining an experiment with modelling of the experiment. Two methods for assessment of friction are presented, an optimization approach and an analytical approach. The experimental data have been taken from the single-action cold pressing of a cylinder-shaped component. A procedure to estimate friction also in low density regime is presented. In modelling the hot isostatic pressing (HIP) of powder materials, different deformation mechanisms are active during the consolidation process. The early stage of consolidation is dominated by granular behaviour. In order to account for these early granular deformation mechanisms in the numerical analyses, a combined elasto-plastic and elasto-viscoplastic material model has been developed and tested. Analysis with the combined material model shows improved characterization of the initial deformations compared with results from using the elasto-viscoplastic model only.