Densification of sintered molybdenum during hot upsetting: experiments andmodelling

The densification behaviour of sintered molybdenum is investigated experime ntally and by modelling using a pressure dependent plasticity model. Theref ore the yield condition of Gurson, extended by Tvergaard is used. The uniax ial compression test is applied to determine the evolution of the density a s well as the stress-strain curves for the porous metal. Powder metallurgic al molybdenum exhibits closed porosity after consolidation due to sintering with nearly spherical shaped pores. The experimental results show that the densification, especially during the first stage of deformation, is differ ent from that of powder compacts or partially consolidated powder materials with open porosity. During hot upsetting, the pores change their size and shape. This behaviour strongly affects the densification rate. For an accur ate prediction of the evolution of the density using Gurson's model, the pa rameters q(1) and q(2) introduced by Tvergaard, will be defined as internal variables. The use of internal variables is justified by the fact that the pores change their shape during deformation, although the link between the internal variables and the pore shape is not explicitly established in thi s paper. If the loading is proportional (which means that the ratio of the stress-components does not change with plastic strain), the pore shape can be associated with the applied plastic strain. With this association the pa rameters qi can be defined as a function from the invariant quantity equiva lent plastic strain, which can be used as the internal variable in the fini te element simulation. The influence of the porosity on the flow stress at different levels of plastic strain will also be investigated and is used as a second information to fit both parameters q(1) and q(2). (C) 1999 Elsevi er Science S.A. All rights reserved.