Issue 37

G. Cricrì et alii, Frattura ed Integrità Strutturale, 37 (2016) 333-341; DOI: 10.3221/IGF-ESIS.37.44 333 Modelling the mechanical behaviour of metal powder during Die compaction process G. Cricrì, M. Perrella University of Salerno –Department of Industrial Engineering, Via Giovanni Paolo II 132, 84084. Fisciano (SA), Italy, gcricri@unisa.it ; mperrella@unisa.it A BSTRACT . In this work, powder compaction process was investigated by using a numerical material model, which involves Mohr-Coulomb theory and an elliptical surface plasticity model. An effective algorithm was developed and implemented in the ANSYS finite element (FEM) code by using the subroutine USERMAT. Some simulations were performed to validate the proposed metal powder material model. The interaction between metal powder and die walls was considered by means of contact elements. In addition to the analysis of metal powder behaviour during compaction, the actions transmitted to die were also investigated, by considering different friction coefficients. This information is particularly useful for a correct die design. K EYWORDS . Cap-cone model; Die compaction process; FEM simulation; Residual strain. I NTRODUCTION deep knowledge of metal powder behaviour during compaction stage is necessary to predict the final shape and density distribution of formed products, and to avoid damages and breakages, which may occur during the subsequent sintering phase. The design of die and of compaction process should be supported by an adequate simulation analysis of the mechanical behaviour of compacted material during the process steps. In literature, several constitutive, phenomenological and micromechanical models, based on the continuum mechanics approach, have been proposed to this aim. In general, most used constitutive models for compaction consider the powder material as granular or as porous . Soil composed primarily of coarse-grained sand or gravel is an example of granular material, whose behaviour is strongly influenced by the friction phenomenon. Instead, constitutive equations for modelling porous materials are usually extensions of the Von Mises plasticity model, satisfying conditions of symmetry and convexity of plasticity theory. In particular, extensions of plastic flow theory, by introducing additional state variables, are used for modelling different kinds of phenomena, from creep [1] to ductile fracture [2]. For porous materials, the limit function differs from the classical one of Von Mises criterion because of incompressibility of porous solid particles during the compaction stage. Thus, the plastic flow of porous particles is considered function of two terms, at least: the hydrostatic tensor and the second principal invariant of the stress deviator tensor. For these models, the general form of the limit surface is as follows:         2 2 2 1 ij f A J B I C K           (1) A