Thermo-Mechanical Modelling of Indirect Extrusion Process for Magnesium Alloys

Abstract

The complexity of deformation phenomena observed in magnesium and its alloys originates from its hexagonal closed packed crystallographic structure. Mechanical anisotropy and asymmetry in tension-compression are hence observed. A phenomenological model derived by Cazacu and Barlat accounts for the respective phenomena. However, the capabilities of this model for simulation of extrusion are limited since strain rate and temperature dependency on flow behaviour are not considered. Modifications including these phenomena have been implemented as user defined subroutine, VUMAT, into the commercial Finite Element (FE) software, ABAQUS/Explicit. Cowper-Symonds’ overstress model was chosen to capture the rate dependency of plastic deformation. A fully coupled thermo-mechanical analysis, in which the temperature is assumed as an additional degree of freedom, has been used for calculation of the temperature field by considering heat fluxes and inelastic heat resulting in softening. In order to describe the temperature and rate dependency of deformation, sets of upsetting tests were executed at different punch velocities and test temperatures. Simulations of upsetting tests were performed to fit model parameters by comparing with the corresponding experimental results and then to use as input data for simulations of indirect extrusion trials of cylindrical billets performed at different profile speeds. Temperature measurements during the extrusion trials by a thermocouple embedded in the surface of the die were used to calibrate heat transfer properties between the billet and the die. The FE simulations performed with the proposed model aim at a better understanding of evolution of anisotropy during experimental extrusion trials of magnesium alloys.