Thermo-mechanical modelling and simulation of magnesium alloys during extrusion process
Magnesium alloys have become promising materials saving structural weight and consequently reducing fuel consumption especially for transportation industry due to being the lightest metal for structural applications. Such alloys show anomalous mechanical properties such as deformation anisotropy and asymmetry in tension-compression originating from their specific crystallographic structure, which is hexagonal closed packed (hcp). One of the basic metal forming process for semi-finished products is extrusion. Since extrusion involves complex thermo-mechanical and multiaxial loading conditions resulting in large strains, high strain rates and an increase in temperature due to deformation, a proper yield criterion and hardening law is an important issue. A phenomenological model based on the Cazacu-Barlat yield potential has been proposed that takes strain, strain rate and temperature dependency on flow behaviour into consideration. A hybrid methodology of experiment and finite element simulation has been adopted in order to obtain necessary model parameters. Compression tests were executed at different punch velocities and test temperatures in order to describe the temperature and rate dependency on deformation. Simulations of compression tests were performed to fit model parameters by comparing with the corresponding experimental results. The anisotropy/asymmetry in yielding was quantified by tensile and compression tests of specimens prepared from different directions. The identification of the corresponding model parameters was performed by a genetic algorithm. The set of parameters having the minimum error with respect to the experimental results was assigned as the optimised solution. The information obtained from various means of material characterisation was taken as input data for simulations of extrusion trials. A fully coupled thermo-mechanical analysis has been used in extrusion simulations for calculation of the temperature field by considering heat fluxes and heat generated due to plastic deformation. The results of the approach adopted in this study appeared to be successful showing promising predictions of the experiments and thus may be extended to be applicable to other magnesium alloys or even other hcp metals.