Modelling and Simulation of Extrusion of Magnesium Alloys

Extrusion, as one of the bulk-metal forming processes, is of significant importance for the production of semi-finished components. For magnesium and its alloys, the technology for processing is available today, but there is still a fundamental lack in understanding the factors that determine the development of microstructure and mechanical properties during the process. Due to its hexagonal crystallographic structure, deformation mechanisms observed in magnesium alloys are rather different from those in fcc metals such as aluminium alloys. As a result, mechanical anisotropy and tension-compression asymmetry, i.e. unequal compressive and tensile yield stresses are observed. The resulting complexity in the yielding behaviour of such materials cannot be captured by conventional models based on J2 plasticity. A phenomenological model derived by Cazacu and Barlat accounts for the respective phenomena by introducing the third invariant of the stress tensor. However, processes such as extrusion involve complex thermomechanical and multiaxial loading conditions which include large strain, high strain rates and moderate increase in temperature due to deformation. The capability of Cazacu and Barlat model is limited in this regard since strain rate and temperature dependency on flow behaviour were not considered in their original work. The respective modifications to capture these phenomena were performed and implemented successfully as user defined subroutine, VUMAT, into commercial Finite Element software, ABAQUS/Explicit. In order to capture the rate dependency on plastic deformation, Cowper-Symonds overstress model was chosen. Adiabatic conditions were assumed considering the rise of temperature due to plastic deformation.