Processing fine magnesium materials. Numerical simulation and experimental analyses

Abstract

Currently developing elements for a renewable hydrogen storage and transportation is needed and its demand is rapidly increasing. Metal hydrides are among optimum solutions for hydrogen storage in terms of effectiveness and safety. The promising material which is good for such approach is magnesium and its alloys that can reversibly absorb hydrogen in quantities up to 7.6 % by weight which meets the DOE requirement. At first, determining a fast hydrogen saturation of Mg-based alloys has consisted in mechanical grinding of materials up to delivering the micrometric grain size. Increasing markedly the specific surface of the treated powders by plastic deformation processing leads to delivering very reactive samples contrary to bulk materials which are markedly un-reactive. More recently, great improvements of the H-sorption characteristics were demonstrated efficiently when applying the Equal Channel Angular Pressing (ECAP) treatments to bulk Mg-alloys. The implementation of ECAP process entails workpieces to pass the dedicated matrix formed by two channels crossing at a certain angle, i.e. from 90, 105 up to 120 degrees according to the degree of plasticity of the material. Effectively, the stress level delivered to the material depends on the angle between the channels, the applied pressure on the sample, the friction and (or) counter-pressure effects and obviously the physical and mechanical characteristics of the sample versus temperature. As the dimensions of the workpiece in terms of cross section are not changed, the deformation process can be applied several times successively, with the aim to achieve extremely high degrees of stresses and deformation. So, during such a severe deformation process the achievement of a fine-grained microstructure in the magnesium bulk samples is accompanied by the formation of a high density of defects and overall texture. When considering the ECAP process applied to the magnesium alloys, the numerical simulations were developed to anticipate the mechanical behaviour of the workpieces parallel to experimental characterizations using different methods such as structural and texture analyses. The present article reports on the deformation process Mg-based materials by using the numerical simulation method. By using the numerical LS-Dyna package for spatial deformed states we have successively applied ECAP operations in order to determine the optimum conditions of deformation delivering fine-grained Mg-materials with a high level of internal stresses. The results of the calculations are found in good agreement with the experimental data; and make it possible to use the proposed optimised process and adapted tool to up-scale the effective mass production.