As a lightweight material, magnesium is being increasingly used for automotive parts. However, due to a hexagonal-closed-packed (hcp) crystal structure, in which only the basal plane can move, magnesium alloy sheets exhibit a low ductility and formability at room temperature. Press forming of magnesium alloy sheets is conventionally performed at elevated temperatures of 200°C to 250°C and thus is known as energy consumed forming. Therefore, in view of an energy saving forming technology, we study magnesium alloy sheet forming by a rotational incremental sheet forming (RISF) at room temperature, where the rotational tool generates local heat of specimen enough to accelerate plastic deformation. The flow curves of the magnesium alloy sheet are obtained and calculated at elevated temperatures, while the yield loci of the magnesium alloy sheet are measured at room temperature. Using RISF, a square cup of 80-mm width, 80-mm length, and 25-mm height is then formed from a magnesium alloy sheet at room temperature. In addition, the strain distribution is obtained and compared with the forming limit curve (FLC) by considering the effect of the tool radius and is found to effectively predict the forming limit of a magnesium alloy sheet in RISF.

No abstract

In order to predict the ductile fracture of rotational incremental forming for magnesium alloy sheet , a combination of kinematic and isotropic hardening law is implemented and ev aluated from the histories of ductile fracture value (I) by means of finite element analysis. Here, the criterion for a ductile fracture, as developed by OYANE, [J. Mech. Work. Tech. 4 (1980), pp. 65-81], is carried out via a user material, using finite el ement code. To simulate the effect of the large amount of heat generation at elements in the contact area due to friction energy of the rotational toolspecimen interface on equivalent stress-strain evolution in incremental forming, Johnson -Cook model was applied and also compared with equivalent stress -strain curves obtained by tensile test at elevated temperatures. The FE simulation results of ductile fracture was then compared with the experimental results of 80 mm × 80 mm × 25 mm square shape with 45°, 60°, and 80 mm × 80 mm × 20 mm square shape with 70° wall angles. The trend of FE simulation results were quite in good agreement with experiment results. Finally, the effect of process parameters e. g tool down -step and tool radius on the ductile fract ure value and forming limit curve at fracture (F LCF) were investigated using FE simulation results

Using lightweight materials is the emerging need in order to reduce the vehicle's energy consumption and pollutant emissions. Being a lightweight material, magnesium alloys are increasingly employed in the fabrication of automotive and electronic parts. Presently, magnesium alloys used in automotive and electronic parts are mainly processed by die casting. The die casting technology allows the manufacturing of parts with complex geometry. However, the mechanical properties of these parts often do not meet the requirements concerning the mechanical properties (e.g. endurance strength and ductility). A promising alternative can be forming process. The parts manufactured by forming could have fine-grained structure without porosity and improved mechanical properties such as endurance strength and ductility. Because magnesium alloy has low formability resulted form its small slip system at room temperature it is usually formed at elevated temperature. Due to a rapid increase of usage of magnesium sheets in automotive and electronic industry it is necessary to assure database for sheet metal formability and plastic yielding properties in order to optimize its usage. Especially, plastic yielding criterion is a critical property to predict plastic deformation of sheet metal parts in optimizing process using CAE simulation. Von-Mises yield criterion generally well predicts plastic deformation of steel sheets and Hill'1979 yield criterion predicts plastic deformation of aluminum sheets. In this study, using biaxial tensile test machine yield loci of AZ31 magnesium alloy sheet were obtained at elevated temperature. The yield loci ensured experimentally were compared with the theoretical predictions based on the Von-Mises, Hill, Logan-Hosford, and Barlat model.