Study On AZ31 Magnesium Alloy Forging Technology And Numerical Simulation

Magnesium alloys are regarded as the ideal engineering materials in the 21st century because of their low density, high specific strength and rigidity, good heat and electrical conductivity, therefore they have been used several years in the fields of aerspace, astronautic instruments, automobile and household appliances. But magnesium alloy has a hexagonal close-packed (hcp) crystal structure, so they are normally low formability near the room temperature. It is necessary to explore the hot working potential of this kind of materials. However, up to date the research of wrought magnesium alloys mainly focused on the extrusion and rolling. Little information about the deformation behavior under intermediate strain rate and dynamic load of AZ31 magnesium alloys after forging is avaible. In this paper, the deformation mechanics and microstructures evolution of AZ31 by upsetting at high temperatures will be studied and provide theoretical guidance to the technology making.The experimental material was AZ31 magnesium alloy after homogenizing treatment. The experiment contains two parts, hot compression and hot forging. In the first part, the true stress-strain curve were obtained in the temperature range 200～450℃and strain rate range 0.01～5s-1 by hot compress test to prepare for the establishment of AZ31 material model. The second part is the test of hot forging. Some samples were forged to 30% at different temperatures(200～450℃) , and other samples were forged to different strains(5～40%)at 350℃. The microstructures of hot-forged samples were observed by metalloscope, then measure the size and volume fraction of dynamic recrystallized grain. The compression failure tests were carried out with the samples of strains 30%, and the fractures were analyzed under SEM. Finally, deformation mechanism was investigated by using TEM. Besides,finite element simulation was taken to analyze hot forging process by DEFORM.The results showed that as the temperature was rising, the twins decreased but recrystallization fraction increased. At the same temperature, only when the strain more than a certain critical strain, could the recrystallization occur. Recrystallization fraction increased as strain increased. Compressive strength of samples after forged increased to 317Mpa at 350℃then reduced slowly when temperature was rising. Fracture morphology was observed to have characteristics of brittle cleavage fracture. For AZ31 alloy, twinning was the dominate deformation mechanism at middle-low temperature. Dynamic recrystallization occurred during the deformation of the alloy at high temperature with uniform microstructure and deformation mechanism is continuous dynamic recrystallization.Finite element simulation showed that plastic deformation dead zone was the center position of upper and lower end surface when upsetting. Severe deformation zone are the core and diagonal line of samples. The deformation temperature, velocity and friction coefficient have great influence on the upsetting load..