| Magnesium alloys are known as sustainable development of resources and environment green material in the 21st century, have become the focus concerned by all the countries in the world. Due to its hexagonal structure magnesium alloys can be deformed difficultly at room temperature for there are few separate slip system, which results in a low temperature plastic deformation processing. Currently, more than 90% of the magnesium alloys are used in cast state, but aluminum alloys are applied as extruates and sheets. In the future development of magnesium alloys will rely on large-scale production applications of wrought magnesium alloy products. While the extensive use of the wrought magnesium alloy products must rely on fundamental breakthrough in plastic processing technology of magnesium alloy.Deformation capacity,strength and toughness for the traditional extruded rods (profiles) of magnesium alloy is rather poor.Serve plasticity deformation technologies are difficult to be promoted to industrialize, and the processes are complicated, and costs are high. A new type of magnesium alloy composite extrusion method was presented which combines the traditional extrusion and the serve plastic deformation ECAP (equal channel anger pressing), that is to say extrusion and shearing (one or more than one) are combined (referred to ES).The ES may improve the industrialization preparation and processing of magnesium alloy rods (profiles). A kind of technology prototype with low-cost mass production has been developed to refine grains of magnesium alloys and control textures. And a way and new principles to improve the plastic deformation of magnesium have been found. New types of composite forming theories for magnesium alloy have been formed. The research results are as follows:Deformation behaviors of magnesium alloys were systematically studied including the stress state, deformation path as well as the deformation energy by using of modern plastic processing method.Stress states of magnesium alloy in ES deformation process have been analyzed.The differences between the ES extrusion and direct extrusion have been described.The billets are beared compression stress from the four-direction, then sheared by (once or multiple) continuous shearings.The required extrusion force per unit area with considering the deformation energy and friction has been derived.According to energy conservation principle mathematical models of temperature rise has been established in the ES deformation zone.Formulas of Accumulated strain and strain rate in the different phase during ES process have been established. Relationships between the Zener-Hollomon parameters and die structure parameters have been built.In the forward extrusion phase the function relationships Z1 parameter with the extrusion speed v1, extrusion ratioλ, the ingot radius R1, the temperature T as follows:In shearing stage the Z2 parameter function related to extrusion speed v2, rod radius R2, the temperature T, shear-channel angleβ, the fillet angleψis as follows:ES devices suitable for thermal simulation instrument Gleeble1500D have been designed and manufactured. A new recrystallization mode has been found out which was different from the traditional recrystallization process and known as the"the dual level dynamic recrystallization"in this paper. AZ31 magnesium alloy rods have been prepared at different temperatures based on thermal simulation devices. It was proved that ES extrusion was feasible. Microstructures have been observed and analyzed. The cast original 200μm can be refined into the 2μm, 4μm respectively with extrusion ratio 4 and preheat temperature 300℃, and 350℃respectively. And mathematical models of two-stage recrystallization have been established in ES process. In the extrusion stage, the accumulation strain value is smaller, dynamic recrystallization is mainly discontinuous recrystallization.Recrystallization mechanism in the shearing zones is continuous dynamic recrystallization.The relationship between Z parameters and grain size was established in ES deformation process based on thermal simulation experiment: InZ1=0.36-0.002Ind; InZ2=0.81-0.004Ind.According to ES thoughts and the deformation inhomogeneity in thermal simulation, characteristics of multi-channel channel and continuous extrusion process in industrial ES on the horizontal extruder, some ES combination dies used in industrial extrusion machine were designed and manufactured. Experimental and pilot production processes were carried out. AZ31 magnesium alloy billets had been extruded at different temperatures and different compression ratios. Pilot production was successful. The extrusion experiments with 18 extrusion ratio were successful, but the die was used only a few times.The analysis for stress state of the billets showed ES billet extrusion process by the four direction compressive stress, and billets were exerted continuous shear stress after the direct extrusion.The ES extrusion limit diagram was established preliminary for the ES extrusion process parameters with results of computer simulation and laid the foundation for the process choice. It was found from the results of ES extrusion that the limit diagram of the ES process can guide the ES process certainly.Finite element models were established according to ES process. It was found that there were many similarites between finite element simulation results and experimental results.1-2μm grain size of recrystallization grains can be obtained with extrusion ratio 32.1 and extrusion temperature 420℃. Microstructures were uniform with extrusion temperature of 450℃, but the grains grow rapidly, recrystallization grain size of about 6.3μm was obtained.Microstructures of ES and direct extrusion in pilot productions were compared. ES can not only refine grains on the surface effectively but the centers of the rods. ES extruded recrystallized fine grain size was obtained with ES extrusion temperature 370℃and 400℃, and the dynamic recrystallization volume fraction was much greater than direct extrusion.Typical recrystallization microstructures were appeared. But with the temperature increasing, the recrystallization and original grains grow rapidly at the extrusion temperature 420℃.In the ES during hot deformation, there are two obvious dynamic recrystallization stages, known as"the dual level dynamic recrystallization". Discontinuous dynamic recrystallization happened mainly in the initial stages of ES extrusion. Continuous dynamic recrystallizations occur during continuous shearings.ES extrusion with low temperature can improve hardness (strength) obviously. ES can enhance magnesium alloy compression performance while raise yield strength, tensile strength.The second phase was sheared gradually and became granular during ES extrusion, and they were turned into 2-3μm particles dispersed in the Mg matrix. Two shearings made grain orientation change and basal plane and the coexistence of non-basal reorientate. There were several types of texture after extrusion, (0002) basal texture of the dominant position was reduced.
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