Study On Microstructures And Properties Of Magnesium Alloy AZ31 Processed By Compound Channel Double Shear Extrusion

In this society, the superiority of deformed magnesium alloy has been recognized by many scholars. In this study, due to the poor plastic deformation of magnesium alloy at room temperature and the limitations of magnesium alloy extrusion methods, we combined forward extrusion, equal channel angular extrusion and change channel angular extrusion chich called compound channel double shear extrusion ( hot extrusion ratio was 16:1) to improve microstructure and properties of magnesium alloy AZ31.Large deformation can been made on magnesium alloy AZ31 which has been widely used, is the selection of this subject. The main topic contents of this experiment:①We carried out this extrusion at different temperatures, the microstructure and texture evolution of AZ31 magnesium alloy during hot compound DSE were researched, and we analyzed its mechanism during hot deformation.②Mechanical properties of as-extruded AZ31 magnesium alloy were tested at room temperature, and the deformation behavior ananalysis of its mechanism at room temperature was discussed.③S imultaneously, the fracture behaviors of different states of AZ31 magnesium alloy were researched which in order to determine the failure behavior.④As to as-extruded AZ31 magnesium alloy was carried out annealing process, at the same time, we analyze its microstructure and mechanical properties in order to get the most reasonable annealing process parameters about this new extrusion.⑤Finally, the compound DSE deformation process was simulated by the finite element software, and the analysis of deformation process parameters were discussed, such as extrusion pressure and the stress-strain of this extrusion.Experimental results: AZ31 magnesium alloy deformed by compound DSE shows significant grain refinement. The grain size of AZ31 magnesium alloy with deformation temperature decreases. The compressed yield strength and hardness with the grain refinement are improved. Due to the impact of texture and residual stress, the compressed rate changed little, but on the whole the strength and toughness of the AZ31 magnesium alloy worked by hot compound DSE press are integrated to improve. An innovation point of this experiment: inhomogeneity is a characteristic when magnesium alloy pressed by many extruded ways, that is to say there are obvious long grains along the extrusion direction. The uniformity of magnesium alloy microstructure has been significantly improved, which results from equal channel angular pressing after equal channel angular part. As to as-extruded at 350℃samples were carried out different annealing processes. We found that the best annealing process is 350℃for 2h, long original and mixed grains took place abundant recrystalliton, changed into equal-axis grains, Simultaneously, the thermal stress caused by squeezing and organizational stress were eliminated.Because magnesium alloy has relatively low stacking fault energy, so the magnesium alloys prone to dynamic recrystallization in hot extrusion process, and the grain refinement reduces as the extrusion temperature increasing. When the extrusion temperature and speed were 350℃and 2mm/s, the average grain size was 5μm, mic-hardness was 76.3, compressive yield stress was 156MPa, UCS was 390MPa, total strain-to-fracture rate was 18.1%, so the strength and toughness of magnesium alloy could effectively improved. Deformation mechanism of magnesium alloy at room temperature is mainly twinning. With increasing strain, the number of twins increased and the twin morphology and mode changed that the thick-lenticular shape{10—12}<10—11> tension twins were generated at onset of plastic deformation and narrow-band {10—11}<101—2> compression twins were formed at a late or final stage of deformation. Under the same strain, compression produced more twins because of the basal plane {0002} fiber texture. Simultaneously, the {0002} fiber texture affects tension/compression asymmetry of magnesium alloy.In addition, we use finite element software Deform-3D to simulate the extrusion pressure and the stress and strain distribution of the DSE. From the simulation process, the extrusion temperature and extrusion parameters such as mold play an important role. Our objectives are: to ensure both the mold and materials from destruction, but also provide laboratory equipment to meet the extrusion pressure conditions, the experimental material got refinement. Therefore, further simulation experiments provide some reference value and a good direction.