| Magnesium and magnesium alloys can stand out in the huge metal material family because of their internal advantages of light quality and excellent performance.In recent years,they have become the ideal choice of lightweight research direction in engineering applications,so there has been a research upsurge in many fields.However,hexagonal close-packed(hcp)structure determines the magnesium alloy poorer plastic deformation ability and lower strength at room temperature,which is difficult to meet the actual manufacturing requirements of engineering application.Therefore,in order to prepare high performance magnesium alloys,a lot of research has been carried out in recent years.In the extrusion process,the deformation zone has three directional compressive stress state and a large shear deformation at the core die orifice,which makes the processed products obtain fine grain structure and excellent mechanical properties.Therefore,the process is especially suitable for the processing of low plasticity magnesium alloy.How to realize the precise control of magnesium alloy microstructure has become magnesium alloy research bottleneck and deeper goals which to improve the production efficiency,comprehensive performance and practical range of magnesium alloy.Based on this,alternate forward extrusion(AFE)method is proposed,which innovative attempt was made to discrete design of punch structures in conventional extrusion(CE)and alternately downward loading.On the basis of early exploration and research,the optimum design of the die structure(especially the core die)was carried out to ensure the practicability and operability of the AFE.The unique feature of the AFE process is that the alternate loading of the split punch changes the flow behavior and sequence rule of the metal in the container,and produces additional shear deformation at the interface of the split punch.In this paper,the AFE process experiment of as cast AZ31 magnesium alloy was carried out,and the microstructure and deformation mechanism of the extrudates were analyzed by means of optical microscopy(OM),electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM).The results showed that the unique loading mode made metal flow sequence and behavior significantly changed during AFE.The additional shear deformation produced by the double-split punch structures resulted in a refining effect on the microstructure of the blank,which was then further refined during flow through the die orifice owing to shear deformation.Compared with the CE,the recrystallization process in the AFE produced grains that were smaller and more homogeneous in size.The recrystallization process was more abundant and the dislocation density was significantly increased,which is undoubtedly beneficial to the improvement of the comprehensive performance of the extrudates.The microstructure experimental results of AFE extrudates at different temperatures indicated that homogeneous fine-grained structure was obtained after AFE at 573 K.In the 573-673 K range,the yield strength,tensile strength and elongation of the composite mechanical properties are reduced accordingly with the increase of the forming temperature.The fracture morphology mainly changed from ductile fracture to brittle fracture,and the small grain obtained by dynamic recrystallization(DRX)gradually grew up and the dislocation density in the crystal decreased obviously.The proportion of the large-angle grain boundaries decreased significantly,which led to a decrease in tensile strength.When the temperature rises to 673 K,the dispersion of different shapes precipitated particles in the sample is also the other inducement to reduce the comprehensive mechanical properties.In conclusion,the AFE process could achieve fine-grained strengthening and load reduction,which provided technical support and scientific guidance for the engineering application of magnesium alloy extrusion forming technology. |
PDF Full Text Request