| Magnesium alloy which known as the “green material” at the age of sustainable development in 21 st century, is also the lightest metal structural materials with high specific strength and stiffness. Widespread application of magnesium alloy in the field of bio-medicine, 3C products, transportation and aerospace has been reported. Therefore, the magnesium alloy has become the focus of attention and research in the world. However, the magnesium has not enough independent slip system under the room temperature deformation condition due to its close packed hexagonal structure, so the plastic of magnesium alloy is poor and the deformation id difficult at room temperature. In order to improve the plasticity and process performance of magnesium alloy, it is necessary to adopt the composite technology of large plastic deformation to refine grain of magnesium alloy effectively. So as to promote the wide application of wrought magnesium alloy in various fields.The traditional extruded magnesium alloy have the poor deformability and obdurability, in order to improve the deformability of its, nowadays, the large plastic deformation technology of equal channel angular extrusion(ECAP), which can achieve the large plastic deformation of bulk metallic material by a large pure shear deformation through the two intersecting equal channel, can adopt to prepare ultra-fine grain structure material effectively. However, it is needs multi-pass ECAP to obtain the high performance materials with uniform organization, and it is also prone to cracking during the ECAP process. Therefore, ECAP process is difficult to be applied in the industrial promotion. Based on the defect of ECAP process, we put forward and designed a composite cumulative deformation process which is combined equal channel angular extrusion with forward extrusion(hereinafter referred to as ECAP-FE), that means a forward extrusion mould with a certain ratio is connect after the ECAP cavity. Thus, a continuous extrusion cavity which set a variety of deformation process in one comes into being to achieve the cumulative plastic deformation of materials. Therefore, a new way to significantly improve the plasticity of magnesium alloy is found.First all, the material model of AZ31 magnesium alloy is established by secondary development technology in the software of MSC. Marc. And then, the program flow diagram of the microstructure prediction system of AZ31 magnesium alloy is established, and programming the microstructure model of AZ31 magnesium alloy by FORTRAN language, thus, the transformation process of grain size and the volume fraction of dynamic recrystallization of magnesium alloy are predicted successfully. The numerical simulation of AZ31 magnesium alloy model by ECAP-FE process is carried out. Finally, the extrusion press, the stress field, the strain field and the strain rate of AZ31 magnesium alloy during the ECAP-FE extrusion simulation process is analyzed by the finite element. The simulation results show that the ECAP-FE composite extrusion deformation process can improve the amount of deformation of materials and refine grain effectively, compared with the single ECAP process. In addition, the average strain is increased about 2 times during the ECAP-FE composite process, the equivalent non-uniform coefficient decrease greatly and the effective strain distribution is axisymmetric. Therefore, the ECAP-FE process can achieve the higher cumulative plastic deformation. Moreover, during the ECAP-FE composite process, the die surface of forward extrusion can provide effective back pressure for ECAP process, to reduce the shear zone area, making the deformation form closer to the ideal simple shear. The maximum strain rate after EACP-FE process is 0.9916. It is increased remarkable compare with the ECAP process. Thus, the finer and more uniform grain of AZ31 magnesium alloy can obtain by ECAP-FE composite cumulative plastic deformation process. The results provide a theoretical basis for the further study of ECAP-FE composite extrusion process. |
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