| The density of pure metallic magnesium is about1.7g/cm3. The density of the magnesium alloy is only2/3lighter than the density of aluminum alloy. Magnesium alloys have relatively high specific strength, specific stiffness and modulus of elasticity. Its cutting performance is good. Magnesium alloys have broad application prospects in the automotive, computer, appliance, aerospace and other fields. However, magnesium alloys are close-packed hexagonal structure. Its ductility at room temperature is poor, which seriously hampered the development of magnesium alloys. Equal channel angular processing was proposed in the1970s, and in recent years has been extensively studied as one of severe plastic deformation methods. Compared with other severe plastic deformation methods, materials without holes can be obtained by equal channel angular processing. The ECAP process is simple. Without changing the cross-sectional dimensions of the workpiece grains can be refined and material properties can be improved. The superplastic materials eventually can be produced. ECAP has a very high scientific value.In this paper the finite element simulation and experimental study have been done about ECAP of the square workpiece whose material is AZ31magnesium alloy. The aim of the finite element simulation is to understand the characteristics of equal channel angular processing, to obtain the optimization of the process parameters and to design die and experiment based on this. After the completion of the experiment, the microstructure changes of the workpiece before and after extrusion were observed using an optical microscope. The hardness tests and tensile tests were done and changes in the mechanical properties of workpiece of magnesium alloys before and after extrusion were analyzed by varying hardness values and stress-strain curves. Finally, it was summarized that the influence rules of ECAP in magnesium alloys and the reasonable parameters were determined.It was found that by finite element simulation the load-stroke curve in single-pass extrusion can be divided into three stages, namely the initial stage of the extrusion, the load increase stage and the load stable stage. By analyzing the distribution of effective strain(including the size and the uniformity of the equivalent strain)of the workpece, it was found that when the die angle was near90°and the die corner angle was near37°, a workpiece with more uniform deformation could be obtained and extrusion load size was more reasonable. The friction, as a harmful factor, should try to avoid. In the course of the experiment lubrication between the workpece and die should be done in order to improve die life.Through observation of microstructure of and mechanical properties test, it was found that equal channel angular processing can refine grains effectively improve the workpiece plastic. But it was difficult to achieve the excellent effect of the extrusion after one single pass. Evenly distributed organization could be obtained when deformation was enough after multiple passes. The hardness, tensile strength and yield strength declined and the elongation increased with the increase of the extrusion pass. When the pass was to a certain number, crystal grains refinement effect was no longer significant changes between adjacent passes. Different paths could also cause different effect. Through experimental analysis, when the workpiece gain refinement and the uniformity of tissue in the path B were good. Grains refined in the path A were elongated. There was no practical value. There were not only elongated grains but also equiaxed grains. The refinement effect was between path A and path B. |
PDF Full Text Request