Effect Of Back Pressure On Equal Channel Angular Pressing Of A Magnesium Alloy

Magnesium alloys are among the lightest metallic materials for structural applications. Their high specific strength and rigidness makes them attractive for a broad range of technical applications, particularly in automotive and aircraft industries as well as in electronic devices. However, magnesium alloys have poor formability and limited ductility at room temperature rooted in their hexagonal close-packed (HCP) crystal structure.Studies have shown that the low ductility of magnesium alloys at room temperature could be improved by grain refinement through equal channel angular pressing (ECAP). This process, however, concurrently results in decrease in yield strength especially in the early passes (-4P) inevitably when extruded rod with a strong base plane fibrous texture is used for ECAP, and limited increase of strength can only be observed after high passes of ECAP. On the other hand, for magnesium alloys, higher ECAP processing temperature can avoid cracks formation and propagation, and promote recovery and recrystallization which lead to less stacking in crystal defects. This adversely affects the ability of ECAP to refine grains as it depends on accumulated dislocations and other defects to form ultrafine grains. Therefore, grain refining effect is limited.Structural application requires both high strength and ductility which drives the development of new processes. Applying a back pressure during ECAP (BP-ECAP) could be a good candidate. Due to the hydrostatic pressure, difficult-to-work metals can be deformed at lower temperatures without cracking, resulting in a finer grain structure.Experiment and computer simulation were used in this paper. In experimental part, the constitutive equation is from regression of hot compression data; the deformation and fracture behaviors of magnesium alloy in lower temperature were obtained trough ECAP; both high strength and ductility in magnesium alloy were obtained at the same time by BP ECAP; microstructure evolution were studied by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) to establish the deformation mechanism of magnesium alloy during BP-ECAP; the mechanical properties of magnesium alloy after ECAP and BP-ECAP were tested by tensile and compression testing machine; the texture evolution and its effect on the microstructure and mechanical properties also were investigated by X-ray diffraction (XRD). On the other hand, the effect back pressure on deformation characteristics and the evolution of damage during equal channel angular pressing of a Mg-3A1-1Zn alloy were analyzed using finite element method (FEM). From this work, the following conclusions are drawn:(1) The application of hydrostatic pressure (backpressure) during ECAP of as-extruded AZ31 magnesium alloy results in the split of (0002) dominant texture component from one strong peak into two weak peaks. Transmission electron microscopic observation confirms the activation of slip leads to the split of texture peak. This demonstrates that different deformation mechanism is activated by the application of hydrostatic pressure, which will help to open new window to understand the essence of plastic deformation under hydrostatic pressure, and which will help to open up a new way to improvement of plastic deformation capacity of magnesium alloys.(2) Compare with ECAP without backpressure, the strength and ductility in as-extruded AZ31 magnesium alloy were both obviously increased after ECAP with back pressure. The strong soft texture in the case of ECAP without backpressure is changed to a much weak texture with some transformed to hard orientation, and together with the stronger grain refinement strengthening effect, contributes to the higher strength and ductility in samples ECAP processed with backpressure. The problem that the strength of the as-extruded magnesium alloy samples after ECAP without back pressure always decrease can be solved by ECAP with back pressure, which will help to open up a new way to increasing the strength and ductility of magnesium alloys.(3) FEM in ECAP and BP-ECAP, especially the pressing of the crack generation and propagation was successfully carried out by establishing the constitutive equation and criteria for damage and fracture of AZ31 alloy, which provide strong support for the BP-ECAP experiments.(4) According to experimental results and simulation results, the ECAP pressing window of AZ31 alloy is established. The simulation results demonstrate that the application of hydrostatic pressure can effectively control the propagation of crack, even the formation of initial crack, and can actualize the ECAP deformation of AZ31 alloy at lower temperatures without cracking.(5) At the same deformation temperature and deformation velocity, compare with ECAP without backpressure, the application of hydrostatic pressure during ECAP of AZ31 alloy can remarkably enhance the grain refining effective, which can be attribute to the activation of slip, promote the accumulation of dislocations and other defects in magnesium alloy.