Given the limitations of traditional hot extrusion methods in improving the microstructure and mechanical properties of magnesium(Mg) alloys, this paper attempts to treat AZ31 Mg alloy billet by pre-upsetting continuous variable cross-section direct extrusion (U-CVCDE). The effects of dynamic recrystallization behavior and slip system activity on texture evolution and mechanical properties of CVCDE Mg alloys with different pre-upsetting amounts were systematically analyzed. The results indicate that the introduction of the pre-upsetting process promotes dynamic recrystallization during the CVCDE process. The recrystallization proportion shows a trend of first rising and then decreasing with the increase of the pre-upsetting amount. Among them, the proportion of recrystallization grains in the U2-CVCDE-formed structural parts is as high as 88.3%. The average grain sizes of U1-CVCDE, U2-CVCDE, and U3-CVCDE were 6.01 µm, 4.90 µm, and 10.45 µm, respectively. In addition, following U-CVCDE, the pyramidal slip of each forming component consistently maintains a high level of activation and opening and dominates, making more grains deflect in the axial extrusion direction of C to varying degrees, which is conducive to the uniform distribution of stress in more grains during plastic deformation. The synergistic effect of dynamic recrystallization behavior and the high activity of the pyramidal slip system significantly weakened the (0001) basal texture strength, and the maximum basal texture strength showed a gradually decreasing trend, among which the base surface texture strength of U3-CVCDE formed parts was only 9.9. The U-CVCDE process is employed to achieve deep modification of Mg alloy, and excellent comprehensive mechanical properties are obtained; among them, the yield and tensile strength of U2-CVCDE are as high as 243.4 MPa and 317.5 MPa, respectively, and the elongation after breaking is up to 21.3%. This study introduces a practical new idea for investigating the extrusion forming technology of high-performance Mg alloys.

In this paper, the work hardening and softening behavior of AZ31 magnesium alloy sheets by hard plate accumulative roll bonding (HP-ARB) process in a specific temperature range was studied for the first time, and the cyclic stress relaxation test, EBSD, TEM and other characterization methods were used. When the rolling temperature is 350 ℃, the grain size of magnesium sheets is refined to 4.32 (±0.36) µm on average, and it shows an excellent combination of strength and plasticity. The tensile strength reaches 307 (±8.52) MPa and the elongation is 12.73 (±0.84)%. At this time, the curve of work hardening rate decreases smoothly and the degree of hardening is the lowest, and the amplitude of stress drop Δσ_p in work softening test is the smallest with the increase of cycle times, which shows that the well coordination between work hardening and softening behavior has been achieved. Research has found that the combined effect of grain boundary strengthening and fine grain strengthening enhances the yield and tensile strength of magnesium sheets after three passes HP-ARB process at 350 ℃. This is attributed to the high degree of dislocation slip opening in the pyramidal surface 〈a〉 and 〈c + a〉, which not only coordinates the c-axis strain of the entire grain, but also promotes the slip transfer of dislocations in the fine-grained region, significantly improving the elongation of the sheets. This study provides a new idea for the forming and manufacturing of high performance magnesium alloy sheets.

The changes in strain gradient induced by grain boundaries are crucial for enhancing the plasticity of gradient magnesium (Mg) alloys. The change of strain distribution influence by grain boundaries during plastic deformation of the gradient structure was examined. In this paper, the gradient structure AZ31 Mg-alloy plate with the surface fine grain (FG) to the center coarse grain (CG) was fabricated using hard plate rolling (HPR). The microstructure and strain distribution of Mg-alloy with a gradient structure were analyzed by electron backscatter diffraction (EBSD) and Digital image correlation (DIC) during uniaxial tensile. The findings indicate that the gradient structure sample (GS sample) displays a uniform strain distribution during the tensile process. Coarse-grain sample (CG sample) have obvious strain concentration, which leads to premature fracture. Based on EBSD characterization, low-angle grain boundaries (LAGBs) accumulates in the CG during plastic deformation. Orientation of CG tends to the (0001) basal. At the same time, the density of geometrically necessary dislocations (GNDs) inside CG has changed, which improves the Heterogeneous deformation induced (HDI) stress of gradient structure. During the uniaxial tensile, LAGBs accumulates in CG and changes the strain distribution of the gradient structure, which induces the accumulation of GNDs, and hence improving the properties of the GS Mg-alloy. These findings unveil the mechanism of strength-plasticity synergism of GS alloys from a new perspective and offer insights into the application of GS in Mg-alloys.

Accumulative alternating back extrusion was a potential fine-grain modification method. In this paper, it was an innovative attempt to develop high-performance magnesium alloy sheet by this process. Under the condition of 350 K, commercial AZ31 magnesium alloy was made into billet by accumulative alternating back extrusion, and then extruded into fine-grain magnesium alloy sheet. Through a systematic study of its microstructure and mechanical properties, the results showed that the initial state had an important influence on the evolution of the structure during extrusion. After accumulative alternating back extrusion to produce the billet, the grain size of the sheet obtained by extrusion was significantly refined, which was related to the accumulation of deformation and grain refinement during the alternating loading process. Grain refinement caused the proportion of dynamic recrystallization inside the sheet with 2 cycles of accumulative alternating back extrusion to drop to 27%. With the increase of extrusion cycles from 2 to 4, the high density of dislocations led to an increase in the proportion of dynamic recrystallization and finer grains. The texture changed from strong basal texture to weak bimodal texture. The results of uniaxial tensile test show that due to grain refinement and texture change, the yield strength was significantly reduced, and the plasticity was significantly improved. It was verified that accumulative alternating back extrusion was meaningful for subsequent processing, and it also provided scientific guidance for the development of fine-grained magnesium alloy sheet.

The interactive alternating forward extrusion (AFE) method can realize the change of texture type and the weakening of texture strength. Taking AZ31 magnesium alloy as an example, the texture evolution of interactive AFE was studied. The results show that all kinds of dynamic recrystallization (DRX) behaviors can weaken the texture to vary degrees. The weakening effect of twinning-induced recrystallization (TDRX) behavior was particularly significant. During the interactive AFE process, the c-axis of most grains rotated under the external force, and tended to be 90° angle with the ED direction, forming a stable fiber texture. In addition, with the increase of loading passes, the starting of {0001} <11–20> basal slip system became more and more difficult. The (10–10) texture formed by {10–10} <11–20> prismatic slip system after sixth passes was the main texture type. With the increase of forming temperature, the starting ability of {10–10} <11–20> prismatic slip systems increased, and the (10–10) texture formed by prismatic slip system above 623 K dominated.