Structure-activity relationship on tensile properties under DRX and slip system activity of U-CVCDEed AZ31 magnesium alloy

• The U-CVCDE process can significantly reduce the grain size of AZ31 magnesium alloy. • The introduction of the pre-deformation process promotes the dynamic recrystallization in the CVCDE process and makes the recrystallization more fully. • The dynamic recrystallization behavior and the high activity of the cone slip system weaken the texture strength of the (0001) basal plane. • The deep modification of magnesium alloy was achieved by U-CVCDE process, and excellent comprehensive mechanical properties were obtained. 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.