镁合金板材弯曲工艺冷冲压英文文献和中文翻译(3)

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3. Results

3.1. Mechanical properties

Fig. 4 shows that the true stress–strain curves of the as-received specimens and the RUB processed specimens in the tensile directions of RD, 45◦ and TD. Compared with the as-received specimens, the RUB processed specimens exhibit larger in-plane anisotropy, and the significant differences can be observed from the true stress–strain curves at the beginning stage of the tensile deformation. The work-hardening effects are stronger for the tensile specimens in the tensile directions of RD, 45◦ and TD after the yield deformation. The yield strength, tensile strength and the fracture elongation are shown in

Fig. 5. The tensile strengths of the RUB processed specimens are nearly the same as that of the as-received specimens regardless of the tensile directions. While yield strength of the RUB processed specimens is significantly lower than that of the as-received specimens especially in the RD. These results indicate that the RUB process has a strong effect on the yield strength but not the tensile strength. Additionally, the fracture elongations of the RUB processed specimens are improved in the tensile directions of RD, 45◦ and TD in comparison with those of the as-received specimens, especially in the RD with the largest increase from 19.2% to 26.7%. These are mainly due to the RUB processed specimens with stronger work-hardening effects which contribute to the increase in the fracture elongation. Above all, the inclination of the c-axis toward the RD lowers the yield strength but elevates work-hardening effects which contribute to improve the uniform elongation.

The r-value and the n-value of the as-received specimens and the RUB processed specimens are shown in Fig. 6. Compared with the as received specimens, the RUB processed specimens show a much smaller r-value and a larger n-value especially in the RD, which decreases from 2.15 to 0.92 and increases from 0.20 to 0.29, respectively. The difference between r-values as well as that between n-values of the as received specimens and the RUB processed specimens decreases with increasing the tensile angle. The average r-value (r ¯ = (rRD + 2r45◦ + rTD)/4) falls from 2.45 to 1.36, and the average n-value (n ¯ = (nRD + 2n45◦ + nTD)/4) rises from 0.175 to 0.225 in comparison with those of the as-received specimens. The decrease in r ¯ indicates that it is easier to reduce or increase the thickness of sheet during the plastic deformation. Furthermore, the improvement in the fracture elongation was mainly due to the high n ¯ which resulted in a low sensitivity to strain localization in the form of necking.

Fig. 7 shows cold deep drawn cups of the as-received specimens and the RUB processed specimens for DR = 1.5. The as-received specimens fractured at the punch shoulder, and the drawing depth was only 7.2 mm. However, the drawn cup of the RUB processed specimens showed a good appearance at a drawing depth of 11.8 mm. Compared with the as-received specimens, the RUB processed specimens show better stamping formability. These are mainly due to the RUB processed specimens with a tiled texture, which contribute to the increase in the drawing depth. If the drawing depth went up to 14.8 mm, the fracture occurred at the edge of the flange for the RUB processed specimens during deep drawing. Yang et al. (2008)

investigated die as shown in Fig. 8(a), the force was not applied onto the edge using the flat blank holder. To apply the force onto the edge even in passing though the die corner, the blank holder was exchanged for that having a ring-shaped projection in an intermediate stage of the deep drawing as shown in Fig. 8(b) (Mori and Tsuji, 2007). Additionally, for magnesium alloy sheets, the fracture happened in the top of the cup during bending–unbending as the material passes over the die radius. Those previous observations point out that compared with aluminum-alloy sheets (including AA2024, 6061,7075), magnesium alloys exhibit poor bending ductility due to their strong in-plane anisotropy and mechanical twinninginduced tension–compression strength asymmetry in two sides of the bending blank (Agnew et al., 2006). The blank holder with a ring-shaped projection is employed instead of the flat bank holder after the edge of the flange breaking out of the flat bank holder, which is helpful to improve unbending ductility of the sheet in the die corner.