镁合金板材弯曲工艺冷冲压英文文献和中文翻译(4)

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Fig. 9 shows cold deep drawn cup using the blank holder with a ring-shaped projection in an intermediate stage of the deep drawing as shown in Fig. 8(b). The LDR of the RUB processed specimens is 1.5 under present experimental conditions. However, compared to a circular cup deep drawing, the depth of cell phone housing is only 6 mm, thus the subsequent cold stamping process of cell phone housing is carried out using one-step and flat blank holder.

Fig. 10 shows the thickness strain at the angles of 0◦ (RD), 45◦ and 90◦ (TD) to the rolling direction of cold deep drawn cup for the RUB processed specimens. The valleys of the curves represent the sections of the cup corners. Despite of the different r-values in the three directions, the values at the cup corners are approximately the same. It is well known that the stresses in the hoop directions around the flange of the cup resulted in the increase in thickness during deep drawing. For the RUB processed sheets with a tilted basal texture, the thickness strain can be generated by basal <a> slip.

3.3. Cold stamping of cell phone housings

Preliminary experimental results demonstrate that the RUB process has an important influence on the stamping formability of AZ31 magnesium alloy sheets. Fig. 11 shows the results of cold stamping of cell phone housings. The as-received specimen was drawn unsuccessfully, as shown in Fig. 11(a). It can be found that the critical section at the punch shoulder was broken before the flange of the specimen was fully dragged into the die cavity. While the RUB processed specimen was drawn successfully, the critical section at the punch shoulder and the flange was excellent, as shown in Fig. 11(b). The experimental results show that the RUB process improved the shallow drawing formability of magnesium alloy sheets. Besides, certainly, cell phone housings can be obtained successfully in crank press using the RUB processed AZ31 specimens by the cold stamping process.

4. Discussion

G.S. Huang et al. (2009) revealed that mechanical properties and stretch formability of magnesium alloy sheets with a tilted basal

texture obtained by the RUB process were improved at room temperature. Agnew and Duygulu (2005) and Koike et al. (2003) have noted that for magnesium alloy sheets with a very strong basal texture, the width strain εw can be generated by prismatic <a> slip, while the thickness strain is generated by pyramidal <c + a> slip and twinning. Therefore, this led to the high r-value and the poor deformation capability of sheet thinning for the as-received sheets in the work of X.S. Huang et al. (2009). In contrast, the thickness strain of magnesium alloy sheets, with a tilted basal texture obtained by the RUB process, can be generated by basal <a> slip, which resulted in a lower r-value. It is generally expected that high r values favor sheet formability and will lead to higher limiting drawing ratios (Lee,1984). However, the RUB processed sheet exhibits a lower r value and better drawability at room temperature. The results indicate that the relationship between the r values and sheet formability of magnesium alloys should be interpreted in a different way than is usually done for cubic metals. The lower r value means the tendency of increase in the thickness strain, which favors the formability of the drawn cup corners. Previous studies (Cheng et al., 2007; Yi et al.,2010) have reached the same conclusion, but the relation between drawability and a lower r value is unclear and further research is needed. It is reported that the sheets with a favored texture for the basal <a> slip exhibited a superior formability in both stretch forming (G.S. Huang et al., 2009) and deep drawing (Cheng et al., 2007). Therefore, formability of magnesium alloy sheets can be improved by the RUB process weakening basal texture of the sheet. Compared with the as-received sheets, the Erichsen values of the RUB processed sheets increased to 5.90 from 3.53, which increased by 67% at most. The LDR of the RUB processed sheets can reach 1.5 from 1.2 which was proved in other study of Chino et al. (2006) at room temperature. The larger Erichsen values for the RUB processed sheets were attributed to the larger n-value and the smaller r-value, which enhanced the capability of sheet thinning during the stretch forming. For the as-received sheets, that formability is lower may be due to its smaller uniform elongation and larger rvalue, which restricted the plastic deformation accompanied with the increase in thickness at the flange during deep drawing (X.S. Huang et al., 2009). However, the RUB processed sheets exhibited excellent drawability because of the larger uniform elongation and the lower r-value. The superior formability of the RUB processed sheets at room temperature was also demonstrated in the present study by punching out cell phone housings successfully in crank press.