3.2 Effect of RUB process on shallow drawing

Preliminary experimental results showed that RUB process had an important influence on the stamping

formability of AZ31 magnesium alloy sheets. In this work, the previous conclusion was also verified by FE simulation. Fig.7 shows the FE results of shallow drawing of cell phone shell. It can be found that critical section, the punch-nose round corner as shown in Fig.7(a), was broken, before the flange of the as-received alloy sheet which produced a defect of wrinkling was fully dragged into the die cavity. However, due to the fact that thickness reduction at the punch-nose corner as shown in Fig.7(b) exceeded the fracture limit, the sheet underwent RUB process was finally drawn successfully. The FE results showed that the RUB process improved shallow drawing formability of magnesium alloy sheets. And cell phone houses can be obtained by cold stamping for the sheets underwent RUB process.

 Fig.7 FE simulation results of cold stamping for AZ31B magnesium alloy sheets: (a) As-received sample; (b) Sample underwent RUB process

3.3 Effect of blanking way on shallow drawing

Fig.8 shows the cold stamping parts of the AZ31 magnesium alloy sheets underwent RUB process in

different blanking ways. The quality of cell phone house shown in Fig.8(a) was excellent, by the way of blanking along planes coinciding with the RD (0˚). While critical sections of cell phone houses as shown in Fig.8(b) and Fig.8(c) were broken, by the ways of the other two kinds of blanking ways. It is anisotropy of magnesium alloy sheets underwent RUB process that made the material flow into the die cavity at different rates during the shallow drawing, which resulted in different thickness strains. 

 Fig.8 Cold stamping parts of AZ31 magnesium alloy sheets underwent RUB in different blanking ways: (a) Blanking along planes coinciding with rolling direction (0°); (b) Blanking along planes at angles of 45°; (c) Blanking along planes transverse (90˚) to rolling direction

The thickness strain distribution plays an important role in the research of fracture of magnesium alloy sheet during cold stamping process. When the thickness reduction exceeds the fracture limit, it is bound

to trigger fracture. Stamping parts shown in Fig.8 were cut along the diagonal, and the thickness was measured. Fig.9 shows the thickness strain distribution of cell phone shell in different blanking ways.

 Fig.9 Distributions of wall thickness strain of cell phone house obtained in different blanking ways: (a) Blanking along planes coinciding with rolling direction (0˚); (b) (c) Blanking along planes at angles of 45˚; (d) Blanking along planes transverse (90˚) to rolling direction

The simulated thickness is consistent with the experimental data. It can also be observed that the thickness of magnesium alloy sheets mainly reduces at the punch corner region. When the thickness strain exceeds 8%, the billets here are pulled to fracture. While a large region of the sheet thickness, in contact with the flat top of punch, remains invariable basically during cold deep drawing. The material inflows in the die cavity are mainly restricted by die structure and anisotropy of material, and the material reduced cannot be supplied rapidly by metal flow. It is easy to find that the effect of the blanking ways on shallow drawing of magnesium alloy of cell phone house is significant. Lubrication can also affect the quality of the sheet forming. Then, cold stamping was investigated by FE analysis and experiments, showing that specimens used for cold drawing process with animal oil lubrication were cut according to the way of blanking along planes coinciding with the RD, consequently cell phone houses could be successfully obtained during deep drawing. 

4 Conclusions

1) RUB process makes most of grains c-axis around the normal direction (ND) have a larger incline towards the RD. Basal texture is weakened. The formability is improved greatly at room temperature.