This paper presents the work-hardening behaviors of a rolled AZ31B magnesium alloy sheet during in-plane cyclic loading. The overall trend of the stress-strain curve was as follows. As established before, the yield stress under compression was considerably less than that under tension, and an inflected shape was observed in the stress-strain curve during the subsequent tension. Furthermore, an asymmetric evolution of work-hardening was observed as follows. The rate of work-hardening in the late stage of compression became gradually large with an increase in the number of cycles. Owing to this increase in the rate of work-hardening, the stress at the end of compression increased as the number of cycles increased. On the other hand, the rate of work-hardening in the late stage of tension became small as the number of cycles increased, yielding a decrease in the stress at the end of tension with the increase in the number of cycles. The results were almost the same when the cyclic loading test was carried out after tensile strain was applied to the sheet. On the other hand, when the cyclic loading test was carried out after compressive strain was applied, the increase in the rate of work-hardening in the late stage of compression was significantly more pronounced, whereas the inflected shape of the curve during tension was considerably less pronounced. The mechanisms of the above macroscopic behaviors were investigated in terms of twinning.

In this study, we carried out a two-dimensional draw-bending test on an AZ31B magnesium alloy sheet at various forming temperatures and blank holding forces, and the springback characteristics of the Mg alloy sheet were systematically examined. The amount of springback decreased with increasing temperature and blank holding force. The decrease in the amount of springback caused by the increase in temperature was much larger than that caused by the increase in blank holding force, which indicated that increasing temperature was much more effective for decreasing the amount of springback than increasing blank holding force. The amount of springback became negligible at 200 C and above. This result was attributable to the following factors: (a) flow stress decreased rapidly as temperature increased, (b) reverse bending on the sidewall arose at 150 C and above, and (c) fine grains due to dynamic recrystallization were formed at 200 C and above. Microstructure evolution during the draw-bending test was also observed with particular focus on twinning, and its effects on springback characteristics were studied in detail.