Rolling speed dependence of cryogenic rollability of bimodal non-basal textured AZ31 Mg alloy sheet

Abstract

The rolling speed dependence of cryogenic rollability of bimodal non-basal textured AZ31 Mg alloy sheet has been investigated through multi-pass rolling experiments with a constant 5% thickness reduction per pass and characterization experiments via X-ray diffractometer (XRD) and electron backscatter diffraction (EBSD) techniques. The chosen rolling speeds include 30 mm/s (low-speed), 300 mm/s (mediate-speed) and 600 mm/s (high-speed). Rolling results show that the accumulated thickness reduction (ATE) of low-speed rolled sheet reaches up to about 22.6%, which is 58.0% bigger than that (14.3%) of high-speed rolled sheet. This means that the rollability of studied sheet under high-speed rolling is obviously worse than that under low-speed rolling. Characterization results confirm that high-speed rolled sheet possesses the smallest tilted angle (about 16.3°) between the maximum intensity point in (0002) pole figure and normal direction (ND) by comparison with other two sheets, leading to the enhancement of difficulty to accommodate the thickness plastic strain during rolling at liquid-nitrogen temperature. Moreover, high-speed rolling can not only realize the saturation of {10-12} extension twin (ET) nucleation at the beginning of cryogenic rolling, but also promote the growth of {10-12} ET nuclei. This issue benefits in improving the frequency of occurrence of Ortho-position (OP) -typed {10-12} ET variant interaction, which would block the motion of dislocation slip effectively. The deteriorated rollability of high-speed rolled sheet is mainly attributed to these two issues.