Study On Hot-rolling Cogging Process Of AZ31 Magnesium Alloy By Cold Rollers

Magnesium alloys can only deform through basal slip providing only two independent slip systems at room temperature due to its hcp structure and low stacking fault energy characteristics. As a result, it has poor plastic deformation capacity at room temperature and is not fit for cold deformation. The deformation temperature must be above the recrystallization temperature (above 250 ℃) for the production of magnesium alloy plate whether in cogging or finishing rolling. In addition, the rollers must be heated during hot rolling. If hot-rolling cogging with cold rollers is realized, the roller heating in cogging mill will be eliminated, the equipment and process will be simplified and the plate profile will be controlled well and the production efficiency will be improved, which will be meaningful for high efficient rolling of magnesium alloy sheets with low cost.In this study, AZ31B magnesium alloy was taken as the research object. The real-time continuous temperature detection and microstructure observation of rolled sheets were used to research the effects of technological parameters such as reduction, rolling temperature and rolling speed to temperature variation, microstructure evolution and mechanical properties of rolled sheets during cogging rolling process. The main conclusions of this research are as follows:(1) During cogging process of magnesium alloy, a large temperature drop appeared in the surface while the temperature increased in the 1/2H and 1/4H place in most cases at the sheet rolling moment. However the temperature in 1/2H and 1/4H place declined and the temperature in the surface increased until a minimum constant value due to the presence of inside thermal conduction after rolling;(2) The temperature change during rolling process was sensitive to both reduction and initial rolling temperature. The maximum instantaneous temperature rise during rolling process and the overall temperature rise after temperature uniformity both increased with both increased reduction and initial rolling temperature; At the same initial rolling temperature, the maximum instantaneous temperature rise during rolling process was not sensitive to roller temperature for relatively small reduction (<25～-35%), while was much less during cold roller rolling for relatively heavy reduction (>25-35%); The cold-roller rolling can decrease the finishing rolling temperature and this effect enhanced with increased reduction, especially when reduction is larger than 30～40%;(3) During hot-rolling cogging with both hot rollers and cold rollers, the overall temperature-rise technology zone and temperature-drop technology zone both appear which is affected by reduction and rolling temperature. This indicates that those two rolling technology both can achieve thermostatic rolling while a larger critical reduction for cold-roller rolling. At cogging temperature higher than 350℃, the temperature-rise technology zone decreases and the critical reduction increases with increased cogging temperature; At cogging temperature lower than 350℃, as for hot-roller rolling, the temperature-rise technology zone is not sensitive to cogging temperature and the critical reduction remains unchanged. On the other hand, for cold-roller rolling, the temperature-rise technology zone decreases and the critical reduction increases with increased cogging temperature while the ascending range is less than that of high-temperature region;(4) The comparison of temperature variation between model calculation and experiment data shows that the agreement of maximum instantaneous temperature rise during rolling is quite satisfactory, while the model-calculation values of overall temperature rise after temperature uniformity are larger than the experiment values, especially for large reduction rolling;(5) Pre-vertical compression before rolling can significantly restrain edge crack, increase microstructure homogeneity, refine grains and sharply weaken basal texture of rolled sheets. For high speed rolling, the microstructure homogeneity of rolled sheet weakens with increased rolling speed. The rolling speed has no significant effects on tensile strength and elongation, however affects yield strength and this effect depends on the rolling reduction distribution.