Research On Special Rolling Techniques Of Wrought Magnesium Alloy Sheet

Being the lightest metallic structural materials, magnesium alloys are considered as the environment-friendly engineering material with the greatest prospects for development in the 21st century. Especially, wrought magnesium alloy plates have broad application prospects due to their excellent combined properties. However, magnesium alloy plates prepared by the traditional preparation & plastic processing technologies always exhibit high fabrication cost, undesirable mechanical properties and forming ability at low temperatures due to the intrinsic close-packed hexagonal crystal structure and thus their devolpment is restricted to a great extent. Therefore, research and development of wrought magnesium alloy plates with low cost and high performance is a hot topic in the field of materials research today.The thought that improving the forming ability of magnesium alloys can be fundamentally achieved by grain refinement, grain orientation modification and control of dynamic recrystallization & twinning and the high-performance magnesium alloy plates can be prepared by the optimum rolling process in the combination with the pretreatment mentioned above is proposed to improve the rate of finished products and their ambient temperature plasticity. The AZ31 magnesium alloy is selected for the study on the special rolling technologies of magnesium alloy plates in the present study.Firstly, the conventional rolling technique of the extrusions prepared from the AZ31 ingots is studied in the consideration that the crystal orientation of original grains, dynamic recrystallization and twinning play the important role during the rolling process of magnesium alloy plates. The microstructural feature and the compression deformation behavior of the as-extruded AZ31 magnesium alloy are analyzed. The effects of the rolling parameters such as rolling temperature, reduction in each pass and crystal orientation of the original plate on the microstructure and mechanical properties of the AZ31 plate are also investigated. Moreover, the evolution of twinning during the hot rolling process as well as the microstructural feature and the mechanism of dynamic recrystallization is explored. In addition, the evolution regularities of the microstructure and mechanical properties of the as-rolled AZ31 plates are revealed during the subsequent annealling. The results show that:(1) The as-extruded AZ31 magnesium alloy exhibits a strong (0002) basal preferential orientation and an obvious ambient-temperature anisotropy and its rolling ability is evidently improved as the angle between the basal plane and the rolling surface increases. Grain refinement, higher ambient-temperature ultimate tensile strength and higher elongation are achieved with the increasing reduction in each pass. The hot-deformed microstructure of the AZ31 alloy is very sensitive to the original microstructure and exhibits rather an obvious microstructure hereditary effect. The properly gradual increase of reduction in each pass is beneficial to achieve the fine, homogenous dynamic recrystallization grains during the multiple pass hot rolling.(2) The interfaces of the{1012} tensile twins are liable to expand and deformation through dislocation slip is dominant within these twins. The interfaces of the {1011} compression twins are stable and these twins can refine grains by acting as the nucleation sites of dynamic recrystallization. The twin-induced dynamic recrystallization is dominant during rolling at low temperatures. In essence, it is dynamic recrystallization in the{1011} compression twins. However, the twin-induced dynamic recrystallization in the combination with the continuous dynamic recrystallization is dominant during rolling at medium temperatures, while the non-continuous dynamic recrystallization is dominant during rolling at high temperatures.(3) The AZ31 plates rolled at low temperatures exhibit the best microstructure after annealing at 200℃for 12min. Annealing below 400℃brings about the primary recrystallization and the latter has no obvious influence on the basal textures. However, annealing at 450℃induces the secondary recrystallization and the latter weakens the basal textures.Secondly, the different speed rolling technique of the extrusions prepared from the AZ31 ingots is studied to prohibit the formation of the basal rolling textures since the conventional rolling brings about the severe basal texture and the latter is harmful to the subsequent plastic processing. However, extrusion before rolling can improve the hot rolling ability of the AZ31 ingots. The deformation features and regularities of the different speed rolling with a small differece in the rolling speed and multiple passes are also analyzed. Moreover, the effects of the different speed rolling parameters on the microstructure and mechanical properties of the as-rolled plates are also disclosed. The results show that:(1) The different speed rolling with a small differece in the rolling speed and multiple passes can not only achieves the bigger accumulation of shear strain effectively, but also the equivalent strain distribution is more uniform than that of the highly different speed rolling.(2) The rotation ability of the basal textures is weakened and the forming ability is reduced with the increasing reduction in each pass during the different speed rolling with a small differece in the rolling speed and multiple passes. The intensity of the basal textures is weakened with the increasing total strain. The formation mode of new grains is varied from dynamic recrystallization to the deformation band and the intensity of the basal textures is reduced gradually. The best mechanical properties and forming ability can be achived by rolling along the D path. The intensity of the basal textures of the as-rolled plate is obviously reduced after annealing at 300℃for 60min and thus its forming ability at ambient temperature is further improved.(3) The AZ31 plates prepared by the optium different speed rolling process exhibits the elongation at ambient temperature up to 31.7%, about 49% higher than that of the plate prepared by conventional rolling. Its Erichsen value at ambient temperature is high up to 6.14mm, about 3 times higher than that of conventional rolling.Thirdly, the rolling technique of the directionally solidified AZ31 ingot is developed on the basis of the understanding of the influences of the microstructure and the grain orientation of the ingot on the rolling deformation to modify the microstructure especially the grain orientation of the ingot since the conventional rolling after extrusion exhibits the limitation for the preparation of the large-size plates and the traditional magnesium alloy castings are characteristic of coarse grains and coarse plate-like eutectics along the grain boundaries, which have a detrimental effect on the rolling. The microstructure, grain orientation, hot compression and hot rolling deformation features of the AZ31 ingots prepared under the different directional solidification conditions are investigated. On the basis of the understanding mentioned above, the constitutional equation of the directionally solidified AZ31 ingot during hot compression deformation and its hot rolling procedure are established. The results show that:(1) Directional solidification can inhibit the precipitation of the brittle phases along the grain boundaries in the traditional castings. The preferential orientations of the (1010), (1120) prismatic planes and the (1011), (1012) pyramidal planes are existent in the planar directionally solidified AZ31 ingots.(2) The strain rate sensitivity exponent m of the directionally solidified AZ31 ingot during hot compression is 0.19, much higher than that of the traditional casting reported in the literature (0.14) and comparable to that of the as-extruded state.(3) The rolling technique of the directionally solidified ingot can improve the hot rolling ability of the AZ31 alloy, shorten the process flow of rolling, lower the tendency of cracking and increase the rate of the finished AZ31 plate.