【作者】 夏伟军；

【导师】 陈振华；

【作者基本信息】 湖南大学 ， 材料加工工程， 2010， 博士

【摘要】 作为最轻的金属结构材料,镁合金被誉为“二十一世纪最具发展前景的绿色工程材料”,特别是变形镁合金板材以其优异的的综合性能展现出极其广阔的应用前景。但受到镁合金自身晶体结构的限制,传统的制备和加工技术生产镁合金板材时存在成本偏高、力学性能和低温成形性能不够理想等局限,从而很大程度上制约了变形镁合金的发展。因此,低成本、高性能变形镁合金板材的研制已成为当今材料领域研究的热点。为提高镁合金板材的成品率并进一步改善其室温塑性,本文采用AZ31镁合金为研究对象,提出了通过细化晶粒、调整晶粒取向和控制动态再结晶及孪生行为等前期处理,从根本上改善镁合金成形性能,再结合优化的轧制工艺制备高性能镁合金板材的思想,开展了镁合金板材的系列特殊轧制技术研究。首先,基于镁合金初始晶粒取向、动态再结晶和孪生行为在板材轧制过程中的重要作用,论文开展了AZ31镁合金铸锭挤压坯轧制技术的研究,分析了挤压坯的显微组织特征和压缩变形行为,研究了轧制温度、变形量和板坯初始取向对AZ31镁合金板材微观组织和力学性能的影响规律,探讨了热轧过程中孪晶的演变以及动态再结晶组织特征和机制,揭示了板材在退火过程中的组织、性能的演变规律,结果表明：(1)AZ31镁合金挤压坯具有很强的(0002)基面择优取向和明显的室温各向异性,挤压坯的轧制成形性能随基面与轧面之间倾角的增大而明显改善；随着道次变形量的增大,板材的晶粒组织细化,室温抗拉强度和伸长率增大；镁合金热变形组织对初始晶粒组织非常敏感,具有较明显的组织遗传效应；多道次热轧时,合理地逐步增大道次变形量有利于获得细小均匀的动态再结晶晶粒组织。(2){1012}拉伸孪晶的界面容易扩展,其内部主要发生滑移变形；{1011}压缩孪晶的界面稳定,可以作为再结晶的形核质点而细化晶粒。轧制过程中的动态再结晶在低温时以基于孪生的再结晶为主要机制,其实质为{1011}压缩孪晶内的动态再结晶；中温轧制时的动态再结晶机制包括基于孪生的动态再结晶和连续动态再结晶；高温轧制时为非连续动态再结晶。(3)低温轧制板材在200℃退火120min可获得最佳的晶粒组织,在400℃以下温度退火发生初次再结晶对板材织构影响不大,在450℃退火时发生二次再结晶使基面织构弱化。其次,基于铸锭先挤后轧虽可显著改善镁合金铸锭的热轧开坯性能,但挤压坯常规轧制时,板材仍形成强烈的基面织构而对后续成形极为不利的事实,以抑制轧制过程中基面织构的形成为目的,论文开展了挤压坯异步轧制技术的研究,分析了小异速比多道次异步轧制的变形特点和规律,揭示了异步轧制工艺参数对板材组织及性能的影响规律,结果表明：(1)小异速比多道次异步轧制不仅能有效地获得大剪切应变积累,而且等效应变分布比大异速比轧制时更为均匀。(2)小异速比多道次异步轧制时,随着道次变形量的增大,形成倾转基面织构的能力减弱,成形性能降低；随着总变形量的增大,基面织构的强度有所减弱；随着异步轧制温度的升高,新晶粒的形成机制逐渐由动态再结晶转变为形成变形带,基面织构强度逐渐降低；沿D路径轧制时可以获得最佳的综合力学性能和成形性能。对异步轧制板材在300℃下经60min退火处理后,基面织构强度显著降低,板材的室温成形性能进一步提高。(3)采用优化工艺制备的异步轧制AZ31镁合金板材的室温伸长率达31.7%,比普通轧制的提高了约49%;室温Erichsen值高达6.14mm,比普通轧制的高出3倍。第三,基于先挤后轧技术在制备大尺寸板材时的局限性,以及轧制成形受坯件显微组织和晶粒取向影响的认识,同时针对传统铸造镁合金晶粒组织粗大、易于在晶界析出粗大片状共晶相等对板材轧制的不利影响,以控制铸锭微观组织特别是晶粒取向为目的,论文提出了镁合金定向凝固坯轧制技术,研究了不同定向凝固方式下AZ31镁合金铸锭的晶粒组织、晶粒取向、热压缩和热轧制变形特点,建立了定向凝固铸件热压缩变形本构方程,制定了定向凝固坯热轧工艺规程,结果表明：(1)定向凝固能有效抑制传统铸造坯晶界脆性相的析出,在平面定向凝固镁合金中同时存在(1010)、(1120)棱柱面和(1011)、(1012)锥面择优取向。(2)定向凝固AZ31镁合金热压缩时的应变速率敏感指数m值为0.19,明显大于文献报道的普通铸锭的0.14,并且与挤压态的相当。(3)定向凝固坯轧制技术能明显改善镁合金的热轧成形性能,缩短轧制工艺流程,减小开裂倾向,提高板材成品率。更多还原

【Abstract】 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.更多还原