【作者】 胡红军；

【导师】 张丁非；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2010， 博士

【摘要】 镁合金被誉为21世纪资源与环境可持续发展的绿色材料,已成为世界各国普遍关注的焦点。镁合金由于其具有的六方晶体结构的特点,在室温变形条件下独立的滑移系少,导致室温塑性低,变形加工困难。目前,90%以上的镁合金是以铸件的形式获得应用,而不是像铝合金那样大部分以挤压材和板材的形式获得应用。未来镁合金的发展必将依靠变形镁合金产品的大规模生产应用,而变形镁合金产品的广泛应用必须依靠镁合金塑性加工技术的根本突破。主要针对传统的镁合金挤压棒材的变形能力比较差和强韧性差,大变形技术(如ECAP)又难以工业化推广,而且工艺复杂、成本高等常见问题,提出了一种新型的镁合金复合挤压方法,就是将传统的挤压(Extrusion)和大塑性变形方法等通道挤压(ECAP)相结合,也就是将压缩变径挤压(Extrusion)和剪切(Shearing)(一次或者连续二次)结合起来(简称ES)。发展了一种低成本变形镁合金的挤压技术原型,对镁合金棒材进行晶粒细化及织构控制,找到一种提高镁合金塑性变形的新途径,形成一些新型的镁合金复合成形理论。所取得的成果如下:采用现代塑性加工方法从应力状态、变形路径以及变形能等方面对镁合金变形行为进行了研究。ES挤压不仅具有一般挤压的特点,而且在局部受到四向压力,而且承受连续剪切力。建立了镁合金ES变形过程应力状态模型和滑移场模型,推导出了考虑摩擦和不考虑摩擦的包含一次压缩减径挤压和n次连续剪切的挤压力模型。根据能量守恒原理建立了ES变形过程中变形区的温度场温升数学模型。确立了ES变形过程中累积应变,建立了Zener-Hollomon参数和模具结构的关系。在正挤压阶段Z1参数与挤压速度v1、挤压比λ、铸锭半径R1温度T之间的关系为:在一次剪切阶段Z2参数和二次剪切Z3与挤压速度v2、棒料半径R2、温度T、剪切通道转角β、夹角ψ之间的关系为:根据ES变形的思想,设计并制造了适合于热模拟仪Gleeble1500D的一次剪切的ES挤压装置。基于Gleeble1500D热模拟测试,证明了ES挤压是可行的。从ES热模拟挤压过程的应力-应变曲线和挤压力曲线的特点,ES热模拟实验中镁合金发生了与一般动态再结晶过程不一样的再结晶过程,具有明显的两个动态再结晶阶段,被称为“双级动态再结晶”。在300℃、350℃挤压速度为2mm/s时,经过ES热模拟设备挤压后动态再结晶尺寸分别为2μm、4μm。在正挤压阶段,累积应变的值较小,动态再结晶的方式主要是不连续再结晶。在剪切阶段主要为连续动态再结晶机制。根据热模拟实验建立了ES变形过程中每个阶段Z参数(压缩减径阶段lnZ1和剪切阶段lnZ2 )和晶粒尺寸的关系: InZ1=0.36-0.002Ind ;InZ2=0.81-0.004Ind。借鉴多道次等通道挤压工艺的特点,设计并制造了多付适合工业卧式挤压机上的ES变形组合凹模(挤压比为32.1、18、11.6)。进行了ES工艺实验和中试生产。中试生产在挤压温度为420℃、400℃和370℃挤压速度为20mm/s时取得了成功,由于挤压机的挤压能力的局限,使得在350℃下没有挤压成功。对坯料的应力状态进行了计算机模拟分析,发现ES挤压过程局部坯料受到四向压应力,坯料所承受的压力和剪切力比普通挤压大,因此可以更有效的细化晶粒。初步利用计算机模拟的结果建立了ES挤压极限图,为ES挤压工艺参数的选择奠定了基础。针对ES挤压实验留存在ES模具内部的棒料(挤压比为32.1、18)进行了微观组织观察和计算机模拟。结果表明在较低温度下ES挤压可以得到尺寸很小的动态再结晶晶粒,挤压比增大可以有效的细化晶粒,挤压温度升高虽然可以提高再结晶的体积分数,但使得再结晶晶粒长大。挤压比32.1、挤压温度420℃的工艺可以得到小到1-2μm的细小晶粒;温度为450℃组织较均匀,但晶粒长大迅速,最终组织较粗大。针对中试生产(挤压比为11.6)的ES挤压和普通挤压棒料的不同位置进行了微观组织观察,发现在370℃和400℃的ES挤压可以有效的细化晶粒,不仅可以细化棒材表层晶粒,心部也得到了细化。在对于420℃下的ES挤压效果比普通挤压效果要差,主要原因是高温下ES挤压的温升比普通挤压高,使得晶粒长大。在具有有二次连续剪切的ES热变形过程中由模拟计算的挤压力-时间曲线,可以发现双级动态再结晶的现象,在ES挤压的起初阶段主要是不连续动态再结晶,在挤压压缩变径和转角剪切阶段为连续动态再结晶。ES挤压可以在一定程度上提高屈服强度、抗拉强度。ES挤压前块状的第二相在剪切后逐步变成弥散分布在Mg基体上的小颗粒。挤压和连续两次剪切使更多的晶粒取向发生改变,使得基面与非基面取向共存。更多还原

【Abstract】 Magnesium alloys are known as sustainable development of resources and environment green material in the 21st century, have become the focus concerned by all the countries in the world. Due to its hexagonal structure magnesium alloys can be deformed difficultly at room temperature for there are few separate slip system, which results in a low temperature plastic deformation processing. Currently, more than 90% of the magnesium alloys are used in cast state, but aluminum alloys are applied as extruates and sheets. In the future development of magnesium alloys will rely on large-scale production applications of wrought magnesium alloy products. While the extensive use of the wrought magnesium alloy products must rely on fundamental breakthrough in plastic processing technology of magnesium alloy.Deformation capacity、strength and toughness for the traditional extruded rods (profiles) of magnesium alloy is rather poor.Serve plasticity deformation technologies are difficult to be promoted to industrialize, and the processes are complicated, and costs are high. A new type of magnesium alloy composite extrusion method was presented which combines the traditional extrusion and the serve plastic deformation ECAP (equal channel anger pressing), that is to say extrusion and shearing (one or more than one) are combined (referred to ES).The ES may improve the industrialization preparation and processing of magnesium alloy rods (profiles). A kind of technology prototype with low-cost mass production has been developed to refine grains of magnesium alloys and control textures. And a way and new principles to improve the plastic deformation of magnesium have been found. New types of composite forming theories for magnesium alloy have been formed. The research results are as follows:Deformation behaviors of magnesium alloys were systematically studied including the stress state, deformation path as well as the deformation energy by using of modern plastic processing method.Stress states of magnesium alloy in ES deformation process have been analyzed.The differences between the ES extrusion and direct extrusion have been described.The billets are beared compression stress from the four-direction, then sheared by (once or multiple) continuous shearings.The required extrusion force per unit area with considering the deformation energy and friction has been derived.According to energy conservation principle mathematical models of temperature rise has been established in the ES deformation zone.Formulas of Accumulated strain and strain rate in the different phase during ES process have been established. Relationships between the Zener-Hollomon parameters and die structure parameters have been built.In the forward extrusion phase the function relationships Z1 parameter with the extrusion speed v1, extrusion ratioλ, the ingot radius R1, the temperature T as follows:In shearing stage the Z2 parameter function related to extrusion speed v2, rod radius R2, the temperature T, shear-channel angleβ, the fillet angleψis as follows:ES devices suitable for thermal simulation instrument Gleeble1500D have been designed and manufactured. A new recrystallization mode has been found out which was different from the traditional recrystallization process and known as the“the dual level dynamic recrystallization“in this paper. AZ31 magnesium alloy rods have been prepared at different temperatures based on thermal simulation devices. It was proved that ES extrusion was feasible. Microstructures have been observed and analyzed. The cast original 200μm can be refined into the 2μm, 4μm respectively with extrusion ratio 4 and preheat temperature 300℃, and 350℃respectively. And mathematical models of two-stage recrystallization have been established in ES process. In the extrusion stage, the accumulation strain value is smaller, dynamic recrystallization is mainly discontinuous recrystallization.Recrystallization mechanism in the shearing zones is continuous dynamic recrystallization.The relationship between Z parameters and grain size was established in ES deformation process based on thermal simulation experiment: InZ1=0.36-0.002Ind; InZ2=0.81-0.004Ind.According to ES thoughts and the deformation inhomogeneity in thermal simulation, characteristics of multi-channel channel and continuous extrusion process in industrial ES on the horizontal extruder, some ES combination dies used in industrial extrusion machine were designed and manufactured. Experimental and pilot production processes were carried out. AZ31 magnesium alloy billets had been extruded at different temperatures and different compression ratios. Pilot production was successful. The extrusion experiments with 18 extrusion ratio were successful, but the die was used only a few times.The analysis for stress state of the billets showed ES billet extrusion process by the four direction compressive stress, and billets were exerted continuous shear stress after the direct extrusion.The ES extrusion limit diagram was established preliminary for the ES extrusion process parameters with results of computer simulation and laid the foundation for the process choice. It was found from the results of ES extrusion that the limit diagram of the ES process can guide the ES process certainly.Finite element models were established according to ES process. It was found that there were many similarites between finite element simulation results and experimental results.1-2μm grain size of recrystallization grains can be obtained with extrusion ratio 32.1 and extrusion temperature 420℃. Microstructures were uniform with extrusion temperature of 450℃, but the grains grow rapidly, recrystallization grain size of about 6.3μm was obtained.Microstructures of ES and direct extrusion in pilot productions were compared. ES can not only refine grains on the surface effectively but the centers of the rods. ES extruded recrystallized fine grain size was obtained with ES extrusion temperature 370℃and 400℃, and the dynamic recrystallization volume fraction was much greater than direct extrusion.Typical recrystallization microstructures were appeared. But with the temperature increasing, the recrystallization and original grains grow rapidly at the extrusion temperature 420℃.In the ES during hot deformation, there are two obvious dynamic recrystallization stages, known as“the dual level dynamic recrystallization”. Discontinuous dynamic recrystallization happened mainly in the initial stages of ES extrusion. Continuous dynamic recrystallizations occur during continuous shearings.ES extrusion with low temperature can improve hardness (strength) obviously. ES can enhance magnesium alloy compression performance while raise yield strength, tensile strength.The second phase was sheared gradually and became granular during ES extrusion, and they were turned into 2-3μm particles dispersed in the Mg matrix. Two shearings made grain orientation change and basal plane and the coexistence of non-basal reorientate. There were several types of texture after extrusion, (0002) basal texture of the dominant position was reduced.更多还原