AZ31镁合金交替挤压成形规律研究

发布时间：2018-01-28 18:09

本文关键词： AZ31镁合金交替挤压微观组织工艺优化　出处：《哈尔滨理工大学》2017年硕士论文　论文类型：学位论文

【摘要】：镁合金具有低密度、高比强度及良好的再循环利用特性等系列优点,在轻量化、节能、环保等诸多方面受到青睐。室温条件下,密排六方的晶体结构,导致镁合金塑性较差,严重限制了镁合金的应用。挤压可以令金属处于三向压应力状态,能够充分发挥它们的塑性,进而获得较细的晶粒组织结构和很好的力学性能。因此,挤压工艺特别适于镁合金的制备和加工。节能减排、提高制品性能是挤压工艺不断改进及创新过程中长期关注的研究热点方向之一,为此,本文提出了一种新型挤压方法-交替挤压(AE)。交替挤压工艺突破了传统金属挤压工装制造中冲头为整体的设计理念,将其制成分体结构形式,即用两个或两个以上的分体冲头代替传统的整体结构在挤压过程中交替下行加载。尽管不同分体冲头交替下行加载对生产效率有些影响,但却收到了降低载荷和细化晶粒的实效。本文采用数值模拟与工艺实验相结合法,剖析了交替挤压与常规挤压在流动行为、变形特征及载荷变化等方面的异同;利用金相显微镜(OM)、扫描电子显微镜(SEM)以及电子背散射技术(EBSD)等手段,深入研究了不同挤压条件下晶粒尺寸的变化、断口形貌等。结果表明:交替挤压能显著降低挤压过程中的载荷值;并可改变挤压过程中各部位金属的流动顺序及模式,促使挤出流动均匀性的提高;经过交替挤压工艺后,织构的类型发生了改变,同时织构具有减弱的趋势,有利于削弱材料的各向异性。在挤压温度350°C、总挤压比为6和11的情况下,交替挤压可以显著细化晶粒,将原始坯料的平均晶粒尺寸,从385μm分别细化至6.5μm和5.3μm,并且晶粒细化能力随着挤压比的增加而增大;与此同时,经过交替挤压后,原始坯料的断裂方式发生改变,由脆性断裂向韧性断裂方向转变。交替挤压工艺还能使晶粒的位向发生改变,大角度晶界数量增加,有利于非基面滑移系的激活,有助于提高镁合金的塑性。尽管交替挤压中单道次内实际的挤压比减小了,但不同冲头交替加载时在交界面处可产生附加剪切应力,对微观组织及综合性能有重要影响。最后利用响应面方法,优化AZ31镁合金交替挤压成形工艺,并验证了模型的准确性。可以预知,随着该工艺理论的逐渐成熟,有望为形/性一体化挤压成形新方法研究提供新思路。
[Abstract]:Magnesium alloys have a series of advantages such as low density, high specific strength and good recycling characteristics. They are favored in many aspects, such as light weight, energy saving, environmental protection and so on. The plasticity of magnesium alloys is poor, which seriously limits the application of magnesium alloys. Extrusion can make metals in the state of three dimensional compressive stress, and can give full play to their plasticity. Therefore, the extrusion process is especially suitable for the preparation and processing of magnesium alloys, energy saving and emission reduction. Improving the properties of products is one of the hot research directions in the process of extrusion process improvement and innovation for a long time. In this paper, a new extrusion method, alternating extrusion (AEN), is proposed. The alternating extrusion process breaks through the traditional design concept of the punch as a whole in the manufacture of metal extrusion tools, and it is made into the form of split structure. That is to say, two or more split punch heads are used to replace the traditional integral structure in the process of extrusion alternately downlink loading, although different split punch alternately downlink loading has some effect on production efficiency. However, the effectiveness of reducing load and refining grain has been obtained. In this paper, the similarities and differences between alternate extrusion and conventional extrusion in flow behavior, deformation characteristics and load variation are analyzed by combining numerical simulation with technological experiments. The changes of grain size under different extrusion conditions were studied by means of optical microscope, scanning electron microscopy (SEM) and electron backscattering (EBSD). The results show that alternating extrusion can significantly reduce the load value in the extrusion process. The order and mode of metal flow in different parts of extrusion process can be changed, and the uniformity of extrusion flow can be improved. After the alternate extrusion process, the texture type has changed, and the texture has the tendency of weakening, which is favorable to weaken the anisotropy of the material, at the extrusion temperature of 350 掳C. When the total extrusion ratio is 6 and 11, the grain size can be significantly refined by alternating extrusion. The average grain size of the original billet is refined from 385 渭 m to 6.5 渭 m and 5.3 渭 m, respectively. The grain refinement ability increases with the increase of extrusion ratio. At the same time, after alternating extrusion, the fracture mode of the original billet changed from brittle fracture to ductile fracture. The increase in the number of large angle grain boundaries is conducive to the activation of non-basal slip systems and helps to improve the plasticity of magnesium alloys, although the actual extrusion ratio within a single pass decreases during alternating extrusion. However, additional shear stress can be produced at the interface between different punches under alternating loading, which has an important effect on the microstructure and comprehensive properties. Finally, the AZ31 magnesium alloy alternate extrusion process is optimized by using the response surface method. The veracity of the model is verified. It can be predicted that with the maturity of the process theory, it is expected to provide a new idea for the study of the new method of shape / sex integrated extrusion forming.
【学位授予单位】：哈尔滨理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG379

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