基于Marc的ECAP-FE累积塑性变形工艺数值模拟研究

发布时间：2019-01-25 19:08

【摘要】：镁合金被誉为21世纪可持续发展的“绿色材料”,也是目前最轻的金属结构材料,具有很高的比强度,主要应用于医学、3C电子产品、汽车、航空航天等各个领域,已然成为世界各国关注和研究的焦点。由于镁合金具有密排六方结构的特点,在室温变形条件下独立的滑移系较少,因此室温下塑性差,变形加工困难,需要采用大塑性变形复合工艺才能有效的细化镁合金晶粒,以提高镁合金的塑性,从而改善其加工性能。以促进变形镁合金产品在各个领域的广泛应用。由于传统的镁合金挤压棒材的变形能力和强韧性差,采用等通道转角挤压大塑性变形技术(ECAP),即由两个相交的等径通道组成的模具并通过纯剪切方式实现块体金属材料大塑性变形的成形工艺,它能够有效的制备超细晶结构材料,但是,需要多道次等通道转角挤压成形才能获得组织均匀的高性能材料,并且在多道次挤压过程中还容易出现开裂现象,因此,这种大塑性变形工艺难以进行工业化推广。本文基于等通道转角挤压工艺的缺陷,提出并设计了等通道转角挤压与正挤压相复合的累积变形工艺(以下简称ECAP-FE),即在等通道转角挤压型腔后直接连接一个具有一定挤压比的正挤压芯模,由此形成一个集多种变形工艺于一体的连续挤压型腔,从而实现对材料的累积塑性变形。由此找到一种提高镁合金塑性的新途径。本文首先采用二次开发技术在MSC.Marc软件中建立了AZ31镁合金的材料模型,然后构建了AZ31镁合金微观组织预测系统的程序开发流程图,并利用FORTRAN语言对微观组织模型进行编程,成功的预测了挤压过程中AZ31镁合金的晶粒尺寸和动态再结晶体积分数的变化过程。将构建成功的AZ31镁合金的ECAP-FE挤压过程有限元分析模型进行数值模拟。最后对AZ31镁合金在ECAP-FE挤压模拟过程中的挤压力、等效应力场、等效应变场、等效应变速率进行有限元分析。模拟结果表明:与单一的ECAP挤压工艺相比,ECAP-FE复合挤压变形工艺能有效的提高材料的变形量,使材料产生累积变形,细化晶粒;并且,ECAP-FE复合挤压变形工艺所获得的平均等效应变提高了约2倍,等效不均匀系数有很大幅度的下降,且等效应变呈轴对称分布,因此ECAP-FE能实现较高的累积塑性变形;而且,在ECAP-FE复合挤压工艺中,正挤压的模面能够为ECAP挤压过程提供有效背压,减小剪切带面积,使之更接近理想的简单切变;使得经ECAP-FE挤压后的最大等效应变速率为0.9916,最大等效应变速率显著提高。由此表明ECAP-FE复合累积塑性变形工艺可获取更加细小、均匀的晶粒组织。此结果为ECAP-FE复合挤压工艺的进一步深入研究提供了理论依据。
[Abstract]:Magnesium alloys are praised as "green materials" for sustainable development in the 21st century. They are also the lightest metal structural materials at present. They have high specific strength and are mainly used in medicine, 3C electronic products, automobiles, aerospace and other fields. Has become the focus of attention and research around the world. Because magnesium alloy has the characteristic of dense hexagonal structure and there are few independent slip systems under room temperature deformation, it is difficult to deform and deform at room temperature, so it is necessary to use large plastic deformation composite process to refine magnesium alloy grain effectively. In order to improve the plasticity of magnesium alloys and improve their processing properties. In order to promote the wide application of wrought magnesium alloy products in various fields. Due to the poor deformation ability and strength and toughness of the traditional magnesium alloy extruded bar, (ECAP), is used to extrude large plastic deformation technology at equal channel angle. That is, the mould composed of two intersecting equal channels and the forming process of large plastic deformation of bulk metal material by pure shear, it can effectively prepare ultrafine crystal structure material, but, High performance materials with uniform microstructure can be obtained by multi-pass secondary channel angular extrusion forming, and cracking is easy to occur in multi-pass extrusion process. Therefore, this kind of large plastic deformation process is difficult to be popularized in industry. Based on the defects of the equal channel angular extrusion process, the cumulative deformation process of equal channel angular extrusion and forward extrusion (hereinafter referred to as ECAP-FE) is proposed and designed in this paper. That is, a positive extrusion core die with a certain extrusion ratio is connected directly behind the equal channel angular extrusion cavity, and a continuous extrusion cavity with multiple deformation processes is formed, thus the cumulative plastic deformation of the material is realized. Therefore, a new way to improve the plasticity of magnesium alloy is found. In this paper, the material model of AZ31 magnesium alloy is established by using secondary development technology in MSC.Marc software, then the program development flow chart of AZ31 magnesium alloy microstructure prediction system is constructed, and the microstructure model of AZ31 magnesium alloy is programmed by FORTRAN language. The change of grain size and dynamic recrystallization integral number of AZ31 magnesium alloy during extrusion was successfully predicted. The finite element analysis model of ECAP-FE extrusion process of AZ31 magnesium alloy was numerically simulated. Finally, the extrusion force field, equivalent strain rate and equivalent strain field of AZ31 magnesium alloy during ECAP-FE extrusion simulation are analyzed by finite element method. The simulation results show that compared with the single ECAP extrusion process, the ECAP-FE composite extrusion process can effectively increase the amount of deformation of the material, make the material produce cumulative deformation and refine the grain. Moreover, the average equivalent strain obtained by ECAP-FE composite extrusion process has increased by about 2 times, and the equivalent inhomogeneity coefficient has been greatly reduced, and the equivalent strain has been distributed symmetrically, so ECAP-FE can achieve higher cumulative plastic deformation. Moreover, in the process of ECAP-FE composite extrusion, the die face of forward extrusion can provide effective back pressure for ECAP extrusion process, reduce the area of shear band, and make it closer to the ideal simple shear. The maximum equivalent strain rate after ECAP-FE extrusion is 0.9916, and the maximum equivalent strain rate increases significantly. It is shown that ECAP-FE composite cumulative plastic deformation process can obtain more fine and uniform grain structure. The results provide a theoretical basis for the further study of ECAP-FE composite extrusion process.
【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】：2016
【分类号】：TG379

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