【作者】 张小娜；

【导师】 华林；

【作者基本信息】 武汉理工大学 ， 材料加工工程， 2012， 硕士

【摘要】 等径角挤压工艺(ECAP)是基于大塑性变形(SPD)理论的一种制备大块体超细晶材料的先进技术,由于其特殊优点,已引起众多学者的研究。随着汽车轻量化和高性能的发展,迫切需要轻质材料的使用。目前,作为结构件的镁合金,具有密度轻、比强度和比刚度高的优点,现已应用于汽车、航空航天等领域。因此,研究镁合金的ECAP晶粒细化,积极探索改善镁合金力学性能和成形性能的途径,对于推动镁合金材料的应用有重要意义。本文主要以变形镁合金中强度最高的ZK60镁合金棒材为研究对象,从DEFORM-3D有限元模拟和实验的角度分别探索了等径角挤压这种晶粒细化的新工艺,为ECAP晶粒细化工艺的工业化生产及应用提供了理论和技术依据。具体的研究内容与成果如下：(1)本文基于ECAP三维实体几何模型和三维塑性变形有限元力学模型,通过解决ZK60镁合金的材料模型、网格划分、边界条件等关键建模技术,在DEFORM-3D中建立了准确的ECAP变形三维刚塑性有限元模拟模型。(2)基于ZK60镁合金的ECAP三维有限元模型,揭示了试样在ECAP变形过程中的载荷、等效应力、应变和速度的基本变形规律；探究了各工艺参数(模具内角、挤压温度、摩擦因子、挤压速度、挤压道次等)对ECAP变形的影响；并进一步重点模拟分析了试样的直径尺寸变化(φ6-40mm)对载荷、应变以及变形不均匀性的影响,为ECAP实验提供了理论依据。模拟得到的主要结论为：镁合金ZK60的ECAP挤压应使用Φ=90°的模具,在T=250℃且较低的挤压速度下,采用Bc路径在润滑良好的条件下进行多道次挤压获得的挤压效果最好,并且小尺寸试样的变形比大尺寸试样的变形程度大且均匀。(3)对铸态和热处理后的ZK60试样分别进行了ECAP实验,结果表明：均匀化热处理后的试样变形相对较好,晶粒细化效果更明显。所以,对ZK60镁合金铸态试样进行均匀化退火处理是进行ECAP变形实验的必要前提条件。(4)在不同温度下(T=180,200,250,300℃)对ZK60分别进行ECAP实验,结果表明：ZK60试样低于200℃挤压时会发生裂纹甚至断裂；综合结果,当T=250℃挤压后的晶粒分布最均匀,细化效果相对较好。(5)对ZK60试样连续进行了4道次ECAP实验,分析表明：随着挤压道次的增加,晶粒细化程度越好,小尺寸细晶数量不断增多,晶粒分布更为均匀。1道次挤压后ZK60的晶粒尺寸由～110μm细化到～11μm,细化程度很大；4道次挤压后细化到～5μm,说明ECAP法是一种有效的晶粒细化手段。(6)用自行设计的两套ECAP模具对比研究了径向尺寸分别为φ10mm与φ30mm的ZK60镁合金试样前两道次ECAP变形,得到的平均晶粒尺寸分别为-6gm和～8μm。微观组织观察结果表明：小尺寸φ10mm试样的晶粒细化程度较大,细化后的晶粒尺寸和形状的分布更为均匀；而大尺寸Φ30mm试样局部的细晶数量较多,但大尺寸粗晶的数量也比较多,晶粒尺寸分布很不均匀。(7)对比了ECAP变形后的两种尺寸试样横截面上有代表性区域的晶粒细化情况。结果表明：ZK60试样横截面上晶粒细化程度与模拟中的等效应变分布有正相关性,试样维氏硬度值的变化与变形程度是一致的。所以,利用有限元模拟的等效应变分布情况可以解释实验结果中的晶粒细化状态和硬度变化。(8)有限元模拟得到的挤压后试样形状和ECAP实验结果是一致的,并且模拟中等效应变的分布和晶粒细化结果相对应,从而验证了在DEFORM中建立的模拟模型的准确性,说明了模拟结果具有一定的可靠性。更多还原

【Abstract】 Equal channel angular pressing (ECAP) is an advanced technique based on the theory of severe plastic deformation (SPD) which to producing bulk ultrafine-grained materials (UFG). With its special advantages, the scientists have made a lot of research about this technology. With the development of the cars’lightweight and high performance, the using of lightweight materials is in urgent demand. At present, the magnesium alloy as the structural components, with the advantage of light density, high strength and rigidity, has been used in automotive and aerospace field. Therefore, it is important to study the magnesium alloy’s grain refinement processed by ECAP and explore active approach of improving the mechanical properties and formability, thus to push the application of magnesium alloy.The research object of this paper is ZK60magnesium alloy which has the highest strength. The new ECAP technology is explored via finite element (FE) simulation and experimental method. The research results provide useful guidance for the future successful incorporation of the ECAP process into commercial manufacturing and applying operations. The contents and results of this paper are generally shown as following:(1) The sound3D rigid-plastic FE model of the ECAP process is established under the DEFORM-3D platform by solving several key modeling techniques such as geometrical and material models of ZK60, meshing and boundary conditions.(2) Based on the FE models, the law of forming loads, equivalent stress and strain during the ECAP process is revealed. And the effects of multi-parameters, such as die angle, pressing temperature, friction factor, pressing speed and passes are explored. Furthermore, the ZK60FE models with channel diameters fromφ6-40mm are built. Through these simulations, the distributions of the load, equivalent strain and the nonuniformity of deformation with the increase of diameter are analyzed in detail. The simulation results provide a theory basis for the ECAP experiments. The main conclusion of the simulations shows that: the ECAP of ZK60should be conducted under the die with the inner angle Φ=90°, T=250℃, a lower pressing speed, route Bc, more passes and in good lubrication. And the deformation of the specimen with a small size is larger and more uniform than that with a large size.(3) Do the ECAP experiments of ZK60with different state of as-cast and after heat treatment. The results show that, the deformation of ZK60specimen after heat treatment is better and the grain refinement is more obvious. Thus, doing heat treatment is the necessary primise for the ECAP of ZK60specimen.(4) The grain refinement of ZK60alloy is discussed after pressing under different temperature (T=180,200,250,300℃). The ZK60specimen will craze when pressing under the temperature below200℃. The grain distribution is uniform and grain refinement is better when pressing at T=250℃.(5) The specimens of ZK60alloy are subjected to4-passes ECAP. The results show that, with the increase of passes, the grain refinement is getting better, the number of fine grain with small size is larger, and the grain distribution is more uniform. The grain size from～110μm refined to～11μm after1pass, and refined to～5μm after4passes. Thus ECAP process is an effective method of grain refinement.(6) The pressing results of ZK60specimens with different diameters of φ10mm and φ30mm after2-passes in two ECAP die-sets with different channel diameters is comparatively studied, the grain size respectively refined to～6μm and～8μm. The grain refinement ofφ10mm is bigger than that of φ30mm.(7) The grain refinement at different regions on the cross section of ZK60with two different diameters after1and2passes are compared. The grain refinement and the equivalent strain of simulation have positive correlation. And the change of vicker hardness and the deformation of the specimen is consistant. So the grain refinement and the vicker hardness of experiment can be explained by the equivalent strain of simulation.(8) The shape of ZK60specimen from FE simulation is the same with the experimental result. The grain refinement is consistant with the equivalent strain of simulation. Thus the FE models are verified by the ECAP experiments and the simulation results are reliable.更多还原