【作者】 任国成；

【导师】 赵国群；

【作者基本信息】 山东大学 ， 材料加工工程， 2013， 博士

【摘要】 镁及镁合金是迄今在工程应用中最轻的金属结构材料,在航空航天、交通运输以及3C产品制造领域具有广阔的应用前景,但密排六方的晶格结构决定了其塑性变形能力差,因此提高镁合金塑性变形能力已成为镁合金研究的热点之一。细化晶粒是提高材料性能的有效手段,当材料的微观结构达到纳米量级时,便有可能使该材料的强度和塑性同时增加。等通道转角挤压(Equal Channel Angular Pressing,ECAP)是一种有效制备超细晶乃至纳米晶结构材料的大塑性变形方法,已广泛应用于各种块状金属超细晶材料的制备。采用该工艺细化镁合金晶粒的效果同变形过程中应力、应变、温度等场量的大小及分布密切相关,而这些场量分布又取决于ECAP模具结构及变形工艺参数设置,因此全面研究不同参数对挤压过程的影响规律对于采用ECAP方法制备镁合金超细晶材料具有重要意义。本文采用数值模拟与实验分析相结合的方法对镁合金的ECAP变形工艺进行研究,首先采用塑性成形有限元软件DEFORM-3D对不同条件下的ECAP工艺进行数值模拟,分析模具几何形状,变形工艺参数对挤压过程的影响规律,研究镁合金ECAP挤压过程的变形机理,优化变形工艺参数。采用空间转换法实现了ECAP工艺多道次挤压有限元分析中的数据继承,得到了不同变形路径下的应变累积规律。在分析数值模拟结果的基础上,设计了镁合金ECAP挤压实验模具并确定了挤压工艺方案,实现了AZ31镁合金的多道次挤压变形,并采用室温拉伸实验、微观硬度测试、金相光学显微镜、扫描电子显微镜和X射线衍射等分析手段,获得了不同变形条件下镁合金ECAP挤压件微观组织、变形织构以及宏观力学性能的演化规律。工件变形均匀性一直是等通道转角挤压过程中影响材料性能的主要原因之一模具结构是影响试样变形均匀的重要因素。本文通过对不同模具外角Ψ,内角半径r值以及背压条件下ECAP变形过程的有限元模拟,研究获得了不同模具结构AZ31镁合金ECAP变形时的等效应变分布。结果表明当模具外角Ψ等于20°,内角半径r为2mm时工件可以获得均匀变形。微观组织观察以及硬度测试表明,AZ31镁合金经过一道次ECAP挤压后,微观组织显著细化,力学性能明显改善,但是平均晶粒尺寸及微观维氏硬度在工件横截面上分布不均匀,且实验所得变化趋势与有限元分析所得等效应变分布的变化趋势基本一致,等效应变分布的不均匀性是导致材料微观组织和力学性能不均匀的主要因素之一。变形温度是影响镁合金变形行为的重要因索。本文通过建立镁合金等通道转角挤压过程的热力耦合有限元分析模型,对不同挤压速度及摩擦条件下ECAP变形过程中的温度场及应变场分布进行了分析,获取了变形工艺条件对镁合金变形行为的影响规律。研究结果表明,等通道挤压过程中试件温度分布不均匀,在模具转角剪切部位温度显著升高,且存在明显的温度梯度。XRD分析和微观组织观察显示,AZ31镁合金变形后锥面衍射强度显著增强,并且镁合金的再结晶速度随着变形温度的升高而显著加快。结合变形过程中温度场的分布状况,建议AZ31镁合金等通道转加挤压的合理变形温度为250℃。变形路径对多道次ECAP载荷及平均等效应变影响较小,但对变形的应变分布有一定影响。工件经过不同路线ECAP变形后横截而上等效应变分布相似,纵截面存在较大差异,不同路径工件纵截面上的等效应变分布基本继承了首道次ECAP变形特点,而横截面上的等效应变分布随变形路径及变形道次的不同而不同。在经过四道次ECAP工艺处理后,A路径(相邻道次工件相对位置不变)和Ba路径(相邻道次工件沿轴向交错旋转90°)挤压工件的主变形区内存在较为明显的梯度,而Bc路径(相邻道次工件沿轴向依次旋转90°)和C路径(邻道次工件沿轴向依次旋转180°)处理的工件可以获得良好的等效应变分布。从工件主变形区横截面上不均匀变形系数来看,Bc路径挤压所获得的应变累积最为均匀。经过不同路径四道次ECAP工艺处理后,AZ31镁合金的室温抗拉强度、伸长率较原工件均有明显的提高,采用A路径和Ba路径挤压时,可以获得较高的的抗拉强度,而采用Bc路径和C路径时工件的延伸率较高。SEM断口观察显示ECAP挤压使合金拉伸断口形貌山原始态的解理断裂特征转变为延性韧窝断裂特征。综合抗拉强度和延伸率两方面因素,发现采用Bc路径挤压可以获得较为理想的力学性能。有限元模拟及实验分析结果表明通过合适的变形条件可以获得细小而均匀的微观组织,当材料的应变累积均匀时,其力学性能也较好。更多还原

【Abstract】 As the lightest metal structure material, magnesium and magnesium alloys has a broad application prospect in aerospace, transportation and3C products manufactur-ing, but its plastic deformation ability is poor because the hexagonal close packed structure. Constrained by poor deformation ability has became a major obstacle for magnesium alloy to wider application. So, it has became a hot spot in the study of magnesium alloys that how to improve the magnesium alloy plastic deformation ca-pacity. The material’s strength and plasticity can be increased at the same time when its grain size refine to nanometer range. As one of servere plastic deformation method for effective produce ultrafine grain and even nanocrystal structure materials, equal channel angular pressing has been widely used in making ultra-fine grained material for many kinds of metals. The grain refining effect of magnesium alloy is closely related to the field distribution during ECAP process, such as stress, strain, tempera-ture and etc. These fields distribution depends on the Settings of ECAP die structure and deformation process parameter. It will be of great importance to comprehensive study the rule of different parameters influence on the extrusion effect in making ul-tra-fine grained magnesium alloy by ECAP process.ECAP deformation process of magnesium alloy was investigated by Numerical simulation and experimental analysis together in this paper. ECAP deformation process was simulated by the finite element software, DEFORM-3D under different conditions at the first. The influence of different parameters on the extrusion process, such as the die geometry, process parameters, deformation routs and etc was analyzed generally. It was acquired that the deformation mechanism and appropriate process parameters about the ECAP process of magnesium alloy. Four processing routes are analyzed in detail by using finite element method with spatial switching method through rotating three-dimensional model in multi-pass pressing. The accumulated effective strain distribution of the work-piece processed by ECAP for four pressing routes are obtained respectively through finite element simulation. On the basis of the simulations, the ECAP experiments are carried out by the optimized design of mould geometry that suite for magnesium alloy. And the finally pressed workpieces are ob-tained in the experiments. The microstructure evolutions of the grain refinement me-chanisms and micro-structural characters for different multi-pass ECAP processing routes are verified by using OM (optical microscope), SEM and XRD analysis. The mechanical properties was obtained room temperature tensile experiment and micro-hardness test.The homogeneous deformation is the main factor that influences the material performance during ECAP process. The die structure is an important factor that af-fecting homogeneous deformation of samples. The strain size and distribution of AZ31magnesium deformed by ECAP for different die corner angle Ψ and inner cor-ner radius r were investigated through the finite element simulation. When the die out corner angle is equal to20°and inner corner radius is equal to2mm, the results show that the effective strain distribution in the samples is more homogeneous than those of others. The effect of the stain distribution in pressed workpiece on the micro-structure and mechanical properties of the pressed workpiece was explored by using optical observation and micro-hardness testing. The result show that the microstruc-ture is significantly refined and the mechanical property is improved, but the distribu-tion of the grain size and micro-hardness in the cross-section of the pressed samples is inhomogeneous. The inhomogeneous distribution of the effective strain is one of the main factors that lead to the uneven microstructure and property.The temperature is an important factor that affects the deformation of magne-sium alloy. A thermomechanical coupled finite element model was established and the temperature distribution of the magnesium work-piece during Equal Channel Angular Pressing at different extrusion speed and friction condition was analysed in this paper. The result showed that the distributing of temperature is not uniform during equal channel angular pressing, there exists obvious temperature gradient and the tempera-ture increased significantly in the mold Angle shear part. The effect of temperature on deformation of magnesium alloy was acquired by XRD analysised and microscopic examination. XRD analysis and microscopic examination shows that the pyramidal face diffraction intensity enhanced significantly, the recrystallization rate of magne-sium alloy increases obviously with the rise of deformation temperature. The optimal deformation temperature of250℃was obtained according to the temperature distri-bution.The impact on average equivalent strain and extrusion load is limited during multi-pass ECAP process, but there are certain influence on the strain distribution. The equivalent strain distribution at cross section is similarity after multi-pass ECAP deformation through different route, and there is bigger difference at longitudinal sec-tion. The equivalent strain distribution at cross section inherited the first pass ECAP deformation characteristics for the most part, but different at the longitudinal section through different route. There is obvious gradient distribution at main deformation area by A and Ba routes, the distribution is well when deformation by Be and C routes. From the non-uniform deformation coefficient at the main deformation zone cross section, strain accumulation obtained by Be route is the most uniform.The tensile strength and elongation under room temperature were improved ob-viously for AZ31magnesium alloy processed by ECAP four passes through different routes. A higher tensile strength can be obtained through A or Ba routes, elongation is higer when processed through Be or C routes. It can be obtained from SEM fracto-graphic analysis that the as-received magnesium alloy shows brittle cleavage fracture while the alloy processed by ECAP exhibits dimple-like fracture characteristics. The better processing route for AZ31magnesium ECAP processed is acquired through comparing the results of Simulation and experimental study. Finite element simulation and experimental analysis results show that small and uniform microstructure can be obtained Through the appropriate deformation condition. Mechanics performance of magnesium is preferably when strain accumulation uniform.更多还原