【作者】 崔晓辉；

【导师】 莫健华；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2013， 博士

【摘要】 本文结合金属板料单点渐进成形和电磁脉冲成形工艺，提出电磁脉冲渐进成形新方法。该技术的基本原理是放电线圈代替单点渐进成形装置中的刚性工具头。放电线圈在计算机控制下按照一定的三维空间轨迹逐次移动到一个大型工件的各个局部位置，并通过线圈放电和磁场力使工件分布变形。最终这些局部变形累加成整个大型零件。另外，电磁脉冲成形涉及到一个多物理场相互耦合过程。数值模拟提供了一种手段去解决这种耦合问题。但是大多数的模拟方法只针对简单的2D轴对称模型。对于实际的工业应用，需要分析非轴对称和曲面复杂的成形零件，这都必须设计相应的3D有限元模型。因此，本文的研究内容可概括为：为了提供能够适用于复杂3D模型的强耦合模拟方法，采用顺序耦合法对板料电磁自由胀形过程进行3D有限元模拟分析。对空气网格采用任意拉格朗日欧拉算法（ALE）或者网格随移技术，实现空气3D网格能够与工件变形随动而不产生畸变。动态而无畸变的六面体网格和不同时刻工件上的磁场力数据能够在磁场分析模块与塑性成形模块间准确传递数据，既提高了运算速度又可获得精确计算结果。与过去学者所发表文献的实验数据比较证明，本文的模拟方法能合理的描述工件3D电磁动态变形过程。对于复杂线圈结构，提出采用网格重划技术实现空气网格随工件的变形而更新。建立含有均压力线圈的3D电磁成形有限元模型，分析均压力线圈的工作原理、板料上的磁场力分布情况。采用ANSYS/MECHANICAL软件分析板料在均压力线圈作用下的变形规律。通过分析线圈的受力情况，提出均压力线圈容易发生失效的原因。采用松散耦合法和顺序耦合法分析了脉冲电流第二半波对成形的影响。研究发现：随着电流频率的增加，第二半波对电磁脉冲成形结果影响加剧。如果仅考虑第一半波对成形的影响和高的电流频率条件下，松散耦合的计算精度有可能会高于顺序耦合法。但是当第二半波对成形的影响被考虑，无论在高的电流频率还是低电流频率，顺序耦合法计算结果都与实验结果接近。另外，采用顺序耦合法研究了线圈与凹模尺寸的比值和电流频率对板料电磁脉冲成形的影响。模拟结果发现板料电磁自由胀形存在两个明显的厚度减薄区域。当线圈与凹模尺寸的比值接近于1时，板料会得到最大的厚度减薄量。同时存在最佳的电流频率使板料的变形量最大。模拟结果发现最佳电流频率会受到板料厚度和电流衰减系数显著影响。因此，不能依据传统观点（即趋肤深度应该小于或者接近工件的壁厚）来选择最佳电流频率。结合实验和2D、3D有限元分析手段，深入研究了电磁渐进成形方法对大尺寸板坯件和长直壁管件的成形问题。针对大尺寸板坯件成形，研究发现：板料高速变形并与凹模贴合时，板料变形时的空气阻力随着放电电压的增加而增加。当放电电压大于某一临界值（U=1700V），由于空气阻力的原因，导致板料表面有凹坑出现。为了提高板料与模具的贴合质量，需要在一个放电位置连续两次放电成形。另外本文实验得到：当最佳的放电区域个数N=4时，相邻两次放电区域存在最佳的重叠率，使板料最终成形质量最好。针对长直臂管件均匀变形的研究，采用“生死单元法”间接描述线圈移动放电成形过程。与一次放电、两次放电和三次放电实验结果相比，模拟与实验结果吻合。进一步采用数值模拟方法研究了放电区域的重叠系数、放电电压、成形顺序和模具尺寸对管件变形均匀性的影响。结果证明电磁渐进成形工艺在大型板材和长直臂管件成形上的可行性。更多还原

【Abstract】 In this paper, a new method named electromagnetic incremental forming (EMIF) hasbeen proposed based on the single point incremental sheet forming and electromagneticforming (EMF). The principle of the new method can be described that the working coil isused to replace the rigid tool in the incremental sheet forming. The working coil is movedalong a special trajectory and small discharge energy to cause workpiece localdeformation in a high speed by magnet force. Finally, all local deformations accumulateinto large parts. In addition, EMF is a complex Multi-physics coupling forming process.Numerical simulation offers an opportunity to overcome the problem. However, modelingapproaches found for EMF process in the literature are mainly restricted to2D model. Forreal industrial applications, the modeling of3D forming operation becomes crucial for aneffective process design. Thus, the whole paper can be divided into the following sections:In order to propose a new strong coupling method which can be used for complex3Dmodels, three-dimensional finite element models are established and sequential couplingmethod are used to analyze electromagnetic sheet free bulging. The ArbitraryLagrangian-Eulerian (ALE) algorithm or the Morphing technologies are used in airmeshes to make them change regularly with the deformation of the workpiece and avoidthe occurrence of distorted meshes. The dynamic and distortionless hexahedral meshes canensure the accuracy of date transfers between the magnetic field module and structurefield module. Therefore, the computational accuracy and computational efficiency havebeen improved. The comparison between the simulation results and the experimental onesindicates the deformation laws in simulation results agree with the experimental one. Forthe complex coil structure, the remeshing technology is used to make the air mesheschange with the workpiece deformation. Then, the3D finite element model is establishedto analyze the distribution of the magnetic forces and work principle of the uniformpressure electromagnetic actuator. The magnetic forces on the sheet are input into thesoftware, ANSYS/MECHANICAL, which is used to analyze the deformation law of thesheet.And, the reason for coil failure has been proposed based on the force on coil.The loose coupling method and sequence coupling method are both used toinvestigate how the second half current wave affects the simulation result. It is found that the effect of the second current pulse on sheet deformation increases with the increasing of the discharge frequency. The sequential coupling method may or may not obtain higher simulation accuracy than the loose coupling method if the first current pulse just be considered. However, the sequential coupling method always can obtain accurate simulation results whatever in a high or low current frequency if the second current are also considered. In addition, the sequential coupling method is used to analyze the effect of the ratio of coil to die dimension and current frequency on sheet forming. It exist two regions with great thickness thinning in the final sheet shapes. If the ratio of coil and die dimention is close to1, the sheet could obtain the maximum value thickness thinning. And there exists optimum frequency which corresponding to the maximum displacement in sheet forming. However, the effect factors, such as sheet thickness and current damping exponent, play a very great role in choosing the optimum current frequency. Thus, it is not reasonable to choose the optimum frequency according to the basic experience that the skin depth should be equal to or slight below the wall thickness.Based on the experiment and the2D and3D finite element models, it is deeply analyzed the forming process of electromagnetic incremental sheet and tube forming. To produce large sheet parts using EMIF, it is found that:1) the forming quality of sheet side surface is better in a high discharge voltage than that in the low discharge voltage. However, a high air pressure is created to hinder sheet deformation during the forming process, which will cause the appearance of concave on the sheet if the discharge voltage exceeds a critical value (U=1700V);2) Two consecutive discharges in a fixed position are needed to produce large parts using EMIF technology;3) There exists optimum number of discharge regions (N=4), which corresponds to the best overlap ratio in two adjacent discharge regions and best forming quality. For the deformation of large-straight tube, the technology liked "birth-death element" is used to indirectly describe the movement of the coil. The simulaiton results are better agreement with the the experimental values of single, twice and triple discharging. Then the effect factors named overlap ratio of discharge regions, discharge voltage, forming sequence and the dimensional of die structure on tube homogeneous deformation are analyzed. The results demonstrate that this new technology is feasible to produce large sheet and tube parts.更多还原