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5052铝合金板材磁脉冲辅助冲压成形变形行为及机理研究

Deformation Behavior and Mechanism of Electromagnetically Assisted Sheet Metal Stamping of 5052 Aluminum Alloy

【作者】 刘大海

【导师】 李春峰;

【作者基本信息】 哈尔滨工业大学 , 材料加工工程, 2010, 博士

【摘要】 近年来,节能和环保的要求使得以铝合金为代表的轻质结构材料在先进制造领域得到广泛应用。但是,与传统钢材相比,铝合金的室温成形性差,采用传统冲压工艺很难直接成形形状复杂的零件。磁脉冲辅助冲压成形(Electromagnetically assisted sheet metal stamping, EMAS)把高速率磁脉冲成形(EMF)技术的优点有机地结合到准静态冲压成形中,为以铝合金为代表的轻质、难成形板材复杂形状零件的室温成形提供了新的加工途径。由于该工艺尚处于初步工艺实现阶段,其变形机理还没被系统研究过。本文采用理论分析、耦合场数值模拟、工艺试验及微观分析相结合的方法对5052铝合金板材磁脉冲辅助冲压成形的变形行为和机理进行了系统研究。基于多物理场有限元分析软件ANSYS,结合磁脉冲辅助冲压成形中板坯变形的基本特点,给出了准静态冲压成形—高速率磁脉冲成形顺序耦合求解的有限元理论基础,包括板料冲压成形弹塑性理论、磁脉冲成形的电磁场基础理论、结构场的弹塑性理论和电磁-结构耦合场理论等,并在理论分析的基础上提出了基于ANSYS分析平台ANSYS Multiphysics/LS-DYNA分析模块的“多工步”松散耦合的磁脉冲辅助冲压成形有限元分析方案及流程,通过编制用户子程序实现连续变形过程的动态连接。针对磁脉冲辅助冲压成形中板坯的准静态—动态复合变形特点,建立了典型应变状态(单向拉伸、平面应变和双等拉伸)下的准静态—动态复合成形极限试验方案,并对准静态—动态复合加载下材料的动态响应进行了系统研究。结果表明:准静态—动态复合加载下,5052铝合金板材表现出显著高于准静态成形而略高于动态成形的塑性水平。复合成形过程塑性的提高归因于动态变形过程,且随着准静态预变形水平增加,动态变形过程的增塑效果增强。采用理论分析和微观分析相结合的方法对磁脉冲辅助冲压成形中动态变形行为和塑性失稳机理进行了研究。结果表明:动态变形中,惯性力的影响起主要作用。惯性力对板坯的结构失稳具有抑制作用,从而使板坯的塑性提高并产生分散失稳。微观机制表明:5052铝合金动态变形和准静态变形表现出相似的变形性质,不会产生特殊的组织结构,塑性变形微观机制均为位错滑移机制。准静态变形以均匀单系位错滑移为主,断裂伴随着位错的缠结和攀移;而动态变形位错滑移趋于多系开动,在大面积区域出现明显的交滑移现象,且滑移带较准静态成形时窄且密,位错组态更均匀。动态变形的多系滑移和位错均化作用可在比准静态变形高的多的塑性应变水平下形成,从而使材料具有较高的塑性和强度。针对筒形件传统拉深成形中存在的成形极限问题(底部圆角处破裂)进行了分析,建立了筒形件磁脉冲辅助拉深成形方案。采用提出的有限元分析方案对筒形件磁脉冲辅助拉深成形进行了仿真,分析了板坯的复合变形行为和规律,并结合试验研究和验证了磁脉冲辅助冲压成形在改善板材成形性方面的优势和特点。结果表明:提出的磁脉冲辅助冲压成形有限元分析方案,能成功实现磁脉冲辅助冲压成形中板坯复合变形行为的描述,变形规律与试验结果吻合较好。磁脉冲辅助冲压成形的思想能充分发挥磁脉冲成形在改善板材成形性方面的优势,并能实现与普通冲压成形工艺的有机结合。板坯变形过程中,高速变形的惯性效应和板坯与模具冲击作用显著,能抑制失效,分散变形。基于磁脉冲辅助冲压成形的思想,建立了U形件磁脉冲辅助弯曲成形工艺试验,分析了脉冲磁场力对弯曲回弹控制的作用,结果表明:将脉冲磁场力应用到弯曲角部能有效减小回弹。脉冲力的回弹控制作用主要表现为两个方面:脉冲力改善弯曲角部位应力应变分布的作用和脉冲力作用下形成的板坯与模具的冲击作用。两个过程均能实现对弯曲回弹的有效控制。弯曲变形过程中,采用小能量多次放电不仅可以控制回弹,而且可以有效地改善弯曲件角部的变形分布。

【Abstract】 In recent years, the need to improve fuel economy and protect environment has led to extensive application of lightweight structural materials as represented by aluminum alloys in the field of advanced manufacturing. Unfortunately, the viability of the press forming of complex-shape aluminum parts is hindered by the fact that aluminum has much lower formability compared with steel at room temperature. Electromagnetically assisted sheet metal stamping (EMAS) integrates the high-speed forming advantages of electromagnetic forming (EMF) into the conventional quasi-static stamping process, which has provide a new way for room-temperature processing of complex-shape parts from lightweight, hard-forming materials as represented by aluminum alloys. As this technique is still in laboratory of initial realization, the deformation behavior and mechanism have not been well systematically studied. In this dissertation, the deformation behavior and mechanism of EMAS of 5052 aluminum alloy sheets are systematically investigated by theoretical analysis, coupled-field numerical simulation, experiments and microanalysis.Based on the basic characteristics of EMAS, the theoretical basis of FEA of stamping-EMF process is presented, including the elastic-plastic theory of sheet metal stamping, both the electromagnetic-field theory and the elastic-plastic theory of structural filed of EMF, and the coupled-field theory of electromagnetic and structural fields. A“multi-step”loose coupling numerical scheme for EMAS is proposed based on the ANSYS Multiphysics/LS-DYNA platform. The dynamic links of the successive deformation process is realized through establishing user-defined subroutines.Based on the hybrid quasi-static-dynamic deformation characteristics of sheets in EMAS, a series of quasi-static-dynamic forming limit experiments are established under the typical strain states (uniaxial tension, plane strain and equi-biaxial tension) of conventional FLDs, and the dynamic behaviors of material under complex-loading conditions are systematically investigated in traditional FLD space. Results show that the formability of 5052 aluminum alloy sheet undergoing a quasi-static-dynamic loading process is dramatically increased beyond that exhibited in quasi-static deformation process, and a little higher than that obtained in the fully dynamic EMF process. Analyzing from the loading conditions, we may reasonably attribute the hyperplasticity effect of quasi-static-dynamic process to high-velocity deformation. And with the increasing of quasi-static pres-training, the hyperplasticity effect of dynamic process increases.The plastic instability mechanism and deformation behavior of dynamic deformation process in EMAS are investigated by theoretical analysis and microanalysis. Results show that inertia force plays an important role in dynamic forming, which has the suppression effect on structural instability and thus improves the formability of sheet and spreads instability. Micromechanism shows that the nature of dynamic deformation is much similar with that of quasi-static deformation and no special deformation structures arise in dynamic process for 5052 aluminum alloy sheets. The deformation mechanism of both processes is dislocation slip mechanism. For quasi-static deformation, the dislocations show a uniform single-slip pattern, fracture combined with dislocation tangling and climbing. While for dynamic deformation, dislocation system tends to more slips, large areas showing clear cross-slip structures. The dislocation bands are more narrow and dense than those shown in the quasi-static process, and a much more uniform dislocation configuration is also exhibited after pulsed magnetic loadings. The characteristics of multi-slips and uniform effect of dislocations under pulsed magnetic loading conditions will result in much higher plasticity and strength of materials. In order to analyze the deformation behaviors of typical EMAS technology process, a limit forming problem (fracture at bottom corner) in conventional cylindrical deep drawing process is analyzed and the electromagnetically assisted cylindrical deep drawing scheme is established accordingly. On this basis, the former proposed FEA scheme of EMAS is firstly applied to investigate the hybrid deformation behaviors and deformation laws of sheets in EMAS, and then the efficacy and characteristics of EMAS on improving the sheet formability are established and validated experimentally. Results show that the proposed FEA scheme can successfully simulate the successive deformation process of EMAS, and the deformation law is validated by the experimental results. The forming limit experiments of EMAS of cylindrical parts shows that the idea of EMAS can successfully exploit the advantages of improving room-temperature formability of EMF, and the EMF phenomenon can successfully be integrated into conventional stamping process. In the deformation process of sheets metal, the effects of inertia and tool-sheet interaction are very remarkable, which can suppress damage evolution and stabilize deformation.Based on the idea of EMAS, the technology experiments of electromagnetically assisted bending of U-shape parts are established, and the involved springback-control effect of pulsed magnetic force is analyzed. Results show that the way of applying the pulsed magnetic force to the bending area can effectively reduce the springback. The springback control of magnetic force in EMAS is mainly presented in two aspects: the role of magnetic force changing the strain distributions of bending area and the role of tool-sheet interaction effect of high-speed loading. Both processes can successfully reduce the spingback. During the bending process, the way of many times of small discharges can not only control the springback, but also improve the deformation quality of bending area.

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