摘要
航空航天、汽车等工业领域轻量化要求的不断提高，促进了轻质、高强度材料的应用。但是，由于这些材料的成形性能差、回弹不易控制等问题，导致冲压成形难度大。采用控制压边力大小、优化拉深筋形状、改善润滑条件、补偿模具型面等方法，通常需要长时间的试模和修模才能获得成功，导致模具质量降低、成本上升。而一旦调试不成功，就会造成极大的经济损失。实际上，在冲压成形过程中，成形速度及成形载荷能够有效地改善材料的受力状态和变形行为，从而在无需改变模具的情况下，即可降低材料的成形回弹，提高材料的成形性能，有效降低冲压成本。因此，深入研究变速/变载对冲压成形的影响规律，探讨相关机理，对完善冲压成形技术，提高冲压件的质量具有重要的意义。本文将重点围绕成形速度及成形载荷对板料成形极限和回弹的影响规律，采用塑性理论、有限元数值模拟和实验研究相结合的方法，进行系统深入的研究。
首先，基于塑性力学的基本理论，建立拉深成形过程的虚功率方程，从变形功的角度，探讨成形速度与成形载荷提高材料成形性能的可行性。在此基础上，进一步采用有限元数值模拟方法，研究了成形速度对主变形区等效应力和等效应变速率的影响。模拟结果表明，通过成形速度的变化，可以改变主变形区的总变形功率，从而能够有效地提高材料的成形性能。
在冲压成形过程中，速度对成形过程的影响主要通过摩擦来实现。为此，针对拉深成形，分别采用平板滑动摩擦实验和拉弯摩擦实验，研究了变速/变载条件下板料在法兰处和凹模圆角处的摩擦行为。实验结果表明：随法向载荷的增大，滑动摩擦系数减小；而随速度的增大，滑动摩擦系数增大，在运动稳定阶段，滑动摩擦系数与速度成线性关系；速度对法兰处摩擦的影响远大于对圆角处摩擦的影响，且速度对圆角处摩擦的影响不显著；另外，通过合理设置变速成形方式，能有效地降低初始摩擦力和摩擦力随速度变化的幅值。在上述实验结果基础上，为了进一步研究速度对摩擦影响的机理，采用分形理论，建立了基于板料表面形貌的弹塑性接触模型，研究了板料和模具间实际接触面积和法向载荷之间的定量关系，并首次建立了考虑成形速度、法向载荷、材料力学性能和板料表面形貌的摩擦模型，该模型能够定量计算法兰处的滑动摩擦系数，对于进一步揭示冲压成形过程中的摩擦行为，以及提高成形过程数值模拟精度都具有重要的意义。
针对影响冲压成形质量的弯曲回弹，分别采用V形和U形弯曲实验，研究了速度、载荷、凸模行程和弯曲次数等参数对回弹的影响。实验结果表明，成形速度对回弹的影响较小，而成形载荷、凸模的行程和弯曲成形次数对回弹有较大影响，尤其是通过成形载荷的变化能有效地控制回弹。为此，在考虑应力-应变分布、受力边界条件、弹塑性材料模型和Hill屈服准则的基础上，建立了成形载荷控制下的弯曲回弹预测模型，该模型与实验结果相吻合，克服了传统经验公式仅能预测自由弯曲回弹的不足，可用于指导难成形材料的校正弯曲成形工艺设计。
在上述研究成果基础上，采用圆筒件拉深工艺实验进一步研究了速度影响拉深成形性能的规律。实验研究发现，采用不同的拉深速度，筒形件的拉深深度及壁厚均匀性都存在较大的差异。这表明速度是影响拉深成形性能和质量的一个重要因素。当采用阶段变化的速度模式成形时，速度的转变应在板料完全进入圆角区之后进行，还需使拉深初始速度小于材料的极限成形速度。这些现象主要是由于成形过程中的摩擦随速度变化而引起的。为此，采用塑性力学理论和前面提出的摩擦模型，以及拉深实验结论，首次建立了变速/变载条件下，材料的极限拉深系数与材料性能参数和拉深速度之间的关系式，该表达式能够判断变速/变载成形工艺的可行性，并指导变速变载条件下的拉深成形工艺参数的设计。
综上所述，采用变速变载的方法，能够有效地提高材料的成形性能，改善冲压成形质量。本文的研究成果为今后采用变速变载方法，解决难成形材料和复杂零件的成形奠定了初步的理论基础。
Due to a strong effort for light-weight stamped parts in automotive and aerospace industry, materials with the characteristics of light weight and high strength show their widely application foregrounds. However, the stamping process is faced with a particular challenge because of poor formability and more springback for difficult-to-deform materials. Some trial-and-error efforts for dies and moulds are used to deal with problems by controlling blank-holder force, optimizing the design of draw-bead, reducing friction and correcting springback-related problems, but these methods are costly and time consuming. Actually, during the stamping process, variable forming speed and forming loading are able to effectively improve formability and reduce springback and cost for light-weight alloys and high-strength steel on the conditions of no-modifying the dies and moulds. So further research on the forming laws and forming mechanism during variable forming speed/forming loading stamping process is significant to improve servo pressure forming technology and advance the forming quality. In this dissertation, a systematical and thorough investigation on variable forming speed/forming loading influencing forming limits and springback of sheet metals has been carried out by using the method combining plasticity theory, finite element numerical simulation and experimental study.
A virtual power equation of deep-drawing of sheet metals is established based on plastic mechanics theory. On this foundation, the study on formability affected by forming speed and forming loading is made according to the change of total deformation power. And then the influences of forming speed on the equivalent stress and equivalent stain rate in the deformation zone are studied by finite element simulation. It is found that reasonable variable forming speed can change the total deformation power so as to improve the formability of sheet metals effectively.
During the stamping process, friction shows its importance on forming speed influencing forming process. The friction behaviors in the flanges area and die corner radius area during the variable forming speed/forming loading stamping process are studied by flat plate friction test and stretch bending friction test. It is found that, the friction coefficient decreases with the increase of the normal pressure, while increases with the increase of the forming speed and especially friction coefficient is linear increasing with forming speed at the stable stage. The effects of forming speed on friction in die corner radius area are smaller than the one in the flanges area, and initial friction force and fluctuation amplitude of friction force are reduced by reasonably setting the variable forming speed. The above conclusions provide an important guide for the design of variable forming speed/forming loading process. So on the basis of above experimental results, a elastic-plastic contact model between sheet metal and die surface is established based on the surface topography of sheet metals, and the relationships between real contact area and normal contact loading are quantitatively obtained to improve the calculation accuracy of shear friction force. An advanced friction model considering forming speed, normal loading, materials properties and surface topography of sheet metals is developed based on elastic-plastic contact model and results of friction experiments, which quantitatively predicts the friction coefficient in the flanges area. The friction model clearly shows its potential of studying the friction behaviors and improving FE simulations during the stamping process of sheet metals.
To study the effects of forming speed, forming loading, displacement and multiple-hits bending times on springback, the experiments of V-bending and U-bending are performed. It is found that, the effects of forming speed on springback are small, and springback could be reduced greatly by increasing the punch displacement or multiple-hits bending times, especially the effects of forming loading on the springback could make it effectively. Based on a general analysis of stress-strain state, loading boundary conditions, a elastic-plastic materials model and Hill's yield criterion, a springback prediction model is established, which agrees with experiments. The model confirms the quantitative relationship of forming loading and springback, and overcomes the disadvantages of traditional empirical formula only predicting springback for free bending.
Based on the above experimental results, a forming scheme is designed to study the effects of forming speed on formability of sheet metals during the deep drawing process. It is found that, according to forming limit analysis and thickness distribution of formed parts, variable forming speed/forming loading technical scheme for improving the formability is obtained, so forming speed shows its importance on the effects of formability and forming equality during the deep drawing process. When forming speed is designed for different phases, the turning point of forming speed variations should be lied on the place after where the sheet metals have completely turned into the fillet areas, and initial forming speed is not larger due to friction varying with forming speed during the deep drawing process. Therefore, based on plastic theory, friction model and experimental results of deep drawing, the quantitative relationships among critical drawing ratio of sheet metals, materials parameters and forming speed are obtained on conditions of variable forming speed/forming loading of deep drawing. The equation could be used to verify the feasibility of the variable forming speed/forming loading process and to provide a theoretical guide for parameters design of deep drawing.
In a word, the processing technology of sheet metals for variable forming speed/forming loading not only improves the formability of sheet metals, but also advances the forming equality of stamping parts, so the results could provide a theory basis for stamping production of poor formability material and complex features of parts.
引文
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