镁合金及先进高强钢电脉冲辅助塑性成形性能研究

发布时间：2018-08-11 11:53

【摘要】：近年来为满足汽车轻量化的要求,使用镁合金、先进高强度钢(Advanced High Strength steel,AHSS)等轻型材料来制造汽车零部件越来越受到各大车企的重视。然而,镁合金为密排六方的晶格结构,室温塑性变形能力较差;先进高强钢较高的流变应力,导致其成形力较大;同时,镁合金较低的弹性模量、先进高强钢较高的屈服强度,导致二者弯曲变形时回弹严重。这些问题严重制约了这两种轻型材料在汽车工业中的应用。迫切需要针对这些轻型材料开发新的冲压工艺。电塑性成形技术是指材料变形时在其塑性变形区施加电流,以降低材料的变形抗力、提高材料塑性的成形技术。该技术已经成功地应用在拔丝、轧制等成形工艺上,并取得了良好的效果,同时电流对材料内部的晶粒细化、损伤修复以及表面质量的改善具有积极作用。因此,将电塑性成形工艺应用于轻型材料的冲压工艺中,有着很好的应用前景。尽管电塑性成形技术已经过多年的研究,然而相关的理论模型和实际应用的研究尚不够充分。理论模型方面,有学者根据材料的物理机理提出了一些流动应力的模型,但该类模型旨在描述电塑性效应的物理意义,不便于工程应用。另一方面,电塑性冲压工艺的探索也刚刚起步,相关理论和技术都有待研究。针对以上问题,本文开展了以下工作:(1)通过AZ31B镁合金和DP980 AHSS的电脉冲单向拉伸实验和等温无电单向拉伸实验,研究了脉冲电流对材料流变行为的影响,证实了电塑性效应能降低AZ31B镁合金的流动应力、改善其塑性,发现了DP980 AHSS的“反电塑性”效应;基于微观组织分析,解释了观察到的现象,阐明了脉冲电流对材料微观组织的影响。通过修改Johnson-Cook流动应力模型,建立了AZ31B镁合金考虑脉冲电流影响的电塑性流动应力模型,并通过实验验证了模型的正确性。(2)通过AZ31B镁合金和QP980 AHSS的电脉冲应力松弛实验和等温无电应力松弛实验试验,研究了脉冲电流对材料应力松弛行为的影响,分析了温度对QP980高强度钢板应力松弛行为的影响,指出了适合该材料电脉冲松弛的合适温度;通过金相分析,解释了脉冲电流对镁合金应力松弛的作用机理;基于蠕变力学基本理论,推导并建立了考虑脉冲影响的AZ31B镁合金和QP980AHSS的应力松弛模型,通过实验检验了模型的正确性。(3)采用平面应变假设,基于考虑脉冲影响的AZ31B镁合金和QP980AHSS的应力松弛模型与Mises屈服准则,将单轴应力状态下得到的应力松弛模型扩展到多轴应力状态,建立了电脉冲辅助的V形弯曲回弹角的预测模型,并通过实验验证了模型的准确性。通过微观组织分析,对电脉冲抑制回弹的机理进行了探讨。(4)基于脉冲电流有利于降低流动应力、改善材料的塑性等优点,研发了若干脉冲电流辅助的典型塑性成形工艺,包括:电脉冲辅助的圆筒形件拉深工艺、滚轮包边工艺和扩孔工艺。工艺考虑了成形动作的实现、脉冲电流的流通路径和绝缘等设计要素,成功地将脉冲电流引入材料的塑性变形区,明显地改善了材料塑性变形能力。上述工艺为塑性相对较差或者难成形材料的塑性成形提供了新的思路,具有较好的应用前景。
[Abstract]:In recent years, in order to meet the requirements of automobile lightweight, the use of magnesium alloys, advanced high strength steel (AHSS) and other light materials to manufacture automobile parts has attracted more and more attention of automobile enterprises. At the same time, the lower elastic modulus of magnesium alloy and the higher yield strength of advanced high strength steel lead to serious springback during bending deformation. These problems seriously restrict the application of these two light materials in automotive industry. The technique is a forming technique in which a current is applied to the plastic deformation zone of a material to reduce the deformation resistance and improve the plasticity of the material.The technique has been successfully applied to wire drawing, rolling and other forming processes and has achieved good results.At the same time, the current refines the grains in the material, repairs the damage and improves the surface quality. Therefore, the application of electroplastic forming technology in the stamping process of light materials has a very good application prospect. Although electroplastic forming technology has been studied for many years, the related theoretical model and practical application research are not enough. On the other hand, the exploration of electroplastic stamping process is just beginning, and the related theories and techniques need to be studied. In view of the above problems, the following work has been carried out in this paper: (1) through the AZ31B magnesium alloy and DP980 AHSS pulse. The effects of pulse current on the rheological behavior of AZ31B magnesium alloy were studied by uniaxial tensile test and isothermal uniaxial tensile test. It was proved that the electroplastic effect could reduce the flow stress and improve the plasticity of AZ31B magnesium alloy. By modifying Johnson-Cook flow stress model, the electroplastic flow stress model of AZ31B magnesium alloy considering the effect of pulse current was established, and the correctness of the model was verified by experiments. (2) The electro-pulse stress relaxation experiment and isothermal non-electric stress relaxation experiment of AZ31B magnesium alloy and QP980 AHSS were carried out. The effect of pulse current on stress relaxation behavior of QP980 high strength steel sheet was studied by experiment, and the effect of temperature on stress relaxation behavior of QP980 high strength steel sheet was analyzed. The stress relaxation model of AZ31B magnesium alloy and QP980AHSS considering the effect of pulse is deduced and established, and the correctness of the model is verified by experiments. (3) Based on the stress relaxation model and Mises yield criterion of AZ31B magnesium alloy and QP980AHSS considering the effect of pulse, the stress relaxation model under uniaxial stress state is obtained by using plane strain assumption. The model is extended to the multi-axial stress state, and the V-shaped bending springback angle prediction model assisted by electric pulse is established. The accuracy of the model is verified by experiments. The mechanism of restraining springback by electric pulse is discussed through microstructure analysis. (4) Based on the advantages of pulse current, such as reducing flow stress and improving plasticity of materials, the research and development are carried out. Several typical pulse current assisted plastic forming processes, including electric pulse assisted cylindrical drawing, roller edge wrapping and hole reaming, have been developed. The process considers the realization of forming action, the flow path of pulse current and insulation. The pulse current is successfully introduced into the plastic deformation zone of the material and is improved obviously. The above process provides a new idea for plastic forming of materials with relatively poor or difficult plasticity and has a good application prospect.
【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG386

【相似文献】