【作者】 马宁；

【导师】 董湘怀；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2005， 硕士

【摘要】 随着微型化趋势的迅速发展,以形状尺寸微小或者操作尺寸极小为特点的微细加工技术已经成为人们在认识微观领域和改造客观世界的一种高新技术。从移动电话到计算机技术,从微型数码设备到医药卫生等各个方面,微细零件具有广阔的应用空间,同时应用传统的加工技术来加工这些微细零件也成为可能。但是,由于微细零件的尺寸非常小,在加工过程中材料会表现出不同于宏观加工的特性。在宏观塑性加工过程中,根据实验数据和已有的经验,已经建立起了一套完整的加工体系,但在微细塑性加工中,许多因素都发生了变化,因此必须考虑微型化对加工过程的影响。研究发现,当不均匀塑性变形尺度在微米量级时,材料就将表现出强烈的尺度效应(Size effects),本文系统地介绍了两种为解释第I、II 类尺度效应而提出的典型模型,一为表面层模型,即考虑金属表面晶粒对金属屈服应力的影响的模型; 另一个为考虑应变梯度效应的模型,该模型在已有的材料特征基础上引入了尺度效应。目前,对微细塑性成形技术的研究还没有成形的理论体系和系统的加工方法,因此本文采用理论分析、计算机模拟和实验分析相结合的方法,针对微细零件的拉延成形的基本工艺开展研究,分析微细塑性成形理论方法及确定工艺参数方法,将传统确定圆筒拉延问题的参数设计转化为无量纲的形式,通过确定极限拉应力和极限周向应力,绘制成图表以确定成形可行区域。由于采用无量纲形式,因此认为可将这种方法运用于微细拉延问题。本文给出了运用该方法确定成形可行区域的实例。最后结合有限元模拟技术,在分析中加入新的本构关系,分别分析了两种模型对材料成形性能的影响。结果表明,表面层的弱化降低了拉延件的抗起皱能力,但另一方面却提高了其抗破裂能力; 应变梯度对材料的硬化降低了拉延件的成形性能。参考已有的实验数据及利用ABAQUS 定性地分析了所提方法的正确性和实用性。尽管目前提出的理论还不成熟,但是随着研究的不断深入,这些理论将会逐步完善。更多还原

【Abstract】 The production of miniature parts is becoming more and more important due to the trend of miniaturization, which is increasingly determining the development of product ranging from mobile phones, computers and micro digital camera to medical products and so on. The application of conventional manufacturing processes for the production of such micro-parts is possible, but there are some problems that result from the small dimensions. During micro-forming materials will behave some characters that are different from normal size processing. Up to now, the systematical research for micro-forming have not come into being. In conventional forming processing, a systematical processing technology has been developed, but in the micro range, the processing parameters have changed greatly. Many experiments have shown that materials display strong size effects when the characteristic length scale associated with non-uniform plastic deformation is in the order of microns. Two kinds of size effects on micro deep drawing are investigated, which can be explained by surface layer model and strain gradient model. With the increase of the thickness ratio of surface grain layer to the whole sheet, the bending moment of the sheet decreases, leading to decrease of feasible zone in the feasibility diagram. This paper also systematically studies the processing parameters and the size effect of micro-plastic-forming. The formulation concerning cylindrical deep drawing parameters is rewritten in a dimensionless form. A diagram, in which a feasible zone is drawn to bound both the maximal allowable tension and compression stress during the deep drawing process, is established. Since it is presented in a dimensionless form, it may be applied for both conventional and micro deep drawing. In this paper, cylindrical cup deep drawing is taken as an example to show the dimensionless process design method. Numerical simulations are carried out to compare the strain distribution with or without consideration of size effect. It is found that in micro deep drawing, surface layer softening may have dual effects on micro formability: it decreases bending moment and the wrinkling resistance capability, but strain distribution tends to be more uniform, which is advantageous for preventing fracture. Strain gradient hardening may lead to heavier strain concentration and worse micro formability. The feasibility and practicability of the proposed method is qualitatively verified by comparison with exsisting experimental results and finite element simulation.更多还原