【作者】 周健；

【导师】 郭斌；

【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,金属箔板在电子工业、微机电系统、医疗以及新能源领域中的应用日渐广泛。塑性微成形技术以其高效率、高质量和低成本等优点成为微型零件批量化生产的首选。然而,金属箔板的力学性能和断裂行为都与宏观尺寸的板材有着很大的区别,目前这方面的研究工作不多,这制约了金属箔板微成形技术的发展。因此,研究金属箔板在塑性变形过程中产生的尺寸效应十分必要。本文以纯铜箔和黄铜箔为实验材料,通过对不同厚度和晶粒尺寸的铜箔试样进行单向拉伸实验,研究箔板的屈服强度、抗拉强度和延伸率等参数的尺寸效应。为了提高拉伸试验的应变测量精度,研制了基于图像测量技术的非接触式视频引伸计,建立了可靠的适用于金属箔板的拉伸实验方法。拉伸实验结果表明:试样宽度对铜箔试样力学性能的影响不大,屈服强度的大小同时受到厚度和晶粒尺寸的影响,但与其没有明显的比例关系;其变化规律既不同于金属薄板的“越小越弱”,也与金属薄膜的“越小越强”有区别,表现出复杂的尺寸效应。从位错理论出发分析了板材屈服强度尺寸效应的机理:表面层晶粒的流动应力的下降造成了屈服强度“越小越弱”的现象,而“越小越强”则是应变梯度强化作用的体现,其影响随着厚度的减小快速增强。由于箔板的厚度介于薄板和薄膜之间,因此屈服强度对厚度的变化很敏感,其变化规律也表现为两者之间的“过渡”。从实验结果来看,两种铜箔的实验规律几乎一致:厚度约为特征长度的10倍时是个临界点,对于铜箔,若厚度大于40μm,屈服强度随厚度的变化主要表现为“越小越弱”,而小于40μm则会表现为“越小越强”。综合表面层晶粒和应变梯度的影响,以Hall-Petch公式为基础建立了包含厚度和表层晶粒比例的屈服强度关系式,此关系式可以较好地描述箔板屈服强度的尺寸效应。铜箔的应变硬化指数与晶粒尺寸和厚度成正比。抗拉强度与厚度成正比,与晶粒尺寸成反比,但在厚度晶粒尺寸比很小时与厚度成正比,而在t/d很大时成反比关系。延伸率随着晶粒尺寸的增长而增大,且随着厚度的减小急剧下降,这完全不同于金属薄板的变化规律。采用球体孔洞模型和基于应变梯度的本构方程对孔洞的长大率进行了计算,结果表明应变梯度对孔洞的长大有明显的抑制作用。拉伸试样的断口分析揭示了铜箔延伸率的尺寸效应机理:在应变梯度的影响下,厚度小于40μm的铜箔的断裂机制是沿晶断裂,因此延伸率较低;而厚度较大的铜箔的断裂机制是韧窝-微孔聚集型断裂,变形中产生的孔洞也延缓了断裂的发生,使得延伸率较高。使用有限元软件DYNAFORM进行了不同条件的微拉深成形模拟。设计加工了多套微拉深模具,采用不同条件的坯料进行了微拉深成形实验。结果表明:随着拉深件直径的减小,无量纲拉深力和极限拉深比都随之降低,但与此同时,摩擦力的影响却逐渐增大,使直径1mm拉深件的极限拉深比在坯料晶粒尺寸较大时出现了与延伸率成反比的现象;微拉深件的表面质量主要取决于晶粒尺寸。微拉深成形的尺寸效应主要是由铜箔力学性能的改变造成的,摩擦力对其也有重要的影响。最后,采用落料拉深复合模,成形出了直径1mm到8mm的,表面质量良好的拉深件。更多还原

【Abstract】 In recent years, metal foil is wide applied in electronics industry, micro electromechanical systems, medical and new energy fields. Plastic microforming technology becomes the first choice of mass production of miniature parts for its advantages including high efficiency, high quality and low cost. However, the mechanical properties and fracture mechanism of metal foil differ significantly from sheet in macroscopic size, and the current research in this area is infrequent, which limits the development of metal foil microforming technology. Therefore, the study of size effects of metal foil during plastic deformation is necessary.Tensile tests of pure copper foil and brass foil with various thickness and grain size were performed at room temperature to investigate the size effects on yield strength, tensile strength and elongation. A set of non-contact video extensometer based on image measuring technique was developed to improve the strain measurement accuracy of the tensile test. A reliable tensile test method for metal foil was established.The results showed that the influence of the sample width on the mechanical properties of copper foil is inconspicuous. The yield strength affected by the thickness and grain size, but with no obvious proportional relations between them, its variation was different from the "smaller is weaker" of the metal sheet, but also from the "smaller is stronger" of the metal film, presented the complicated size effect. The size effect mechanism of yield strength was analyzed from the dislocation theory, the "smaller is weaker" phenomenon of yield strength was caused by the surface layer, and the "smaller is stronger" was the result of the strengthening effect of strain gradient, which rapidly increased with decreasing thickness. As the thickness of foil was intervenient between sheet and film, so the yield strength was sensitive to the change of thickness, which also showed the variation of "transition". The experimental rules of two kinds of copper foil were almost identical: the value about 10 times of the characteristic length was a critical point for the thickness. For the copper foil, the variation of yield strength would exhibit the "smaller is weaker" if the thickness was greater than 40μm, but otherwise it showed as "smaller is stronger". Integrated the influences of the surface layer and strain gradient, a modified equation of yield strength contained the thickness and the ratio of surface layer grain based on Hall-Petch equation was established. This relationship can describe the yield strength size effect of foil properly.The strain hardening exponent of copper foil was proportional to the grain size and thickness. The tensile strength was proportional to the thickness, and inversely proportional to the grain size. Elongation increased with the increasing grain size, and decreased sharply with the decreasing thickness. The variation of copper foil in elongation was entirely different from sheet metal. The calculated results based on spherical cavity model and strain gradient constitutive equation showed that the growing rate of hole is significantly restrained by the strain gradient. The fracture surface analysis of tensile specimen revealed the size effect mechanism of elongation. The elongation of copper foil which thickness less than 40μm was lower because its fracture mechanics was intergranual crack. As the fracture mechanics was dimple fracture, and the holes postponed the fracture, the elongation of the thicker copper foil was well.The simulation of micro deep drawing was performed by finite element software DYNAFORM. Multiple sets of micro deep drawing dies were designed and fabricated. Micro deep drawing experiment with billets under different conditions was carried out to study the size effects in microforming. The results showed that the nondimensional drawing load and limit drawing ratio were decreased with the reduction in the size of deep drawing parts, but at the same time, the impact of friction escalated. Influenced by the increasing force of friction, the LDR of 1mm drawing parts with larger grain size was inversely proportional to the elongation. The surface quality of the micro deep drawing parts depended on the grain size. Finally, drawing parts of 1mm to 8mm in diameter with good surface quality were drawn by blanking-deep drawing compound dies.更多还原