【作者】 孟庆当；

【导师】 董湘怀；

【作者基本信息】 上海交通大学 ， 材料工程， 2012， 硕士

【摘要】 随着近现代工业的发展,各行业对微型零件的需求也大大增加,而微型零件的加工技术中,微塑性成形技术以其高效率、高精度和低成本为主要特点,越来越受到广大研究者的重视。微塑性成形过程中,由于材料的晶粒尺寸越来越趋近零件的几何尺寸等,导致材料的力学性能呈现出与常规成形不同的尺寸效应现象。关于金属薄板尺寸效应方面的研究工作尚不够深入,制约了金属薄板成形技术的发展。因此本文针对尺寸效应开展研究。首先,将不同板厚304不锈钢薄板在不同温度下进行了热处理,然后进行了单向拉伸实验。由于表面韧性钝化膜具有类似于晶界的强化作用,导致板料的屈服应力随板料的减薄而增加,即表现出所谓“越薄越强”的尺寸效应现象,在经典霍尔-佩奇公式中添加相对厚度项,对不同板厚屈服应力进行了较好的预测;由于板料厚向晶粒数减少,使变形的不均匀性增加,导致板料的延伸率和抗拉强度随板料的减薄而降低,即表现出所谓“越薄越脆”的尺寸效应现象。其次,对不同条件下热处理后的304不锈钢薄板进行了微弯曲实验,观察到板料的回弹角和无量纲弯矩随板厚减薄而增加的“越薄越强”的尺寸效应现象,采用修正的Nix-Gao应变梯度硬化模型预测了这种现象,所得结果与实验吻合。最后,进行了304不锈钢薄板的微拉深实验,实验结果表明:板料的极限拉深比随板厚和筒形件尺寸的减薄而降低,表现出“越薄越脆”的尺寸效应现象。分别从极限拉深比计算公式和凸模在拉深过程的作用方面解释了这种现象。建立含有径向应变梯度项的最大拉深力计算公式,较好的预测了不同板厚材料拉深过程中的的最大拉深力。更多还原

【Abstract】 With the development of modern industry, the demands of micro-parts in various fields have been greatly increased, in metal microforming process, plastic microforming technology is investigated by more and more researchers recently for its advantages including high efficiency, high quality and low cost. Size effect is found during plastic microforming process for the grain size is much closer to the micro-parts size. However, the current research in the metal foil of this area is infrequent, which limits the development of metal foil microforming technology. Therefore, the study of this paper is based on size effect to do research.Firstly, 304 stainless steel foils of various thicknesses are heated at different temperature, and uniaxial tensile tests are investigated. the results show that yield stress increases with the foil thickness decreasing, i.e. a size effect“smaller is stronger”, there is a layer of passive film out of the foil surface, the role of this passive film is similar to the enhanced role of the grain boundary, in this paper, the Hall-Petch equation was modified by introducing the influence of foil relative thickness, and applied to calculate yield stress with better agreement with test results; The results also show that limit elongation and tensile stress decreases with the thickness decreasing, i.e. a size effect“smaller is crispier”, the reason is that the numbers of the grain through the foil decrease with the foil thickness decreasing, this makes the forming process more unevenly.Secondly,microbending tests of 304 stainless steel foils are investigated, the results show that springback angle and non-dimensi- onal bending moment increase with the foil thickness decreasing, i.e. a size effect“smaller is stronger”, this phenomenon can be explained by modified Nix-Gao strain gradient hardening model, and the results agree with the experimental data.At last, micro-deep drawing tests of 304 stainless steel foils are investigated, the results show that limit drawing rations(LDR) decrease with the foil thickness and cylinder diameter decreasing, this can be explained by LDR equation and the role of punch in forming separately. An formula with radial strain gradient is established to forecast the drawing force, and the results agree with the experimental data.更多还原