Study On Modeling And Size Effects Of Micro Bulk Forming

With the rapid developments of electronics industry and precision devices, the minimization of products has become one of important development tendency of manufacturing industry. As one of the mass production methods among all kinds of the micro-manufacturing technique, micro-forming process has recently attracted attention of the manufacturers and researchers because of its low cost, low energy consuming, good mechanical property and high precision. However, due to the so called "size effect", the mature theory applicable to traditional macro-forming is not applied to micro-forming directly, which restrained the development of micro manufacturing industry.In this dissertation, the micro bulk forming process was selected as the research object and the scale effects in material and friction behaviors of micro bulk forming process were sharply studied. Firstly, the material model and friction model which are applied to micro bulk forming process were proposed by introducing scale parameters. Then experiments of the micro upper setting process and coining process with micro cylinder were designed and perform to study on the scale effect in micro bulk forming process. Finally, the FEM simulation of coining process with micro cylinder was carried out to investigate the forming mechanism and scale effects in coining process in relation to the experimental results. The main contents are as follows:The essential cause of scale effect in material behavior of micro bulk forming was revealed by dislocation pile up theory, surface model theory and metal crystal plasticity theory. Then a composite material model considering grain-boundary strengthening and surface model theory was established by introducing scale parameters, which was verified by experimental data from literatures. Meanwhile, a region model considering grain-boundary strengthening and surface model theory was proposed, in which the object is divided into grain region and grain-boundary region according to the shape and orientation of the grains; the model was also used for simulation and study on the micro upper setting process and the comparison of simulation and experimental results verify the reasonableness of the region mode. In this thesis, the dry friction behavior in micro-forming process was studied in detail. The constant shear and Coulomb friction models based on Wanheim/Bay friction model were deduced respectively, which are both applied to the dry friction behavior or friction behavior with solid lubrication in micro forming. Those two friction models were also verified by experimental data from literatures. The micro upper setting process without lubrication was simulated by the Coulomb friction models based on Wanheim/Bay friction model and the region model considering grain-boundary strengthening and surface model theory. The size effects caused by the distribution of grain and the size of workpiece in dry friction behavior of micro bulk forming were studied in detail by the simulation results.The experiment of micro upper setting process was designed and performed to analyze the size effects caused by average grain size and specimen size in material behavior. Meanwhile, the solving methods of material constitutive relationship of copper were proposed according to the upper setting data of specimens in different scale by using the composite material model considering grain-boundary strengthening and surface model theory.The experimental system of coining process with micro-cylinder was built. This system was employed to analyze the influence of friction condition, specimen size, forming load on the formability of micro-cylinder. Moreover, the size effect due to the micro-hole size, grain distribution and grain orientation were mainly studied.The material constitutive model suitable for closed micro forming process was deduced by using the modified Hall-Petch Equation according the mechanism of closed micro forming process. The feasibility of employment of material constitutive model based on experimental data and constant shear friction model in FEM simulation of closed micro forming process was proved by analyzing the numerical example. Then the effects of workpiece thickness, friction condition and micro-hole size on the formability of micro-cylinder were investigated based on the simulation results of coining process with micro-cylinder. According to the characteristic of coining process with micro-feature, a region model considering the grain orientation and grain-boundary strengthening was established. Then the simulation of coining process with micro-cylinder was performed by using this region model and the size effects caused by average grain size and micro-hole size were studied in detail according to the simulation and experimental results.