Modeling, Analysis And Experimental Study Of Micro/Meso Scale Sheet Forming

Recent years, there is an increasing trend of more and more minimized products. The micro products are widely used in cell phone, laptop, micro navigator, IC encapsulation, medical appliances and micro sensors. It is estimated that the whole industry brought by micro products will reach billions of dollars annually. The trend of micro products needs new micro/meso manufacturing technology.Micro/meso forming process is an important technique, which is economical and suitable for mass production. However, the micro/meso scale forming process theory and technology are not mature, especially for the special phenomena called size effects, which are totally different from conventional manufacturing process. Thus these so called size effects make mature traditional forming process design theory not applicable directly in micro/meso scale. In this thesis, taking micro/meso stamping process as the research object, two different kinds of size effects (size effects in material behavior and size effects in micro/meso friction) are studied in details. By introducing a scale effect factor, scale dependent material model and friction model are established, which can be used to analyze the size effects in micro/meso scale forming process. Moreover, FEM simulations and experiments of micro/meso sheet stamping process with soft punch are conducted to unveil the characters of this kind of micro/meso sheet forming process. Meanwhile, the friction size effects are investigated during forming process. Main research of this thesis focuses on the flowing aspects:1) Investigation of material size effects in micro/meso forming process and its constitutive modelingIn this thesis, the reason why the size effects occur in micro/meso scale is studied examining the deformation theory of grain and grain boundary, the grain structure of the material can be classified into two different kinds of grain: surface grain and inner grain. Therefore the whole deformation behavior consists of surface grain deformation and inner grain deformation. The material constitutive model in micro/meso scale can be established by using the scale effect factor, which is used to depict the ratio of surface grains to inner grains. Moreover the constitutive model is verified by some experimental results cited from literatures as well as our own experiments. Different thickness metal sheets with same grain size are prepared as tensile specimen and also tensile experiments are performed to study the material deformation behavior and to verify the deueloged constitutive model.2) Investigation and modeling of friction size effects for micro/meso forming processIn micro/meso manufacturing, another special character is the friction size effects besides material size effects, which are totally different from that in conventional manufacturing process. By adopting an open lubricant model and a close lubricant model, the friction size effects with decreasing of manufacturing scale can be unveiled. The ratio and change of real contact surface, as well as the lubricant models on the contact surface between material and tools are analyzed based on the Wanheim/Bay general model. Then, a scale effect factor is introduced and the friction model in micro/meso scale is established as the function of normal pressure. Moreover, the micro friction model has been applied into micro ring press process simulation, with which the influence of friction size effect on micro/meso forming process can be studied.3) Numerical modeling and analysis for micro/meso sheet forming process with soft punchUsing the Mooney-Rivlin super plastic material model to describe the behavior of soft material and applying incompressible super plastic element, the numerical simulation mode for micro/meso sheet forming process with soft punch is established. Furthermore, the material model, element, contact, load and boundary condition definitions are also discussed in detail. Three different simulation models with different types of ratio of depth to width h /w are established and the influence of different process parameters( material, hardness of soft punch and friction condition) on the micro/meso sheet forming process are detailedly investigated. And the effect of features of die channel on the sheet formability in micro/meso forming process is studied with different h /w.4) Experimental study of micro/meso sheet forming process and friction behivourDifferent types of dies with various channel section (different geometric parameters) and different roughness on the contact surface of the dies are designed and construted. Using these dies, the forming experiments are done at various pressures 20KN, 60KN and 100KN to study the forming process. By these experiments, the numerical simulation models proposed in this study can be validated. Furthermore, three different types of specimen with different width are applied in the tensile tests and forming experiments. By comparing numerical simulation results and experimental results, the Equivalent Friction Coefficients (EFC) is obtained for different friction contact scale and various roughness. Therefore the friction size effects in micro/meso scale are investigated.This thesis research focuses on two different special phenomena: material size effects and friction size effect. Material model and friction model considering the size effects in micro/meso scale are established and a simulation model for micro/meso sheet forming process with soft punch is developed to study the key process parameters. Besides that, forming experiments are done to investigate the characters of micro/meso sheet forming process and friction with the decreasing of manufacturing scale. Research work of this thesis will be helpful for further work in this new field.