Research On Microforming Mechanism And Precision Micro Bulk Forming Apparatus

With the rapid development of micro-electro-mechanical systems and micro-system technology, a high requirement is taken out for the micropart manufacture technology, which gives a chance of development for the microforming technology. The technology is a process to manufacture micropart by plastic deformation, and it is very suitable for the low-cost and mass production of micropart. Since part dimension or typical dimension reaches the same order of magnitude as sub-millimeter or micron, and the size effect occurs obviously, microforming process is different from the traditional plastic deformation. And the forming mechanism and deformation law change, then micrforming process becomes a new research area.This dissertation founds a polycrystal model based on crystal plasticity, and the microforming mechanism is disclosed. Using the developed special precision microforming apparatus that is drove by a PZT, the experimental investigation of size effect in microforming process is carried out, and the forming process of micropart is studied by using the typical microgear as the forming object. The key technology that constrains microforming process is settled.From the viewpoint of multi-grain structure of material, a polycrystal model is founded based on the crystal plasticity theory. A deformable body is divided into two parts based on the difference of the dislocation pile-up degree and deformation restriction of grain, and they are single crystal area on the free surface and polycrystal area in the internal portion of deformable body. The areas are analyzed by the crystal plasticity and traditional plastic deformaton theory respectively. The upsetting deformation of micro cylindrical specimen is simulated by the founded model, and the effect of specimen size and grain original direction on the flow stress and surface topography is investigated. The result shows that the flow stress decreases with the decreasing of the specimen size, the size effect occurs obviously. And the deformation ununiformity is caused by the anisotropy of the grain, which leads to irregularity surface of deformed specimen.To carry out experimental investigation of microforming process, a precision micro bulk forming apparatus is developed with a series-wound structure using a PZT as the punch driver. A position adjustment device is designed to adjust the position of the PZT, and it can also meet the requirement of the large displacement in some processes. A PZT driver based on a PID control method is selected, and a process controller is designed for the microforming process. The test and analysis of the micro bulk forming apparatus show that it is suitable for the experimental research of the microforming process.The experimental investigation is carried out by the upsetting test. The effect of specimen size, grain size, deformation degree and temperature on the flow stress is studied, and the mechanism of size effects is analyzed from the viewpoint of polycrystal structure. Using the metallography, the effect of grain size and strain on deformation ununiformity are investigated, and the mechanism is analyzed from the viewpoint of the anisotropy of grain.The cavity dimension also leads to size effect in microforming process. A coining process is used in the dissertation, and the effect of the die cavity dimension (b), grain size (L) and their ratio L/b on the filling ability are investigated. The result shows that ratio L/b is the main effect factor.Based on the above research, the investigation of microforming process of typical microgear is carried out. Using the developed floating die device, the forming procedure of micogear is studied. Effect of punch speed, load and forming temperature on the microforming process is studied, and the filling law is analyzed from the viewpoint of stress station and polycrystal structure of the specimen. The distribution of flow lines in the longitudinal section and the cross section is analyzed, and the measurement of surface roughness and nano-hardness in cross section of the microgear is implemented. The results show that the formed microgear owns fine surface quality and compositive capability.