| As the’sun-rise metal’in the twenty-first century, magnesium alloys are a kind of attractive lightweight structural materials with excellent inherent properties of low density, high specific strength, high specific stiffness, easy recovery and so on, so they have become the promising candidates in industry. In recent years, with the increasing demand in lightweight structure and environmental protection, the applications of magnesium alloys in the automotive and aerospace fields have been further promoted. However, their poor plasticity and weak formability induced by the hexagonal close-packed structure seriously limit their widespread applications in the industry. Incremental forming (IF) is a flexible and near net-shape forming technology, and the workpiece is locally deformed successively to improve the formability, which has been an important development direction in the advanced manufacturing fields. As the application of high-energy field in plastic forming, the electro-plastic effect (EPE) can improve the formability of hard-to-form materials, so it has attracted widespread attention. However, the high demand in current density limit its application. Coincidentally, due to the characteristic of point-by-point forming, there is a very small contact area during IF, which can meet the demand of current density relatively easily. The combined technology with EPE and IF can improve the formability of magnesium alloys and forming the quality effectively to promote the improvement of forming technology for magnesium alloys, which has an important theoretical significance and engineering value.In this study, in order to improve the formability of magnesium alloys, the EPE was applied to IF, and the electro-plastic incremental forming (EPIF) technology was proposed. Based on experimental investigation and theoretical analysis, the processing technique, processing mechanism and EPE mechanism were discussed. The main contents and conclusions of this thesis are shown as follows:(1) The electro-plastic incremental forming (EPIF) technology was proposed, and its feasibility and technical advantages were demonstrated by theoretical analysis. Based on the theoretical analysis, the self-design experimental platform was built. Then, the friction mechanism combined with the working condition was investigated, and the results show that the friction mode in EPIF is a current-carrying and hybrid friction, and the MoS2 coating is an appropriate lubricant to improve the formability in EPIF. In addition, the forming procedure and forming quality were investigated by theoretical analysis, which can provide a theoretical guidance.(2) By means of the response surface methodology, the influence rules and significance of different processing parameters on formability of AZ31B sheet were investigated. It was found that, the electropulse parameters is the most significant factor, and the forming limit of AZ31B sheet increases obviously with the increase of electropulse parameters. With the increase of feed rate and step size, the forming limit decreases gradually, and they have an equal significance. In addition, the electropulse parameters interact with the feed rate and tool diameter, and so is the feed rate and step size. Based on the orthogonal test, the influence rules and significance of different processing parameters on the forming quality of workpieces were investigated. The results showed that, the most significant influencing factor on forming quality is the electropulse parameters, and with the increase of electropulse parameters, the forming quality of workpieces are improved obviously. In addition, the lower feed rate, medium step size and tool diameter can decrease the errors in forming height, and the springback angle increases when the tool diameter is too small because of the excessive accumulation of elastic deformation. For the surface hardness of workpiece, only the medium tool diameter can cause a relatively obvious increase, while other factors are unsignificant. By the comprehensive analysis, both of the formability of sheets and forming quality of workpieces are relatively high when the electropulse parameters, feed rate, step size and tool diameter were set in 80V/400Hz,800mm/min,0.2mm and 8mm, respectively. Compared to the workpiece without electropulse at room temperature, the formability of AZ31B sheet with aforementioned electropulse parameters are greatly improved.(3) Based on the research results of processing technique, the reasonable processing parameters were selected, and the EPIF experiments were carried out. The temperature, forming force and strain state in forming process were investigated by the experimental investigation and theoretical analysis. It is found that, the temperature in a fixed point of workpiece is determined by the real-time positon and the temperature of forming tool. The temperature of the fixed point is relatively high only when the point is near the current forming zone, and the duration at high temperature is very short. In addition, the temperature-rise rate depends on the electropulse parameters, tool diameter and forming angle, rather than the feed rate and step size. The higher electropulse parameters can cause a higher temperature-rise rate, while the influencing trends of tool diameter and forming angle on higher temperature-rise rate are negative. The varying pattern of forming force was investigated, and the results showed that, with the increase of forming deep for workpieces, the vertical force increases in the early stages, then it tends to be stable, and the decreasing trends emerges in the later stage due to the continuous effect of electropulse. The horizontal force of tool is generated only in the tangential direction of tool motion instead of radial direction. The horizontal forces in the direction of X-axis and Y-axis present alternative variation as sine wave with time, and the phase difference is π/2. Furthermore, the electropulse parameters, step size and forming angle have a significant effect on forming force, while the tool diameter is not significant. The higher electropulse can decrease the maximum axial force, but the maximum vertical force will increase when the feed rate, step size, tool diameter are higher. The research results of strain state show that, the linear loading mode of rectangular pyramid presents the plane strain state, while the circular loading mode in cone model has a tendency of biaxial stretch. The forming limit of EPIF in an appropriate processing parameter combination can reach a very high forming limit.(4) By means of the IF tests and tensile tests with different electropulse parameters, the influence rules of electropulse parameters on EPE were studied, and combined with the theoretical model, the EPE pattern was discussed. It was found that, The EP effect of magnesium alloy depends mainly on the root mean square (RMS) current density. With the increase of current parameters, the materials flow stress decreases gradually, while the elongation to fracture almost keeps unchanged after it reaches a certain value, and even shows a decreasing trend. There should be an optimal electropulse parameter for EP tensile deformation. The athermal effect in EP effect is proved to exist, and the peak current density decides the athermal effect in electro-plastic effect. The higher peak current density can lead to a more potent EP effect when the RMS current density keeps approximate. Furthermore, it seems to have a current density threshold for EP effect, and the promotion effect can be enhanced sharply after the threshold while the effect is little before the value.(5) The microstructural mechanism of electropulse on improving the formability of magnesium alloys was investigated. The results showed that, electropulse can reduce the dynamic recrystallization (DRX) temperature and accelerate the DRX progress, moreover, the crack growth can be restrained, which improve the formability of magnesium alloys. The rapid heating pattern in EPIF restrains the grains growth and the fine grains are obtained, which makes the grain boundary sliding mechanism operable. The electropulse with a high parameter can lead to an inverse eutectic reaction (a+P=L) at the necking fracture zone because of the transient high temperature. The presence of appropriate amount of inverse-eutectic liquid phase can optimize the deformation mechanism and improve the materials ductility. However, the ductility will be deteriorated when the grain boundaries are too weak due to the overreaction of inverse eutectic process. |
PDF Full Text Request