Research On Bending Processes Of Lightweight Structures

Lightweight is the long-term goal of aerospace, rail transport and other industries. As the typical lightweight structures, the integral panel skins of aluminum alloys and complex cross-section profile of magnesium alloys have been widely used in the launch vehicles, missiles, aircrafts and automobile traffic. These lightweight structures are often bent to appearance with certain curvature in order to acquire the overall structure of superior performance and improve the aerodynamic appearance. However, these lightweight structures are typical components with multi-ribs. And because of their complex structures, lightweight structures that manufactured by traditional bending process are prone to producing various defects and have the poor geometric accuracy. Therefore, it is necessary to research the advanced bending forming process of lightweight structures. According to the analysis of specific lightweight structures, the forming processes of mechanical milling-filling roll bending of integral panel skin of aluminum alloys and warm tension-rotation bending of magnesium alloy profiles are proposed. By means of theoretical analysis, finite element numerical simulation and experiments, the filling roll bending process and warm tension-rotation bending process are researched. The main contents and results are as follows:(1) The bending mechanics of the integral panel skins and profiles are studied. The forming principle of integral panel skin filling aided roll bending is analyzed systematically; the mechanical state, geometrical relationship and springback rule of rib and skin of integral panel skin with rib-inward and rib-outward are researched. The forming principle of warm tension-rotation bending of magnesium alloy is studied; the mechanical state and springback rule of different feature positions on the inside and outside of the profile are investigated. The theoretical analysis provides theoretical foundations for the mathematical model of bending process.(2) The stress-strain curves are gained through uniaxial tensile tests. The three-dimensional elasto-plastic finite element model of aluminum alloy integral panel skin filling aided roll bending is established based on MSC.Marc package. The changes of stress and strain and springback law of integral panel skin during roll bending process are analyzed. The role of filler in bending process is revealed. The three-dimensional elasto-plastic thermo-mechanical coupled finite element model of magnesium alloy profile warm tension-rotation bending is established. The evolution rules of stress, strain, temperature, springback and geometric dimensions of the profile during warm tension-rotation bending process are sumarized. (3) A roll bending equipment and data acquisition system for forming technical parameter are developed. The rubber and plastic filler are fabricated, and filling aided roll bending experiments are carried out in order to investigate the influence of filler and process parameters on surface quality and geometry accuracy of integral panel skins. The results indicate that the filler can meet the need of integral panel skins during roll bending process. Filler can change stress state of panel which generates uniform plastic deformation and improves the deformation harmonization between rib and skin. The radius of integral panel skin increases with increasing the top-roller reduction. When the reduction is small, the radius decreases sharply and the decrease amplitude is big. However, the decreasing trend becomes slower with the reduction increases. The empirical formula based on the Least Squares Regression shows the relationship between bending radius and reduction is the power exponential function.(4) A tension-rotation bending equipment for magnesium alloy profiles is developed, and some warm tension-rotation bending experiments are conducted on it. The effects of forming temperature, bending angle and pre-tension amount on springback angle and geometric accuracy are analyzed. Moreover, the comparison between experimental results and simulation results is'conducted. Microstructure and microtexture of the profile are investigated by using an optical microscopy, X-rays diffraction and EBSD analysis system. The results show that the simulation results agree well with the experimental results. The effects of process parameters on springback and geometric dimension are coincident for both methods. Therefore, the simulation results can be used to conduct experiments. The evolution of microstructure and texture shows that during the warm tension-rotation bending process, a big number of twins emerge, which changes the initial orientations of grains, accommodates deformation of the c-axis and improves the plastic property of the profile.