The Drawability Optimization And Springback Analysis Of High Strength Steel Based On Controllable Drawbead

The pace of lightweight automobile has been quickened under the dual pressures of energy shortage and greenhouse effect. Lightweight automobile can be realized and the safety performance of automotive can be improved by using the high strength steel(HSS). So the lightweight material such as HSS for the autobody parts has been widely used year by year. These defects such as crack, wrinkle and springback will appear in the stamping process of HSS because of its high strength and low plasticity. So the new forming technology which will improve the formability of HSS is becoming the research focus.Sheet metal forming is a complicated deformation problem, which involves material nonlinearity, geometrical nonlinearity and boundary nonlinearity. The factors including blankholder force, the shape and layout of drawbead, lubrication have great influence on the quality of stamped parts. The drawbead, which provides the larger drawing resistance in the conditions of smaller blankholder force, is a important method to control the material flow. The reasonable design of drawbead is the key to success or failure of drawing. Up till now, although the scholars at home and abroad have done a large amount of research work on studying the influence law of drawbead in sheet metal forming, their researches are mostly limited to the fixed drawbead whose geometry size and drawing resistance is almost invariant in sheet metal forming. The research shows that part optimal strain path will be obtained and the drawability will be improved by the variable darwing resistance in sheet metal forming. Now the scholars at home and abroad mostly are always adopting the variable blankholder force to obtain the variable darwing resistance, but the research on the variable flow resistance by adjusting the height of drawbead has been seldom studied.Because the height has great influence on darwing resistance and can be controlled easily, a new controllable drawbead technology was put forward to improve the formability of HSS by the real-time height adjustment of drawbead based on the low formability of HSS.The JAC590Y HSS was selected as research object, and its drawability and springback laws were systematically studied based on the controllable drawbead technology by the method of finite element analysis and physics experiment combined with the key technologies such as numerical simulation, optimization algorithm and hydraulic and electric control technology. Three following ascepts were mainly investigated.①The effect of controllable drawbead technology on drawability of HSS and the optimization of controllable drawbead trajectory1)Firstly, the height step-like change and fixed drawbead of HSS box was simulated and experimental verification. It shows that the controllable drawbead has the comprehensive advantage in the aspects of improving the drawability of HSS and reliability of simulation results are verified by physics experiment.2)The trajectory of drawbead height variation matching punch travel has great influence on the drawability of HSS. Three different types of height continuous change drawbead trajectories with six different drawbead forms were designed accroding to the need of forming force in the drawing process. The test schemes were determined by orthogonal test design and the processes were simulated by the software Dynaform. The drawing depth, strain distribution, material inflow are analyzed in different trajectories. The optimal type trajectory, ascent——halt——descent, is determined and the major influencing factors of drawing depth and the optimal combination of each factor level are obtained.3)The GA-BP neural networks of controllable drawbead trajectory factors—limite drawing depth with six drawbead forms are constructed based on the simulation results of the optimal drawbead trajectory. Through the verification of test samples, the GA-BP neural networks have good prediction ability. So the GA-BP neural networks can replace the real model to predict the effects of factors on the drawability of HSS. And the response surfaces composed of main influence factors H1&H2 and limit drawing depth with the optimal controllable drawbead trajectory are established based on the GA-BP neural networks and Latin Hypercube.4)The regional separation optimization strategy of controllable drawbead was proposed. The simulation results show that the limite drawing depth of HSS will be further improved by long drawbead increasing later than short drawbead or long drawbead decreasing earlier than short drawbead. It should adpot the same velocity of the long drawbead and the short drawbead to ensure the convenient control and material flow stability.②Study on springback of HSS based on the controllable drawbead technologyFirstly, the influences of different fixed drawbead heigth on springback of HSS were studied. The results show drawbead heigth has great influence on springback of HSS and springback decreased with drawbead heigth increased. Then three different controllabe drawbead trajectories similar to the trajectories in the drawing process were designed. The influence of these trajectories on springback of HSS U-shaped part was studied by numerical simulation. The results show controllabe drawbead trajectories have great influence on springback of HSS. The trajectory of ascent——halt can decrease springbak, but the trajectory of ascent——halt——descent which improve the drawability of HSS is worst on decreasing springbak.③Design of experiment device and control systemThe HSS box drawing experimental device with the controllable drawbead driven by hydraulic and electric control system was designed based on the controllable drawbead movement principle and the conventional structure of drawing die. The tests of step-like changed in drawbead height and fixed drawbead were carried out by the experimental device. The driving mode with the Leverage and hydraulic unit is chosen by comparing the advantages and disadvantages of different motion mechanisms. The corresponding hydraulic and electrical control principle diagrams are designed.The controllable drawbead technology has great potential for improving the drawability of light materials, such as HSS, which are difficult to deform. The technology provide a new optimization method for the forming of HSS. And the research results laid a technology foundation for further application of controllable drawbead technology in practical production. Furthermore, the research on the controllable drawbead can give a new development in technology of HSS and has both important academic meaning and engineering value to enlarge application range of HSS and AHSS.