The Research On Numerical Simulation Of Multi-point Digital Stretch Forming Process

The basic principles of Multi-point forming (MPF) is dividing the curved surface of traditional stamping dies into ordered, high adjustable element group, and then the three-dimensional shape of sheet metal is formed by the curved surface of elements(element group forming surface). Tools and sheet mental, means of contact is point in forming process. In another word, tool's 3D surface is formed by ordered points. Multi-point forming technology combining with CAD / CAE / CAM as one of modern processing technologies, is a flexible digital processing method for 3D surface of sheet metal forming. Currently MPF is extensively applied in many realms, such as shipbuilding, orbit passenger car, medical science engineering, the large stadium building construction and also the aviation aerospace. Its prospects are very broad.So far, multi-point punching process is the most important application of multi-point forming technology, that the up and down elements group squeeze each other. Sheet mental is formed only by normal force in the punching process, but the multi-point stretch forming of aircraft skin need only down element group, sheet mental is formed not only by normal force but also by tangential pulling force of clamps. On the other hand, the multi-point punching process will be finished in a short time. Generally, because tools velocity> 80mm / s and time step is small, it is suitable for dynamic explicit algorithm which have stable calculation and high efficiency. The multi-point stretch forming velocity of aircraft skin is very slow, and the tools velocity<8mm / s, it belongs to quasi-static forming process which have unstable calculation. So the forming principle of multi-point stretching is more complex than that of multi-point punching, and the simulation calculation is hard. The aircraft skin multi-point stretching also need high forming efficiency. The research at home and abroad is still in laboratory studies.In modern production practice, Numerical analysis technology of sheet metal forming is an essential tool to optimize die design, forecast forming defects and analysis forming quality. Finite element technology is the most valid method in the simulation methods. In recent years, ANSYS is applied more and more extensively in sheet forming. It is not only used to typical process analysis and simulation of forming process, but also used to forecasting wrinkling, dimpling defects , calculating spring-back and original blank design. Because the contact state between the elements and blank is changing all the time, and the complex loading conditions, adopting dynamic explicit finite element calculation has apparent advantage on the process of multi-point stretching for aircraft skin.Therefore, numerical simulation analysis on the process of multi-point stretching for aircraft skin is finished by dynamic explicit finite element calculation, and then the results is used to simulate spring-back process of the work piece by implicit calculation.The main contents and conclusions are as follows:(1) Building up the finite element model of multi-point stretching for aircraft skin, simulating the wrinkling defect of the sheet mental under different horizontal pulling force. By analyzing the simulation results, Shaping effects reach the best when vertical pulling force F=0.80～0.95σbS. In the actual process of aircraft skin stretch forming, vertical pulling force is determined by empirical formula F=0.9σbS. By comparison, the simulation results are basically consistent with the practical values which verify the accuracy of the algorithm.(2) Building up the finite element model of multi-point stretching with Polyurethane (PU) elastic cushion for aircraft skin and simulating the stretching process. The simulation result shows that, using PU elastic cushion on the process of multi-point stretching can suppress the dimpling defect effectively, and can get work piece with smooth surface.(3) Building up the finite element model of repeated multi-point stretching for aircraft skin and simulating the process for the first time. The simulation result shows that,repeated multi-point stretching can suppress the dimpling defect effectively and prevent the sheet wrinkling simultaneously. This method is suitable for thin sheet (thickness less than 1 mm), the first stage in multi-stage stretching process, and can improve the efficiency obviously.(4) Simulating the wrinkling defect on the process of multi-point stretching, discussing the causes and solutions of the wrinkling defect through vertical pulling force,forming region size and material parameter etc. Simulation result shows that, the wrinkling trend is decreasing as the blank thickness and vertical pulling force increase , but too large horizontal pulling force can result in cracking defect. Making use of the hard aluminum blank of LY12 takes better result than using industry of L2Y2 aluminum blank and wrinkle appear easier as the stretching coefficient increase. Proposing the method combining flexible clamps with multi-point stretching, the simulation result shows that the method can improve formability of the stretching process obviously.(5) Simulating the dimpling defect on the process of multi-point stretching, discussing the causes and solutions of the dimpling defect, comparing several different thickness of the elastic cushion having an effect on dimpling defect. Simulation result shows that, the thickness of the elastic cushion does not affect the dimpling defect apparently. But when the elastic cushion is thick, the forming accuracy increases. Using PU elastic cushion has greater influence on stretching accuracy. Calculating the several different thickness of sheet metal corresponds to the best thickness of elastic cushion.(6) Simulating spring-back of the work piece after unloading .The spring-back has an influence on the forming accuracy. Comparing the spring-back amount of cylindrical work piece and saddle shaped one, which is formed by traditional stretching dies and multi-point stretching method, and then analyzing process features of the spring-back. Through calculating and comparing the spring-back amount of cylindrical work piece with different thickness and in different loading ways, the saddle shaped work piece strain before and after spring-back, the following conclusions are: the spring-back amount of the work piece by multi-point stretching method is less than the amount by traditional stretching dies; The spring-back amount by both stretching methods reach the maximum as the thickness of blank increase to about 1.5 mm; When the thickness exceeds 1.5mm, the spring-back significantly reduces.(7) Based on the oretical analysis and numerical simulation, this paper systematically studies the forming accuracy of multi-point stretching process for aircraft skin; put forward the method of correcting stretching surface of elements to compensate elastic cushion's thickness, inhomogeneous deformation and spring-back of sheet. The method's essential is the advanced manufacturing technology combining numerical virtual forming with closed loop forming. The convergence rate of numerical virtual computation is very fast; commonly the result can reach higher accuracy of stretch forming by twice virtual computation revise. To verify the effectiveness of compensation, multi-point stretch forming experiments are carried out. Experimental proof, the revise compensation method can form higher accuracy's aircraft skin.The system studies the multi-point digital stretching process for aircraft skin, simulates all kinds of possible process which have an influence on the quality of the work piece. Focus on the wrinkling defect, the dimpling defect and the spring-back defect. Propose the methods of suppressing defects, improving the formability and forming quality. Experimental results show the accuracy of the methods.