Numerical Simulation Research On Forming Defects And Their Suppression Methods In Profile Multi-point Bending

Multi-point forming (MPF) is a flexible forming technology for 3-D surface parts. Its basic idea is to make the traditional die discrete, using the ordered and height adjustable elements instead of the traditional die, which make punches formed the required envelope surface by controlling the relative displacements of elements. Compared with the traditional die forming, MPF can save a lot of time and cost in manufacturing die and achieve a plan that is one machine can have several usages. As a new-style flexible forming process, MPF has been widely applied to the industrial production. With the development of modernization in current society, energy conservation is publicly known as the survival foundation for modern enterprises. Therefore, in the manufacture, such as aircraft, automobile and shipbuilding, people pay more and more attention to the structural design of lightweight in which a kind of frame style structure is usually adopted. While the bending parts of profile not only have the lightweight structure, but also can satisfy the requirement of higher stiffness, therefore, they will have an extensive future. For this reason, the bending process of profile has become one of hot topics in the material processing field.The multi-point bending process of profile combines MPF with profile forming, using the flexible features of MPF press to realize the flexible and efficient forming for the bending parts of profile. It is particularly suitable for producing the multi-item and small-lot products and developing new products, meanwhile, it also widens the application of MPF technology. With the further development of computer technology and finite element method, the technology of numerical simulation has become one of the important methods for studying the profile bending. In this paper, finite element method was utilized to analyze the influence of the technological parameter on formed parts in profile multi-point bending, discussing the deformation characters during the forming process, detecting formed parts by a 3-D measuring instrument, comparing simulation results with experimental results and seeking the effectiveness method to suppress and remove the forming defects.The main contents and conclusions are as follows:1. Study on building the finite element model of multi-point bending process for profileBased on the numerical simulation of MPF for sheet, the multi-point bending process of profile was put forward, and the relative positions between the punch and the profile were discussed. The punch was carried out the optimal selection. The element adjustment program of the multi-point bending process for profile was compiled. The forming mode of profile multi-point bending was discussed. The finite element model of multi-point bending for profile was built. The numerical simulation of the process of multi-point bending for profile was achieved, obtaining the defects from the results of finite element simulation, such as dimple, cross-section distortion and springback.2. Based on the finite element method, study the dimple in sheet and tube multi-point formingDimple is a particular forming defect in the process of MPF. In this paper, the reasons for dimples on formed parts and the types of dimples were analyzed in MPF. Based on a new material constitutive relation, simulation analysis of the micro multi-point sheet forming was carried out, discussing the influence of size effect on the surface quality of formed parts and obtaining the influence rules of sheet thickness, grain size and punch size on surface quality. Meanwhile, the numerical simulation of the process of tube multi-point bending was carried out. By analyzing the results of numerical simulation, the influence of the technological parameters, including deformation degree, the thickness of tube wall and tube diameter, on dimple was discussed. The results show that when the deformation degree is smaller, the tube wall is thicker and the tube diameter is smaller, then the dimple of formed parts is not obvious; on the contrary, the dimple of formed parts is obvious. With the forming device in my lab, the simulation results of tube multi-point bending, which are nearly the same as the experimental results, were verified. In order to suppress dimpling and improve the surface quality of formed parts, a method that is to use an elastic medium to increase the contact area was put forward. The influence rule of the material and thickness of elastic medium and its decrement in forming process on dimple was analyzed.3. Research on the cross-section distortion in tube multi-point bending by using numerical simulation methodCross-section distortion is a special forming defect in profile bending forming. In this paper, the reasons for cross-section distortion were analyzed and the cross-section distortion rate was defined as the criterion to measure the deformation degree of cross-section. Without elastic medium, the cross-section deformation of tube multi-point bending was simulated, which analyzed the influence rule of several forming parameters on cross-section distortion. The change rule between the cross-section distortion rate and the parameters which include the thickness of tube wall, tube diameter and deformation degree was obtained. The results show that the cross-section distortion rate increases as the thickness of tube wall and the target curvature radius decreases, while the cross-section distortion rate increases as the tube diameter increases. Meanwhile, a method, which is to fill the tube with liquid to support tube wall, was put forward to suppress the cross-section distortion of tube, but in the simulation, the liquid was replaced by the uniform pressure, and the edge cross-section distortion rate was analyzed under different uniform pressure and different elastic medium decrements. According to the deformation degree of the edge cross-section, the relationship between the tube diameter and the minimum bending radius was given before and after improving technology.4. Research on the process of multi-point bending for angle profileRecounted the multi-point bending modes of angle profile simply, the deformed process and the forced situation were analyzed under different bending modes. The process of multi-point bending for angle profile was simulated under different cross-section dimensions, material parameters and deformation degrees and the influence of different technological parameters on the lateral movement value and torsional angle of boom plate was obtained by analyzing simulation results. The change rules which describes the influence of different cross-section dimensions, material parameters and target curvature radii on the precision of formed parts was concluded. According to these curves, the variation tendency of the lateral movement value and torsional angle of boom plate can be determined as the technological parameter changed. In order to improve the precision of formed parts in multi-point bending for angle profile, the symmetrical forming method which made two angle profiles formed simultaneously was put forward to suppress the lateral movement value and torsion of boom plate. According to the numerical simulation and forming experiment, the effectiveness and feasibility of the symmetrical forming method were verified. At the same time, based on the section assumption and the uniaxial stress assumption, the forming force in multi-point bending for angle profile was analyzed and the computing method of forming force was deduced from the view of the elastic-plastic mechanics and the finite element contact analysis. The influence rules of several factors on forming force was obtained by analyzing simulation results, that is, the required forming force increases as the rate of boom plate to web plate and the deformation degree of formed parts increases; the required forming force in"flange in"bending is larger than that in"flange out"bending; the forming force in symmetrical forming is more than twice as large as that in separate forming.5. Study on springback and its suppression method for profile The causes for generating springback were studied after bending profile and the distribution rule of stress in the formed parts of profile was analyzed after springback. According to the average curvature of profile before and after springback, the computing formula of springback amount of formed parts was deduced. The feasibility, with the explicit-standard algorithm to carry out the analysis of springback, was discussed. The process of the bending springback for tube and angle profile was simulated systematically and the influence rule of the thickness of profile, cross-section dimension and deformation degree on springback amount of formed parts was analyzed. The change rule between different technological parameters and springback amount was concluded. It results that when the thickness of profile and the cross-section dimension are larger and the target curvature radius is smaller, then the springback amount is smaller. The forming and springback experiments of tube multi-point bending were carried out. The precision of springback analysis model for tube bending parts was verified by comparing the experimental results with the simulation results. The reiterative forming method was put forward to reduce springback, and the basic ideas and concrete step of reiterative forming process were given. The effect of the compensation quantity on the forming curvature was obtained by numerical simulation, and the precision of formed parts was analyzed under different compensation quantities. It results that the compensation quantity is inversely proportional to the curvature radius after springback; the forming error decreases gradually as the compensation quantity increases. The mutual transformation between the explicit results and the standard results was realized, and the numerical simulation of reiterative bending process for profile was achieved. With a fixed compensation quantity, the influence of reiterative forming time on the precision of formed parts was analyzed. It results that the forming error gradually decreases as the reiterative forming time increases, and the shape of formed parts is more and more close to the target shape.