Several Pivotal Multi-point Forming Process And Its Numerical Simulation Study

Multi-point forming (MPF) is a flexible forming process for 3D sheet metal. Its basic idea is to make traditional mold discrete, using a series of height adjustable elements (little punch) to combine forming surface. By CAD/CAM system, MPF can rapidly achieve required surface, save a lot of modeling time and cost compared with traditional mold, realize the purpose of one equipment for multi-use. Multi-step MPF, multi-point press forming (MPPF) and sectional MPF are the concentrated expression of MPF flexible characteristics. Multi-step MPF and MPPF can control and optimize deformation path of sheet metal, improving forming quality and limit. Sectional MPF can achieve the whole deformation by regional deformation so as to use small equipment forming large workpieces. MPF has been applied to vehicles covering parts, medical engineering (titanium sheet forming), architectural decoration and other fields. And in ship hull plate, pressure vessels, aircraft skin, urban sculpture and many other fields have broad application prospects. Using sectional MPF technology, MPF equipment has been successfully used in the torsion plates forming of Beijing Stadium (nest project) for 2008 Olympic Games.As for the use of discrete elements, it is easier to produce more dimples and wrinkles. The use of elastic cushion and flexible MPF technology can effectively eliminate dimples and inhibit wrinkles. In-depth research on the characteristics and laws is very important for the further development and practical use of MPF. With rapid development of computer technology and numerical simulation method, various defects in forming process of sheet metal has been able to accurately forecast. CAE technology has been an important means to avoid defects and optimize mold design. Study of forming process by the use of simulation method can save a lot of time and costs, increasing efficiency. Moreover, in-depth study of deformation characteristics and laws can be carried out, which can not be studied by experiment currently.The main content and conclusions are as follows:1. Study of MPF surface modeling method and elements position adjustment calculationSurface in IGES is parsed in detail and trimmed surface is searched and retrievaled as an example. Contact point calculation method between elements and surface is discussed. Fast contact point calculation arithmetic for analytic surface such as spherical surface, saddle surface and cylindrical surface is realized. Arithmetic for NURBS surface based on IGES and surface reconstruction study based on point clouds are carried out. Reconstruction arithmetic and surface continuity problem are explored. A number of issues in program development are discussed. Application examples show that surface modeling method based on IGES realizes data exchange of MPF system with other CAD system. It has great significance to the extensive application of MPF.2. Study of numerical simulation method for MPF and development of simulation parametric preprocessorFinite element method for MPF and material constitutive relations are studied and dynamic analysis equations and related issues are described. Issues such as elements, contact, friction, material models, virtual forming speed and springback simulation method etc. are discussed. Paremetric preprocessor program for MPF numerical simulation is studied and developed based on dynamic analysis software ANSYS/LS-DYNA. The process simultion of MPF technology can be rapidly parametric modeling by the paremetric preprocessor program, which makes the simulation results more accurate and efficient.3. Simulation study and experimental verification of MPF process with elastic cushion, Development of composite elastic cushion model and its simulation studyForming process is studied by comparing different elastic modulus, thickness and compression ratio of elastic cushion. Results of numerical simulation and experimental verification show that elastic cushion can completely eliminate dimples and effectively inhibit wrinkles. Small elastic modulus and soft material can not inhibit wrinkles effectively. Increasing thickness can only reduce wrinkling degree. Small compression ratio can not effectively inhibit wrinkles either, while too large ratio will form gentle dimples. Study of the impact of the use of elastic cushion on accuracy shows that forming errors increase along with the increase of thickness and compression ratio. To improve accuracy, elements adjustment compensation methods based on thickness and springback are put forward and studied. Results show that thickness compensation can effectively reduce errors with the use of elastic cushion and springback compensation can improve accuracy. New composite elastic cushion is put forward and studied. Results show that the use of composite elastic cushion can completely avoid wrinkling and improve accuracy. By the use of elastic cushion constituted by common elastic cushion and steel plate, simulation study on the forming process of titanium plate shows that the use of composite elastic cushion can also achieve good effect even forming rigid material. Finally workpiece photos in practical application are given.4. Simulation study and experimental verification of multi-step MPF and optimization method for forming processSimulation on the process of multi-step MPF are studied by comparing the impact of forming steps, deformation process configuration, surface shape and such factors. Results show that multi-step forming can effectively inhibit wrinkles and improve forming quality and limit of sheet metal. With the increase of forming steps, wrinkles gradually reduce. Uniform distribution of deformation can inhibit defects the greatest extent. Forming steps of saddle surface is fewer than that of spherical surfaces, which shows that uniform flow of sheet metal is conducive to good deformation. Based on the smallest plastic work principle and NURBS technology, uniform deformation path configuration design method is put forward and relations about forming steps, materials, thickness and deformation are explored. Results show that the method can improve forming quality and efficiency. Comparative impact study of different forming process, material and thickness on springback shows that multi-step MPF can effectively reduce springback. With the increase of forming steps, springback gradually distributes uniformly and its value gradually declines. Stronger deformation resistance and bigger thickness lead to smaller springback.5. Simulation study of MPPF technology and the impact of deformation path on sheet metal formingForming process of MPPF is analysed and restriction and load state is compared with MPDF. The impact of forming process on wrinkles, springback and forming limit is comparatively studied. Results show that deformation path of MPPF can make every regional deformation uniform and improve forming quality and limit a larger extent than other methods. Wrinkles have no birth and development process. Springback is smaller than other methods. Study by comparing once forming with MPPF of spherical and saddle surface shows that forming limit ascends with the increase of thickness and dispersive flow of sheet metal. However, the impact of thickness and flow of sheet metal on forming limit gradually weakens, which means that forming limit closes with material itself better along good deformation path. Finally comparison of multi-step MPF with MPPF shows that multi-step MPF can approximately realize MPPF with the increase of forming steps.6. Simulation study of sectional MPF and transition region design method, MPF process of dish head and experimental verificationSectional MPF process is analysed and region deformation characteristics is explored, according to which technique processes for previous and subsequent forming are developed. Three transition region design methods are analysed. Two direction sectional forming is discussed firstly. Comparative study on sectional MPF of cylinder shows that smooth transition region can prevent local severe plastic deformation, inhibit wrinkles, realize uniform deformation. Good stamping direction can reduce the impact of subsequent forming on previous deformation and help improving accuracy. Comparative study on the impact of overlap area on accuracy shows that overlap area can effectively improve accuracy and 15 to 20 percent overlap area of the previous effective forming area is more suitable. The impact of sheet metal material, thickness and surface shape on severe plastic deformation of transition region is studied and principle of transition region smooth design is analysed. Based on NURBS and COONS technology, optimal design methods of transition region are put forward and experimental verification on sectional MPF of saddle surface is done. Results show that optimal design of transition region can effectively reduce the uneven deformation and eliminate defects.Finally MPF process for dish head is studied, in which the processes of multi-step forming for bottom and sectional forming for edge are determined. Discussions on the impact of forming region and thickness of sheet metal are carried out and reasonable forming region is determined. Numerical simulation and experimental verification show that reasonable forming process can achieve good forming of dish head. MPF technology has broad application prospects.