| With the rapid development of digitization and information technology,advanced manufacturing concepts such as industry 4.0,intelligent manufacturing and cloud computing have been put into practice in the fields of automobile,aviation,aerospace,ship and architecture.Higher requirement are put forward for appearance,comfort and intelligence of the newly developed products,which lead to the speed of product development and upgrading in various industries are promoted.Multi-point forming technology is an advanced method of manufacturing sheet metal.It combines flexible manufacturing ideas and computer control systems successfully,and has the characters of flexible,digital and mold-free.After more than 30 years of research and development,multi-point forming technology has been widely used in the forming of 3D curved surfaces.The flexible wrinkles suppression mehthod in multi-point forming is a novel wrinkles suppression method based on flexible thinking.If this wrinkles suppression method is used,the mold manufacturing cost and cycle can be greatly reduced without increasing the blank size and the number of processes.In this paper,three kinds of flexible wrinkles suppression methods of sheet metal multi-point forming,namely using a discrete steel pad,using a strip steel pad and using a composite elastic pad,are proposed.The corresponding finite element model of sheet metal forming process is established by using the finite element analysis software of Abaqus.Three kinds of flexible wrinkles suppression methods were applied to the multi-point forming process of spherical parts and several aspheric parts with double curvature,respectively.Meanwhile,the effects of three kinds of flexible wrinkles suppression methods on the formability of sheet metal were studied by means of numerical simulation and experiment.In addition,the influence of key process parameters in the flexible wrinkles suppression process on the sheet metal forming results and the influence of the flexible wrinkles suppression process using multi-layer steel cushion on the sheet metal forming properties were studied.The main contents and conclusions of this paper are as follows:1.Analysis of the principle and characteristics of flexible wrinkles suppression methods in multi-point formingBased on the idea of flexible forming,a method of using flexible wrinkles suppression process to restrain wrinkling defect in multi-point forming was proposed.The forming principles and structural forms of three flexible wrinkles suppression methods,namely discrete steel pad,strip steel pad and composite elastic pad,were introduced in detail.Based on the structural characteristics and working principle of three kinds of steel cushion,the principle of three kinds of flexible steel cushion restraining wrinkling and their advantages in multi-point forming were compared and analyzed.The comparative analysis results are as follows: from the perspective of the overall flexibility of the steel pad,the discrete steel pads and composite elastic pads are “point-like” discrete,and both of them have better flexibility.In terms of improving the condition of the die surface,the strip steel "linear" discrete way makes the continuity and stiffness of the steel pad relatively good.The common process structure forms of three kinds of flexible steel cushion are introduced,and the application status,advantages and disadvantages of each process are discussed.2.The finite element models of three different types of flexible wrinkles suppression methods used in multi-point forming are establishedThe solution algorithm of finite element simulation,the basic theory of plate and shell element and the elastoplastic constitutive relation of materials are discussed.The problems of element selection,material model selection,contact and friction treatment,boundary condition and mesh setting in numerical modeling were studied.Some key problems in numerical simulation are analyzed and the countermeasures adopted in this paper are described.The finite element numerical models of five multi-point forming methods,namely forming without pad,forming with polyurethane elastic pad,forming with composite elastic pad,forming with discrete steel pad and forming with strip steel pad were established.3.Research on flexible wrinkles suppression multipoint forming of spherical partsThe finite element models of three kinds of flexible wrinkles suppression methods were established with spherical curved part as the target forming part.Compared with the numerical simulation results of multi-point forming without pad and using polyurethane elastic pad,the forming effect of using three kinds of flexible wrinkles suppression methods in multi-point forming were studied,respectively.The results show that the use of the flexible wrinkles suppression process can significantly improve the distribution of stress and strain of the forming part during the forming process,enhance the degree of plastic deformation,and inhibit the defects of wrinkling,dimpling and straight edge.The ability of three kinds of flexible wrinkles suppression methods to restrain the wrinkling defects of sheet metal and to improve the forming accuracy of sheet metal were compared.The results show that,among the three kinds of flexible steel pads,the discrete steel pad has the best flexible,the strip steel pad has the strongest ability to restrain the wrinkling of the plate,and the composite elastic pad has the widest application range.The forming experiments of three kinds of flexible wrinkles suppression methods were carried out,and the results were contrasted between the test and the numerical simulation.The test results were completely consistent with the numerical simulation results,which verified the accuracy and effectiveness of the numerical simulation.4.Research on flexible wrinkles suppression forming of other curved partsThe geometric features of the petal-shaped parts,saddle-shaped parts and ellipsoidal parts are analyzed.According to the characteristics of three flexible wrinkles suppression processes,the composite elastic cushion,discrete steel cushion and strip steel cushion were used in the multi-point forming process of a melon petal shaped part,a saddle part and a ellipsoid part,respectively.The structural parameters of the wrinkles suppression methods are designed according to the structural dimensions of the forming parts,and the multi-point forming process of three kinds of forming parts is simulated by finite element method.The numerical simulation results show that the rational selection of the flexible wrinkles suppression method according to the characteristics of the forming parts can effectively improve the forming quality of the plates,restrain the wrinkling defects and maximize the forming accuracy.5.Influence of flexible wrinkles suppression method parameters on sheet metal forming resultsBased on the structural characteristics of the flexible steel pad,the influence of the key parameters of the flexible wrinkles suppression method on the sheet forming results was studied.The results show that by optimizing the arrangement of the steel pad,the stress and strain on the surface of the forming part can be significantly improved,and the forming quality and precision of the plate can be further improved,too.Compared with the multi-point forming method of without cushion,the average errors of forming parts can be reduced by 70.88%,72.44% and 74.70% respectively by using conventional,staggered and interactive forms of discrete steel pad.Compared with conventional permutations,staggered and interactive permutations further reduced the average error by 13.10% and 8.18%.The effect of the thickness of polyurethane sheet on the formability of strip steel pad was studied by numerical simulation.The results show that choosing the right thickness of polyurethane cushion can improve the forming precision of strip steel cushion.The effect of the thickness of two layers of polyurethane in composite elastic pad on the properties of composite elastic cushion was studied.The results show that the thickness of polyurethane board on the upper and lower sides of the steel pad can significantly affect the sheet forming results.The thickness of polyurethane board close to the blank needs to be relatively thin,usually set at 5 mm,while the thickness of polyurethane board close to the punch unit needs to be determined according to empirical data and numerical simulation results.6.Influence of multi-layer steel cushion on sheet metal forming resultsThe effect of multilayer steel cushion on the forming result of sheet metal was studied.The structures of the three flexible wrinkles suppression methods with multi-layer steel pad were designed,and the corresponding finite element model were established.Comparative analysis was carried out on the strain,spring-back and forming precision of the formed parts formed by the three forming processes.The results show that compared with the discrete steel pad with single layer steel pad,the use of the discrete steel pad with double layer steel pad can further enhance the homogeneity of stress and strain on the surface of the formed parts,increase the degree of plastic deformation of the blank,and improve the forming accuracy.With the increase of the number of steel layers,the overall stiffness of the strip steel pad increases gradually,which can further enhance the ability of the strip steel pad to inhibit the wrinkling.However,increasing the number of steel layers cannot completely eliminate the defects of forming effect is directional.In addition,it is found that too many layers of steel bars will cause the flexible decline of strip steel pads,and forming the part with small local characteristics will become difficult.Compared with no cushion forming,the composite elastic cushion with double steel cushion can greatly improve the stress and strain uniformity on the surface of the plate,reduce the tendency of wrinkles on the plate,improve the forming precision and reduce the spring-back.Compared with the composite elastic cushion with single layer steel cushion,the spring-back of the formed part is reduced by 35.11% and the maximum error is reduced by 13.2% after increasing the steel pad of the composite elastic cushion to two layers. |
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