Investigation To The Springback Prediction And Springback Control Strategies In Spatial Tube Forming

Spatial tubes are widely used in aviation,aerospace,automobile,CNC machines,robots,shipbuilding and other high-tech fields,which are related to the livelihood of national economy.Due to the limitation of design performance and installation environment,this kind of component often has complex shape and high precision.In the traditional manufacturing process,the spatial tubes are forming by tube blanks,which are bent by the flexible bending machines.However,metalic tube bending is a complex elastic-plastic forming process which integrates material nonlinearity,geometric nonlinearity and boundary condition nonlinearity.After unloading,due to the contraction of the stretch part on the outer side and the expansion of the extrusion part on the inner side of the tube,there will be obvious springback phenomenon(mainly the increase of bending angle and bending radius).Springback is inevitable and can reduce the forming accuracy of tubes and lead to assembly problems.In serious cases,it will lead to the scrapping of parts and equipment.Therefore,it is of great theoretical value and economic benefit to study the springback mechanism and control strategy of spatial tubes.At present,the research on bending process of spatial tubes are inadequate,and there is no effective strategy to control springback.In this paper,the bending and twisting composite forming technology of spatial pipe is proposed.Based on the theory of elastic-plastic mechanics,the bending-twisting model and the numerical model based on ABAQUS are constructed,and an effective and practical prediction method for the springback of spatial tubes is established.Additionally,this paper also focuses on the springback control methods.New springback compensation methods are proposed which are based on forming knowledge database(FKD),elements connecting and clamp compensation.The full content of this paper is summarized as follows:(1)According to the different loading conditions of bending-twisting,the bending-twisting process is divided by three modes(synchronous loading mode,asynchronous loading mode and combined loading mode).For synchronous loading mode,two springback prediction models are established.The first model is based on linear loading mode and elastic-perfectly plastic model without hardening response.The second model is based on non-linear hardening mode and elastic plastic hardening material model.Influences of forming parameters,material properties and loading paths on the final springback magnitudes are analyzed.Besides,the differences of springback rules among synchronous loading mode,asynchronous loading mode and combined loading mode are discussed.(2)Based on ABAQUS,a FEM model of bending-wisting process is established.For planar bending instances,the characters of curvatures distribution along bending line is analyzed,and the relationship between mobile die position and stable bending radii is established.For spatial bending instances,the relationship between torsion of tube axis and the clamp angles are investigated.The variation tendency of torsion adjustment index is also analyzed.Finally,the forming procedures of typical spatial products are performed and forming parameters are predicted numerically.(3)The defects of forming prarameters compensation strategies in existing literatures are summarized.Element modifying methods are applied for tube forming.The concepts of stable elements and unstable elements are announced.For planar tube products forming,the characters of curvatures distributions are investigated and parameterized description method is established.For spatial tube products,a geometrical collection is proposed for describing the characters of unstable elements.And then,the relationship between geometrical collections and forming parameters collections are established,which is stored in a knowledge base.To convert a complex shape into manufacture shape,unstable elements in knowledge base are used for reconnecting and convergencing the tube axis.By compensating the motion of clamp,forming parameters for typical spatial tube products are finally acquired.Through this study,the purpose is to realize the springback prediction of tube element in bending-twisting under different composite conditions,and obtain the influences of forming parameters,material characteristics and loading path on tube element springback.The influencing factors and rules of the inconsistency between the clamp angle and the forming torsion in bending-twisting process are revealed.By using the springback compensation method established in this paper,the springback compensation of the complex spatial tubes is realized,and the ideal forming process parameters are obtained.The bending twisting tests on typical spatial tube products are carried out to verify the effectiveness of the springback compensation strategy.This paper has a positive effect on the springback investigation of spatial tube precisely forming,which may also have theoretical meanings and economic profits to some extent.