Research On Theory And Forming Limit Of Tube Bending Without Mandrel

As an important part of fluid conveying system,metal material bent tube is widely used in aviation,aerospace,automobile and boiler industries.In view of the urgent need for miniaturization and lightweight of fluid conveying system,it is urgent to develop bent tube with relative bending radius less than 1.5.Nowadays,the tube bending process with mandrel can produce the bent tube with relative bending radius less than 1.5,but it has some shortcomings in operation conditions,mandrel consumption and production efficiency.Bending without mandrel can avoid the shortcomings of the above technology,but due to the relatively backward theory and technology of it,this technology can only produce bent tube with relative bending radius of about 2.0.So this subject uses tube bending process without mandrel,mainly for the systematic study of the0Cr18Ni9 tube cold bending without mandrel process.The main research contents and corresponding conclusions are as follows:(1)Forming process and experimental study of tube bending without mandrelIn view of the tube bending without mandrel forming state,the causes of defects in the deformation process are revealed through the theoretical of forming process and experimental study.A three-arc groove pressure die was developed to overcome the flattening or even collapse of cross-section during bending process.The forming quality of bending process without mandrel,especially under the limit forming condition,was further improved by adding internal and external detachable aid-push dies and matching the internal and external aid-push dies,pressure die and bending tangent velocity.(2)Establishment of mathematical analytical model for tube bending process without mandrelMathematical analytical model for tube bending process without mandrel is established,which covers the influence of bending strain neutral layer offset and multiple deformation indices.The mathematical expressions of some important deformation indices,such as force and energy parameters,wall thickness variation rate,cross-section flattening rate and unloading springback angle,are derived.The correctness and reliability of the analytical models are verified by experimental results.(3)Establishment of finite element numerical model of tube bending process without mandrel,and in-depth study of the influence of bending parameters on the forming quality of tube bendingA fully parameterized finite element numerical model covering geometric parameters of tools and dies,material and bending parameters of workpiece,and external boundary conditions was established.The influence of these key parameters on the forming quality of bent tube,such as wall thickness variation,cross-section distortion and springback,was revealed.(4)Research on the forming limit of tube bending process without mandrelAiming at the multiple unstable defects in the process of tube bending without mandrel,a multi-defect constrained forming limit diagram was constructed with the relative tube diameter as the basic design parameter.Aiming at the forming limit of tube bending without mandrel,a stepwise regression method was proposed to determine the bending without mandrel limit of tube,and the correctness and reliability of the method were verified by experiments.Finally,the bent tube with relative bending radius of 1.0 was bent by this method and the developed bending process.Aiming at the deformation state of metal tube during bending process,the theory,numerical simulation and experimental research of tube bending process without mandrel are carried out.The bending deformation mechanism of tube,especially the deformation characteristics under extreme bending state,are revealed.The feasible tool and die is designed and put forward matching mode of boost speed in bending process.The key problems of excessive thinning of the outer convex side wall thickness,excessive thickening of the inner concave side wall thickness,even unstable wrinkling and excessive flattening of the cross section were solved when the relative bending radius was reduced,and the qualified tube with relative bending radius of 1.0 was finally bent.The subject provides theoretical support and practical preparation for tube bending process without mandrel of small bending radius tube and its popularization and application.