| Flexible roll forming is a novel continuous flexible forming technology for three-dimensional(3D)surface of sheet metal.This technique applies the multi-point control theory to the traditional two roll rolling process,so that the smooth continuous work roll can bend with small deflection.A curved and uneven roll gap is constructed.A 3D surface is rapidly formed by continuous rolling through such a roll gap.The flexible rolling forming method has the advantages of high material utilization,good quality of forming surface,simple production process,rapidity and flexibility.It is suitable for rapid production of single,small and large batches of 3D surface parts.Therefore,the technology has high scientific research value and broad market application prospect.At present,research of continuous flexible roll forming is still in its infancy.Although some research results have been obtained,there are still many problems should be resolved in theoretical analysis,forming process control and optimization of process parameters.A large number of theoretical and experimental studies are still needed.In this paper,an finite element model is established according to the principle of flexible rolling.The forming law of surface is studied.The influence of process factors on the formed surface is analyzed.The forming force of flexible rolling is deduced.A process improvement method that can effectively restrain the end effect of the formed surface is proposed.The main research contents and conclusions are as follows:1.Flexible rolling forming principle and equipmentThe forming principle of flexible rolling for 3D surface is elaborated.The transverse deformation of the formed surface is based on bending theory,and the longitudinal deformation is based on uneven thinning rolling theory.The decisive factors of surface rolling forming are analyzed,that is,the centerline of the roll gap is in bending state,and the roll gap is non-uniform distribution.The forming theory and control method of two typical surfaces,convex surface and saddle surface are discussed.The functions for roll shape design of two typical surfaces are deduced.The structure and composition of flexible rolling forming equipment are introduced.The equipment debugging method,the work roll adjustment method and surface rolling forming process are described.2.Finite element model establishment of flexible rollingThe finite element model of flexible rolling is established based on the idea of discretization.The problem that the work roll can be bent and controlled to rotate around its own bending axis is solved.The modeling process is illustrated.The key problems involved in the establishment of the model and the corresponding treatment measures are expounded.Two typical 3D surface parts are simulated by using the established model.The simulation results consistent with the experimental deformations are obtained.It proves that the model is correct and the parameters are reasonable.3.Deformation analysis of flexible rolling surfaceThe stress-strain state of the deformation zone of the flexible rolling is analyzed based on the plastic deformation theory.The equivalent stress and equivalent strain formula of any point in the deformation zone are given.The equivalent stress and plastic strain field of convex surface and saddle surface are analyzed by numerical simulation.The strain and deformation distributions of the two typical surfaces in transverse,longitudinal and thick directions are analyzed.The relation between roll gap distribution and strain and deformation is discussed.The forming law of surface is analyzed.The center line of the roll gap is the decisive factor of transverse curvature,but it is affected by longitudinal curvature.The relative reduction is the decisive factor of the longitudinal curvature,but it is affected by transverse curvature.Transverse and longitudinal curvature interacts with each other in the opposite direction.4.Influence factors analysis of flexible rolling formingTaking convex surface as an example,the influence of process parameters on forming result in the forming process is studied.The research shows that the shape error of the work roll has a greater influence on the surface quality of the formed surface,and the formed surface contacting with the error shape roll appears striped indentation defect in the longitudinal direction.The displacement error of the upper roller has a significant influence on the longitudinal deformation of the formed surface,and has little influence on the transverse deformation.Roll shape and roll gap adjustment method which can eliminate the elastic deformation error,structural clearance error,roll shape error and reduction error of the upper roll in the forming experiment are given.The influence of forming process parameters on the stress,strain and deformation of formed surface is analyzed.The forming speed has a significant influence on the formed surface,and the slower the forming speed is,the better the surface is.The influence of the diameter of the working roll on formed surface is contradictory.Balancing the primary and secondary contradictions,under the condition that the work roll meets the hardness and torque requirements of the premise,the smaller the diameter is,the better the forming effect is.The influence of friction coefficient on formed surface is limited.The influence degree of each influence factor on the longitudinal deformation is greater than that of the transverse deformation.5.Research on forming force of flexible rollingThe composition of the forming force of flexible rolling is analyzed.The forming force is composed of bending force and rolling force,and they exist simultaneously.Based on the assumption that the bending force and the rolling force do not affect each other,a theoretical formula for calculating the forming force of flexible rolling is derived.The distribution law of forming force is analyzed by means of numerical simulation.The forming force of plastic deformation of sheet metal mainly comes from the forming force of y direction(i.e.thick direction of sheet).The peak value of forming force occurs when the sheet thickness reaches the maximum reduction.The influence of process parameters on forming force is analyzed,in which the relative reduction is the most important factor affecting the forming force.Decreasing the relative reduction,working roll diameter and friction coefficient help to reduce the forming force.6.Research on repeated forming method of flexible rollingForming accuracy of flexible rolled surface is analyzed.The end effect of the surface formed by flexible rolling is found.The mechanism of the end effect is analyzed.An improved method of forming process,that is repeated forming method,is put forward.The results show that two steps repeated forming method has the best influence on suppressing the end effect of the formed surface when the reduction ratio is 1.3. |
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