Research On Three-dimensional Surface Flexible Rolling Based On Rigid Arc-shaped Rollers

Large three-dimensional curved surface products of different shapes and sizes are widely used as military and civilian products such as ships,aircrafts,spacecrafts,vehicles,large containers,and construction and decoration.The small batch and diversified characteristics of three-dimensional surface products make conventional mold manufacturing face fatal problems such as high equipment costs and long processing cycles,and because each product needs to develop corresponding molds for production,mold forming is not suitable for producing different types of large threedimensional curved surface parts.Thus,there is an urgent need to develop new flexible forming methods to meet the development needs of advanced manufacturing.Three-dimensional surface rolling with rigid arc-shape rollers(TSRRAR)is a novel forming method for 3D surface sheet metal parts.This method adopts the deformation principle of non-homogeneous rolling,and includes three contents as the new design to produce 3D surface metal sheets: rigid rollers,the variable cross-section diameters of rollers,and the generatrix of the roller belonging to an arc.A flat plate is continuously deformed when fed along the rolling direction under the action of the frictional force of the two rollers rotating in opposite directions.The plate is subjected to the bending action of the rollers in the transverse direction,and the longitudinal deformation is produced by the additional stress caused by the non-homogeneous deformation.After the whole plate passes through the roll gap between the rollers,it is processed into a double-curvature curved shape with bending in both directions.Based on the analysis of the advanced manufacturing market's demand for continuous forming methods for forming three-dimensional surface sheet metal parts,this paper proposed a novel threedimensional surface flexible rolling method based on rigid arc-shaped rollers.Numerical simulation methods and self-developed 3D surface rolling equipment are used to verify the feasibility and practicability of this method for producing 3D surface parts.The deformation laws of the formed parts under different conditions were analyzed,and the accuracy of the formed curved surfaces was studied.The main research contents and conclusions of this paper are as follows:1.Theoretical discussion of TSRRAR.The axial radius of roller is processed according to the transverse curvature radius of the target curved surface,and the unevenly distributed roll gap is adjusted by controlling the displacement of the upper roll during the forming process,so that the sheet metal has inhomogeneous thickness thinning in the transverse direction.This non-uniform compression effect causes the metal fibers at different positions in the sheet to produce uneven elongation in the longitudinal direction,and the resulting additional stress makes the flat plate deform into a three-dimensional curved piece.Based on the analysis of the mathematical expressions of equal curvature spherical parts,convex surface parts,and saddle parts that can be processed,the curved surface rolling process is studied.And according to the principle of plastic deformation with constant volume before and after deformation,the corresponding relationships between the longitudinal strain,the longitudinal arc length and the roll gap are described ignoring the elastic deformation after forming,which proves that the longitudinal strain field is a process variable to realize the control of the plate shape.2.Finite element modeling method of TSRRAR.The finite element model of the forming process is established and the specific modeling parameters are determined according to the key process parameters based on the finite element analysis software ANSYS/LS-DYNA and the characteristics of the three-dimensional surface flexible rolling process based on rigid arc-shaped rollers.0.6 mm is selected as the mesh size of the sheet metal and the roller after the mesh refinement process both considering the calculation time and the accuracy of the forming result.The obtained thicknessdirection strain distribution of the formed part is consistent with the rolling principle of curved surface rolling with non-uniform deformation.The dimensions and process parameters of rollers are designed,and the forming parts with two typical threedimensional surfaces(spherical and saddle)and different shapes are obtained.The reliability of the finite element model is verified by forming tests,and the feasibility of TSRRAR for forming 3D surface sheet metal parts is proved.3.Analysis of simulated results of TSRRAR.The deformation characteristics of TSRRAR are described from the perspective of mechanics,which belongs to the compression-elongation compound deformation type.Two typical three-dimensional surface parts are numerically simulated,and their internal stress and strain fields are analyzed.The thickness-direction strain is distributed in strips and continuous along the longitudinal direction of the formed part,and the strain values gradually decreases from the middle to the two sides,which proves the stability of the forming process.The analysis of the longitudinal stress field of the formed part shows that the additional stress is the reason of the non-uniform deformation of the metal inside the plate.At the same time,it restricts the free change of the metal when the uneven deformation occurs,which proves that the longitudinal non-uniform additional stress of the spherical part is the mechanical characteristic of longitudinal deformation of the formed part.4.Deformation analysis of TSRRAR.The influence of a single process parameter and related process parameters on the longitudinal deformation of the formed part is discussed.It is found that the initial sizes of the sheet metal and processing parameters not only affect the deformation of the formed part,but the effects are related to each other,such as the effect of the difference in the axial radius of roller and the sheet width on the longitudinal deformation of the formed part is opposite,so there is a problem of mutual matching between them.The forming process parameters of the spherical part with equal curvature after increasing the plate width are obtained through numerical calculation,and the maximum reduction is deduced by the calculation formula between the process parameters and the curvature of the spherical part,which is consistent with the maximum reduction given by the numerical simulation.5.Process study on saddle parts of TSRRAR.The stress and strain distribution characteristics of saddle parts in different forming stages,different surfaces and different regions are analyzed,and the characteristics of plastic strain distribution in saddle parts deformed by TSRRAR process are obtained.The effect of asynchronous rolling on improving the accuracy of the formed surface of the saddle part is discussed,which shows that the reasonable arrangement of asynchronous rolling can improve the uniformity of the longitudinal deformation of the formed part.The forming process of a saddle part connected end to end under a certain technological condition is simulated.The simulation results of continuous and uniform changes in the thickness-direction strain distribution show that the forming process is stable.6.Experimental study on TSRRAR.A self-developed forming device is used for experimental research.The influence of the main process parameters on the flexibility of the forming method is studied,and the results show that the longitudinal curvature radius of the formed part is very sensitive to the change of the maximum reduction.The forming force can be effectively reduced by reasonable selection of the difference in axial radius of rollers,and if increasing the difference in axial radius of rollers,the required reduction amount is smaller when the curvature of the target surface is the same,which demonstrates this forming method has the characteristics of flexibility,and the forming process is controllable and easy to control.Experiments are performed without changing rollers,and the curved surface parts of different shapes are obtained,which shows that it is feasible to obtain three-dimensional curved surface parts with different curvatures only by adjusting the amount of thickness thinning in the processing of practical curvature.In addition,it is verified that when the roll combination is different,both spherical and saddle surfaces can be obtained;and the key factor determining the surface type of the forming part is the difference in the reduction amount between the center and both sides of the formed part.