Research On Forming Technology And Simulation Of Flexible Rolling For Surface Parts

Flexible rolling is a novel forming process for forming three-dimensional parts, whichcombines the rolling process with multi-point forming technology. This forming processemploys a pair of rolls as a forming tool. By controlling the roll gap between the upper andlower rolls, the sheet metal is non-uniformly thinned in the thickness; residual stress caused bythe longitudinal non-uniform elongation of sheet metal makes the sheet metal generate bendingdeformation. With the rotations of the bendable rolls, the sheet metal is continuously deformedand a three-dimensional part could be obtained. Convex and saddle surfaces could be obtainedby reasonably controlling the roll gap distribution. Flexible rolling process is characterized thatonly a small region of the sheet metal is deformed, and the local deformation is graduallymoving over the entire product. So far, some achievements have been obtained on investigatingflexible rolling process. In the paper, the forming formulas of flexible rolling are modified; theshape errors of forming parts are analyzed through simulation results; and the improved methodis provided for improving the forming precision of surface parts. The main investigation contentsand conclusions are listed as follows:(1) Theoretical research of flexible rollingBy controlling the contour curves of the upper and lower rolls and roll gap, the sheet metalis non-uniformly thinned in the thickness; residual stress caused by the longitudinal non-uniformelongation of sheet metal makes the sheet metal generate bending deformation. Based on thelongitudinal elongation is different in the transverse direction of the target three-dimensionalparts, the relationship of thickness reduction and longitudinal elongation of the sheet metal isinvestigated, and the formulas of flexible rolling are established. The formulas of roll gapdistribution and the contour curves of the upper and lower rolls could be obtained respectivelyfor forming three-dimensional parts.(2) Finite element modeling method of flexible rollingBabsed on the forming process of flexible rolling and the Finite element modeling methodof continuous multi-point forming process, Finite element model of flexible rolling is establishedand the related process parameters are determined. The influences of contour curves and rotation speeds of the upper and lower rolls are investigated with a convex part as an example, the plasticstrain distribution of forming part is well in condition that the contour curves are theoreticalcurves; and the rotation speeds of the upper and lower rolls are100rad/s according to the stressdistributon of the forming part. Convex and saddle parts are simulated and correspondingexperiments are carried out. Simulated results are well agreement with the experimental results,which verifies the correctness of modeling method.(3) Analysis of simulated results of flexible rollingConvex and saddle parts are simulated, stress and strain distributions of the simulatedresults are analyzed. The relationship of roll gap and stress and strain distributions of thesimulated results are investigated. Taking a saddle part as an example, the characteristics ofstress and strain distributions are investigated in the transverse, thinkness and longitudinaldirections, and stress and strain distribution of the saddle surface in the upper, lower and middlesurfaces are investigated. The upper and lower surfaces of forming part are firstly yielded, thenthe middle layer is yielded; thus, the stress and strain value of upper and lower surfaces is higherthan that of middle surface of forming part. The software of Geomagic Qualify is used tomeasure the forming precision of simulated results and the reason of shape errors of simulatedresults are analyzed in the head and trail regions.(4) Improvement of shape control formulas of flexible rollingThe transverse bending deformation of forming rolls is consistent with target transversebending deformation of three-dimensional parts in previous curved surface formulas. As theforming rolls can only bear small bending deformation, previous curved surface formulas is onlysuitable for three-dimensional parts with small curvatures. The improved method of curvedsurface formulas is to reduce the bending deformation of forming rolls in condition that the rollgap distribution is constant. The improved curved surface formulas are suitable formanufacturing three-dimensional parts including the large width of the sheet metal withrelatively small transversal curvature radius, and the processing range is relatively extensive forthree-dimensional parts. Finite element model is established, and stress and strain distributionsof the simulated results are analyzed. The distributed regularities of stress and strain of formingparts are consistent with that of the roll gap distribution. The transverse bending deformation ofdifferent regions is investigated in the forming process with a convex part as an example. Thetransverse bending deformation of forming region is gradually larger away from the formingrolls, and the transverse bending deformation of forming region is unchanged after a certaindistance. The forming precision of simulated results is presented. The shape errors of formingparts in middle region are smaller than those in head and trail regions. (5) Analysis of process parameters of flexible rollingThe process parameters of roll adjusting radius and roll gap distribution are important inflexible rollinf process. The relationship of roll adjusting radius and roll gap distribution areinvestigated, and the comparison of forming results is investigated in stress and straindistributions and forming precision. The transverse and longitudinal bending deformation islarger with decreasing roll adjusting roll. With enlarging the length of sheet metal, the lengths offorming part in the head and tail regions are unchanged, and the length of effective formingregion of forming part increases gradually. The change trend of roll adjusting radius andcompression ratio is opposite in order to form the same target surface.(6) Improvement of transition region and experiment of flexible rollingThe roll adjusting radius is much larger than the target transverse curvature radius of theforming surface in flexible rolling process. The length of the transition zone in the head and trailis relatively long, the improved method of flexible rolling is introduced by utilizing the methodsof unequal speeds of the upper and lower rolls at the beginning of the forming process andenlarging the difference value of the maximum and minimum of the roll gap at the end of theforming process. Experiment is carried out using the large-scale flexible rolling divice. Convexand saddle surfaces with different process parameters are presented. The forming precision ofexperimental results is obtained.