Research On Multi-point Roll Forming For 3-D Surface Of Sheet Metal

Three-dimensional (3-D) surface parts are widely applied to many domains of industry, because they have the characteristics of light weight, good streamline surface and reasonable stress state. 3-D surface parts are manufactured by die forming method in the mass production sectors. For the small batch production sectors and the sectors which unfit the die forming, they manufacture 3-D surface parts by flexible forming methods. At present, there are some flexible forming methods for sheet metal, such as flame bending, incremental forming, laser forming, shot peen forming and multi-point forming. With the development of economy and society, the requirement of products trends to diversification and individuation, so the flexible manufacturing methods with better quality, higher efficiency and lower cost are needed to bring forward for modern industry, therefore to the forming of sheet metal, new-style flexible forming technology is needed to be invented.Multi-point roll forming (MPRF) is a new-style flexible forming technology for 3-D surface parts, it can be considered as the combination of conventional continuous local forming (roll bending, spinning) and multi-point forming. MPRF adopts bendable roller which is controlled by multi-point adjusting machine as forming tool to make sheet metal generate 3-D deformation. MPRF can manufacture spherical surface part, saddle surface part, twisty surface part, disc-shape part, tubelike shape part and arbitrary surface part by dieless, high efficient and continuous method, it is provided with higher engineering applied value and academic research value.The main contents and conclusions are as follows:1. Researh on MPRF equipment and forming processThe two types of MPRF equipment are studied, which are flexible roller type and rigid roller type. Design scheme of flexible roller type equipment is discussed, including the selection of flexible roller, the structure of adjusting element and the structure of the equipement; the related experimental validation is finished. The rigid roller type equipment is developed, including design of working element, design of the equipment; the related experimental research is finished, the impression and fold on the surface of workpiece are restrained by polyurethane layer, and the good 3-D surface parts are obtained. The typical processes are summarized, which are one-time adjusting forming process, increamental adjusting forming process and real-time adjusting forming process. The advantages and disadvantages of two types of MPRF are compared. 2. Research on the finite element modeling technology of MPRFThree kinds of finite element modeling methods in ANSYS/LS-DYNA environment are put out, which are continuous and dependent on adjusting element controlling modeling method, continuous and dependent on rigid body driving modeling method and discrete modeling method. Compare the three methods: the first mothed can reflect the flexible roller adjustment state and the stress state of the components, but it is not suitable for simulating the forming process of sheet metal; the second method can realize the simulation of sheet forming process in a certan extent, but it still has great limitation due to the restriction of flexible roller curvature radius and other reasons; the third method gets rid of all the problems when the flexible roller adopts elastomer, and achieves the simulation of sheet forming process successfully, so it is the best one. The continuity and the convergence of the third method are analyzed. The simultation results of the third method are presented, and they are nearly the same as experimental results.3. Numerical analysis of stress and strain fields in MPRF processThe forming characteristic of MPRF is described from the view of mathematics, and the mathematical expressions of spherical surface part, similar spherical surface part, saddle surface part, tubelike shape part and arbitrity surface part are presented. The forming zones of MPRF process are divided. The stress and strain fields in the forming process of spherical surface part are analyzed, including the average equivalent stress and plastic strain fields at start stage, middle stage and end stage of forming process, the average equivalent stress and plastic strain fields of lower layer, middle layer and upper layer, the stress and strain fields of load zone, and the reasonable explanation is made. The average equivalent stress and plastic strain fields of saddle surface part and twisty surface part are analyzed, and the reasonable explanation is given.4. Numerical simulation of forming force in MPRF processBased on the Euler-Bernouli section assumption, the uni-direction stress state assumption, the same stress-strain curve assumption for tension and compression and the ideal elastic-plastic material assumption, the calculated method of local transverse bending force, local longitudinal bending force and total forming force are deduced. The calculation method of forming force in FEA analysis process is discussed. Take spherical surface part of 08Al as the research object, the variational trend of x-force, y-force, z-force and resultant force are analyzed by numerical simulation method. The y-force is the main force which makes sheet metal plastic formed, and the z-force is the main force which drives sheet metal continuous movement. The influence of upper flexible roller press displacement, thickness of sheet metal, material properties and curvature radius of flexible roller are studied, and the simulation results are in accord with practical situation.5. Numerical simulation of forming defects in MPRF processSpringback and wrinkling are the main forming defects in MPRF process. The key points in springback simulation are elaborated, including springback theory, calculation of springback ratio and simulation method. Take the spherical surface part as research object, and the springback phenomenon is analyzed by explicit-implicit algorithm. The simulation results indicate: after springback, the equivalent stress of sheet metal part is released, and it generates deformation along the reverse direction of its bending direction; the springback ratio decreases with the increasing of thickness and press displacement; these simulation results are in accord with practical situation. Take the spherical surface part as research object again, the wrinkling is simulated. The simulation results indicate: wrinkling is on the both ends and both sides of the part, and the reasons of wrinkling are explained by principal strain state of shell element and the law of stress-strain. The situation that thickness and press displacement influence the wrinkling of both ends is analyzed. The situation that flexible roller curvature radius and rolling times influence the wrinkling of both sides is analyzed, and the simulation results are explained from the view of principal strain state, tangential compressive stress, flowing state of metal, mechanics, energy and geometric deformation. No-wrinkling critical graph of both sides is obtained. The simulation results are in accord with experimental results. The methods that restrain wrinkling are brought forward.6. Numerical simulation of MPRF for rotary surfaceCompare MPRF with conventional spinning, the advantage that line forming method replaces point forming method is indicated. FEA model of disc-shape part is established, forming process of disc-shape part is simulated, and the differences between simulation process and experimental process are indicated. The equivalent stress and plastic strain fields at start stage, half circle stage and whole circle stage are analyzed, and the reasonable explanation is made. Wrinkling of disc-shape part is analyzed. The wrinkles are on the edge of disc-shape part. At the beginning, the quantities of wrinkes are few, and the amplitudes of wrinkles are large; with the increasing of rotation times, the quantities of wrinkles increase, and the amplitudes of wrinkles decrease; finally, the wrinkles disappear. The simulation result of disc-shape part is nealy the same as exiperimental result. FEA model of concave tubelike shape part is established, the simulation result is presented, and it is in accord with experimental result.