Research On Flexible Stretch Forming Technology For Sheet Metal Based On Discrete-gripper And Multi-point Die

Stretch forming technology is mainly used in manufacturing of aircrafts skin and due to its low material utilization and relatively high cost,application of stretch forming technology is limited in other industry field. However, stretch forming technology has the advantages of small springback and high precision of formed parts,which make it still applicable in many other fields that have small demands and variant products.To solve the problem of low material utilization and to broaden the application range for stretch forming technology,a new technology is needed. To meet this need, a flexible stretch forming technology is developed for fabricating sheet metals. Flexible stretch forming technology is realized by interaction of the discrete-gripper with multi-point die. This technology combines self-coordination of the discrete-gripper with reconfigurability of the multi-point die, which can meet the demand in high flexibility, high productivity and low cost, respond swiftly to the increasingly demands of individualized products, quickly improve the ability level of fabricating sheet metals in plastic processing field.The discrete-gripper flexible stretch forming technology is a new technology for forming sheet metals. With the hydraulic drive, the multiple grippers can work in a self-coordination mode, instead of the mode that gripper moving as a whole in traditional stretch forming process. In clamping process, the multiple grippers are arranged in a line and hold tightly at both sides of the sheet metal; during the stretching forming, the multiple grippers, based on the variation of the die configuration, can automatically adjust displacements and rotations of each gripper jaws correspondingly to make the clamped edges curvatures in accordance with the die-face. So this process can make the sheet metal easily attach the die with less stretching margin and have high material utilization. In this study, numerical simulations on sheet metal flexible stretch forming process were performed by LS-DYNA software, the influences of gripper structure and transition zone length to the quality of formed parts were investigated, springback and contact force were discussed and relative verification experiments were carried out. The main contents and conclusions of this study are as follows:1. Research on sheet metal discrete-gripper flexible stretch forming process and its forming deviceThe difference and connection between discrete-gripper flexible stretch forming process and current sheet metal stretch forming process was studied, and the technical features of discrete-gripper flexible stretch forming process were pointed out. Mechanics analysis of flexible stretch forming process was done by using analytical method with derived equations for computing the stress, strain, tensile force and thickness reduction in stretching process. The composition, working principle and structural characteristics of discrete-gripper flexible stretch forming device were elaborated and the technical advantages compare with traditional stretch forming equipment were given, for example, lower equipment building cost and higher material utilization.2. Building finite element model of flexible stretch forming processBased on relative theories of sheet metal plastic forming, the finite element model of flexible stretch forming process was established, and dynamic explicit algorithm was chosen for numerically simulating its stretching process and static implicit algorithm for analyzing the springback of formed parts. Problems in establishing the finite element model of flexible stretch forming process such as element selection, meshing, constraint conditions,contact and friction were elaborately discussed. Constitutive relationships of different materials were analyzed, in which sheet metal is taken as bilinear isotropic hardening elastoplastic model, polyurethane elastic cushion is taken as hyper-elastic material model and the die along with grippers is taken as rigid model, these results are foundation of numerical simulations in flexible stretch forming process.3. Numerical simulations on the quantity and configuration of discrete grippersTaken spherical parts as investigating objects, numerical simulations on flexible stretching process by finite element method were done. And the influences of discrete gripper's quantity to the stress, strain distributions, thickness reduction, shape of clamped parts sides, and material flow in sheet metal were studied. The results show: with the flat integral-gripper, the stress and strain distributions of formed parts are uneven, and accompany with serious thickness reduction; while, with the discrete-gripper, the stress and strain distributions are relatively even, moreover, the more the grippers, the more even the stress and strain distributions are, the closer the curvatures of the parts clamped sides approach the die and the better quality the formed parts has. The influences to the configuration of grippers on the parts stress and strain distributions, thickness reduction and the shape of clamped sides of the parts were analyzed, and the results show that the circular grippers can not only improve the distributions of stress, strain and thickness within the transition zones, but also prevent the parts crack. With the circular grippers, the curvatures of clamped sides of parts are closer to the die and the forming quality of the parts is better. Relative experiments were done to verify the simulation results.4. Numerical simulations on different transition zone lengthBy means of finite element method, numerical simulations were done for flexible stretching process of spherical parts and saddle parts with different transition lengths, and the influences of both flat integral-gripper and discrete-gripper on sheet metal attaching die, forming defects and material utilization were studied. The results show: the flat integral-gripper need a long transition length for forming, which can cause multiple wrinkles in the transition zones of spherical parts and single wrinkle in the forming areas of saddle parts; the discrete -gripper need a relatively shorter transition length and the formed parts have smooth surface with no wrinkle, so the process margin is reduced and the material utilization is improved. Moreover, the dimpling of the discrete-gripper on the formed parts are less deep than that of the flat integral-gripper, and the surface defects of formed parts can be prevented by using a thinner elastic cushion accompany with decreased forming errors. The impacts of deformation quantity, thickness of sheet metal, and the influence of transition zone length on the thicknesses of the spherical and saddle parts were investigated. Obviously, when other conditions are same, the results are: the greater the curvature, the smaller the maximum thinning rate of the parts thickness is; the thicker the sheet metal, the smaller the maximum thinning rate; the longer the transition zone, the smaller the maximum thinning rate. Verification experiments were done by using both the flat integral-gripper and discrete-gripper forming machines to form spherical parts, and the experimental results are basically agree with the simulated results.5. Research on springback simulation and the method of correcting springback by die surface compensationThe springback of spherical parts in both flat integral-gripper and discrete-gripper stretch forming processes were investigated by using explicit-implicit algorithm, and the impacts to springback quantity and shape errors caused by clamping mode, length of transition zone, stretching increment and materials were studied. The results show: springback and forming errors in the flat integral-gripper stretch forming mode are bigger than that in the discrete-gripper stretch forming mode; springback quantity and average shape error of formed parts are gradually increasing as the length of transition zone increase; springback gradually become smaller as the magnitude of pre-strecth increasing, and the corresponding average shape error is slowly reduced. Once the magnitude of pre-stretch exceeds1%, it has no influence on springback and the shape error, so 1% of pre-stretch is a priority; as sheet metal gets thicker, springback is reduced and the shape error is also gradually reduced; the greater the elastic modulus of the material, the smaller the springback of the formed parts gets after unloading, and the less the average shape error is. A method for correcting springback by multi-point stretching die surface compensation was presented, and this method was practiced by numerical simulation and experiment, which validates the feasibility and practicability of the method.6. Numerical simulations on the contact forces between sheet metal, grippers and punch Numerical simulations were performed for cylindrical, spherical and saddle parts of different materials and different thicknesses by using finite element method. The distribution of contact force between sheet metal and grippers was studied. The results show: the greater the elastic modulus and the yield strength of the material, the greater the deformation resistance of the sheet metal has, and the bigger the contact force is needed between sheet metal and grippers in stretching process. The thickness of sheet metal has a relative bigger influence on contact force: the thicker the sheet metal is, the more difficult it is for the material to deform, and the bigger the contact force is needed between sheet metal and grippers. The situation of contact force between sheet metal and punch was analyzed by simulating the stretching process of forming spherical parts with different materials, different thicknesses and different deformation. The results are: the contact force applied to the punch tends to increase as the material elastic modulus and yield strength increase; given the same dimensions of the same material, the thicker the sheet metal, the greater the contact force; the bigger the target curvature of the parts, the smaller the contact force.