High Flexible Stretch Forming Process And Its Numerical Simulation For Complex Parts

Stretch forming has the advantages of small springback and high forming precision,and is the main forming method for aircraft skin. With the rapid development of aviation industry, the updating of airplane models is speeding up. The demands for skin parts are increasing and the shapes are more and more personalized and complex. Traditional stretch forming process has not been able to meet the requirements of modern production because of low material utilization, poor conformability and difficulty in forming complex surface parts. Therefore, it is urgent for more advanced stretch forming process to be developed.High flexible stretch forming(HFSF) process applies discrete idea into forming die and loading mechanism, which makes them equipped with high flexibility simultaneously. By replacing the solid die with multi-point digitized die, various 3D surface parts can be formed with only one set of die. With conventional stretch forming device replaced by multi clamps flexible stretch forming machine, sheet metal can attach the die more easily; crack and wrinkle tendency can be reduced. Smaller transition zone, higher material utilization rate and better forming quality can be obtained. Complex surface parts can be formed. In this paper, traditional stretch forming and HFSF process for complex surface parts were compared and analyzed by numerical simulation. Effects of process conditions on HFSF results were discussed. Effects of the radius of punch unit hemispherical end, unit forms(fixed and swinging), and elastic cushion type(solid and discrete) on HFSF were studied. The corresponding experiments were carried out with HFSF machine.The main contents and conclusions of this study are as follows:1. Illustrate principle and characteristics of HFSFThe principle of traditional stretch forming was discussed, and its application scope and limitation were pointed out. The structure and forming principle of HFSF machinewere studied; and the flexible characteristics of forming die and loading mechanism were discussed. The stress-strain relation and tensile force in HFSF process were studied.By HFSF process, various surface parts could be formed with only one set of die and the sheet metal could attach the die more easily. HFSF process had the advantages of high forming quality, wide forming range and high material utilization rate.2. Establishment of the finite element model of HFSFThe elastoplastic finite element theory for sheet deformation in HFSF was studied.The explicit time integration of dynamic explicit algorithm and its stability were expounded. Issues of element selection, material model, contacts and friction condition were also discussed. And the finite element model of HFSF was eventually established.3. Research on HFSF for complex surface partsNumerical simulations of traditional stretch forming and HFSF were carried out by taking twisted parts, trapezoidal profile parts and hyperbolic top parts as research objects. The results showed that sheet metal attached the die more easily by HFSF than that by traditional stretch forming process for twisted parts and trapezoidal profile parts,and the forming quality was also higher. With the same stretching force, hyperbolic top parts by HFSF could attach the die without crack; while it couldn't attach the die completely and even cracks appeared by traditional stretch forming. It demonstrated that HFSF process could be used to form complex surface parts. Experiments were done using HFSF machine for above three complex surface parts, and the forming effects were good, which verified the correctness of numerical simulations.4. Research on effects of process condition on HFSF resultsEffects of clamp number, clamp gap, clamping block fillet radii, vertical hydraulic cylinder location and transition zone on forming results of HFSF for trapezoidal profile parts were numerically investigated. The results indicated that increasing clamp number and clamping block fillet radii was beneficial to obtain uniform stress distribution and smaller strain value in effective forming area. As clamp gaps increased, the stress and strain decreased firstly and then begin to increase when clamp gap was 10 mm. The smaller the distance between vertical hydraulic cylinder and clamp jaw was, the more uniform stress distributed, the smaller strain was, and the greater thickness became.Decreasing transition zone contributed to attach the die with the needed stretching force decreased. The stress distributed more uniformly; besides, the strain value and thickness thinning rate were smaller. Experiments were conducted for trapezoidal profile parts by HFSF machine to validate numerical simulations. It was proved that HFSF process was capable of forming non rectangular sheet metal with small transition zone.5. Research on effects of punch unit on HFSF resultsThe reasons for dimple during stretch forming process of sheet metal were analyzed.The swinging unit structure and forming principle were discussed. Effects of the radius of punch unit hemispherical end, fixed unit and swinging unit on HFSF for spherical and parabolic spherical parts were numerically investigated. The results showed that increasing the radius of hemispherical end contributed to reducing surface dimple, and the distribution of stress, strain and thickness were more uniform. But excessive the radius of hemispherical end would reduce forming precision. When forming with swinging units, surface dimple could be reduced; the forming quality with swinging units was obviously better than that of fixed units. Employing swinging units could reduce elastic cushion thickness, which further illustrated the superiority of swinging units. HFSF experiments with fixed units and swinging units were carried out, and the results had a good agreement with numerical simulations.6. Research on effects of elastic cushion on HFSF resultsEffects of elastic cushion thickness on HFSF with fixed units and swinging units were compared and analyzed. Numerical simulations of stretch forming for spherical and parabolic spherical parts were performed. The results showed that forming effect was improved both for fixed units and swinging units by using solid elastic cushion. As thickness of solid elastic cushion increased, the stress, strain and thickness of formed parts distributed more uniformly. And thickness thinning rate and dimple were reduced;while springback of formed parts increased. HFSF with fixed units could obtain surface parts without any dimple when elastic cushion thickness was 40 mm, but it was only20 mm with swinging units. It demonstrated that HFSF with swinging units could obtain better forming effect than fixed units with the same elastic cushion thickness.Comparative analysis for effects of discrete elastic cushion thickness on HFSF with fixed units and swinging units were done. The results showed that as discrete elastic cushion thickness increased within 20 mm, the stress, strain and thickness distribution were more uniform; thickness thinning rate decreased gradually and forming effect was improved. In summary, discrete elastic cushion can only improve forming effect in a certain range of thickness, so reasonable thickness should be selected according to specific conditions in actual production. Using discrete elastic cushion can help to reduce handling intensity of elastic cushion and improve production efficiency.Employing swinging units and discrete elastic cushion simultaneously can reduce solid elastic cushion thickness and even omit its use, further improving forming effect.