Forming Technology And Numerical Simulation Study Of Multi-Clamp Type Flexible Stretch-forming Machine

Sheet metal forming is widely used in many fields, such as aviation, aerospace,automotive, high-speed trains and construction. These parts has a variety of complex andchanging shape, small batch or single-piece production, such as fuselages, wings and fairingsof aircrafts, cover panels of high-speed trains as well as the appearance decorations ofbuildings. The stretch forming machine is appropriative equipment for skin part forming,mainly used in the field of aircraft manufacturing at the present stage. The stretch formingmachine used by the manufacturers of China’s aircraft manufacturers and high-speed trainsare imported from abroad because of monopolized by foreign corporations such as Erie PressSystems、ACB&Cyril Bath and Ducommun Incorporated. Stretch forming technology ofaircraft fuselages, wings and fairings based entirely on foreign production experience，whichseriously restricts the development of China’s aviation industry. There is high demand forstretch forming machine in many key projects and key industries along with the wide use oflarge-scale three-dimensional sheet metal high-speed rail, ships, modern architectures as wellas other industries. Therefore, the R&D work carried out with independent intellectualproperty rights of stretch forming machine has an extremely important strategic andeconomic significance.In this paper, the principle and the main advantages of the stretch forming wereintroduced as well as the effect in sheet metal production. The status of stretch formingequipment of a variety of types was described and structural features and the advantages anddisadvantages of stretching devices at home and abroad. The clamp discretized design ofMulti-Clamp Type Flexible Stretch-Forming Machine formed on this basis, and theadvantages of discrete clamps were shown through theoretical analysis. By simulating avariety of shapes sheet metal forming with finite element software, the influence of loadingpath on forming results were studied, the advantages and disadvantages of transversestretching and longitudinal stretching were compared, and the relationship between theclamp swing angle and the difficulty degree of stretching was pointed out. Finally,Multi-Clamp Type Flexible Stretch-Forming Machine was applied to multi-point stretchforming and the simulation analysis and experimental research on production instance was carried out.The study content and conclusions of this paper can be summarized as follows：1. The development work of Multi-Clamp Type Flexible Stretch-Forming Machine wascarried out and completed based on analyzing flexible forming theory. The main structureincludes rack, three rows of hydraulic cylinders (horizontal cylinder, vertical cylinder andtilt cylinder) discrete clamps and universal joint. Three rows of hydraulic cylinders ofstretch forming machine are able to offer tensions of different directions and sizes. Thediscretized design of clamps and universal joint is capable of reducing the transition zone,improving material utilization and forming quality because the independence of the clampaction ensures that the clamps swing with the surface shape during forming.2. A finite element model was established to simulate forming quality of multi-clampflexible stretch forming with the finite element simulation technology. The analysis wasperformed specifically for these saddle-shaped aircraft skin pieces which are common anddifficult to form for traditional stretch forming machine. A variety of programs to optimizethe load path was proposed, and the influence of different loading paths on the formingquality was discussed. The influence of loading-curve power on stretching action researchedto confirm the stress and strain of formed saddle-shaped pieces was evenly distributedchanged smoothly under the loading of three power curve, which proved the loading of threepower curve has a significant role in improving forming quality.3. longitudinal and transverse stretch forming were adopted respectively for thespherical surface parts with different length and width to simulate the influence of differentstretch forming on stress and strain distribution，thinning and forming difficulty. Therelationship of the angle between adjacent clamps and the forming of three-dimensionalsurface was discussed and determined, which proved the principle that stretch forming alongthe length direction is easier with the same radius and provided guidance for the follow-upsimulations and experiments.4. A three-dimensional target surface of a building was selected to research with bothfinite element simulations and stretch forming experiments. The influence of differentmaterial and stretch-forming manner on forming quality was simulated to guide theproduction practice. Multi-point stretch forming experiments were conducted based onsimulation results and in turn used to prove simulation results.