Study On Flexible Forming Of Sheet Metal Using Plasma Arc

Bending forming of sheet metal is an advanced processing technique for sheet metal developed in recent three years, which is newly emerging and promising. The core of the technology is that it takes plasma arc as heat source to form a controllable temperature gradient and an inhomogeneous thermal stress field through controlling arc-power, scanning path and scanning speed reasonably. Thus, the expected goal can be satisfied. The flexible forming without mould and external force is one of the advantages. The traditional technology does not only cost a lot of time and money, but also take up considerable human resource. So, the new technology will do goods to reduction of production costs and shortening developing time of new product, especially the production of multi-type and small batch of the large-scale sheets such as cars.The research status domestic and overseas of flexible forming of sheet metal with plasma arc and the mechanism and features of plasma arc were introduced in the paper. Two basic forms of the FFUPA, bending towards and away from plasma arc, were stressed. Temperature gradient mechanism, buckling mechanism and upsetting mechanism and causes induced were also analyzed. Further more, technology problem of the flexible forming using plasma arc was brought forward.In order to show the inner relations among part accuracy, arc characteristics and scanning path, and solve the technology problems such as power, scanning path and scanning speed, finite element method analytical model of thermo-elasto-plastic stress during plasma arc scanning was established. The plasma arc was treated as a jumping heat source with small steps, which was loaded on the small metal, according to its process of floating scanning. Transient responses of stress and elasto-plastic large deformation were solved. Distribution of temperature field, stress state and result of deformation were calculated. The thesis developed a 3-D FEM simulation model of plasma arc forming which included a nonlinear thermal-structural analysis and the calculates of the temperature field and stress- strain field of metal plate under different series of process parameter. It had also analyzed the influence of process parameters on temperature field and stress- strain field.The stress- strain field induced by the forming of doubly curved surface was calculated using large-deformation elastic FEM to make sheet-metal form better comply with the object shape. Scanning paths of heat source were designed, including spherical-crown and saddle-shaped surfaces. The mechanisms of edge effects induced in the experiments had been discussed and the solutions were brought forward to form the proper sheet metals through forming experiments and numerical analysis.The principle of selection of scanning interval is determined to improve the forming quality of smooth continuous arc surface on the base of simulation computation and experiment. The minimized scanning space can obtain strain-plastic fields with good continuity. The geometrical relationship between circular arc bending part and V-shape was investigated. In this paper, the author advanced the relationship between a parameterγand the degree of circular arc smoothness firstly. A simulation program of circular arc flexible forming of sheet metal using plasma arc was programmed using the ANSYS Parametric Design Language (APDL). The numerical simulation and experiments adopted increased forming method using plasma arc. The results gained were consistent with the required shapes after several V-shape bends.For angular effects in the experiment the mechanism analysis and numerical calculations are performed. The warp is mainly induced by the smaller compression strain caused by geometric constraint and different shrinkage between top and bottom surface. The solution of angular effects is brought forward and the satisfied sheets are formed.Based on the above analysis, the experimental facility of bending forming using plasma arc was built up. In order to form complex 3D surfaces, flexible forming with plasma arc and CNC were combined to realize the adjustment of plasma arc heat loading under the conditions of speed variation. Thus, the spherical-crown and saddle-shaped surfaces were demonstrated. Consequently, the basic rules and key technology of bending forming using plasma arc in practical work were obtained and the application level of the flexible forming using plasma arc had been further improved. The effects of the main parameters including power, scanning speed, scanning distance and scanning sequence had been investigated.Metallurgical analysis of the formed sheet metal was executed on the basis of mass of experimental data, and the change of microstructure of sheet metal after hot working could be mastered to make sure that the grain refining were induced by bending forming. Accordingly, the material microstructure was improving and the mechanical properties were superior to original materials.BP neural network was applied to flexible forming with plasma arc on the base of MATLAB. Inputs and outputs to the neural network were properly chosen. The mathematical model between the forming effect and experimental parameter was established, which was used of the forecast of bending angle and scanning times. The prediction error of bending angle is less than 5% and that of scanning times is less than 2%. The grey system theory and time series analysis was applied to forecast the dynamic deformability of angle. When observation sequence is the latest, it can keep the short-term prediction with a high accuracy. The prediction accuracy is.less than 3%.