Study On Gas-Liquid Hydroforming Process Of Thin-Walled Tube

The design and application of hollow variable cross-section tubular part is an important way to realize lightweight.There are two main ways to form thin-walled tublar part with little change of section perimeter,Pressure Sequence Hydroforming(PSH)and Liquid Impact Forming(LIF).These two processes both use liquid as the filling medium to support the pipe wall,which inevitably leads to the problems of large pressure fluctuation and difficulties to seal during the forming process.In order to solve this problem,the gas-liquid mixture is used as the filling medium to support the tube forming.Firstly,geometric analysis and stress state analysis of each stage during the process of flattening and pressing circular section to rectangle section are carried out.The relationship between the volume of tube and the displacement of upper die in the process of “vertical and horizontal combined loading” and “vertical and horizontal sequential loading” is given;The force state and stress state of the tube under different constraints are analyzed,and we can get the conclusion that the position of maximum equivalent stress is on the transition point.Secondly,the compression laws of pure liquid,pure gas and gas-liquid mixture are deduced;comparing the characteristics of compression of these three filling media,it can be obtained that the compression curves of gas-liquid mixture is more adjustable.Combining the compression laws of pure gas and gas-liquid mixture with the geometric analysis of typical pressing processes,the "pressure-displacement" curves are obtained.According to the analysis of compression curves of gas-liquid mixture,the method to select process parameters is obtained;The sensitivity analysis of the compression curves show that the deviation of the volume ratio of the initial gas has a great influence on the pressure.When selecting the process parameters,the larger the volume ratio of the initial gas is,the smaller the pressure fluctuation caused by the deviation is.Thirdly,because of the particularity of gas-liquid hydroforming that the pressure inside the tube need to be coupled with the shape of the tube,the development of simulation software is necessary,two methods of manual simulation and semi-automatic simulation are developed,and the simulation results are consistent;The simulation software is used to carry out the forming with different filling media and different initial conditions.By comparing the forming results,it can be concluded that for the same parts,the ranges of process parameters of gasliquid hydro-forming is wider,and the forming conditions are easier to get;Under the same initial pressure condition,different loading curves can be obtained by compressing gas-liquid mixture.Finally,the dimension analysis and forming scheme of a tubular part with variable crosssections are carried out,and the initial parameters are determined by the selected loading curve through simulation.A special device for gas-liquid hydroforming was developed,which can realize the input and sealing of gas and liquid media,measure the pressure in the tube,and obtain the "pressure-displacement" curves.The die can fix tubes in the process of pressure establishment and forming to prevent tube moving or warping;The forming experiments of the tubular part with variable cross-sections were carried out and the experimental results were compared with the simulation,the results of wall thickness distribution are consistent.