Researches On High Pressure Hot Gas Forming Technology For Thin-wall Tube

In order to meet the needs of auto industry for reduction of the weight of car bodies, energy consumption and gas emission, the high temperature gas tube forming technology was proposed for manufacturing thin-wall tube parts of ultrahigh strength steel. This technology employs the method of high temperature forming, in which the ultrahigh strength steel tube billets are heated in the forming dies to the temperature above the austenite phase transform point, and then the billets are bulged to the desired forms by internal high pressure gas. After forming, the tube parts are quenched rapidly in the dies to increase the strength of the tube material up to 1200MPa by the quench hardening effect of the ultrahigh strength steel. In this project, a simulating device for high temperature gas tube forming was manufactured and typical mild steel tubes were adopted to conduct plenty of experimental research. Combined with numerical simulation, the influences of various technology parameters in the forming process were investigated, the results of which will provide guidance for further research of equipment upgrade and experimental research of ultrahigh strength steel.The main contents and conclusions of this research are as follows:(1) Study on the development of experimental device for gas tube forming. This project conducted the research on several key institutions of the gas forming device such as the water cooling system, sealing structure, high pressure gas supply system and heating device and so on. Also, STEP7 and WinCC were utilized to achieve the automation control of the equipment and HMI design, respectively.(2) Study on the numerical simulation of the tube gas forming process. The finite element analysis software ANSYS 13.0 was adopted in this paper to establish the finite element model of forming die. The simulation results indicate that both of the maximum stress and strain appear in the transition fillet of the die. Apart from that, finite element model of the forming process considering the mechanical and thermal was conducted by LS-DYNA 971. The influences of various technology parameters in the simulation process were investigated. The simulation results show that the higher temperature and gas pressure, the more thinning the wall; the faster gas inlet velocity, the greater axial force, the less thinning the wall.(3) Study on the experiments of high pressure gas forming for thin-wall tube. A vast number of experiments were conducted by adopting typical mild steel tubes and the influence of different process parameters on tube geometry shape and wall thickness distribution were also studied. The experimental results show that with a certain range of temperature and pressure, the higher temperature and gas pressure, the better geometry shape, the more thinning the wall; the faster gas inlet velocity, the more uniform bulging; the holding time have no effect on the geometry shape and wall thickness distribution; under 900 ℃, large quantity of bainite phase appeared and the maximum Vickers hardness increased from 130 before forming to 310 after forming. The tube with best geometry shape was obtained under the process scheme of 900℃ and 19MPa.