Fast Gas Forming Assisted By Current Heating And Properties Control For Titanium Alloy Thin-walled Tubular Components

The aerospace industry develops rapidly and has led to a dramatic increase in the demand for complex thin-walled structural parts made of titanium alloys.However,the existing forming processes need to heat the die and material at the same time.The low forming efficiency,high energy consumption and high cost seriously limit the large-scale application of titanium alloy thin-walled components.There is an urgent to develop a high-efficiency,environmentally friendly and low-cost titanium alloy forming process.In this paper,a new process of current heating and fast gas forming for titanium alloy thin-walled tubular components is proposed.The tube is heated by high-density current,and tube temperature raised to theβtransus temperature in tens of seconds,and then the high-pressure gas quickly filled into the tube.Under the action of internal pressure,the material contacted with the room temperature die at a higher strain rate to complete the in-die quenching,and integratedly and efficiently control the microstructure,properties and dimensional accuracy of the component.However,during the forming process,nonisothermality of the die and tube blank,high strain rate of the material,and the non-equilibrium state of the microstructure present a enormous challenge to controlling the forming process,microstructure and properties.Therefore,in this paper titanium alloy thin-walled tubular components are taken as the research object and conducted forming process research on near-αtype TA18 titanium alloy andα+βtype TC31 titanium alloy,which are commonly used in aviation thin-walled structural components.It provides new idea for efficient forming manufacturing of titanium alloy thin-walled tubular components with high property.The electro-thermal theory analysis shown when the tube was heated by current,the circumferential temperature distribution of the tube was not affected by its initial circumferential wall thickness distribution.While the axial temperature distribution was closely related to its axial wall thickness distribution.Therefore,the tube was no longer heated by current during the gas forming process.Through the electro-thermal coupling simulation,it was found that the length of the axial uniform temperature zone of the tube increased rapidly with the increase of the heating current density.With the increase of the heating temperature,the length of the uniform temperature zone increased slowly and then decreased rapidly.After solving the engineering problems such as accurate control of tube temperature,insulation between tube and die,high-pressure gas seal in tube,and current introduction electrode method,the current heating and fast gas forming experimental device was built,and the linkage control of each operation step was realized through integrated control.Through the combination of thermal-mechanical coupling simulation and gas pressure forming experiment,the reasonable pressurization rate of 20MPa/s should be used for tube gas forming was firstly determined.Then,under the condition of rapid pressurization,based on the influence of forming temperature and forming pressure on the forming accuracy and property of thin-walled tubular components,the fast gas forming process window of nearαtype TA18 titanium alloy thin-walled tubular components was established.When the forming temperature was 895°C and the forming pressure was 30MPa,the tubular components have the best comprehensive mechanical properties.The yield strength and tensile strength at room temperature increased from 599MPa and 706MPa to 758MPa and 813MPa,an increase of 26.5%and 15.2%respectively.The elongation only decreased slightly,still reaching 16.2%.In addition,the microhardness and microstructure of the bulging zone of the thin-walled tubular component were very similar,and the deviation of microhardness was less than 3%,indicating that the new process can obtain uniform microstructure and properties.Through the high temperature tensile test of TC31 dual phase titanium alloy at850°C-1000°C and 0.001s^-1-0.1s^-1.It was found that the flow stress curves were mainly characterized by a speedy increase to the peak stress in a small range of strain,and then decreased approximately linearly with the further increase of strain,which shown the characteristics of continuous softening.At a small strain rate of 0.001s^-1,the material exhibited slight dynamic hardening behavior when the forming temperature was above 950°C.The main softening mechanism during deformation was dynamic recrystallization and material damage in the later stage of deformation.The dynamic hardening at low strain rate was mainly caused by grain growth during deformation.Based on the influence of process parameters such as heating rate,heating temperature,holding time and cooling rate on the microstructure and mechanical properties ofα+βtype TC31 high temperature titanium alloy during non-equilibrium heat treatment,the reasonable process parameters of current heating and fast gas forming for TC31 titanium alloy thin-walled tubular components were obtained.The components with excellent high-temperature property was formed,which effectively broadened the application range of the new process.When the forming temperature is1040°C,the TC31 titanium alloy tube had the best comprehensive mechanical properties at the service temperature of 650°C.Compared with the original material,the yield strength of the formed part at 650°C increased by 34.0%from 574MPa to769MPa,and the tensile strength increased by 29.8%from 645MPa to 837MPa.At the same time,the fracture elongation still remained 21.5%.When the formed component was heated at 650°C for 1h,the yield and tensile strength at 650°C increased from769MPa and 837MPa to 913MPa and 963MPa respectively.The fracture elongation only decreased from 21.5%to 18.5%due to the gradual precipitation of secondaryαphase and silicide.Based on the microstructure characterization,the microstructure evolution of titanium alloy thin-walled components during the current heating and fast gas forming process are as follows:the forming temperature of the component is close to theβtransus temperature,a large number ofαphase toβphase transition occured.Subsequently,rapid deformation and in-die quenching were completed under the high pressure gas and room temperature die.The cooling rate exceeded 100°C/s,forming a large number of fine martensite,which greatly improved the strength of the formed components.At the same time,due to the large heating rate,strain rate and cooling rate,the time of the material at high temperature is particularly short,which effectively avoid the excessive coarsening of theβphase,so that the formed component still has good plasticity.