| For the purpose of promoting development of titanium alloy gas forming process,shortening the forming time of thin-walled titanium alloy components and improving their microstructure,properties and dimensional accuracy,this paper proposed high pressure gas forming process.This process was carried out under the circumstance of reducing the forming temperature and increasing the forming pressure.It can provide technical support for manufacturing of titanium alloy parts with high efficiency and broaden the application of titanium alloys.The mechanical properties and constitutive model of titanium alloy change after reduc ing forming temperature and increasing forming pressure,thus the effects of various parameters on forming process are more complicated.Therefore,high pressure gas forming laws of titanium alloy should be further investigated.Through hot tension testing,gas forming experiments,coupled thermal-mechanical simulation and microstructure analysis,this paper investigated high pressure gas forming laws and microstructure of Ti-3Al-2.5V titanium alloy parts.High temperature mechanical properties of Ti-3Al-2.5V titanium alloy tube was investigated through the uniaxial tensile test.In the temperature range of 650?~800? and strain rate range of 0.001s-1~0.1s-1,the softening behavior resulting from dynamic recovery and dynamic recrystallization appears with temperature increasing or strain rate reducing.And this behavior can increase the fracture elongation.Besides,the work hardening behavior results from strain hardening and strain rate hardening during high temperature deformation of titanium alloy.But the work hardening rate reduces with temperature increasing at the same strain rate.The constitutive equation which depicts the stress versus strain laws of work hardening and softening at the same time was also constructed.On the base of the high temperature deformation behavior and formability of Ti-3Al-2.5V alloy,high pressure gas forming device was designed and developed.The key technologies including gas pressurization,transmission and closed-loop control were solved.The maximum pressure of this device is 70 MPa.The precise control of pressure loading path is achieved through input-output coordination control with variable flow.Meanwhile,the deformation and temperature can be measured on real time.High pressure gas forming experiments of the components with square cross-section was conducted.It was found that the circumferential temperature difference of the tube appeared when the high pressure gas flew into the tube fastly.When the pressurization rate increases,the temperature difference also increases.When the pressurization rate is above 1MPa/s,the corner radius changes with time linearly at the pressurization stage,but it changes with time exponentionally at the constant pressure stage.The circumferential temperature difference could be reduced through reducing pressurization rate and preheating gas.As a result,the maximum temperature difference could be reduced within 3 ? and the isothermal forming could be achieved under specific loading path.With these methods,the effect of circumferential temperature difference on the thickness distribution was investigated.It was found that the thinning ratio of straight wall area was higher than that of corner area when the temperature difference exceed ed specific value.And when the temperature difference was overlarge,the overlarge strain at the straight wall center led to crack.Adopting stepwise loading path with lower pressurization rate and lower forming pressure can decrease the temperature difference and strain rate at the initial forming stage.The local strain at the straight wall reduces obviously and the crack can be avoided.And then,through increasing pressure rapidly in the subsequent stage,the corner forming rate increases again,which improves the uniformity of wall thickness effectively and reduces the forming time of corner.The coupled thermal-mechanical numerical simulation was also conducted.Compared with the isothermal condition,the effective strain rate of straight wall outer region improved,but the effective strain rate of corner outer region reduced under the non-isothermal condition.This result demonstrated that deformation concentrated on the straight wall when large temperature difference exist ed.Whether under the isothermal condition or under the non-isothermal condition,the corner outer region is subjected to coupled-tensile stress.But for the corner inner region,it is subjected to tensile stress in the circumferential and axial direction and compressive stress in the thickness direction in the initial stage o f forming process,and then it is subjected to three-dimentional compressive stress subsequently.This illustrates that corner outer region is apt to deform during corner forming.The microstructure of Ti-3Al-2.5V alloy components after high pressure gas forming was investigated through optical microscope and EBSD.In the temperature range of 650?~800?,the deformation mainly attributes to grain elongation under the high strain rates and the microstructure mainly depends on the plastic deformation.At 700 ? and with the maximum circumferential temperature difference of 19?,larger deformation occurs in the straight wall inner center and corner outer center,so the grain undergoes obvious elongation deformation.Nevertheless,less deformation occurs in the corner inner center,and the grain remains equiaxed.Under the isothermal condition,the deformation conformity of corner outer center,corner inner center and straight wall inner center improves,so the difference of grain morphology reduces.The overlarge circumferential temperature difference resulting from improper loading path contribute s to rapidly increasing of local strain in the region of straight wall center,and then the voids appear in the initial forming stage.The voids get together and grow,which results in cracks.And thus void volumn fraction mainly depends on the plastic deformation. |
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