Research On The Forming And Springback Of TC4 Titanium Alloy For The Pile Sheet Hot Stamping With Cold Die Process

Titanium alloy is widely used in aerospace because of its high specific strength,high temperature resistance and corrosion resistance.However,titanium alloy is difficult to be formed at room temperature.The hot forming process is used to manufacture the titanium alloy parts,the traditional hot forming process includes hot creep forming,superplastic forming,isothermal forming and so on.The traditional forming process has insurmountable problems which are the low production cycle,high production cost and high energy consumption.Although the single-layer sheet hot stamping with cold die process can improve the production efficiency and reduce the production cost,it also has the disadvantages of poor formability and low forming precision.Therefore,pile sheet hot stamping with cold die process is proposed to study the formability and springback of titanium alloy sheet.Based on the uniaxial hot tensile tests,a constitutive model coupled with soften mechanism and damage evolution was established.The constitutive model reflects the internal relationship between macro deformation and microstructure evolution.The effects of deformation temperature and strain rate on the deformation behaviors,phase transformation behaviors and damage evolution of titanium alloy are analyzed.The high temperature deformation law and damage evolution mechanism of titanium alloy are revealed.The results show that the volume fraction of βincreases with the deformation temperature increasing.It is found that the damage mainly occurs at the junction of two phases by observing the microfracture morphology.The prediction ability of the model is evaluated by statistical parameters,and the results show that the change of flow stress can be predicted accurately by the model.The evoluation of microstructure also can be accurately predicted.Then,the plate-tool was used to study the interfacial heat transfer behaviors of titianium alloy sheet under the pile sheet hot stamping conditions.The interfacial heat transfer coefficient(IHTC)between the titanium alloy sheet and steel sheet is calculated by the Thermal Balance Methodology.The mechanical property and microstructure were measured and obversed.The results show that the heat loss of titanium alloy sheet is less during transferring and deforming,and titanium alloy sheet can be deformed at high temperature during the pile sheet hot stamping process.When the contact pressure is greater than 12 MPa,it can ensure that the titanium alloy sheet has good mechanical properties.It is found that the IHTC increases with the increase of contact pressure.When the contact pressure is greater than 19 MPa,the IHTC tends to be a stable value.A finite element(FE)model is established to verify the accuracy of IHTC.It is found that the IHTC can accurately describe the temperature change of titanium alloy sheet.Thirdly,based on the established constitutive model and IHTC,the hot tensile FE model and pile sheet hot stamping FE model were established to analyze the effects of deformation temperature,stamping speed,blank holder force,steel thickness and friction coefficient on the formability of TC4 alloy sheet.The change of sheet temperature,stress,and thickness was analyzed in simulation.The results show that the formability of TC4 alloy increases with the increase of deformation temperature,stamping speed,and steel sheet thickness.On the contrary,the increasing of blank holder force reduces the formability of the sheet.The thickness uniformity of sheet also can be improved with the increase of deformation temperature,stamping speed and steel sheet thickness,and the decrease of blank holder force.The titanium alloy parts obtained by pile sheet hot stamping have deeper stamping depth and greater forming limit.The stamping depth is 33mm at pile sheet hot stamping,increases by 135.7%comparing with the single-layer sheet hot stamping process.The forming limit obtained by pile sheet hot stamping increases by 64.8%comparing with the single-layer sheet hot stamping process.The FE simulation results are in good agreement with the experimental results,which shows that the FE simulation model can accurately describe the deformation process of titanium alloy sheet in pile sheet hot stamping process.Based on the stress relaxation theory,the Hyperbolic-Sine stress relaxation constitutive equation was established and embedded into the FE simulation model of U-shaped parts to predict the springback of titanium alloy sheet under the pile sheet hot stamping conditions.The effects of different process parameters(deformation temperature,die radius,steel sheet thickness and holding time)on the springback of titanium alloy are analyzed,and the deformation mechanism of reducing the springback of titanium alloy under hot deformation is revealed.The results show that the decrease of stress and the increase of strain are the main reasons for the decrease of springback.The FE model coupled with stress relaxation constitutive equation can better reflect the stress dropping and strain increasing of titanium alloy sheet during holding.The U-shaped part experiments are carried out to verify the accuracy of the springback prediction results in the pile sheet hot stamping process.Finally,a map which can decribe the forming process under the pile sheet hot stamping with cold die process is drawn,and the best forming parameters are determined.The titanium alloy pile sheet hot stamping with cold die process was applied to manufacture the typical aviation thin-walled components.The change of sheet thickness and springback was measured to analyze the formability and springback of the pile sheet hot stamping with cold die process.The results show that the aviation thin-walled components obtained by pile sheet hot stamping process have the uniform thickness distribution and small springback.The research on forming and springback of titanium alloy can provide theoretical guidance for the application of titanium alloy pile sheet hot stamping process.