Numerical Simulation Of Stretch Straightening Process Assisted By Resistance Heating For Titanium Alloy Extrusion Profile

Titanium alloy has lots of advantages such as high strength to weight ratio,excellent corrosion resistance and composite compatibility,which is widely used in aerospace.Titanium profiles,with special cross-section shapes,are important structural material in aircraft.Hot extrusion is the main production process of titanium alloy profiles,but the features of hot extrusion are high extrusion temperature and fast extrusion rate,thus,titanium profile products accompanied by bending and twisting deformations inevitably.The stretch straightening is an effective strategy that bending and twisting deformations can be simultaneously improved by single straightening process,which has high production efficiency and flexibility.However,the straightening production of titanium alloy profiles faces following difficulties at present:titanium alloy has high deformation resistance and low elastic modulus which would cause large springback after straightening;the cross-section of profile is complicate and multiplicate,there are large temperature difference during heating and cooling leads to uneven straightening stress and deformation;the shape of the profile to be straightening is irregular,generally includes bending and twisting that the straightening quality under the same straightening process is unstable.In order to improve the precision and properties of titanium alloy profile products,and breakthrough the key technology of straightening.This study mainly focused on the stretch straightening process of titanium alloy profile assisted by resistance heating.The temperature distribution law of resistance heating was researched and the hot deformation material parameters of profiles was obtained.On this basis,the numerical simulation model of stretch straightening was established,and the method and process parameters of straightening were optimized by numerical simulation,which provides a theoretical reference for the straightening production.The major research contents and conclusions are as follows:The heat exchange law between profile and environment was researched through aircooling experiment and the thermal boundary conditions were determined by using finite element inverse analysis method.The emissivity of profile is 0.7 and the air heat convective coefficient is 7 W/(m2·℃).Then,the thermal-electrical coupling numerical simulation model and theoretical analysis model resistance heating process were established.The results show that the shape and size of the profile’s cross-section are the key factors affecting temperature distribution,which mainly depends on the ratio between area and surface perimeter(S/C)of different subregions accordingly.Under the same loading voltage,the larger the S/IC value is,the higher the equilibrium temperature of the cross-section area is.The hot tension behavior and creep behavior of TA15,TC4 and TC18 titanium alloy profiles was researched by high temperature tensile tests and stress relaxation tests respectively.Flow stress of material is mainly influenced by hardening and softening behavior which depends on temperature and strain rate.Based on the flow stress curves,the modified Johnson-Cook constitutive equation was established.Comparing with the original equation,the modified constitutive equation has wider range of application and higher agreement degree,which can accurately characterize the tensile behavior of material.The stress relaxation curves show that as the temperature increase,the creep rate increases and the relaxation limit stress decreases.The Arrhenius creep constitutive model was obtained according to stress relaxation curves and verified by numerical simulation;the simulation results agree well with experiment results.Based on the above temperature model and material model,the numerical simulation model of hot stretch straightening process of titanium alloy profile was established in ABAQUS software.The numerical simulation results show that when the profile is directly exposed to air for resistance heating stretch straightening,the non-uniform temperature field on the cross section would affect the stress distribution of the profile after straightening.The non-uniform stress on cross-section will lead to inconsistency of springback in different regions along the length direction,thus resulting in a certain amplitude of single arc bending after cooling and unloading.If the non-uniform temperature distribution is improved that under isothermal condition,the straightening parameters include temperature,stretch rate and stretch strain.The temperature and stretch rate would directly affect the internal stress of the profile.The temperature should be no less than 700℃,and the stretch rate should be controlled at the order of 10-4 s-1,so that the internal stress of the profile does not exceed 300 MPa,a good straightness can be obtained(deflection/length≤2‰).The stretch strain mainly affects the value of plastic strain after straightening.The optimum stretch strain is the strain corresponding to the peak point in the flow stress curve.An excessive stretch strain will cause cross-section necking on the area near clamp.The conventional straightening process has a poor effect on the twisting deformation of profile.Therefore,the stress relaxation straightening process was developed,and the stress relaxation straightening simulation model of bending-twisting composite deformed profiles was established.The residual stress and springback can be greatly reduced through holding straightening temperature and strain constant for a period after twist-stretch straightening.Then,the resistance heating stretch straightening experiments of titanium alloy profiles with various cross-sections were carried out on the basis of 100 t stretch straightening machine.The law of temperature distribution during resistance heating and the forming rules of stretch straightening were verified.Combined with the numerical simulation results and practical condition,the problem of nonuniform temperature and difficult straightening of composite deformation were solved.Applying the straightening process optimized by numerical simulation to the industrial production of TC4 alloy profiles,the straightening effect is excellent and the production stability is high.The deflection/length and angle/length of profile after straightening does not exceed 1.5‰and 2%‰°/mm respectively.Finally,the microstructure observation and mechanical properties test were carried out to research the microstructure evolution of titanium alloy profiles during extrusion and straightening,and compare the mechanical properties of profile under different state.The results show that annealing and straightening process will not significantly change the microstructure and properties of profile.Under the optimized straightening process,the finished profile product retains the ideal microstructure and mechanical properties while improving the straightness and torsion degree,which can meet the application requirements of the profile.