Research On The Tension-Compression Asymmetry Of Commercially Pure Titanium For Grade 2

The commercially pure titanium(CP-Ti)is widely used in high-end technology for its excellent comprehensive performance,and the deformation behaviour and microstructural evolution are of great significance for achieving the precise forming.The tension-compression asymmetry(TCA)is obvious in the HCP metal,and it is very important to control this phenomenon effectively.In this paper,the asymmetry of initial yield and strain hardening of CP-Ti is studied systematically at room and higher temperature.The research provides experimental data for the precise forming technology and process development of the CP-Ti,and provides experimental data for the precise numerical simulation of the CP-Ti.The asymmetric and anisotropic mechanical behavior and microstructure evolution of the CP-Ti were studied in uniaxial tension and compression at ambient temperature.The results show the obvious TCA and planar anisotropy(PA)at room temperature.The initial yield strength ratios for 0°,45°and 90°to the rolling direction were 1.21,1.08,1.04,and the ratio of the strain hardening exponent were 0.557,0.561,0.548,respectively.The yield asymmetry is caused by a large number of dominant twins.The strain hardening of CP-Ti is the result of the combined effect of the Hall-Petch resulting from the separation of grain by the twins boundary and the texture hardening because of rotation lattice.Due to the large number of twins,the strain hardening rate increased significantly in the second stage,resulting in the asymmetry of strain hardening.By comparing the differences of the cylindrical deep drawing experimental results and the simulations using the tensile and compressive true stress-true strain curve as hardening criterion,the effects of TCA on the deep drawing were studied.The PA of the CP-Ti causes the earings,and the earing behavior is some contribution of both the PA and TCA.The evolution of TCA will reduce the thickness of the deep drawing parts,increase the earing height and influence the drawing force.The effects of deformation temperature and strain rate on the asymmetry of the CP-Ti were studied by analyzing the experimental results of uniaxial thermal tension and compression in the rolling direction.The high temperature constitutive model of rheological stress under different stress states were established.The yield asymmetry decreases with the deformation temperature increasing.The strain hardening rate decreases,and the second increasing hardening stage gradually decreases and disappears with the increasing temperature.The asymmetry of strain hardening is weakened.At the same temperature,the initial yield strength of uniaxial tension and compression and the ratio increase with the increasing strain rate.The yield strength ratio increased significantly with the increase of strain rate under the temperature of600?,and the strain hardening rate changes significantly.The dominated hardening mode changed from dislocation slipping to twinning.The microstructural analysis showed that the temperature and strain rate sensitivity of the twins type and numbers caused the changes of TCA.In addition,the thermal activation energy of the tensile stress state(Q~T=250.2645KJ/mol)is significantly less than that of the compressive stress state(Q~C=380.096KJ/mol).That is,the latter stored more energy under the same strain,which is indicated that compressive deformation is more prone to recrystallization than the tensile deformation,and is controlled by dislocation slip and twinning under higher temperature.The twin boundary should contribute to the recrystallization.