Hot Deformation Behavior And Microstructure Evolution Of As-forged Titanium Alloy TC27

TC27 is a new type of titanium alloy with high strength high tenacity and high hardenability of Chinese self-developed research and development, can be used as important bearing parts in the aerospace industry and the weapons. In this paper, have analyzed thermal deformation behaviour of the forging state TC27 titanium alloy by the isothermal compression experiment at constant strain rate, have constructed the constitutive relationships of the titanium alloy by adopting multivariate linear regression and internal variable methods, have studyed its microstructural evolution rules. The main conclusions are as follows:The flow stress of TC27 alloy rise rapidly with the rising of strain, after reaching the maximum then slowly decreased and present the flow softening phenomenon, the lower the deformation temperature the more obvious the flow softening phenomenon. The curve appeared fluctuating at each of the deformation temperature, strain rate for 70 s- 1 because of the alternative action by working hardening and softening.Multiple linear regression to build the constitutive equation within the range of 700 ~ 850 ℃, the error is less than 10% of the data points to account for around 93.1% of all data points; in the range of 900 ~ 1150 ℃, error less than 10% of the data to account for around 97.4% of all data points. Based on internal variable(dislocation density) to build the constitutive equation within the range of 700 ~ 800 ℃, the error is less than 10% of the data points to account for around 89.1% of all data points; in the range of 850 ~ 1150 ℃, the error less than 10% of the data to account for around 92.2% of all data points.Before compression, the specimen at 850 ℃ lower than the α+β/β transition temperature has β one-phase grain with the holding time for 12 min, 900 ℃ specimen higher than α+β/β transition temperature has a certain amount of α+β equiaxed structure with the holding time for 2 minAt the deformation temperature of 850 ℃, 70s-1, rolling reduction 30% ~ 50%, the primary α phase in the organization becomes unequiaxed gradually and has certain direction. At the deformation temperature of 900 ℃, rolling reduction 25%~50%, β phase grain get squashed obviously. Dynamic recrystallization have occurred at the deformation temperature of 900℃, 0.001s-1, rolling reduction 15% ~35%, the grain size of β decrease further, but when the rolling reduction was 50%, the grain size have grew up.At the strain rate 0.001s-1 ~70s-1, rolling reduction 50%, 700 ℃, the alloy microstructure were similar with the original samples without deformation, the volume fraction of α phase in organization is more than those at other temperature. But in the high strain rate, such as the 70s-1, local flow and adiabatic shear can be observed in large deformation area. 850 ℃, the volume fraction of primary α phase compared to other temperature is least. deformation temperature from 900 ℃ to 1150 ℃, in the case of low strain rate, with increasing deformation temperature, the original β grain size increase further and the dynamic recrystallization were also more likely to happen in the interior of the grains, at last tiny recrystallization grain gradually replace the original rough grain. The suitable hot working parameters about forging state TC27 titanium alloy is: the deformation temperature from 700 ℃ to 1150 ℃, strain rate 0.001s-1, 0.01s-1and0.1s-1.while the instability area is easy happened at high strain rate(10s-1and70s-1) at various temperatures.