Research On Microstructure Evolution And Dynamic Recrystallization Behavior Of Titanium Alloy TB6 And TA15 During β Forging

In recent years, as the aeronautical materials with traditional strength design criterion to modern damage tolerance design criterion, titanium alloy has to be meet the requirement of the performance of damage tolerance. In order to adopt this change of material selection criterion, the beta forging on titanium alloy is a major technical way. Since titanium alloy in beta forging is heated upon the beta phase transition temperature, the diffusion coefficient of alloy elements and impurity element in the beta phase is big. Beta grain growth tendency is particularly obvious and leads to the subsequent "organization hereditary". Dynamic recrystallization occurred in titanium alloy is one of the effective means of refining beta grains. In this paper, the hot dynamic recrystallization behavior and microstructure evolution of titanium alloy TB6 with different original microstructures, namely as-cast and as forged, and of titanium alloy TA15 with original duplex microstructure, have been studied during the isothermal forging in theβphase field. The microstructure evolution models have also been estab1ished. The results are of great theoretical significance and practical application for grains refinement in beta forging of titanium alloys TB6 and TA15 and beta forging development.In this paper, dynamic recrystallization behavior of titanium alloy TB6 (as-forging and as-cast) and TA15 (double beta state) has been studied systematically during theβforging process. The results show that titanium alloy TB6 with original microstructures as-forging only nucleates near the grain boundaries, and the operation of discontinuous dynamic recrystallization as predominant mechanisms is anticipated correspondingly. Titanium alloy TB6 with original microstructures as-cast has two typical nucleation sites, namely, grain interiors and near grain boundaries, the operation of discontinuous dynamic recrystallization and continuous dynamic recrystallization as predominant mechanisms is anticipated correspondingly. Titanium alloy TA15 with original duplex microstructure has two typical nucleation sites, namely, deformation band and near grain boundaries, and nucleation mechanism is of grain boundaries bulging and subgrain rotation correspondingly. For titanium alloy TB6 with as-forging microstructures, dynamic recovery of appears andβgrain elongate with strain rate higher than 1s^-1. Dynamic recrystallization appears with strain rate lower than 0.1s^-1, and with strain rate range from 0.01s^-1 to 0.1s^-1 and deformation temperature lower than 950℃, since dynamic recrystallization are sufficient and dynamic recrystallization grain growth is not apparent, deformation microstructures are of refinement. With strain rate lower than 0.001s^-1 and deformation temperature higher than 1050℃, although dynamic recrystallization is sufficient, dynamic recrystallization grains grow up dramatically. Thus the feasible strain rates are not more than 0.1s^-1 and feasible deformation temperatures are not higher than 950℃as viewed from obtaining fined recrystallized microstructures. For titanium alloy TB6 with As-cast microstructure, dynamic recrystallization are sufficient and nucreate in grain interiors, so deformation microstructures are of refinement in the strain rates no more than 0.001s^-1 and deformation temperatures no lower than 950℃. In the deformation conditions of stran rates higher 1s^-1, dynamic recovery occurs withβgrain enlongating. In other deformation condition, partial dynamic recrystallization appears, and the value of dynamic recrystallization volume fraction is not more than 20%. Increasing deformation temperature and decreasing strain rate can promote dynamic recrystallization of titanium alloy TA15 with double state microstructures. In strain rate no more than 0.01^-1, discontinuous dynamic recrystallization occurs and dynamic recrystallization microstructures are non-uniform. In strain rate no lower than 0.1s^-1, continuous dynamic recrystallization occurs with high nucleation rate and not obvious grain growth, which presents dynamic recrystallization related to nucleation primarilyDue to the composition segregation and the inner stress in deformed grains, the deformation-induced martensite which was produced from as-casted TB6 Ti alloy in the hot working and following rapid solidification process was testified. In the research, although there are two microstructural morphology in the deformation-induced martensite: needle shaped and serrated martensite, the structure of both two type of martensite belong to the orthorhombic martensite whose lattice parameters are a=0.301nm,b=0.491 nm,c=0.463 nm. The production mechanism of deformation-induced martensite could be concluded with following steps. Firstly, the parallel wicker shaped or needle shaped main dendrites were formed in the hot working and following rapid solidification process. Then, the branch dendrites were gradually grown from the main dendrites, traverse and overlap with each other. In the dendrites of deformation-induced martensite, twins could be found. The precipitate quantity of deformation-induced martensite increase with the deformation temperature, then decrease gradually. The influence of strain rate on precipitate quantity of deformation-induced martensite is closely related to the deformation temperature region. When the deformation temperature region is between 800℃-900℃, the relationship of precipitate quantity of martensite with strain rate present a single-peak characteristics and reach the peak value at 0.1s^-1. When the deformation temperature region is between 925℃^-1000℃, the relationship of precipitate quantity of martensite with strain rate present a bipeaks characteristics and reach the peak value at 0.01s^-1 and 1s^-1. When the deformation temperature is higher than 1000℃, the precipitate quantity will increase firstly with strain rate and then decrease with the raise of strain rate. The minimum value of precipitate quantity will appeare when the strain rate is 1s^-1. However, with the strain rate further increasing, the precipitate quantity will increase obviously once again. In all deformation conditions, in this research, when the deformation conditions are 925℃and 1s^-1, the maximum precipitate quantity of martensite with volume fraction of about 50% could be obtained. The feasibility of determining dynamic recrystallization critical strain using work hardening rate was dicussed, and the relevant data processing techniques and methods were proposed. The results show that the inflection point and the minimum value will appear inθ^σcurve and–?θ/?σ^σcurve respectively, when the dynamic recrystallization of titanium alloys occur in the deformation process. Based on the inflection point inθ^σcurve that could be used as evidence to judge the critical strain of dynamic recrystallization, the critical strains of dynamic recrystallization in deformed TB6 and double state TA15 Ti alloy were confirmed. With the increase of strain rate and decrease of deformation temperature, the critical strain of dynamic recrystallization the two kind of Ti alloy increase obviously. For the critical strain of dynamic recrystallization, it is related to the peak strain that can be shown by some equations:εc/εp=0.62 for deformed TB6 Ti alloy andεc/εp=0.504 for double state TA15 alloy, respectively. In order to further inflect the relationship between critical strain and deformation conditions for both two Ti alloy, Z parameter can be used in the research. And the function between the two parameter can be shown as the following equations:εc=1.7129×10-2Z0.156 for deformed TB6 Ti alloy andεc=1.72×10-2Z0.0605 for double state TA15 alloy, respectively.The behavior of dynamic recrystallization kinetic and the evolution model of dynamic recrystallized grain size have been studied. The results show that dynamic recrystallization volume fraction of dynamic recrystallized grains for titanium alloy TB6 with as-forging and TA15 with double state increase with increase of temperature and decrease of strain rate. For both the two alloys, the kinetic curve of dynamic recrystallization looks like"S". The kinetic model of dynamic recrystallization can be contributed to the structure of Avrami kinetic model, for titanium alloy TB6 with original as-forging microstructures: 1.1848(1 92R3T0.8) and titanium alloy TA15 with original double state: and by analysing the errors, the model of dynamic recrystallization kinetic shows a high accuracy. In this research, the drive force for the growth of dynamic recrystallized grains in Ti alloy was assayed in theory. Then, the theory that the driving force provided by interface energy and strain energy could stimulate grain growth, while the equivalent non-driving force restrain the grain growth is also be announced. By this basis, the evolution model of dynamic recrystallized grain size was established, which is of and the parameters in this model were confirmed by using inherit calculation method. By analysing the errors, the model of grain size shows a high accuracy .