Dissertation
Dissertation > Industrial Technology > Metallurgy and Metal Craft > Metal pressure processing > Forging, forging and blacksmith > Forging process

Research on Hot Deformation Behavior and Optimization of Forging Process of Titanium Alloy TC11

Author LiZuo
Tutor LuShiQiang
School Nanjing University of Aeronautics and Astronautics
Course Materials Processing Engineering
Keywords titanium alloy TC11 Dynamic Materials Model processing map criteria of flow stability or instability constitutive equation microstructure optimization of thermomechanical parameter deformation mechanism
CLC TG316
Type PhD thesis
Year 2009
Downloads 623
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TC11 is anα+βtitanium alloy and belongs to difficult-to-deform materials, which has been widely used in aerospace industry. Processing map technology based on Dynamic Materials Model (DMM) is a tool for design and optimization of hot deformation processes of metals, with which not only flow instability regimes can be avoided but also optimized ranges of temperature and strain rate can be identified. In this paper, the hot deformation behavior of two kinds of titanium alloy TC11 with different original microstructures, namely equiaxed and lamellar, has been studied by hot compression tests, and the forging processes for these two kinds of titanium alloy TC11 have been optimized by using processing map technology. The studied results are of great guidance significance in theory and practical application value for making reasonable forging processes to manufacture the forgings free from deficiencies with excellent and uniform microstructures as well as properties on a repeatable basis in a manufacturing environment.The theoretical basis and development of DMM and its processing map technology were here reviewed, and the basis and principle of both irreversible thermodynamics and dissipative structure related to various criteria for identifying the regimes of flow stability or instability as well as the criteria’s deduction processes were introduced in detail. The advantage and disadvantage, including available situations, of the criteria based on DMM were theoretically compared and analyzed. Analysis in theory indicated that the essence of Gegel criterion is identical to that of Malas criterion. These two criteria not only take mechanistic stability and thermodynamic stability of investigated materials into account, but also make the deformation process have self-modification to outside disturbance. If the value of m of investigated material is not a constant, Malas criterion is more reasonable than Gegel criterion. When using these two criteria to identify the flow stability regimes, the feasible ranges of thermomechanical parameters for hot working of material may be reduced because of more limiting conditions. The essence of Prasad criterion is identical to that of Murty criterion, and the latter is more reasonable when the value of m of investigated material is not a constant.The effects of deformation thermomechanical parameters on flow stresses and deformed microstructures of two kinds of titanium alloy TC11 with different original microstructures have been studied systematically. The results indicated that the flow stresses of these two kinds of titanium alloy TC11 decrease with increasing temperatures and decreasing strain rates. The stress-stain curves are type of strain softening at high strain rates and type of steady-state at low strain rates. As viewed from resistance of deformation, these two kinds of titanium alloy TC11 are feasible to be worked at low strain rates. If the temperatures increase, the strain rates may be increased aptly, thus the feasible range of strain rates will get wider. For these two kinds of titanium alloy TC11, the deformation uniformity will degenerate with decreasing temperatures and increasing strain rates. For titanium alloy TC11 with lamellar microstructure, inα+βphase field, the globularization ofαlamellas will occur when the strain rates are lower than and equal to 0.01s-1, thus the feasible strain rates are lower than and equal to 0.01s-1 as viewed from deformation uniformity and obtaining globularized microstructures. For titanium alloy TC11 with lamellar microstructure, in near-βandβphase field, theβgrains will undergo relatively full dynamic recrystallization when the strain rates are lower than and equal to 0.01s-1, thus the feasible strain rates are lower than and equal to 0.01s-1 as viewed from obtaining recrystallized microstructures. For titanium alloy TC11 with equiaxed microstructure, the morphology ofαphase changes just a little generally inα+βphase field. If the strain rates are higher, the deformation uniformity will degenerate, thus this kind of titanium alloy TC11 is feasible to be worked at low strain rates as viewed from deformation uniformity. For titanium alloy TC11 with equiaxed microstructure, the relatively full dynamic recrystallization ofβgrains occurs at 0.01s-10.1s-1 with fined grains inβphase field. Thus the feasible strain rates are in the strain rate range of 0.01s-10.1s-1 as viewed from obtaining fined recrystallized microstructures, which are higher than that of titanium alloy TC11 with lamellar microstructure an order of magnitude.The constitutive equations for these two kinds of titanium alloy TC11 have been studied. The results indicated that the hyperbolic sine equation of Arrhenius type and the modified power function equation of Arrhenius type may be regarded as the constitutive relationship models of titanium alloy TC11 with lamellar microstructure in near-βandβphase field andα+βphase field respectively, and the modified power function equation of Arrhenius type may also be regarded as the constitutive relationship model of titanium alloy TC11 with equiaxed microstructure in entire temperature regime. Error analysis indicated that these constitutive equations constructed have high precision. The average differences between the calculated and experimental flow stresses for titanium alloy TC11 with lamellar microstructure in near-βandβphase field andα+βphase field are 5.04% and 5.57%, respectively. The average difference between the calculated and experimental flow stresses for titanium alloy TC11 with equiaxed microstructure in entire temperature regime is 5.20%. The approach that constructing the constitutive model by modifying power function equation of Arrhenius type has a universal applicability and may be used for construction of constitutive equation for other materials.The optimization of forging processes for these two kinds of titanium alloy TC11 has been studied by using processing map technology for the first time, and the processing maps of these two kinds of titanium alloy TC11 have been plotted by using various criteria of flow stability or instability, respectively. The applicability of various criteria of flow stability or instability to these two kinds of titanium alloy TC11 has been analyzed and compared. The results indicated that the processing maps based on Murty criterion are in general more reasonable than those based on Prasad criterion or Malas criterion in predicting the regimes of flow stability, predicting the regimes of flow instability and optimizing forging thermomechanical parameters. The predicted results by using the processing maps based on Murty criterion are the following. For titanium alloy TC11 with lamellar microstructure, inα+βphase field, the regimes of flow instability are in the temperature ranges and the strain rate ranges of 750℃875℃and 0.005s-110.0s-1, 875℃1000℃and 0.2s-110.0s-1 with corresponding manifestations of flow instability including macro cracks, adiabatic shear bands and priorβboundary cavities. The better thermomechanical parameters for forging are in the temperature ranges and the strain rate ranges of 750℃900℃and 0.001s-10.005s-1, 900℃1000℃and 0.001s-10.03s-1, and the corresponding main deformation mechanism in these two domains is globularization ofαlamella. The optimum thermomechanical parameters for forging lies in the temperature range of 840℃980℃and near 0.001s-1. For titanium alloy TC11 with lamellar microstructure, in near-βandβphase field, the regimes of flow instability are in the temperature ranges and the strain rate ranges of 1000℃1100℃and 1.0s-110.0s-1, 1075℃1100℃and 0.001s-10.003s-1 with corresponding manifestations of flow instability including elongate ofβgrains, break ofβgrain boundaries, mixed microstructures like“necklace”and dynamic growth ofβgrains. The better thermomechanical regime for forging is in the temperature range of 1000℃1100℃and the strain rate range of 0.001s-10.05s-1 (except a little regime of 1075℃1100℃and 0.001s-10.003s-1) with corresponding deformation mechanism of dynamic recrystallization. The optimum thermomechanical parameter for forging lies near 1050℃and 0.001s-1 at stains below 0.4 and near 1050℃and 0.016s-1 at strains above 0.4, respectively. For titanium alloy TC11 with equiaxed microstructure, inα+βphase field, the regimes of flow instability are in the temperature ranges and the strain rate ranges of 780℃850℃and 0.008s-170.0s-1, 850℃927℃and 0.01s-170.0s-1, 927℃1008℃and 0.1s-170.0s-1 with corresponding manifestations of flow instability including cracks and cavities inβphase, adiabatic shear bands and flow localization. The better thermomechanical parameters for forging are in the temperature ranges and the strain rate ranges of 780℃850℃and 0.001s-10.008s-1, 850℃940℃ and 0.001s-10.01s-1, 940℃1008℃and 0.001s-10.01s-1, and the corresponding main deformation mechanism in these three domains is superplasticity. The optimum thermomechanical parameter for forging lies near 900℃and 0.001s-1. For titanium alloy TC11 with equiaxed microstructure, inβphase field, the regime of flow instability is in the temperature range of 1008℃1080℃and the strain rate range of 4.0s-170.0s-1 with corresponding manifestations of flow instability including elongate ofβgrains and break ofβgrain boundaries. The better thermomechanical parameters for forging are in the temperature range of 1030℃1080℃and the strain rate range of 0.001s-10.1s-1 at strains below 0.7, in the temperature range of 1020℃1060℃and the strain rate range of 0.004s-10.6s-1 at strains above 0.7, and the corresponding main deformation mechanism in these two domains is dynamic recrystallization. The optimum thermomechanical parameters for forging lies in the temperature range of 1060℃1080℃and near 0.001s-1 at strains below 0.7, in the temperature range of 1040℃1050℃and the strain rate range of 0.016s-10.07s-1 at strains above 0.7.The superplastic deformation behavior of titanium alloy TC11 with equiaxed microstructure has been investigated preliminarily here. The results indicated that this alloy would not exhibit superplasicity inβphase field, but exhibit superplasicity inα+βphase field. The optimum superplastic deformation temperature is near 900℃and the optimum superplastic strain rate is the lowest in the investigated strain rate range, which is in agreement with that predicted by using processing map. At 900℃and 0.001s-1, the elongation reaches 1215%. The volume ratio ofαandβhas a great effect on the superplasticity. When the volume fraction of primaryαphase is about 70%, this alloy exhibits optimum superplasticity. During superplastic deformation, dynamic recrystallization, diffusion creep, intracrystalline deformation and interface sliding operate together, and interface sliding mainly occurs atα/βphase interface.

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