In this section, TA15 titanium alloy was taken as the research object, and the mechanical properties of TA15 titanium alloy at low temperature were obtained through a series of cryogenic tensile tests. The temperature term of J-C constitutive model of TA15 titanium alloy was modified at low temperatures, and the cryogenic constitutive equation of TA15 titanium alloy was obtained.
2.1 Test materials
The tensile test material was Ti-6AL-2Zr-1Mo-1V titanium alloy, which was treated by solution and aging. The mass fraction is shown in Table 2.1。
Table 2.1
Chemical composition of TA15 titanium alloy
Element | Al | N | H | Fe | C | Mo | V | Ti |
Content(%) | 6.55 | 0.009 | 0.01 | 0.2 | 0.08 | 0.94 | 0.82 | Bal. |
2.2 Cryogenic tensile test
The titanium alloy specimens had a nominal diameter of 5 mm, a nominal length of 27 mm (Fig. 2.1), and a testing rate of 2 mm/min (nominal strain rate of approximately 1.333e-4s− 1). The tensile tests were performed on the electronic universal testing machine (MTS SANS CMT5000, USA) at 25°C, 0°C, -50°C, -100°C, -150°C, and − 196°C. The temperature test system was KEITHLEY 2000 MULTIMETER, and the CryoLab low-temperature system was used to maintain the temperature inside the cryogenic box. The tensile test platform is shown in Fig. 2.2.
The engineering stress-strain curve can be obtained from the load and displacement data in the tensile process, and the engineering stress S and engineering strain e are defined as
$$e=\frac{{l - {l_0}}}{{{l_0}}}$$
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Where is the engineering stress, is the applied axial load, \({A_0}\) is the initial cross-sectional area of the specimen, is the current gauge length of the specimen, and \({l_0}\) is the initial gauge length of the specimen.
Through the above calculation, the engineering stress-strain curves of TA15 titanium alloy in the tensile process at test temperatures can be obtained, as shown in Fig. 2.3. It can be seen from the figure that TA15 titanium alloy has no obvious yield platform at all temperatures. However, with the decrease in temperature, the strength of TA15 titanium alloy increases and the fracture strain decreases, indicating that the plasticity of TA15 decreases with the decrease in temperature.
However, to accurately describe the dynamic mechanical properties of materials and obtain accurate constitutive equations of materials, it is necessary to convert engineering stress and strain into true stress and strain data. True stress is defined as:
Where is the current cross-sectional area of the specimen;
True strain is defined as.
$$d\varepsilon =\frac{{dl}}{l}$$
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Namely.
$$\varepsilon =\int\limits_{{{l_0}}}^{l} {\frac{{dl}}{l}} =\ln (\frac{l}{{{l_0}}})$$
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The relationship between the real strain and the engineering strain is
$$\varepsilon =\ln (1+e)$$
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The relationship between real and engineering stresses can be obtained from the incompressible theory of plasticity mechanics:
However, the use of this transformation formula should meet the incompressible conditions. In the uniaxial tensile test, the deformation concentration of the specimen after necking occurs near the necking area, and the elongation detected in the experiment is the entire gauge length section. Therefore, the relationship between the true stress and the engineering stress after necking cannot be obtained through this relationship.
Through the above relationship, the true stress and strain during the tensile process of the specimen from the yield stage to the tensile strength stage can be obtained. The true stress-strain curves at various temperatures are shown in Fig. 2.4. According to the stress-strain curve, the yield strength and tensile strength at each temperature can be obtained in Table 2.2.
Table 2.2
Yield strength and tensile strength at different temperatures.
Test temperature (℃) | Yield strength (MPa) | Tensile strength (MPa) |
25 | 608.2 | 658.6 |
0 | 700.8 | 734.8 |
-50 | 741.7 | 808.6 |
-100 | 889.9 | 930.1 |
-150 | 989.9 | 1041.9 |
-196 | 1021.5 | 1176.7 |
2.3 Johnson-Cook constitutive model parameter identification
J-C constitutive model is a material constitutive model proposed by Johnson and Cook for large strain, high strain rate, and high temperature of materials, which can comprehensively describe the strain hardening, strain rate hardening, and temperature softening effect of materials. Since the cutting process involves large strain, high strain rate, and, high temperature, the model is often used to describe the mechanical behavior of materials in the cutting process. The function model is as follows:
$$\sigma =(A+B*{\varepsilon ^n})*[1+C*\ln (\frac{{\mathop \varepsilon \limits^{.} }}{{\mathop {{\varepsilon _0}}\limits^{.} }})]*[1 - {(\frac{{T - {T_0}}}{{{T_m} - {T_0}}})^m}]$$
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Where \(\sigma\) is the flow stress, is the yield stress of the material at the reference temperature, and is the strain reinforcement factor. is the work hardening index, is the strain rate strengthening factor, \(\mathop \varepsilon \limits^{.}\) is the equivalent plastic strain rate, \(\mathop {{\varepsilon _0}}\limits^{.}\) is the reference equivalent plastic strain rate, is the temperature softening factor, is the test temperature, \({T_0}\) is the reference temperature, and \({T_m}\) is the material melting point.
To obtain the relevant parameters of the J-C constitutive model, the constitutive equation is first simplified to \(\sigma =(A+B*{\varepsilon ^n})\), and the parameters , and are fitted through the tensile tests at reference strain rate and reference temperature; then the equation was simplified to\(\sigma =A*[1+C*\ln (\frac{{\mathop \varepsilon \limits^{.} }}{{{\varepsilon _0}}})]\), and the strain rate hardening coefficients is fitted by Hopkinson compression bar tests at different strain rates (high strain rate) under reference temperature; finally, the constitutive equation is simplified to\(\sigma =A*[1 - {(\frac{{T - {T_0}}}{{{T_m} - {T_0}}})^m}]\), and the temperature softening coefficient is fitted by tensile tests at different temperatures under reference strain rate. Hongchao Ji [17] et al. modified the strain hardening and temperature softening terms of the J-C model of TA15 titanium alloy at high temperature and high strain rate through experiments, and obtained the following model:
$$\sigma =(279.579 - 122.153\varepsilon - 14.103{\varepsilon ^2})[1+0.152\ln (\frac{{\mathop \varepsilon \limits^{.} }}{{\mathop {{\varepsilon _0}}\limits^{.} }})][ - 0.011+0.001\ln (\frac{{\mathop \varepsilon \limits^{.} }}{{\mathop {{\varepsilon _0}}\limits^{.} }})(\frac{{T - {T_0}}}{{{T_m} - {T_0}}})]$$
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By analyzing the real stress-strain curve, it can’t be effectively fitted because the strengthening stage of TA15 titanium alloy is too short at-196°C. Therefore, − 150°C was selected as the reference temperature to fit the strain hardening term of the TA15 titanium alloy constitutive model, and A = 989.3, B = 910.9, n = 0.75466 were obtained. The fitting curve is shown in Fig. 2.5.
Since the J-C constitutive model is mainly used to describe the dynamic mechanical properties of the material at high temperature, the temperature softening coefficient fitted at low temperature cannot accurately reflect the relationship between material properties and temperature. Therefore, the temperature term of the J-C constitutive model is modified. The modified temperature term is\(\sigma =A*[1 - {m_1}{(\frac{{T - {T_0}}}{{{T_m} - {T_0}}})^{{m_2}}}]\), and the fitting results are \({m_1}\)= 3.8772, \({m_2}\) = 0.9862. The results show that the modified J-C constitutive temperature term can better characterize the influence of temperature on the mechanical properties of TA15 titanium alloy. The fitting curve is shown in Fig. 2.6.
For the strain rate strengthening coefficient of the constitutive model, the strain rate term of the J-C model of TA15 titanium alloy obtained by Hongchao Ji [17] et al. can be selected. where the strain rate strengthening coefficient = 0.152. Finally, the J-C constitutive model is sorted out, and the cryogenic constitutive model of TA15 titanium alloy is obtained as follows:
$$\sigma =(989.3+910.9{\varepsilon ^{0.75466}})[1+0.152\ln (\frac{{\mathop \varepsilon \limits^{.} }}{{\mathop {{\varepsilon _0}}\limits^{.} }})][1 - 3.872{(\frac{{T - {T_0}}}{{{T_m} - {T_0}}})^{0.9862}}]$$
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where the reference strain rate\(\mathop {{\varepsilon _0}}\limits^{.}\) = 0.0013s− 1, the reference temperature \({T_0}\)= 123K, and the TA15 melting point temperature \({T_m}\)= 1800K.