Process method for multi-pass forming of TC11 titanium alloy

Abstract

This invention relates to the field of titanium alloy material preparation technology, and particularly to a multi-pass forming process for TC11 titanium alloy, comprising the following steps: Step 1, preparing titanium alloy bars; Step 2, heating the titanium alloy bars to 950℃ at a heating rate of 5℃ / s; Step 3, after reaching the set temperature, holding the titanium alloy bars at that temperature for 300 s to fully eliminate the temperature gradient inside the sample; Step 4, performing three compression deformations on the titanium alloy bars using a press, with a strain rate of 10 s. ‑1 The maximum deformation is 70%; a 10-second isothermal interval is set between each two compression passes; in step five, after deformation, rapid cooling is immediately achieved using water quenching. This invention achieves the technical goal of optimizing the hot forming process of titanium alloys and producing high-performance titanium alloy products.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy material preparation technology, and in particular to a process method for multi-pass forming of TC11 titanium alloy. Background Technology

[0002] Titanium alloys, due to their excellent specific strength, good corrosion resistance, and outstanding thermal stability, have become one of the important materials for structural components in the aerospace field. In various key components such as engine compressor blades, landing gear, and braking system components, titanium alloys are gradually replacing steel and aluminum alloys. Early torsion cylinder structural components in my country were mainly manufactured using direct machining or die forging processes of titanium alloys. Although these processes can achieve the basic shape and performance of the components, they suffer from problems such as low material utilization, high manufacturing costs, and short service life. In direct machining, a large amount of titanium alloy material is removed, leading to significant material waste, while also being difficult to machine and causing rapid tool wear. In die forging, the forming accuracy and internal microstructure uniformity of the components are limited, making it difficult to meet the stringent requirements of high-performance aircraft for key structural components. With the rapid development of my country's aviation industry, new-generation aircraft and re-entry-orbit spacecraft have placed higher standards on key structural components, and traditional manufacturing processes are no longer sufficient to meet their demands for lightweight, high performance, and long service life. Summary of the Invention

[0003] The problem solved by this invention is to provide a process method for multi-pass forming of TC11 titanium alloy, thereby achieving the technical goal of optimizing the hot forming process of titanium alloy and producing high-performance titanium alloy products.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: A process for multi-pass forming of TC11 titanium alloy includes the following steps: Step 1: Prepare titanium alloy rods. Step 2: Heat the titanium alloy bar to 950℃ at a heating rate of 5℃ / s. Step 3: After reaching the set temperature, the titanium alloy rod is held at that temperature for 300 seconds to fully eliminate the temperature gradient inside the sample. Step four: The titanium alloy bar is subjected to three compression deformations using a press, with a strain rate of 10 s. -1 The maximum deformation is 70%; a 10-second isothermal interval is set between every two compression passes. Step 5: After deformation is complete, immediately use water quenching for rapid cooling.

[0005] As an improved technical solution, the TC11 titanium alloy bar in step one is obtained by three vacuum arc melting processes, multiple upsetting and drawing processes, and finally double annealing.

[0006] As an improved technical solution, in steps three and four, the temperature control accuracy during the heat preservation and deformation processes is ±1 ℃. As an improved technical solution, in steps two through five, the titanium alloy rods are processed under a high-purity argon protective atmosphere.

[0007] As an improved technical solution, in step four, the first compression deformation is 30% of the height of the titanium alloy bar, corresponding to a true strain of approximately 0.35.

[0008] As an improved technical solution, in step four, the second compression deformation is 50% of the height of the titanium alloy bar, corresponding to a true strain of approximately 0.69.

[0009] As an improved technical solution, in step four, the third compression deformation is 70% of the height of the titanium alloy bar, corresponding to a true strain of approximately 1.21.

[0010] As an improved technical solution, in step four, a layer of high-temperature lubricant is uniformly coated on the surfaces of the upper and lower pressure heads of the compressor.

[0011] As an improved technical solution, in step four, graphite sheets and tantalum sheets are placed between the upper and lower pressure heads of the compressor and the titanium alloy rod, respectively.

[0012] As an improved technical solution, in step four, the single-pass compression is a one-time compression deformation of the sample immediately after reaching the set temperature and holding it at that temperature, with the true strain controlled at 1.21. In the three-pass multiple compression, the first compression deformation is 30% of the original sample height, corresponding to a true strain of approximately 0.35; the second and third compression deformations are 50% and 70% of the original height, respectively, with the true strains accumulating to 0.69 and 1.21, respectively. A 10-second isothermal interval is set between every two compression passes.

[0013] The beneficial effects of the present invention are as follows: By subjecting TC11 titanium alloy to three compression cycles at a deformation temperature of 950℃ and a strain rate of 10s, -1Adiabatic shear bands were generated during the second compression pass, while the microstructure recovered to equiaxed structure after the third compression pass. EBSD and TEM analysis revealed that the higher the strain rate of hot deformation, the higher the dislocation density within the microstructure. Furthermore, the generation of adiabatic shear bands consumed some dislocations within the microstructure. Within the shear bands, the β grains exhibited unique 60° and 90° orientations. It was found that the recovery of the shear bands to an equiaxed microstructure was due to the recrystallization of the β grains, resulting in a large amount of α-phase equiaxed microstructure. This achieved the technical goal of optimizing the hot forming process of titanium alloys and producing high-performance titanium alloy products. Attached Figure Description

[0014] Figure 1 This is a cross-sectional view of the original microstructure of a titanium alloy bar from a hot compression test. Figure 2 It includes single-pass hot compression test routes and three-pass continuous hot compression test routes; Figure 3 This is a schematic diagram of the parameters in equations (1)-(4); Figure 4 This is the actual stress-strain curve of TC11 titanium alloy after friction correction; Figure 5 This describes the relationship between temperature and strain under different hot compression parameters. Figure 6 It is the actual stress-strain curve after correction for both friction and temperature; Figure 7 It is a comparison between experimental and calculated values; Figure 8 It is a fit between experimental values ​​and predicted values; Figure 9 The TC11 titanium alloy was subjected to a strain rate of 0.1 s⁻¹ within the temperature range of 850–900℃. -1 up to 10 s -1 Microstructural evolution characteristics under certain conditions; Figure 10 This corresponds to the display of the average size, aspect ratio, and α phase content variation under different temperature and strain rate conditions for the α phase. Figure 11 This demonstrates the evolution of the metallographic structure of TC11 titanium alloy under different strain rates in three hot compressions at 950℃. Figure 12 The effect of three deformation passes at different temperatures and strain rates on the α-phase content; Figure 13 It is 950 ℃ and strain rate 0.1 s. -1 EBSD analysis results after three passes of deformation under the given conditions.

[0015] Figure 14 It is 950 ℃, strain rate 1 s -1EBSD results under the given conditions; Figure 15 It is 950 ℃, strain rate 10 s -1 EBSD results under the given conditions; Figure 16 The images are TEM characters after the second compression (a)(b) and the third compression (c)(d) at 950℃ and 10s⁻¹. Detailed Implementation

[0016] A process for multi-pass forming of TC11 titanium alloy includes the following steps: Step 1: Prepare titanium alloy bars. The TC11 titanium alloy bars are obtained by three vacuum arc melting processes, multiple upsetting and drawing processes, and finally double annealing.

[0017] Step 2: Heat the titanium alloy bar to 950℃ at a heating rate of 5℃ / s. Step three: After reaching the set temperature, the titanium alloy bar is held at that temperature for 300 seconds to fully eliminate the temperature gradient inside the sample. The temperature control accuracy during the holding process is ±1℃ to ensure a uniform and stable overall temperature field, guaranteeing consistent stress conditions during hot deformation. The temperature control accuracy throughout the entire holding and deformation process is ±1℃. Step four: The titanium alloy bar is subjected to three compression deformations using a press, with a strain rate of 10 s. -1 The maximum deformation was 70%; a 10-second isothermal interval was set between each two compression passes; the temperature control accuracy during the deformation process was ±1℃; a layer of high-temperature lubricant was uniformly coated on the surfaces of the upper and lower end pressure heads of the compressor. Graphite sheets and tantalum sheets were placed between the upper and lower end pressure heads of the compressor and the titanium alloy rod, respectively. Graphite sheets have good thermal conductivity and lubrication properties, which can effectively reduce the frictional stress concentration caused by the temperature rise of the contact surface; tantalum sheets can prevent the sample from directly contacting the pressure head at high temperatures and causing adhesion, thereby ensuring a more uniform stress distribution during deformation and reducing the interference of friction on the rheological stress measurement. The first compression deformation was 30% of the height of the titanium alloy rod, corresponding to a true strain of approximately 0.35. The second compression deformation was 50% of the height of the titanium alloy rod, corresponding to a true strain of approximately 0.69. The third compression deformation was 70% of the height of the titanium alloy rod, corresponding to a true strain of approximately 1.21.

[0018] Step 5: After deformation is complete, immediately use water quenching for rapid cooling. This effectively freezes the high-temperature deformed structure.

[0019] To prevent oxidation of TC11 titanium alloy under high-temperature conditions, the titanium alloy bars in the above forming process are all formed under a high-purity argon protective atmosphere.

[0020] To verify the superiority of the above-mentioned process method, hot compression tests were conducted on TC11 titanium alloy using a Gleeble-3500 thermal simulation testing machine. The high-temperature rheological behavior was studied through single-pass hot compression tests, and the rheological stress under different deformation processes was predicted using a strain-compensated modified Arrhenius constitutive equation. Simultaneously, multi-pass hot compression tests were performed, and the alloy was characterized using optical microscopy, scanning electron microscopy, electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The influence of deformation processes on its microstructure was systematically elucidated, providing a theoretical basis for guiding the forming process of TC11 titanium alloy and laying the foundation for finite element numerical simulation.

[0021] Before the experiment, a layer of high-temperature lubricant (usually graphite-based lubricant) is uniformly coated on the surfaces of the upper and lower indenters. A graphite sheet and a tantalum (Ta) sheet are placed between the sample and the indenter, respectively. The graphite sheet has good thermal conductivity and lubrication properties, which can effectively reduce the concentration of frictional stress caused by the temperature rise at the contact surface; the tantalum sheet can prevent the sample from directly contacting the indenter at high temperatures and causing adhesion, thereby ensuring a more uniform stress distribution during deformation, reducing the interference of friction on the rheological stress measurement, and improving the reliability of the experimental results.

[0022] The hot compression test of TC11 titanium alloy includes the following steps: Step 1: Prepare titanium alloy rods. Machine the titanium alloy rods into Φ8x12mm cylindrical specimens using wire cutting technology. The cross-section is shown below. Figure 1 As shown, Step two: The experimental temperature is 850℃~1000℃, with a temperature point set every 50℃; the strain rate range is 0.01 s. -1 0.1 s -1 1 s -1 10 s -1 The maximum deformation is 70%. Step 3: After reaching the set temperature, the sample is kept at that temperature for 300 s to fully eliminate the temperature gradient inside the sample, so that the overall temperature field is uniform and stable, and the stress conditions are consistent during the thermal deformation process.

[0023] Step four involves subjecting the heated titanium alloy bars to single-pass compression and three-pass multiple compression. These two deformation paths, single-pass compression and three-pass multiple compression, are used to examine the differences in microstructure evolution and mechanical response under different deformation processes. Figure 2 (left) shows the single-pass hot compression experimental route. After reaching the set temperature and holding it at that temperature, the sample undergoes a single compression deformation, with the actual strain controlled at 1.21. This deformation path is used to analyze the rheological stress characteristics of the TC11 alloy under single deformation. Figure 2 The right side shows the experimental route for a three-pass continuous hot compression test. The first compression deformation was 30% of the original sample height, corresponding to a true strain of approximately 0.35. The second and third compression deformations were 50% and 70% of the original height, respectively, with accumulated true strains of 0.69 and 1.21. A 10-second isothermal interval was set between each two compression passes to eliminate local temperature fluctuations and stress concentration effects, allowing the microstructure to stabilize and simulating the actual heating history and strain accumulation effect of the material during multi-pass hot forging. This multi-pass deformation mode better reflects the dynamic recovery, dynamic recrystallization, and phase transformation characteristics of TC11 titanium alloy under actual process conditions.

[0024] Step 5: After deformation, to preserve the characteristics of the deformed microstructure at high temperature as much as possible, the sample was immediately and rapidly cooled by water quenching, which effectively froze the high-temperature deformed microstructure. The deformed sample was then cut along the compression axis and polished with sandpaper from 320# to 2000#, followed by polishing with SiO2. Subsequently, it was etched with Kroll reagent, and the point of maximum strain was selected as the observation area.

[0025] Step six: Through statistical analysis and processing, the experimental and calculated values ​​show good agreement under different temperatures, strains, and strain rates. The accuracy of the model can be evaluated by calculating the correlation coefficient R and the mean absolute relative error (AARE). The formulas for calculating R and AARE are shown below: , , In the formula This is the experimental stress value. To calculate the stress value, For total data, , The calculated average stress is used for both experiments and calculations. The correlation coefficient R is 0.98, and the mean absolute relative error (AARE) is 10.5%, meaning that the calculated value can accurately predict the flow stress changes during the high-temperature deformation process of TC11 titanium alloy.

[0026] The temperature control accuracy of the entire heat preservation and deformation process is ±1 ℃. In order to prevent the TC11 titanium alloy from undergoing oxidation reaction under high temperature conditions, all experiments were carried out under a high-purity argon protective atmosphere.

[0027] The high-temperature rheological behavior of TC11 titanium alloy was studied. During the hot deformation process, friction between the materials and between the materials and the indenter caused varying degrees of bulging in the samples, resulting in uneven stress distribution and making the calculated stress higher than the actual flow stress. Therefore, friction correction is needed to ensure the accuracy of the data [43-45]. The corrected flow stress σf can be obtained from the formula: (1); In the formula: σf is the flow stress after friction correction; ε is the strain; σ is the experimentally measured flow stress value; is the friction correction coefficient; h0 is the initial height of the sample; r0 is the initial radius of the sample.

[0028] From equation (1), we can see that the friction correction coefficient is... The friction correction coefficient is a key factor for obtaining accurate flow stress values ​​and can be calculated using equation (2): (2); In the formula, r represents the average radius of the deformed specimen; h is the height of the deformed specimen; and b is the geometric structure factor. The geometric structure factor b and the average radius r are determined by equations (3) and (4), respectively: (3), (4); In the formula: ΔH is the height difference of the specimen before and after deformation; ΔR is the difference between the radius of the rear end face of the specimen after deformation and the maximum radius. The parameters in formulas (1)-(4) can be as follows: Figure 3 As shown; Reference Figure 4 As shown, the comparison results of the actual stress-stress curves of TC11 titanium alloy before and after friction correction at different deformation temperatures and strain rates can be seen. It can be seen that TC11 titanium alloy exhibits typical work hardening and dynamic recrystallization competition behavior during high-temperature deformation.

[0029] In the initial stage of deformation, due to the rapid increase in dislocation density, significant work hardening occurs within the material, causing a sharp rise in flow stress. The stress peak is reached when the work hardening rate balances with the dynamic recovery and dynamic recrystallization softening rates. As deformation continues, dynamic recrystallization dominates the softening process, dislocation density decreases significantly, and flow stress shows a decreasing trend, eventually reaching a steady state; this stress is called "steady-state stress." The comparison results at different strain rates show that the strain rate has a significant impact on flow stress. As the strain rate increases from 0.01 s⁻¹... -1 Increased to 10 s -1The peak stress increases significantly. This is because at high strain rates, the dislocation accumulation rate is faster than the dynamic recrystallization and recovery process, leading to a dominance of work hardening. Conversely, at low strain rates, the material has more time for dynamic recrystallization, resulting in sufficient softening and a significant reduction in flow stress.

[0030] Temperature also has a significant effect on flow stress. As the temperature increases from 850 ℃ to 1000 ℃, the flow stress curve shifts downward overall, and the stress peak value decreases significantly. This is because under high temperature conditions, atomic diffusion is enhanced, dislocations are more likely to slip and annihilate, thereby accelerating the dynamic recovery and recrystallization process, reducing deformation resistance. In addition, high temperature also promotes the α-phase to β-phase transformation, which further reduces deformation resistance and significantly improves the material's plasticity.

[0031] Comparing the original stress curve with the friction-corrected curve reveals that the corrected stress values ​​are generally lower than those of the original curve, and the difference increases with increasing strain rate and decreases with increasing temperature. This pattern indicates that the friction effect is more significant at high strain rates and low temperatures, because the material's plasticity is lower and the interfacial shear resistance is higher, leading to an overestimation of the originally measured stress value. Conversely, at high temperatures, plasticity increases and frictional resistance decreases, resulting in a smaller stress correction. Furthermore, as strain increases, the contact area between the specimen and the indenter increases, enhancing the interfacial friction effect and causing the difference between the original and corrected curves to gradually widen.

[0032] Besides the increased stress caused by friction, the heat generated during the hot compression test cannot dissipate outward due to the internal temperature rise caused by plastic deformation, leading to a localized temperature increase and thus affecting the flow stress. Further temperature correction is needed for the friction-corrected flow stress, considering the temperature rise during compression. T) can be obtained from equation (5): (5); In the formula, is the thermal conversion efficiency, which is generally taken as 0.95; is the adiabatic factor, and its value range is shown in the following formula; ρ is the material density; Cp is the specific heat capacity of TC11 titanium alloy; (6); Depend on Figure 6 It can be seen that under different strain rates, the temperature rise of TC11 titanium alloy continuously increases with the increase of actual strain, and the temperature rise amplitude significantly increases with the increase of strain rate. When the strain rate is 0.01 s⁻¹... -1 At this stage, the temperature rise is relatively low, with a maximum of approximately 8 °C, and the deformation process is approximately isothermal; when the strain rate increases to 0.1 s⁻¹... -1 At this point, the temperature rises to approximately 22 °C, indicating that some of the plastic work has been converted into heat energy; the strain rate further increases to 1 s. -1At that time, the temperature rose significantly to 60 ℃; while in 10 seconds... -1 Under high-speed deformation, the temperature rise reached a maximum of 75 °C. This indicates that at high strain rates, heat cannot dissipate quickly enough, the insulation effect inside the material is significantly enhanced, and most of the deformation work is accumulated in the sample as heat energy.

[0033] Furthermore, the temperature rise decreases with increasing initial temperature, indicating that thermal conductivity and thermal softening effects dominate at high temperatures. The temperature rise is most pronounced in the 900–950 ℃ range, while the temperature rise difference is smaller in the 950–1000 ℃ range. This suggests that TC11 titanium alloy approaches the α+β to β phase transformation region near 900–950 ℃, where thermal softening and microstructure evolution absorb more plastic work, thus inhibiting further temperature rise. Related studies indicate that TC11 titanium alloy exhibits significant dynamic recrystallization behavior near the β transformation region, leading to a rapid decrease in flow stress and an increase in plasticity. The flow stress value caused by the adiabatic effect is: (7); where is the set temperature for the experiment.

[0034] The stress values ​​after temperature correction are generally lower than those after friction correction alone, especially under high strain rate conditions. This indicates that if the effect of adiabatic heating is ignored, the flow stress will be overestimated. Temperature correction eliminates the "pseudo-strengthening" part caused by the conversion of plastic work into heat energy, making the corrected curve closer to the real material response under isothermal conditions [5]. This rule is consistent with the results of hot compression tests of various titanium alloys and high-temperature alloys. For example, Castellanos et al. [6] also found that temperature rise correction can significantly improve the accuracy of constitutive models when studying the hot deformation behavior of Ti–6Al–4V. In summary, Figure 5 and Figure 6 Together, these findings reveal the multi-factor coupling characteristics of the high-temperature flow behavior of TC11 titanium alloy: under low-speed deformation, the temperature rise effect is negligible, and the rheological behavior is mainly controlled by work hardening and dynamic softening; under high-speed deformation, adiabatic heating becomes the dominant factor, significantly altering the material's true stress level. Therefore, when establishing a high-temperature constitutive model for TC11 alloy, both friction and temperature corrections must be considered simultaneously to obtain accurate flow stress data, providing a reliable basis for subsequently establishing the high-temperature deformation constitutive equations for TC11 titanium alloy.

[0035] The constitutive equation for high-temperature deformation of TC11 titanium alloy is currently used to establish constitutive models for material hot deformation, such as Arrhenius and JC type. The Arrhenius constitutive equation is widely used due to its high accuracy, and it was originally proposed by Sellars and McTegart. The constitutive equation is used to describe the relationship between flow stress and hot deformation related parameters, and its equations are shown below (8)-(10): (8); (9); (10); In the formula: For stress, For strain rate, Let Z be the deformation activation energy, R be the gas constant, T be the deformation temperature (K), and A, A1, A2, n, n1, α, and β be material constants. The relationship between strain rate and deformation temperature T can be expressed by the parameter Z, as shown in the equation: (11) Based on equations (8)-(11), we can derive: (12); (13); (14); (15); where α, n, and Q can be determined by calculating the average slope, and by taking the logarithm of both sides of equation (11), we obtain the following equation: (16); Determined by calculating the average intercept value.

[0036] Figure 6 When the strain is 0.2, - , - , - , - The fitted graph can be obtained by taking the average value of the fitted curve and equation (16). , , , The calculated values ​​are 0.099, 3.33, 624.17 and 60.65, respectively. Figure 6 Functional relationship when strain is 0.2 (a) - (b) - (c) - , (d) - .

[0037] pass , , , The flow stress at a strain of 0.2 can be calculated from this value, expressed as follows: (17) In the formula .

[0038] This formula expresses the relationship between flow stress, strain temperature, and deformation rate at a fixed strain of 0.2. However, in actual deformation processes, there is also a relationship between stress and strain. Therefore, using the above method, for strains of 0.05-1.1, considering that the peak stress changes rapidly with strain (0.05 strain interval for 0-0.2 and 0.1 strain interval for 0.2-1.1), we calculate the values ​​of , , and , and obtain the relationship between each parameter and strain by fitting a polynomial of no more than three degrees: (18); (19); (20); (twenty one); By combining equations (15) and (18)-(21), a constitutive model for strain compensation correction is established: (twenty two) By using a strain-compensated constitutive model, the stress-strain curves of the TC11 high-temperature deformation process can be quickly calculated, as shown below. Figure 7 As shown in the figure, the experimental and calculated values ​​at different temperatures, strains, and strain rates show good agreement. The accuracy of the model can be evaluated by calculating the correlation coefficient R and the mean absolute relative error AARE. The formulas for calculating R and AARE are shown in (23) and (24): (twenty three) (twenty four) In the formula, represents the experimental stress value, represents the calculated stress value, represents the total data, and and represent the average stress of the experiment and the calculation. The calculation results are as follows: Figure 8 As shown, the correlation coefficient R is 0.98 and the average absolute relative error AARE is 10.5%, which means that the calculated value can accurately predict the change of flow stress during the high-temperature deformation process of TC11 titanium alloy.

[0039] The effects of deformation passes, temperature, and strain rate on microstructure evolution; the effects of temperature and strain rate on microstructure evolution under single-pass compression, refer to... Figure 9 and Figure 10As shown, systematic observation reveals that deformation temperature and strain rate significantly influence the morphology and distribution of the isometric primary α phase (αp) in the alloy. Generally, with increasing deformation rate and temperature, the morphology of the α phase gradually transforms from isometric to lamellar, accompanied by an enhanced tendency for spheroidization. Specifically, at 850℃ and a strain rate of 0.1 s⁻¹, the morphology of the α phase increases. -1 Under low-speed deformation conditions, the morphology of the isoaxial αp phase in TC11 alloy does not change much and still maintains a relatively complete isoaxial morphology. Obvious lamellar α phase precipitation can be observed in the microstructure, and these lamellae are mostly arranged parallel to the compression direction.

[0040] When the strain rate increases to 1 s -1 At this time, a significant thinning of the α phase and marked elongation along the compression direction can be clearly observed, exhibiting typical fibrous characteristics. This indicates that the higher deformation rate promotes the dynamic recovery and recrystallization process of the microstructure, causing some αp phases to undergo deformation coordination within the β phase matrix, thus elongating and refining. Simultaneously, the lamellar α phase is compressed, sheared, and exhibits a tendency to fracture, with fine fragmented structures appearing in localized areas, indicating that stress concentration at high strain rates promotes the fragmentation and rearrangement of the microstructure. Through this study, researchers have proposed grain boundary separation and lath shear spheroidization models to explain this phenomenon.

[0041] When the strain rate is further increased to 10 s -1 At high strain rates, the material struggles to transfer heat effectively, resulting in a significant localized adiabatic temperature rise. This causes the strip-shaped α phase to further split under shear stress, revealing a large number of fine granular α phases in the microstructure, with sizes significantly larger than those observed at 1 s⁻¹. -1 The strain rate decreases significantly under certain conditions, exhibiting a high degree of spheroidization. This characteristic suggests that the intense plastic flow at high strain rates not only induces dynamic recovery and recrystallization but may also promote the redissolution and reprecipitation of some α phases, leading to a finer and more homogeneous microstructure. Similar deformation mechanisms have also been reported in the literature. Researchers have pointed out that under high strain rate hot compression, the adiabatic heating inside titanium alloys can raise the local temperature above the β transformation point, thereby inducing the α→β phase transformation and forming finer secondary α phases during cooling.

[0042] When the deformation temperature increases from 850℃ to 900℃, the microstructure evolution pattern is basically the same as at 850℃, but the overall α-phase content decreases. Figure 10It is evident that as temperature increases, the activation energy of phase transformation decreases, making phase transformation more likely to occur. Specifically, the volume fraction of the α phase decreases from approximately 45% to around 38%; simultaneously, the average length of the α phase decreases from 12.4 μm to 9.8 μm, the width decreases from 6.7 μm to 5.7 μm, and the aspect ratio decreases from 1.9 to 1.7, indicating that deformation at high temperatures is more conducive to the spheroidization and homogenization of the α phase. This is consistent with the conclusions drawn by Ge et al. through their study of the dynamic spheroidization mechanism of Ti-6Al-2Mo-2V-1Fe alloy during hot deformation. Further heating to 950℃ reveals even more complex changes in the microstructure. At a strain rate of 0.1 s⁻¹... -1 At this point, the α-phase content further decreased to approximately 20% compared to 900℃, indicating that some of the α-phase had already transformed. As the strain rate increased to 1 s... -1 The α phase forms a distinct adiabatic shear band along the compression direction, and the α phase within the tissue is compressed, fractured, or even dissolved, causing its content to plummet to approximately 10%. When the strain rate is further increased to 10 s⁻¹, the α phase... -1 At this temperature, the α-phase content actually increased to approximately 26%, while the average length of the α-phase decreased to 7.8 μm, the width to 4.9 μm, and the aspect ratio to 1.58, indicating that the increased temperature led to more complete spheroidization. This phenomenon may be due to the significant strain rate dependence of the microstructure evolution of TC11 titanium alloy at 950℃. When the strain rate is 1 s⁻¹... -1 At this rate, a distinct shear band characteristic appeared in the microstructure, primarily due to the localized adiabatic temperature rise effect under thermo-mechanical coupling. At this rate, part of the plastic work generated by the material is converted into heat, but the heat dissipation rate is slower than the heat generation rate, leading to a rapid increase in temperature and thermal softening in localized areas. This causes strain to concentrate in narrow, band-like regions, forming a typical adiabatic shear band (ASB). The strain and temperature peaks within the shear band are significantly higher than those in the surrounding matrix, causing the strip-shaped α phase to be strongly stretched, fractured, and even dissolved. When the strain rate is further increased to 10 s⁻¹… -1 At this point, the microstructure no longer exhibits obvious shear bands, but instead displays a more uniform, fine, and nearly equiaxed α phase. This anomalous characteristic can be attributed to a stronger adiabatic heating effect, causing the temperature over a wide range to rapidly exceed the β transformation point, thereby inducing partial α→β ​​phase transformation and high-temperature dissolution. Subsequently, during the unloading and cooling stages, a large number of fine secondary α phases recrystallize from the β phase, forming a uniformly distributed granular microstructure. Therefore, at 950℃, TC11 titanium alloy undergoes a 1 s... -1 The shear band dominates deformation in the 10 s direction. -1 The essence of recrystallization refinement transformation is the result of the coupling of heat accumulation, phase transformation activation, and tissue reconstruction behavior.

[0043] The effect of temperature and strain rate on microstructure evolution under three compression cycles, refer to Figure 11 and Figure 12 As shown, observe Figure 12 As shown in (a) and (b), at 850℃ and 900℃, the α-phase content does not change significantly with strain rate and strain, indicating that the microstructure is mainly controlled by the deformation morphology. In 12(c), the trend of α-phase content with deformation pass and strain rate at 950℃ can be observed, clearly showing that strain rate has a significant impact on microstructure evolution and α-phase content under high-temperature conditions. At low strain rates (0.1-1 s⁻¹), the α-phase content... -1 In the first deformation pass, the α phase content is generally lower than that after the second deformation pass. This phenomenon is mainly due to the fact that 950℃ is close to the β transformation temperature of TC11 titanium alloy, and some of the original α phase undergoes α→β phase transformation during the high-temperature holding stage. Simultaneously, the plastic deformation during the first deformation pass slightly increases the local temperature, further promoting the α phase to β phase transformation, resulting in a significant reduction of the α phase in the microstructure after the initial deformation. With the progress of the second deformation pass, strain accumulation may intensify dynamic recovery and recrystallization, releasing the deformation energy and prompting the recrystallization of new fine α phases during the cooling stage, thus causing a slight increase in the α phase content after the second deformation pass. After the third deformation pass, the α phase content in the microstructure decreases to varying degrees at both strain rates. Figure 11 In (g) and (h), obvious linear flow traces can be seen in the internal structure. This is because the increase in local strain causes the material to flow violently perpendicular to the compression direction, resulting in local heating and leading to partial α-phase transformation or even the formation of... Figure 11 (h) contains an adiabatic shear band, thus reducing the α-phase content after the third compression. When the strain rate is further increased to 10 s⁻¹... -1 The microstructure evolution exhibits more complex characteristics. In the first deformation pass, due to the high strain rate limiting the complete α→β phase transformation, the initial α phase content is approximately 25%, higher than 0.1–1 s⁻¹. -1 The values ​​under the given conditions. High strain rates cause plastic deformation to occur primarily within a short time, preventing sufficient heat dissipation within the material and creating localized insulating zones. During the second deformation pass, due to the enhanced insulating effect, significant localized shear bands are generated within the microstructure, leading to a decrease in the internal α-phase content to approximately 10%, compared to 1 s. -1 The third shear band effect is similar, but occurs in an earlier stage of deformation. It is noteworthy that at 10 s... -1 Under the given conditions, after the third deformation pass, the α-phase content actually rebounded to around 36%. This paper presents a detailed study of the phenomenon occurring during deformation at 950℃.

[0044] Reference Figure 11 and Figure 12 As shown, in order to verify and explain the deformation mechanism at 950℃, Figures 13 to 15The microstructure of EBSD after multiple compressions at 950℃ and 0.1s⁻¹-10s⁻¹ was shown. The microstructure variation was observed by examining the IPF, KAM, and orientation difference distribution diagrams. In the IPF diagram, the white line represents the low-angle grain boundary (LAGB) with an angle range of 2° to 15°, while the black line represents the high-angle grain boundary (HAGB) with an angle exceeding 15°. KAM represents the residual stress and dislocation density after processing to a certain extent

[72] , and its density increases gradually from blue to red. In addition, we selected grains smaller than 1.5 μm as recrystallized grains.

[0045] Reference Figure 13 As shown, from Figure 13 As shown in (a) and (d), with the increase of deformation passes, the recrystallization volume fraction inside the material gradually increases from 17% to 23%, while the grain size refines from 3.5 μm to 2.9 μm, indicating that significant grain refinement occurs at lower strain rates. Due to the lower strain rate and sufficient thermal activation time during low-speed deformation, dislocation climb and slip are more efficient, promoting the continuous formation and evolution of subgrain boundaries into high-angle grain boundaries, thereby driving continuous dynamic recrystallization (CDRX). Figure 13 As shown in (c) and (f), the orientation difference distribution decreases slightly at 60°, indicating that some high-angle grain boundaries are replaced by subgrain boundaries, and the local orientation tends to be more uniform. With the progress of the third deformation pass, as... Figure 13 As shown in (g), numerous strip-shaped grains appeared in the microstructure, the dynamic recrystallization volume fraction further increased to 28%, and the grain size continued to decrease to approximately 2.5 μm. Figure 13 As shown in (i), the distribution of orientation difference near 60° is significantly enhanced again. This may be due to the phase transformation caused by excessive local temperature during deformation. During the cooling process, the α phase precipitated from the β phase exhibits a unique 60° and 90° orientation distribution [73, 74].

[0046] Figure 14 The figure shows a temperature of 950 °C and a strain rate of 1 s. -1 EBSD results under the given conditions. (Compared to...) Figure 13 (a) As can be seen from the comparison, 1 s -1 After deformation, the grain morphology becomes more complex, with internal grains exhibiting a mixed distribution of strip-like, needle-like, and equiaxed grains. From Figure 14 As can be seen in (a), some grains have been elongated into needle-like morphologies, while others retain an equiaxed shape, indicating that recrystallization and deformation structures coexist at this stage. Combined with... Figure 11 (a), (b) and Figure 12 (c) It can be seen that 1 s -1Under these conditions, the α phase content was relatively low and a large number of needle-like α phases were generated. This structural change may be due to the high deformation energy input to the system at this strain rate, but its energy is largely dissipated as heat

[75] , leading to intensified phase transformation during deformation. At high temperatures, the metastable β phase easily precipitates as needle-like α phase during the cooling stage, while residual stress and thermal effects after deformation jointly promote the nucleation and growth of the α phase. The orientation difference distribution results show that the orientation angle is mainly concentrated at 60° and 90°. The volume fraction of recrystallization in the second pass increased from 17% to 25%, and the grain size decreased from 3.8 μm to 2.5 μm, indicating that recrystallization occurred more fully under medium-speed conditions. Figure 11 From (d) and (e), it can be inferred that at a strain rate of 0.1-1 s⁻¹, the increase in α-phase content in the second pass is a result of recrystallization: the newly formed recrystallized grains provide more grain boundaries and dislocation walls for α-phase precipitation, which is conducive to the nucleation and growth of the α-phase. When entering the third pass of deformation, as... Figure 14 As shown in (g), the internal grains exhibit obvious needle-like orientation deflection and kinking structure, and typical adiabatic shear bands appear in the microstructure (such as...). Figure 14 (as shown in (h)), indicating severe local strain concentration. At this point, the needle-like grains can be identified as β-phase grains, with consistent orientation along the shear direction, indicating that under high temperature and high strain conditions, the β-phase grains were elongated and reoriented along the principal slip system. Simultaneously, the dynamic recrystallization fraction decreased to 20%, and the grain size increased to approximately 4.4 μm, indicating that in the later stage of this pass, the recrystallization growth caused by local temperature rise exceeded the grain refinement effect, exhibiting typical grain coarsening behavior. Furthermore, the orientation difference distribution shows distinct 60° and 90° orientation angle peaks. Further analysis... Figure 14 (i) with Figure 15 (h) It can be observed that the orientation difference within the shear band is also distributed between 60° and 90°, similar to the orientation of the needle-like β grains. Figure 14 The orientation relationship shown in (j) is very close, indicating that the shear band and the acicular β grains have a certain orientation inheritance during deformation, suggesting that the formation of the shear band is closely related to the deformation orientation of the β phase. However, due to the fact that... Figure 14 (c) No shear bands were generated. Therefore, the level of orientation difference aggregation at 60° and 90° can only indicate the degree of β phase transformation into needle-like structure, and cannot quantitatively indicate whether adiabatic shear bands are generated inside the material.

[0047] Reference Figure 15 As shown, compared to the first two groups, the microstructure evolution at high strain rates is more dramatic. From Figure 15 (a) It can be seen that the recrystallization volume fraction in the first pass is as high as 25%, and the α phase content is about 27%, which is significantly higher than that in the 1 s pass. -1The α-phase content under these conditions. This may be because at high strain rates, the deformation time is short, and energy cannot be completely dissipated as heat. Some deformation energy is used to drive lattice distortion and dislocation accumulation, thereby promoting recrystallization nucleation and resulting in a higher recrystallization volume fraction than under medium-speed conditions. On the other hand, 0.1 s -1 The content of the lower α phase and 10 s -1 The similarity indicates that the temperature rise caused by the high strain rate and the high recrystallization rate can cancel each other out. The temperature rise promotes the dissolution of the α phase, while the high recrystallization fraction increases the precipitation of the α phase, thus making the α content similar to that at 0.1 s⁻¹. After the second deformation pass, as... Figure 15 As shown in (d), the tissue contains... Figure 14 (g) Similar β-acicular grain structure Figure 14 (f) also shows a clear adiabatic shear band, indicating that local strain concentration at this point leads to the rearrangement of β grains along the shear direction. And from... Figure 14 (h) and Figure 15 As can be seen in (e), the formation of shear bands absorbs a certain amount of dislocations, thus reducing the dislocation density within the tissue. Figure 15 As shown in (e), the dislocation density decreases significantly. This may be because, under high strain rate conditions, the externally applied plastic work cannot diffuse quickly enough through heat conduction, causing a large amount of plastic work to accumulate as heat in local areas, forming adiabatic shear bands. The strain and strain rate of these shear bands are much higher than those of the surrounding matrix, leading to stress concentration and local temperature rise. Dislocations accumulate in large numbers here, but as the local temperature rises, the thermal activation ability of dislocations increases, climb and cross-slip accelerate significantly, and a large number of dislocations are annihilated or absorbed. As deformation continues, after the third compression, as... Figure 15 (g) It can be clearly observed that a large number of fine equiaxed recrystallized grains have formed in the microstructure. The recrystallization volume fraction has increased significantly from 19% to 43%, and the grain size has decreased significantly from 4.2 μm to 2.1 μm. At the same time, the α phase content has increased from 12% to 36%. This significant recrystallization indicates that under high strain rates, the large strain in the third pass generates a large dislocation density, such as... Figure 15 As shown in (i), sufficient driving force was provided for the microstructure, promoting dynamic recrystallization and α-phase precipitation. The presence of numerous strip-shaped β-phases during deformation, and the faster dynamic recrystallization of the β-phase compared to the α-phase, is one reason for the increased α-phase content. The microstructure recovers from the second-pass shear band structure to a fine equiaxed microstructure, as shown in (i). Figure 14 As shown in (i).

[0048] Reference Figure 16 As shown, Figure 16 As shown in (a), the second shear band region is mainly composed of strip-shaped β grains with a low surrounding dislocation density, while the dislocations are mainly concentrated near the α grain boundaries. Figure 16As shown in (b), local dislocation walls are formed, and these high-density dislocation walls provide nucleation conditions for subsequent recrystallization. With a significant decrease in the number of strip-shaped β grains after the third compression pass, the microstructure mainly transforms into fine equiaxed α grains, and the number of dislocation walls increases and the dislocation density significantly improves. This indicates that strong dynamic recrystallization occurred during the third deformation pass, a phenomenon consistent with the increased recrystallization degree and rising KAM value in the IPF results from the EBSD analysis.

[0049] Hot compression tests were conducted on TC11 titanium alloy to investigate its performance at deformation temperatures of 850-1000℃ and strain rates of 0.1-10 s⁻¹. -1 The actual stress variation under a strain of 1.21 was investigated, and based on this, friction and temperature corrections were successively applied to establish the Arrhenius-type constitutive equation for TC11 titanium alloy. Furthermore, three compression passes were conducted at 850-950℃ with strains of 0.35, 0.69, and 1.21 to further study the microstructure evolution. The conclusions are as follows: (1) TC11 alloy exhibits a significant work hardening effect in the early stage of deformation. Subsequently, dynamic recovery and dynamic recrystallization compete with each other, causing the stress to gradually decrease after reaching its peak and tend to a steady state. With increasing strain rate, the dislocation accumulation rate is faster than the recrystallization softening rate, resulting in a significant increase in peak stress. With increasing temperature, diffusion enhancement promotes dislocation annihilation, thus reducing flow stress. The results of friction and temperature rise correction further show that the friction effect and adiabatic heating under high strain rate and low temperature conditions have a significant impact on flow stress, and the strain difference increases with increasing strain.

[0050] (2) Based on the true stress-strain curves after friction and temperature correction, the constitutive equation of TC11 titanium alloy was established according to the Arrhenius-type hyperbolic sine equation. Its accuracy was verified; the AARE was 10.5% and the R² was 0.98, providing a theoretical basis for actual forging. An equation with strain as the independent variable was established through linear regression.

[0051] (3) Through three-pass compression of TC11 titanium alloy, it was found that when the total deformation of the three-pass compression was 70% at a deformation temperature of 950℃ and a strain rate of 1s⁻¹, the microstructure produced adiabatic shear bands, which were the same as those produced when the deformation was 70% in a single compression. Adiabatic shear bands were produced in the second compression at a strain rate of 10s⁻¹, while the microstructure recovered to equiaxed structure after the third compression. EBSD and TEM analysis showed that the higher the strain rate of hot deformation, the higher the dislocation density in the microstructure, and the formation of adiabatic shear bands consumed some dislocations in the microstructure. In the shear bands, β grains exhibited unique 60° and 90° orientations. The study found that the shear bands recovered to equiaxed structure because of the recrystallization of β grains, producing a large amount of α-phase equiaxed structure. This indicates that the modified process can optimize the hot forming process of titanium alloys and produce high-performance titanium alloy products.

Claims

1. A process for multi-pass forming of TC11 titanium alloy, characterized in that, Includes the following steps: Step 1: Prepare titanium alloy rods. Step 2: Heat the titanium alloy bar to 950℃ at a heating rate of 5℃ / s. Step 3: After reaching the set temperature, the titanium alloy rod is held at that temperature for 300 seconds to fully eliminate the temperature gradient inside the sample. Step four: The titanium alloy bar is subjected to three compression deformations using a press, with a strain rate of 10 s. -1 The maximum deformation is 70%; a 10-second isothermal interval is set between each two compression passes. Step 5: After deformation is complete, immediately use water quenching for rapid cooling.

2. The process method for multi-pass forming of TC11 titanium alloy according to claim 1, characterized in that, As an improved technical solution, the TC11 titanium alloy bar in step one is obtained by three vacuum arc melting processes, multiple upsetting and drawing processes, and finally double annealing.

3. The process method for multi-pass forming of TC11 titanium alloy according to claim 1, characterized in that, In steps three and four, the temperature control accuracy during the heat preservation and deformation process is ±1 ℃.

4. The process method for multi-pass forming of TC11 titanium alloy according to claim 1, characterized in that, In steps two through five, the titanium alloy rods are processed under a high-purity argon protective atmosphere.

5. The process method for multi-pass forming of TC11 titanium alloy according to claim 1, characterized in that, In step four, the first compression deformation is 30% of the height of the titanium alloy bar, corresponding to a true strain of approximately 0.

35.

6. The process method for multi-pass forming of TC11 titanium alloy according to claim 1, characterized in that, In step four, the second compression deformation is 50% of the height of the titanium alloy bar, corresponding to a true strain of approximately 0.

69.

7. The process method for multi-pass forming of TC11 titanium alloy according to claim 1, characterized in that, In step four, the third compression deformation is 70% of the height of the titanium alloy bar, corresponding to a true strain of approximately 1.

21.

8. The process method for multi-pass forming of TC11 titanium alloy according to claim 1, characterized in that, In step four, a layer of high-temperature lubricant is uniformly coated on the surfaces of the upper and lower pressure heads of the compressor.

9. The process method for multi-pass forming of TC11 titanium alloy according to claim 1, characterized in that, In step four, graphite sheets and tantalum sheets are placed between the upper and lower pressure heads of the compressor and the titanium alloy rod, respectively.

Images