Machining method for remarkably improving impact toughness of TC32 titanium alloy bar

Abstract

The invention belongs to the field of metal material scientific and technical processing, and relates to a processing method for remarkably improving the impact toughness of a TC32 titanium alloy bar. According to the method, the microscopic structure type of the TC32 titanium alloy is a double-state structure, a primary alpha phase is converted into an equiaxial shape or a long rod shape under the action of dynamic recrystallization and static recrystallization under the action of 845-890 DEG C thermal deformation and double annealing, the volume fraction is kept at 15%-35%, a beta conversion matrix comprises two alpha phases with different sizes, and the size of the beta conversion matrix is kept at 10%-20%. Wherein the alpha laths of the coarse lamella form an alpha cluster, the toughness of the alloy is improved, and the fine needle-shaped alpha laths are woven into a mesh basket structure, so that the strength of the alloy is improved. Besides, through the radial forging mode of single-pass small deformation and accumulated large deformation, fine grain uniform forging deformation of the alloy is achieved, the strength level of the bar can be maintained to be 1000 MPa or above, and the impact toughness can stably reach 80 J / cm < 2 > or above. In addition, the machining method is high in efficiency, the size precision is easy to control, batch production can be achieved, and the structure performance stability of different furnace batches is improved.

Description

Technical Field

[0001] The invention belongs to the field of metal material science and technology processing, and relates to a processing method for significantly improving the impact toughness of a TC32 titanium alloy bar. Background Art

[0002] Titanium and titanium alloys are widely used in aviation, aerospace, ships, weapons, chemicals, biomedicine, marine engineering, sports and leisure and other fields due to their high specific strength, high plasticity, high temperature resistance, corrosion resistance, and weldability. Especially in the field of aviation, due to the weight reduction requirements of advanced aircraft, they are widely used in the manufacture of key load-bearing structural parts such as frames, beams, joints and landing gear. In order to achieve one or more functions of these key load-bearing structural parts, parts are often processed into discontinuous structural features such as various keyways, arcs, oil holes, threads and cross-sectional changes. Under the action of external loads, these features are prone to stress concentration. In addition, the load-bearing parts in these key positions generally have harsh service environments and complex forces, so the strength and toughness of titanium alloy materials are very high. Among them, the impact toughness of titanium alloy materials is an important parameter for evaluating the reliability and safety of key components, because the impact toughness reflects the ability of titanium alloy materials to absorb plastic deformation work and fracture work under impact loads, and is an important mechanical performance indicator in engineering.

[0003] Similar to other metal materials, as the strength of titanium alloy materials increases, their impact toughness will significantly decrease. Therefore, the strength of titanium alloy materials with high impact toughness is generally low or medium strength. For example, for the most widely used (α+β) type TC4 titanium alloy, when the tensile strength is at the level of 960MPa, its room temperature impact toughness can reach 50J / cm 2 When the tensile strength of the α-type TA15 titanium alloy is at the level of 970MPa, its impact toughness can reach up to 60J / cm 2 From actual production experience, the impact toughness level of this alloy is mostly around 50J / cm 2 For Ti-6-22-22S and TC21 titanium alloys with a high degree of alloying, when the tensile strength reaches 1035MPa, the impact toughness level can also be maintained at 50J / cm 2 For aging-hardened high-strength titanium alloys with higher alloying degree and strength level, such as TC18, TB6 and TB17, the impact toughness water is mostly reduced to 30J / cm 2 It is also reported that the room temperature impact toughness of low temperature titanium alloy CT20 can reach up to 90J / cm 2level, but its tensile strength is only maintained at about 650MPa. From the existing research experience, due to the inverse relationship between strength and toughness, when the strength of titanium alloy material is maintained above 1000MPa, in order to maintain extremely high impact toughness (for example, greater than 70J / cm 2 ), there are still considerable challenges. Summary of the invention

[0004] The purpose of the present invention is to provide a processing method for significantly improving the impact toughness of TC32 titanium alloy bars, thereby improving the strength and toughness matching capability of the alloy and being used for manufacturing parts with high requirements for impact resistance.

[0005] TC32 titanium alloy is a new type of high comprehensive performance and low cost titanium alloy successfully developed in my country. β The temperature of the alloy is 915±5℃. The notable feature is its wide thermal processing window, especially its great potential for matching strength and toughness. It can significantly improve toughness while maintaining high strength, and its comprehensive performance is higher than that of TC4 and TA15 titanium alloys.

[0006] There are many methods to improve the strength of titanium alloys, such as solid solution strengthening, precipitation (precipitation) strengthening, dispersion strengthening, fine grain strengthening, deformation strengthening and phase transformation strengthening. Obviously, from the perspective of strengthening essence, some strengthening methods, such as precipitation and dispersion strengthening, can improve the strength of the alloy, but often at the expense of the plasticity of the material, while some strengthening methods can significantly improve the strength of the material, but sharply damage the toughness of the material. For example, the nanoscale ω phase obtained by phase transformation strengthening in titanium alloys belongs to this category, so it needs to be avoided in engineering. However, reasonable solid solution strengthening, fine grain strengthening and deformation strengthening methods can improve the strength and toughness of titanium alloys at the same time, so they have also received great attention. In addition, from the perspective of engineering application, a single strengthening method generally cannot effectively exert the strength and toughness potential of the material, and combining multiple composite strengthening and toughening methods is a more realistic choice. For TC32 titanium alloy, the solid solution strengthening effect of its main alloying elements Al, Mo, Cr and Zr on the α phase and β phase is the key to the alloy's high strength and toughness matching ability. Therefore, the floating range of the main elements of the alloy is also clearly restricted in the GB / T 3620.1-2016 specification.

[0007] The interstitial strengthening element O is extremely sensitive to the strength and toughness of the alloy. Even a very small change in content can have an important impact on the performance. Generally, as the content increases, the strength increases significantly, but the impact toughness decreases significantly. Therefore, the alloy specification generally requires a content of ≤0.2%. In order to obtain higher impact toughness of TC32 titanium alloy and ensure sufficient strength at the same time, its content is further limited to 0.4% to 0.12%. More importantly, a reasonable hot working method is also the most effective means to make full use of alloy fine grain strengthening and deformation strengthening methods to improve strength and toughness. Therefore, the present invention particularly adopts a radial forging process method. The forging process has the following significant characteristics: first, multi-directional forging is realized, and second, pulse forging can promote the titanium alloy to deform under a three-dimensional compressive stress state, thereby improving the toughness of the alloy.

[0008] The technical scheme adopted by the present invention is: a processing method for significantly improving the impact toughness of TC32 titanium alloy rods, the raw materials of the TC32 titanium alloy rods are 3-10 ton ingots, which are smelted in a vacuum consumable arc furnace, and the chemical composition content of the ingots is as follows: Al content 4.5%-5.5%, Mo content 2.5%-3.5%, Cr content 2.5%-3.5%, Zr content 0.5%-1.5%, Si content 0.1%-0.2%, Fe content ≤0.3%, C content ≤0.08%, N content ≤0.05%, H content ≤0.0125%, O content ≤0.2%, and the balance is Ti; the method comprises the following steps:

[0009] Step 1: Select the above-mentioned TC32 titanium alloy intermediate bar blank, grind away visible cracks on the surface of the blank, and chamfer the end surface of the blank;

[0010] Step 2, heating and keeping the blank, the heating temperature is 845°C to 890°C, and the holding time is 40min to 240min;

[0011] Step 3, radial forging is performed by a multi-pass deformation method, wherein the total deformation ε is controlled at 40% to 70%, the deformation of the penultimate pass is controlled at 5% to 15%, and the deformation of the remaining passes is controlled at 15% to 35%;

[0012] Step 4: Perform double annealing heat treatment on the radially forged bar to obtain a finished bar product.

[0013] The O content of the intermediate bar blank is controlled in the range of 0.4% to 0.12%.

[0014] During the radial forging, the rotation angle of each strike is 10° to 35°, the striking frequency is 300 times / min to 600 times / min, and the pulling speed is controlled at 2500mm / min to 3500mm / min.

[0015] During the radial forging, the number of passes is controlled within 3 to 7 times, and the total forging time is controlled within 6 minutes.

[0016] When the radial forging is completed, the final forging temperature of the finished bar product is ≥700°C.

[0017] During the double annealing, the first annealing heat treatment temperature adopts the heating temperature during radial forging, and the second annealing temperature is controlled at 500°C to 600°C.

[0018] After the radial forging and double annealing heat treatment are completed, the bars are placed in the air at a spacing of not less than 200 mm for dispersion cooling.

[0019] The finished rod product has a diameter Φ of 20 mm to 120 mm.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] Through the above technical scheme, the microstructure type of TC32 titanium alloy is a dual-state structure, and its main feature is that it is composed of primary α phase and β transformation matrix. In particular, under the action of hot deformation at 845℃~890℃ and double annealing, the primary α phase is transformed into equiaxed or long rods under the action of dynamic recrystallization and static recrystallization, and the volume fraction is maintained at 15%~35%. The β transformation matrix contains two α phases of different scales, in which the α laths of the coarse lamellar layer constitute α bundles, which improve the toughness of the alloy, and the fine needle-like α strips are woven into a basket structure, which improves the strength of the alloy. In addition, the fine-grained uniform forging deformation of the alloy is achieved by the radial forging method of single-pass small deformation and cumulative large deformation, in which the original β grain size is controlled in the range of 8μm~15μm, and the average grain size of the α phase is ~0.6μm, which can also improve the strength and toughness of the alloy at the same time. In addition, the O content in the range of 0.4%~0.12% is also one of the key factors for the maximum potential to obtain the toughness of TC32 alloy. After testing and analysis, the strength level of the rod can be maintained above 1000MPa, and the impact toughness can be stably reached 80J / cm 2 In addition, the processing method is highly efficient, easy to control dimensional accuracy, and can be mass-produced, thus improving the stability of organizational performance of different furnace batches. DETAILED DESCRIPTION

[0022] The present invention is further described below in conjunction with embodiments:

[0023] A processing method for significantly improving the impact toughness of TC32 titanium alloy rods, the (α+β) type two-phase titanium alloy with the Chinese material grade TC32, the chemical composition meets the requirements of "Titanium and Titanium Alloy Grades and Chemical Compositions (GB / T 3620.1-2016)", and the main chemical element content (weight percentage) is: Al content 4.5% to 5.5%, Mo content 2.5% to 3.5%, Cr content 2.5% to 3.5%, Zr content 0.5% to 1.5%, Si content 0.1% to 0.2%, Fe content ≤0.3%, C content ≤0.08%, N content ≤0.05%, H content ≤0.0125%, O content 0.4% to 0.12%, and the balance is Ti. The method comprises the following steps:

[0024] Step 1: Select the above-mentioned TC32 titanium alloy intermediate bar blank, grind away visible cracks on the surface of the blank, and chamfer the end face of the blank; the O content of the intermediate bar blank is controlled in the range of 0.4% to 0.12%.

[0025] Step 2: Heat and keep the blank at 845℃~890℃ for 40min. ～ 240min;

[0026] Step 3: radial forging is performed by multi-pass deformation, wherein the total deformation ε is controlled at 40% to 70%, the deformation of the penultimate pass is controlled at 5% to 15%, and the deformation of the remaining passes is controlled at 15% to 35%; during the radial forging, the number of passes is controlled at 3 to 7 times, and the total forging time is controlled within 6 minutes. During the radial forging, the rotation angle of each strike is 10° to 35°, the striking frequency is 300 times / min to 600 times / min, and the pulling speed is controlled at 2500mm / min to 3500mm / min.

[0027] When the radial forging is completed, the final forging temperature of the finished bar product is ≥700°C.

[0028] Step 4: Perform double annealing heat treatment on the radially forged rod to obtain a finished rod. The finished rod has a diameter Φ of 20 mm to 120 mm.

[0029] During the double annealing, the first annealing heat treatment temperature adopts the heating temperature during radial forging, and the second annealing temperature is controlled at 500°C to 600°C.

[0030] After the radial forging and double annealing heat treatment are completed, the bars are placed in the air at a spacing of not less than 200 mm for dispersion cooling.

[0031] Example 1: Taking the preparation of TC32 titanium alloy Φ90 mm rod as an example, the O content is 0.72% and the phase transition point Tβ 917℃. First, select the intermediate billet bar with a diameter of Φ140mm, grind off the visible cracks on the billet surface, and chamfer the billet end face; heat the billet and keep it warm, the heating temperature is 868℃, and the insulation time is 120min; after the insulation, radial forging is carried out, the total deformation is 59%, and 4 passes are used for radial forging, the deformation of each pass is 26.5%, 23.4%, 18.1%, 10.2%, each strike rotation angle is 23°, the strike frequency is 450 times / min, the pulling speed is controlled at 3200mm / min, and the hot deformation is completed. Dispersed air cooling; then the bar is subjected to double annealing heat treatment, the specific system is 868℃ / 2h, dispersed air cooling + 540℃ / 6h, dispersed air cooling; the bar spacing is not less than 200mm during forging and heat treatment cooling. According to the mechanical properties test, the average tensile strength of the finished bar is 1032MPa, and the average U-shaped impact toughness is 85J / cm 2 .

[0032] Example 2: Taking the preparation of TC32 titanium alloy Φ120 mm rod as an example, the O content is 0.85%, the phase transition point T β 915℃. First, select the intermediate billet bar with a diameter of 200mm, grind off the visible cracks on the billet surface, and chamfer the billet end face; heat the billet and keep it warm, the heating temperature is 886℃, and the insulation time is 160min; after the insulation, radial forging is carried out, the total deformation is 64%, and 4 passes are used for radial forging, the deformation of each pass is 31.9%, 28.0%, 20%, and 7.8%, respectively. The rotation angle of each strike is 18°, the striking frequency is 350 times / min, and the pulling speed is controlled at 3000mm / min. After the hot deformation is completed, it is dispersed and air-cooled; then the bar is subjected to double annealing heat treatment, the specific system is 886℃ / 2h, dispersed air cooling + 520℃ / 6h, dispersed air cooling; the bar spacing during forging and heat treatment cooling is not less than 200mm. According to the mechanical properties test, the average tensile strength of the finished bar is 1042MPa, and the average U-shaped impact toughness is 83J / cm 2 .

[0033] Example 3: Taking the preparation of TC32 titanium alloy Φ50mm rod as an example, the O content is 0.65%, and the phase transition point T β913℃. First, select the intermediate billet bar with a diameter of Φ80mm, grind off the visible cracks on the billet surface, and chamfer the billet end face; heat the billet and keep it warm, the heating temperature is 852℃, and the insulation time is 70min; after the insulation, radial forging is carried out, the total deformation is 60.9%, and 5 passes are used for radial forging, and the deformation of each pass is 19.0%, 18.5%, 20.3%, 16.4%, 11.0% respectively. The rotation angle of each strike is 28°, the striking frequency is 550 times / min, and the pulling speed is controlled at 2700mm / min. After the hot deformation is completed, it is dispersed and air-cooled; then the bar is subjected to double annealing heat treatment, the specific system is 852℃ / 2h, dispersed air cooling + 535℃ / 6h, dispersed air cooling; the bar spacing is not less than 200mm during forging and heat treatment cooling. According to the mechanical properties test, the average tensile strength of the finished bar is 1008MPa, and the average U-shaped impact toughness is 82J / cm 2 .

[0034] Example 4: Taking the preparation of TC32 titanium alloy Φ32 mm rod as an example, the O content is 0.45%, the phase transition point T β 918℃. First, select the intermediate billet bar with a diameter of Φ45mm, grind off the visible cracks on the surface of the billet, and chamfer the end face of the billet; heat the billet and keep it warm, the heating temperature is 863℃, and the insulation time is 50min; after the insulation, radial forging is carried out, the total deformation is 49%, and three passes are used for radial forging, the single pass deformation is 28.6%, 24.5%, and 5.9% respectively, the rotation angle of each strike is 12°, the striking frequency is 310 times / min, the pulling speed is controlled at 2800mm / min, and the air cooling is dispersed after the thermal deformation is completed; then the bar is subjected to double annealing heat treatment, the specific system is 863℃ / 2h, dispersed air cooling + 560℃ / 6h, dispersed air cooling; the distance between the bars during forging and heat treatment cooling is not less than 200mm. According to the mechanical properties test, the average tensile strength of the finished bar is 1055MPa, and the average U-shaped impact toughness is 89J / cm 2 .

[0035] Example 5: Taking the preparation of TC32 titanium alloy Φ85mm rod as an example, the O content is 0.63% and the phase transition point T β915℃. First, select the intermediate billet bar with a diameter of Φ150mm, grind off the visible cracks on the billet surface, and chamfer the billet end face; heat the billet and keep it warm, the heating temperature is 875℃, and the insulation time is 120min; after the insulation, radial forging is carried out, the total deformation is 67.9%, and 6 passes are used for radial forging, and the deformation of each pass is 19%, 21%, 23.4%, 26%, 23.2%, and 14.6%, respectively. The rotation angle of each strike is 20°, the striking frequency is 380 times / min, and the pulling speed is controlled at 2500mm / min. After the hot deformation is completed, it is dispersed and air-cooled; then the bar is subjected to double annealing heat treatment, the specific system is 875℃ / 2h, dispersed air cooling + 550℃ / 6h, dispersed air cooling; the bar spacing during forging and heat treatment cooling is not less than 200mm. According to the mechanical properties test, the average tensile strength of the finished bar is 1036MPa, and the average U-shaped impact toughness is 84J / cm 2 .

Claims

1. A processing method for significantly improving the impact toughness of TC32 titanium alloy bars, characterized in that: The raw material of the TC32 titanium alloy bar is a 3-10 ton ingot, which is smelted in a vacuum consumable arc furnace. The chemical composition content of the ingot is as follows: Al content 4.5%-5.5%, Mo content 2.5%-3.5%, Cr content 2.5%-3.5%, Zr content 0.5%-1.5%, Si content 0.1%-0.2%, Fe content ≤0.3%, C content ≤0.08%, N content ≤0.05%, H content ≤0.0125%, O content ≤0.2%, and the balance is Ti; characterized in that the method comprises the following steps: Step 1: Select the above-mentioned TC32 titanium alloy intermediate bar blank, grind away visible cracks on the surface of the blank, and chamfer the end surface of the blank; Step 2, heating and keeping the blank, the heating temperature is 845°C to 890°C, and the holding time is 40min to 240min; Step 3, radial forging is performed by a multi-pass deformation method, wherein the total deformation ε is controlled at 40% to 70%, the deformation of the penultimate pass is controlled at 5% to 15%, and the deformation of the remaining passes is controlled at 15% to 35%; Step 4: Perform double annealing heat treatment on the radially forged bar to obtain a finished bar.

2. A processing method for significantly improving the impact toughness of TC32 titanium alloy bars according to claim 1, characterized in that: The O content of the intermediate bar blank is controlled in the range of 0.4% to 0.12%.

3. A processing method for significantly improving the impact toughness of TC32 titanium alloy bars according to claim 1, characterized in that: During radial forging, the rotation angle of each strike is 10° to 35°, the striking frequency is 300 times / min to 600 times / min, and the pulling speed is controlled at 2500mm / min to 3500mm / min.

4. A processing method for significantly improving the impact toughness of TC32 titanium alloy bars according to claim 1, characterized in that: During radial forging, the number of passes is controlled within 3 to 7 times, and the total forging time is controlled within 6 minutes.

5. The processing method for significantly improving the impact toughness of TC32 titanium alloy rod according to claim 1, characterized in that: When radial forging is completed, the final forging temperature of the finished bar is ≥700℃.

6. A processing method for significantly improving the impact toughness of TC32 titanium alloy rod according to claim 1, characterized in that: During double annealing, the first annealing heat treatment temperature adopts the heating temperature during radial forging, and the second annealing temperature is controlled at 500℃~600℃.

7. The processing method for significantly improving the impact toughness of TC32 titanium alloy rod according to claim 1, characterized in that: After radial forging and double annealing heat treatment, the bars are placed in the air for cooling at a spacing of not less than 200 mm.

8. The processing method for significantly improving the impact toughness of TC32 titanium alloy rod according to claim 1, characterized in that: The diameter Φ of the finished bar is controlled between 20mm and 120mm.