A method of ultrafine-grained Ti-6Al-4V titanium alloy

Abstract

The application discloses a method for preparing ultra-fine grain Ti-6Al-4V titanium alloy. The method comprises the following steps: pretreating a coarse grain Ti-6Al-4V titanium alloy plate, processing the plate into a square block with a specific proportion, and performing surface smoothing treatment. Then, forging deformation is performed through multi-directional isothermal closed-die forging, the forging direction, the pressure and the deformation pass are controlled, and the mixed crystal Ti-6Al-4V titanium alloy with an ultra-fine grain structure is prepared. After the forging, the sample is subjected to surface cleaning treatment, and is prepared for rolling at a proper rolling temperature, and the thickness of the sample is reduced through rolling in multiple passes. Finally, the obtained ultra-fine grain Ti-6Al-4V titanium alloy has the following characteristics: the maximum grain size is less than 5 microns, the average grain size is less than 1 micron, and the proportion of grains with a size of 1 micron or less is more than 75%. The method effectively improves the uniformity of the material structure, prolongs the service life of the die, optimizes the process efficiency and surface quality control, and significantly improves the mechanical properties of the material, and provides a new technical approach for the preparation of titanium alloy materials.

Description

Technical Field

[0001] The invention relates to a method for preparing an ultrafine-grained Ti-6Al-4V titanium alloy, belonging to a method for preparing titanium alloy materials. Background Art

[0002] Titanium and its alloys are widely used in aerospace, military, automotive, biomedical, and everyday applications due to their high specific strength, excellent corrosion resistance, and good biocompatibility. Ti-6Al-4V titanium alloy, the earliest and most widely used titanium alloy, despite its many excellent properties, suffers from high deformation resistance at room temperature and a narrow processing window, limiting its formability and making it difficult to meet high-precision and application requirements. Existing methods for preparing ultrafine-grained titanium alloys, such as equal channel angular pressing (ECAP), high pressure torsion (HTP), and multi-directional isothermal closed-die forging, can achieve grain refinement and improve performance, but they also present challenges in practical applications. In particular, multi-directional isothermal closed-die forging, while capable of achieving grain refinement while maintaining the material's shape and size, suffers from poor overall microstructure uniformity during the preparation of ultrafine-grained materials, according to previous studies. To obtain samples with good microstructure uniformity, multiple deformation passes are required, which not only increases process complexity but also places higher demands on mold design.

[0003] In view of these limitations of multi-directional isothermal closed-die forging technology, combined with the advantages of rolling technology, this study aims to explore a new preparation technology to optimize the production process of ultrafine-grained titanium alloys. Rolling technology plays a key role in the metal processing industry with its advantages such as improved material properties, improved production efficiency, dimensional accuracy, flexibility and diversity, and mature process. This technology can achieve grain refinement, uniform structure, precise control of product size, adapt to different materials and production requirements, and provide efficient, economical and environmentally friendly solutions for modern industrial production. Therefore, this study intends to combine multi-directional isothermal closed-die forging and rolling technology to develop a new composite plastic deformation technology, aiming to overcome the shortcomings of existing technologies, achieve efficient and uniform structure refinement of ultrafine-grained titanium alloys, and meet the production needs of high-performance materials. Summary of the Invention

[0004] The present invention aims to overcome the uniformity and mold design problems encountered in the prior art titanium alloy preparation process and provides a new method for preparing ultrafine-grained Ti-6Al-4V titanium alloy. The method of the present invention comprises the following steps:

[0005] (1) Pretreatment of coarse-grained Ti-6Al-4V titanium alloy plates.

[0006] (2) After the coarse-grained Ti-6Al-4V titanium alloy plate treated in step (1) and the isothermal closed-die forging die are respectively subjected to heat preservation treatment, the coarse-grained Ti-6Al-4V titanium alloy plate is placed in the isothermal closed-die forging die for isothermal closed-die forging in three different directions. After completing one forging pass, water quenching is immediately performed at room temperature. The forging temperature is kept consistent with the heat preservation temperature to obtain a mixed-grained Ti-6Al-4V titanium alloy.

[0007] (3) Surface treatment and heat preservation treatment are performed on the mixed crystal Ti-6Al-4V titanium alloy obtained in step (2).

[0008] (4) The mixed crystal Ti-6Al-4V titanium alloy treated in step (3) is rolled, the rolling temperature is kept consistent with the holding temperature in step (3), and water quenching is immediately performed after each rolling pass to obtain an ultrafine-grained Ti-6Al-4V titanium alloy.

[0009] Preferably, the pretreatment in step (1) includes: processing the coarse-grained Ti-6Al-4V titanium alloy plate into a shape that meets the size of the isothermal closed-die forging die, and polishing the surface of the fine-grained Ti-6Al-4V titanium alloy.

[0010] Preferably, the holding temperature in step (2) is 500° C. to 700° C., and the holding time is 5 min to 120 min.

[0011] Preferably, the isothermal closed-die forging in three different directions in step (2) includes: a horizontal direction, a vertical direction and an inclined direction, wherein there is no special requirement for the inclination angle of the inclined direction.

[0012] Preferably, the medium temperature closed die forging conditions in step (2) are: the total strain is at least 1.2, the strain is 0.4 in each pass, and the pressing speed is between 0.1 mm / s and 1 mm / s.

[0013] Preferably, in step (2), the coarse-grained Ti-6Al-4V titanium alloy plate and the mold are lubricated with a lubricant.

[0014] Preferably, the holding temperature and holding time in step (3) are consistent with those in step (2).

[0015] Preferably, the total deformation of rolling in step (4) is 30% to 90% of the total deformation, the average reduction of each pass is maintained between 10% and 20%, and the deformation pass is determined based on the total deformation and the reduction of each pass.

[0016] Preferably, the rotation speed during the rolling process in step (4) is in the range of 50 to 300 rpm.

[0017] Preferably, in step (4), the mixed crystal Ti-6Al-4V titanium alloy and the rolling roller are lubricated with a lubricant.

[0018] Beneficial effects of the present invention

[0019] (1) Through the multi-directional isothermal closed-die forging and rolling composite plastic deformation technology, the microstructure of Ti-6Al-4V titanium alloy was precisely controlled, thereby significantly improving the mechanical properties of the material.

[0020] (2) The process provided by the present invention has high efficiency and repeatability, is suitable for industrial large-scale production, and reduces production costs. By optimizing forging and rolling parameters, the present invention not only improves material properties but also reduces energy consumption and material waste, meeting the requirements of environmentally friendly production. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the EBSD structure of the mixed crystal Ti-6Al-4V titanium alloy obtained after three multi-directional isothermal compression closed die forging in Example 1.

[0022] Figure 2 This is the TEM structure after three multi-directional isothermal compression closed die forgings and 50% rolling composite deformation in Example 1.

[0023] Figure 3 This is the EBSD structure of the Ti-6Al-4V titanium alloy obtained after 12 multi-directional isothermal compression closed die forging in Comparative Example 1.

[0024] Figure 4 The engineering stress-engineering strain curves of the initial samples of Example 1 and Comparative Examples 1-3 are shown.

[0025] Figure 5 This is the engineering stress-engineering strain curve of Example 1.

[0026] Figure 6 This is the engineering stress-engineering strain curve of Comparative Example 1.

[0027] Figure 7 This is the engineering stress-engineering strain curve of comparative example 2.

[0028] Figure 8 This is the engineering stress-engineering strain curve of comparative example 3. DETAILED DESCRIPTION

[0029] The technical solution of the present invention will be further described below with reference to the accompanying drawings and through specific embodiments. However, the following embodiments are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention shall be subject to the claims. Example

[0030] In this embodiment, the process of preparing ultrafine-grained Ti-6Al-4V titanium alloy by using multi-directional isothermal closed-die forging and rolling composite plastic deformation technology is as follows:

[0031] (1) In order to adapt to the size of the mold cavity (10mm x 10mm x 45mm), the Ti-6Al-4V titanium alloy sheet was processed into 10mm x 10mm x 15mm rectangular blocks. Subsequently, the surfaces of these blocks were polished to ensure that the sample surface was smooth and met the finish requirements.

[0032] (2) Start the heating device of the isothermal closed mold process system and accurately place the lower mold in it to ensure that the lower mold can be heated evenly. Set the preset temperature of the lower mold to 550°C. At the same time, start another heating furnace and set the preset temperature to 550°C. When the furnace is heated to 550°C, place the Ti-6Al-4V titanium alloy block processed in step (1) into the furnace for insulation treatment. The insulation time is set to 10 minutes according to actual needs.

[0033] (3) The sample preheated in step 2 was accurately placed in the center of the lower die heated to 550°C. Then, three forging passes were performed in three different directions, one forging pass in each direction, with the same strain of 0.4 applied in each pass to ensure that the accumulated strain reached 1.2 during the entire forging process. During the entire forging process, a constant forging temperature of 550°C and a reduction speed of 0.15 mm / s were maintained. Between each forging pass, the sample was water quenched and a high-temperature BN lubricant was used to reduce friction and wear.

[0034] (4) The Ti-6Al-4V titanium alloy sample prepared in step 3 was characterized by electron backscatter diffraction (EBSD) technology. The results are as follows: Figure 1 As shown in the figure, the structure contains a distribution of large and small grains. Grains smaller than 1 μm account for approximately 53.6%, the largest grain size is approximately 9.64 μm, and the average grain size is approximately 0.85 μm. This result meets the requirements of a mixed-grain structure.

[0035] (5) The Ti-6Al-4V titanium alloy sample that has been multi-directionally isothermal closed-die forged in step (3) is subjected to surface cleaning treatment to remove any scale, oil stains or other impurities that may be generated during the forging process to avoid affecting the surface quality of the material during the rolling process.

[0036] (6) During the rolling process of the Ti-6Al-4V titanium alloy after the treatment in step (5), the preheating temperature was determined to be 550°C according to the forging temperature, and the holding time was also set to 10 minutes. During the rolling process, the Ti-6Al-4V titanium alloy block subjected to multi-directional isothermal closed die forging was subjected to a total deformation of 50%, with an average reduction of 10% per pass. During the implementation process, a total of 6 deformation passes were implemented at a speed of 100 rpm. After each rolling pass, water quenching was performed to fix the microstructure, and high-temperature BN was used as a lubricant.

[0037] The microstructure of the Ti-6Al-4V titanium alloy sample prepared in step (6) was characterized using transmission electron microscopy (TEM). Figure 2 As shown in the figure, the structure is relatively uniform, with grains smaller than 1 μm accounting for more than 80% and an average grain size of less than 1 μm. This result meets the requirements of an ultrafine-grained structure.

[0038] Comparative Example 1

[0039] In this comparative example, the process of preparing ultrafine-grained Ti-6Al-4V titanium alloy by multi-directional isothermal closed-die forging technology is as follows:

[0040] (1) In this example, in order to fit the size of the mold cavity (10mm x 10mm x 45mm), the Ti-6Al-4V titanium alloy sheet was processed into rectangular blocks of 10mm x 10mm x 15mm. Subsequently, the surfaces of these blocks were polished to ensure that the sample surface was smooth and met the finish requirements.

[0041] (2) Start the heating device of the isothermal closed mold process system and accurately place the lower mold in it to ensure that the lower mold can be heated evenly. Set the preset temperature of the lower mold to 550°C. At the same time, start another heating furnace and set the preset temperature to 550°C. When the furnace is heated to 550°C, place the Ti-6Al-4V titanium alloy block processed in step (1) into the furnace for insulation treatment. The insulation time is set to 10 minutes according to actual needs.

[0042] (3) The sample preheated in step (2) is accurately placed in the center of the lower die heated to 550°C. Next, three forging passes are performed in three different directions, one forging pass in each direction, and the same strain of 0.4 is applied in each pass to ensure that the accumulated strain reaches 1.2 during the entire forging process. A constant forging temperature of 550°C and a reduction speed of 0.15 mm / s are maintained throughout the forging process. Between each forging pass, the sample is water quenched and a high-temperature BN lubricant is used to reduce friction and wear.

[0043] Comparative Example 2

[0044] In this comparative example, the process of preparing ultrafine-grained Ti-6Al-4V titanium alloy by multi-directional isothermal closed-die forging technology is as follows:

[0045] (1) In this example, in order to fit the size of the mold cavity (10mm x 10mm x 45mm), the Ti-6Al-4V titanium alloy sheet was processed into rectangular blocks of 10mm x 10mm x 15mm. Subsequently, the surfaces of these blocks were polished to ensure that the sample surface was smooth and met the finish requirements.

[0046] (2) Start the heating device of the isothermal closed mold process system and accurately place the lower mold in it to ensure that the lower mold can be heated evenly. Set the preset temperature of the lower mold to 550°C. At the same time, start another heating furnace and set the preset temperature to 550°C. When the furnace is heated to 550°C, place the Ti-6Al-4V titanium alloy block processed in step (1) into the furnace for insulation treatment. The insulation time is set to 10 minutes according to actual needs.

[0047] (3) The sample preheated in step (2) was accurately placed in the center of the lower die heated to 550°C. Next, 12 forging passes were performed in three different directions, 4 forging passes in each direction, and the same strain of 0.4 was applied in each pass to ensure that the accumulated strain reached 4.8 during the entire forging process. A constant forging temperature of 550°C and a reduction speed of 0.15 mm / s were maintained throughout the forging process. Between each forging pass, the sample was water quenched and a high-temperature BN lubricant was used to reduce friction and wear.

[0048] (4) The Ti-6Al-4V titanium alloy sample prepared in step (3) was characterized by electron backscatter diffraction (EBSD) technique. The results are as follows: Figure 3 As shown in the figure, the microstructure shows a distribution of large and small grains. Grains smaller than 1 μm account for approximately 87.8%, the largest grain size is approximately 4.05 μm, and the average grain size is approximately 0.14 μm. This result meets the requirements for an ultrafine-grained microstructure.

[0049] Comparative Example 3

[0050] In this comparative example, the process of 50% rolling of the initial Ti-6Al-4V titanium alloy is as follows:

[0051] (1) To contrast with Example 1, the Ti-6Al-4V titanium alloy plates in Comparative Example 2 were still processed into rectangular blocks of 10 mm x 10 mm x 15 mm. Subsequently, the surfaces of these blocks were polished to ensure that the sample surfaces were smooth and met the finish requirements.

[0052] (2) During the rolling process of the Ti-6Al-4V titanium alloy after the treatment in step (1), the preheating temperature was determined to be 550°C according to the forging temperature, and the holding time was also set to 10 minutes. During the rolling process, the initial Ti-6Al-4V titanium alloy block was subjected to a total deformation of 50%, and the average reduction of each pass was 10%. During the implementation process, a total of 6 deformation passes were implemented at a speed of 100 rpm. After each rolling pass, water quenching treatment was performed to fix the microstructure, and high-temperature BN was used as a lubricant.

[0053] Performance test: In order to evaluate the tensile properties of the Ti-6Al-4V titanium alloy prepared in Example 1 and Comparative Examples 1-3, the tensile performance tests were carried out on the samples in Example 1 and Comparative Examples 1-3, respectively. The specific tensile performance test results are shown in FIG. Figures 4 to 8 and Table 1.

[0054] Table 1 Yield strength (YS), tensile strength (UTS) and total elongation at break (YS) of the Ti-6Al-4V titanium alloy material of the present invention )

[0055]

[0056] As shown in Table 1, the yield strength of the Ti-6Al-4V titanium alloy sample in Comparative Example 1, after three passes of multi-directional isothermal closed-die forging, increased by 170 MPa, or 17.8%, from 955 MPa to 1125 MPa. The tensile strength increased by 140 MPa, or 13.7%, from 1023 MPa to 1163 MPa. The elongation after fracture was 13.98%.

[0057] In Comparative Example 2, the yield strength of the Ti-6Al-4V titanium alloy sample after 12 passes of multi-directional isothermal closed-die forging deformation increased from 955 MPa to 1217 MPa, an increase of 262 MPa, an increase of 27.4%, and the tensile strength increased from 1023 MPa to 1247 MPa, an increase of 224 MPa, an increase of 21.9%, and the elongation at break was 13.17%.

[0058] In Comparative Example 3, after 50% rolling, the yield strength of the Ti-6Al-4V titanium alloy sample increased from 955 MPa to 1026 MPa, a 7% increase of 71 MPa. The tensile strength increased from 1023 MPa to 1136 MPa, an 11% increase of 113 MPa. The elongation at break was 14.7%.

[0059] In Example 1, after undergoing three passes of multi-directional isothermal closed-die forging followed by 50% rolling, the yield strength of the Ti-6Al-4V titanium alloy sample increased from 955 MPa to 1300 MPa, an increase of 345 MPa, or 36.1%. The tensile strength increased from 1023 MPa to 1330 MPa, an increase of 307 MPa, or 30%. The elongation at break was 12.98%.

[0060] The above analysis shows that the combined process of three-pass multi-directional isothermal closed-die forging followed by 50% rolling significantly improves the mechanical properties of Ti-6Al-4V titanium alloy. The increases in yield strength and tensile strength are both greater than those achieved with the single process, while the elongation at break, while decreasing, remains at a high level, indicating that the material maintains a certain degree of plasticity while achieving higher strength.

[0061] While the target ultrafine-grained structure was successfully achieved in Comparative Example 2, the mold experienced significant wear and deformation, significantly shortening its service life. This not only inconveniently impacted subsequent material processing but also placed greater demands on mold design and material selection.

[0062] Comparing Example 1 with Comparative Example 2, Example 1's advantage lies in its use of a more complex and sophisticated composite process, which allows for more effective control of grain size and distribution, improving material uniformity. Furthermore, Example 1 also excels in process efficiency, surface quality control, and mold durability. Most importantly, this process significantly improves the material's mechanical properties.

Claims

1. A method for preparing an ultrafine-grained Ti-6Al-4V titanium alloy, characterized in that: The steps include: (1) Pretreatment of coarse-grained Ti-6Al-4V titanium alloy plates; (2) After the coarse-grained Ti-6Al-4V titanium alloy plate and the isothermal closed-die forging die treated in step (1) are respectively subjected to heat preservation treatment, the coarse-grained Ti-6Al-4V titanium alloy plate is placed in the isothermal closed-die forging die for isothermal closed-die forging in three different directions, and immediately water quenched at room temperature after completing one forging pass, with the forging temperature being kept consistent with the heat preservation temperature, to obtain a mixed-grained Ti-6Al-4V titanium alloy; In step (2), the holding temperature is 500° C. to 700° C., and the holding time is 5 min to 120 min; The medium temperature closed die forging conditions in step (2) are: a total strain of at least 1.2, a strain of 0.4 in each pass, and a pressing speed between 0.1 mm / s and 1 mm / s; (3) performing surface treatment and heat preservation treatment on the mixed crystal Ti-6Al-4V titanium alloy obtained in step (2); (4) rolling the mixed crystal Ti-6Al-4V titanium alloy treated in step (3), wherein the rolling temperature is kept consistent with the holding temperature in step (3), and water quenching is performed immediately after each rolling pass to obtain an ultrafine grained Ti-6Al-4V titanium alloy; The total deformation of rolling in step (4) is 30% to 90% of the total deformation, the average reduction of each pass is maintained between 10% and 20%, and the deformation pass is determined based on the total deformation and the reduction of each pass.

2. The method for preparing the ultrafine-grained Ti-6Al-4V titanium alloy according to claim 1, characterized in that: The pretreatment in step (1) includes: processing the coarse-grained Ti-6Al-4V titanium alloy plate into a shape that meets the size of the isothermal closed-die forging die, and polishing the surface of the fine-grained Ti-6Al-4V titanium alloy.

3. The method for preparing the ultrafine-grained Ti-6Al-4V titanium alloy according to claim 1, characterized in that: The isothermal closed-die forging in three different directions in step (2) includes: a horizontal direction, a vertical direction and an inclined direction.

4. The method for preparing the ultrafine-grained Ti-6Al-4V titanium alloy according to claim 1, characterized in that: In step (2), the coarse-grained Ti-6Al-4V titanium alloy plate and the mold are lubricated with a lubricant.

5. The method for preparing the ultrafine-grained Ti-6Al-4V titanium alloy according to claim 1, characterized in that: The holding temperature and holding time in step (3) are the same as those in step (2).

6. The method for preparing the ultrafine-grained Ti-6Al-4V titanium alloy according to claim 1, characterized in that: The rotation speed during the rolling process in step (4) is in the range of 50 to 300 rpm.

7. The method for preparing the ultrafine-grained Ti-6Al-4V titanium alloy according to claim 1, characterized in that: In step (4), the mixed crystal Ti-6Al-4V titanium alloy and the roller are lubricated with a lubricant.

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