Method for preparing ultra-fine grain Ti-6Al-4V titanium alloy

Abstract

The invention discloses a method for preparing an 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 coarse-grain Ti-6Al-4V titanium alloy plate into square blocks with a specific proportion, and carrying out surface smoothing treatment; and then, forging deformation is implemented through multidirectional isothermal closed die forging, and the forging direction, the pressure and the deformation pass are controlled, so that the mixed crystal Ti-6Al-4V titanium alloy with the ultra-fine grain structure is prepared. After forging, the surface of a sample is cleaned, rolling preparation is conducted at the proper rolling temperature, and the thickness of the sample is reduced through rolling of multiple passes. The maximum grain size is smaller than 5 micrometers, the average grain size is smaller than 1 micrometer, and the proportion of grains within 1 micrometer reaches 75% or above. According to the method, the uniformity of a material structure is effectively improved, meanwhile, the service life of a mold is prolonged, the process efficiency and surface quality control are optimized, the mechanical property of the material is remarkably improved, and a new technical approach is provided for preparation of the titanium alloy material.

Description

Technical Field

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

[0002] Titanium and titanium alloys have been widely used in aerospace, military, vehicle engineering, biomedical engineering and daily life due to their high specific strength, excellent corrosion resistance and good biocompatibility. Ti-6Al-4V titanium alloy is the earliest and most widely used titanium alloy. Although it has many excellent properties, its high deformation resistance and narrow processing window at room temperature limit its forming performance and it is difficult to meet high precision and application requirements. Existing methods for preparing ultrafine-grained titanium alloys, such as large plastic deformation technologies such as equal channel angular pressing (ECAP), high pressure torsion (HTP) and multi-directional isothermal closed die forging, can achieve grain refinement and performance improvement, but there are some problems in practical applications. In particular, the large plastic deformation technology of multi-directional isothermal closed die forging can achieve grain refinement while maintaining the appearance shape and size of the material unchanged, but it was found in previous studies that this technology has the problem of poor overall microstructure uniformity in the process of preparing ultrafine-grained materials. In order to obtain samples with good overall microstructure uniformity, multiple deformation passes are required, which not only increases the complexity of the process, but also puts higher requirements 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 of improving material properties, improving production efficiency, dimensional accuracy, flexibility and diversity, and mature process. This technology can achieve grain refinement, uniform organization, and accurately control 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 organization 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 titanium alloy preparation process in the prior art, 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 sheet.

[0006] (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 insulation 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, and the forging temperature is kept consistent with the insulation temperature to obtain a mixed crystal Ti-6Al-4V titanium alloy.

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

[0008] (4) rolling the mixed crystal Ti-6Al-4V titanium alloy treated in step (3), wherein the rolling temperature is kept consistent with the insulation temperature in step (3), and water quenching is performed immediately 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 grinding the surface of the fine-grained Ti-6Al-4V titanium alloy.

[0010] Preferably, in step (2), the insulation temperature is 500° C. to 700° C., and the insulation 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 insulation temperature and insulation 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 a single 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 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 is 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.

[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 is further described below in conjunction with the accompanying drawings and through specific implementation methods. However, the following embodiments are only simple 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.

[0030] Example 1

[0031] In this embodiment, the process of preparing ultrafine-grained Ti-6Al-4V titanium alloy by multi-directional isothermal closed-die forging and rolling composite plastic deformation technology is as follows:

[0032] (1) In order to adapt to the size of the mold cavity (10 mm x 10 mm x 45 mm), the Ti-6Al-4V titanium alloy sheet was 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 surface was smooth and met the finish requirements.

[0033] (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 evenly heated. 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.

[0034] (3) The sample preheated in step 2 is accurately placed in the center of the lower die heated to 550°C. Then, three forging passes are performed in three different directions, one forging pass in each direction, and each pass implements the same strain of 0.4 to ensure that the accumulated strain in the entire forging process reaches 1.2. During the entire forging process, a constant forging temperature of 550°C and a pressing speed of 0.15 mm / s are maintained. Between each forging pass, the sample is water quenched and a high-temperature BN lubricant is used to reduce friction and wear.

[0035] (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 1 As shown in the figure, there is a distribution of large grains and small grains in the structure. Among them, the proportion of grains smaller than 1μm is about 53.6%, the largest grain size is about 9.64μm, and the average grain size is about 0.85μm. This result meets the requirements of mixed crystal structure.

[0036] (5) The Ti-6Al-4V titanium alloy sample subjected to multi-directional isothermal closed die forging 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.

[0037] (6) During the rolling process of the Ti-6Al-4V titanium alloy after the treatment in step (5), the preheating temperature is determined to be 550°C according to the forging temperature, and the holding time is also set to 10 minutes accordingly. During the rolling process, the Ti-6Al-4V titanium alloy block subjected to multi-directional isothermal closed die forging is subjected to a total deformation of 50%, and the average reduction of each pass is 10%. During the implementation process, a total of 6 deformation passes are implemented, and the rotation speed is 100 rpm. After each rolling pass, water quenching treatment is performed to fix the microstructure, and high-temperature BN is used as a lubricant.

[0038] 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, the organization in the organization is relatively uniform, the proportion of grains within 1 μm reaches more than 80%, and the average grain size is less than 1 μm. This result meets the requirements of the ultrafine grain organization structure in claim 5.

[0039] Comparative Example 1

[0040] 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:

[0041] (1) In this example, 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.

[0042] (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 evenly heated. 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.

[0043] (3) Place the sample preheated in step (2) accurately in the center of the lower die heated to 550°C. Next, perform three forging passes in three different directions, one forging pass in each direction, and implement the same strain of 0.4 in each pass to ensure that the accumulated strain in the entire forging process reaches 1.2. During the entire forging process, maintain a constant forging temperature of 550°C and a pressing speed of 0.15mm / s. Between each forging pass, the sample is water quenched and a high-temperature BN lubricant is used to reduce friction and wear.

[0044] Comparative Example 2

[0045] 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:

[0046] (1) In this example, 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.

[0047] (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 evenly heated. 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.

[0048] (3) Place the sample preheated in step (2) accurately in the center of the lower die heated to 550°C. Next, perform 12 forging passes in three different directions, 4 forging passes in each direction, and implement the same strain of 0.4 in each pass to ensure that the accumulated strain in the entire forging process reaches 4.8. During the entire forging process, maintain a constant forging temperature of 550°C and a pressing speed of 0.15mm / s. Between each forging pass, the sample is water quenched and a high-temperature BN lubricant is used to reduce friction and wear.

[0049] (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, there is a distribution of large grains and small grains in the organization. Among them, the proportion of grains smaller than 1 μm is about 87.8%, the largest grain size is about 4.05 μm, and the average grain size is about 0.14 μm. This result meets the requirements of the ultrafine grain structure in claim 5.

[0050] Comparative Example 3

[0051] In this comparative example, the process of 50% rolling of the initial Ti-6Al-4V titanium alloy is as follows:

[0052] (1) To contrast with Example 1, the Ti-6A1-4V titanium alloy plate in Comparative Example 2 was still processed into rectangular blocks of 10 mm x 10 mm x 15 mm, and then the surfaces of these blocks were polished to ensure that the sample surface was smooth and met the finish requirements.

[0053] (2) During the rolling process of the Ti-6Al-4V titanium alloy after the treatment in step (1), the preheating temperature is determined to be 550°C according to the forging temperature, and the holding time is also set to 10 minutes accordingly. During the rolling process, the initial Ti-6A1-4V titanium alloy block is subjected to a total deformation of 50%, and the average reduction of each pass is 10%. During the implementation process, a total of 6 deformation passes are implemented, and the rotation speed is 100 rpm. After each rolling pass, water quenching treatment is performed to fix the microstructure, and high-temperature BN is used as a lubricant.

[0054] Performance test: In order to evaluate the tensile properties of the Ti-6A1-4V titanium alloy prepared in Example 1 and Comparative Examples 1-3, the tensile properties of the samples in Example 1 and Comparative Examples 1-3 were tested respectively. The specific tensile properties test results are shown in Figures 4 to 8 and Table 1.

[0055] Table 1 Yield strength (YS), tensile strength (UTS) and total elongation at break (ε t )

[0056] YS(MPa) UTS(MPa) <![CDATA[ε t (%)]]> Initial sample 955 1023 14.87 Example 1 1300 1330 12.98 Comparative Example 1 1125 1163 13.98 Comparative Example 2 1217 1247 13.17 Comparative Example 3 1026 1136 14.7

[0057] As shown in Table 1, the yield strength of the Ti-6A1-4V titanium alloy sample after three passes of multi-directional isothermal closed die forging deformation in Comparative Example 1 increased from 955MPa to 1125MPa, an increase of 170MPa, an increase of 17.8%. The tensile strength increased from 1023MPa to 1163MPa, an increase of 140MPa, an increase of 13.7%, and the elongation after fracture was 13.98%.

[0058] In Comparative Example 2, after 12 passes of multi-directional isothermal closed-die forging deformation, the yield strength of the Ti-6Al-4V titanium alloy sample increased from 955MPa to 1217MPa, an increase of 262MPa, an increase of 27.4%, the tensile strength increased from 1023MPa to 1247MPa, an increase of 224MPa, an increase of 21.9%, and the elongation after fracture was 13.17%.

[0059] In comparative example 3, after 50% rolling, the yield strength of the Ti-6A1-4V titanium alloy sample increased from 955MPa to 1026MPa, an increase of 71MPa, an increase of 7%. The tensile strength increased from 1023MPa to 1136MPa, an increase of 113MPa, an increase of 11%, and the elongation after fracture was 14.7%.

[0060] In Example 1, after three passes of multi-directional isothermal closed die forging deformation + 50% rolling composite deformation, the yield strength of the Ti-6Al-4V titanium alloy sample increased from 955MPa to 1300MPa, an increase of 345MPa, an increase of 36.1%. The tensile strength increased from 1023MPa to 1330MPa, an increase of 307MPa, an increase of 30%, and the elongation after fracture was 12.98%.

[0061] From the above analysis results, it can be seen that the composite processing technology of 3-pass multi-directional isothermal closed die forging deformation + 50% rolling has the most significant improvement on the mechanical properties of Ti-6Al-4V titanium alloy. The growth rate of yield strength and tensile strength is higher than that of a single processing technology, and although the elongation at break has decreased, it still remains at a high level, indicating that the material has maintained a certain plasticity while obtaining higher strength.

[0062] In Comparative Example 2, although the target ultrafine grain structure was successfully obtained, the mold wear and deformation problems were significant during the experiment, which greatly shortened the service life of the mold. This not only brought many inconveniences to the subsequent material processing, but also put forward more stringent requirements on mold design and material selection.

[0063] Compared with Example 1 and Comparative Example 2, the advantage of Example 1 is that it adopts a more complex and sophisticated composite process, which can more effectively control the grain size and distribution and improve the uniformity of the material. In addition, Example 1 also performs better in terms of process efficiency, surface quality control and mold durability. Most importantly, this process significantly improves the mechanical properties of the material.

Claims

1. A method for 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 pass of forging, and the forging temperature is kept consistent with the heat preservation temperature to obtain a mixed crystal Ti-6Al-4V titanium alloy; (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 crystal Ti-6Al-4V titanium alloy.

2. The method of claim 1, wherein: 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 grinding the surface of the fine-grained Ti-6Al-4V titanium alloy.

3. The method of claim 1, wherein: In step (2), the insulation temperature is 500° C. to 700° C., and the insulation time is 5 min to 120 min.

4. The method of claim 1, wherein: The isothermal closed-die forging in three different directions in step (2) includes: a horizontal direction, a vertical direction and an inclined direction.

5. The method of claim 1, wherein: The medium temperature closed die forging conditions in step (2) are: the total strain is at least 1.2, and each pass implements a strain of 0.4, wherein the pressing speed is between 0.1 mm / s and 1 mm / s.

6. The method of claim 1, wherein: In step (2), the coarse-grained Ti-6Al-4V titanium alloy plate and the mold are lubricated with a lubricant.

7. The method for preparing 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).

8. The method of claim 1, wherein: 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 a single pass.

9. The method of claim 1, wherein: The rotation speed during the rolling process in step (4) is in the range of 50 to 300 rpm.

10. The method for preparing 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 rolling roller are lubricated with a lubricant.

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