A superplastic forming process for making large size titanium alloy components

Abstract

The application provides a superplastic forming process for preparing a large-size titanium alloy component, and belongs to the field of welding, heat treatment and superplastic forming of metal materials. The process is characterized in that: two or more titanium alloy plates are spliced together by using a friction stir welding technology under low heat input, fine equiaxed crystal organization is obtained at the welding core, and then the large-size titanium alloy plate after splicing is subjected to heat treatment to further control the microstructure. By using the principle that the coarse rate of the fine organization is greater than that of the coarse organization during the heat treatment, the holding temperature and the holding time are adjusted, and finally the microstructure of the base material and the welding core is uniform. Then, under a specific process, the large-size titanium alloy plate with the weld is subjected to overall superplastic forming, synchronous superplastic deformation of the weld and the base material is realized, and a high-performance large-size titanium alloy integral component is prepared.

Description

Technical fields:

[0001] This invention relates to the fields of welding, heat treatment and superplastic forming technology of metallic materials, and specifically to a superplastic forming process for preparing large-size titanium alloy components. Background technology:

[0002] Titanium alloys have wide applications in aerospace, chemical, and marine industries due to their high specific strength, good corrosion resistance, and good biocompatibility. Globally, titanium materials account for more than half of applications in the aerospace sector, primarily used in aircraft structural components and engines. With the further development of my country's aircraft manufacturing industry, titanium alloy structural components are gradually showing a trend towards larger size, greater integration, and increased complexity.

[0003] Currently, superplastic forming / diffusion joining technology is a relatively mature method for preparing large and complex titanium alloy components. However, due to the low production efficiency and difficulty in determining the weld ratio of diffusion welding, it is difficult to meet the current production needs of large integral components. Although fusion welding, represented by laser welding, can improve production efficiency, the microstructure obtained in the molten zone is very coarse, which seriously damages the excellent superplastic properties of the original titanium alloy sheet (Chen S, Huang J, Cheng D, Zhang H, Zhao X. Superplastic deformation mechanism and mechanical behavior of a laser-welded Ti–6Al–4V alloy joint. Materials Science and Engineering: A. 2012; 541: 110-9.). Studies have shown that hydrogen placement in the laser weld joint can alleviate the problem of excessive difference in superplasticity between the base material and the weld nugget. However, hydrogen is extremely easy to escape in titanium alloys, which will lead to unstable superplastic properties of the hydrogen-placed joint. Moreover, hydrogen stored in titanium alloys will greatly deteriorate the service performance of titanium alloys after forming. Therefore, hydrogen removal is necessary in a timely manner after forming. However, the hydrogen removal process before welding and after forming of titanium alloys is complex and costly. In addition, this method is difficult to implement on large-sized titanium alloy plates, and the hydrogen content is very easy to be uneven. Moreover, hydrogen is easy to remain in the titanium alloy and is not easy to remove completely, which often leads to a sharp deterioration in the service performance of the titanium alloy. (Jiang X, Chen Y, Wang M, Cheng D. Investigation on microstructural homogeneity in laser beam welding joint superplastic deformation. Transactions of the China Welding Institution. 2017; 38(8):28-32.).

[0004] Friction stir welding (FSW), as a solid-state joining method, can produce fine grains, thus achieving excellent superplastic properties. However, there is a problem of mismatch between the superplastic properties of the base material and the weld nugget. During the superplastic deformation process, the part with lower flow stress undergoes severe local deformation due to the inconsistency of the initial flow stress between the base material and the weld nugget, while the part with higher flow stress hardly deforms. This leads to severe thinning or even tearing in local areas during the superplastic forming of the overall component, which reduces the service performance of the component (Edwards P, Ramulu M, Sanders DG. Superplastic behavior and microstructure of titanium (Ti-6Al-4V) friction stir welds made under a variety of processing conditions. Key Engineering Materials. 2010; 433:169-76. Fu Mingjie, Li Xiaohua, Han Xiuquan, Gao Zhiyong, Li Jizhong. Study on superplastic forming performance of TC4 titanium alloy based on friction stir welding. Aerospace Manufacturing Technology. 2015(17):79-81.).

[0005] By controlling the friction stir welding process, fully lamellar, bimodal, and fully equiaxed microstructures can be obtained at the weld nugget. A fully lamellar microstructure indicates a welding temperature above the β transformation point, a bimodal microstructure indicates a welding temperature near the β transformation point, and an equiaxed microstructure indicates a welding temperature below the β transformation point. Our previous research found that lamellar and equiaxed microstructures are difficult to achieve superplasticity similar to the base material. However, bimodal microstructures exhibit superplasticity similar to rolled annealed base material and show potential for uniform deformation of the entire titanium alloy component containing the weld joint (Invention title: Friction Stir Welding Process and Superplastic Forming Process for Achieving Uniform Superplastic Forming of Titanium Alloy Welded Joints, Application No. 202110259444.7). However, obtaining bimodal microstructures requires extremely demanding friction stir welding parameters, necessitating control of the welding temperature near the β transformation point while experiencing intense plastic deformation in the stir zone. Furthermore, the size of the base material microstructure is also subject to strict limitations; if the size of the base material microstructure is slightly larger or smaller than a specified size, uniform deformation of the entire joint is difficult to achieve. Summary of the Invention:

[0006] The purpose of this invention is to provide a superplastic forming process for preparing large-size titanium alloy components. This preparation method can ensure the consistency of the base material and the weld nugget structure, and the process is simple, has a short process flow, and is low in cost.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A superplastic forming process for preparing large-size titanium alloy components, the process comprising the following steps:

[0009] (1) The dual-phase titanium alloy rolled plate was subjected to friction stir welding using a friction stir tool. During the welding process, the temperature of the weld nugget area was controlled to be 50-200℃ lower than the β phase transformation point, and argon gas was introduced for purging. After welding, a fine equiaxed grain structure was obtained in the weld nugget area.

[0010] (2) Static annealing treatment is performed on the equiaxed grain structure in the two-phase region of titanium alloy. The static annealing temperature is 800-980℃ and the holding time is 0.5-10h. By controlling the annealing temperature and annealing time, the weld nugget and the base material structure are made to be consistent.

[0011] (3) Perform overall superplastic molding on large-size plates after static annealing.

[0012] In step (1), the thickness of the duplex titanium alloy rolled plate is 1 to 5 mm.

[0013] In step (1), the base material, the duplex titanium alloy rolled plate, is in the rolled annealed state, and the base material microstructure size is 3-10 μm; the fine equiaxed crystal microstructure obtained after friction stir welding has a size of 200-800 nm.

[0014] In step (1), the material of the friction stir tool is a high-temperature resistant and wear-resistant material such as W-Re alloy, W-La2O3, or Co-based alloy. When using a W-Re alloy or W-La2O3 tool, the friction stir welding parameters range is: tool rotation speed 150-400 rpm, travel speed 50-150 mm / min; when using a Co-based alloy stirring tool, the welding parameters range is: rotation speed 130-250 rpm, travel speed 25-150 mm / min.

[0015] In step (1), the friction stir tool is a conical stirring needle with a shoulder diameter of 6-20 mm, a top diameter of 4-8 mm, and a root diameter of 5-12 mm. During the friction stir welding process, the axial direction of the stirring needle is at an angle of 1-5° to the normal direction of the rolled surface of the titanium alloy plate.

[0016] In step (1), when purging with argon gas, the argon gas flow rate is 0.5-15 L / min. The argon gas protects the weld nugget area from oxidation, and the cooling rate of the weld nugget area (after welding) is adjusted by controlling the argon gas flow rate. After welding, argon gas is continued to be introduced for 3-8 minutes to prevent the stirring head and titanium plate from reacting with gaseous impurities in the air.

[0017] In step (2), as the annealing time increases, the equiaxed grain structure of the weld nugget and the rolled structure of the base material coarsen simultaneously, but the coarsening rate of the fine structure at the weld nugget is higher than that of the base material; by controlling the annealing temperature and annealing time, the size of the weld nugget and the base material structure are made consistent.

[0018] In step (2), the static annealing process is carried out under an inert atmosphere or vacuum conditions to prevent oxidation of the titanium alloy sample.

[0019] In step (3), superplastic molding is carried out in an inert gas atmosphere, wherein the deformation temperature is 800-980℃ and the gas pressure is 0.1-10MPa.

[0020] The advantages of this invention are:

[0021] 1. Compared with fusion welding, the equiaxed microstructure obtained by the present invention is smaller in size, and compared with the coarse lamellar microstructure obtained by fusion welding, the fine weld nugget microstructure can greatly reduce the initial rheological stress at the weld nugget, which helps to obtain rheological stress at the weld nugget that matches the base material.

[0022] 2. Compared to hydrogen-containing laser welding of titanium alloys, hydrogen-containing welding of the weld joint can alleviate the problem of excessive difference in superplasticity between the base material and the weld nugget. However, hydrogen easily escapes from titanium alloys, leading to unstable superplastic properties of the hydrogen-containing joint. Moreover, hydrogen stored in titanium alloys will greatly deteriorate the service performance of the titanium alloy after forming. Therefore, timely hydrogen removal is necessary after forming. However, the hydrogen-containing process before welding and the hydrogen removal process after forming are complex and costly. In addition, this method is difficult to implement on large-sized titanium alloy plates, the hydrogen content is prone to unevenness, and hydrogen is easily retained in the titanium alloy, making it difficult to remove completely, often resulting in a sharp deterioration in the service performance of the titanium alloy.

[0023] 3. Compared to the lamellar microstructure obtained under high heat input parameters in friction stir welding, the fine equiaxed microstructure under low heat input parameters exhibits lower rheological stress during superplastic forming. Furthermore, the equiaxed microstructure avoids the instability, spheroidization, and rheological softening that occur in lamellar microstructures during superplastic forming, resulting in a more stable high-temperature deformation process in the welded joint. Subsequent annealing of the equiaxed microstructure achieves microstructure consistency with the base material, simplifying the process and reducing costs.

[0024] 4. Compared to the bimodal microstructure obtained by friction stir welding, the equiaxed microstructure has a wider welding window and is easier to achieve. Furthermore, the superplastic deformation mechanism of the bimodal microstructure is more complex, involving not only the coarsening mechanism of the equiaxed microstructure but also the spheroidizing mechanism of the lamellar microstructure, making precise control of its coordination with the base metal microstructure difficult. In contrast, the superplastic deformation mechanism of the equiaxed microstructure is simpler, exhibiting only a single equiaxed α-microstructure coarsening phenomenon. Therefore, the deformation mechanism of the equiaxed microstructure is more clearly defined, easier to coordinate with the deformation of the base metal, and more convenient to control, thus making it easier to achieve uniform superplastic forming of the entire joint. Attached image description:

[0025] Figure 1 These are the key processes of the method of this invention; wherein: (a) friction stir welding; (b) annealing treatment.

[0026] Figure 2 The present invention provides a friction stir welding joint for the present invention; wherein: (a) the surface of the friction stir weld; (b) the friction stir weld joint; (c) the equiaxed α structure at the friction stir weld nugget (in the figure, Ti and Al elements are aggregated in the α phase, and V elements are aggregated in the β phase, indicating that the β phase is distributed at the α phase interface after friction stir welding).

[0027] Figure 3 The microstructures obtained after static annealing are shown below: (a) Microstructure of the base material after holding at 900℃ for 5 min; (b) Microstructure of the weld nugget after holding at 900℃ for 5 min; (c) Microstructure of the base material after holding at 900℃ for 30 min; (d) Microstructure of the weld nugget after holding at 900℃ for 30 min; (e) Microstructure of the base material after holding at 900℃ for 300 min; (f) Microstructure of the weld nugget after holding at 900℃ for 300 min. This demonstrates that as the annealing time increases, the microstructures of the weld nugget and the base material gradually become more consistent.

[0028] Figure 4 The changes in microstructure and dimensional properties of the base material and weld nugget after holding at 900℃ for different times.

[0029] Figure 5 When holding the friction stir welded joint at 900℃ for 75 minutes, it was subjected to a temperature of 1×10⁻⁶. -3 s -1 Macroscopic stretching diagram at strain rate. Detailed implementation method:

[0030] This invention provides a superplastic forming process for preparing large-size titanium alloy components. First, the friction stir welding temperature is controlled below the β phase transformation point, thus the process window for obtaining an equiaxed microstructure is significantly larger than the welding window for obtaining a biphasic microstructure. Second, static holding allows for simultaneous coarsening of the base material and the weld nugget microstructure. The annealing process ensures greater flexibility in the grain size range of both the base material and the weld nugget. In other words, even with some fluctuation in grain size, the grain growth can still be controlled by varying holding times or temperatures, thereby ensuring the consistency of the microstructure in both the base material and the weld nugget. This contributes to obtaining uniformly superplastically formed integral components.

[0031] The friction stir tool used in the following embodiments is a conical stirring pin with a shoulder diameter of 6-20 mm, a top diameter of 4-8 mm, and a root diameter of 5-12 mm. During the friction stir welding process, the axial direction of the stirring pin is inclined at an angle of 1-5° to the normal direction of the rolled surface of the titanium alloy plate.

[0032] Example 1:

[0033] Friction stir welding (SSW) was performed on 2mm thick rolled and annealed Ti-6Al-4V plates using a truncated cone stirrer with a shoulder diameter of 12mm and a W-5wt% Re material. The tool rotation speed was 300 rpm, the travel speed was 100 mm / min, and the tilt angle was 2.5°. Argon gas was simultaneously introduced to purge the processing area at an average flow rate of 3 L / min to prevent oxidation of the weld nugget and to regulate its cooling rate. The resulting processed area was complete and defect-free, with minimal deformation of the plate after welding. The microstructure was fine, consisting of equiaxed grains with a grain size of 300 nm. Annealing the joint at 850-950℃ revealed that the difference between the weld nugget and the base material gradually decreased with time. After annealing at 900℃ for 300 min, the grain size of the entire joint became almost uniform.

[0034] Comparative Example 1:

[0035] A triangular pyramidal stir bar with a shoulder diameter of 16mm and pcBN as the tool material was used for friction stir welding of 4mm thick Ti-6Al-4V plates. The tool rotation speed was 500 rpm and the tool travel speed was 100 mm / min. After friction stir machining, a full lamellar structure could be obtained in the stirring zone. However, a significant tool wear zone appeared in the stirring zone. The wear zone was mainly due to the poor high-temperature wear resistance of the stirring tool under the specified parameters. During the machining process, some small particles were detached from the stirring tool and remained in the stirring zone, introducing some impurities and seriously affecting the subsequent annealing process and further research.

[0036] Comparative Example 2:

[0037] Friction stir welding was performed on 2mm thick rolled annealed Ti-6Al-4V plates using a truncated cone stirrer made of W-5wt% Re material with a 20mm shoulder diameter. The stirrer needle had a top diameter of 5mm and a root diameter of 15mm. During welding, the tool rotation speed was 300rpm and the travel speed was 100mm / min. Argon gas was simultaneously introduced to purge the processing area at a flow rate of 1L / min to prevent oxidation of the weld nugget and to regulate its cooling rate. During welding, due to the excessively large root diameter of the stirrer needle, the shoulder could not effectively drive the material flow, resulting in a noticeable tunnel in the stirrer zone and preventing the formation of a complete weld joint.

[0038] Comparative Example 3:

[0039] Friction stir welding was performed on 2mm thick rolled and annealed Ti-6Al-4V plates using a truncated cone stir bar made of W-5wt% Re material with a 12mm shoulder diameter. The stir bar had a tip diameter of 4mm and a root diameter of 7mm. During welding, the tool rotation speed was 400rpm, the travel speed was 100mm / min, and argon gas was simultaneously introduced to purge the processing area at a flow rate of 0.1L / min. Due to insufficient argon gas flow, the titanium alloy could not be completely protected during welding, resulting in a dark gray surface and the introduction of impurities into the weld joint, severely affecting its mechanical properties.

[0040] Example 2:

[0041] Friction stirring was performed on 3mm thick rolled and annealed Ti-6Al-4V plates using a truncated cone stirrer made of W-5%Re material with a shoulder diameter of 15mm. The tool rotation speed was 200rpm, the travel speed was 50mm / min, and the tilt angle was 3°. Argon gas was simultaneously introduced to purge the processing area at a flow rate of 3L / min to prevent oxidation of the weld nugget and to regulate the cooling rate of the weld nugget. An equiaxed microstructure with a width of 300-500nm was obtained. Subsequently, the microstructure was annealed at 900℃. When the holding time reached 100min, the grain size of the base material and the weld nugget reached the same level.

[0042] Superplastic tensile testing was performed on the friction stir welded joint after annealing, and it was found that the joint could deform relatively uniformly, and the entire welded joint achieved a high degree of superplasticity of over 400% without fracture.

[0043] Titanium alloy sheets that had been friction stir welded and annealed were subjected to superplastic forming at 900℃ and a gas pressure of 0.2-2.0 MPa. The test results showed that the overall deformation of the entire sheet exceeded 200%, the superplastic formed specimens were intact and free from defects such as cracks, and the deformation transition between the weld nugget and the base material was uniform during the superplastic forming process.

[0044] Comparative Example 4

[0045] A truncated cone stir bar with a shoulder diameter of 12mm and W-5Re as the tool material was used to perform friction stir machining on a 3mm thick Ti-6Al-4V plate. The tool rotation speed was 500 rpm and the tool feed speed was 100 mm / min. Friction stir welding was then performed, directly obtaining a full-layer microstructure. However, regardless of the holding time, it was consistently impossible to obtain a base material and weld nugget microstructure with similar dimensions after heat treatment at different temperatures.

[0046] Comparative Example 5:

[0047] Friction stirring was performed on 3mm thick rolled and annealed Ti-6Al-4V plates using a truncated cone stirrer made of W-5%Re material with a shoulder diameter of 12mm. The tool speed was 300rpm and the travel speed was 50mm / min. Argon gas was simultaneously introduced to purge the processing area at a flow rate of 4L / min to prevent oxidation of the weld nugget and to regulate the cooling rate of the weld nugget. An equiaxed microstructure with a width of 300-500nm was obtained. Subsequently, the microstructure was held at 750℃ for 300min. After holding at 750℃ for 300min, the size of the weld nugget increased, and the size of the base material microstructure also increased slowly. However, the grain size of the weld nugget and the base material remained inconsistent, thus failing to meet the requirement of uniform superplastic forming of the overall joint.

[0048] Comparative Example 6

[0049] Laser welding was performed on rolled and annealed titanium alloy sheets using a 4 kW laser at a welding speed of 3 m / min. The base material exhibited a fine equiaxed structure, while the weld nugget zone, due to melting, displayed a coarse lamellar structure with an aspect ratio greater than 10. Subsequently, the entire weld joint was subjected to superplastic tensile testing at 900℃. After tensile testing, the molten zone showed minimal deformation, while the base material zone exhibited severe deformation. The overall weld joint displayed uneven deformation, failing to meet the requirements for integral superplastic forming of large-sized components.

[0050] Comparative Example 7

[0051] Laser welding was performed on rolled annealed titanium alloy sheets using a 4 kW laser at a welding speed of 3 m / min. After welding, the weld joint was subjected to hydrogen annealing. Before hydrogen annealing, the base metal exhibited an equiaxed structure, while the molten zone showed a lamellar structure. At a hydrogen annealing concentration of 0.2 wt%, a certain proportion of lamellar structure appeared in both the base metal and the molten zone. At a hydrogen annealing concentration of 1.0 wt%, both the base metal and the weld nugget exhibited coarse lamellar structures with an aspect ratio greater than 10. Although the microstructure of the base metal and weld nugget was similar when the hydrogen annealing concentration was 1.0 wt%, the overall elongation of the joint decreased significantly during superplastic stretching at 900°C, failing to demonstrate superplasticity and thus not meeting the requirements for superplastic forming of components. Furthermore, when the hydrogen-annealed joint was placed in air, hydrogen gradually escaped from the titanium alloy, and the hydrogen content decreased over time. Especially during high-temperature stretching in a vacuum and inert gas atmosphere, hydrogen escaped rapidly, leading to unstable superplastic properties of the joint microstructure.

Claims

1. A superplastic forming process for preparing large-size titanium alloy components, characterized in that: The process includes the following steps: (1) The duplex titanium alloy rolled plate was subjected to friction stir welding using a friction stir welding tool. During the welding process, the temperature of the weld nugget area was controlled to be 50-200℃ lower than the β phase transformation point, and argon gas was introduced for purging. After welding, a fine equiaxed crystal structure was obtained in the weld nugget area. The thickness of the duplex titanium alloy rolled plate was 1-5 mm. The duplex titanium alloy rolled plate as the base material was in the rolled annealed state, and the base material structure size was 3-10 μm. The fine equiaxed crystal structure obtained after friction stir welding had a size of 200-800 nm. (2) Static annealing treatment was performed on the equiaxed crystal structure in the two-phase region of the titanium alloy. The static annealing temperature was 800~980℃ and the holding time was 0.5~10 h. By controlling the annealing temperature and annealing time, the microstructure of the weld nugget and the base material were made to be consistent. As the annealing time was extended, the equiaxed crystal structure of the weld nugget and the rolled microstructure of the base material coarsened at the same time, but the coarsening rate of the fine microstructure at the weld nugget was higher than that of the base material. By controlling the annealing temperature and annealing time, the microstructure of the weld nugget and the base material were made consistent. (3) Perform overall superplastic molding on large-size plates after static annealing.

2. The superplastic forming process for preparing large-size titanium alloy components according to claim 1, characterized in that: In step (1), the material of the friction stir welding tool is W-Re alloy, W-La2O3 or Co-based alloy.

3. The superplastic forming process for preparing large-size titanium alloy components according to claim 2, characterized in that: The parameters for friction stir welding when using W-Re alloy or W-La2O3 tools are: tool rotation speed 150~400 rpm, travel speed 50~150 mm / min; the parameters for welding when using Co-based alloy stirring tools are: rotation speed 130-250 rpm, travel speed 25~150 mm / min.

4. The superplastic forming process for preparing large-size titanium alloy components according to claim 2, characterized in that: In step (1), the friction stir welding tool is a conical stirring pin with a shoulder diameter of 6~20 mm, a top diameter of 4~8 mm, and a root diameter of 5~12 mm. During the friction stir welding process, the axial direction of the stirring pin is inclined at an angle of 1~5° to the normal direction of the rolled surface of the titanium alloy plate.

5. The superplastic forming process for preparing large-size titanium alloy components according to claim 1, characterized in that: In step (1), when argon gas is introduced for purging, the argon gas flow rate is 0.5-15 L / min. The argon gas protects the weld nugget area from oxidation, and the cooling rate of the weld nugget area after welding is adjusted by controlling the argon gas flow rate.

6. The superplastic forming process for preparing large-size titanium alloy components according to claim 1, characterized in that: In step (2), static annealing is performed under an inert atmosphere or vacuum to prevent oxidation of the titanium alloy sample.

7. The superplastic forming process for preparing large-size titanium alloy components according to claim 1, characterized in that: In step (3), superplastic molding is carried out in an inert gas atmosphere, wherein the deformation temperature is 800~980℃ and the gas pressure is 0.1-10 MPa.

Images