A method suitable for low temperature diffusion bonding of TiAl alloys

Abstract

The application relates to a method suitable for low-temperature diffusion connection of TiAl alloy, which comprises the following steps: firstly, surface plastic deformation is performed on the surface of a TiAl plate to be diffusion-connected through surface mechanical attrition treatment; and then, vacuum diffusion connection is performed on the surface according to connection requirements, so that a diffusion connection joint with good structure of the TiAl alloy is obtained. The application proposes a method suitable for surface mechanical attrition treatment of the TiAl plate and specific attrition parameters, realizes surface plastic deformation of the TiAl base material, and thus promotes atomic diffusion on both sides of the diffusion connection interface; through regulation and control of diffusion connection parameters, a high-quality diffusion connection joint without holes can be obtained at 1000 DEG C, the diffusion connection cost is reduced, and the application of the diffusion connection technology in the forming process of TiAl alloy components is facilitated.

Description

Technical Field

[0001] This invention belongs to the field of diffusion bonding technology of TiAl intermetallic compounds, and specifically relates to a method for low-temperature diffusion bonding of TiAl alloys. Background Technology

[0002] TiAl intermetallic compounds are characterized by their low density (3.9–4.2 g / cm³). -3 High specific yield strength, high specific stiffness, excellent high-temperature oxidation resistance, and good high-temperature creep performance make TiAl the only candidate material for weight reduction in the upper limit of the service temperature range (600℃) and the lower limit of the service temperature range (900℃) of high-temperature alloys. In the preparation and repair of TiAl intermetallic compounds, the problem of TiAl joining is unavoidable. Due to the high hot cracking sensitivity and strong elemental reactivity of TiAl intermetallic compounds, when traditional fusion welding technology is used for joining, deformation cracks and solidification defects easily form at the joint after welding. Diffusion joining technology is a solid-state joining method that achieves atomic-level bonding between two or more parts through surface contact under certain temperature and pressure. Using diffusion joining technology on TiAl intermetallic compounds can effectively avoid defects such as hot cracking and can achieve solid-state joining with other high-temperature structural materials. It has been applied to the joining of fairings and supports for hypersonic aircraft and the joining of thin-walled hollow multilayer structures of TiAl, showing significant application prospects.

[0003] Currently, diffusion bonding of TiAl plates faces several challenges. First, the inherent brittleness of TiAl intermetallic compounds necessitates stringent process conditions (T>1100℃, t>2h, and P>20MPa) for successful bonding. The relatively long welding time and the high temperatures and pressures in a vacuum result in high equipment costs. Second, oxygen contamination inevitably leads to the formation of a hard and brittle α2 phase at the diffusion bonding interface, affecting the stability of the joint's mechanical properties. These drawbacks limit the size and manufacturing cost of diffusion-bonded components. Therefore, it is necessary to explore new interface strengthening methods and improve diffusion bonding process parameters to achieve stable microstructure and performance of the bonded joint. Extensive research has been conducted by scholars both domestically and internationally in this regard.

[0004] Song Xiaoguo of Harbin Institute of Technology proposed a method for diffusion bonding of high-niobium TiAl alloys using composite metal foils in his published invention patent (publication number CN105798449A). This method uses a 20μm thick Ti foil and a 15μm thick Nb foil, arranged in a Ti / Nb / Ti sequence as an interlayer. Diffusion bonding is performed at 1200℃ / 20MPa / 2h. The addition of the interlayer facilitates element diffusion at the interface, resulting in a good metallurgical bond and improving the quality of the high-niobium TiAl alloy diffusion bonding joint. However, the introduction of incompletely diffused and residual Ti and Nb interlayers at the interface leads to inhomogeneity in the microstructure and mechanical properties between the interface and the base material, making it prone to crack initiation at the interface.

[0005] Qi Xiansheng et al. from Northwestern Polytechnical University studied the effect of shot peening on the microstructure evolution of the diffusion-bonded interface in high-niobium TiAl alloys. Their research showed that after high-energy shot peening at 0.4 MPa for 15 min, a deformation zone of approximately 50 μm thickness was generated on the surface to be bonded. While the microstructure of the deformed surface did not show significant refinement, a large number of stacking faults and dislocation networks were generated at a distance of 20 μm from the surface. Following shot peening with diffusion bonding at 1100℃ for 45 min at 30 MPa, compared to direct diffusion bonding, resulted in a higher degree of recrystallization and improved joint strength at the diffusion-bonded interface. The crystal defects generated on the shot-peening surface can serve as atomic diffusion channels, promoting atomic diffusion on both sides of the interface. Furthermore, the residual distortion energy generated by shot peening is conducive to interface recrystallization, all of which contribute to the evolution of the interface microstructure and thus improve the joint strength. However, the interface did not undergo significant refinement after shot peening, and the diffusion channels provided in the subsequent diffusion bonding process were limited, resulting in a limited increase in the recrystallization volume fraction at the interface. The shear performance was only improved by about 50 MPa, which needs to be further improved (Qi Xiansheng, et al. Influence of surface plastic deformation on the evolution of the microstructure of diffusion bonding interface of high Nb-TiAl alloy [J]. Journal of Plasticity Engineering, 2014, 21(04):23-7.).

[0006] Currently, the main methods for strengthening diffusion-bonded joints in TiAl alloys focus on adding intermediate layers and surface modification. Among these methods, the method of surface plastic deformation followed by diffusion bonding has been shown to have an important positive effect on the evolution of the diffusion bonding interface microstructure. On the other hand, surface mechanical grinding can generate gradient nanostructures from the grinding surface to the matrix. This surface nanostructure has significant potential for reducing the diffusion bonding temperature and strengthening diffusion-bonded joints. Summary of the Invention

[0007] To address the issues of high diffusion bonding temperature, uneven joint microstructure evolution, and poor diffusion bonding quality in the vacuum diffusion bonding process of TiAl alloys, this invention proposes a method suitable for large-area low-temperature diffusion bonding of TiAl alloy plates.

[0008] To achieve the above objectives, the present invention adopts the following solution: a method for performing surface plastic deformation using Surface Mechanical Grinding Treatment (SMGT) followed by diffusion bonding, the specific process of which is as follows:

[0009] Step 1: Mechanical grinding treatment of the surface to be joined: Press the semi-circular WC cutter head into the surface to be joined of the wire-cut TiAl plate with a set external load of 100-300N, and grind along the original rolling direction of the TiAl plate for a set number of passes. The feed rate V1 is 0.1-0.3m / s. After completing 100-300 passes, the cutter head moves a distance d = 50-150μm at a speed of V2 = 0.002m / s. Then, the grinding is repeated until the set number of passes is completed.

[0010] Step 2: Polishing treatment of the surface to be joined: Polish the TiAl plate surface to be joined obtained by mechanical grinding in Step 1 until the grinding marks of the cutting tool are eliminated, then mechanically polish and clean and store.

[0011] Step 3: Vacuum diffusion bonding: The TiAl plate samples to be bonded after polishing in Step 2 are vacuum diffused bonded according to the bonding requirements to obtain a well-structured TiAl alloy diffusion bonding joint.

[0012] As a further explanation of the present invention, before mechanical grinding in step 1, the surfaces of the TiAl plates to be joined after wire cutting are successively polished with 240# and 400# sandpaper and then cleaned with alcohol to remove surface dirt.

[0013] As a further explanation of the present invention, in step 2, 1.5# diamond polishing paste is used for mechanical polishing.

[0014] As a further explanation of the present invention, in step 2, the mechanically polished TiAl plate is immersed in anhydrous ethanol for ultrasonic cleaning for 5 minutes, and then stored in anhydrous ethanol.

[0015] As a further explanation of the present invention, step 3, the vacuum diffusion connection process specifically includes:

[0016] After the TiAl plate samples polished in step 2 are assembled according to the connection requirements, they are placed into a vacuum diffusion welding machine. After closing the furnace door, the vacuum degree of the vacuum diffusion welding machine furnace cavity is evacuated to 5×10⁻⁶. -3Below Pa; the furnace temperature is raised to 950-1100℃ at a heating rate of 5-15℃ / min, wherein the furnace temperature is held for 10 minutes at 300℃ and 600℃ respectively before the temperature is raised further. When the furnace temperature reaches 950-1100℃, an axial pressure of 25-40MPa is applied to the TiAl plate sample to be joined through the graphite pressure head of the vacuum diffusion welding machine and held for 50-90 minutes. After the holding and pressure holding are completed, the TiAl plate sample is cooled to room temperature with the furnace to obtain a diffusion joint of TiAl alloy with good structure.

[0017] As a further explanation of the present invention, the vacuum degree of the vacuum diffusion welding machine is maintained during the vacuum diffusion connection process.

[0018] Compared with existing graphene-titanium composite materials, the present invention has the following beneficial technical effects:

[0019] 1. Due to the intrinsic brittleness of its intermetallic compounds, TiAl alloys exhibit poor weldability. The diffusion bonding temperature required to obtain a good diffusion-bonded joint is generally above 1100℃, placing high demands on diffusion bonding equipment and resulting in high production costs. To reduce the diffusion bonding temperature of TiAl alloys and obtain high-quality TiAl alloy diffusion-bonded joints, this invention designs a process combining surface mechanical grinding and vacuum diffusion bonding. After surface mechanical grinding, a gradient microstructure is formed from the surface layer to the substrate, resulting in nanoscale grains in the outermost layer and the formation of a plastic deformation influence zone of a certain thickness. On one hand, the nanoscale fine-grained structure of the outermost layer and the substructure near the surface provide numerous diffusion channels for atomic diffusion during the subsequent diffusion bonding process, promoting element diffusion on both sides of the interface and thus promoting interface microstructure evolution and improving the weldability. On the other hand, the large amount of distortion energy stored in the surface layer by surface mechanical grinding provides a driving force for dynamic recrystallization during the diffusion bonding process, promoting dynamic recrystallization at the interface and improving joint performance. By combining surface mechanical grinding treatment, this invention can obtain a high-quality diffusion bonding joint without pores at 1000℃, which reduces the cost of diffusion bonding and is beneficial to the application of diffusion bonding technology in the forming process of TiAl alloy components.

[0020] 2. The surface mechanical grinding surface pretreatment adopted in this invention can be used as a surface pretreatment method for diffusion bonding of large-size plates, which can more effectively bond large-size surfaces and is applicable to diffusion bonding of large thin-walled hollow structures.

[0021] 3. The microstructure of the diffusion-connected joint obtained by this invention is shown in the attached figure. Figure 4 As shown, with the attached Figure 6Compared with the microstructure of the joint prepared by the direct diffusion bonding process shown, the diffusion bonding joint prepared by the present invention is free from defects such as cracks and pores, and has a fine microstructure, high degree of recrystallization, and complete microstructure evolution at the weld interface, resulting in a high-quality diffusion bonding joint. Attached Figure Description

[0022] Figure 1 The flowchart illustrates the method for large-area low-temperature diffusion bonding of TiAl alloy plates provided by this invention.

[0023] Figure 2 This is a schematic diagram of surface mechanical grinding in this invention.

[0024] Figure 3 This is a graph showing the vacuum diffusion bonding process in this invention.

[0025] Figure 4 This is a backscattered electron scanning image of the microstructure of the TNM alloy diffusion joint obtained in Example 1 of the present invention, where the white short lines represent the weld seam.

[0026] Figure 5 This is a backscattered electron scanning image of the microstructure at the diffusion connection interface of Embodiment 1 of the present invention, with higher magnification. The white short lines represent the weld seam.

[0027] Figure 6 This is a backscattered electron scanning image of the joint microstructure obtained in Comparative Example 1 of the present invention, where the white short lines represent the weld seam. Detailed Implementation

[0028] To better understand the above-mentioned objects, features, and advantages of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0030] The TiAl alloy used in the three sets of embodiments and one set of comparative examples provided by this invention is a TNM alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B. In each embodiment, the surface of a TNM rolled plate with a size of 50mm×90mm×2mm was mechanically ground, and then cut into plates with a size of 8mm×60mm×2mm and 4mm×60mm×2mm and vacuum diffusion bonded together.

[0031] Comparative Example 1

[0032] The specific process of this comparative example is as follows:

[0033] Step 1: Surface treatment of the sample to be joined. The surfaces to be joined are polished sequentially with 240#, 400#, 600#, 800#, 1000#, 1500# and 2000# sandpaper to remove obvious marks. Then, mechanical polishing is performed with 1.5# diamond polishing paste to achieve a mirror finish. The sample is then immersed in anhydrous ethanol for ultrasonic cleaning for 5 minutes and subsequently stored in anhydrous ethanol.

[0034] Step 2: Vacuum Diffusion Bonding. Assemble the samples to be bonded according to the bonding requirements and place them into the vacuum diffusion welding machine. After closing the furnace door, evacuate the vacuum level of the diffusion welding machine chamber to 5 × 10⁻⁶. -3 Below Pa. The furnace temperature was raised to 1000℃ at a heating rate of 10℃ / min, with holding at 300℃ and 600℃ for 10 minutes before further heating. When the furnace temperature reached 1000℃, an axial pressure of 30MPa was applied to the bonding sample using the graphite indenter of the vacuum diffusion welder and held for 60 minutes. After the holding and pressure holding were completed, the bonding sample was cooled to room temperature with the furnace. This completed the vacuum diffusion bonding, obtaining a well-structured TNM alloy diffusion bond. Throughout the vacuum diffusion bonding process, the vacuum level of the diffusion welder was maintained.

[0035] The TNM alloy diffusion-bonded specimens obtained in the above comparative examples were cut along a direction perpendicular to the bonding interface, and the microstructure at the interface was observed. The obtained diffusion-bonded interface microstructure is shown in the attached figure. Figure 6 As shown, the interface obtained by direct diffusion bonding is flat and obvious, and there are a large number of unclosed pores and defects at the interface, resulting in a limited welding rate and poor mechanical properties.

[0036] Example 1

[0037] The specific process of this embodiment is as follows:

[0038] Step 1: Mechanical grinding of the surfaces to be joined. After wire cutting, the surfaces of the TNM sheet to be joined are successively ground with 240# and 400# sandpaper, then cleaned with alcohol to remove surface dirt. (See attached image) Figure 2 This is a schematic diagram of surface mechanical grinding. The original rolling direction of the material is RD. A semi-circular WC cutter head with an external load of 300N is pressed into the material surface. The cutter head diameter is 10mm. Grinding is performed 50 times along the RD direction at a feed rate V1 of 0.2m / s. After completing the set number of passes, the cutter head moves a distance d = 100μm at a speed of V2 = 0.002m / s, and then the grinding is repeated until the set number of passes is completed.

[0039] Step 2: Polishing the surface to be joined. After the surface obtained by mechanical grinding is sanded with 2000# sandpaper until the grinding marks of the surface tool are removed, mechanical polishing is performed with 1.5# diamond polishing paste to make the surface to be joined have a mirror effect. Then, the sample is immersed in anhydrous ethanol for ultrasonic cleaning for 5 minutes, and then stored in anhydrous ethanol.

[0040] Step 3: Vacuum Diffusion Bonding. Assemble the samples to be bonded according to the bonding requirements and place them into the vacuum diffusion welding machine. After closing the furnace door, evacuate the vacuum level of the diffusion welding machine chamber to 5 × 10⁻⁶. -3 Below Pa. The furnace temperature was raised to 1000℃ at a heating rate of 10℃ / min, with holding at 300℃ and 600℃ for 10 minutes before further heating. When the furnace temperature reached 1000℃, an axial pressure of 30MPa was applied to the bonding sample using the graphite indenter of the vacuum diffusion welder and held for 60 minutes. After the holding and pressure holding were completed, the bonding sample was cooled to room temperature with the furnace. This completed the vacuum diffusion bonding, obtaining a well-structured TNM alloy diffusion bond. Throughout the vacuum diffusion bonding process, the vacuum level of the diffusion welder was maintained.

[0041] The obtained TNM alloy diffusion-bonded specimens were cut along a direction perpendicular to the bonding interface, and the microstructure at the interface was observed. The obtained diffusion-bonded interface microstructure is shown in the attached figure. Figure 4 and attached Figure 5 As shown. (From the appendix) Figure 4 and attached Figure 5 It can be seen that after surface mechanical grinding followed by diffusion bonding, the joint structure consists of a gradient structure formed by surface mechanical grinding and a recrystallized fine-grained layer. (See attached...) Figure 6 As shown in the comparison of the direct diffusion bonding interfaces, the TNM alloy diffusion bonding joints obtained by this method are free of defects such as cracks and pores, have complete interface microstructure evolution, and have a welding rate close to 100%, thus obtaining diffusion bonding joints with excellent performance.

[0042] Example 2

[0043] The specific process of this embodiment is as follows:

[0044] Step 1: Mechanical grinding of the surfaces to be joined. After wire cutting, the surfaces of the TNM sheet to be joined are successively ground with 240# and 400# sandpaper, then cleaned with alcohol to remove surface dirt. (See attached image) Figure 2This is a schematic diagram of surface mechanical grinding. The original rolling direction of the material is RD. A semi-circular WC cutter head with an external load of 200N is pressed into the material surface. The cutter head diameter is 10mm. Grinding is performed 100 times along the RD direction at a feed rate V1 of 0.3m / s. After completing the set number of passes, the cutter head moves a distance d = 150μm at a speed of V2 = 0.002m / s, and then the grinding is repeated until the set number of passes is completed.

[0045] Step 2: Polishing the surface to be joined. After the surface obtained by mechanical grinding is sanded with 2000# sandpaper until the grinding marks of the surface tool are removed, mechanical polishing is performed with 1.5# diamond polishing paste to make the surface to be joined have a mirror effect. Then, the sample is immersed in anhydrous ethanol for ultrasonic cleaning for 5 minutes, and then stored in anhydrous ethanol.

[0046] Step 3: Vacuum Diffusion Bonding. Assemble the samples to be bonded according to the bonding requirements and place them into the vacuum diffusion welding machine. After closing the furnace door, evacuate the vacuum level of the diffusion welding machine chamber to 5 × 10⁻⁶. -3 Below Pa. The furnace temperature was raised to 1100℃ at a heating rate of 15℃ / min, with holding at 300℃ and 600℃ for 10 minutes before further heating. When the furnace temperature reached 1100℃, an axial pressure of 25MPa was applied to the bonding sample using the graphite indenter of the vacuum diffusion welder and held for 90 minutes. After the holding and pressure holding were completed, the bonding sample was cooled to room temperature with the furnace. This completed the vacuum diffusion bonding, obtaining a well-structured TNM alloy diffusion bond. Throughout the vacuum diffusion bonding process, the vacuum level of the diffusion welder was maintained.

[0047] The obtained TNM alloy diffusion-bonded specimens were cut along the direction perpendicular to the bonding interface, and the microstructure at the interface was observed. The results show that the diffusion-bonded interface obtained in this embodiment is also composed of recrystallized fine-grained layers and a gradient structure formed by grinding. The interface microstructure evolution is complete, the degree of interface recrystallization is high, and the welding rate can reach 100%, resulting in a TNM diffusion-bonded joint with excellent performance.

[0048] Example 3

[0049] The specific process of this embodiment is as follows:

[0050] Step 1: Mechanical grinding of the surfaces to be joined. After wire cutting, the surfaces of the TNM sheet to be joined are successively ground with 240# and 400# sandpaper, then cleaned with alcohol to remove surface dirt. (See attached image) Figure 2This is a schematic diagram of surface mechanical grinding. The original rolling direction of the material is RD. A semi-circular WC cutter head with an external load of 100N is pressed into the material surface. The cutter head diameter is 10mm. Grinding is performed 150 times along the RD direction at a feed rate V1 of 0.2m / s. After completing the set number of passes, the cutter head moves a distance d = 50μm at a speed of V2 = 0.002m / s, and then the grinding is repeated until the set number of passes is completed.

[0051] Step 2: Polishing the surface to be joined. After the surface obtained by mechanical grinding is sanded with 2000# sandpaper until the grinding marks of the surface tool are removed, mechanical polishing is performed with 1.5# diamond polishing paste to make the surface to be joined have a mirror effect. Then, the sample is immersed in anhydrous ethanol for ultrasonic cleaning for 5 minutes, and then stored in anhydrous ethanol.

[0052] Step 3: Vacuum Diffusion Bonding. Assemble the samples to be bonded according to the bonding requirements and place them into the vacuum diffusion welding machine. After closing the furnace door, evacuate the vacuum level of the diffusion welding machine chamber to 5 × 10⁻⁶. -3 Below Pa. The furnace temperature was raised to 950℃ at a heating rate of 10℃ / min, with holding at 300℃ and 600℃ for 10 minutes before continuing the heating. When the furnace temperature reached 950℃, an axial pressure of 40MPa was applied to the bonding sample through the graphite pressure head of the vacuum diffusion welder and held for 60 minutes. After the holding and pressure holding were completed, the bonding sample was cooled to room temperature with the furnace. This completed the vacuum diffusion bonding. Throughout the vacuum diffusion bonding process, the vacuum level of the diffusion welder was maintained.

[0053] The obtained TNM alloy diffusion-bonded specimens were cut along the direction perpendicular to the bonding interface, and the microstructure at the interface was observed. The results show that the diffusion-bonded interface obtained in this embodiment is also composed of recrystallized fine-grained layers and a gradient structure formed by grinding. The interface microstructure evolution is complete, the degree of interface recrystallization is high, and the joint is basically free of defects such as cracks and pores. The interface microstructure evolution is complete, and a high-quality TNM alloy diffusion-bonded joint is obtained.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for low-temperature diffusion bonding of TiAl alloys, characterized in that, Includes the following steps: Step 1: Mechanical grinding treatment of the surface to be joined: Press the semi-circular WC cutter head into the surface to be joined of the wire-cut TiAl plate with a set external load of 100-300N, and grind along the original rolling direction of the TiAl plate for a set number of passes. The feed rate V1 is 0.1-0.3m / s. After completing 100-300 passes, the cutter head moves at a speed of V2=0.002m / s along a direction perpendicular to the original rolling direction of the TiAl plate by a distance d=50-150μm. Then, the grinding is repeated until the set number of passes is completed. Step 2: Polishing treatment of the surface to be joined: Polish the TiAl plate surface to be joined obtained by mechanical grinding in Step 1 until the grinding marks of the cutting tool are eliminated, then mechanically polish and clean and store. Step 3: Vacuum diffusion bonding: The TiAl plate samples to be bonded after the polishing treatment in Step 2 are vacuum diffused bonded according to the bonding requirements to obtain a well-structured TiAl alloy diffusion bonding joint. The vacuum diffusion connection process specifically includes: After the TiAl plate samples polished in step 2 are assembled according to the connection requirements, they are placed into a vacuum diffusion welding machine. The furnace door is then closed, and the vacuum level in the furnace chamber is evacuated to [a certain value]. The furnace temperature is raised to 950–1100°C at a heating rate of 5–15°C / min. The furnace temperature is held for 10 minutes at 300°C and 600°C respectively before further heating. When the furnace temperature reaches 950–1100°C, an axial pressure of 25–40 MPa is applied to the TiAl plate sample to be joined using the graphite pressure head of the vacuum diffusion welding machine, and the pressure is held for 50–90 minutes. After the holding and pressure holding are completed, the TiAl plate sample is cooled to room temperature with the furnace, resulting in a well-structured diffusion-bonded joint of TiAl alloy.

2. The method for low-temperature diffusion bonding of TiAl alloys according to claim 1, characterized in that: Before mechanical grinding in step 1, the surfaces of the TiAl plates to be joined after wire cutting are sanded with 240# and 400# sandpaper in sequence, and then cleaned with alcohol to remove surface dirt.

3. The method for low-temperature diffusion bonding of TiAl alloys according to claim 1, characterized in that: In step 2, 1.5# diamond polishing paste is used for mechanical polishing.

4. The method for low-temperature diffusion bonding of TiAl alloys according to claim 1, characterized in that: In step 2, the mechanically polished TiAl plate is immersed in anhydrous ethanol for ultrasonic cleaning for 5 minutes, and then stored in anhydrous ethanol.

5. The method for low-temperature diffusion bonding of TiAl alloys according to claim 1, characterized in that: During the vacuum diffusion bonding process, the vacuum level of the vacuum diffusion welding machine is maintained.

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