CN113182632B - Method for connecting C/C composite material by adopting high-entropy alloy brazing - Google Patents

Abstract

The invention discloses a method for connecting C/C composite materials by high-entropy alloy brazing. The C/C composite material is firstly subjected to surface treatment, and the high-entropy alloy AlCoCrFeNi _{x is} cut and polished to a thickness of 70 μm (±10 μm). The brazing filler metal foil is made; the C/C composite material and the brazing filler metal foil are assembled and placed in a vacuum brazing furnace for heating and activation treatment, that is, the preparation is completed. In the method of the present invention, the high-entropy alloy brazing filler metal used has excellent wettability to the C/C composite material, and the wetting is mainly realized by the action of Ni and the dissolution-diffusion reaction. During the welding process, the residual stress between the base metal and the joint is fully relieved by the formation of M ₇ C ₃ , the increase of the BCC phase and the formation of the composite structural layer, and the joint strength can reach up to 21.93 MPa. In addition, the high-entropy structure in the weld is preserved, which ensures the stability of the high-temperature strength of the joint, which has important research prospects.

Description

A method for connecting C/C composites by brazing with high-entropy alloys

technical field

The invention relates to the field of brazing, in particular to a method for connecting C/C composite materials by brazing using high-entropy alloys.

Background technique

C/C composite material has excellent high temperature stability and corrosion resistance, and is the only structural material that can be used above 2500 °C. In addition, C/C composites have low density, high specific strength, high specific modulus, excellent heat transfer performance and wear resistance, and are widely used in aerospace and other industrial fields. For example, the leading edge of wings, turbine engine parts, aircraft front cover and brake pads, as well as bearings, sealing rings and other precision wear parts. In order to meet the service requirements under different working conditions, the self-connection of C/C composite materials is usually required to obtain C/C composite materials of different sizes and complex structures. At present, the solders used for the connection of C/C composites are still mainly Ag-based, Ni-based and Ti-based, and the wetting and reliable connection of C/C composites is achieved by the active elements in the solder. However, the currently used solder has the remarkable characteristics of large linear expansion coefficient and insufficient high-temperature mechanical properties. It is urgent to develop a method for connecting C/C using a solder with a small thermal expansion coefficient and excellent high-temperature performance. In the AlCoCrFeNi _2.1 high-entropy alloy, Ni and Cr elements have good wettability to the C-based material, and compared with the above traditional brazing filler metals, the high-entropy alloy has a smaller thermal expansion coefficient and high-temperature mechanical properties, using high-entropy alloys Connecting C/C composites will effectively relieve the stress involved in the joint and improve the high temperature strength of the joint.

SUMMARY OF THE INVENTION

The present invention provides a method for brazing and connecting C/C composite materials using high-entropy alloys, and aims to improve the problems of difficult wetting, large residual stress and poor high-temperature mechanical properties of joints currently used for brazing C/C composite materials. Reliable C/C composite welded joints.

The concrete technical scheme adopted in the present invention is as follows:

The method for connecting C/C composite materials by high-entropy alloy brazing is characterized by comprising the following steps:

1) Surface treatment of the C/C composite material to be brazed, that is, grinding, cleaning and drying the surface of the C/C composite material;

2) Stack the surface-treated C/C composite material in step 1) in a graphite mold, and place a piece of high-entropy alloy brazing material foil between the two C/C composite materials to form a welded assembly; on each welded assembly Place the molybdenum block for weighting, so that the two C/C composite materials are in close contact with the high-entropy alloy brazing filler metal foil in the middle, and then transferred to the vacuum brazing furnace together;

3) The welding assembly obtained in step 2) put into the vacuum brazing furnace is heated and activated, so that the to-be-welded surfaces of the two C/C composite materials are connected by the high-entropy alloy brazing filler metal foil, and then the temperature is lowered and cooled , and finally obtained the C/C composite material connected by high-entropy alloy brazing.

The method for connecting C/C composite materials by high-entropy alloy brazing is characterized in that in step 1), the process of surface treatment of the C/C composite material to be brazed is: grinding the C/C composite material with sandpaper The to-be-welded surface of the composite material was then placed on a polishing machine, polished with a diamond suspension with a particle size of 1 μm to no traces, then placed in acetone for ultrasonic cleaning, and finally dried to complete the surface treatment.

The method for connecting C/C composite materials by high-entropy alloy brazing is characterized in that in step 2), the high-entropy alloy brazing filler metal foil is AlCoCrFeNix, and the atomic ratio of each element in the high-entropy alloy is Al :Co:Cr:Fe:Ni=1:1:1:1:2.1.

The method for connecting C/C composite materials by high-entropy alloy brazing is characterized in that the preparation method of high-entropy alloy brazing filler metal foil AlCoCrFeNi _2.1 , the steps are as follows:

S1: According to the atomic ratio of each metal raw material in the high-entropy alloy AlCoCrFeNi _2.1 , each metal raw material powder is mixed together according to the ratio to form a powder raw material;

S2: suspend smelting the powder raw material obtained in step S1 to make a brazing alloy ingot;

S3: The solder alloy ingot is cut into thin slices by wire cutting;

S4: The sheet is polished by hand to obtain a high-entropy alloy brazing filler metal foil AlCoCrFeNi _2.1 .

The method for connecting C/C composite materials by high-entropy alloy brazing is characterized in that after the powder raw material is smelted into a solder alloy ingot in step S2, the smelting is repeated 3 to 5 times to obtain a uniform composition. Brazing alloy ingot; in step S2, the melting temperature is 1300-1400°C, preferably 1350°C.

The method for connecting C/C composite materials using high-entropy alloy brazing is characterized in that in step S3, the brazing alloy ingot is cut into thin slices with a thickness of 0.2 to 0.4 mm, so as to facilitate later grinding and reduction. Thinning process; in step S4, the sheet is polished to 2000# with SiC sandpaper to remove cutting marks and oxide films on all surfaces of the sheet, and thin to a thickness of 60 μm-80 μm, finally obtaining a high-entropy alloy brazing filler metal foil AlCoCrFeNix.

The method for connecting C/C composite materials by high-entropy alloy brazing is characterized in that in step 3), the process of heating and activation treatment is as follows: firstly, the temperature is raised from room temperature to a temperature of 10-20° C./min. 250~350°C, hold for 8~15 minutes; then heat up to 1150~1250°C at a rate of 10~20°C/min, hold for 8~15 minutes; then heat up to the brazing temperature at a rate of 8~12°C/min, keep warm After a certain period of time; finally, the temperature is lowered to 250~350°C at a speed of 8~12°C/min, and then naturally cooled to room temperature with the furnace.

The method for connecting C/C composite materials by high-entropy alloy brazing is characterized in that the brazing temperature is 1350-1440°C, preferably 1410-1440°C, and the temperature is raised to the holding time after the brazing temperature. It is 8~15 minutes.

As a new type of structural material, high-entropy alloys have excellent properties such as high temperature stability, high hardness, and strong corrosion resistance. Among them, the dual-phase high-entropy alloy is composed of FCC and BCC phases, and has good plasticity and strength at the same time. In addition, the BCC phase in the high-entropy alloy has a small thermal expansion coefficient, which is beneficial to alleviate the problem of large residual stress caused by the mismatch of thermal expansion coefficients between materials. Therefore, the present invention adopts the dual-phase high-entropy alloy brazing to connect the C/C composite materials, which is beneficial to relieve the residual stress of the joint and improve the high-temperature service performance of the joint.

Compared with the prior art, the present invention has the following beneficial effects:

1) In the embodiment of the present invention, AlCoCrFeNi _2.1 high-entropy alloy is used as the brazing filler metal. Through the wetting effect of Ni element on the C/C composite material and the strong dissolution and diffusion reaction between the brazing filler metal and the base metal, the C/C composite material is realized. Good wetting of composites. There is no continuous M ₇ C ₃ reaction layer formed at the material interface, breaking the phenomenon of wetting through the reaction layer in traditional active brazing.

2) Since M ₇ C ₃ and BCC phases have small thermal expansion coefficients, during the welding process, the formation of M ₇ C ₃ in the weld and the increase of BCC phases are beneficial to relieve the residual stress of the joint. In addition, the joint obtained in the present invention has a composite structure mixed with graphite and metal phases, which can realize a smooth transition of CTE between the base metal and the weld, and effectively reduce the residual stress of the joint and the base metal.

3) AlCoCrFeNi _2.1 high-entropy alloy has excellent high-temperature performance and still has a yield strength of 500 MPa at 700 °C, so the use of high-entropy alloy can be beneficial to improve the high-temperature strength of C/C composite joints.

Description of drawings

Figure 1 shows the microstructure and morphology of the joint of the C/C composite brazed by AlCoCrFeNi _2.1 high-entropy alloy in Example 1.

FIG. 2 shows the microstructure and morphology of the joint of the brazed C/C composite material using AlCoCrFeNi _2.1 high-entropy alloy in Example 4. FIG.

Detailed ways

The present invention will be further described below with reference to specific embodiments, but the protection scope of the present invention is not limited thereto.

Example 1

A method for brazing C/C composites with AlCoCrFeNi _2.1 high-entropy alloy, the steps are as follows:

1) Use 800# and 2000# SiC sandpaper in turn to grind the surface of the C/C composite material to be brazed step by step, and place it on a polishing machine. Ultrasonic cleaning in acetone, and finally drying, that is, the surface treatment is completed;

2) According to the atomic ratio of each metal raw material in the AlCoCrFeNi _2.1 high-entropy alloy, the metal raw material powders are mixed together according to the proportion to form powder raw materials; the powder raw materials are smelted at a temperature of 1350 ℃, and the powder raw materials are smelted into brazing materials. After the alloy ingot is cast, the smelting is repeated 4 times to make a brazing alloy ingot with uniform composition;

3) Cut the brazing alloy ingot obtained in step 2) into 0.3 mm thick slices by wire cutting, and hand-polished to 2000# with SiC sandpaper to remove cutting marks and oxide films on all surfaces of the slices, and thin them to 60 μm -80 μm thick;

4) Take two pieces of C/C composite material after surface treatment in step 1) and stack them in a graphite mold, and place a piece of AlCoCrFeNi _2.1 sheet obtained in step 3) between the two C/C composite materials to form a welded assembly; on the welded assembly Place a 5 g molybdenum block for weighting, so that the two C/C composite materials are in close contact with the high-entropy alloy brazing filler metal foil in the middle, and then put into a vacuum brazing furnace;

5) The vacuum degree of the vacuum brazing furnace was controlled at 2×10 ^-3 Pa; firstly, the temperature was raised from room temperature to 300°C at a heating rate of 10°C/min, and kept for 10 minutes; then the temperature was raised to 1200 at a rate of 10°C/min. ℃, hold for 10 minutes; then raise the temperature at a rate of 10℃/min to the brazing temperature (the brazing temperature is 1350℃), and keep the temperature for 10 minutes; finally, cool down to 300℃ at a rate of 10℃/min, and then naturally cool to room temperature with the furnace, that is, A joint for brazing C/C composites using AlCoCrFeNi _2.1 high-entropy alloy was prepared.

Example 1 After the brazing is completed, the backscattered microstructure morphology of the formed joint after brazing is shown in Figure 1 (Figure 1 is a schematic diagram of the microstructure obtained by schematic depiction according to the results of the backscattered electron image under the scanning electron microscope. ). It can be seen from Figure 1 that there are no holes, cracks and other phenomena at the weld interface, and the structure is uniform. Combined with EDS and XRD characterization, the joints were determined to be composed of graphite, FCC, BCC and M ₇ C ₃ phases.

Example 2

Using AlCoCrFeNi _2.1 high-entropy alloy brazing C/C composite method, the operation steps are repeated in Example 1, the only difference is that "in step 5), the brazing temperature is replaced from 1350 ℃ to 1380 ℃ ", other conditions Similar to Example 1, a joint of C/C composite material brazed by AlCoCrFeNi _2.1 high-entropy alloy is finally prepared.

Example 3

Using AlCoCrFeNi _2.1 high-entropy alloy brazing method for C/C composite materials, the operation steps are repeated in Example 1, the difference is only that "in step 5), the brazing temperature is replaced from 1350 ℃ to 1410 ℃ ", other conditions Similar to Example 1, a joint of C/C composite material brazed by AlCoCrFeNi _2.1 high-entropy alloy is finally prepared. The shape of the joint is shown in Figure 2.

Example 4

Using AlCoCrFeNi _2.1 high-entropy alloy brazing method for C/C composite materials, the operation steps are repeated in Example 1, the only difference is that "in step 5), the brazing temperature is replaced from 1350 ℃ to 1440 ℃ ", other conditions Similar to Example 1, a joint of C/C composite material brazed by AlCoCrFeNi _2.1 high-entropy alloy is finally prepared.

Example 4 After the brazing is completed, the backscattered microstructure and morphology of the formed joint after brazing is shown in Figure 2 (Figure 2 is a schematic diagram of the microstructure obtained by schematic depiction according to the results of the backscattered electron image under the scanning electron microscope. ). It can be seen from Figure 2 that increasing the brazing temperature will be beneficial to the uniform mixing of various structures in the joint, thereby further reducing the residual stress of the joint.

The brazed joints obtained in the above Examples 1-4 were placed on a universal testing machine to test their shear strength, and the results are shown in Table 1.

It can be seen from Table 1 that the shear strength of the brazed joints obtained in Examples 1-4 increases in turn, that is, the joint strength presents an upward trend with the increase of the brazing temperature, and is the highest in the embodiments of the present invention. Can reach 21.93MPa. However, when the brazing temperature increased from 1410 °C to 1440 °C, the joint strength did not increase significantly; it was inferred from the microstructure analysis that the degree of mixing of graphite and metal phases in the joint was already very intense at 1440 °C. The performance impact is small; in addition, due to technical and equipment limitations, it is difficult to achieve further increases in brazing temperature, and it will consume higher costs. Therefore, in the present invention, it is considered that a temperature of 1410° C.-1440° C. is a suitable welding temperature.

The invention improves the problems of low high-temperature mechanical properties and large residual stress of joints of brazing filler metals currently used for C/C composite materials, and discloses a new wettability mechanism of C/C composite materials. During the welding process, due to the delayed diffusion effect of the high-entropy alloy, the active element Cr cannot diffuse to the interface between the base metal and the solder at high temperature. In addition, the C element in the base metal has a fast diffusion rate at high temperature, and it diffuses sufficiently before the solder melts, and reacts with the Cr element to form a uniformly distributed M ₇ C ₃ phase in the brazing joint. And due to the consumption of C element, a porous graphite structure is formed at the C/C interface. Further, due to the precipitation of Cr element, the originally Cr-rich FCC phase is transformed into a Cr-poor BCC phase. Continue to heat up to the brazing temperature, the brazing filler metal liquefies and gradually flows into the C/C porous interface. Due to the interaction between the Ni element and the base metal, the reaction with C/C is further deepened during the heat preservation process, thereby realizing the effect of the base metal. Good wetting. Finally, a reaction layer in which graphite and metal phases coexist was formed during the cooling process, and the weld mainly contains FCC, BCC, M ₇ C ₃ and graphite phases.

To sum up, the weld obtained by the present invention has a matrix of FCC+BCC high-entropy structure. Since the high-entropy alloy has good high-temperature strength, compared with the traditional alloy brazing filler metal used in the prior art, it is beneficial to improve the C/C joint. High temperature mechanical properties. Since the M ₇ C ₃ and BCC phases in the brazing joint have low thermal expansion coefficients, the formation of the two phases is beneficial to relieve the residual stress in the joint due to thermal mismatch. In addition, due to the formation of a composite layer in which graphite and metal phases coexist between the base metal and the brazing filler metal, the smooth transition of the thermal expansion coefficient from the brazing seam to C/C is realized, which greatly reduces the residual stress at the interface, which is beneficial to the improvement of the joint strength.

The content described in this specification is only an enumeration of the realization forms of the inventive concept, and the protection scope of the present invention should not be regarded as being limited to the specific forms stated in the embodiments.

Claims (9)

1. a method for adopting high-entropy alloy brazing to connect C/C composite material, is characterized in that comprising the following steps: 1) Surface treatment of the C/C composite material to be brazed, that is, grinding, cleaning and drying the surface of the C/C composite material; 2) Stack the surface-treated C/C composite material in step 1) in a graphite mold, and place a piece of high-entropy alloy brazing material foil between the two C/C composite materials to form a welded assembly; on each welded assembly Place the molybdenum block for weighting, so that the two C/C composite materials are in close contact with the high-entropy alloy brazing filler metal foil in the middle, and then transferred to the vacuum brazing furnace together; 3) The welding assembly obtained in step 2) put into the vacuum brazing furnace is heated and activated, so that the to-be-welded surfaces of the two C/C composite materials are connected by the high-entropy alloy brazing filler metal foil, and then the temperature is lowered and cooled , and finally obtained the C/C composite material connected by high-entropy alloy brazing; In step 2), the high-entropy alloy brazing filler metal foil is AlCoCrFeNix, and the atomic ratio of each element in the high-entropy alloy is Al:Co:Cr:Fe:Ni=1:1:1:1:2.1. 2. A method of using high-entropy alloy brazing to connect C/C composite materials according to claim 1, wherein in step 1), the process of surface treatment of the C/C composite materials to be brazed is: The surface to be welded of the C/C composite material was polished with sandpaper, then placed on a polishing machine, polished with a diamond suspension with a particle size of 1 μm to no traces, then placed in acetone for ultrasonic cleaning, and finally dried to complete the surface treatment. 3. a kind of method that adopts high-entropy alloy brazing to connect C/C composite material according to claim 1, it is characterized in that the preparation method of described high-entropy alloy brazing filler metal foil AlCoCrFeNix, step is as follows: S1: According to the atomic ratio of each metal raw material in the high-entropy alloy AlCoCrFeNix, each metal raw material powder is mixed together according to the ratio to form a powder raw material; S2: suspend smelting the powder raw material obtained in step S1 to make a brazing alloy ingot; S3: The solder alloy ingot is cut into thin slices by wire cutting; S4: The sheet is polished by hand to obtain a high-entropy alloy brazing filler metal foil AlCoCrFeNix. 4. a kind of method that adopts high entropy alloy brazing to connect C/C composite material according to claim 3, it is characterized in that after powder raw material is smelted into brazing alloy ingot in step S2, smelting 3～5 times again Second, a brazing alloy ingot with uniform composition is obtained; in step S2, the melting temperature is 1300-1400°C. 5 . The method for connecting C/C composite materials by high-entropy alloy brazing according to claim 4 , wherein in step S2 , the melting temperature is 1350° C. 6 . 6. A method for connecting C/C composite materials by high-entropy alloy brazing according to claim 3, characterized in that in step S3, the brazing alloy ingot is cut into thin slices with a thickness of 0.2-0.4 mm, so as to have a thickness of 0.2-0.4 mm. It is beneficial to the later grinding and thinning process; in step S4, the sheet is polished to 2000# with SiC sandpaper to remove the cutting marks and oxide film on all surfaces of the sheet, and thin to a thickness of 60μm-80μm, and finally obtain high-entropy alloy brazing Material foil AlCoCrFeNix. 7. A method of using high-entropy alloy brazing to connect C/C composite materials according to claim 1, characterized in that in step 3), the process of heating and activation treatment is: first from room temperature to 10 ~ 20 ℃ / The heating rate was heated to 250~350 °C for 1 min, and the temperature was maintained for 8~15 min; then the temperature was increased to 1150~1250 °C at a rate of 10~20 °C/min, and the temperature was maintained for 8~15 min; then the temperature was increased at a rate of 8~12 °C/min. To brazing temperature, keep for a certain period of time; finally, cool down to 250~350°C at a speed of 8~12°C/min, and then naturally cool to room temperature with the furnace. 8. a kind of method that adopts high entropy alloy brazing to connect C/C composite material according to claim 7, it is characterized in that described brazing temperature is 1350～1440 ℃, and the holding time after warming up to brazing temperature is 8~15 minutes. 9 . The method for connecting C/C composite materials by high-entropy alloy brazing according to claim 8 , wherein the brazing temperature is 1410-1440° C. 10 .

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