Method for preparing Cr2Nb / Cu composite material through selective laser melting in-situ reaction

Abstract

The invention discloses a method for preparing a Cr2Nb / Cu composite material through selective laser melting in-situ reaction. The method specifically comprises the following steps that 1, Cu-Cr alloy powder, Nb powder and Cu powder are weighed in proportion; 2, the Cu-Cr binary powder and Nb powder are mixed, then Cu powder is added for mixing, and the obtained mixed powder is dried; 3, constructing a test piece model read by the 3D printing equipment, and importing the constructed test piece model into the 3D printing equipment; and 4, the dried powder is loaded into 3D printing equipment to be printed, and the deposited Cr2Nb / Cu composite material is obtained. And 5, heat treatment is conducted on the Cr2Nb / Cu composite material, and the Cr2Nb / Cu composite material is obtained. According to the preparation method, the problems of high cost and low yield in the existing preparation process of the Cu, Cr and Nb ternary composite material are solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of copper-based composite material additive manufacturing, and relates to a method for preparing Cr2Nb / Cu composite material through laser selective melting in-situ reaction. BACKGROUND

[0002] The Cr2Nb / Cu composite material has become the first choice of a new generation of space engine combustion chamber liners due to high strength, high electrical conductivity and good thermal stability at high temperatures. As a new technology in recent years, selective laser melting (SLM) gradually replaces traditional manufacturing and becomes the preferred technology for core components of high-end equipment due to its complex structure integration forming capability and more than 95% material utilization. The Cr2Nb / Cu composite material improves the comprehensive performance of the material by relying on the dispersion distribution and small size of the Cr2Nb reinforcing phase; the SLM technology generates a small molten pool and a heat-affected zone by laser beam scanning, and at the same time provides a very high cooling rate (10 4 ~ 10 6 K / s) for the material, so that the size of the reinforcing phase Cr2Nb phase is small, which can greatly improve the mechanical properties of the material. Therefore, it is the future development trend to integrate the SLM and the Cr2Nb / Cu composite material to form a rocket engine combustion chamber liner.

[0003] However, SLM needs a powder with high sphericity and good fluidity as a raw material. At present, the Cr2Nb / Cu composite material powder is prepared by the gas atomization method, but due to the large difference in melting points of Cu, Cr and Nb, the high-melting-point Nb component is difficult to melt completely in the smelting process, so a large superheat is needed to melt the raw materials and make the melt have good fluidity. However, too high superheat of the melt will cause serious burning loss of the material, and the solidification time of the melt will also increase, resulting in high cost of the preparation of the Cu-Cr-Nb ternary powder; in addition, the molten droplets are easy to adhere to each other during flight, increasing the probability of satellite powder and other irregular powder, which is not conducive to powder forming. Therefore, it is difficult to mass-produce the composite material powder, and the yield is low. SUMMARY

[0004] The purpose of the present application is to provide a method for preparing Cr2Nb / Cu composite material through laser selective melting in-situ reaction, which solves the problems of high preparation cost and low yield of Cu-Cr-Nb ternary powder in the preparation process of Cr2Nb / Cu composite material.

[0005] The technical scheme adopted by the present application is a method for preparing Cr2Nb / Cu composite material through laser selective melting in-situ reaction, which specifically includes the following steps: Step 1, weighing Cu-Cr alloy powder, Nb powder and Cu powder according to the proportion; Step 2, the Cu-Cr binary powder is mixed with the Nb powder, and then the Cu powder is added for mixing, and the obtained mixed powder is subjected to drying treatment; Step 3, a test piece model read by a 3D printing device is constructed, and the constructed test piece model is imported into the 3D printing device; Step 4, the dried powder is loaded into the 3D printing device for printing, and a deposited Cr2Nb / Cu composite material is obtained; Step 5, the Cr2Nb / Cu composite material is subjected to heat treatment, and a Cr2Nb / Cu composite material is obtained.

[0006] The application also has the characteristics that: In step 1, 10-40wt.% Cu-Cr powder, 1.5-6wt.% Nb powder, 10-15wt.% Cu powder with a particle size of 1-2μm, and 39-73.5wt.% Cu powder with a particle size of 15-45μm are weighed according to the weight percentage.

[0007] In step 1, the density of the Cr element in the Cu-Cr is the same as that of Nb, the Cu-Cr powder with a particle size of 20-53μm is screened, the Nb powder with a particle size of 20-53μm is taken, the Cu powder with a particle size of 1-2μm is taken, and the Cu powder with a particle size of 15-45μm is taken.

[0008] The specific process of step 2 is as follows: First, the Cu-Cr binary powder is mixed with the Nb powder, and the Cu powder with a particle size of 15-45μm is divided into several parts for mixing, each time for 2-4h, and finally the Cu powder with a particle size of 1-2μm is added for mixing for 6-8h, and the mixed powder is heated in a vacuum box at a heating temperature of 80-150℃.

[0009] The specific process of step 3 is as follows: A cuboid three-dimensional model is drawn and imported into a slicing software for two-dimensional slicing and layering, the single-layer thickness is 25-30μm, the printing order is set, a file with a format of.STL is generated, a test piece model read by a 3D printing device is obtained, and the constructed test piece model is imported into the 3D printing device.

[0010] The specific process of step 4 is as follows: Step 4.1, the dried powder is loaded into the 3D printing device for powder feeding, the cabin door is closed for gas washing, argon is introduced, and the oxygen in the forming cabin is excluded, after the gas washing is completed, the base plate is preheated, and the fan is turned on after the preheating temperature is reached; Step 4.2, before printing, a layer of powder is pre-laid on the substrate for scanning and melting, first melting by using a laser with a power of 430-450w, without continuing to lay powder, and then melting by using a laser with a power of 360-380w again; after repeated laser scanning, a layer of powder is laid again until the printing is completed, and finally a deposited Cr2Nb / Cu composite material is obtained.

[0011] In step 4.2, the printing process is as follows: the laser scanning speed is 600-700mm / s; the single-layer printing thickness is 25-30μm; the repeated printing process is as follows: the laser scanning speed is 1000-1500mm / s; and the scanning mode is a stripe mode.

[0012] The specific process of step 5 is as follows: a muffle furnace is used for heating, the sample is placed in the muffle furnace after the temperature reaches the preset temperature, and then the sample is aged for a certain time, and then the sample is cooled to room temperature by air cooling, the aging temperature is 450-600℃, and the aging time is 1-5h.

[0013] The beneficial effects of the present application are as follows: 1) The Cu-Cr density is calculated theoretically, the Cu-17Crwt.% alloy powder is self-made, and the Cu-Cr alloy powder and the Nb powder are ensured to be 8.57g / cm 3 , and the density difference between the three powders is within 0.04%, so that the powders will not be layered due to the density difference during mixing. The Cu-Cr alloy powder is prepared by gas atomization, which is relatively simple, and the current process is relatively mature, so that the raw material process is simple and the cost is low.

[0014] 2) SLM first uses a high-energy-density laser to ensure that the Nb powder is melted, and the Nb element and the Cr element react in situ to generate Cr2Nb particles, but at this time the melt has a high superheat, and the generated Cr2Nb particles are relatively large. Since the melting point of Cr2Nb particles (1870℃) is lower than that of Nb powder (2468℃), a lower energy density is used for secondary scanning to melt the Cr2Nb particles generated once again. Due to the low superheat and fast solidification speed during melting, the Cr2Nb particles generated during the secondary solidification process are more dispersed and fine, which improves the microstructure of the composite material and improves the comprehensive performance of the material. At the same time, the secondary low-temperature remelting also reduces the size of the molten pool, ensuring that the formed sample has good surface quality. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 The process flow chart of the method for preparing the Cr2Nb / Cu composite material by laser selective melting in-situ reaction of the present application is shown in the figure. Figure 2 The sample physical picture prepared by the method for preparing the Cr2Nb / Cu composite material by laser selective melting in-situ reaction of the present application is shown in the figure. Figure 3 SEM image of the Cr2Nb / Cu composite material prepared by the method for preparing Cr2Nb / Cu composite material in-situ reaction by laser selective melting according to the present application in Example 1; Figure 4 Aging state stress-strain curve of the Cr2Nb / Cu composite material prepared by the method for preparing Cr2Nb / Cu composite material in-situ reaction by laser selective melting according to the present application in Example 1. DETAILED DESCRIPTION

[0016] The present application will be described in detail below with specific embodiments.

[0017] The method for preparing Cr2Nb / Cu composite material in-situ reaction by laser selective melting according to the present application is shown in the flow chart as Figure 1 The Cu-Cr powder is prepared by using the gas atomization method, the Cu-Cr binary powder, the Nb powder and the Cu powder are mixed by SLM, the proportion of the Cu-Cr binary powder is calculated, the specific powder mixing method, the specific printing parameters and the heat treatment process are used to realize the manufacturing of the second phase small and dispersed distribution and high performance Cu-Cr-Nb. Specifically, the steps include: Step 1, raw material calculation, specifically: Step 1.1, the proportion of Cr in the Cu-Cr powder is calculated to make the density the same as that of Nb, so as to ensure that the powder mixing process will not cause stratification.

[0018] Step 1.2, the Cu-17Crwt.% powder is self-made by using the gas atomization method, the Cu-Cr powder with a particle size of 20-53μm is screened, the Nb powder with a particle size of 20-53μm is taken, and the Cu powder with a particle size of 1-2μm is taken.

[0019] Step 2, powder preparation, specifically: 10-40wt.% Cu-Cr powder, 1.5-6wt.% Nb powder, 10-15wt.% Cu powder with a particle size of 1-2μm, and 39-78.5wt.% Cu powder with a particle size of 15-45μm are weighed according to the weight percentage. The use of part of the Cu powder with a particle size of 1-2μm can fill the gap between the Cu-Cr powder and the Nb powder, ensure the uniformity of the powder in the mixing stage, and improve the density of the powder bed during subsequent powder laying.

[0020] Step 3, powder mixing, specifically: Step 3.1, the Cu-Cr binary powder and the Nb powder in step 2 are mixed by using a three-dimensional mixer, the mixer process: power is 0.55KW, rotation speed is 20rpm, filling rate is 35%. The three-dimensional mixer realizes mixing by using gentle tumbling motion, reduces mechanical stress on the powder, and avoids damaging the sphericity.

[0021] Step 3.2, the Cu powder with particle size of 15-45 μm in step 1 is divided into 3 parts and mixed, and 1 part is added each time and mixed for 2-4 h.

[0022] Step 3.3, 1-2 μm Cu powder is added and mixed for 6-8 h.

[0023] Step 3.4, the mixed powder is obtained and heated in a vacuum box. The vacuum oven is extracted to a vacuum of ≤-0.1, the heating temperature is 80-150 ℃, and the holding time is 4 h.

[0024] Step 4, modeling, specifically: Step 4.1, a three-dimensional model of a cuboid with a length of 20 mm, a width of 60 mm and a height of 10 mm is drawn; Step 4.2, the two-dimensional slicing layering is performed in the slicing software, the single-layer thickness is 25-30 μm, the printing order is set, the file in the format of.STL is generated, and the test piece model readable by the 3D printing equipment is obtained.

[0025] Step 4.3, the built slicing layering file model is imported into the 3D printing equipment.

[0026] Step 5, printing, specifically: Step 5.1, the powder after drying in step 3 is loaded into the powder supply cabin of the 3D printing equipment, the cabin door is closed for gas washing, argon is introduced, the oxygen in the molding cabin is excluded, and the oxygen content is less than 400 ppm during the printing process, after the gas washing is completed, the substrate preheating is started, the preheating temperature is 100 ℃, so as to reduce the thermal stress in the molding process, after heating to the preheating temperature, the fan is turned on, and the wind speed is 55 mm 3 / h; Step 5.2, a layer of powder is laid on the substrate before printing and scanned and melted, first, high energy density (laser power 430-450 w) laser melting is adopted, powder laying is not continued, then, low energy density (laser power 360-380 w) laser melting is adopted. After repeated laser scanning, a layer of powder is laid again, and the steps (powder laying, first scanning, second scanning, powder laying again, first scanning, second scanning) are repeated, and finally the as-deposited Cr2Nb / Cu composite material is obtained. The printing process is: the laser scanning speed is 600-700 mm / s; the single-layer printing thickness is 25-30 μm; the repeated printing process is: the laser scanning speed is 1000-1500 mm / s; the scanning strategy is the stripe mode, and the interlayer deflection angle is 67°.

[0027] Step 6, aging, specifically: Step 6.1, a muffle furnace is used for heating, after the temperature reaches the preset temperature, the sample is put in for aging treatment. The aging temperature is 450-600 ℃, and the aging time is 1-5 h.

[0028] Step 6.2, after the aging time is reached, air cooling is used to cool to room temperature.

[0029] Example 1 Step 1, the proportion of Cr in the Cu-Cr alloy powder is calculated so that the density difference between the Cu-Cr alloy powder, the Nb powder and the Cu powder is between 0.04%. The Cu-17Crwt.% powder is self-made by gas atomization method, the Cu-Cr powder with a particle size of 20-53 μm is screened, the Nb powder with a particle size of 20-53 μm is taken, and the Cu powder with a particle size of 1-2 μm and the Cu powder with a particle size of 15-45 μm are taken.

[0030] Step 2, 25wt.% Cu-Cr powder, 3.75wt.% Nb powder, 10wt.% Cu powder with a particle size of 1-2 μm and 61.25wt.% Cu powder with a particle size of 15-45 μm are weighed according to the weight percentage.

[0031] Step 3, the Cu-Cr binary powder is mixed with the Nb powder, the Cu powder with a particle size of 15-45 μm is divided into three parts and added three times for mixing, each time for 1h, for a total of 3h. Finally, the Cu powder with a particle size of 1-2 μm is added and mixed for 6h after mixing, to obtain the mixed powder which is heated in a vacuum box. The mixing machine process: power is 0.55kw, rotating speed is 20rpm, filling rate is 35%. The vacuum oven: vacuum degree is ≤-0.1Pa, heating temperature is 80℃, holding time is 4h.

[0032] Step 4, a three-dimensional model of a rectangular cuboid with a length of 20mm, a width of 60mm and a height of 10mm is drawn and imported into a slicing software for two-dimensional slicing and layering, the single layer thickness is 30μm, the printing order is set, and a file with a format of.STL is generated to obtain a test piece model readable by a 3D printing device. The built slicing and layering file model is imported into the 3D printing device.

[0033] Step 5, the powder in step 3 is loaded into the powder supply cabin of the 3D printing device, the cabin door is closed for gas washing. After the gas washing is completed, the substrate preheating is started, the fan is turned on after the preheating temperature is reached, and the printing is started. The powder is loaded into the powder supply cabin of the 3D printing device, the cabin door is closed for gas washing, high-purity (99.9%) argon gas is introduced, the oxygen in the forming cabin is excluded and the oxygen content is less than 400ppm during the printing process, after the gas washing is completed, the substrate preheating is started, the fan is turned on, the wind speed is 55mm 3 / h, preheating temperature to 100℃ after starting printing. In the printing process, a layer of powder is first laid on the substrate, and then high-power laser scanning is used to melt, followed by low-power high-speed laser scanning to melt again. The above steps are repeated, and finally the Cr2Nb / Cu composite material is prepared. The printing process is: laser power 440w; laser scanning speed 650mm / s; single layer printing thickness 25-30μm; repeated printing process: laser power 370w; laser scanning speed 1300mm / s; scanning strategy is stripe mode, and the interlayer deflection angle is 67°.

[0034] Step 6: The obtained as-deposited sample is subjected to aging treatment. The aging temperature is 500℃, and the aging time is 1h. The aging method: using a muffle furnace to heat, after the temperature reaches the preset temperature, the sample is put in for aging treatment, and after the aging time is reached, the temperature is cooled to room temperature by air cooling.

[0035] The electrical and mechanical properties of the Cr2Nb / Cu composite material prepared in Example 1 are tested, and the conductivity is 60.5%IACS, and the tensile strength is 688MPa.

[0036] Figure 2 The finished product of the Cr2Nb / Cu alloy prepared in Example 1 is shown in the figure, and the surface roughness is good without warping deformation. Figure 3 The SEM image of the Cr2Nb / Cu alloy prepared in Example 1 is shown in the figure, and the grains are alternately distributed with typical columnar grains and equiaxed grains prepared by laser, and the grain size is small, providing a large amount of fine grain strengthening. Figure 4 The tensile diagram of Example 1 is shown in the figure, and the tensile strength is 688MPa.

[0037] Example 2 In this embodiment, in step 2, 40wt.% Cu-Cr powder, 6wt.% Nb powder, 15wt.% Cu powder with a particle size of 1-2μm, and 39wt.% Cu powder with a particle size of 15-45μm are weighed according to weight. The printing process in step 5 is: laser power 450w; laser scanning speed 600mm / s; single layer printing thickness 30μm; repeated printing process: laser power 360w; laser scanning speed 1000mm / s; scanning strategy is stripe mode, and the interlayer deflection angle is 67°. The aging process is 600℃ / 1h. The other steps are the same as those in Example 1.

[0038] The electrical and mechanical properties of the Cr2Nb / Cu composite material prepared in Example 2 are tested, and the conductivity is 65%IACS, and the tensile strength is 660MPa.

[0039] Example 3 In this embodiment, in step 2, 10wt.% Cu-Cr powder, 1.5wt.% Nb powder, 15wt.% Cu powder with a particle size of 1-2 μm, and 73.5wt.% Cu powder with a particle size of 15-45 μm were weighed according to weight. In step 5, the printing process laser power was 430w; the laser scanning speed was 700mm / s; the single-layer printing thickness was 30 μm, and the repeated printing process was as follows: the laser power was 360w; the laser scanning speed was 1500mm / s; the scanning strategy was stripe mode, and the interlayer deflection angle was 67°. The aging process was 450℃ / 1h. The other steps were the same as those in examples 1 and 2.

[0040] The electrical and mechanical properties of the Cr2Nb / Cu composite prepared in example 3 were tested, and the conductivity was 73.8% IACS, and the tensile strength was 581.8MPa.

[0041] Example 4 In this embodiment, in step 2, 25wt.% Cu-Cr powder, 3.75wt.% Nb powder, 10wt.% Cu powder with a particle size of 1-2 μm, and 61.25wt.% Cu powder with a particle size of 15-45 μm were weighed according to weight. In step 5, the printing process laser power was 440w; the laser scanning speed was 700mm / s; the single-layer printing thickness was 25 μm, and the repeated printing process was as follows: the laser power was 370w; the laser scanning speed was 1300mm / s; the scanning strategy was stripe mode, and the interlayer deflection angle was 67°. The aging process was 500℃ / 3h. The other steps were the same as those in examples 1, 2 and 3.

[0042] The electrical and mechanical properties of the Cr2Nb / Cu prepared in example 4 were tested, and the conductivity was 63% IACS, and the tensile strength was 665MPa.

[0043] Example 5 In this embodiment, in step 2, 40wt.% Cu-Cr powder, 6wt.% Nb powder, 10wt.% Cu powder with a particle size of 1-2 μm, and 44wt.% Cu powder with a particle size of 15-45 μm were weighed according to weight. In step 5, the printing process laser power was 430w; the laser scanning speed was 600mm / s; the single-layer printing thickness was 25 μm, and the repeated printing process was as follows: the laser power was 380w; the laser scanning speed was 1000mm / s; the scanning strategy was stripe mode, and the interlayer deflection angle was 67°. The aging process was 600℃ / 3h. The other steps were the same as those in examples 1, 2, 3 and 4.

[0044] The electrical and mechanical properties of the Cr2Nb / Cu composite prepared in example 5 were tested, and the conductivity was 60% IACS, and the tensile strength was 650MPa.

[0045] Example 6 In this embodiment, 10wt.% Cu-Cr powder, 1.5wt.% Nb powder, 15wt.% Cu powder with a particle size of 1-2 μm, and 73.5wt.% Cu powder with a particle size of 15-45 μm are weighed in step 2. In step 5, the printing process laser power is 440w; the laser scanning speed is 600mm / s; the single-layer printing thickness is 25 μm, and the repeated printing process is as follows: the laser power is 360w; the laser scanning speed is 1500mm / s; the scanning strategy is the stripe mode, and the interlayer deflection angle is 67°. The aging process is 500℃ / 3h. The other steps are the same as those in examples 1, 2, 3, 4, and 5.

[0046] The electrical and mechanical properties of the Cr2Nb / Cu composite material prepared in example 6 are tested, and the conductivity is 75.4% IACS, and the tensile strength is 560.3MPa.

[0047] The application uses self-made Cu-Cr alloy powder with specific components, Nb powder, and Cu powder as raw materials, designs a powder mixing process, adopts a specific printing method and printing parameters, and successfully prepares a Cr2Nb / Cu composite material with dispersed distribution of reinforcing phases and small size through in-situ reaction, solves the problems of high cost and limited source of Cr2Nb / Cu composite material powder in SLM, and provides a solution for the Cr2Nb / Cu composite material for the integrated forming of the rocket engine combustion chamber liner.

Claims

1. A method for preparing Cr2Nb / Cu composite materials by laser selective melting in situ reaction, characterized in that: Specifically, the steps include the following: Step 1: Weigh out Cu-Cr alloy powder, Nb powder, and Cu powder according to the specified proportions; Step 2: Mix Cu-Cr binary powder with Nb powder, then add Cu powder and mix again. Dry the resulting mixed powder. Step 3: Construct the specimen model for the 3D printing equipment and import the constructed specimen model into the 3D printing equipment; Step 4: Load the dried powder into a 3D printing device for printing to obtain a deposited Cr2Nb / Cu composite material; Step 5: Heat-treat the Cr2Nb / Cu composite material to obtain the Cr2Nb / Cu composite material.

2. The method for preparing Cr2Nb / Cu composite materials by laser selective melting in situ reaction according to claim 1, characterized in that: In step 1, 10-40 wt.% Cu-Cr powder, 1.5-6 wt.% Nb powder, 10-15 wt.% Cu powder with a particle size of 1-2 μm, and 39-73.5 wt.% Cu powder with a particle size of 15-45 μm are weighed according to their respective weight percentages.

3. The method for preparing Cr2Nb / Cu composite materials by laser selective melting in situ reaction according to claim 2, characterized in that: In step 1, the density of Cr in Cu-Cr is the same as that of Nb. Cu-Cr powder with a particle size of 20-53 μm is screened out, and Nb powder with a particle size of 20-53 μm, Cu powder with a particle size of 1-2 μm, and Cu powder with a particle size of 15-45 μm are also taken.

4. The method for preparing Cr2Nb / Cu composite materials by laser selective melting in situ reaction according to claim 3, characterized in that: The specific process of step 2 is as follows: First, Cu-Cr binary powder and Nb powder are mixed. Cu powder with a particle size of 15-45μm is divided into several equal parts and mixed for 2-4 hours each time. Finally, Cu powder with a particle size of 1-2μm is added and mixed for 6-8 hours to obtain a mixed powder. The powder is then heated in a vacuum chamber at a temperature of 80-150℃.

5. The method for preparing Cr2Nb / Cu composite materials by laser selective melting in situ reaction according to claim 4, characterized in that: The specific process of step 3 is as follows: Draw a rectangular 3D model and import it into slicing software for 2D slicing with a single layer thickness of 25-30μm. Set the printing order and generate a .STL file to obtain the specimen model that the 3D printing equipment can read. Import the completed specimen model into the 3D printing equipment.

6. The method for preparing Cr2Nb / Cu composite materials by laser selective melting in situ reaction according to claim 5, characterized in that: The specific process of step 4 is as follows: Step 4.1: Load the dried powder into the powder supply chamber of the 3D printing equipment, close the chamber door and perform gas washing. Introduce argon gas to remove the oxygen from the molding chamber. After the gas washing is completed, turn on the substrate preheating. After heating to the preheating temperature, turn on the fan. Step 4.2: Before printing, a layer of powder is pre-laid on the substrate for scanning and melting. First, a laser with a power of 430-450w is used for melting, and the powder is not further laid. Then, a laser with a power of 360-380w is used for melting. After the laser scan is repeated, another layer of powder is laid until the printing is completed, and finally, the deposited Cr2Nb / Cu composite material is obtained.

7. The method for preparing Cr2Nb / Cu composite materials by laser selective melting in situ reaction according to claim 6, characterized in that: In step 4.2, the printing process is as follows: laser scanning rate 600-700 mm / s; single-layer printing thickness 25-30 μm; repetitive printing process is as follows: laser scanning rate 1000-1500 mm / s; scanning method is stripe pattern.

8. The method for preparing Cr2Nb / Cu composite materials by laser selective melting in situ reaction according to claim 7, characterized in that: The specific process of step 5 is as follows: heating is carried out in a muffle furnace. After the temperature reaches the preset temperature, the sample is placed in for aging treatment. After the aging time is reached, the sample is cooled to room temperature by air cooling. The aging temperature is 450-600℃ and the aging time is 1-5h.

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