A brazing method for improving high temperature welding performance of nickel-based alloy joints and prepared multi-layer welded joints

Abstract

The present invention belongs to the technical field of nickel-based alloy connection, and specifically relates to a brazing method for improving the high-temperature welding performance of nickel-based alloy joints and a prepared multi-layer welded joint. First, the brazing material is processed into an amorphous brazing foil, and then the amorphous brazing foil is pre-placed between the joint surfaces of the workpieces to be welded of the Inconel625 nickel-based alloy, and then the workpieces to be welded are fixed and placed in a vacuum brazing furnace for brazing. After the brazing is completed, the workpiece group is cooled to below 100°C and then solution treated in a vacuum brazing furnace. The amorphous foil-shaped brazing material used in the present invention has a small difference in melting point from the parent material, which is conducive to capillary action and can optimize the formation and performance of the brazed joint; the solution treatment can improve the distribution and morphology of brittle phases such as borides in the brazing seam. The present invention can be used as a special brazing method for multi-layer brazing joints or Inconel 625 nickel-based alloys, and has great application prospects.

Description

Technical Field

[0001] The invention belongs to the technical field of nickel-based alloy connection, and in particular relates to a brazing method for improving the high-temperature welding performance of a nickel-based alloy joint and a prepared nickel-based alloy multilayer welded joint. Background Art

[0002] Nickel-based alloys can withstand large complex stresses in high-temperature environments above 600°C, have good organizational stability and excellent high-temperature mechanical properties, and are therefore widely used in the forming and connection of equipment such as aircraft engines, gas turbines, and high-temperature fuel cell heat exchange systems.

[0003] Compared with fusion welding, the brazing connection technology used in the production process of aircraft engine parts, such as blades, turbine disks, and flame tubes, can reduce internal stress near the joints, reduce structural deformation, and can also repair the shape of complex surfaces; high-temperature zone heat exchangers are key core equipment in the thermal balance system of solid oxide fuel cells (SOFCs). Brazed plate or plate-fin heat exchangers have compact structures, high heat transfer efficiency, small pressure drop, and low production costs, making them very suitable for SOFC application scenarios with high medium temperatures but low pressures.

[0004] Inconel 625 nickel-based alloy, despite its high price, has excellent high-temperature oxidation resistance and corrosion resistance, and good service reliability at high temperatures, making it one of the preferred materials for industrial equipment such as SOFC high-temperature heat exchangers. However, 625 alloy brazing technology is a high-temperature brazing technology, which is more difficult than stainless steel brazing, and the technology is not mature at present.

[0005] In order to lower the melting point, silicon and boron are commonly added to nickel-based brazing filler metals. Improper brazing process control may cause the presence of intermetallic compounds and brittle phases in the brazed joint. Under the action of internal stress after welding, there is a risk of microcracks. At the same time, the aggregation and uneven distribution of brittle phases cause the mechanical properties of the brazed joint to deteriorate. In addition, the industry currently generally uses boron-containing solder paste as the brazing material for brazing nickel-based high-temperature alloys, such as the brazing method disclosed in patent CN 1147697718A, which also has the following disadvantages: First, the plate-type gap pre-set solder paste can be roller-coated, dispensed, or screen-printed, but the control of its thickness deviation is very different from that of industrial product μm-level foil solder, and the gap effect of nickel-based high-temperature alloy brazing is more sensitive than other alloys; second, the solder paste requires a solder carrier, which has strong activity and affects the performance of the brazing seam during the brazing process, easily causing brazing seam holes and reducing the brazing bonding rate; third, the solder paste component contains higher boron, which causes greater erosion of the base material, produces more borides, and has poor high-temperature performance, especially under conditions above 800°C and high-temperature long-term conditions.

[0006] Therefore, for the high-temperature brazing of Inconel 625 nickel-based alloy, it is a technical problem that needs to be solved urgently to find a brazing method for high-performance nickel-based alloy joints with high reliability, uniform brazing seam after welding, and more sufficient diffusion effect between the base material and the brazing filler metal. Summary of the invention

[0007] In order to solve the above technical problems, the present invention first provides a brazing method for improving the high-temperature welding performance of nickel-based alloy joints, which can significantly improve the performance disadvantages caused by paste solder, and can also improve the distribution state of its compounds through heat treatment to improve the high-temperature performance of the brazing seam.

[0008] The technical solution adopted by the present invention is as follows:

[0009] A brazing method for improving the high temperature welding performance of a nickel-based alloy joint comprises the following steps:

[0010] S1. Processing the selected solder into an amorphous solder foil, wherein the thickness of the amorphous solder foil is between 30 and 50 μm, and the formula of the selected solder is: by mass fraction, 14.5% to 15.5% chromium, 1.1% to 1.6% boron, 7.0% to 7.5% silicon, 0% to 1% iron, 0% to 1% cobalt, and the balance is nickel;

[0011] S2. Pre-place a layer of the amorphous brazing foil between the required bonding surfaces of the workpieces to be welded, wherein the workpieces to be welded are made of Inconel625 nickel-based alloy;

[0012] S3. Fix the workpiece to be welded, and then place it in a vacuum brazing furnace for brazing. After the brazing is completed, the workpiece group is cooled to below 50°C at a rate of 15°C ~ 20°C / min;

[0013] S4. Place the workpiece group in a brazing furnace for heat preservation and solution treatment: first evacuate the brazing furnace to below 0.005Pa, then heat it to the solution temperature of 1100~1150℃ at a rate of 10~20℃ / min, keep it warm for 150 min, then use high-purity nitrogen or argon to cool the workpiece group to 700℃ at a rate of 15℃ / min, and then cool it to room temperature with the furnace at a rate of 15~20℃ / min.

[0014] Preferably, a pre-welding cleaning step is also included. Before pre-setting the amorphous brazing foil, the surface of the workpiece to be welded is firstly polished to remove the oxide layer, and then the polished workpiece to be welded is ultrasonically cleaned in anhydrous ethanol, then rinsed with clean water, and dried for later use.

[0015] Preferably, in step S3, the brazing procedure is to evacuate the vacuum brazing furnace to below 0.001 Pa, then heat it to 600°C at a rate of 10-20°C / min, and keep it warm for 20 min for preheating; then heat it to 900°C at a rate of 10-15°C / min, and keep it warm for 20 min for uniform heating; finally heat it to 1150-1195°C at a rate of 5-8°C / min, and keep it warm for 30-50 min.

[0016] min for brazing.

[0017] Preferably, in step S3, the brazing temperature is 1180° C. and the temperature is kept for 40 min.

[0018] Preferably, in step S3, the workpiece group is cooled using high-purity nitrogen or argon.

[0019] The present invention also provides a nickel-based alloy multilayer welded joint, the welded joint having at least two brazing seams, the welded joint being obtained by the brazing method as described above, and the brazing filler metal formula of the welded joint being: in terms of mass fraction, 14.5% to 15.5% chromium, 1.1% to 1.6% boron, 7.0% to 7.5% silicon, 0% to 1% iron, 0% to 1% cobalt, and the remainder being nickel.

[0020] Preferably, the material of the weld joint is Inconel625 nickel-based alloy.

[0021] The beneficial effects of the present invention are:

[0022] The brazing method provided by the present invention particularly uses an amorphous foil brazing filler metal with a melting point of 1030-1130°C. Compared with the conventional brazing filler metal in the form of powder, paste or tape, the amorphous brazing foil is evenly distributed on the brazing joint surface and has a uniform thickness, which is extremely beneficial to the control of the brazing gap and can optimize the formation and performance of the brazing joint. The present invention is particularly suitable for the brazing of multi-layer welding joints.

[0023] Nickel-based amorphous brazing foil is a micron-sized foil strip mainly in an amorphous state formed by liquid metal splashing onto an extremely cold copper roller (target surface). At present, it is mainly used in the manufacture of high-frequency transformers, mutual inductors, and inductors by utilizing its excellent magnetic properties. Traditionally, non-metallic brittle elements such as B and Si are added to nickel-based metals to form a multi-phase structure of eutectic or hypoeutectic, and the alloy elements are unevenly distributed. However, since amorphous alloys do not have a crystalline structure, the distribution of alloy elements is uniform, the fluidity is good, and the bonding strength is high, so they can be used as a high-quality brazing material. The amorphous brazing foil used in the present invention is conducive to the uniform distribution of elements such as boron and silicon in the brazing seam, reduces the aggregation of borides and silicides, and is conducive to the diffusion of elements. More importantly, it does not contain an adhesive carrier, eliminates contamination residues, reduces gas escape in the brazing seam area, and reduces the generation of micro defects such as post-welding holes.

[0024] Compared with the commonly used Ni2 solder, the solder used in the present invention has a higher melting point and a smaller difference with the melting point of the base material. This smaller and more uniform gap control is conducive to capillary action, making the diffusion between the base material and the solder more sufficient, so that the brazed joint has better performance.

[0025] The present invention sets a brazing insulation temperature of 1150-1195° C. during the brazing process, which is matched with a insulation time of 30-50 min, thereby ensuring sufficient mutual diffusion of elements between the brazing material and the nickel-based alloy parent material and avoiding the melting corrosion of the nickel-based alloy parent material by the brazing diffusion-affected zone structure.

[0026] The present invention sets a solution treatment step after the brazing procedure, and adopts a solution treatment at 1100-1150°C, which is beneficial to the dispersion strengthening of the second phase particles in the alloy, eliminates the intermetallic compounds inside the brazing seam and the brazing diffusion affected zone (DAZ), and improves the distribution and morphology of brittle phases such as borides in the brazing seam. It can not only improve the mechanical properties of the brazed joint, such as tensile strength and shear strength; but also greatly improve the high temperature creep performance and endurance strength of the brazed joint, and increase the service life of the brazed joint.

[0027] Experiments show that the tensile strength of the alloy joint prepared by the method of the present invention can reach 342MPa at a high temperature of 700°C, and the high temperature creep resistance time under a stress of 150MPa at a service temperature of 650°C can reach 3000 h, indicating that the brazing method of the present invention has an excellent application effect.

[0028] The method of the present invention is a one-time brazing method with high welding efficiency, suitable for mass production, and is a stable and reliable high-temperature connection technology. It can be used as a reliable brazing method for multi-layer welded joints and can also be used as a special brazing method for Inconel 625 nickel-based alloy, and has great application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The microstructure diagram of the workpiece group obtained in step S3 of Example 1. In the figure, a is the overall microstructure diagram of the multi-layer brazing joint; b is the microstructure diagram of a randomly selected single-pass brazing seam; c is the SEM diagram of the single-pass brazing joint; d is the scanning microstructure diagram of the center surface of the position selected by the red frame in figure c.

[0030] Figure 2 for Figure 1 The distribution diagram of each element scanned by the center plane of the position selected by the red frame in Figure c.

[0031] Figure 3The microstructure diagram of the brazed workpiece obtained in step S4 of Example 1. In the figure, a is the overall microstructure diagram of the multi-layer brazed joint; b is the microstructure diagram of a randomly selected single-pass brazing seam; c is the SEM diagram of the single-pass brazed joint; d is the scanning microstructure diagram of the center plane of the position selected by the red frame in figure c.

[0032] Figure 4 for Figure 3 The distribution diagram of each element scanned by the center plane of the position selected by the red frame in Figure c.

[0033] Figure 5 This is a scanning diagram of the bonding rate of the tensile fracture (brazing surface) of Example 1 of the present invention.

[0034] Figure 6 The microstructure diagram of the welding joint obtained in Example 2 of the present invention, in which a is the overall microstructure diagram of the multi-layer brazing joint; b is the fiber structure diagram of a single brazing joint.

[0035] Figure 7 This is an EDS spectrum analysis diagram of the welding joint obtained in Example 2 of the present invention, where a is a SEM image of a single brazing joint; b is an EDS analysis diagram of the selected position (+ sign) in a.

[0036] Figure 8 3 is a microstructure diagram obtained in Example 3 of the present invention, wherein a is an overall microstructure diagram of a multi-layer brazing joint; and b is a microstructure diagram of a randomly selected single-pass brazing seam. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is described in more detail below in conjunction with the embodiments.

[0038] Unless otherwise specified, the terms used herein have the meanings commonly understood by those skilled in the art.

[0039] Example 1

[0040] A multi-layer welded joint is prepared according to the following steps:

[0041] S1. The selected solder is processed into an amorphous solder foil with a thickness of 40 μm. The formula of the solder is shown in Table 1.

[0042] Table 1 Chemical composition of solder (wt.%)

[0043] ;

[0044] S2. Select Inconel625 alloy 0.6 mm thin plate as the base material (composition see Table 2), process it into a single sheet of 250 mm long × 150 mm wide, and then clean it before welding: use manual or mechanical grinding to remove the oxide layer, then put it in anhydrous ethanol for ultrasonic cleaning for 15 min to remove surface oil and other impurities, rinse with water after cleaning and dry; stack

[0045] 8 pieces, with a layer of amorphous brazing foil from step S1 pre-placed between the bonding surfaces of every two pieces to form a workpiece to be welded;

[0046] Table 2 Chemical composition of base material (wt.%)

[0047] ;

[0048] S3. Assemble and position the workpiece to be welded, fix it between two frame plates, and use studs to pre-tighten and fix the frame plates; place the assembled workpiece to be welded in a vacuum brazing furnace, evacuate to below 0.001Pa, then heat it up to 600℃ at a rate of 10~20℃ / min, and keep it warm for 20 minutes for preheating; then heat it up to 900℃ at a rate of 10~15℃ / min, and keep it warm for 20 minutes.

[0049] The workpieces are then heated evenly for 20 minutes; finally, the temperature is raised to 1180°C at a rate of 5-8°C / min and kept at this temperature for 45 minutes for brazing; after welding, high-purity nitrogen or argon is used to cool the workpiece assembly to below 50°C at a cooling rate of 15-20°C / min before being taken out of the furnace.

[0050] S4. Place the workpiece group in a vacuum brazing furnace, evacuate to below 0.005Pa, then heat to the solution temperature of 1150℃ at a rate of 10~20℃ / min, keep warm for 150 min, then use high-purity nitrogen or argon to cool the workpiece group to 700℃ at a rate of 15℃ / min, and then cool to room temperature with the furnace at a cooling rate of 15℃~20℃ / min to obtain the brazed workpiece.

[0051] The microstructure of the workpiece group obtained in step S3 and the microstructure of the brazed workpiece after the solid solution treatment in step S4 were observed using a scanning electron microscope. The results are as follows: Figure 1-Figure 4 shown.

[0052] from Figure 1 As can be seen in Figure a, a high bonding rate brazing workpiece group was obtained by brazing with amorphous brazing foil. The 7 brazing seams are mainly solid solution structures, and there are intermittent strip-shaped compounds in the center of the brazing seams. When a single brazing seam is enlarged, it is obvious from Figure b that the compounds are mainly concentrated at the grain boundary position; the distribution of the main elements in the box in Figure c at the selected position is scanned, as shown in Figures e and Figure 2, nickel, chromium and other elements are obviously locally aggregated, and the location of the aggregation is consistent with the location of the compound in Figure d. The results of EDS analysis show that the compound is mainly boride.

[0053] from Figure 3 As can be seen in Figure a, after solution treatment, the 7 brazing seams are mainly solid solution structures, and no obvious compound aggregation is seen. After zooming in to see a single brazing seam, in Figure b, the distribution of the boride brittle phase is more discrete, uniform, small, and equiaxial in shape, and there is no obvious aggregation on the grain boundary. Further scanning of the distribution of the main elements in the box in Figure c at the selected position, see Figures d and Figure 4 No obvious aggregation was found. This shows that the solid solution treatment improves the distribution and morphology of brittle phases such as boride in the brazing seam and improves the mechanical properties of the brazed joint.

[0054] The performance tests were performed on the workpiece group obtained in step S3 and the brazed workpiece after the solid solution treatment in step S4, respectively, and three groups were used for the test in parallel.

[0055] 1. Carry out high temperature tensile test at 700℃, and the test method is carried out in accordance with GB / T228. The tensile fracture (brazing surface) bonding rate scanning diagram of the welded sample is shown in Figure 5 The depth scanning result of the fracture shows that the ratio of its effective bonding area to the cross-sectional area is 93.67%. When the amorphous brazing foil provided by the present invention is used for brazing, the actual bonding rate is above 93%.

[0056] The tensile strength results are shown in Table 3.

[0057] Table 3 High temperature tensile test strength at 700℃ (MPa)

[0058] ;

[0059] It can be seen from Table 3 that the tensile strength after solution treatment is slightly higher than that after welding, and the average value increases from 318.97MPa to 336.43MPa. This shows that after the entire brazing seam is solid-solution treated, the compound distribution is more uniform and the dispersion strengthening effect is stronger.

[0060] 2. At 650℃, 150Mpa stress was applied to carry out high temperature creep test. The test method was carried out in accordance with GB / T2039. The results are shown in Table 4.

[0061] Table 4 High temperature creep test (h)

[0062] ;

[0063] It can be seen that the creep rupture time of the brazed workpiece after solution treatment is significantly longer than that of the welded sample, which further indicates that the solution treatment improves the mechanical properties of the welded joint.

[0064] Example 2

[0065] On the basis of Example 1, the same base material and brazing material were used, the same brazing steps were adopted, and the brazing time was adjusted to 30 min. The brazed joint obtained was as follows: Figure 6-Figure 7 shown.

[0066] Figure 6 In the figure, Figure a is the overall microstructure of the multi-layer brazing joint; Figure b is the enlarged microstructure of a randomly selected single brazing seam. Figure 7 Figure a is the SEM image of a randomly selected single brazing seam; Figure b is the selected position (+ sign) in Figure a

[0067] EDS analysis diagram of the selected points. Figure 7 As shown in Table 5, the intermetallic compound has a high boron content. Therefore, although the intermetallic compounds in the brazed joint with a brazing time of 30 min are more concentrated and larger in size than those in the process of Example 1 (45 min), a well-bonded brazed joint can still be obtained.

[0068] Table 5 EDS analysis results of brazing seam structure

[0069] .

[0070] Example 3

[0071] In order to verify the advantages of using amorphous brazing foil, Example 3 was carried out using the same base material and the same brazing process as Example 1, but using a paste brazing filler metal. The effective ingredients of the paste brazing filler metal were consistent with those of the amorphous brazing foil. The obtained brazed joint and its microstructure were as follows: Figure 8 shown.

[0072] Figure 8 Figure a is the overall microstructure of the multi-layer brazing joint; Figure b is the fiber structure of a randomly selected single brazing seam after enlargement. It can be clearly seen that in the brazing joint obtained by using brazing paste for multi-layer brazing, the gap between the brazing seam plates is unevenly controlled, the proportion of cold welding is significantly increased, and there are un-escaped pores in the brazing seam. This shows that the brazing joint obtained by using amorphous brazing foil brazing material is better than that by using brazing paste brazing material under the brazing process selected by the present invention.

[0073] The above are only preferred practical examples of the present invention and are not intended to limit the invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, a person of ordinary skill in the art should understand that any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A brazing method for improving the high temperature welding performance of nickel-based alloy joints, characterized in that: The following steps are involved: S1. Processing the selected solder into an amorphous solder foil, wherein the thickness of the amorphous solder foil is between 30 and 50 μm, and the formula of the selected solder is: by mass fraction, 14.5% to 15.5% chromium, 1.1% to 1.43% boron, 7.0% to 7.5% silicon, 0% to 1% iron, 0% to 1% cobalt, and the balance is nickel; S2. Pre-place a layer of the amorphous brazing foil between the required bonding surfaces of the workpieces to be welded, the workpieces to be welded are multi-layer welded joints, and the material of the workpieces to be welded is Inconel625 nickel-based alloy; S3. Fix the workpiece to be welded, and then place it in a vacuum brazing furnace for brazing. After the brazing is completed, the workpiece group is cooled to below 50°C at a rate of 15°C ~ 20°C / min; S4. Place the workpiece group in a brazing furnace for heat preservation and solution treatment: first evacuate the brazing furnace to below 0.005Pa, then heat it to the solution temperature of 1100~1150℃ at a rate of 10~20℃ / min, keep it warm for 150 min, then use high-purity nitrogen or argon to cool the workpiece group to 700℃ at a rate of 15℃ / min, and then cool it to room temperature with the furnace at a rate of 15~20℃ / min.

2. A brazing method for improving high temperature welding performance of nickel-based alloy joints according to claim 1, characterized in that: The method also includes a pre-welding cleaning step, in which the surface of the workpiece to be welded is firstly polished to remove the oxide layer before pre-setting the amorphous brazing foil, and then the polished workpiece to be welded is ultrasonically cleaned in anhydrous ethanol, then rinsed with clean water, and dried for later use.

3. A brazing method for improving high temperature welding performance of nickel-based alloy joints according to claim 1, characterized in that: In step S3, the brazing procedure is to evacuate the vacuum brazing furnace to below 0.001 Pa, then heat it to 600°C at a rate of 10-20°C / min, and keep it warm for 20 min for preheating; then heat it to 900°C at a rate of 10-15°C / min, and keep it warm for 20 min for uniform heating; finally, heat it to 1150-1195°C at a rate of 5-8°C / min, and keep it warm for 30-50 min for brazing.

4. A brazing method for improving high temperature welding performance of nickel-based alloy joints according to claim 3, characterized in that: In step S3, the brazing temperature is 1180° C. and the temperature is kept at 45 min.

5. A brazing method for improving high temperature welding performance of nickel-based alloy joints according to claim 1, characterized in that: In step S3, the workpiece group is cooled using high-purity nitrogen or argon.

6. A nickel-based alloy multilayer welded joint, characterized in that: The weld joint has at least two brazing seams, and the weld joint is obtained by the brazing method described in any one of claims 1 to 5. The brazing material formula of the weld joint is: in terms of mass fraction, 14.5% to 15.5% chromium, 1.1 to 1.43% boron, 7.0 to 7.5% silicon, 0 to 1% iron, 0 to 1% cobalt, and the balance is nickel.

7. The nickel-based alloy multilayer welded joint according to claim 6, characterized in that: The material of the welding joint is Inconel625 nickel-based alloy.

Images