Method for joining dissimilar metals

Abstract

The present application belongs to the field of dissimilar metal connection, and particularly relates to a dissimilar metal connection method. The dissimilar metal connection method comprises: (1) roughening the surface of a high-hardness metal base to be connected to introduce surface protrusions on the surface to be connected; and then introducing dissimilar elements on the surface to be connected of the high-hardness metal by a chemical treatment method; (2) combining the high-hardness metal base and a low-hardness metal base, and then heating and pressurizing the combination in a vacuum or a protective atmosphere to connect the dissimilar metals together through the surface protrusions of the high-hardness metal and the in-situ self-generated compound particles. The protrusions on the surface layer of the high-hardness metal surface to be connected and the in-situ self-generated compound particles embedded in the low-hardness metal in the heating and pressurizing process realize the direct connection between the dissimilar metals with large differences in physical and chemical properties, and a high-strength and high-reliability dissimilar metal direct connection interface can be obtained.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of dissimilar metal connection, and particularly relates to a dissimilar metal connection method. BACKGROUND

[0002] The melting point (~3410℃), hardness (~3430HV), Young's modulus (~410GPa) and thermal conductivity (~173W×m -1 ×K -1 ) of tungsten are high, and the thermal expansion coefficient (~4.5×10 -6 K -1 ), sputtering rate, vapor pressure and tritium retention rate are low, so the performance advantages are obvious in the fields of high temperature resistance, ablation, erosion, corrosion, friction, etc., and the application is wide. The melting point (~1083℃), hardness (~350HV) and Young's modulus (~110GPa) of copper are low, the plasticity is good, the thermal expansion coefficient (~16.5×10 -6 K -1 ), thermal conductivity (~400W×m -1 ×K -1 ) and electrical conductivity (~57×10 6 S / m) are high, so the performance advantages are obvious in the fields of heat conduction, electrical conduction, plastic forming, etc., and the application is wide. Therefore, in the fields of plasma confinement of nuclear fusion reactors, super-high voltage electrodes, X-ray tube target materials, etc., in order to fully exert the performance advantages of tungsten and copper, the two are usually welded together to form a combined part for use. At this time, tungsten mainly undertakes the service conditions of high temperature, sputtering, erosion, friction, size stability, etc., and copper mainly undertakes the service conditions of heat conduction, electrical conduction, bearing, etc.

[0003] The performance differences of the melting point, hardness, Young's modulus, thermal expansion coefficient, etc. of tungsten and copper are huge, and the two do not react or solid solution at any temperature. Therefore, a filler layer or an adaptation layer (Cu, Nb, Ni, Ti, CuMn, AgCu, AuCu, FeW, WCu, etc. filler or adaptation layer materials used in the form of foil, film, plating layer, cladding layer, powder layer, etc.) is generally added between tungsten and copper, and the two are connected together by brazing, diffusion welding, etc.

[0004] However, after adding the filler layer or the adaptation layer between the tungsten and copper, the probability of the occurrence of brittle compounds, defects, failure positions, etc. in the welding interface area will be increased to a certain extent, and the risk of damage and failure of the connection surface will be increased. Therefore, it still has important theoretical significance and practical application value to study and develop high-reliability direct connection technology of dissimilar metals such as tungsten / copper. When tungsten and copper are directly connected, the two do not react or solid solution, and the interdiffusion of two kinds of atoms is extremely limited even at high temperature. Therefore, it is difficult to form a high-strength and high-reliability connection interface by relying only on the diffusion of dissimilar atoms to directly connect tungsten and copper. SUMMARY

[0005] The present application aims to provide a method for connecting dissimilar metals to solve the problem that it is difficult to form a high-strength and high-reliability connecting interface when directly connecting dissimilar metals such as tungsten / copper.

[0006] To achieve the above object, the technical solution adopted by the present application is as follows:

[0007] A method for connecting dissimilar metals, which connects a high-hardness metal base and a low-hardness metal base by the following steps:

[0008] (1) performing surface roughening treatment on the connecting surface of the high-hardness metal base to introduce surface protrusions on the connecting surface, and then introducing dissimilar elements including oxygen on the connecting surface of the high-hardness metal by chemical treatment;

[0009] (2) combining the high-hardness metal base and the low-hardness metal base obtained in step (1), and then heating and pressurizing under vacuum or protective atmosphere to connect the dissimilar metals together through the surface protrusions of the high-hardness metal and the in-situ self-generated compound particles.

[0010] The method for connecting dissimilar metals of the present application does not need to add a filler layer or an adaptation layer between dissimilar metals such as tungsten / copper, and does not rely on the mutual diffusion of dissimilar metal atoms; by embedding the protrusions on the surface layer of the connecting surface of the high-hardness metal and the in-situ self-generated compound particles into the low-hardness metal, the direct connection between dissimilar metals with large differences in physical and chemical properties and which do not react or do not solid-solve (or are difficult to react or solid-solve) at any temperature can be achieved, and a high-strength and high-reliability direct connecting interface of dissimilar metals can be obtained.

[0011] Preferably, the chemical treatment is a static treatment in an air environment below 400°C. Through the above chemical treatment, appropriate amounts of dissimilar elements such as oxygen can be introduced on the connecting surface of the high-hardness metal through slow oxidation and other chemical reactions of metal surface atoms.

[0012] Preferably, the static treatment is placed in a room temperature air environment for 24-48 hours, or in a 300-400°C air environment for 1-10 minutes. The in-situ reaction is spontaneously carried out at the connecting interface during the heating and pressurizing process. Through the above in-situ reaction process, oxide and other compound particles can be obtained, which are single crystals or polycrystals, have an equiaxed or long rod shape, and have a particle size of about 0.5-100 μm.

[0013] Preferably, the surface roughening treatment is surface grinding using a grinding tool, and the surface roughness R a of the high-hardness metal after grinding is 0.6-1.4 μm; and the surface roughness Ra The surface of the high-hardness metal layer is protruded. The protrusion height, shape, number, distribution and other morphological characteristics of the high-hardness metal surface layer can be adjusted and controlled by different mesh or different type and specification of grinding wheel grinding process; the high-hardness metal surface after grinding can have different surface roughness according to specific use requirements.

[0014] Preferably, the temperature of the heating and pressurization is 950-1000℃, and the pressure is 7.5-12.5MPa. The heating and pressurization under the above process parameters can effectively realize the direct connection between the dissimilar metals, and the average interface shear strength can reach more than 100MPa.

[0015] In step (2), the high-hardness metal base and the low-hardness metal base are combined to form a fitting part, which can be pressed by a specific tool or by the hydraulic system of the hot press furnace. The pressure can be applied before heating or after heating to a certain temperature. Preferably, the holding time at 950-1000℃ is 1-3h; the pressurization is at least performed when heated to 950-1000℃. Further preferably, the pressure is pressurized to 7.5-12.5MPa before the heating.

[0016] Preferably, the high-hardness metal and the low-hardness metal do not react or do not form solid solution, or are difficult to react or form solid solution. More preferably, the low-hardness metal is Cu, and the high-hardness metal is W. In this way, the direct connection between the dissimilar metals such as tungsten / copper, which have large differences in physical and chemical properties and do not react or form solid solution (or are difficult to react or form solid solution) with each other, can be realized without adding a filler layer or an adaptation layer material between the dissimilar metals such as tungsten / copper, and without relying on the mutual diffusion of dissimilar metal atoms.

[0017] To effectively reduce the internal stress of the connection interface, preferably, the furnace is cooled after heating, and the pressure is unloaded after furnace cooling.

[0018] In summary, the dissimilar metal connection method of the present application has the following characteristics:

[0019] (1) According to specific use requirements, the protrusion morphology of the high-hardness metal surface layer and the content of oxygen and other dissimilar elements can be controlled by pretreatment, combined with appropriate pressure, temperature, holding time and other hot pressing processes, to further control the strength and reliability of the dissimilar metal connection surface.

[0020] (2) When directly connecting dissimilar metals such as tungsten / copper, the dissimilar metal connection surface does not need to be polished or deoxygenated.

[0021] (3) It can realize direct connection between dissimilar metals such as tungsten and copper with large differences in physicochemical properties and which do not react or dissolve in each other (or are difficult to react or dissolve in each other), without the need to add a solder layer or adapter layer material between dissimilar metals such as tungsten and copper, and does not rely on the mutual diffusion of dissimilar metal atoms.

[0022] (4) The disclosed method can also be used for the connection of other dissimilar metal mating parts (Mo / Cu, Re / Cu, Ta / Cu, etc.) with similar physicochemical properties to the above-mentioned tungsten / copper, or has important reference value for the research and development of the latter.

[0023] (5) The method is simple, highly operable, low-cost, and easy to carry out industrial production. Attached Figure Description

[0024] Figure 1 This refers to the tungsten protrusion morphology in Embodiment 1 of the present invention, which is embedded in the copper substrate on the shearing surface of the metallic copper and broken off during the shearing process;

[0025] Figure 2 These are the groove marks left on the sheared surface of the metallic copper in Embodiment 1 of the present invention due to the scratching, friction, and pulling-out processes caused by the protrusions and oxide particles on the tungsten surface.

[0026] Figure 3 The morphology and distribution of in-situ self-generated oxide particles on the shear surface of tungsten metal in Example 1 of the present invention;

[0027] Figure 4 for Figure 3 Energy dispersive spectroscopy (EDS) analysis results of tungsten matrix;

[0028] Figure 5 for Figure 3 Energy dispersive spectroscopy (EDS) results for medium and large particles. Detailed Implementation

[0029] To achieve direct connection of dissimilar metals such as tungsten and copper without adding a solder layer or adapter layer and without relying on the mutual diffusion of dissimilar metal atoms, this invention proposes a dissimilar metal connection method that can obtain a direct dissimilar metal connection interface with high strength and reliability.

[0030] The technical concept of this invention is as follows:

[0031] Between the two metals to be joined, no intermediate transition layer is needed. Instead, the two metals are joined by direct mechanical interlocking formed by the surface protrusions of the high-hardness metal surfaces to be joined and the compound particles spontaneously generated in situ during hot pressing. The compound particles spontaneously generated in situ during hot pressing increase the position and degree of mechanical interlocking. Under load, they enhance the bonding strength and reliability of the interface through pull-out, scratching, and friction. See details... Figure 2 andFigure 3 Moreover, the generation of the compound particles does not require a complex processing procedure and includes the existence of a certain amount of heterogeneous elements such as oxygen on the surface of the high-hardness metal (W), thereby reducing the requirement for the purity of the surface of the metal to be connected.

[0032] The present application is to realize the direct connection between heterogeneous metals with large differences in physical and chemical properties and which do not react or are difficult to react and solid-solve (or react and solid-solve) at any temperature by embedding the protrusions on the surface layer of the high-hardness metal to be connected and the in-situ self-grown compound particles into the low-hardness metal, which can be implemented by the following technical solutions:

[0033] (1) The heterogeneous metals such as tungsten and copper are subjected to surface grinding, cleaning and pretreatment.

[0034] First, the surface of the metal is ground by using different types and specifications of diamond grinding wheels and other grinding tools to obtain a high-hardness metal substrate with different surface roughness and a low-hardness metal substrate with lower roughness, so that the surface layer of the high-hardness metal to be connected has a certain number and form of protrusions. The surface roughness of the low-hardness metal is as small as possible, generally Ra≈0.2-0.6 μm.

[0035] The height, shape, number and distribution of the protrusions on the surface of the high-hardness metal to be connected can be adjusted and controlled by grinding with different mesh or different types and specifications of grinding wheels (or sandpaper); the surface of the high-hardness metal after grinding can have different surface roughness according to specific use requirements. When the interface bonding strength is required to be high, the surface roughness of the high-hardness metal is generally Ra≈0.6-1.0 μm; when the interface bonding strength is required to be low, the surface roughness of the high-hardness metal is generally less than about 0.4 μm or Ra>1.2 μm. a a a

[0036] Then, the metal substrate after surface grinding is soaked and ultrasonically cleaned with solvents such as ethanol (or acetone), dilute hydrochloric acid and deionized water (or distilled water) to remove oil stains and impurities on the surface of the metal.

[0037] This step is to clean the surface with a solvent after surface roughening to remove oil stains and impurities on the surface of the metal and reduce the possibility of the existence of the above-mentioned substances which are not conducive to the interface bonding.

[0038] Finally, the high-hardness metal substrate is placed in an air furnace below 400°C for static treatment to appropriately increase the content of oxygen and other heterogeneous elements in the surface layer of the metal to be connected.

[0039] (2) The pretreated heterogeneous metals are combined, and the combined parts are placed in a heating furnace and subjected to a certain pressure.

[0040] ​​​The pre-processed tungsten, copper and other dissimilar metals are stacked together with their connecting surfaces in contact to form a tungsten / copper and other dissimilar metal assembly; a specific tool is used or the hydraulic system of the hot press is used to apply a pressure of 7.5-12.5 MPa to the assembly. Specifically, the pressure can be applied before heating or after heating to a certain temperature.

[0041] (3) The assembly is heated and kept warm, and after cooling in the furnace, the pressure is unloaded, and after subsequent treatments such as surface cleaning and machining, a dissimilar metal connecting piece is obtained.

[0042] The assembly is heated to 950-1000℃ at a heating rate of about 15℃ / min in a vacuum environment or under inert gas protection, and kept warm at the temperature for 1-3 hours; during the heating and pressing process, oxide and other compound particles are spontaneously generated in situ at the connecting interface of the dissimilar metals; after the warm keeping is completed, the heating is stopped, and the assembly is cooled to room temperature in the furnace; the pressure is unloaded, the assembly is taken out, and subsequent treatments such as surface cleaning and machining are performed to obtain a tungsten / copper and other dissimilar metal connecting piece.

[0043] The vacuum environment generally has a vacuum degree less than 1×10 -2 Pa. The inert gas (or protective atmosphere) can generally be selected from inert gases such as argon and helium, and the gas purity is greater than 99.9%, and the atmosphere pressure is 1×10 -2 Pa to 1×10 5 Pa.

[0044] The implementation process of the present application will be described in detail below in combination with specific examples. In the following examples, the experimental methods, raw materials, chemical reagents and the like, unless otherwise specified, are conventional methods or conventional materials and reagents, which can be purchased from the market.

[0045] First, the dissimilar metal connecting method of the present application is as follows:

[0046] Example 1

[0047] The dissimilar metal connecting method of the present application includes the following steps:

[0048] (1) A tungsten plate with a size of 45×45×2.5 mm is ground using a diamond grinding wheel, and the surface roughness R a of the W plate after grinding is about 0.9 μm. The ground tungsten plate and a hot-rolled copper plate with a size of 45×45×2.5 mm (R a ≈0.4-0.6 μm) are sequentially immersed in acetone, dilute hydrochloric acid (concentration of 5 wt%) and deionized water for 5 min each (for the purpose of removing oil, dirt and impurities); after drying, the copper plate is stored in a vacuum plastic bag, and the tungsten plate is placed in an air environment at 25℃ for 24 hours.

[0049] (2) The pretreated tungsten plate and copper plate are stacked together with the surfaces to be connected contacting each other to form a fitting; the non-connection surface of the fitting is brushed with hexagonal boron nitride powder, the fitting is placed on the lower press head of a hot press furnace, and graphite paper is placed between the fitting and the upper and lower press heads; and the fitting is subjected to a pressure of 12.5 MPa by the hydraulic system of the hot press furnace.

[0050] (3) After the furnace door is closed, the furnace chamber is vacuumed to a vacuum degree lower than 3x10 1 Pa, and then is filled with argon (purity 99.9%) to 1x10 5 Pa; the above vacuuming and argon filling operation is repeated once.

[0051] (4) The fitting is heated to 950°C at a heating rate of 15°C / min, and is kept for 3 hours; after the keeping, the heating power is turned off, and the fitting is cooled to room temperature with the furnace, and then the pressure is unloaded.

[0052] (5) The furnace door is opened, and the fitting is taken out and cleaned; different types of grinding wheels are used to grind the non-connection surface of the fitting to a thickness meeting the requirements; and then the fitting is cut into a required shape and size by wire cutting process, to obtain a tungsten / copper dissimilar metal direct connection fitting.

[0053] Example 2

[0054] The dissimilar metal connection method of the present example comprises the following steps:

[0055] (1) A tungsten plate with a size of 45x45x2.5 mm is ground by a diamond grinding wheel, and the surface roughness R a of the W plate after grinding is about 0.9 μm. The ground tungsten plate and a hot-rolled copper plate (same as in Example 1) with a size of 45x45x2.5 mm are sequentially immersed in ethanol, dilute hydrochloric acid (concentration 5 wt%) and deionized water, and are ultrasonically cleaned for 10 min each; after drying, the copper plate is stored by vacuum packaging, and the tungsten plate is placed in an air environment at 25°C for 48 hours.

[0056] (2) The pretreated tungsten plate and copper plate are stacked together with the surfaces to be connected contacting each other to form a fitting; the non-connection surface of the fitting is brushed with hexagonal boron nitride powder, the fitting is placed on the lower press head of a hot press furnace, and graphite paper is placed between the fitting and the upper and lower press heads; and the fitting is subjected to a pressure of 12.5 MPa by the hydraulic system of the hot press furnace.

[0057] (3) After the furnace door is closed, the furnace chamber is vacuumed to a vacuum degree lower than 3x10 1 Pa, and then is filled with argon (purity 99.9%) to 1x10 5 Pa; the above vacuuming and argon filling operation is repeated once.

[0058] (4) heating the matched piece to 950℃ at a heating rate of 15℃ / min, holding for 3 hours; after the holding, turning off the heating power and waiting for the matched piece to cool down to room temperature with the furnace, and then unloading the pressure.

[0059] (5) opening the furnace door, taking out the matched piece, cleaning the surface of the matched piece, grinding the non-connecting surface of the matched piece to the required thickness by using different types of grinding wheels, and then cutting the matched piece into the required shape and size by using the wire cutting process, thereby obtaining the tungsten / copper dissimilar metal direct connecting piece.

[0060] Example 3

[0061] The dissimilar metal connecting method of the present example is basically the same as that of Example 2, except that in step (2), the matched piece is subjected to a pressure of 7.5 MPa by the hydraulic system of the hot-pressing furnace.

[0062] Example 4

[0063] The dissimilar metal connecting method of the present example is basically the same as that of Example 2, except that in step (4), the holding time of the W / Cu matched piece at the target temperature is 2 hours.

[0064] Example 5

[0065] The dissimilar metal connecting method of the present example is basically the same as that of Example 2, except that in step (4), the holding time of the W / Cu matched piece at the target temperature is 1 hour.

[0066] Example 6

[0067] The dissimilar metal connecting method of the present example is basically the same as that of Example 1, except that in step (1), the surface roughness R a ≈0.6 μm of the W plate after grinding.

[0068] Example 7

[0069] The dissimilar metal connecting method of the present example is basically the same as that of Example 1, except that in step (1), the surface roughness R a ≈1.4 μm of the W plate after grinding.

[0070] Example 8

[0071] The dissimilar metal connecting method of the present example is basically the same as that of Example 1, except that in step (1), the W plate is subjected to holding for 3 min in an air furnace at 400℃.

[0072] II. Experimental Examples

[0073] Experimental Example 1 Interface Shear Strength

[0074] The average interface shear strength of the tungsten / copper dissimilar metal connections of Examples 1-8 was detected in this experimental example. The method used in the detection was as follows: 6 shear strength test samples were cut from the W / Cu composite plate after connection by wire cutting process, the length of the samples was about 5 mm, and the width was about 14.7 mm. The 6 test samples were cleaned with anhydrous ethanol, and the size was accurately measured before being placed on the fixture in turn. The Cu plate side of the test sample was supported by a steel fixture, and the W plate side was suspended. A steel plate-shaped pressure head with the same thickness and length as the W plate was used to apply pressure to the W plate. The maximum pressure load was recorded through the compression load-displacement curve, and the pressure was stopped after the W plate and the Cu plate were completely separated. The interface shear strength of the test sample = maximum pressure load / shear area, and the interface shear strength of the W / Cu composite plate was equal to the average value of the shear strength of the 6 test samples. The test results are shown in Table 1.

[0075] Table 1 Average interface shear strength of tungsten / copper dissimilar metal connections obtained in each example

[0076] Example number Average interfacial shear strength, MPa Example 1 131.8 Example 2 119.8 Example 3 114.5 Example 4 104.0 Example 5 96.1 Example 6 129.1 Example 7 82.6 Example 8 68.7

[0077] As can be seen from the test results in Table 1, the average interface shear strength of the tungsten / copper dissimilar metal connections obtained in Examples 1, 2, 3, 4 and 6 all reached more than 100 MPa, and had high interface connection reliability.

[0078] Interface bonding morphology analysis of experimental example 2

[0079] The morphology of the copper shear surface of the tungsten / copper dissimilar metal connection obtained in Example 1 after shear failure is shown in Figure 1 (backscattered electron image) and Figure 2 (second electron image). Figure 1 The tungsten surface layer protrusion morphology embedded in the copper matrix and broken by shear is shown. Figure 2 The grooves left by the tungsten surface layer protrusion and the oxide particles broken and pulled out are shown.

[0080] The morphology of the tungsten shear surface of the tungsten / copper dissimilar metal connection obtained in Example 1 after shear failure is shown in Figure 3 (second electron image). Figure 3 The size, morphology and distribution of the oxide particles generated in situ during the hot-pressing connection process of the tungsten connection surface layer are shown. Figure 4 and Figure 5 The energy spectrum analysis results shown in Figure 3 were collected from the tungsten matrix and oxide particles shown in Figure 3The large, spontaneously generated particles in situ contain both tungsten and a significant amount of oxygen, suggesting they are tungsten oxide particles, while the tungsten matrix contains only metallic tungsten. This indicates that the oxygen introduced into the surface of the high-hardness tungsten plate through chemical treatment spontaneously generates tungsten oxide grains at the interface to be joined through diffusion, migration, solid solution, and reaction during the heating and pressurization process. These grains gradually grow to form large-sized tungsten oxide particles.

[0081] in addition, Figure 3 The size, shape, and quantity of the numerous regular single-crystal tungsten oxide particles present on the W substrate are related to... Figure 2 The size, shape, number, and other morphological features of the grooves etched on the Cu substrate shown are very well matched, therefore Figure 2 The numerous regularly etched trenches on the Cu substrate shown are clearly formed by… Figure 3 The large tungsten oxide particles shown are the result of this process. During the scratching, pulling out, and friction of single-crystal tungsten oxide particles, a significant amount of external force is inevitably consumed. Therefore, the large-sized compound particles spontaneously generated in situ play a crucial role in enhancing the interfacial shear strength.

[0082] From the above Figures 1-5 Analysis shows that the protrusions on the surfaces to be joined by the hard metal (W) and the spontaneously generated oxide particles in situ during hot pressing effectively improve the interfacial connection between dissimilar metals, thereby increasing the connection strength and reliability between them.

Claims

1. A method of joining dissimilar metals, characterized by, A method for joining a high hardness metal substrate W and a low hardness metal substrate Cu includes the following steps: (1) roughening the surface of the high hardness metal substrate to be joined to introduce surface protrusions, and then introducing a heterogeneous element on the surface of the high hardness metal substrate to be joined by a chemical treatment method, the heterogeneous element including oxygen; the chemical treatment method is a static treatment in an air environment below 400℃; The surface roughening treatment is surface grinding using a grinding tool, and the surface roughness R a is 0.6 to 1.4 μm; Surface roughness R of the low-hardness metal surface to be connected a Not higher than 0.6 μm; (2) combining the high hardness metal substrate and the low hardness metal substrate obtained in step (1), and then heating and pressurizing at 950-1000℃ in a vacuum or a protective atmosphere to join the heterogeneous metals together through the high hardness metal surface protrusions and in-situ self-generated compound particles; the pressure of the heating and pressurizing is 7.5-12.5MPa; the in-situ self-generated compound particles are oxide particles generated in-situ spontaneously during the heating and pressurizing.

2. The method of joining dissimilar metals according to claim 1, wherein The static treatment is placed in a room temperature air environment for 24-48h.

3. The method of joining dissimilar metals of claim 1, wherein The static treatment is placed in a 300-400℃ air environment for 1-10min.

4. The method of joining dissimilar metals of claim 1, wherein The surface roughening treatment is surface grinding using a grinding tool, and the surface roughness R a is 0.6 to 1.0 μm; Surface roughness R of the low-hardness metal surface to be connected a is 0.2 to 0.6 μm.

5. The method of joining dissimilar metals of claim 1, wherein The holding time at 950-1000℃ is 1-3h; the pressurizing is performed at least when heated to 950-1000℃.

6. The method of joining dissimilar metals of claim 5 wherein, The pressurizing to 7.5-12.5MPa is performed before the heating.

7. The method of joining dissimilar metals of claim 4, wherein Surface roughness R of the high hardness metal after polishing a is 0.6 to 0.9 μm.

8. The method of joining dissimilar metals according to any one of claims 1 to 7, wherein After the heating is completed, furnace cooling is performed, and the pressure is unloaded after the furnace cooling.

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