Metallographic corrosion liquid and metallographic corrosion method for dissimilar metal welded joint

Abstract

The invention relates to a metallographic etchant for a dissimilar metal welded joint and a metallographic etchant method. The metallographic etchant comprises a metallographic etchant A and a metallographic etchant B, the metallographic etchant A comprises the following components: ferric trichloride, perchloric acid and water; the metallographic etchant B comprises the following components: nitric acid, glacial acetic acid and ethanol; the metallographic phase corrosive liquid A is used for corroding the oxygen-free copper side of a Kovar alloy and oxygen-free copper welded joint, and the metallographic phase corrosive liquid B is used for corroding the Kovar alloy side of the Kovar alloy and oxygen-free copper welded joint. The components of the metallographic etchant are improved, so that one metallographic etchant only reacts with metal on one side, and double-liquid directional corrosion is realized. According to the step-by-step abrasion method, it is ensured that primary battery reaction is not formed in the corrosion process of the two kinds of metal based on the corrosion electrochemical principle, when the metal on one side is corroded, the metal on the other side is not corroded, the metallographic interface structure morphology can be clearly observed, the preparation quality of the sample is remarkably improved, and the preparation cost is reduced. And a high-precision microscopic characterization basis is provided for process optimization.

Description

Technical Field

[0001] The present invention belongs to the technical field of microscopic analysis of metal materials, and particularly relates to a metallographic etching solution and a metallographic etching method for dissimilar metal welded joints. Background Art

[0002] Kovar alloy is widely used in the fields of electronic packaging, aerospace, etc. due to its low coefficient of thermal expansion, good welding performance, and excellent airtightness. The composite material welded by Kovar alloy and oxygen-free copper is mainly used for the transition welding and packaging of low coefficient of thermal expansion materials, and is applied to the packaging bases of semiconductor power modules and high-frequency microwave components. To guide the research and development of Kovar alloy / oxygen-free copper composite materials, it is necessary to study the metallographic morphology of the composite materials. Metallographic morphology analysis can understand the fusion line continuity of Kovar alloy / oxygen-free copper welded joints, evaluate whether the bimetal is welded, whether the Kovar alloy generates phase change microstructure characteristics, and detect defects such as pores and inclusions therein, which are the key steps to improve the welding process.

[0003] In the existing technology, the metallographic etching solutions for Kovar alloy are mostly nitric acid alcohol solution, nitric acid, alcohol mixture, aqua regia, etc., and the metallographic etching solutions for oxygen-free copper are mostly aqua regia solution, ammonia water solution, etc. Although these etching solutions can reveal the microstructure, the traditional etching solutions have technical bottlenecks.

[0004] Because the activities of the two metals are different, a primary battery reaction is easily formed, and galvanic corrosion causes the interface to be distorted. For example, when 5% nitric acid alcohol solution is used for Kovar alloy / oxygen-free copper joints, due to the difference in standard electrode potentials between the two (4J29: -0.25V vs SHE, Tu1: +0.34V), a primary battery with a potential difference of 0.59V will be formed at the contact interface, resulting in the accelerated corrosion of oxygen-free copper as the cathode, while the anodic region of Kovar alloy only corrodes at 0.3μm / min, causing the interface morphology to be distorted. Corrosion with double etching solutions also has certain limitations. Different etching solutions are likely to cause excessive corrosion of the metal on the side with stronger activity, a decrease in grain boundary recognition, and an impact on the metallographic quality. In addition, the strong acidity of the metallographic etching solution easily passivates the surface of the Kovar alloy, making it difficult to show the grain boundaries. So far, there has been no report on a special metallographic etching solution and a step-by-step etching method for Kovar alloy / oxygen-free copper dissimilar metal welded joints. Summary of the Invention

[0005] Aiming at the above technical problems, the present invention provides a metallographic etching solution and a metallographic etching method for dissimilar metal welded joints to solve problems such as over-corrosion of the interface, blurred grain boundaries, and poor operation consistency caused by the difference in electrochemical activity of traditional single metallographic etching solutions. Through the dual-liquid directional etching mechanism and the synergistic corrosion inhibition system, high-precision characterization of the grain structure at the welding interface is achieved, providing a reliable basis for process optimization.

[0006] Note that the description of these objectives does not preclude the existence of other objectives. One embodiment of the present invention does not need to achieve all of the above objectives. Objectives other than the above can be extracted from the descriptions in the specification, drawings, and claims.

[0007] The present invention achieves the above technical objectives through the following technical means.

[0008] In the present invention, by regulating the composition of the metallographic etching solution, the metallographic etching solution A only reacts with oxygen-free copper, and the metallographic etching solution B selectively etches kovar alloy. The present invention mixes glacial acetic acid into the solution to balance the acidity while reducing the corrosiveness. The present invention blocks the bimetallic electrical contact through stepwise etching, eliminates the galvanic current, and avoids the occurrence of a primary battery reaction between the metallographic etching solution and oxygen-free copper. For the metallographic etching of oxygen-free copper, the present invention uses a ferric chloride-perchloric acid solution. The chloride ions provided by perchloric acid can indirectly achieve the corrosion effect of aqua regia, and due to the low concentration and weak acidity, the corrosiveness is not strong and it is not easy to cause over-etching. The synergistic effect of the above several components enables the metallographic etching solution to have an excellent corrosion effect on the welded joint sample of kovar alloy / oxygen-free copper material.

[0009] A metallographic etching solution for a dissimilar metal welded joint, comprising a metallographic etching solution A and a metallographic etching solution B;

[0010] The metallographic etching solution A comprises the following components: ferric chloride, perchloric acid, and water;

[0011] The metallographic etching solution B comprises the following components: nitric acid, glacial acetic acid, and ethanol;

[0012] The metallographic etching solution A is used for the corrosion of the oxygen-free copper side of the kovar alloy and oxygen-free copper welded joint, and the metallographic etching solution B is used for the corrosion of the kovar alloy side of the kovar alloy and oxygen-free copper welded joint.

[0013] In the above solution, the metallographic etching solution A comprises the following components by volume percentage: 25%-35% of an aqueous ferric chloride solution, 25%-35% of a perchloric acid solution, and the balance is water;

[0014] The concentration of the aqueous ferric chloride solution is 1%; the concentration of the perchloric acid solution is 70%.

[0015] Further, the volume ratio of the aqueous ferric chloride solution to the perchloric acid solution in the metallographic etching solution A is 1:0.8 to 1:1.2.

[0016] In the above solution, the pH value of the metallographic etching solution A is 1.5 to 2.5.

[0017] In the above solution, the metallographic etching solution B comprises the following components by volume percentage: 25%-35% of nitric acid, 20%-40% of glacial acetic acid, and the balance is ethanol;

[0018] The concentration of the nitric acid is 4%; the concentration of the glacial acetic acid is 100%.

[0019] Furthermore, the volume ratio of nitric acid to glacial acetic acid in the metallographic etching solution B is 1:0.5 to 1:1.5;

[0020] In the above scheme, the pH value of the metallographic etching solution B is 2.0-3.0.

[0021] In the above scheme, the Kovar alloy is 4J29 alloy; the oxygen-free copper is Tu1 copper; and the welded joint of the Kovar alloy and the oxygen-free copper is a vacuum-sealed welded structure of an electronic packaging device or an aerospace device.

[0022] A metallographic etching method for a dissimilar metal welded joint based on the metallographic etching liquid for dissimilar metal welded joints comprises the following steps:

[0023] Pretreatment: The metallographic specimens of the welded joint of Kovar alloy and oxygen-free copper are subjected to wire cutting, inlaying, rough grinding, fine grinding, polishing, ultrasonic cleaning, alcohol washing and drying in sequence to obtain the welded joint of Kovar alloy and oxygen-free copper after pretreatment;

[0024] Prepare metallographic etching solution: prepare metallographic etching solution A and metallographic etching solution B respectively;

[0025] Step-by-step etching: using the metallographic etching liquid A to wipe the oxygen-free copper side of the pretreated Kovar alloy and oxygen-free copper welding joint until the surface turns dark yellow, and the etching is finished; using the metallographic etching liquid B to wipe the Kovar alloy side of the pretreated Kovar alloy and oxygen-free copper welding joint until the surface turns silver-gray, and the etching is finished;

[0026] Post-treatment: The samples of the welded joints of Kovar alloy and oxygen-free copper were cleaned with deionized water and ethanol in turn and dried.

[0027] In the above scheme, the ultrasonic cleaning in the pretreatment step is washing with alcohol in an ultrasonic cleaning machine.

[0028] In the above scheme, the preparation of the metallographic etching solution A in the step of preparing the metallographic etching solution comprises the following steps:

[0029] Put ferric chloride into a container filled with deionized water and stir thoroughly until the ferric chloride is dissolved to obtain a ferric chloride aqueous solution;

[0030] Then, the ferric chloride aqueous solution and the perchloric acid solution are mixed and stirred thoroughly;

[0031] Let it stand to obtain metallographic etching solution A and bottle it;

[0032] The preparation of metallographic etching solution B includes the following steps:

[0033] Add the nitric acid solution to the ethanol solution to obtain a mixed solution, and an esterification reaction occurs to produce ethyl nitrate;

[0034] Then mix the glacial acetic acid solution with the mixed solution and stir well;

[0035] Let it stand, obtain the metallographic etching solution B and bottle it.

[0036] Furthermore, in the step of preparing the metallographic etching solution, the preparation of the metallographic etching solution A is to measure ferric chloride, perchloric acid, and water to prepare the metallographic etching solution A, which specifically includes the following steps:

[0037] Put ferric chloride into a container filled with deionized water and stir well until the ferric chloride is dissolved to obtain a 1% aqueous solution of ferric chloride;

[0038] Then mix the 1% aqueous solution of ferric chloride with the 70% perchloric acid solution and stir well;

[0039] Let it stand, obtain the metallographic etching solution A and bottle it; preferably, the standing time is after 30 min;

[0040] The preparation of the metallographic etching solution B is to measure nitric acid, glacial acetic acid, and ethanol to prepare the metallographic etching solution B, which specifically includes the following steps:

[0041] Add the 4% nitric acid solution to the ethanol solution to obtain a mixed solution, and an esterification reaction occurs to produce ethyl nitrate;

[0042] Then mix the 100% glacial acetic acid solution with the mixed solution and stir well;

[0043] Let it stand, obtain the metallographic etching solution B and bottle it. Preferably, the standing time is after 30 min.

[0044] Furthermore, the steps of step-by-step etching and post-treatment are specifically as follows: Lay the kovar alloy and oxygen-free copper welded joint after pretreatment flat, use cotton dipped in the prepared metallographic etching solution A to wipe the oxygen-free copper side until the surface turns dark yellow, then rinse the wiped metal with deionized water and clean it with an ethanol solution; use another group of cotton dipped in the prepared metallographic etching solution B to wipe the kovar alloy side until the surface turns silver-gray, and rinse the kovar alloy and oxygen-free copper welded joint with deionized water and ethanol in sequence and dry it, then a metallographic specimen with a clear surface can be obtained.

[0045] Furthermore, the drying can be carried out by blowing with an electric hair dryer to avoid oxidation of the metal joint.

[0046] Compared with the prior art, the beneficial effects of the present invention are:

[0047] 1. The present invention improves the composition of the metallographic etching solution, enabling a metallographic etching solution to react only with one side of the metal, thereby achieving dual-liquid directional etching.

[0048] 2. In the metallographic etching solution A of the present invention, perchloric acid is used to replace hydrochloric acid in the original ferric perchloride solution, achieving mild etching while avoiding the passivation reaction of the metal.

[0049] 3. The present invention realizes a synergistic corrosion inhibition system. In the metallographic etching solution B, nitric acid alcohol plus glacial acetic acid is used to replace the original nitric acid alcohol. Glacial acetic acid serves as a pH buffer, inhibiting the violent esterification reaction of ethyl nitrate. While stabilizing the corrosion rate, it enhances the corrosion effect, making the contrast between the interior and grain boundaries of the bimetal grains obvious and the metallographic quality higher.

[0050] 4. The step-by-step rubbing etching method of the present invention ensures that no primary battery reaction occurs between the two metals during the corrosion process based on the principle of corrosion electrochemistry. When etching one side of the metal, the other side of the metal is not etched, enabling clear observation of the interfacial microstructure of the metallography and guiding subsequent production improvements.

[0051] Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not necessarily have all of the above effects. Effects other than the above can be obviously seen and extracted from the descriptions in the specification, drawings, claims, etc. Description of the Drawings

[0052] Figure 1 Metallographic photograph of Example 1 using the metallographic etching solution with the minimum concentration;

[0053] Figure 2 Metallographic photograph of Example 2 using the metallographic etching solution with the maximum concentration;

[0054] Figure 3 Metallographic photograph of Comparative Example 1 using aqua regia solution and ferric perchloride solution for corrosion;

[0055] Figure 4 Metallographic photograph of Comparative Example 2 using nitric acid alcohol and ferric perchloride solution for corrosion. Detailed Description of the Embodiments

[0056] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following detailed description of the specific embodiments of the present invention will be given in conjunction with the drawings. Without conflict, the features in the embodiments of the present invention can be combined with each other. In addition, in the preparation processes in the following embodiments, if not otherwise specified, they are all conventional means in the prior art, so they will not be described in detail; all raw materials used in the following embodiments are commercially available products.

[0057] Example 1

[0058] A metallographic etching solution for a kovar alloy and oxygen-free copper welded joint, comprising a metallographic etching solution A and a metallographic etching solution B;

[0059] The metallographic etching solution A comprises the following components: ferric chloride, perchloric acid and water;

[0060] The metallographic etching solution B comprises the following components: nitric acid, glacial acetic acid and ethanol;

[0061] The metallographic etching solution A is used for etching the oxygen-free copper side of the kovar alloy and oxygen-free copper welded joint, and the metallographic etching solution B is used for etching the kovar alloy side of the kovar alloy and oxygen-free copper welded joint.

[0062] The components of the metallographic etching solution A include: 25 mL of an aqueous solution of ferric chloride with a concentration of 1%, 25 mL of a perchloric acid solution with a concentration of 70%, and 50 mL of deionized water;

[0063] The components of the metallographic etching solution B include: 25 mL of nitric acid with a concentration of 4%, 20 mL of glacial acetic acid with a concentration of 100%, and 55 mL of an ethanol solution with a purity of 99%;

[0064] The glacial acetic acid is of analytical purity.

[0065] The pH value of the metallographic etching solution A is 1.5.

[0066] The pH value of the metallographic etching solution B is 2.5.

[0067] The kovar alloy grade is 4J29, and the oxygen-free copper grade is Tu1

[0068] The kovar alloy and oxygen-free copper welded joint is a vacuum sealing welding structure of an electronic packaging device or an aerospace device.

[0069] A metallographic etching method for a kovar alloy and oxygen-free copper welded joint based on the metallographic etching solution for the kovar alloy and oxygen-free copper welded joint, comprising the following steps:

[0070] Pretreatment: The kovar alloy and oxygen-free copper welded joint specimen is successively subjected to wire cutting, embedding, rough grinding, fine grinding, polishing, ultrasonic cleaning and drying;

[0071] Prepare the metallographic etching solution: Prepare the metallographic etching solution A and the metallographic etching solution B respectively;

[0072] Step-by-step etching: Wipe the kovar alloy side with the metallographic etching solution B until the surface becomes silver-gray; wipe the oxygen-free copper side with the metallographic etching solution A until the surface becomes dark yellow;

[0073] Post-treatment: Wash the kovar alloy and oxygen-free copper welded joint specimen with deionized water and ethanol in sequence and dry.

[0074] The preparation of the metallographic etching solution A includes the following steps:

[0075] Put 2.5 g of ferric chloride into a container containing 25 mL of deionized water and stir well until the ferric chloride dissolves to obtain a 1% aqueous solution of ferric chloride.

[0076] Then mix the 1% aqueous solution of ferric chloride with a 70% perchloric acid solution and stir well until evenly mixed.

[0077] After standing for 30 min, the metallographic etching solution A is obtained and bottled.

[0078] The preparation of the metallographic etching solution B includes the following steps:

[0079] Add a 4% nitric acid solution to a 99% ethanol solution to obtain a mixed solution, and an esterification reaction occurs to produce ethyl nitrate.

[0080] Then mix a 100% glacial acetic acid solution with the mixed solution and stir well until evenly mixed.

[0081] After standing for 30 min, the metallographic etching solution B is obtained and bottled.

[0082] Specifically, wire-cut the kovar alloy and oxygen-free copper welded joint with a cutting speed of 1 mm / min and a size of 10*10*5 mm. Use epoxy resin for inlaying, and the ratio of epoxy resin to curing agent is 3:1, that is, 3 parts of epoxy resin are mixed with 1 part of curing agent, and stand at room temperature for 2 h until it cures. Mechanical polishing is carried out on a grinding and polishing machine. Select 600# - 2000# sandpaper to process the sample, and grind the sample from coarse to fine in sequence until the surface is smooth and there are no obvious scratches. Add a diamond suspension with a particle size of 2.5 μm, and use a flannel polishing cloth to process the sample. Add a diamond suspension with a particle size of 1.0 μm, and use a flannel polishing cloth to process the sample until a mirror effect is achieved. Use 90% ethanol for fine cleaning in an ultrasonic cleaning machine, then rinse with ethanol, and dry the sample with a hair dryer or drying oven.

[0083] Lay the pretreated kovar alloy and oxygen-free copper welded joint flat, dip a cotton ball in the prepared metallographic etching solution A and wipe the oxygen-free copper side for 1 min until the surface turns dark yellow, then rinse the wiped metal with deionized water and clean it with an ethanol solution; dip another group of cotton balls in the prepared metallographic etching solution B and wipe the kovar alloy side for 1 min until the surface turns silver gray. Rinse the kovar alloy and oxygen-free copper welded joint with deionized water and ethanol in sequence and dry it with a hair dryer to avoid oxidation of the metal joint, and then a metallographic sample with a clear surface can be obtained.

[0084] The metallographic etching solution prepared in Example 1 is a low-concentration metallographic etching solution. Figure 1Figure of the metallographic microstructure of the Kovar alloy and oxygen-free copper welded joint after being etched by the metallographic etching solution in Example 1. It can be seen that both the Kovar alloy side and the oxygen-free copper side are etched well, without incomplete etching or over-etching, and there are no corrosion pits on the metal surface. The low-concentration metallographic etching solution can achieve precise etching of the interface.

[0085] Example 2

[0086] The difference between this example and Example 1 is that the components of the metallographic etching solution A are: 35 mL of an aqueous solution of ferric chloride with a concentration of 1%, 35 mL of perchloric acid with a concentration of 70%, and 30 mL of deionized water; the components of the metallographic etching solution B are: 35 mL of nitric acid with a concentration of 4%, 40 mL of glacial acetic acid with a concentration of 100%, and 25 mL of ethanol with a purity of 99%, and the glacial acetic acid is of analytical purity.

[0087] The metallographic etching solution prepared in Example 2 is a high-concentration metallographic etching solution. Figure 2 Figure of the metallographic microstructure of the Kovar alloy / oxygen-free copper welded joint specimen after being etched by the metallographic etching solution in Example 2. It can be seen that both the Kovar alloy side and the oxygen-free copper side of the fusion line are etched well, without incomplete etching or over-etching, and the high-concentration metallographic etching solution significantly enhances the grain boundary contrast.

[0088] Comparative Example 1

[0089] Etch the Kovar alloy with aqua regia prepared by mixing 1 part of nitric acid with a concentration of 68% and 3 parts of hydrochloric acid with a concentration of 38%, and etch the oxygen-free copper with a ferric perchloride solution with a concentration of 1%. The figure of the metallographic microstructure obtained is shown in Figure 3 . The remaining steps are the same as in Example 1. It can be seen from Figure 3 that serious over-etching occurs, and parts such as the fusion line are difficult to identify. This is because when aqua regia etches oxygen-free copper, due to the too high Cl- concentration, reaching 6.5 mol / L, intergranular corrosion is triggered, and corrosion grooves are formed at the grain boundaries, resulting in the failure of the metallography.

[0090] Comparative Example 2

[0091] Etch the Kovar alloy with a nitric acid alcohol solution with a concentration of 4%, and etch the oxygen-free copper with a ferric perchloride solution with a concentration of 1% to etch the Kovar alloy and oxygen-free copper welded joint. The figure of the metallographic microstructure obtained is shown in Figure 4 . The remaining steps are the same as in Example 1. It can be seen that it only acts on the oxygen-free copper side, and the metal passivation phenomenon occurs on the surface of the Kovar alloy due to the nitric acid alcohol solution, making it difficult to further etch.

[0092] In view of the problems in the prior art such as galvanic cell reaction, uncontrollable corrosion rate, over-corrosion at the interface, and poor metallographic consistency caused by the electrochemical activity difference between kovar alloy and oxygen-free copper, a dual-etchant directional etching system is adopted. The metallographic etchant A includes the following components by volume percentage: 25% - 35% of an aqueous solution of ferric chloride with a concentration of 1%, 25% - 35% of a perchloric acid solution with a concentration of 70%, and the balance is water. It replaces traditional aqua regia through mild oxidation by chloride ions and is dedicated to the corrosion of the oxygen-free copper side. The metallographic etchant B includes the following components by volume percentage: 25% - 35% of nitric acid with a concentration of 4%, 20% - 40% of glacial acetic acid with a concentration of 100%, and the balance is ethanol. Glacial acetic acid buffers the pH value to 2.0 - 3.0, inhibits passivation and stabilizes the corrosion rate at 0.8 microns per minute, and is dedicated to the corrosion of the kovar alloy side. Through the step-by-step rubbing method, two independent wiping processes are used to directionally etch the kovar alloy and oxygen-free copper respectively, avoiding the galvanic effect when the two metals are in contact. For the kovar alloy and oxygen-free copper welded joints etched by the metallographic etchant of the present invention, in the metallographic photos, the grain boundaries and grain contrast are clear, and there is no over-corrosion or passivation phenomenon at the interface. It is applicable to the metallographic analysis of the welded joints of 4J29 kovar alloy and Tu1 oxygen-free copper, providing a reliable basis for process optimization. The present invention significantly improves the preparation quality of metallographic specimens and is applicable to the analysis of the fusion line morphology, phase transformation layer, and defects of dissimilar welded joints in semiconductor packaging and aerospace devices, providing a basis for high-precision microscopic characterization for process optimization.

[0093] It should be understood that although this specification is described according to various embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0094] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present invention, and they are not intended to limit the protection scope of the present invention. Any equivalent embodiments or modifications made without departing from the technical spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A metallographic etching solution for dissimilar metal welded joints, characterized in that, It includes metallographic etching solution A and metallographic etching solution B; The metallographic etching solution A includes the following components: ferric chloride, perchloric acid and water; The metallographic etching solution B includes the following components: nitric acid, glacial acetic acid and ethanol; The metallographic etching solution A is used for etching the oxygen-free copper side of the kovar alloy and oxygen-free copper welded joint, and the metallographic etching solution B is used for etching the kovar alloy side of the kovar alloy and oxygen-free copper welded joint.

2. The metallographic etching solution for dissimilar metal welded joints according to claim 1, characterized in that, The metallographic etching solution A includes the following components by volume percentage: 25%-35% ferric chloride aqueous solution, 25%-35% perchloric acid solution, and the balance is water; The concentration of the ferric chloride aqueous solution is 1%; the concentration of the perchloric acid solution is 70%.

3. The metallographic etching solution for dissimilar metal welded joints according to claim 2, characterized in that, The volume ratio of the ferric chloride aqueous solution to the perchloric acid solution in the metallographic etching solution A is 1:0.8 to 1:1.

2.

4. The metallographic etching solution for dissimilar metal welded joints according to claim 1, characterized in that, The pH value of the metallographic etching solution A is 1.5 to 2.

5.

5. The metallographic etching solution for dissimilar metal welded joints according to claim 1, characterized in that, The metallographic etching solution B includes the following components by volume percentage: 25%-35% nitric acid, 20%-40% glacial acetic acid, and the balance is ethanol; The concentration of the nitric acid is 4%; the concentration of the glacial acetic acid is 100%.

6. The metallographic etching solution for dissimilar metal welded joints according to claim 5, characterized in that, The volume ratio of the nitric acid to the glacial acetic acid in the metallographic etching solution B is 1:0.5 to 1:1.

5.

7. The metallographic etching solution for dissimilar metal welded joints according to claim 1, characterized in that, The pH value of the metallographic etching solution B is 2.0 to 3.

0.

8. The metallographic etching solution for dissimilar metal welded joints according to claim 1, characterized in that, The kovar alloy is 4J29 alloy; the oxygen-free copper is Tu1 copper.

9. A metallographic etching method for dissimilar metal welded joints using the metallographic etching solution for dissimilar metal welded joints according to any one of claims 1 to 8, characterized in that, It includes the following steps: Pretreatment: Wire cut, inlay, rough grind, fine grind, polish, ultrasonically clean, alcohol rinse and dry the kovar alloy and oxygen-free copper welded joint specimen in sequence to obtain the pretreated kovar alloy and oxygen-free copper welded joint; Prepare the metallographic etching solution: Prepare the metallographic etching solution A and the metallographic etching solution B respectively; Step-by-step etching: Use the metallographic etching solution A to wipe the oxygen-free copper side of the pretreated kovar alloy and oxygen-free copper welded joint until the surface turns dark yellow; use the metallographic etching solution B to wipe the kovar alloy side of the pretreated kovar alloy and oxygen-free copper welded joint until the surface turns silver gray; Post-treatment: Wash the kovar alloy and oxygen-free copper welded joint specimen with deionized water and ethanol in sequence and dry.

10. The metallographic corrosion method for dissimilar metal welded joints according to claim 9, characterized in that, In the step of preparing the metallographic etching solution, the preparation of the metallographic etching solution A includes the following steps: Put ferric chloride into a container filled with deionized water and stir well until the ferric chloride dissolves to obtain a ferric chloride aqueous solution; Then mix the ferric chloride aqueous solution with the perchloric acid solution and stir well evenly; Let it stand to obtain the metallographic etching solution A and bottle it; The preparation of the metallographic etching solution B includes the following steps: Add the nitric acid solution to the ethanol solution to obtain a mixed solution, and an esterification reaction occurs to generate ethyl nitrate; Then mix the glacial acetic acid solution with the mixed solution and stir well evenly; Let it stand to obtain the metallographic etching solution B and bottle it.

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