Corrosive liquid for microcosmic detection of 6082 aluminum alloy

Abstract

The invention belongs to the field of metal material microstructure detection, and particularly relates to a corrosive liquid for microcosmic detection of 6082 aluminum alloy, and the corrosive liquid comprises sodium hydroxide, sodium fluoride, acetone and water. By constructing a single-phase corrosion system which does not contain strong acid, is stable in component and controllable in reaction, the technical problems that the existing Keller reagent and a double-liquid step-by-step corrosion system have blurred grain boundaries, spot-shaped pits cover the grain boundaries, corrosion-resistant phase interference exists, the operation safety is low and the like in 6082 aluminum alloy metallographic structure display are solved. The corrosive liquid is based on the selective dissolution of NaOH on an aluminum matrix, the efficient breaking of NaF on a surface oxide film and the cooperative regulation and control effect of acetone on the reaction rate and the interfacial tension, and the cooperative regulation and control of low-temperature impact brought by the rapid cooling of dry ice particles on the interfacial tension and the reaction rate during corrosion are utilized; and the definition, the corrosion uniformity and the detection reproducibility of crystal grains and crystal boundaries are obviously improved.

Description

Technical Field

[0001] This invention relates to the field of metal material microstructure detection technology, specifically to a etching solution for microstructure detection of 6082 aluminum alloy. Background Technology

[0002] 6082 aluminum alloy belongs to the 6-series (Al-Mg-Si) heat-treatable aluminum alloys. It possesses moderate strength, good weldability, and corrosion resistance, and is widely used in transportation and structural engineering. In metallographic microstructure analysis, the composition and ratio of the etching solution directly affect the visualization of grain boundaries and grains. Conventional 6-series aluminum alloys are often etched using Keller's reagent (1 ml HF + 1.5 ml HCl + 2.5 ml HNO3 + 95 ml H2O). However, due to the unique composition and heat treatment conditions of 6082 alloy, problems such as unclear grain boundary visualization, uneven etching, or over-etching frequently occur, affecting microstructure observation and performance analysis.

[0003] Patent CN114778249A discloses a metallographic etching solution and method for 6082 aluminum alloy. The etching solution consists of a first etching solution (0.4%–0.6% sodium hydroxide solution by mass) and a second etching solution (6%–10% nitric acid solution by mass). This method achieves clear grain boundaries and intact grains through a two-step etching process, and is simple to operate with low equipment requirements. However, this etching system uses an alkaline pre-etching combined with an acidic final etching method. In practical applications, the etching time window is quite sensitive. If the alkaline etching time is slightly longer, it can easily cause excessive dissolution of the surface, resulting in blurred grain boundaries; if the alkaline etching is insufficient, the subsequent nitric acid etching will not effectively highlight the grain boundaries, and the overall reproducibility is limited by the operator's experience.

[0004] Patent CN116288355A discloses a metallographic etching solution and method suitable for 6-series aluminum alloys. The first etching solution contains 1%–4% sodium fluoride and 1%–4% sodium hydroxide, while the second etching solution is a 5%–8% nitric acid solution. This method also employs a two-liquid stepwise etching process, claiming to obtain a clear and complete grain structure. However, the coexistence of sodium fluoride and sodium hydroxide in this etching system easily generates hydrofluoric acid and releases fluoride ions. This can lead to localized pitting or selective dissolution during the etching process. Especially in 6082 aluminum alloys, due to the uneven distribution of the silicon phase, inconsistent etching can easily occur, affecting the true reflection of the microstructure. Furthermore, it places higher demands on the operating environment and wastewater treatment.

[0005] In summary, while existing metallographic etching techniques for 6082 aluminum alloy have improved the microstructure display to some extent, there is still room for improvement in terms of etching uniformity, operational tolerance, and fidelity of microstructure. There is an urgent need to develop a special etching solution with stable composition, controllable etching process, and suitable for routine laboratory conditions to meet the precise and efficient requirements of microscopic detection of 6082 aluminum alloy. Summary of the Invention

[0006] The purpose of this invention is to provide an etching solution for microscopic inspection of 6082 aluminum alloy, so as to solve the above-mentioned problems.

[0007] Firstly, an etching solution for microscopic inspection of 6082 aluminum alloy comprises sodium hydroxide, sodium fluoride, acetone, and water.

[0008] Further, the mass ratio of sodium hydroxide, sodium fluoride, acetone and water is (4.5~5.5):(4.5~5.5):(4.5~5.5):(48~52).

[0009] Furthermore, the mass ratio of sodium hydroxide, sodium fluoride, acetone and water is 5:5:5:50.

[0010] Secondly, the method for detecting the microstructure of 6082 aluminum alloy using the etching solution includes the following steps:

[0011] S1: Take a 6082 aluminum alloy sample and machine it into sample one. The test surface is the rolled surface or a specified section.

[0012] S2: Grind the sample step by step, and then perform mechanical polishing to obtain an aluminum alloy sample;

[0013] S3: Prepare a corrosive solution by mixing sodium hydroxide, sodium fluoride, acetone and water evenly;

[0014] S4: Immerse the aluminum alloy sample vertically into the etching solution, add dry ice particles during the etching process, and assist etching in a megaphonic environment;

[0015] S5: Immediately after corrosion is complete, remove the aluminum alloy sample, wash it with anhydrous ethanol and deionized water in sequence, and then blow it dry.

[0016] S6: Place the aluminum alloy sample after step S5 under an optical metallographic microscope or a scanning electron microscope to observe the grain morphology.

[0017] Furthermore, in step S4, the etching time is 160~190s.

[0018] Furthermore, in step S4, a megasonic cleaning tank is used for megasonic-assisted corrosion, with a megasonic frequency of 2.77MHz.

[0019] Furthermore, in step S4, the dry ice particles with a diameter not exceeding 0.8 cm are added in an amount of 3-5 g per liter of corrosive liquid.

[0020] Furthermore, in step S4, the use of ultrasonic-assisted corrosion is prohibited.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] This invention addresses the technical problems of existing Keller reagent and two-liquid stepwise etching systems in the metallographic display of 6082 aluminum alloys, such as grain boundary blurring, pitting obscuring grain boundaries, interference from corrosion-resistant phases, and low operational safety, by constructing a single-phase etching system that is free of strong acids, has stable composition, and allows for controllable reactions. This etching solution utilizes the selective dissolution of the aluminum matrix by NaOH, the efficient removal of the surface oxide film by NaF, and the synergistic regulation of reaction rate and interfacial tension by acetone. Furthermore, it leverages the low-temperature impact of dry ice particles during etching to further regulate interfacial tension and reaction rate, ultimately forming a dedicated etching system suitable for high-silicon-content 6082 aluminum alloys. This significantly improves the clarity of grains and grain boundaries, etching uniformity, and detection reproducibility. Attached Figure Description

[0023] Figure 1 This is a metallographic observation image of Example 1;

[0024] Figure 2 This is a metallographic observation image of Example 2;

[0025] Figure 3 This is a metallographic observation diagram of Comparative Example 1;

[0026] Figure 4 This is a metallographic observation diagram of Comparative Example 4. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings.

[0028] Example 1

[0029] S1: Take a 6082-T6 state aluminum alloy ingot and machine it into a rectangular sample with dimensions of 20mm×20mm×5mm using a wire cutting machine. The test surface is the cross-section in the rolling direction to ensure that the observation area contains typical recrystallized structures. The sample edges are chamfered by 0.5mm to avoid stress concentration.

[0030] S2: A unique identification code, such as "6082-20240501-001", is engraved in the center area of ​​the non-test surface (i.e., the back side) of the sample using a fiber laser engraving device with a wavelength of 1064nm. The laser power is set to 8W, the scanning speed to 500mm / s, the pulse frequency to 20kHz, and the marking depth to 0.08mm. Metallurgical microscopy confirms that the heat-affected zone does not extend to within 0.5mm below the test surface.

[0031] S3: Fix the sample in a stainless steel metallographic fixture, ensuring the clamping force does not cause elastic deformation. Grind the sample sequentially on an automatic polishing machine using 180#, 400#, 800#, and 1200# silicon carbide sandpaper. Before each sandpaper change, rinse the sample and fixture with deionized water to remove residual abrasive debris. Set the grinding pressure to 15N, the rotary table speed to 300 rpm, and the grinding time for each pass to 2 minutes. Ensure that the grinding directions of adjacent sandpapers are perpendicular (e.g., 180# along the X-axis, 400# along the Y-axis, and so on). After grinding, proceed to the mechanical polishing stage: first, use a 3μm diamond polishing slurry with a synthetic fabric polishing cloth, applying a pressure of 25N, a polishing disc speed of 200 rpm, and a polishing time of 3 minutes; then switch to a 1μm diamond polishing slurry and continue polishing for another 3 minutes under the same parameters. The final test surface should exhibit mirror reflection, with no visible scratches, orange peel, or plastic deformation layer. The surface roughness Ra should be ≤0.02μm as measured by a white light interferometer, thus obtaining an aluminum alloy sample that meets the requirements.

[0032] S4: Prepare the etching solution: NaOH 5.000 g, NaF 5.000 g, acetone 5.000 g, deionized water 50.000 g, stir for 4 min.

[0033] S5: Etching Process: The aluminum alloy sample was vertically placed into a megasonic cleaning tank containing an etching solution. At the start of etching, dry ice particles with a diameter not exceeding 0.8 cm were added to the tank at a rate of 3-5 g per liter of etching solution. The megasonic frequency was maintained at 2.77 MHz for 180 seconds, followed by ethanol cleaning for 5 seconds and water rinsing for 10 seconds. After rinsing, the sample was dried with a hair dryer. Metallographic observation showed continuous and intact grain boundaries, an average grain size of 120 μm, no pitting, and uniform bright spots in the Mg2Si phase.

[0034] The test results for this example can be found in [link to example]. Figure 1 The grains etched with the optimized etching solution show clear grain boundaries, allowing for rapid identification of grain size and dimensions.

[0035] Example 2

[0036] Same as Example 1, but with an etching time of 170 s. The grain boundaries were slightly shallower, but the continuity was good, suitable for observing fine-grained structures (<80 μm). The test results for this example are shown below. Figure 2 .

[0037] Example 3

[0038] Same as Example 1, but with an etching time of 190 s. The grain boundary trenches are slightly deeper, suitable for coarse-grained (>150 μm) or low-contrast samples.

[0039] Example 4

[0040] Same as Example 1, but with a etching time of 160 s. A clear equiaxed grain structure was obtained, with no grain boundary fractures.

[0041] Comparative Example 1

[0042] Using conventional Keller's reagent (2.5 mL HNO3 + 1.5 mL HCl + 1.0 mL HF + 95 mL H2O), aluminum alloy samples were etched for 25 s. The results showed blurred grain boundaries, obscuring the true grain boundaries. See [link to Keller's reagent]. Figure 3 .

[0043] Comparative Example 2

[0044] The aluminum alloy sample was etched for 180 seconds using a single NaOH solution (5g NaOH + 50 mL H2O). The surface showed uniform corrosion without grain boundaries, indicating that the oxide film was not effectively removed.

[0045] Comparative Example 3

[0046] In this example, the megasonic cleaning tank was replaced with an ultrasonic cleaning tank; otherwise, it remained the same as in Example 1. It was found that numerous strong cavitation bubbles impacted the already formed grain boundary microgrooves, causing structural collapse or artifacts.

[0047] Therefore, for this invention, the use of ultrasonic cleaners is prohibited throughout the entire process; only megasonic cleaning tanks can be used.

[0048] Comparative Example 4

[0049] Compared to Example 1, the etching process in this example did not involve adding dry ice particles to the megaacoustic cleaning tank; all other aspects remained the same. As a result, grain boundaries were not observed. See [link to example]. Figure 4 .

[0050] Comparative Example 5

[0051] Compared to Example 1, in this example, ice particles were introduced into the mega-sound cleaning tank instead of dry ice particles in the etching process; all other aspects remained the same. The result was blurred grain boundaries, obscuring the true grain boundaries.

[0052] Comparative Example 6

[0053] The acetone in Example 1 was replaced with ethanol, and everything else remained the same.

[0054] Comparative Example 7

[0055] The acetone in Example 1 was replaced with propanol, and everything else remained the same.

[0056] Comparative Example 8

[0057] The acetone in Example 1 was replaced with butanol, and everything else remained the same.

[0058] Comparative Example 9

[0059] When the aluminum alloy in Example 1 was replaced with 6085 aluminum alloy, all other aspects remained the same, and the result was found to be blurred grain boundaries.

[0060] The corrosion effects of the above embodiments and comparative examples were quantitatively evaluated. The grain boundary continuity rate (continuous grain boundary length / total grain boundary length × 100%) was calculated using image analysis software (ImageJ), and the image quality was blind-rated by three metallographic analysts (1-5 points, 5 being the best). The results are shown in Table 1:

[0061] Table 1

[0062]

[0063] As shown in Table 1, the grain boundary continuity rate of the embodiments of the present invention is higher than 97%, the image quality score is close to full marks, and the pitting is almost invisible; while in comparative examples 6 to 8, due to the different polarities of the organic solvents, the micro-pitting corrosion is different under mega-acoustic action.

[0064] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0065] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A etching solution for microscopic inspection of 6082 aluminum alloy, characterized in that: It includes sodium hydroxide, sodium fluoride, acetone, and water.

2. The etching solution for microscopic inspection of 6082 aluminum alloy according to claim 1, characterized in that: The mass ratio of sodium hydroxide, sodium fluoride, acetone and water is (4.5~5.5):(4.5~5.5):(4.5~5.5):(48~52).

3. The etching solution for microscopic inspection of 6082 aluminum alloy according to claim 1, characterized in that: The mass ratio of sodium hydroxide, sodium fluoride, acetone and water is 5:5:5:

50.

4. The method for microstructure detection of 6082 aluminum alloy using the etching solution as described in any one of claims 1 to 3, characterized in that, Includes the following steps: S1: Take a 6082 aluminum alloy sample and machine it into sample one. The test surface is the rolled surface or a specified section. S2: Grind the sample step by step, and then perform mechanical polishing to obtain an aluminum alloy sample; S3: Prepare a corrosive solution by mixing sodium hydroxide, sodium fluoride, acetone and water evenly; S4: Immerse the aluminum alloy sample vertically into the etching solution, add dry ice particles during the etching process, and assist etching in a megaphonic environment; S5: Immediately after corrosion is complete, remove the aluminum alloy sample, wash it with anhydrous ethanol and deionized water in sequence, and then blow it dry. S6: Place the aluminum alloy sample after step S5 under an optical metallographic microscope or a scanning electron microscope to observe the grain morphology.

5. The method according to claim 4, characterized in that, In step S4, the etching time is 160~190s.

6. The method according to claim 4, characterized in that, In step S4, a megasonic cleaning tank is used for megasonic-assisted corrosion, with a megasonic frequency of 2.77MHz.

7. The method according to claim 4, characterized in that, In step S4, the dry ice particles with a diameter not exceeding 0.8 cm are added at a rate of 3-5 g per liter of corrosive liquid.

8. The method according to claim 4, characterized in that, In step S4, ultrasonic-assisted corrosion is prohibited.

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