A method for measuring the depth of micro-scale pitting pits on the surface of aluminum alloy

Abstract

The present invention relates to a method for measuring the depth of microscale pitting pits on the surface of aluminum alloy, belonging to the technical fields of failure analysis and microscopic analysis. First, a wedge-shaped block with a specific shape is prepared, and its shape is measured as the basis for size calibration. Secondly, the aluminum alloy specimen is cut perpendicular to the specimen surface, and the cut surface and the wedge-shaped block are simultaneously inlaid into a metallographic specimen. Subsequently, the specimen is ground, polished and thinned layer by layer, and the size of the wedge-shaped block, as well as the width and depth of the pitting pit, are recorded after each thinning. Since the size of the wedge-shaped block changes linearly with the thinning size, the thinning size can be calibrated by using the change in the size of the wedge-shaped block. Finally, through the metallographic measurement data, the least squares method is used to fit the pitting pit contour, and the depth of the pitting pit is calculated. The present invention realizes obtaining the depth of microscale pitting pits on the surface of aluminum alloy, and at the same time evaluates the metallographic structure of the material near the pitting pit, thereby facilitating the prediction of the service life of aluminum alloy components and ensuring their reliability.

Description

Technical Field

[0001] The present invention belongs to the technical field of failure analysis and microscopic analysis, and relates to a method for measuring the depth of micro-scale pitting pits on the surface of aluminum alloy. Background Art

[0002] Aluminum alloy has the advantages of high strength, high stiffness, low density, low cost, easy forming and processing, etc., and is widely used in various aerospace structures. However, aluminum alloy materials are relatively sensitive to corrosive media. Especially in coastal, rainy and humid areas, pitting is very likely to occur on the surface of aluminum alloy. Therefore, the corrosion problem on the surface of aluminum alloy components has become a difficult problem that designers, process engineers and inspection analysts must face.

[0003] Pitting is likely to occur on the surface of aluminum alloy under the action of corrosive media. Under the action of corrosive media such as S and Cl, pitting pits are formed on the surface of aluminum alloy, and cracks are likely to initiate, propagate and cause structural failure at the shoulders and bottoms of the pitting pits. Aluminum alloy materials widely used in aerospace, especially 2000 series and 7000 series aluminum alloy materials, are very sensitive to corrosion in environmental media. If the pitting pits on the surface of aluminum alloy are not controlled, stress corrosion and corrosion fatigue crack sources are likely to be formed, resulting in catastrophic consequences. Therefore, measuring the depth of the corrosion pits is of great significance for predicting the life and ensuring the reliability of aluminum alloy components.

[0004] At present, the methods for measuring the depth of pitting pits include direct measurement method, film covering measurement method, three-dimensional reconstruction measurement method, zoom measurement method, etc. Among them, the direct measurement method refers to directly measuring the depth of pitting pits by means of vernier calipers and other tools for specimens with larger pitting pit sizes and sufficient measurement spaces; the above method is applicable to pitting pits with large sizes (≥2 mm), and the measurement accuracy is relatively low. The film covering measurement method refers to a method of filling surface traces with materials such as room temperature curable silicone rubber and then removing them to measure the depth; the above method is simple and intuitive, but has poor effects on complex and rough traces, and is often used to measure the depth of relatively simple mechanical impact marks. The three-dimensional reconstruction measurement method refers to a method of reconstructing surface traces in three-dimensional space by using a super-depth-of-field video microscope or a laser confocal microscope and measuring the depth of pitting pits; the above method is an advanced method developed in recent years, which can obtain a large amount of information and has a relatively high measurement accuracy. However, if a large amount of corrosion products accumulate inside the pitting pits, the depth cannot be effectively measured. The zoom measurement method has a principle similar to that of the three-dimensional reconstruction measurement method and can be completed by using equipment such as an optical microscope. It is a method of measuring the depth of pitting pits by measuring the position difference between two focal points by respectively focusing on the bottom of the pitting pit and the surface of the product; the above method is simple to test, but has poor measurement effects on pitting pits with corrosion products accumulated inside.

[0005] In failure analysis work, it is often necessary to test the metallographic structure near the pitting pits. If the depth of the pitting pits can be obtained simultaneously during the metallographic analysis process, it undoubtedly has obvious value and significance. At the same time, a large amount of corrosion products often accumulate inside the pitting pits, and other means besides metallography cannot effectively eliminate the influence of the corrosion products on the measurement results of the pitting pits. Therefore, this project studies the measurement method for the depth of micro-scale pitting pits on the surface of aluminum alloys based on metallographic analysis. However, the metallographic test method is an indirect measurement method. Only the size of the pitting pits on a certain cross-section can be obtained by using the metallographic method, and it is impossible to judge whether this cross-section is located at the deepest part of the pitting pits. To sum up, there is currently no accurate and simple method to measure the depth of pitting pits in the range of 50μm - 500μm. Summary of the Invention

[0006] The technical problem solved by the present invention is: overcoming the deficiencies of the prior art, providing a measurement method for the depth of micro-scale pitting pits on the surface of aluminum alloys, realizing the acquisition of the depth of micro-scale pitting pits on the surface of aluminum alloys, and at the same time evaluating the metallographic structure of the material near the pitting pits, so as to facilitate the prediction of the service life of aluminum alloy components and ensure their reliability.

[0007] The technical solution adopted by the present invention is:

[0008] A measurement method for the depth of micro-scale pitting pits on the surface of aluminum alloys includes:

[0009] Prepare a wedge block, and measure the inclination angle θ and the long side dimension L of the wedge block as the basis for dimension calibration;

[0010] The pitting pits are located on the upper surface of the aluminum alloy specimen; perpendicular to the upper surface of the aluminum alloy specimen, cut the aluminum alloy specimen to form a vertical aluminum alloy cutting surface; and place the wedge block on the side wall of the aluminum alloy specimen, and ensure that the plane where the long side L of the wedge block is located is in the same plane as the aluminum alloy cutting surface; fix the wedge block to the aluminum alloy specimen.

[0011] Perpendicular to the plane where the long side L of the wedge block is located and the aluminum alloy cutting surface, perform horizontal grinding and polishing in the direction of the pitting pits, and perform grinding and polishing on the plane where the long side L of the wedge block is located and the aluminum alloy cutting surface at the same time.

[0012] When the aluminum alloy cutting surface is ground and polished to the edge position of the pitting pits, record the cross-sectional depth D0 of the pitting pits and the long side dimension L0 of the wedge block at this time.

[0013] Continue to perform step-by-step grinding and polishing n times, and record the cross-sectional depth D of the pitting pits after each grinding and polishing in turn i and the long side dimension L of the wedge block i ; i is the number of grinding and polishing times, i = 1, 2,..., n;

[0014] Set the height of the wedge block as H, according to the measured Li Calculate the height reduction dimension ΔH of the wedge block during each polishing and grinding based on the inclination angle θ of the wedge block. i ;

[0015] Establish a least squares fitting equation, substitute D i and ΔH i into the equation to obtain the best fitting parameters;

[0016] Calculate the maximum value according to the least squares fitting equation, which is the depth of the pitting corrosion pit.

[0017] In the above method for measuring the depth of micro-scale pitting corrosion pits on the surface of an aluminum alloy, the long side dimension L of the wedge block is 2 - 20 mm, and the inclination angle θ is 5° - 30°.

[0018] In the above method for measuring the depth of micro-scale pitting corrosion pits on the surface of an aluminum alloy, the horizontal distance between the cutting surface of the aluminum alloy and the edge of the pitting corrosion pit is 0.2 - 0.5 mm.

[0019] In the above method for measuring the depth of micro-scale pitting corrosion pits on the surface of an aluminum alloy, the number of steps n of step-by-step polishing and grinding is not less than 4, and the reduction dimension for each polishing and grinding is 10 - 200 μm.

[0020] In the above method for measuring the depth of micro-scale pitting corrosion pits on the surface of an aluminum alloy, the height H of the wedge block is: the cross-section of the wedge block is triangular, with the long side L of the wedge block as the base of the triangle, and the height H as the height of the triangle, which is the height of the wedge block.

[0021] In the above method for measuring the depth of micro-scale pitting corrosion pits on the surface of an aluminum alloy, the direction of polishing and grinding is along the direction of height H. As the step-by-step polishing and grinding progresses, the height H of the wedge block gradually shortens, and the long side dimension L of the wedge block i also gradually becomes shorter.

[0022] In the above method for measuring the depth of micro-scale pitting corrosion pits on the surface of an aluminum alloy, the calculation method for the height reduction dimension ΔH of the wedge block during each polishing and grinding i is as follows:

[0023] ΔH i = 0.5(L - L i )sin2θ.

[0024] In the above method for measuring the depth of micro-scale pitting corrosion pits on the surface of an aluminum alloy, the least squares fitting equation is:

[0025] y = Ax 2 + Bx + C

[0026] In the formula, A, B, and C are fitting parameters;

[0027] Substitute D i and ΔHi Substitute into the equation to obtain the optimal A, B, and C.

[0028] In the above method for measuring the depth of micro-scale pitting pits on the surface of an aluminum alloy, D i and ΔH i When substituting into the least squares fitting equation, ΔH i is used as x, and D i is used as y.

[0029] In the above method for measuring the depth of micro-scale pitting pits on the surface of an aluminum alloy, the method for calculating the maximum value is as follows:

[0030] s Substitute ΔH i sequentially into x in the least squares fitting equation after determining the optimal fitting parameters, and calculate the maximum y value, which is the maximum value.

[0031] The beneficial effects of the present invention compared with the prior art are as follows:

[0032] (1) The method of the present invention is intuitive, simple, has a wide range of applications, and the obtained results are highly accurate. Compared with other methods, it can measure the depth of pitting pits during metallographic analysis and eliminate the influence of corrosion products on the measurement results of pitting pit depth, having obvious advantages;

[0033] (2) The present invention innovatively uses a wedge block as a reference object. According to the special dimensions of the wedge block, step-by-step grinding and polishing treatment are performed on the long side dimension of the wedge block and the cross-section of the pitting pit to complete the control of grinding accuracy. At the same time, by back-calculating the long side dimension L and inclination angle θ of the wedge block, the depth of the pitting pit of the aluminum alloy sample can be accurately obtained;

[0034] (3) The present invention performs least squares fitting on the array (ΔHi, Di) according to y = Ax2 + Bx + C to obtain the optimal fitting parameters A, B, and C; the maximum value of the pitting pit profile depth is obtained within the range of x ∈ (ΔH1, ΔHn). The least squares fitting method is innovatively incorporated into data processing, and the accuracy of the obtained pitting pit depth result is high. Description of the Drawings

[0035] Figure 1 It is a schematic diagram of the cutting surface of the aluminum alloy of the present invention;

[0036] Figure 2 It is a schematic diagram of the grinding and polishing directions of the wedge block and the aluminum alloy sample of the present invention;

[0037] Figure 3 It is a schematic diagram of the depth of the cross-section of the pitting pit of the present invention;

[0038] Figure 4 It is a schematic diagram of the test data and fitting results of the present invention. Detailed Embodiments

[0039] The present invention will be further described below in conjunction with embodiments.

[0040] The present invention provides a method for measuring the depth of micro-scale pitting pits on the surface of aluminum alloy. The method is intuitive, simple, has a wide range of applications, and the obtained results are highly accurate. Compared with other methods, it can measure the depth of pitting pits during metallographic analysis and exclude the influence of corrosion products on the measurement results of pitting pit depth, having obvious advantages.

[0041] The method for measuring the depth of micro-scale pitting pits on the surface of aluminum alloy specifically includes the following steps:

[0042] Prepare a wedge block, and measure the inclination angle θ and the long side dimension L of the wedge block as the basis for dimension calibration; the long side dimension L of the wedge block is 2 - 20 mm, and the inclination angle θ is 5° to 30°.

[0043] First, prepare a wedge block with a specific shape (generally a right triangle), and measure its inclination angle θ and long side dimension L as the basis for dimension calibration. Generally speaking, for the simplicity of the preparation process, the long side dimension of the wedge block is between 2 mm and 20 mm, and the inclination angle is between 5° and 30°. If the size of the wedge block is too small, it is not easy to manufacture; if the size is too large, it is not easy to inlay. If the inclination angle of the wedge block is too small, it is not easy to manufacture; if the inclination angle is too large, the smaller thinning size change cannot be reflected in the larger long side dimension change, losing the meaning of the wedge block to magnify the thinning size and perform calibration.

[0044] As Figure 1 shown, the pitting pit is located on the upper surface of the aluminum alloy sample; perpendicular to the upper surface of the aluminum alloy sample, cut the aluminum alloy sample to form a vertical aluminum alloy cutting surface. The horizontal distance between the aluminum alloy cutting surface and the edge of the pitting pit is 0.2 - 0.5 mm. Place the wedge block on the side wall of the aluminum alloy sample, and ensure that the plane where the long side L of the wedge block is located is in the same plane as the aluminum alloy cutting surface; fix the wedge block to the aluminum alloy sample, as Figure 2 shown. Horizontally polish in the direction of the pitting pit perpendicular to the plane where the long side L of the wedge block is located and the aluminum alloy cutting surface, and polish the plane where the long side L of the wedge block is located and the aluminum alloy cutting surface simultaneously.

[0045] When the aluminum alloy cutting surface is polished to the edge position of the pitting pit, record the cross-sectional depth D0 of the pitting pit at this time and the long side dimension L0 of the wedge block. The cross-sectional depth of the pitting pit is as Figure 3 shown.

[0046] Continue to perform step-by-step polishing n times, and sequentially record the cross-sectional depth D i of the pitting pit and the long side dimension L i; i is the number of grinding and polishing times, i = 1, 2, ……, n; the number of steps of grinding and polishing n is not less than 4, and the thickness reduction size for each grinding and polishing is 10 - 200 μm.

[0047] Cut the aluminum alloy sample perpendicular to the surface of the sample, and the cutting surface is about 0.2 mm to 0.5 mm away from the pitting corrosion pit size, and inlay the cutting surface and the surface where the long side of the wedge block is located into a metallographic sample at the same time. Grind and polish the sample layer by layer. When grinding to the pitting corrosion pit, measure the long side size of the wedge block and the cross-sectional depth of the pitting corrosion pit, and record them as L0 and D0 respectively; repeat the above grinding and polishing process, and measure the long side size of the wedge block and the cross-sectional depth of the pitting corrosion pit after the first thinning, and record them as L1 and D1 respectively; the grinding and polishing measurement process is carried out at least 4 times to obtain L3 and D3, L4 and D4…

[0048] Set the height of the wedge block as H, and calculate the height reduction size ΔH of the wedge block for each grinding and polishing according to the measured L i and the inclination angle θ of the wedge block i ; the height H of the wedge block is: the cross-section of the wedge block is a triangle, with the long side L of the wedge block as the base of the triangle, and the height H is the height of the triangle, that is, the height of the wedge block, as Figure 2 shown.

[0049] The direction of grinding and polishing is along the direction of height H. As the step-by-step grinding and polishing progresses, the height H of the wedge block gradually shortens, and the long side size L of the wedge block i also gradually becomes shorter.

[0050] The height reduction size ΔH of the wedge block for each grinding and polishing i is calculated as follows:

[0051] ΔH i = 0.5(L - L i )sin2θ.

[0052] Since the size of the pitting corrosion pit is small, it is possible to grind across the entire cross-section of the pitting corrosion pit during the grinding process, resulting in the loss of relevant data. Therefore, during the inlay process, ensure that the data of the long side size of the wedge block and the depth of the pitting corrosion pit can be obtained in no less than 4 grinding and polishing processes. Use the least squares method to perform quadratic polynomial fitting on the (ΔH i , D i ) array to calculate and obtain ΔH i .

[0053] Establish a least squares fitting equation, substitute D i and ΔH i into the equation to obtain the best fitting parameters; the least squares fitting equation is:

[0054] y = Ax 2 + Bx + C

[0055] In the formula, A, B, and C are fitting parameters;

[0056] Substitute D i and ΔH i into the equation to obtain the optimal A, B, and C.

[0057] Substitute D i and ΔH i into the least - squares fitting equation. When substituting, ΔH i is used as x, and D i is used as y.

[0058] According to the least - squares fitting equation, calculate the maximum value, which is the depth of the pitting pit. Substitute ΔH i into x in the least - squares fitting equation after determining the optimal fitting parameters in sequence, and calculate the maximum y value, which is the maximum value.

[0059] Example

[0060] During the inlaying process, ensure that data on the long - side dimension of the wedge block and the depth of the pitting pit can be obtained during at least 4 grinding and polishing processes. The measurement and calculation results are shown in Table 1.

[0061] Table 1 Measurement results of pitting pit depth (μm)

[0062] Number of grinding layers 1 2 3 4 5 <![CDATA[Wedge block size L i > 2534 2455 2428 2229 2167 <![CDATA[Pitting pit depth D i > 21 43 45 55 15 <![CDATA[Thinning dimension ΔH i > 300 318 324 369 383

[0063] Use the least - squares method to perform a quadratic polynomial fitting on the (ΔH i , D i ) array. When calculating, when ΔH i = 343 μm (i.e., when the wedge - block size is 2344 μm), the depth of the pitting pit is 59.3 μm. The fitting result is shown in Equation (1):

[0064]

[0065] By randomly grinding the cross - section of the pitting pit and measuring the corresponding wedge - block size and pitting - pit depth of each layer, the depth of the pitting pit can be measured to be 59.3 μm using the algorithm proposed in the present invention. The fitting result is as Figure 4 shown.

[0066] The method of the present invention is intuitive, simple, has a wide range of applications, and the obtained results are highly accurate. Compared with other methods, it can measure the depth of the pitting pit during metallographic analysis and eliminate the influence of corrosion products on the measurement results of the pitting - pit depth, having obvious advantages;

[0067] The present invention innovatively uses a wedge block as a reference object. According to the special dimensions of the wedge block, a step-by-step grinding and polishing treatment is carried out on the long side dimension of the wedge block and the cross-section of the pitting pit, completing the control of the grinding accuracy. At the same time, by back-calculating the long side dimension L and the inclination angle θ of the wedge block, the pitting pit depth of the aluminum alloy specimen can be accurately obtained; the least squares fitting is performed on the array (ΔHi, Di) according to y = Ax2 + Bx + C to obtain the optimal fitting parameters A, B, and C; the maximum value of the pitting pit profile depth is obtained within the range of x ∈ (ΔH1, ΔHn). The least squares fitting method is innovatively incorporated into the data processing, and the accuracy of the pitting pit depth result is high.

[0068] Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for measuring the depth of micro-scale pitting corrosion pits on the surface of aluminum alloy, characterized in that: Including: Prepare a wedge block, and measure the inclination angle θ and the long side dimension L of the wedge block as the basis for dimension calibration; The pitting corrosion pit is located on the upper surface of the aluminum alloy specimen; Perpendicular to the upper surface of the aluminum alloy specimen, cut the aluminum alloy specimen to form a vertical aluminum alloy cutting surface; and place the wedge block at the side wall of the aluminum alloy specimen, and ensure that the plane where the long side L of the wedge block is located is in the same plane as the aluminum alloy cutting surface; fix the wedge block to the aluminum alloy specimen; Perpendicular to the plane where the long side L of the wedge block is located and the aluminum alloy cutting surface, perform horizontal grinding and polishing in the direction of the pitting corrosion pit, and perform grinding and polishing on the plane where the long side L of the wedge block is located and the aluminum alloy cutting surface at the same time; When the aluminum alloy cutting surface is ground and polished to the edge position of the pitting corrosion pit, record the cross-sectional depth D0 of the pitting corrosion pit and the long side dimension L0 of the wedge block at this time; Continue with step grinding and polishing n times, and sequentially record the cross-sectional depth D of the pitting corrosion pit after each grinding and polishing i and the long side dimension L of the wedge block i ; i is the number of grinding and polishing times, i = 1, 2, ……, n; Set the height of the wedge block as H, and calculate the height reduction dimension ΔH of the wedge block during each grinding and polishing according to the measured L i and the inclination angle θ of the wedge block i ; Establish a least-squares fitting equation and substitute D i and ΔH i into the equation to obtain the best fitting parameters; According to the least squares fitting equation, calculate the maximum value, which is the depth of the pitting corrosion pit.

2. A method for measuring the depth of micro-scale pitting pits on the surface of aluminum alloy according to claim 1, characterized in that: The long side dimension L of the wedge block is 2 - 20 mm, and the inclination angle θ is 5° to 30°; 3. A method for measuring the depth of micro-scale pitting corrosion pits on the surface of aluminum alloy according to claim 1, characterized in that: The horizontal distance between the aluminum alloy cutting surface and the edge of the pitting corrosion pit is 0.2 - 0.5 mm; 4. A method for measuring the depth of microscale pitting corrosion pits on the surface of aluminum alloy according to claim 1, characterized in that: The number of times n of step grinding and polishing is not less than 4, and the thinning dimension for each grinding and polishing is 10 - 200 μm; 5. A method for measuring the depth of microscale pitting pits on the surface of aluminum alloy according to claim 1, characterized in that: The height H of the wedge block is: the cross-section of the wedge block is triangular, with the long side L of the wedge block as the base of the triangle, and the height H is the height of the triangle, that is, the height of the wedge block; 6. The method for measuring the depth of microscale pitting corrosion pits on the surface of an aluminum alloy according to claim 5, wherein: The grinding and polishing direction is along the direction of height H. As the step-by-step grinding and polishing progresses, the height H of the wedge block gradually shortens, and the long side dimension L of the wedge block i also gradually becomes shorter.

7. A method for measuring the depth of micro-scale pitting pits on the surface of aluminum alloy according to claim 6, characterized in that: The height reduction dimension ΔH of the wedge block for each grinding and polishing i is calculated as follows: ΔH i = 0.5(L - L i )sin2θ。 8. A method for measuring the depth of microscale pitting corrosion pits on the surface of aluminum alloy according to claim 7, characterized in that: The least squares fitting equation is: y = Ax 2 + Bx + C Where A, B, and C are fitting parameters; Bring D i and ΔH i into the equation to obtain the optimal A, B, and C.

9. A method for measuring the depth of microscale pitting corrosion pits on the surface of an aluminum alloy according to claim 8, characterized in that: Bring D i and ΔH i into the least squares fitting equation, where ΔH i is taken as x and D i is taken as y.

10. A method for measuring the depth of micro-scale pitting corrosion pits on the surface of aluminum alloy according to claim 9, characterized in that: The method for calculating the maximum value is: Substitute ΔH i into x in the least squares fitting equation after determining the optimal fitting parameters in sequence, and calculate the maximum y value, which is the maximum value.

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