Method of laser restoration of bluing on metal surface

Abstract

FIELD: laser technology; equipment.SUBSTANCE: method for restoration of bluing, a decorative artistic finish on cultural and historical heritage objects made of carbon steel. The method of laser restoration of bluing on a metal surface involves the sequential execution of three technological operations: laser cleaning, laser polishing and local laser heating. Laser cleaning and laser polishing are repeated multiple times, no less than four and no more than ten times. In this case, first all repetitions of the laser cleaning operation are performed, then all repetitions of the laser polishing operation, and finally all repetitions of the laser local heating operation. Cleaning and polishing are performed with a decrease in laser energy density before each repeat operation. Cleaning is carried out at a laser radiation energy density in the range of 1-20 J / cm2, and polishing at a laser radiation energy density in the range of 5-50 J / cm2. Local laser heating is repeated at different scanning angles uniformly distributed within a half-turn at least three times and with constant laser radiation parameters to obtain the required color.EFFECT: increase in the homogeneity of the thickness of the oxide layer and, as a consequence, a reduction in the degree of non-uniformity of the color of the decorative oxide layer created during bluing, and an increase in its protective properties.1 cl, 3 dwg

Description

[0001] This invention relates to laser technology and equipment and is intended for restoring bluing—a decorative artistic finish—on cultural and historical heritage objects made of carbon steel. The invention provides a complete restoration cycle—from cleaning the surface of contaminants and corrosion to achieving the desired color shade, effectively restoring the objects to their original appearance.

[0002] There are several inventions known that can be used as methods for restoring bluing.

[0003] The non-abrasive finishing method described in Russian Patent No. 2287615 dated November 20, 2006, can be used for blueing metal surfaces. This method involves processing the parts in a vibrating device with a chemical solution, which ensures bluing without abrasive action. The disadvantages of this method include poor bluing quality on the treated surface when it is contaminated and corroded due to the lack of a cleaning step, limited effectiveness on hard surfaces, and a lengthy process, resulting in significant time and cost.

[0004] The closest in terms of the set of essential features to the proposed invention is the method described in [M. Salhab, CLB Rajasekara, OS Vasiliev, ON Karpovand VA Parfenov, “Restoration of Original Color Appearance of Decorative Iron-Based Metal Artworks by Laser Treatment,” 2023, pp. 131-134, doi: 10.1109 / FWPE60445.2023.10368532], which consists of laser restoration of bluing on the surface of carbon steel objects by sequentially performing three technological operations: laser cleaning, carried out by removing corrosion centers through laser ablation, laser polishing, carried out by smoothing the surface relief due to surface tension forces during local short-term laser melting, and local laser heating, which allows for the creation of a thin oxide layer by heating the steel in the presence of atmospheric oxygen.

[0005] The main drawback of this method is the uneven thickness of the oxide layer across the surface of the object being restored. This drawback is due to the fact that the scanning process of the treated area uses only one pass of the laser beam at a single fixed scanning angle.

[0006] The problem solved by the invention is to develop a method for restoring bluing that allows improving the quality of its restoration.

[0007] The technical result is an increase in the homogeneity of the thickness of the oxide layer and, as a consequence, a decrease in the degree of non-uniformity of the color of the decorative oxide layer created during bluing and an increase in its protective properties.

[0008] The problem is solved by the proposed method for laser restoration of blued metal surfaces, which, like the known method, involves the sequential execution of three process steps: laser cleaning, laser polishing, and localized laser heating. However, the laser cleaning and laser polishing steps are repeated at least four times and no more than ten times, with the laser cleaning step performed first, followed by the laser polishing step, and finally the localized laser heating step. Cleaning and polishing are performed with a decreasing laser energy density before each repeat step, and cleaning is performed at a laser energy density in the range of 1-20 J / cm. 2 , polishing at laser energy density in the range of 5-50 J / cm 2, and local laser heating is repeated at different scanning angles uniformly distributed within a half-turn at least three times and with unchanged laser radiation parameters to obtain the required color.

[0009] If fewer than four consecutive laser processing operations are performed, the deep-seated contaminant layer is not completely removed, and the required surface roughness is not achieved. If more than ten passes are performed, the improvement in surface quality will be negligible, and excess heat will accumulate, which can lead to the formation of microcracks or undesirable phase changes in the base metal structure. The local laser heating operation is repeated at least three times at different scanning angles uniformly distributed within a half-turn.

[0010] The invention is explained by drawings, where Fig. 1 shows a diagram illustrating the concept of a scanning angle, Fig. 2 shows the mutual orientation of the directions of passage of a laser beam along the surface being processed in the case of a three-fold repetition of the local laser heating operation, Fig. 3 shows microphotographs demonstrating visual changes in the surface of a metal sample at various stages of processing.

[0011] Figure 1 shows a diagram illustrating the concept of a scanning angle. The head of a laser machine 1 generates a laser beam 2 incident on a workpiece 3 located on the table of a laser machine 4. The Cartesian orthogonal coordinate axes X and Y lie in the plane of the machine 4. The spot from the laser beam 2 can shift along the workpiece 3 due to the movement of the head 1 relative to the table 4 (along the X and Y axes). In this case, we speak of the passage of the laser beam 2 along the workpiece 3. The angle θ between one of the axes of the table 4 (in this example, the X axis) and the direction of passage of the laser beam 2 along the surface 3 is called the scanning angle of the workpiece.

[0012] Figure 2 shows the relative orientation of the laser beam's directions of passage across the workpiece in the case of a three-fold repetition of the local laser heating operation. The directions of passage during the first, second, and third repetitions of the local heating operation are designated 1, 2, and 3, respectively. Direction of passage 2 is rotated relative to the direction of passage 1 by 60 degrees. Direction of passage 3 is rotated relative to the direction of passage 2 by 60 degrees.

[0013] Figure 3 shows micrographs demonstrating visual changes in the surface of a metal sample at various stages of processing. Figure 3.1 shows a zone with an original brown surface color without signs of corrosion. Figure 3.2 illustrates a zone with corrosion damage and loss of the original color. Figure 3.3 shows the first example of a zone with uneven color restoration obtained at one energy density for each of the three stages of laser processing. Figure 3.4 shows the second example of a zone with less uneven color restoration obtained at one optimal (experimentally selected) energy density for each of the three stages of laser processing. Figure 3.5 demonstrates a zone with uniform color restoration obtained by the proposed method.

[0014] The first stage of bluing restoration involves laser cleaning of contaminants and corrosion, based on a photothermal ablation mechanism that effectively removes contaminants and corrosion products from metal surfaces. High-energy pulsed laser radiation is directed at the contaminated surface, where it is selectively absorbed by the surface layers of contaminants and corrosion, due to their higher absorption compared to the base metal. This selective absorption leads to rapid heating and evaporation of these unwanted layers.

[0015] A pulse-periodic laser with a wavelength of about 1 μm, a pulse duration of about 100 ns and an energy density in the range of 1-20 J / cm is used for laser cleaning of carbon steel. 2 The studies conducted by the authors show that at energy densities below 1 J / cm 2the ablation process is too weak to effectively remove thick oxide layers and stubborn contaminants, whereas at values ​​≥ 20 J / cm 2 the risk of mechanical damage to the base material increases. Therefore, the range is 1-20 J / cm 2 can be considered as a reliable working range of values ​​of this parameter for selective cleaning of the surface of carbon steel without damaging the substrate.

[0016] Laser cleaning should begin with an initial pass over the entire cleaning area at a high laser energy density to remove the bulk of the corrosion layer. Subsequent passes are then performed at a lower laser energy density to remove residual contaminants. This approach, based on a gradual reduction in energy density, ensures complete removal of corrosion products and other contaminants and minimizes the risk of substrate damage.

[0017] The laser energy density E is determined by the formula:

[0018]

[0019] where P is the laser output power, f is the pulse repetition frequency, A is the area of ​​the light spot in the laser beam impact zone, which is calculated using the following formula:

[0020]

[0021] where d is the laser beam diameter. Using this formula, you can calculate the required energy exposure density for surface treatment.

[0022] The second stage of bluing restoration is laser polishing, which produces a smooth surface. This is achieved by slightly melting the surface layer of the metal under laser radiation. Localized micromelting smooths out surface roughness due to surface tension forces acting on the molten material. Rapid cooling and solidification prevent the growth of a coarse-grained structure, ensuring the preservation of the material's mechanical properties.

[0023] Laser polishing of carbon steel requires the use of a pulsed laser with a wavelength of about 1 μm, a pulse duration of about 50 ns and an energy density in the range of 5-50 J / cm 2 .

[0024] At lower intensity (less than 5 J / cm 2 ) the melting process becomes insufficiently pronounced for noticeable smoothing of the microrelief, while at excessively high intensity (more than 50 J / cm2 ) there is a risk of intense evaporation and deep local melting, which can lead to the formation of microcracks and internal damage to the base metal. Therefore, the range of impact energy density values ​​is in the range of 5-50 J / cm 2 should be considered the working zone for laser polishing of carbon steel. This range provides an optimal balance between achieving a high melting rate of the metal's surface layer and the absence of structural defects. This allows for a high-quality, smooth surface without altering the substrate's internal structure.

[0025] Laser polishing should begin with a high-energy pass over the metal surface to induce localized melting of uneven areas. Surface tension forces will redistribute the molten material, smoothing out the uneven areas. In subsequent stages, the energy density should be gradually reduced to control the cooling rate and reduce internal stress in the surface layer. This will result in the formation of a uniform grain structure and increased hardness and wear resistance of the metal surface.

[0026] Careful control of laser parameters and the sequence of laser processing steps allow for an optimal balance between removing minor defects and smoothing the surface. This also reduces the risk of substrate damage and ensures subsequent restoration of the bluing.

[0027] The proposed laser cleaning and polishing method begins with an initial pass at a higher energy density, focusing on the effective removal of oxides and other stubborn contaminants. During both stages of laser processing, the intensity should be gradually reduced to ensure a more gentle effect during the final cleaning and polishing stages. Experimental studies conducted by the authors show that with fewer than four consecutive laser processing operations, deep-seated contaminants are not completely removed, and the rough surface does not achieve the required degree of smoothing. An insufficient number of passes increases the risk of uneven processing, which leaves residual contaminants. Furthermore, the polishing quality in certain areas may be unsatisfactory.

[0028] On the other hand, if the total number of passes exceeds ten, excessive heat builds up, which can lead to the formation of microcracks or undesirable phase changes in the base metal structure. In this case, the improvement in surface quality will be insignificant, while the time spent, and therefore production costs, increase significantly. Therefore, 4-10 passes is the optimal number to achieve a high degree of cleaning and high-quality polishing without the risk of damaging the material.

[0029] The third stage of bluing restoration is localized laser heating, which involves controlled laser heating in the presence of atmospheric oxygen. This causes oxidation of the metal surface, forming thin oxide films. The thickness of these films depends on the laser radiation parameters and determines their color due to light interference. By carefully adjusting the laser output parameters, a wide range of color shades can be achieved by varying the thickness of the formed oxide layer. The physical properties of the treated surface—brightness, texture, and roughness—are equally important. These determine not only the final color shade but also the uniformity and visual saturation of the resulting coating when implementing the proposed method.

[0030] By adjusting parameters such as energy density, scanning speed and pulse duration, it is possible to precisely control the growth rate and thickness of the oxide layer, which makes it possible to reproduce the required color characteristics, thereby restoring the original appearance of the restored monuments.

[0031] For localized laser heating of carbon steel, it is recommended to use a pulsed laser with a wavelength of approximately 1 µm. Since heating is performed in pulsed mode, the pulse duration directly influences the oxidation process, as it determines the peak power of the laser radiation. The optimal pulse duration is in the range of 1-120 ns, with best results achieved at values ​​around 30 ns.

[0032] To form a uniform oxide layer and a consistent color across the entire metal surface, multiple (at least three) passes are required at different scanning angles using the same laser parameters. With each new pass, the scanning angle increases by 180 / N angular degrees compared to the scanning angle of the previous pass, where N is the number of passes. For example, with three passes (N=3), each subsequent pass direction is rotated by 60 angular degrees (Fig. 2). This is necessary to minimize color unevenness caused by surface texture or geometry.

[0033] With one or two passes, some areas of the surface may not be processed effectively enough due to the orientation of microroughness, leading to variations in the oxide layer thickness. A third pass at a different incident radiation angle compensates for insufficient exposure in certain areas. This ensures a more uniform energy distribution across the entire surface.

[0034] Multiple scanning at different angles ensures uniform growth of the oxide layer. This results in a uniform and aesthetically pleasing metal surface. Furthermore, controlled layer-by-layer oxidation improves corrosion resistance.

[0035] Example of the invention

[0036] To implement the claimed method, carbon steel samples of grade Steel 20 (GOST 1050) were used. These steels contain approximately 98% iron, 0.18% carbon, 0.2% silicon, 0.5% manganese, and trace amounts of arsenic, rubidium, silver, and other elements. This steel was chosen based on its chemical similarity to the metals used in bluing historical artifacts.

[0037] To simulate corrosion damage, the samples were chemically treated. For this purpose, an aqueous solution of hydrochloric acid (HCl) with a concentration of 0.05 to 1 mol / L was applied to their surface. Exposure to chloride ions accelerated the formation of oxide layers, causing varying degrees of surface damage, including loss of the original bluing color.

[0038] The laser processing experiments conducted on the samples utilized a laser processing system based on a pulsed ytterbium-doped fiber laser with a wavelength of 1064 nm. This type of laser system is a commercially available technological solution and is widely used in industry for high-precision marking of metal parts and the creation of oxide coatings.

[0039] The first stage of processing involved laser cleaning, using a laser with a pulse duration of approximately 100 ns. Cleaning was performed in four passes with a successive decrease in laser energy density in the range of 1-20 J / cm. 2 The energy density was reduced at each pass by approximately 50% compared to the previous value. The following values ​​were used: 19, 10, 4, and 2 J / cm 2 .

[0040] The next step was laser polishing using a laser with a pulse duration of approximately 50 ns. Polishing was performed in ten passes with a successive decrease in laser energy density in the range of 5-50 J / cm. 2 With each pass, the energy density was reduced by 10% from the maximum value of 50 J / cm 2 The following values ​​were applied: 50, 45, 40, 35, 30, 25, 20, 15, 10 and 5 J / cm 2 .

[0041] To restore the original color of a metal surface, such as brown, localized laser heating was used. By carefully adjusting the laser output parameters in the presence of atmospheric oxygen, the surface was oxidized, forming thin oxide films. To achieve the brown color, a laser with a pulse duration of approximately 30 ns and an energy density of 8.5 J / cm was used. 2To achieve uniform formation of the oxide layer and a homogeneous color tone, the processing was performed in four passes at scanning angles of 45°, 90°, 135°, and 180°, while the laser radiation parameters remained unchanged.

[0042] The micrographs shown in Figs. 3.1-3.5 demonstrate visual changes in the surface of the metal sample at different stages of processing. Fig. 3.1 shows a zone with the original brown surface color without signs of corrosion. Fig. 3.2 illustrates a zone with corrosion damage and loss of the original color. Fig. 3.3 shows the first example of a zone with uneven color restoration, obtained at one energy density for each of the three stages of laser processing. As can be seen from Fig. 3.3, the bluing has a large number of bright areas with a color (and therefore thickness) different from the required one, in the form of continuous stripes (blue and violet). Fig. 3.4 shows the second example of a zone with less uneven color restoration, obtained at one optimal (experimentally selected) energy density for each of the three stages of laser processing.This corresponds to the method closest in terms of the set of essential features to the proposed invention [M. Salhab, CLB Rajasekara, OS Vasiliev, ON Karpovand VA Parfenov, “Restoration of Original Color Appearance of Decorative Iron-Based Metal Artworks by Laser Treatment,” 2023, pp. 131-134, doi: 10.1109 / FWPE60445.2023.10368532]. As can be seen from Fig. 3.4, the bluing still has a significant number of areas with a color (and therefore thickness) different from the required one (blue and violet segments). Fig. 3.5 shows a zone with uniform color restoration obtained by the proposed method. It is evident that the density of areas with a color (and therefore thickness) different from the required one has decreased, and the saturation of their color has decreased.

[0043] To quantify the difference between two measured colors, the CIE color difference formula was used [K. McLAREN, “XIII-The Development of the CIE 1976 (L* a* b*) Uniform Color Space and Color-difference Formula”, 2008, vol. 92, no. 9, pp. 338-341, doi: 10.1111 / j.1478-4408.1976.tb03301.x], where ΔE* ab - complete color difference.

[0044] A spectrophotometer was used to measure color differences and assess uniformity, averaging the results from multiple measurements taken on different areas of the sample surface. The results demonstrate that the color variation during restoration using the claimed method, which involves multiple laser passes at different angles, is ΔE*. ab = 1.13, which corresponds to the permissible deviations. This confirms the increased color uniformity compared to restoration with a single laser pass, where ΔE* ab = 3.19.

[0045] The description of the proposed method proves the possibility of achieving a technical result - increasing the homogeneity of the thickness of the oxide layer and, as a consequence, reducing the degree of heterogeneity of the color of the decorative oxide layer created during bluing and increasing its protective properties.

[0046] Repeated cleaning, polishing, and localized laser heating operations ensure a gradual and controlled surface treatment of the metal, causing controlled oxidation and the formation of the desired color shade. To demonstrate this, the claimed method for laser restoration of color finishes (blueing) on ​​carbon steel products, which includes sequential stages of laser cleaning, polishing, and localized laser heating, was implemented with the ability to repeat each operation multiple times until the desired degree of cleaning, polishing, and creation of the target color shade were achieved.

[0047] The embodiment described in this example is based on the use of a pulsed ytterbium fiber laser with a wavelength of 1064 nm, specified energy density values ​​with a given percentage reduction (approximately 50% for cleaning and 10% for polishing), a specified number of passes, and scanning angles. However, the implementation of the claimed method is not limited to this embodiment: the use of this specific laser type and the specified parameters is not mandatory. Achieving the stated goal is also possible with other laser types with different technical characteristics, as well as by varying the energy parameters and scanning angles within the limits that ensure the formation of an oxide layer of the desired color shade.

[0048] Thus, the experiment conducted by the authors confirmed the possibility of practical implementation of the declared method of laser restoration of bluing.

Claims

A method for laser restoration of bluing on a metal surface, including the sequential execution of three technological operations: laser cleaning, laser polishing and local laser heating, characterized in that laser cleaning and laser polishing are repeated multiple times - at least four and no more than ten times, with all repetitions of the laser cleaning operation being performed first, then all repetitions of the laser polishing operation and finally all repetitions of the laser local heating operation, with cleaning and polishing being performed with a decrease in the energy density of laser radiation before each repeated operation, cleaning is carried out at a laser radiation energy density in the range of 1-20 J / cm 2 , polishing at laser energy density in the range of 5-50 J / cm 2, and local laser heating is repeated at different scanning angles uniformly distributed within a half-turn at least three times and with unchanged laser radiation parameters to obtain the required color.

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