Aluminum alloy strengthening method using high-frequency pulsed nanosecond laser surface remelting after low-temperature rolling

Abstract

The present invention relates to a method for strengthening an aluminum alloy by surface remelting with a high-frequency pulse nanosecond laser after low-temperature rolling. The preparation method is as follows: first, an aluminum alloy plate is subjected to low-temperature rolling deformation treatment to obtain an aluminum alloy material with a high dislocation density fiber structure to achieve deformation strengthening; then, the surface of the aluminum alloy plate is subjected to a high-frequency pulse nanosecond laser remelting treatment. The high instantaneous energy density and short action time of the high-frequency pulse nanosecond laser are utilized to cause the plate to melt rapidly, thereby obtaining a small molten pool. The small molten pool has a large degree of undercooling, which increases the driving force for crystallization and achieves ultra-rapid solidification. This promotes the formation of submicron equiaxed crystals with a grain size in the range of 0.55 to 0.7 μm on the surface of the aluminum alloy, thereby achieving surface fine grain strengthening. Through the above-mentioned low-temperature rolling and laser remelting process, a special structural aluminum alloy material is prepared, in which the interior presents a hardened high dislocation density structure and the surface presents a submicron equiaxed structure.

Description

Technical Field

[0001] The present invention relates to the technical field of aluminum alloy material production, and in particular to an aluminum alloy strengthening method by high-frequency pulse nanosecond laser surface remelting after low-temperature rolling. Background Art

[0002] Aluminum alloys are widely used in various industries due to their low density, good thermal conductivity, excellent welding properties, and hot working properties. However, when commercial aluminum alloys are faced with certain high-strength requirements or complex and harsh environments, their yield strength often fails to meet their practical application, limiting their in-depth application and widespread expansion in more fields. Work hardening is one of the important ways to increase the yield strength of commercial aluminum alloys. However, this strengthening method is often accompanied by a decrease in the corrosion resistance of aluminum alloys. Given the market's urgent demand for high-performance materials, how to improve the yield strength of aluminum alloys while ensuring that their corrosion resistance increases instead of decreases has become a key problem that needs to be overcome.

[0003] Rolling deformation of aluminum alloys increases dislocation density, leading to enhanced interactions between dislocations and the formation of numerous obstacles such as entangled and immobile dislocations, creating a high-density "dislocation forest" that increases resistance to the movement of remaining dislocations and leads to work hardening. During deformation, dislocations tend to annihilate each other through cross-slip, which indirectly depends on the stacking fault energy. According to research, stacking fault energy decreases with decreasing temperature, and lower stacking fault energy reduces the effective mobility of dislocations, hindering cross-slip. Therefore, compared to rolling at room temperature, rolling at low temperatures has lower stacking fault energy, making cross-slip more difficult, allowing for more dislocation storage, significantly increasing dislocation density, and achieving a more pronounced strengthening effect, thereby improving the mechanical properties of aluminum alloys. However, when aluminum alloys undergo large plastic deformation, lattice distortion occurs, which in turn creates defects in the existing passivation film on the surface, directly weakening the corrosion resistance of the aluminum alloy. This significantly limits the ability of aluminum alloys to operate in complex environments, making it difficult to meet the urgent demand of various industries for aluminum alloys with both high strength and excellent corrosion resistance. In this regard, developing aluminum alloy materials with both high yield strength and excellent corrosion resistance is an important research direction. Summary of the Invention

[0004] This invention addresses the problem that current work hardening, while effectively increasing the yield strength of aluminum alloys, inevitably leads to a decrease in their corrosion resistance. By innovatively proposing a novel aluminum alloy microstructure, this structure features a submicron-scale equiaxed grain structure on the surface and a high-dislocation-density fibrous structure within. Surface fine-grain strengthening improves the material's plasticity, while the submicron equiaxed grain surface compensates for the weak passivation film and less dense microstructure of the aluminum alloy after cold working, ensuring the material's corrosion resistance. Furthermore, by constructing a high-density "dislocation forest" structure, the yield strength of the aluminum alloy is significantly enhanced. Furthermore, the production process involved in this invention requires minimal equipment, consumes little energy, and is easily implemented. The present invention first performs a rolling treatment on the aluminum alloy plate in a low-temperature environment to form a hardened strip-shaped structure with a high dislocation density; then uses a high-frequency pulsed nanosecond laser beam (pulse frequency of KHz level and pulse width of ns level) to perform laser surface remelting treatment on the deformed aluminum alloy. Compared with ordinary millisecond lasers, high-frequency nanosecond lasers have the advantages of high frequency, short action time, concentrated energy density, and fast cooling speed. Therefore, when using high-frequency nanosecond lasers to treat the surface of the material, tiny molten pools will be formed, and the cooling rate in the molten pool is fast. The faster cooling rate inhibits grain growth, and a submicron equiaxed crystal layer with a size ranging from 0.55 to 0.7 μm is formed on the surface of the internal hardened strip-shaped structure.

[0005] In order to achieve the above object, the present invention is achieved by the following means:

[0006] A first aspect of the present invention provides a method for strengthening aluminum alloy by high-frequency pulsed nanosecond laser surface remelting after low-temperature rolling, comprising the following steps:

[0007] (1) An aluminum alloy plate having a thickness of d is subjected to rolling deformation treatment at a deformation temperature of -196 to -20°C and a total deformation of 8% to 42%;

[0008] (2) The surface of the deformed workpiece is laser remelted, and the laser pulse frequency used is 110-440KHz, the pulse width is 15-35ns, the spot diameter is 10-100μm, and the laser power density is 8-65kW / cm 2 , overlap rate is 6-13%, laser scanning speed is 300-750mm / s, light on delay is 20μs, light off delay is 180μs, corner delay is 80μs, laser scanning times is 1-3 times;

[0009] (3) During the laser remelting process, argon is used as the protective gas and the environment is filled with argon to make the oxygen content 120 to 415 ppm.

[0010] Preferably, the aluminum alloy plate in step (1) is a 5 series aluminum alloy with a grade of 5182.

[0011] Preferably, the deformation temperature in step (1) is -196 to -50°C, and the total deformation amount is 10% to 25%.

[0012] Preferably, the thickness d of the aluminum alloy plate in step (1) is 3 to 8 mm.

[0013] Preferably, the thickness d of the aluminum alloy plate in step (1) is 3 to 6 mm.

[0014] Preferably, the pulse frequency in step (2) is 200-340 KHz, the pulse width is 20 ns, the spot diameter is 30-50 μm, and the laser power density is 15-50 kW / cm 2 , the overlap rate is 8-10%, the laser scanning speed is 500mm / s-600mm / s, and the number of laser scanning times is 1-2 times.

[0015] Preferably, the oxygen content in step (3) is 210-350 ppm.

[0016] The second aspect of the present invention provides an aluminum alloy material having a "sandwich" structure with submicron equiaxed crystals on the surface and high dislocation density fiber structure in the interior, which is prepared according to the above-mentioned aluminum alloy strengthening method.

[0017] Preferably, the grain size of the submicron equiaxed crystals in the surface layer of the aluminum alloy material is in the range of 0.55 to 0.7 μm.

[0018] Work hardening can effectively increase the yield strength of aluminum alloy sheets, but for aluminum alloy sheets used in complex environments, achieving this while maintaining excellent corrosion resistance is a technical challenge. Based on the theories of grain refinement and deformation strengthening, the applicant designed a "sandwich" aluminum alloy material with a surface layer of submicron equiaxed crystals and an internal high-dislocation-density fibrous structure. The applicant proposed introducing a large number of dislocations into commercial aluminum alloy sheets to increase the yield strength of the entire sheet. Although the yield strength of the aluminum alloy was significantly improved, the original passivation film on the surface was destroyed during rolling, resulting in reduced corrosion resistance and plasticity. In this regard, in order to maintain and improve corrosion resistance and plasticity, the applicant used laser remelting technology to introduce fine equiaxed crystal structure into the surface of the aluminum alloy material, forming a dense passivation film, and repairing the surface defects that occurred during the rolling process; at the same time, the applicant chose rolling in a low-temperature environment. Under the same deformation amount, low-temperature rolling can obtain a higher dislocation density than room temperature rolling, which causes the internal structure of the material to become a fibrous structure with high dislocation density; high-frequency nanosecond laser remelting treatment was selected, and its advantages of ultra-fast melting and ultra-fast solidification were used to form an ultra-fine equiaxed crystal structure with a size of 0.55 to 0.7 μm without defects, which makes the passivation film denser, thereby ensuring the corrosion resistance of the material and improving the plasticity of the material, while not affecting the yield strength increased by low-temperature rolling inside. Finally, a special structure of aluminum alloy was obtained through the coupling process of low-temperature rolling deformation-laser remelting, so as to achieve a design that improves the yield strength while ensuring the corrosion resistance of the material.

[0019] Implementing the above solution requires completing two phases: (1) low-temperature rolling the aluminum alloy sheet to obtain an internal hardened microstructure; and (2) remelting the aluminum alloy surface to produce a uniform, fine, equiaxed microstructure. Guided by existing theories, the applicant successfully completed these two phases, producing a 5182 aluminum alloy with a submicron equiaxed microstructure on the surface and a high-dislocation-density fibrous microstructure in the hardened state.

[0020] The present invention has the following beneficial effects compared to the prior art:

[0021] (1) The present invention proposes a special structural aluminum alloy with a submicron equiaxed crystal structure on the surface and a fibrous structure with high dislocation density inside, which solves the problem of improved yield strength after rolling but insufficient corrosion resistance, and provides a new idea for the high-strength and corrosion-resistant design of commercial aluminum alloys.

[0022] (2) The present invention innovatively proposes a coupling process of low-temperature rolling deformation and laser remelting, through which the deformation strengthening of the internal fiber structure of the aluminum alloy and the regulation of the submicron equiaxed grain size on the surface can be achieved, providing a new direction for the design of material structure and a new process for the design of high-performance commercial aluminum alloys.

[0023] (3) The present invention effectively improves the corrosion resistance of aluminum alloy without affecting the yield strength significantly improved by low-temperature rolling; moreover, each link process in the process designed in this application is a common process in the field, which has the advantages of simple operation, low cost, and continuous production. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the following examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0025] Example 1

[0026] A strengthening method for an aluminum alloy subjected to surface remelting by high-frequency pulse nanosecond laser after low-temperature rolling comprises the following steps:

[0027] (1) A 3 mm thick aluminum alloy plate was subjected to rolling deformation treatment at a deformation temperature of -50°C and a deformation amount of 10%;

[0028] (2) After the deformation is completed, the workpiece surface is laser remelted with a laser frequency of 220 KHz, a pulse width of 20 ns, and a laser power density of 21.5 kW / cm 2 , the laser scanning speed is 500mm / s, the spot diameter is 50μm, the overlap rate is 10%, the light-on delay is 20μs, the light-off delay is 180μs, the corner delay is 80μs, the scanning path is a serpentine path, and the number of scans is 1;

[0029] (3) During the laser treatment process, argon is used as a protective gas and is filled into the environment so that the oxygen content is 210 ppm.

[0030] The above method can prepare a "sandwich" structure aluminum alloy material with surface submicron equiaxed crystals and internal high dislocation density fiber structure, and the grain size of the surface submicron equiaxed crystals of the aluminum alloy material is in the range of 0.55 to 0.7 μm.

[0031] Example 2

[0032] An aluminum alloy subjected to surface remelting by high-frequency pulse nanosecond laser after low-temperature rolling, wherein the preparation method comprises the following steps:

[0033] (1) A 3 mm thick aluminum alloy plate was subjected to rolling deformation treatment at a deformation temperature of -80°C and a deformation amount of 15%;

[0034] (2) After the deformation is completed, the workpiece surface is laser remelted with a laser frequency of 260 KHz, a pulse width of 20 ns, and a laser power density of 28 kW / cm2 , the laser scanning speed is 525mm / s, the spot diameter is 45μm, the overlap rate is 9%, the light-on delay is 20μs, the light-off delay is 180μs, the corner delay is 80μs, the scanning path is a serpentine path, and the number of scans is 1;

[0035] (3) During the laser treatment process, argon is used as a protective gas and is filled into the environment to make the oxygen content reach 240 ppm.

[0036] The above method can prepare a "sandwich" structure aluminum alloy material with surface submicron equiaxed crystals and internal high dislocation density fiber structure, and the grain size of the surface submicron equiaxed crystals of the aluminum alloy material is in the range of 0.55 to 0.7 μm.

[0037] Example 3

[0038] An aluminum alloy subjected to surface remelting by high-frequency pulse nanosecond laser after low-temperature rolling, wherein the preparation method comprises the following steps:

[0039] (1) A 3 mm thick aluminum alloy plate was subjected to rolling deformation treatment at a deformation temperature of -130°C and a deformation amount of 20%;

[0040] (2) After the deformation is completed, the workpiece surface is laser remelted with a laser frequency of 280 KHz, a pulse width of 20 ns, and a laser power density of 36 kW / cm 2 , the laser scanning speed is 550mm / s, the spot diameter is 40μm, the overlap rate is 8%, the light-on delay is 20μs, the light-off delay is 180μs, the corner delay is 80μs, the scanning path is a serpentine path, and the number of scans is 1;

[0041] (3) During the laser treatment process, argon is used as a protective gas and is filled into the environment to make the oxygen content reach 270 ppm.

[0042] The above method can prepare a "sandwich" structure aluminum alloy material with surface submicron equiaxed crystals and internal high dislocation density fiber structure, and the grain size of the surface submicron equiaxed crystals of the aluminum alloy material is in the range of 0.55 to 0.7 μm.

[0043] Example 4

[0044] An aluminum alloy subjected to surface remelting by high-frequency pulse nanosecond laser after low-temperature rolling, wherein the preparation method comprises the following steps:

[0045] (1) A 3 mm thick aluminum alloy plate was subjected to rolling deformation treatment at a deformation temperature of -196°C and a deformation amount of 25%;

[0046] (2) After the deformation is completed, the workpiece surface is laser remelted with a laser frequency of 300 KHz, a pulse width of 20 ns, and a laser power density of 45 kW / cm 2 , the laser scanning speed is 575mm / s, the spot diameter is 35μm, the overlap rate is 10%, the light-on delay is 20μs, the light-off delay is 180μs, the corner delay is 80μs, the scanning path is a serpentine path, and the number of scans is 1;

[0047] (3) During the laser treatment process, argon is used as a protective gas and is filled into the environment to make the oxygen content reach 300 ppm.

[0048] The above method can prepare a "sandwich" structure aluminum alloy material with surface submicron equiaxed crystals and internal high dislocation density fiber structure, and the grain size of the surface submicron equiaxed crystals of the aluminum alloy material is in the range of 0.55 to 0.7 μm.

[0049] In addition, in other embodiments of the present invention, the deformation temperature can be set to -108°C, -20°C or other values, the total deformation can be set to 8%, 42% or other values, the laser pulse frequency can be set to 110KHz, 440KHz or other values, the pulse width can be set to 15ns, 35ns or other values, the spot diameter can be set to 10μm, 30μm, 100μm or other values, and the laser power density can be set to 8kW / cm 2 、65kW / cm 2 or other values, the overlap ratio is set to 6%, 13%, or other values, the laser scanning speed is set to 300 mm / s, 750 mm / s, or other values, the number of laser scans is set to 2 or 3, and the oxygen content is controlled to 120 ppm, 350 ppm, 415 ppm, or other values. Furthermore, in the embodiment of the present invention, the aluminum alloy plate is a 5-series aluminum alloy with a designation of 5182. Of course, other 5-series aluminum alloys, such as 5005, 5083, 5052, or other series of aluminum alloys, may also be used. Furthermore, the aluminum alloy plate may have a thickness d of 6 mm, 8 mm, or other thicknesses.

[0050] Comparative Example 1

[0051] A 5182 aluminum alloy, the preparation method of which comprises the following steps:

[0052] (1) A 3 mm thick aluminum alloy plate was subjected to rolling deformation treatment at a deformation temperature of 25°C and a deformation amount of 20%;

[0053] (2) After the deformation is completed, the workpiece surface is laser remelted with a laser frequency of 220 KHz, a pulse width of 20 ns, and a laser power density of 21.5 kW / cm2 , the laser scanning speed is 500mm / s, the spot diameter is 50μm, the overlap rate is 10%, the light-on delay is 20μs, the light-off delay is 180μs, the corner delay is 80μs, the scanning path is a serpentine sequence, and the number of scans is 1;

[0054] (3) During the laser treatment process, argon is used as a protective gas and is filled into the environment so that the oxygen content is 210 ppm.

[0055] Comparative Example 2

[0056] A 5182 aluminum alloy millisecond laser surface melting method, the preparation method comprising the following steps:

[0057] (1) The aluminum alloy plate was laser remelted with a laser frequency of 400 Hz, a pulse width of 50 ms, a laser scanning speed of 120 mm / s, a spot diameter of 1 mm, an overlap rate of 10%, a serpentine scanning path, and one scan.

[0058] (2) During the laser treatment process, argon is used as a protective gas and is filled into the environment so that the oxygen content is 210 ppm.

[0059] Comparative Example 3

[0060] A 5182 aluminum alloy, the preparation method of which comprises the following steps:

[0061] (1) An aluminum alloy plate with a thickness of 3 mm was subjected to rolling deformation treatment at a deformation temperature of -50°C and a deformation amount of 20%.

[0062] Comparative Example 4

[0063] A method for improving the corrosion resistance of aluminum alloy by laser surface melting, the preparation method comprising the following steps:

[0064] (1) Use SiC sandpaper to polish the sample surface step by step until it reaches 1200 mesh sandpaper, and use alcohol ultrasonic cleaning to keep the sample surface clean. In order to improve the absorption rate of aluminum alloy to laser, a layer of carbon black is evenly coated on the sample surface;

[0065] (2) Preliminary optimization of laser surface melting process parameters was performed to obtain a preliminary laser surface melting process window. The optimized parameters were: laser wavelength 1.06 μm, average laser power 800 W, scanning speed 8 mm / s, overlap rate 50%, spot diameter 2 mm, and carrier gas flow rate 10 L / min.

[0066] (3) Laser surface melting of aluminum alloy was performed, and the waveform of the laser light source was modulated into a square wave; wherein, the parameters of the square wave were: peak power: 8000W, pulse frequency: 100HZ, duty cycle: 0.5. After the laser melting was completed, a molten layer was formed on the surface of the 5182 aluminum alloy.

[0067] Verification Example 1

[0068] Commercial aluminum plates, aluminum alloys prepared in Examples 1-4 and Comparative Examples 1-4 were respectively taken, and their microstructures, yield strengths and elongations were tested using conventional technical methods in the art. The specific test results are shown in Table 1 below.

[0069] Table 1 Test results of 5182 aluminum plate, Examples 1-4 and Comparative Examples 1-4

[0070]

[0071] By comparing the performance of the aluminum alloys of Examples 1-4 and 5182 aluminum alloy, it was found that under the action of the hardened high-density dislocation fiber structure formed after low-temperature rolling, the yield strength of the aluminum alloy can be greatly improved on the original basis, and since submicron equiaxed crystals are formed on the surface after laser remelting, the corrosion current density of the treated material is smaller than that of the material not treated with high-frequency pulse nanosecond laser through grain refinement, that is, the aluminum alloy plate after laser remelting has better corrosion resistance.

[0072] By comparing and analyzing the results of Examples 1-4 and Comparative Examples 1-4, the following conclusions can be drawn:

[0073] (1) By comparing the examples and comparative examples, it can be seen that compared with room temperature, the yield strength of the 5182 aluminum alloy plate can be improved by rolling at low temperature, and the corrosion resistance of the aluminum alloy can be effectively improved.

[0074] (2) Comparative Example 1 is a room temperature rolling deformation process, Comparative Example 2 is a millisecond laser melting process, Comparative Example 3 does not have a laser remelting process, and Comparative Example 4 is a low-frequency laser melting process. Due to the lack of low-temperature environment rolling in Comparative Example 1, fewer dislocations are introduced into the material, and the improvement of the existing yield strength of commercial aluminum alloys does not achieve the best effect; the millisecond laser processing speed in Comparative Example 2 is slow, the heat-affected zone is large, the molten pool is large, and the molten pool cooling speed is slow. Therefore, after the surface of the material is remelted, the grains are coarse and the surface state is extremely poor, resulting in a decrease in the yield strength and corrosion resistance of the material; Comparative Example 3 has a high deformation temperature and a large deformation amount, so the yield strength improvement is limited, and the surface layer has not undergone laser remelting treatment, lacking surface fine grain strengthening, which greatly reduces the plasticity and corrosion resistance of the material; Although Comparative Example 4 has improved the corrosion resistance of the material through low-frequency pulse laser shock strengthening, the material does not obtain a hardened high dislocation density fiber structure inside the material, so the improvement in the yield strength of the material is small. The above analysis reveals that low-temperature rolling deformation and laser remelting are both indispensable conditions for this design. Without any one of these processes, the 5182 aluminum alloy cannot achieve optimal performance.

[0075] The above detailed description of the analytical methods involved in the present invention provides a detailed introduction. It should be noted that the above description is intended solely to help those skilled in the art better understand the methods and concepts of the present invention, and is not intended to limit the relevant content. Without departing from the principles of the present invention, those skilled in the art may make appropriate adjustments or modifications to the present invention, and such adjustments and modifications shall also fall within the scope of protection of the present invention.

Claims

1. A method for strengthening aluminum alloy by high-frequency pulse nanosecond laser surface remelting after low-temperature rolling, characterized in that: The steps include: (1) An aluminum alloy plate with a thickness of d is subjected to rolling deformation treatment, the deformation temperature is -196~-20℃, and the total deformation is 8%~42%; the aluminum alloy plate is a 5 series aluminum alloy with a grade of 5182; (2) The surface of the deformed workpiece is laser remelted. The laser pulse frequency used is 110~440KHz, the pulse width is 15~35ns, the spot diameter is 10~100µm, and the laser power density is 8~65kW / cm 2 , overlap rate is 6~13%, laser scanning speed is 300~750mm / s, light on delay is 20µs, light off delay is 180µs, corner delay is 80µs, laser scanning times is 1~3 times; (3) During the laser remelting process, argon is used as the protective gas and the environment is filled with argon to make the oxygen content 120~415PPm.

2. The aluminum alloy strengthening method according to claim 1, characterized in that: The deformation temperature in step (1) is -196~-50°C, and the total deformation is 10%~25%.

3. The aluminum alloy strengthening method according to claim 1, characterized in that: The thickness d of the aluminum alloy plate in step (1) is 3-8 mm.

4. The aluminum alloy strengthening method according to claim 3, characterized in that: The thickness d of the aluminum alloy plate in step (1) is 3-6 mm.

5. The aluminum alloy strengthening method according to claim 1, characterized in that: The pulse frequency in step (2) is 200~340KHz, the pulse width is 20ns, the spot diameter is 30~50µm, and the laser power density is 15~50kW / cm 2 , the overlap rate is 8~10%, the laser scanning speed is 500mm / s~600mm / s, and the number of laser scanning times is 1~2 times.

6. The aluminum alloy strengthening method according to claim 1, characterized in that: The oxygen content in step (3) is 210~350PPm.

7. The aluminum alloy strengthening method according to claim 1, characterized in that: The grain size of the surface submicron equiaxed crystals of the aluminum alloy material prepared by this method is in the range of 0.55~0.7µm.

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