A strengthening method for 5 series aluminum alloy

Abstract

The present invention proposes a method for strengthening a 5-series aluminum alloy having surface submicron equiaxed crystals and internal banded structure, the method being as follows: first, the 5-series aluminum alloy plate is subjected to rolling treatment to obtain a banded structure, thereby improving the yield strength of the material; subsequently, the surface is remelted using a nanosecond laser, and the surface remelting treatment is performed on the material by utilizing the characteristics of the nanosecond laser's short action time and high instantaneous energy. The material surface is melted and then rapidly solidified, and the ultrafast cooling rate effectively inhibits grain growth, so that the surface grain size is maintained at 200-400nm. The surface fine grains improve the plasticity and corrosion resistance of the material; ultimately, an aluminum alloy material with submicron equiaxed crystals of 200-400nm on the surface and a banded hardened structure inside is formed, which not only significantly improves the yield strength of the aluminum alloy, but also takes into account its excellent corrosion resistance, thereby solving the technical problem of the difficult balance between the yield strength and corrosion resistance of the aluminum alloy.

Description

Technical Field

[0001] The invention belongs to the technical field of aluminum alloy strengthening production, and in particular relates to a strengthening method for 5 series aluminum alloy. Background Art

[0002] The main alloying elements in 5-series aluminum alloys are aluminum and magnesium. Due to their lightweight, high strength, and good processability, they are widely used in many fields such as marine vessels, framework pipes, and automobile manufacturing. However, 5-series aluminum alloys are non-heat-treatable aluminum alloys, which means that the material strength cannot be improved through precipitation strengthening mechanisms. Therefore, the mechanical properties of 5-series aluminum alloys are limited by their chemical composition and internal structure, and their strength improvement mainly relies on methods such as solid solution strengthening, deformation strengthening, and grain refinement.

[0003] Solid solution strengthening, by increasing the solid solubility of magnesium in aluminum, thereby improving the yield strength of 5 series aluminum alloys. However, the solid solubility of magnesium in aluminum is limited, and when the magnesium content is too high, it will lead to an increase in the intergranular corrosion and stress corrosion sensitivity of the alloy; deformation strengthening, by introducing high-density dislocations into the interior of the material through rolling treatment, the internal structure is transformed into a deformed structure, which greatly improves the yield strength of the material, but rolling destroys the original passivation film structure on the surface of the material, resulting in a decrease in the corrosion resistance of the material; grain refinement strengthening, the material is first subjected to large plastic deformation and then annealed to form small grains, and the mechanical properties of the aluminum alloy are improved by grain refinement, but this method has high requirements for equipment and high energy consumption, and due to large plastic deformation, it has great limitations on the production of thick steel plates. Therefore, there is an urgent need for an economical and effective method that can enhance the yield strength of aluminum alloys while maintaining their good corrosion resistance. Summary of the Invention

[0004] To address the problem that deformation strengthening methods improve the yield strength of 5-series aluminum alloys while simultaneously reducing their corrosion resistance, this paper proposes a new method for preparing high-strength, high-plasticity, and high-corrosion-resistant 5-series aluminum alloys. The aluminum alloy produced by this method features a heterogeneous structure with submicron equiaxed grains on the surface and a banded hardened structure within. This structural heterogeneity leverages surface submicron grain refinement to improve the material's plasticity while simultaneously increasing its yield strength through the banded structure within. Furthermore, high-frequency nanosecond laser remelting of the surface produces defect-free equiaxed grains within the material, ensuring excellent corrosion resistance, achieving improved yield strength without compromising corrosion resistance. The present invention first subjects a 5-series aluminum alloy sheet to a rolling deformation treatment to obtain a banded hardened structure. Subsequently, the surface is remelted using a nanosecond laser beam. Leveraging the rapid solidification and cooling properties of the nanosecond laser, a uniform layer of ultrafine equiaxed grains is formed on the surface, while the banded hardened structure is maintained within the material. This results in an aluminum alloy with equiaxed grains of 200-400 nm on the surface and a banded hardened structure within the material. By utilizing the surface submicron equiaxed crystal structure to enhance the plasticity and corrosion resistance of the aluminum alloy, and with the help of the internal strip-like hardened structure, its yield strength is further improved, achieving the goal of ensuring good corrosion resistance while improving the yield strength.

[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 a 5-series aluminum alloy by nanosecond laser surface remelting, comprising the following steps:

[0007] (1) The 5 series aluminum alloy plate is subjected to rolling deformation treatment, the deformation temperature is 10-70°C, and the total deformation amount is 10-30%;

[0008] (2) After calendering deformation, it is subjected to laser melting treatment. The laser pulse frequency used is 100-400KHz, the pulse width is 10-30ns, the spot diameter is 10-100μm, and the laser power density is 10-90kW / cm 2 , overlap rate 5-15%, laser scanning speed 300-800mm / s, light on delay 20μs, light off delay 180μs, corner delay 80μs, laser scanning times 1-3 times;

[0009] (3) During the laser melting process, argon gas is used to protect the molten pool so that the oxygen content in the environment is 150-330 ppm;

[0010] (4) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate the cooling rate of the molten pool and achieve ultra-fast cooling. The temperature of the low-temperature pad is -90 to -196 °C.

[0011] Preferably, the grade of the 5 series aluminum alloy plate in step (1) is 5182.

[0012] Preferably, the deformation temperature in step (1) is 20-55° C., and the total deformation amount is 10-20%.

[0013] Preferably, the pulse frequency in step (2) is 200-350 KHz, the pulse width is 15-20 ns, the spot diameter is 30-50 μm, and the laser power density is 15-75 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.

[0014] Preferably, the oxygen content in step (3) is 180-300 ppm.

[0015] Preferably, the temperature of the low-temperature pad in step (4) is -110 to -196°C.

[0016] The second aspect of the present invention provides an aluminum alloy material prepared according to the above-mentioned strengthening method, which has submicron equiaxed crystals of 200 to 400 nm on the surface and a strip-shaped hardened structure inside.

[0017] Existing 5-series aluminum alloy strengthening methods, such as solid solution strengthening and deformation strengthening, often reduce the corrosion resistance of the material while increasing the yield strength of the aluminum alloy. Therefore, exploring ways to increase the yield strength of 5-series aluminum alloy while ensuring good corrosion resistance is an important research direction. Based on the theory of deformation strengthening, the applicant proposed using a rolling process to transform the interior of the aluminum alloy into a banded hardened state structure, significantly improving the yield strength of the aluminum alloy material by reducing the number of mobile dislocations inside the material. However, the rolling process will damage the plasticity and corrosion resistance of the material. In response to this, the applicant proposed to introduce submicron equiaxed crystals to the surface of the material to improve the plasticity and corrosion resistance of the material. In order to obtain submicron equiaxed crystals on the surface while maintaining a high-strength hardened state structure inside the material, the applicant used nanosecond laser surface remelting technology, taking advantage of the characteristics of concentrated energy density, small heat-affected zone, and fast cooling rate of nanosecond laser. While the surface of the material is melted by the laser beam, the internal structure of the material will not change due to the heat effect. Combined with a low-temperature pad to achieve ultra-fast cooling of the surface molten pool, the ultra-fast cooling rate forms submicron equiaxed crystals with a size of 200 to 400 nm on the surface of the material, and improves the plasticity of the aluminum alloy through fine grain strengthening. The defect-free submicron equiaxed grains on the surface can ensure good corrosion resistance. The aluminum alloy material with 200 to 400 nm submicron equiaxed crystals on the surface and internal striped hardened state structure constructed by the present invention can improve the yield strength while also ensuring good corrosion resistance of the material through the coupling effect of multiple processes.

[0018] The present invention has the following beneficial effects compared to the prior art:

[0019] (1) The present invention proposes an aluminum alloy with submicron equiaxed crystals of 200 to 400 nm on the surface and a strip-shaped hardened heterogeneous structure inside. The surface fine grain strengthening and defect-free submicron equiaxed crystals improve the plasticity and corrosion resistance of the material, while the internal deformed structure improves the yield strength of the material, solving the problem of ensuring the corrosion resistance of high-strength aluminum alloys and providing new ideas for the design of high-performance aluminum alloys.

[0020] (2) The present invention innovatively proposes a coupled process of rolling deformation-nanosecond laser surface remelting, through which the microstructure of aluminum alloy can be regulated, providing a new method for regulating the microstructure of aluminum alloy materials.

[0021] (3) The present invention effectively improves the yield strength of aluminum alloy without compromising its corrosion resistance; at the same time, each link of the process is easy to operate, cost-effective, and suitable for continuous production, providing a new means for the production of aluminum alloy. DETAILED DESCRIPTION

[0022] 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.

[0023] Example 1

[0024] A method for strengthening a 5 series aluminum alloy comprises the following steps:

[0025] (1) A 5182 aluminum alloy plate with a thickness of 3 mm was rolled at a deformation temperature of 25°C and a total deformation of 20%;

[0026] (2) The deformed workpiece was polished step by step using SiC sandpaper to 2000 grit sandpaper and then ultrasonically cleaned in anhydrous ethanol;

[0027] (3) The polished workpiece was subjected to laser melting treatment. The laser pulse frequency used was 220 kHz, the pulse width was 15 ns, the spot diameter was 30 μm, and the laser power density was 15 kW / cm 2 , overlap rate 8%, laser scanning speed 500mm / s, light on delay 20μs, light off delay 180μs, corner delay 80μs, laser scanning times 1;

[0028] (4) During the laser melting process, argon gas is used to protect the molten pool so that the oxygen content in the environment is 180 ppm;

[0029] (5) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate the cooling rate of the molten pool and achieve ultra-fast cooling. The temperature of the low-temperature pad is -196℃.

[0030] The strengthening method can produce an aluminum alloy material with submicron equiaxed crystals of 200 to 400 nm on the surface and a strip-shaped hardened structure inside.

[0031] Example 2

[0032] A method for strengthening a 5 series aluminum alloy comprises the following steps:

[0033] (1) A 5182 aluminum alloy plate with a thickness of 3 mm was rolled at a deformation temperature of 25°C and a total deformation of 15%;

[0034] (2) The deformed workpiece was polished step by step using SiC sandpaper to 2000 grit sandpaper and then ultrasonically cleaned in anhydrous ethanol;

[0035] (3) The polished workpiece was subjected to laser melting treatment. The laser pulse frequency used was 280 kHz, the pulse width was 20 ns, the spot diameter was 45 μm, and the laser power density was 54 kW / cm 2 , overlap rate 9%, laser scanning speed 550mm / s, light on delay 17μs, light off delay 180μs, corner delay 80μs, laser scanning times 1;

[0036] (4) During the laser melting process, argon gas is used to protect the molten pool so that the oxygen content in the environment is 240 ppm;

[0037] (5) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate the cooling rate of the molten pool and achieve ultra-fast cooling. The temperature of the low-temperature pad is -150℃.

[0038] The strengthening method can produce an aluminum alloy material with submicron equiaxed crystals of 200 to 400 nm on the surface and a strip-shaped hardened structure inside.

[0039] Example 3

[0040] A method for strengthening a 5 series aluminum alloy comprises the following steps:

[0041] (1) A 5182 aluminum alloy plate with a thickness of 3 mm was rolled at a deformation temperature of 25°C and a total deformation of 10%;

[0042] (2) The deformed workpiece was polished step by step using SiC sandpaper to 2000 grit sandpaper and then ultrasonically cleaned in anhydrous ethanol;

[0043] (3) The polished workpiece was subjected to laser melting treatment. The laser pulse frequency used was 330 kHz, the pulse width was 20 ns, the spot diameter was 50 μm, and the laser power density was 75 kW / cm 2 , overlap rate 10%, laser scanning speed 600mm / s, light on delay 20μs, light off delay 180μs, corner delay 80μs, laser scanning times 1;

[0044] (4) During the laser melting process, argon gas is used to protect the molten pool so that the oxygen content in the environment is 240 ppm;

[0045] (5) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate the cooling rate of the molten pool and achieve ultra-fast cooling. The temperature of the low-temperature pad is -110℃.

[0046] The strengthening method can produce an aluminum alloy material with submicron equiaxed crystals of 200 to 400 nm on the surface and a strip-shaped hardened structure inside.

[0047] In addition, in other embodiments of the present invention, the deformation temperature can be set to 10°C, 70°C or other values, the total deformation can be set to 10%, 30% or other values, the laser pulse frequency can be set to 100KHz, 400KHz or other values, the pulse width can be set to 10ns, 30ns or other values, the spot diameter can be set to 10μm, 100μm or other values, and the laser power density can be set to 10kW / cm 2 , 90kW / cm 2 or other values, the overlap rate is set to 5%, 15% or other values, the laser scanning speed is set to 300mm / s, 800mm / s or other values, the number of laser scans is set to 2 or 3 times, the oxygen content is controlled to 150PPm, 330PPm or other values, and the low-temperature pad temperature is set to -90℃, -143℃ or other values. In addition to the 5 series aluminum alloy, other grades of aluminum alloys can also be used.

[0048] Comparative Example 1

[0049] A method for improving the corrosion resistance of aluminum alloy by laser surface melting, the preparation method comprising the following steps:

[0050] (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;

[0051] (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.

[0052] (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.

[0053] Comparative Example 2

[0054] A 5182 aluminum alloy, the preparation method of which comprises the following steps:

[0055] (1) Laser melting treatment of 5182 aluminum alloy plate was performed. The laser pulse frequency used was 220KHz, the pulse width was 20ns, the spot diameter was 30μm, and the laser power density was 8kW / cm 2 , overlap rate 8%, laser scanning speed 500mm / s, light on delay 20μs, light off delay 180μs, corner delay 80μs, laser scanning times 1;

[0056] (2) During the laser melting process, argon gas is used to protect the molten pool so that the oxygen content in the environment is 240 ppm;

[0057] (3) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate the cooling rate of the molten pool and achieve ultra-fast cooling. The temperature of the low-temperature pad is -150℃.

[0058] Comparative Example 3

[0059] A 5182 aluminum alloy, the preparation method of which comprises the following steps:

[0060] (1) A 5182 aluminum alloy plate with a thickness of 3 mm was rolled at a deformation temperature of 25°C and a total deformation of 20%.

[0061] Comparative Example 4

[0062] A 5182 aluminum alloy millisecond laser surface melting method, the preparation method comprising the following steps:

[0063] (1) The aluminum alloy plate was laser remelted with a laser frequency of 400 Hz, a pulse width of 50 ms, a scanning speed of 120 mm / s, a spot diameter of 1 mm, an overlap rate of 10%, a serpentine scanning path, and 1 scan number;

[0064] (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.

[0065] Verification Example 1

[0066] The microstructure, yield strength and corrosion current density of the 5182 aluminum alloy plate, the aluminum alloys prepared in Examples 1-3 and Comparative Examples 1-4 were tested using conventional technical methods in the art. The specific test results are shown in Table 1 below.

[0067] Table 1 Test results of 5182 aluminum alloy plate, Examples 1-3 and Comparative Examples 1-3

[0068]

[0069]

[0070] By comparing the performance of the aluminum alloys of Examples 1-3 with the 5182 aluminum alloy plate, it was found that the aluminum alloy material with submicron equiaxed crystals of 200 to 400 nm on the surface and a strip-like hardened structure inside can effectively improve the yield strength of the commercial 5182 aluminum alloy.

[0071] By comparing and analyzing the results of Examples 1-3 and Comparative Examples 1-4, the following conclusions can be drawn:

[0072] (1) It can be seen from the comparative examples that the submicron equiaxed crystals of 200 to 400 nm and the structure of the internal strip-shaped hardened state organization can effectively improve the yield strength of the commercial 5182 aluminum alloy while ensuring good corrosion resistance.

[0073] (2) Comparative Example 1 is a method for improving the corrosion resistance of aluminum alloy by low-frequency laser surface melting; Comparative Example 2 does not have a rolling deformation process; Comparative Example 3 does not have a laser surface strengthening process; Comparative Example 4 is a millisecond laser melting process. Comparative Example 1 is a low-frequency laser melting technology with a low frequency and large spot diameter, resulting in a large heat-affected zone, a large molten pool, and a slow cooling rate. Although a molten layer is formed on the surface of the material to improve the corrosion resistance of the material, due to the slow cooling rate, submicron equiaxed crystals are not formed on the surface of the material, and the internal structure of the material grows due to the heat during the laser treatment process, which reduces the yield strength of the material. Comparative Example 2, although submicron equiaxed crystals are formed on the surface through laser surface remelting, the effect of improving the yield strength of the aluminum alloy is limited due to the lack of banded hardened structure inside. Comparative Example 3, although the yield strength of the aluminum alloy is improved by introducing a large number of dislocations, the corrosion resistance of the aluminum alloy is seriously reduced. Comparative Example 4, due to the slow millisecond laser processing speed, large heat-affected zone, large molten pool, and slow cooling rate of the molten pool, the grains after surface remelting are coarse and the surface state is extremely poor, resulting in a decrease in the yield strength and corrosion resistance of the material. The above analysis reveals that both rolling deformation and nanosecond laser treatment are essential conditions for this design. Without any of the processes, the 5182 aluminum alloy cannot achieve the optimal performance.

[0074] 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 a 5 series aluminum alloy, characterized in that: The steps include: (1) The 5 series aluminum alloy plate with the grade of 5182 was subjected to rolling deformation treatment, the deformation temperature was 10~70℃, and the total deformation was 10~30%; (2) After calendering deformation, the surface is laser melted. The laser pulse frequency used is 100~400KHz, the pulse width is 10~30ns, the spot diameter is 10~100µm, and the laser power density is 10~90kW / cm 2 , overlap rate 5~15%, laser scanning speed 300~800mm / s, light on delay 20µs, light off delay 180µs, corner delay 80µs, laser scanning times 1~3 times; (3) During the laser melting process, argon gas is used to protect the molten pool so that the oxygen content in the environment is 150~330PPm; (4) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate the cooling rate of the molten pool and achieve ultra-fast cooling. The temperature of the low-temperature pad is -90~-196℃.

2. The strengthening method according to claim 1, characterized in that The deformation temperature in step (1) is 20-55°C, and the total deformation is 10-20%.

3. The strengthening method according to claim 1, characterized in that The pulse frequency in step (2) is 200~350KHz, the pulse width is 15~20ns, the spot diameter is 30~50µm, and the laser power density is 15~75kW / 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.

4. The strengthening method according to claim 1, characterized in that The oxygen content in step (3) is 180~300PPm.

5. The strengthening method according to claim 1, characterized in that The temperature of the low-temperature pad in step (4) is -110~-196℃.

6. The strengthening method according to any one of claims 1 to 5, characterized in that: An aluminum alloy material with submicron equiaxed crystals of 200~400nm on the surface and a strip-like hardened structure in the interior was prepared by an aluminum alloy strengthening method using nanosecond laser surface remelting.

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