Aluminum alloy strengthening method combining cross rolling and nanosecond laser surface remelting

Abstract

The invention relates to an aluminum alloy strengthening method combining cross rolling and nanosecond laser surface remelting. The designed aluminum alloy has the characteristic that isometric crystal wraps a deformation form structure. The strengthening method comprises the following steps that firstly, rolling treatment is conducted on the commercial aluminum alloy plate through the cross rolling technology, the dislocation density is increased after crystal grains are subjected to cross deformation, meanwhile, the internal texture is weakened, and the yield strength of the material can be improved; then, the surface of the material is subjected to remelting treatment through high-frequency pulse nanosecond laser, due to the characteristics that the high-frequency pulse nanosecond laser is short in acting time and high in instant energy, the surface of the material is rapidly solidified after being melted, and grain growth is effectively restrained through the ultra-high cooling rate; the aluminum alloy material with the surface being 320-560 nm submicron equiaxed crystals and the interior being a strip-shaped hardened state structure is formed, the yield strength of the aluminum alloy is remarkably improved, the good corrosion resistance of the aluminum alloy is also considered, and the technical problem that the yield strength and the corrosion resistance of the aluminum alloy are difficult to balance is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of aluminum alloy material production, and in particular relates to an aluminum alloy strengthening method combining cross rolling with nanosecond laser surface remelting. Background Art

[0002] Aluminum alloys are the preferred material for lightweight design due to their low density, high specific strength and excellent processing performance. They are widely used in many fields such as marine ships, frame pipes and automobile manufacturing. However, their low yield strength limits their application in high-strength components. Currently, the commonly used aluminum alloy strengthening methods include solid solution strengthening, precipitation strengthening and deformation strengthening to improve the strength of aluminum alloys.

[0003] Solid solution strengthening, by dissolving alloying elements into the aluminum matrix, causes lattice distortion and pinning of solute atoms, increases resistance to dislocation movement, and improves the strength of the material. However, the solid solution elements will combine with the aluminum matrix to form compounds and precipitate at the grain boundaries, resulting in increased intergranular corrosion sensitivity of the material; precipitation strengthening, by heat treatment, fine and uniformly distributed precipitation phases are formed in the aluminum alloy to improve its mechanical properties, but it is difficult to control the distribution and size of the precipitation phases. Coarse or uneven precipitation phases will form micro-galvanic corrosion with the matrix, reducing the corrosion resistance of the material, and for non-heat-treatable aluminum alloys such as 5 series aluminum alloys, it is impossible to improve the strength of the material through precipitation strengthening; deformation strengthening, by plastic deformation, high-density dislocations are introduced into the material, reducing the number of movable dislocations, and converting the internal structure of the material into a hardened structure, which can greatly improve the yield strength of the material, but plastic deformation destroys the original passivation film structure on the surface of the material, reducing the corrosion resistance of the material. Therefore, how to ensure good corrosion resistance while improving the yield strength of aluminum alloys has become a key issue that needs to be overcome in the development of aluminum alloys. Summary of the invention

[0004] The present invention aims at the problem that the existing aluminum alloy strengthening methods are difficult to take into account the corrosion resistance while improving the yield strength of the material. A cross-rolling combined with nanosecond laser surface remelting aluminum alloy strengthening method is proposed. Submicron equiaxed crystals are prepared on the surface of the strip-like hardened state structure by using nanosecond laser, and the plasticity of the material is improved by surface fine grain strengthening. At the same time, the internal strip-like hardened state structure can greatly improve the yield strength of the material. In addition, the equiaxed crystals with no surface defects can ensure the good corrosion resistance of the material. The present invention firstly performs rolling deformation treatment on the commercial aluminum alloy plate to obtain a strip-like hardened state structure; then, a high-frequency pulse nanosecond laser beam is used to remelt the surface of the material so that a layer of uniform ultrafine equiaxed grains with a size of 320 to 560 nm is formed on the surface of the material, and an aluminum alloy material with a surface size of 320 to 560 nm and an internal strip-like hardened state structure is constructed. The surface submicron equiaxed crystals are used to improve the plasticity and corrosion resistance of the aluminum alloy, and the yield strength is further improved by means of the internal strip-like hardened state structure, so that the corrosion resistance is comprehensively improved while the yield strength is improved.

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

[0006] The first aspect of the present invention provides an aluminum alloy strengthening method by cross rolling combined with nanosecond laser surface remelting, comprising the following steps:

[0007] (1) performing cross-rolling deformation treatment on an aluminum alloy plate with a thickness of d, rotating the aluminum alloy plate by 90° after each rolling pass before performing the next rolling pass, the deformation temperature is 0 to 70°C, and the total deformation amount is 10 to 25%;

[0008] (2) The surface of the deformed workpiece is subjected to laser melting treatment, and the laser pulse frequency used is 170-340KHz, the pulse width is 8-25ns, the spot diameter is 30-80μm, and the laser power density is 5-60kW / cm 2 , overlap rate 4-13%, laser scanning speed 350-850mm / s, light on delay 20μs, light off delay 180μs, corner delay 80μs, laser scanning times 1;

[0009] (3) During the laser melting process, argon is used as the protective gas. By filling the processing environment with argon, the oxygen content in the environment is controlled within the range of 170 to 370 ppm;

[0010] (4) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate heat dissipation and achieve ultra-fast cooling of the molten pool. The temperature of the low-temperature pad is -40 to -150°C.

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

[0012] Preferably, the deformation temperature in step (1) is 15-50° C., and the total deformation amount is 10-20%.

[0013] Preferably, the thickness d of the aluminum alloy plate in step (1) is 3 to 15 mm.

[0014] Preferably, the pulse frequency in step (2) is 220-310 KHz, the pulse width is 12-21 ns, the spot diameter is 40-60 μm, and the laser power density is 10-40 kW / cm 2 , overlap rate 6~9%, laser scanning speed is 450mm / s~700mm / s.

[0015] Preferably, the oxygen content in step (3) is controlled within the range of 200 to 300 ppm.

[0016] Preferably, the temperature of the low-temperature pad in step (4) is -70 to -110°C.

[0017] The second aspect of the present invention provides an aluminum alloy material prepared according to the above aluminum alloy strengthening method, which has submicron equiaxed crystals of 320 to 560 nm on the surface and a strip-shaped hardened structure inside.

[0018] Aluminum alloy strengthening methods such as solid solution strengthening, precipitation strengthening and deformation strengthening will inevitably cause the material's corrosion resistance to decrease while improving the material's yield strength. Therefore, exploring how to improve the yield strength of aluminum alloys without compromising their corrosion resistance is an important research topic. Based on the theory of deformation strengthening, the applicant uses cross-rolling to deform the material in different directions. The entanglement of dislocations in different directions combined with the weakening of the matrix texture by cross-rolling significantly improves the material strength. However, the plasticity and corrosion resistance of the material are greatly damaged after cross-rolling. In this regard, the applicant proposes to use high-frequency pulse nanosecond laser remelting to transform the deformed structure of the material surface into submicron equiaxed crystals, and improve the plasticity and corrosion resistance of the material through fine grain strengthening. Compared with ordinary millisecond lasers, high-frequency pulse nanosecond lasers have concentrated energy, small heat-affected zone, and small molten pool during processing. The small heat-affected zone can avoid the internal deformed structure from being affected by heat during laser processing. The small molten pool has a fast cooling speed of the internal melt, and the low-temperature pad can achieve ultra-fast cooling of the molten pool. Therefore, high-frequency pulse nanosecond lasers can form submicron equiaxed crystals with a size of 320 to 560nm on the surface of the material while ensuring that the material is deformed. The aluminum alloy material with a surface of 320 to 560nm submicron equiaxed crystals and an internal deformed structure is constructed through the above process. Through the coupling of multiple processes, the yield strength can be improved while also ensuring the good corrosion resistance of the material.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The present invention proposes an aluminum alloy material with submicron equiaxed crystals of 320 to 560 nm on the surface and a deformed structure inside. The plasticity of the material is improved by surface fine grain strengthening, while the yield strength of the material is improved by the deformed structure inside. The defect-free submicron equiaxed crystals on the surface can ensure the excellent corrosion resistance of the material, thus solving the problem that the corrosion resistance of high-strength aluminum alloys is difficult to ensure, and providing a new idea for the design of high-performance aluminum alloys.

[0021] (2) The present invention innovatively proposes a coupling process of cross-rolling deformation-high-frequency pulse nanosecond laser surface remelting. Through this process, the grain structure of aluminum alloy can be regulated and structures with different grain sizes and morphologies can be constructed, providing a new method for regulating the structure of aluminum alloy materials.

[0022] (3) The present invention effectively improves the yield strength of aluminum alloy without damaging the corrosion resistance of aluminum alloy; at the same time, all processes in the present application have the advantages of simple operation, low cost, and continuous production, providing a new means for the production of aluminum alloy. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical scheme and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0024] Example 1

[0025] A method for strengthening aluminum alloy by cross rolling combined with nanosecond laser surface treatment comprises the following steps:

[0026] (1) A 5182 aluminum alloy plate with a thickness of 3 mm was cross-rolled, and the aluminum alloy plate was rotated 90° after each rolling process before the next rolling process. The deformation temperature was 24°C and the total deformation was 20%;

[0027] (2) The deformed workpiece was polished step by step using SiC sandpaper to 2000 grit sandpaper, and then ultrasonically cleaned in anhydrous ethanol;

[0028] (3) The surface of the workpiece is subjected to laser melting treatment. The laser pulse frequency used is 220KHz, the pulse width is 12ns, the spot diameter is 40μm, and the laser power density is 10kW / cm 2 , overlap rate 6%, laser scanning speed 450mm / s, light on delay 20μs, light off delay 180μs, corner delay 80μs, serpentine scanning path, laser scanning times 1;

[0029] (4) During the laser melting process, argon is used as the protective gas. By filling the processing environment with argon, the oxygen content in the environment is controlled at 240 ppm;

[0030] (5) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate heat dissipation and achieve ultra-fast cooling of the molten pool. The temperature of the low-temperature pad is -110°C.

[0031] The aluminum alloy strengthening method can prepare an aluminum alloy material with submicron equiaxed crystals of 320 to 560 nm on the surface and a strip-shaped hardened structure inside.

[0032] Example 2

[0033] A method for strengthening aluminum alloy by cross rolling combined with nanosecond laser surface remelting, comprising the following steps:

[0034] (1) A 5182 aluminum alloy plate with a thickness of 3 mm was cross-rolled, and the aluminum alloy plate was rotated 90° after each rolling process before the next rolling process. The deformation temperature was 30°C and the total deformation was 15%;

[0035] (2) The deformed workpiece was polished step by step using SiC sandpaper to 2000 grit sandpaper, and then ultrasonically cleaned in anhydrous ethanol;

[0036] (3) The surface of the workpiece is subjected to laser melting treatment. The laser pulse frequency used is 290KHz, the pulse width is 15ns, the spot diameter is 55μm, and the laser power density is 17.9kW / cm 2 , overlap rate 7%, laser scanning speed 550mm / s, light on delay 20μs, light off delay 180μs, corner delay 80μs, serpentine scanning path, laser scanning times 1;

[0037] (4) During the laser melting process, argon is used as the protective gas. By filling the processing environment with argon, the oxygen content in the environment is controlled at 240 ppm;

[0038] (5) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate heat dissipation and achieve ultra-fast cooling of the molten pool. The temperature of the low-temperature pad is -90°C.

[0039] The aluminum alloy strengthening method can prepare an aluminum alloy material with submicron equiaxed crystals of 320 to 560 nm on the surface and a strip-shaped hardened structure inside.

[0040] Example 3

[0041] A method for strengthening aluminum alloy by cross rolling combined with nanosecond laser surface remelting, comprising the following steps:

[0042] (1) A 5182 aluminum alloy plate with a thickness of 3 mm was cross-rolled, and the aluminum alloy plate was rotated 90° after each rolling process before the next rolling process. The deformation temperature was 50°C and the total deformation was 10%;

[0043] (2) The deformed workpiece was polished step by step using SiC sandpaper to 2000 grit sandpaper, and then ultrasonically cleaned in anhydrous ethanol;

[0044] (3) The surface of the workpiece is subjected to laser melting treatment. The laser pulse frequency used is 310KHz, the pulse width is 21ns, the spot diameter is 60μm, and the laser power density is 40kW / cm 2 , overlap rate 9%, laser scanning speed 700mm / s, light on delay 20μs, light off delay 180μs, corner delay 80μs, serpentine scanning path, laser scanning times 1;

[0045] (4) During the laser melting process, argon is used as the protective gas. By filling the processing environment with argon, the oxygen content in the environment is controlled at 240 ppm;

[0046] (5) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate heat dissipation and achieve ultra-fast cooling of the molten pool. The temperature of the low-temperature pad is -70°C.

[0047] The aluminum alloy strengthening method can prepare an aluminum alloy material with submicron equiaxed crystals of 320 to 560 nm on the surface and a strip-shaped hardened structure inside.

[0048] In addition, in other embodiments of the present invention, the deformation temperature can be set to 0°C, 15°C, 70°C or other values, the total deformation can be set to 10%, 25% or other values, the laser pulse frequency can be set to 170KHz, 340KHz or other values, the pulse width can be set to 8ns, 25ns or other values, the spot diameter can be set to 30μm, 80μm or other values, and the laser power density can be set to 5kW / cm 2 、60kW / cm 2 or other values, the overlap rate is set to 4%, 13% or other values, the laser scanning speed is set to 350mm / s, 850mm / s or other values, the oxygen content is controlled at 170PPm, 200PPm, 300PPm, 370PPm or other values, and the temperature of the low-temperature pad is set to -40°C, -150°C or other values. Moreover, the aluminum alloy plate can also be an aluminum alloy plate with a thickness d of 9mm, 15mm or other thicknesses.

[0049] Comparative Example 1

[0050] A method for improving the corrosion resistance of aluminum alloy by laser surface melting comprises the following steps:

[0051] (1) Use SiC sandpaper to polish the sample surface step by step to 1200 mesh sandpaper, use alcohol ultrasonic cleaning to keep the sample surface clean, and evenly coat a layer of carbon black on the sample surface to improve the laser absorption rate of aluminum alloy;

[0052] (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;

[0053] (3) Laser surface melting of aluminum alloy is performed, and the waveform of the laser light source is modulated into a square wave; wherein the parameters of the square wave are: peak power: 8000 W, pulse frequency: 100 Hz, duty cycle: 0.5. After the laser melting is completed, a molten layer is formed on the surface of the 5182 aluminum alloy.

[0054] Comparative Example 2

[0055] A 5182 aluminum alloy, the preparation method of which comprises the following steps:

[0056] (1) Laser melting treatment of 5182 aluminum alloy plate was performed with a laser pulse frequency of 230 KHz, a pulse width of 15 ns, a spot diameter of 45 μm, and a laser power density of 17.9 kW / cm 2 , overlap rate 5%, laser scanning speed 450mm / s, light on delay 20μs, light off delay 180μs, corner delay 80μs, serpentine scanning path, laser scanning times 1;

[0057] (2) During the laser melting process, argon is used as the protective gas. By filling the processing environment with argon, the oxygen content in the environment is controlled at 200 ppm;

[0058] (3) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate heat dissipation and achieve ultra-fast cooling of the molten pool. The temperature of the low-temperature pad is -90°C.

[0059] Comparative Example 3

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

[0061] (1) A 5182 aluminum alloy plate with a thickness of 3 mm was cross-rolled at a deformation temperature of 24°C and a total deformation of 20%.

[0062] Comparative Example 4

[0063] A 5182 aluminum alloy millisecond laser surface melting method, the preparation method comprising the following steps:

[0064] (1) The 5182 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 a scanning number of 1;

[0065] (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.

[0066] Verification Example 1

[0067] The 5182 aluminum alloy plate, the aluminum alloys prepared in Examples 1-3 and Comparative Examples 1-4 were respectively taken, and their organizational structure, yield strength and corrosion current density were tested using conventional technical methods in the art. The specific test results are shown in Table 1 below.

[0068] Table 1 Test results of 5182 aluminum alloy plate, Examples 1-3 and Comparative Examples 1-4

[0069]

[0070] By comparing the performance of the aluminum alloys of Examples 1-3 and 5182 aluminum alloy, it is found that the aluminum alloy material with submicron equiaxed crystals of 320 to 560 nm on the surface and a strip-like hardened structure inside can effectively improve the yield strength of 5182 commercial 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) By comparing the examples and comparative examples, it can be seen that the submicron equiaxed crystals of 320 to 560 nm and the internal banded hardened structure can significantly improve the yield strength of the 5182 commercial aluminum alloy while still maintaining 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; in Comparative Example 2, no rolling deformation process is set; in Comparative Example 3, no laser surface strengthening process is set; and Comparative Example 4 is a millisecond laser melting process. In Comparative Example 1, carbon black needs to be applied on the surface of the material before processing, and the process is complicated. At the same time, the low-frequency laser melting technology has a low frequency, slow scanning speed, and large spot diameter, resulting in a large heat-affected zone and a long laser beam action time. Although a molten layer is formed on the surface of the material to improve the corrosion resistance of the material, the slow cooling rate makes the grains on the surface of the material coarse, and the internal structure of the material also grows due to the heat, which reduces the yield strength of the material; in Comparative Example 2, submicron equiaxed crystals are formed on the surface of the material through high-frequency pulse nanosecond laser surface remelting, and the internal strip-shaped hardened state structure is lacking. Although good corrosion resistance is guaranteed, the effect of improving the yield strength of the aluminum alloy is limited; Comparative Example 3 introduces a large number of dislocations through rolling, which improves the yield strength of the aluminum alloy, but seriously reduces the corrosion resistance of the aluminum alloy; Comparative Example 4 has a slow millisecond laser processing speed, a large heat-affected zone, a large molten pool, and a slow molten pool cooling rate. Therefore, the grains on the surface of the material are coarse after remelting, 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 cross-rolling deformation and high-frequency pulsed nanosecond laser treatment are both indispensable conditions for this design. Without any one of the processes, the 5182 aluminum alloy cannot obtain the optimal performance.

[0074] In addition, the strengthening method described in the present invention can be applied to not only 5 series aluminum alloys, but also other series aluminum alloys, and is also applicable to other grades of 5 series aluminum alloys.

[0075] The above specific implementation method part specifically introduces the analytical method involved in the present invention. It should be noted that the above introduction is only to help those skilled in the art better understand the method and ideas of the present invention, rather than limiting the relevant content. Without departing from the principle of the present invention, those skilled in the art may also make appropriate adjustments or modifications to the present invention, and the above adjustments and modifications shall also fall within the scope of protection of the present invention.

Claims

1. A method for strengthening aluminum alloy by cross rolling combined with nanosecond laser surface remelting, characterized in that: The steps include: (1) performing cross-rolling deformation treatment on an aluminum alloy plate with a thickness of d, rotating the aluminum alloy plate by 90° after each rolling pass before performing the next rolling pass, the deformation temperature is 0 to 70°C, and the total deformation amount is 10 to 25%; (2) The surface of the deformed workpiece is subjected to laser melting treatment, and the laser pulse frequency used is 170-340KHz, the pulse width is 8-25ns, the spot diameter is 30-80μm, and the laser power density is 5-60kW / cm 2 , overlap rate 4-13%, laser scanning speed 350-850mm / s, light on delay 20μs, light off delay 180μs, corner delay 80μs, laser scanning times 1; (3) During the laser melting process, argon is used as the protective gas. By filling the processing environment with argon, the oxygen content in the environment is controlled within the range of 170 to 370 ppm; (4) During the laser melting process, a low-temperature pad is set at the bottom of the material to accelerate heat dissipation and achieve ultra-fast cooling of the molten pool. The temperature of the low-temperature pad is -40 to -150°C.

2. The aluminum alloy strengthening method according to claim 1, characterized in that: The aluminum alloy plate described in step (1) is a 5 series aluminum alloy with a grade of 5182.

3. The aluminum alloy strengthening method according to claim 1, characterized in that: The deformation temperature in step (1) is 15-50° C., and the total deformation amount is 10-20%.

4. 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 to 15 mm.

5. The aluminum alloy strengthening method according to claim 1, characterized in that: The pulse frequency in step (2) is 220-310 KHz, the pulse width is 12-21 ns, the spot diameter is 40-60 μm, and the laser power density is 10-40 kW / cm 2 , overlap rate 6~9%, laser scanning speed is 450mm / s~700mm / s.

6. The aluminum alloy strengthening method according to claim 1, characterized in that: The oxygen content in step (3) is controlled within the range of 200 to 300 ppm.

7. The aluminum alloy strengthening method according to claim 1, characterized in that: The temperature of the low-temperature pad in step (4) is -70 to -110°C.

8. The aluminum alloy strengthening method according to any one of claims 1 to 7 is used to prepare an aluminum alloy material having submicron equiaxed crystals of 320 to 560 nm on the surface and a strip-shaped hardened structure inside.

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