Aluminum alloy strengthening method for high-frequency pulse nanosecond laser surface remelting after low-temperature rolling

Abstract

The invention relates to an aluminum alloy strengthening method for high-frequency pulse nanosecond laser surface remelting after low-temperature rolling, which comprises the following steps: firstly, carrying out low-temperature rolling deformation treatment on an aluminum alloy plate to obtain an aluminum alloy material with a high dislocation density fiber structure and realize deformation strengthening; afterwards, the surface of the aluminum alloy plate is subjected to high-frequency pulse nanosecond laser remelting treatment, the aluminum alloy plate is rapidly melted by means of the characteristics of high instantaneous energy density, short acting time and the like of high-frequency pulse nanosecond laser, a small molten pool is obtained, the small molten pool has the great supercooling degree, the driving force of crystallization is increased, and ultra-rapid solidification is achieved; and submicron equiaxed crystals with the grain size ranging from 0.55 micrometer to 0.7 micrometer are obtained on the surface of the aluminum alloy, and fine grain strengthening of the surface is achieved. Through the low-temperature rolling and laser remelting processes, the aluminum alloy material with the special structure is prepared, the interior of the aluminum alloy material is of a hardened state high dislocation density structure, and the surface layer of the aluminum alloy material is of a submicron equiaxed structure.

Description

Technical Field

[0001] The 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 performance and hot processing performance. However, when facing certain high-strength requirements or complex and harsh environments, the yield strength of commercial aluminum alloys is often difficult to meet their practical applications, limiting their in-depth application and wide expansion in more fields. Work hardening is one of the important ways to improve the yield strength of commercial aluminum alloys. 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] Aluminum alloys are deformed by rolling, which increases the density of dislocations, strengthens the interaction between dislocations, forms a large number of obstacles such as entanglements and immobile dislocations, and forms a high-density "dislocation forest", which increases the resistance to the movement of the remaining dislocations and achieves work hardening. During the deformation process, dislocations tend to annihilate each other through cross-slip. Cross-slip depends indirectly on stacking fault energy. According to research, stacking fault energy decreases with decreasing temperature, and lower stacking fault energy reduces the effective mobility of dislocations and hinders cross-slip. Therefore, compared with rolling at room temperature, rolling at low temperature has lower stacking fault energy, making cross-slip more difficult, with more dislocations stored, significantly increasing dislocation density, and more obvious strengthening effect, which improves the mechanical properties of aluminum alloy materials. However, when aluminum alloys undergo large plastic deformation, lattice distortion occurs, which in turn causes defects in the original passivation film on the surface, directly weakening the corrosion resistance of aluminum alloys, resulting in great restrictions on the operation of aluminum alloys in complex environments, and it is difficult to meet the urgent needs of various industries for aluminum alloy materials with both high strength and excellent corrosion resistance. In this regard, developing aluminum alloy materials that have both high yield strength and excellent corrosion resistance is an important research direction. Summary of the invention

[0004] The present invention aims at the problem that although the current work hardening can effectively improve the yield strength of aluminum alloy, it inevitably leads to the decline of the corrosion resistance of aluminum alloy. The present invention innovatively proposes a new aluminum alloy structure, which is characterized by its surface layer being a submicron equiaxed grain structure, and the interior being a fiber structure with a high dislocation density. The plasticity of the material is improved by surface fine grain strengthening, while the submicron equiaxed crystal surface makes up for the weakness of the passivation film on the surface of the aluminum alloy after cold working and the lack of dense organization, ensuring the corrosion resistance of the material; and by constructing a high-density "dislocation forest" structure, the yield strength of the aluminum alloy is significantly enhanced; moreover, the production process involved in the present invention has low equipment requirements, low energy consumption, and is easy to implement. The present invention firstly performs 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 pulse 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 process 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 in the range of 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] The first aspect of the present invention provides an aluminum alloy strengthening method by high-frequency pulse nanosecond laser surface remelting after low-temperature rolling, comprising the following steps:

[0007] (1) performing rolling deformation treatment on an aluminum alloy plate having a thickness of d, wherein the deformation temperature is -196 to -20°C and the total deformation is 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 is filled into the environment so that the oxygen content is 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 a "sandwich" structure aluminum alloy material having a surface submicron equiaxed crystal and an internal high dislocation density fiber structure, which is prepared according to the above-mentioned aluminum alloy strengthening method.

[0017] Preferably, 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.

[0018] Work hardening can effectively improve the yield strength of aluminum alloy plates, but it is a technical challenge for aluminum alloy plates used in complex environments to improve the yield strength while maintaining its excellent corrosion resistance. In response to this, the applicant designed a "sandwich" structure aluminum alloy material with submicron equiaxed crystals on the surface and high dislocation density fiber structure inside based on the theory of fine grain strengthening and deformation strengthening, and proposed to introduce a large number of dislocations into commercial aluminum alloy plates to achieve the effect of improving the yield strength of the overall aluminum alloy plates. Although the yield strength of the aluminum alloy is significantly improved, the original passivation film on the surface is destroyed during the rolling process, 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 aluminum alloy materials, 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 a defect-free ultra-fine equiaxed crystal structure with a size in the range of 0.55 to 0.7 μm, 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] To achieve the above solution, two stages of tasks need to be completed: (1) low temperature rolling of the aluminum alloy sheet to obtain an internal hardened structure; (2) remelting the surface of the aluminum alloy into a uniform and fine equiaxed structure. Under the guidance of existing theories, the applicant has completed the above two stages of tasks well and prepared a 5182 aluminum alloy with a submicron equiaxed crystal structure on the surface and a high dislocation density fiber structure in the internal hardened state.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) The present invention proposes a special structural aluminum alloy with a submicron equiaxed crystal structure on the surface and a fiber 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-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 material structure design 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 by the present application is a common process in the field, and has the advantages of simple operation, low cost, and continuous production. DETAILED DESCRIPTION

[0024] 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.

[0025] Example 1

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

[0027] (1) An aluminum alloy plate with a thickness of 3 mm is 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 was used as a protective gas and filled into the environment so that the oxygen content was 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 surface remelted by high-frequency pulse nanosecond laser after low-temperature rolling, the preparation method of which comprises the following steps:

[0033] (1) A 3 mm thick aluminum alloy plate is 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 so that the oxygen content is 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 surface remelted by high-frequency pulse nanosecond laser after low-temperature rolling, the preparation method of which comprises the following steps:

[0039] (1) A 3 mm thick aluminum alloy plate is 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 was used as a protective gas and filled into the environment so that the oxygen content was 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 surface remelted by high-frequency pulse nanosecond laser after low-temperature rolling, the preparation method of which comprises the following steps:

[0045] (1) A 3 mm thick aluminum alloy plate is 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 so that the oxygen content is 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 rate is set to 6%, 13% or other values, the laser scanning speed is set to 300mm / s, 750mm / s or other values, the laser scanning times are set to 2 or 3 times, and the oxygen content is controlled to 120PPm, 350PPm, 415PPm or other values. Moreover, in the embodiment of the present invention, the aluminum alloy plate adopts 5 series aluminum alloy with a grade of 5182; of course, other grades of 5 series aluminum alloys, such as 5005, 5083, 5052, etc., can also be used; or, other series of aluminum alloys can be used. Moreover, the aluminum alloy plate can also adopt an aluminum alloy plate with a thickness d of 6mm, 8mm 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 is 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 was used as a protective gas and filled into the environment so that the oxygen content was 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 scanning path of serpentine order, and a scanning number of 1 time;

[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 of which comprises the following steps:

[0064] (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;

[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 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.

[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 organizational structure, yield strength and elongation were tested using conventional technical methods in the field. 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 is 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 with the 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 under low temperature environment, 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 rolling in Comparative Example 1, less dislocations are introduced into the material, and the improvement of the existing yield strength of commercial aluminum alloys is not achieved to 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 cooling speed of the molten pool is slow, so the grains of the material surface 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; Comparative Example 3 has a high deformation temperature and a large deformation amount, so the yield strength is limited, and the surface layer has not been laser remelted, lacking surface fine grain strengthening, which greatly reduces the plasticity and corrosion resistance of the material; Although Comparative Example 4 has been subjected to low-frequency pulse laser shock strengthening, the corrosion resistance of the material has been improved, but the hardened high dislocation density fiber structure has not been obtained inside the material, so the yield strength of the material has been improved less. The above analysis reveals that low-temperature rolling deformation and laser remelting are both indispensable conditions for this design. Without any one of the processes, the 5182 aluminum alloy cannot obtain the optimal performance.

[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 high-frequency pulse nanosecond laser surface remelting after low-temperature rolling, characterized in that: The steps include: (1) performing rolling deformation treatment on an aluminum alloy plate having a thickness of d, wherein the deformation temperature is -196 to -20°C and the total deformation is 8% to 42%; (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 , the overlap rate is 6-13%, the laser scanning speed is 300-750mm / s, the light-on delay is 20μs, the light-off delay is 180μs, the corner delay is 80μs, and the number of laser scans is 1-3 times; (3) During the laser remelting process, argon is used as the protective gas and is filled into the environment so that the oxygen content is 120 to 415 ppm.

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 -196 to -50°C, and the total deformation amount is 10% to 25%.

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 8 mm.

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

6. 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.

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

8. The aluminum alloy strengthening method according to any one of claims 1 to 7, characterized in that: This method is used to prepare an aluminum alloy material with a "sandwich" structure, which has submicron equiaxed crystals on the surface and high dislocation density fiber structure inside.

9. The aluminum alloy strengthening method according to claim 8, characterized in that: 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.

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