A method for preparing a magnesium alloy heterogeneous nanostructure surface layer by a laser surface remelting technique

Abstract

The application discloses a method for preparing a magnesium alloy heterogeneous nanostructure surface layer by a laser surface remelting technology. The method comprises the following steps: performing laser treatment on upper and lower surfaces of a magnesium alloy plate to obtain a heterogeneous nanostructure remelted layer on the surface of the magnesium alloy plate; and the laser treatment conditions comprise the following: the wavelength of a laser source is 1070 nm, the laser treatment is performed under an argon protective atmosphere, and a cooling medium is liquid nitrogen. The depth of the heterogeneous nanostructure remelted layer prepared on the surface of the magnesium alloy plate by the above laser treatment process can reach 200-1000 microns, and the remelted layer has a multiphase and multiscale heterogeneous nanostructure. The multiphase of the heterogeneous nanostructure refers to that there are magnesium phases and nano precipitated phases in the remelted layer, and the multiscale of the heterogeneous nanostructure refers to that there are micron-scale fine crystals, nano-scale ultra-fine crystals, nanocrystals and nano precipitated phases in the remelted layer, the average scale of the nanocrystals on the surface is not greater than 60 nm, and the average particle size of the nano precipitated phases is not greater than 25 nm.

Description

Technical Field

[0001] This invention relates to the field of metal material processing, and in particular to a method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy. Background Technology

[0002] Magnesium alloys are the lightest known metallic engineering materials, making them ideal for lightweighting components such as electric vehicle battery boxes, steering wheels, steering shaft housings, and seat supports. With the growing trend towards carbon neutrality and the rapid development of the electric vehicle industry, the market application prospects for magnesium alloys are very broad. However, compared to traditional metallic engineering materials such as high-strength steel and aluminum alloys, magnesium alloys have relatively lower strength and poorer plasticity, which limits their practical industrial applications. Grain refinement is one of the most effective means to improve the strength of magnesium alloys; however, while the strength of magnesium alloys increases significantly with decreasing grain size, it is accompanied by a sharp decrease in plasticity, which cannot meet the requirements of industrial applications.

[0003] Currently, introducing heterostructures into metallic materials can simultaneously improve both strength and ductility. Laser processing, with its advantages of high energy density, good controllability, energy saving, and minimal environmental pollution, is one of the effective methods for introducing heterostructures onto magnesium alloy surfaces. However, traditional laser processes for treating magnesium alloy sheets have three drawbacks. First, the depth of the affected zone created by traditional laser processing is limited because excessively high laser power will cause the magnesium alloy surface to be directly burned away and evaporated. Second, after laser treatment, the grains on the surface of the magnesium alloy sheet will be refined, but generally into small columnar crystals or dendritic morphologies. Finally, precipitates on the surface of the magnesium alloy sheet often aggregate at grain boundaries, forming coarse network structures. Therefore, although these magnesium alloy materials prepared by traditional laser processes form heterostructures on the sheet surface, the improvement in material strength is limited, and the coarse, brittle precipitates will significantly reduce the material's ductility.

[0004] Therefore, existing technologies still need to be improved and developed. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for preparing a heterogeneous nanostructure remelted layer on the surface of magnesium alloy, which aims to solve the problem that although the magnesium alloy material prepared by traditional laser process forms a heterogeneous structure on the surface of magnesium alloy sheet, the improvement of the material strength is limited, and the coarse brittle precipitates will greatly reduce the plasticity of the material.

[0006] The technical solution of the present invention is as follows:

[0007] A method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy, wherein the method includes the following steps:

[0008] Magnesium alloy sheets are available.

[0009] The surface of the magnesium alloy sheet is pretreated;

[0010] The pretreated magnesium alloy sheet is placed into a mold on a laser processing platform, and liquid nitrogen is injected until the pretreated magnesium alloy sheet is submerged. Under an argon protective atmosphere, a laser source with a wavelength of 1070 nm is used to perform laser treatment on the pretreated magnesium alloy sheet, resulting in a heterogeneous nanostructure remelted layer on the surface of the magnesium alloy sheet.

[0011] Optionally, the thickness of the magnesium alloy sheet is 2-20 mm.

[0012] Optionally, the pretreatment step of the magnesium alloy sheet surface specifically includes: mechanically polishing the surface of the magnesium alloy sheet with sandpaper, then ultrasonically cleaning it in an alcohol solution, and finally removing and drying it.

[0013] Optionally, the sandpaper is 600-2000 grit sandpaper.

[0014] Optionally, the process parameters of the laser processing include: laser power of 80-200W, scanning speed of 10-100mm / s, laser frequency of 1000-5000HZ, spot diameter of 0.5-2mm, and overlap rate of 0.5.

[0015] Optionally, the depth of the heterogeneous nanostructure remelted layer is 200-1000 μm.

[0016] Optionally, the heterostructure is a multiphase, multiscale heterostructure. The multiphase of the heterostructure refers to the presence of a magnesium phase and a nano-precipitated phase within the remelted layer. The multiscale of the heterostructure refers to the presence of micron-scale fine crystals, nano-scale ultrafine crystals, nanocrystals, and nano-precipitated phases within the remelted layer.

[0017] Beneficial Effects: The heterogeneous nanostructured remelted layer prepared on the surface of magnesium alloy plates by the above-mentioned laser treatment method can reach a depth of 200-1000 μm. This remelted layer has a multiphase, multi-scale heterogeneous nanostructure. The multiphase aspect of the heterogeneous nanostructure refers to the presence of magnesium phase and nano-precipitated phase within the remelted layer. The multi-scale aspect refers to the presence of micron-scale fine grains, nano-scale ultrafine grains, nanocrystals, and nano-precipitated phases within the remelted layer. The average size of the outermost nanocrystals is no greater than 60 nm, and the average particle size of the nano-precipitated phases is no greater than 25 nm. The yield strength and elongation of the alloy after laser treatment by this invention are significantly improved compared to the original alloy, ensuring the alloy's plasticity while improving its strength. Attached Figure Description

[0018] Figure 1 (a) is a schematic diagram of the laser processing in Example 1. Figure 1 (b) is a schematic diagram of the heterogeneous nanostructure on the surface of the plate after laser treatment in Example 1;

[0019] Figure 2 (a) is an electron backscattering diffraction image of the heterostructure with refined remelted layer on the surface of the AZ41 magnesium alloy sheet in Example 1. Figure 2 (b) shows the tensile property test curves of the original AZ41 alloy and the laser-treated alloy in Example 1;

[0020] Figure 3 Image (a) is a transmission electron microscope (TEM) image of the outermost surface of the remelted layer of the WE43 magnesium alloy sheet in Example 2. Figure 3 (b) shows the tensile property test curves of the original WE43 alloy and the laser-treated alloy in Example 1;

[0021] Figure 4 Image (a) is a transmission electron microscope (TEM) image of the outermost surface of the remelted layer of the ZK60 magnesium alloy sheet in Example 3. Figure 4 (b) shows the tensile property test curves of the original ZK60 alloy and the laser-treated alloy in Example 1. Detailed Implementation

[0022] This invention provides a method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0023] This invention provides a method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy, wherein the method includes the following steps:

[0024] Magnesium alloy sheets are available.

[0025] The surface of the magnesium alloy sheet is pretreated;

[0026] The pretreated magnesium alloy sheet is placed into a mold on a laser processing platform, and liquid nitrogen is injected until the pretreated magnesium alloy sheet is submerged. Under an argon protective atmosphere, a laser source with a wavelength of 1070 nm is used to perform laser treatment on the pretreated magnesium alloy sheet, resulting in a heterogeneous nanostructure remelted layer on the surface of the magnesium alloy sheet.

[0027] This embodiment provides a method for preparing a multiphase, multi-scale heterogeneous nanostructure remelted layer on the surface of a magnesium alloy sheet using a modified laser remelting process. The depth of the heterogeneous nanostructure remelted layer prepared on the surface of the magnesium alloy sheet using this laser processing method can reach 200-1000 μm, and this remelted layer possesses a multiphase, multi-scale heterogeneous nanostructure. The multiphase aspect of this heterogeneous nanostructure refers to the presence of a magnesium phase and nano-precipitated phases within the remelted layer. The multi-scale aspect refers to the presence of micron-scale fine grains, nano-scale ultrafine grains, nanocrystals, and nano-precipitated phases within the remelted layer. The average size of the outermost nanocrystals is no greater than 60 nm, and the average particle size of the nano-precipitated phases is no greater than 25 nm.

[0028] Compared to traditional laser processing, this embodiment uses a 1070nm laser source, employs argon as the protective atmosphere, and uses liquid nitrogen as the cooling medium. The energy emitted by the 1070nm wavelength laser is more easily absorbed by the magnesium alloy sheet surface, resulting in better melting, a larger temperature gradient, and a deeper influence zone. Argon protection prevents oxidation of the melt on the magnesium alloy sheet surface or the formation of impurities that could affect the bonding between the molten and unmelted zones. The use of liquid nitrogen increases the cooling rate of the laser remelting zone, and rapid cooling of the melt produces smaller grains and nano-precipitates. This embodiment combines the advantages of a 1070nm wavelength laser source, argon protection, and liquid nitrogen cooling. By adjusting appropriate laser processing parameters, a heterogeneous nanostructure remelted layer of considerable thickness can be formed on the surface of the magnesium alloy sheet. The formation of this heterogeneous nanostructure is due to the combined effect of compositional supercooling caused by solute atoms and thermodynamic supercooling caused by a high cooling rate. On the one hand, the solute elements provide nucleation sites for the solidification of the magnesium alloy melt, forming a compositionally supercooled zone inside the melt. On the other hand, the cooling of liquid nitrogen shortens the solidification time to the millisecond level, resulting in a significant grain refinement effect. Simultaneously, due to the significant temperature gradient created by laser treatment, the cooling rate at the bottom of the melt is two orders of magnitude lower than that at the surface. Therefore, solidification begins from the bottom of the melt, resulting in a significantly higher grain refinement effect on the melt surface compared to the bottom. Consequently, the grain size of this remelted layer gradually decreases from the bottom to the surface, consisting of micron-scale fine grains, nano-scale ultrafine grains, nanocrystals, and nanoprecipitates. As the temperature decreases and magnesium grains nucleate within the melt, the solid solubility of the solute elements in the melt decreases, promoting the precipitation of the second phase. Similarly, the rapid cooling rate slows down the growth and diffusion rates of the precipitated phase, resulting in the precipitated phase remaining at the nanoscale and uniformly distributed within the grains and at grain boundaries. Simultaneously, the pinning effect of the nanoprecipitated phases on the grains also hinders grain growth. Therefore, the heterogeneous nanostructure of the remelted layer on the surface of the magnesium alloy sheet treated by laser in this embodiment is multiphase and multiscale.

[0029] Compared with existing magnesium alloy surface laser treatment technologies, this embodiment has the following advantages:

[0030] 1. Simple to operate, highly safe, high yield, suitable for large-scale production.

[0031] 2. The average grain size of the nanocrystals formed on the outermost surface of the treated magnesium alloy sheet is no greater than 60 nm, and the average particle size of the nanoprecipitates is no greater than 25 nm.

[0032] 3. Magnesium alloy sheets with heterogeneous nanostructures can improve material strength without sacrificing the alloy's plasticity compared to original magnesium alloy sheets.

[0033] In one embodiment, the thickness of the magnesium alloy sheet is 2-20 mm.

[0034] In one embodiment, the magnesium alloy sheet is one of AZ41 magnesium alloy sheet, WE43 magnesium alloy sheet, ZK60 magnesium alloy sheet, etc., but is not limited to this.

[0035] In one embodiment, the step of pre-treating the surface of the magnesium alloy sheet specifically includes: mechanically polishing the surface of the magnesium alloy sheet with sandpaper, then ultrasonically cleaning it in an alcohol solution, and finally removing and drying it.

[0036] In one embodiment, the sandpaper is 600-2000 grit sandpaper, such as 1200 grit sandpaper.

[0037] In one embodiment, the laser processing parameters include: laser power of 80-200W, scanning speed of 10-100mm / s, laser frequency of 1000-5000Hz, spot diameter of 0.5-2mm, and overlap rate of 0.5. Under these laser processing parameters, it can be ensured that the depth of the remelted layer reaches 200-1000μm, the average size of the outermost nanocrystals is no greater than 60nm, and the average particle size of the nanoprecipitated phase is no greater than 25nm.

[0038] This invention provides a magnesium alloy material, comprising a magnesium alloy sheet and a heterogeneous nanostructure remelted layer on the surface of the magnesium alloy sheet, wherein the heterogeneous nanostructure remelted layer is prepared using the method described in this invention.

[0039] The present invention will be further described below through several specific embodiments.

[0040] Example 1

[0041] A 2mm thick AZ41 magnesium alloy sheet was selected for laser processing, such as... Figure 1As shown in (a), the wavelength of the laser source is 1070 nm, the protective atmosphere used in the laser processing is argon, and the cooling medium is liquid nitrogen.

[0042] The specific method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy is as follows:

[0043] (1) Pretreatment of sheet material. The upper and lower surfaces of the 2mm thick AZ41 magnesium alloy sheet are mechanically polished with 600-grit sandpaper, then ultrasonically cleaned in an alcohol solution, and then dried to obtain a smooth AZ41 magnesium alloy sheet.

[0044] (2) Laser processing. For example... Figure 1 As shown in (a), a specially designed aluminum alloy mold is positioned on the laser processing platform using positioning screws. An AZ41 magnesium alloy sheet workpiece is placed into the recess of the mold, and liquid nitrogen is injected through the liquid nitrogen channel until the sheet is submerged. The laser emitter is then activated for laser processing. The laser power is 100W, the scanning speed is 10mm / s, the laser frequency is 5000Hz, the spot diameter is 0.5mm, and the overlap rate is 0.5%. After laser processing on one side, the workpiece is flipped over for laser processing on the other side.

[0045] After laser treatment, the grains of the remelted layer on the surface of the magnesium alloy sheet are significantly refined, and nano-precipitates also appear inside the refined grains, such as... Figure 1 As shown in (b). Figure 2 As shown in (a), the remelted layer thickness on the surface of the AZ41 magnesium alloy sheet prepared by the method of this embodiment can reach 220 μm, and the surface grain refinement effect is obvious, with the outermost grain refined to 60 nm, and the average particle size of the nano-precipitated phase is 23 nm. Figure 2 As shown in Figure (b), tensile mechanical property tests revealed that the yield strength and elongation of the laser-treated alloy increased by 1.32 times and 1.07 times, respectively, compared to the original alloy. This example illustrates that laser treatment can simultaneously improve the strength and ductility of AZ41 magnesium alloy.

[0046] Example 2

[0047] A WE43 magnesium alloy sheet with a thickness of 4mm was selected for laser treatment. The wavelength of the laser source was 1070nm. The protective atmosphere used in the laser treatment process was argon, and the cooling medium was liquid nitrogen.

[0048] The specific method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy is as follows:

[0049] (1) Pretreatment of sheet material. The upper and lower surfaces of the 4mm thick WE43 magnesium alloy sheet material are mechanically polished with 2000 grit sandpaper, and then ultrasonically cleaned in an alcohol solution before being taken out and dried to obtain a smooth WE43 magnesium alloy sheet material.

[0050] (2) Laser treatment. Laser treatment was performed on the upper and lower surfaces of the WE43 magnesium alloy sheet. The laser power was 200W, the scanning speed was 100mm / s, the laser frequency was 1000HZ, the spot diameter was 2mm, and the overlap rate was 0.5.

[0051] The remelted layer thickness of the WE43 magnesium alloy sheet prepared by the method in this embodiment is 1000 μm, and the surface grain refinement effect is obvious, such as... Figure 3 As shown in (a), the outermost grains are refined to 30 nm, and the average particle size of the nano-precipitates in the remelted layer is 20 nm. Figure 3 As shown in Figure (b), tensile mechanical property tests revealed that the yield strength and elongation of the laser-treated alloy increased by 2.68 times and 1.12 times, respectively, compared to the original alloy. This example illustrates that laser treatment can simultaneously improve the strength and ductility of WE43 magnesium alloy.

[0052] Example 3

[0053] A 3mm thick ZK60 magnesium alloy sheet was selected for laser treatment. The wavelength of the laser source was 1070nm. The protective atmosphere used in the laser treatment process was argon, and the cooling medium was liquid nitrogen.

[0054] The specific method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy is as follows:

[0055] (1) Pretreatment of sheet material. The upper and lower surfaces of the 3mm thick ZK60 magnesium alloy sheet are mechanically polished with 1500 grit sandpaper, then ultrasonically cleaned in an alcohol solution, and then dried to obtain a smooth ZK60 magnesium alloy sheet.

[0056] (2) Laser treatment. Laser treatment was performed on the upper and lower surfaces of the ZK60 magnesium alloy plate. The laser power was 80W, the scanning speed was 50mm / s, the laser frequency was 2000HZ, the spot diameter was 1mm, and the overlap rate was 0.5.

[0057] The ZK60 magnesium alloy sheet prepared by the method in this embodiment has a remelted layer thickness of 200 μm, and the surface grain refinement effect is obvious, such as... Figure 4 As shown in (a), the outermost grains are refined to 20 nm, and the average particle size of the nano-precipitates in the remelted layer is 25 nm. Figure 4 As shown in Figure (b), tensile mechanical property tests revealed that the yield strength and elongation of the laser-treated alloy increased by 1.48 times and 1.45 times, respectively, compared to the original alloy. This example illustrates that laser treatment can simultaneously improve the strength and ductility of ZK60 magnesium alloy.

[0058] In summary, this invention provides a method for preparing a heterogeneous nanostructured remelted layer on a magnesium alloy surface. The method involves mechanically polishing the upper and lower surfaces of a 2-20 mm thick magnesium alloy sheet with 1200-grit sandpaper, followed by ultrasonic cleaning in an alcohol solution and drying to obtain a smooth metal surface. Next, laser treatment is applied to the upper and lower surfaces of the magnesium alloy sheet. The laser power is 80-200 W, the scanning speed is 10-100 mm / s, the laser frequency is 1000-5000 Hz, the spot diameter is 0.5-2 mm, the overlap ratio is 0.5, the wavelength of the light source is 1070 nm, the protective atmosphere used during the laser treatment is argon, and the cooling medium is liquid nitrogen. The remelted layer prepared on the sheet surface using this laser treatment process can reach a depth of 200-1000 μm, and this remelted layer possesses a multiphase, multi-scale heterogeneous nanostructure. The multiphase nature of this heterogeneous nanostructure refers to the presence of a magnesium phase and nano-precipitates within the remelted layer. The multiscale nature of this heterogeneous nanostructure refers to the presence of micron-scale fine grains, nano-scale ultrafine grains, nanocrystals, and nano-precipitates within the remelted layer. The average size of the outermost nanocrystals is no greater than 60 nm, and the average particle size of the precipitates is no greater than 23 nm. This invention is simple to operate, highly safe, and has a high yield. The resulting magnesium alloy sheet has a multiphase, multiscale heterogeneous nanostructure on its surface, exhibiting excellent mechanical properties.

[0059] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy, characterized in that, The method includes the following steps: Magnesium alloy sheets are available. The surface of the magnesium alloy sheet is pretreated; The pretreated magnesium alloy sheet is placed in a mold on a laser processing platform, and liquid nitrogen is injected until the pretreated magnesium alloy sheet is completely submerged. Under an argon protective atmosphere, a laser source with a wavelength of 1070nm is used to laser process the pretreated magnesium alloy sheet, and a heterogeneous nanostructure remelted layer is obtained on the surface of the magnesium alloy sheet. The laser processing parameters include: laser power of 80-200W, scanning speed of 10-100mm / s, laser frequency of 1000-5000HZ, spot diameter of 0.5-2mm, and overlap rate of 0.

5. The heterostructure is a multiphase, multiscale heterostructure. The multiphase of the heterostructure refers to the presence of magnesium phase and nano-precipitated phase in the remelted layer. The multiscale of the heterostructure refers to the presence of micron-scale fine crystals, nano-scale ultrafine crystals, nanocrystals and nano-precipitated phases in the remelted layer.

2. The method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy according to claim 1, characterized in that, The thickness of the magnesium alloy sheet is 2-20mm.

3. The method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy according to claim 1, characterized in that, The steps for pre-treating the surface of the magnesium alloy sheet specifically include: mechanically polishing the surface of the magnesium alloy sheet with sandpaper, then ultrasonically cleaning it in an alcohol solution, and finally removing and drying it.

4. The method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy according to claim 3, characterized in that, The sandpaper is 600-2000 grit sandpaper.

5. The method for preparing a heterogeneous nanostructured remelted layer on the surface of a magnesium alloy according to claim 1, characterized in that, The depth of the heterogeneous nanostructure remelted layer is 200-1000 μm.

Images