Method for improving corrosion resistance of magnesium alloy based on ultrasonic-assisted laser modification

Abstract

The invention belongs to the technical field of metal surface treatment, and particularly discloses a method for improving corrosion resistance of magnesium alloy based on ultrasonic-assisted laser modification. The modification method comprises the following steps: under protective gas, carrying out ultrasonic-assisted laser treatment on the surface of the magnesium alloy to obtain the corrosion-resistant magnesium alloy, the equiaxed grain size of the remelting layer is 1 mu m-2 mu m; wherein the ultrasonic power ranges from 60 W to 100 W, and the laser power ranges from 180 W to 200 W. According to the method, a micron-sized grain remelting layer which is good in compactness, uniform in grain size refinement and certain in thickness is formed on the surface of the magnesium alloy in an induced mode through ultrasonic-assisted laser surface melting, the self-corrosion current density of the surface of the magnesium alloy can be reduced through the remelting layer, and the corrosion resistance of the magnesium alloy is remarkably improved. The method for improving the corrosion resistance of the magnesium alloy has the advantages that the treatment process is simple, the forming period of the protective layer is relatively short, popularization and application are facilitated, and the prepared magnesium alloy product is excellent in corrosion resistance, and has a relatively high industrialization prospect.

Description

Technical Field

[0001] The invention belongs to the technical field of metal surface treatment and relates to a method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification. Background Art

[0002] Magnesium alloys have the advantages of low density, high specific strength, and good damping and shock absorption. They have been widely used in engineering fields such as aerospace components, automobile wheels, and engine shafts. However, due to the high chemical activity and low corrosion potential of magnesium alloys, especially in humid environments, galvanic corrosion is prone to occur, which greatly limits the application of magnesium alloys in related fields. Although the dense MgO film formed on its surface can provide certain protection in a dry environment, in a humid environment, especially in an environment containing salt or other corrosive media, MgO will react with water to form loose Mg(OH) 2 , resulting in film breakage. In addition, Mg(OH) 2 It is easy to dissolve, causing the substrate to be continuously exposed to the corrosive environment, eventually forming holes on the surface, leading to product failure.

[0003] In view of the poor corrosion resistance of magnesium alloys, a variety of surface protection methods have been developed, such as plasma electrolytic oxidation and chemical conversion coating. However, these methods generally have shortcomings such as complex pretreatment, long protective layer formation cycle, and increased risk of stress cracking on the surface of magnesium alloys after treatment. Summary of the invention

[0004] In view of the shortcomings of the prior art methods for improving the corrosion resistance of magnesium alloys, the present invention improves the surface stress distribution and grain structure of magnesium alloys through ultrasound-assisted laser modification to achieve the purpose of strengthening magnesium alloys and enhancing their corrosion resistance.

[0005] In order to achieve the above-mentioned object of the invention, the embodiment of the present invention adopts the following technical solution: In the first aspect, the present invention provides a method for improving the corrosion resistance of magnesium alloy based on ultrasonic-assisted laser modification, the method comprising the following steps: under a protective gas, performing ultrasonic-assisted laser treatment on the surface of the magnesium alloy to obtain a corrosion-resistant magnesium alloy; the equiaxed grain size of the remelted layer is 1μm~2μm; wherein the power of the ultrasound is 60W~100W, and the power of the laser is 180W~200W.

[0006] Compared with traditional heating methods, laser control has the advantages of non-contact, local control, and high efficiency. By controlling the laser parameters, the surface structure of magnesium alloy can be precisely controlled. However, magnesium alloy has a low melting point, and the temperature gradient caused by the laser will cause abnormal growth of grains in the vertical direction, resulting in stress concentration and cracking. The present invention uses ultrasonic assisted laser surface melting to improve the corrosion resistance of the magnesium alloy surface. The surface of the magnesium alloy is irradiated with a high-energy laser beam to heat and melt the surface of the material instantly, and then quickly cool it. The surface quality of the laser remelting is significantly improved by ultra-high frequency vibration, which is specifically manifested in shattering coarse growth dendrites, refining grains, promoting molten pool flow, reducing element segregation, and forming a dense remelting layer. The equiaxed grain size of the remelting layer can reach 1μm~2μm. The remelting layer reduces surface defects such as pores and cracks, thereby inhibiting the penetration of corrosive media into the substrate.

[0007] The method for improving the corrosion resistance of magnesium alloy provided by the present invention has a simple processing process, a relatively short protective layer formation period, is conducive to popularization and application, and the corrosion resistance of the obtained magnesium alloy product is significantly improved, thus having a high industrialization prospect.

[0008] Preferably, the thickness of the remelted layer is 34.9 μm to 35.7 μm.

[0009] Preferably, the ultrasonic vibration frequency is 18kHz~22kHz; The protective gas and laser are coaxially output; The protective gas flow rate is 6L / min~8L / min; The protective gas includes at least one of argon, neon or krypton. For example, considering the economic efficiency, the protective gas in the present invention is described by taking argon as an example, and other protective gases can also achieve the same effect as argon.

[0010] The present invention adopts coaxial output of protective gas and laser. The advantage of coaxial output is that the protective gas can always act on the laser remelting area, thereby realizing cooling effect on the area, partially inhibiting the growth of magnesium alloy remelting dendrites, which is beneficial to grain refinement.

[0011] Preferably, in the ultrasound-assisted laser processing process, the laser spot diameter is 28 μm to 50 μm; the laser emission line width is 4 nm to 5.5 nm; the laser frequency is 800 Hz to 1200 Hz; and the laser scanning speed is 8 mm / s to 12 mm / s.

[0012] On this basis, the laser spot pulse width selected in the present invention is 200 μs and the laser wavelength is 1070 nm.

[0013] During the modification process, after the laser completes a single scan from left to right along the X direction, the laser head stops outputting the laser and returns to the left, completing a round trip motion; wherein the laser scanning speed is 8 mm / s to 12 mm / s, and after each round trip, the laser steps a corresponding distance in the Y direction.

[0014] Preferably, during the ultrasound-assisted laser processing, the spot overlap rates of the laser in the X direction and the Y direction are the same.

[0015] Further preferably, the light spot overlap rate is 46.0%-86.0%.

[0016] Preferably, during the ultrasound-assisted laser treatment, the laser beam is set to positive defocus. Exemplarily, the present invention is described by taking a positive defocus of +1 mm as an example, and when the positive defocus is in the range of 0.9 mm to 1.1 mm, a comparable effect can also be achieved.

[0017] Preferably, the magnesium alloy includes any one of AZ31B magnesium alloy, ZE41 magnesium alloy, AZ61 magnesium alloy or WE43 magnesium alloy. For example, AZ31B magnesium alloy is used as an example in the embodiment of the present invention, and other types of magnesium alloys can also achieve comparable effects.

[0018] Preferably, before the ultrasonic-assisted laser treatment, the magnesium alloy is also subjected to pretreatment including grinding and polishing.

[0019] The magnesium alloy is made of a plane-like component. A plane is the best surface for remelting. Since the laser energy is concentrated in the method provided by the present invention, it is allowed that the surface of the component has tiny protrusions, rounded corners, etc.

[0020] In a second aspect, the present invention provides a corrosion-resistant magnesium alloy, which is produced by the method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification provided in the first aspect.

[0021] Compared with conventional magnesium alloys, the surface of the corrosion-resistant magnesium alloy provided by the present invention has a micron-scale grain remelting layer with uniform refinement and good compactness. The thickness of the remelting layer is 34.9μm~35.7μm, and more than 96% of the remelted equiaxed grain sizes are concentrated in 1μm~2μm. The remelting layer can effectively extend the diffusion path of the corrosive medium inside the material. The corrosive medium must pass through more grain boundaries and gaps between small grains to enter the deep part of the material, thereby increasing the diffusion resistance. In addition, the corrosion-resistant magnesium alloy obtained by ultrasonic-assisted laser modification of the present invention has significantly reduced electrochemical activity on the surface, and significantly enhanced the ability to resist corrosion in a humid environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0023] Figure 1 The metallographic micrographs of the magnesium alloy surface before and after the ultrasonic-assisted laser modification in Example 1 of the present invention are compared; wherein, Figure a) is a metallographic micrograph of the pretreated magnesium alloy surface; Figure b) is a metallographic micrograph of the corrosion-resistant magnesium alloy I surface; Figure 2 is a metallographic micrograph of a cross section of the corrosion-resistant magnesium alloy I in Example 1 of the present invention; Figure 3 EBSD image of the cross section of the corrosion-resistant magnesium alloy I in Example 1 of the present invention; Figure 4 This is a metallographic micrograph of the surface of the corrosion-resistant magnesium alloy II in Example 2 of the present invention; Figure 5 This is a metallographic micrograph of a cross section of the corrosion-resistant magnesium alloy II in Example 2 of the present invention; Figure 6 EBSD image of the cross section of the corrosion-resistant magnesium alloy II in Example 2 of the present invention; Figure 7 This is a metallographic micrograph of the surface of the corrosion-resistant magnesium alloy III in Example 3 of the present invention; Figure 8 This is a metallographic micrograph of a cross section of the corrosion-resistant magnesium alloy III in Example 3 of the present invention; Fig. 9 This is a metallographic micrograph of the surface of the laser-modified magnesium alloy in Comparative Example 1 of the present invention; Fig.10 This is a metallographic micrograph of the cross section of the laser-modified magnesium alloy in Comparative Example 1 of the present invention; Fig.11 EBSD image of the cross section of the laser-modified magnesium alloy in Comparative Example 1 of the present invention; Fig.12 These are electron microscope scanning images of the surfaces of magnesium alloys treated differently in the effect examples of the present invention, wherein a) represents the pretreated magnesium alloy, b) represents the laser-modified magnesium alloy, and c) represents the corrosion-resistant magnesium alloy I. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the 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] The laser used in the present invention is produced by senzedlaser and is equipped with a high-speed swing precision welding head (model D24).

[0026] Example 1 This embodiment provides a method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification, the method comprising the following steps: Step 1. The AZ31B magnesium alloy with a size of 10mm×10mm×4mm was pre-treated by grinding with 1000-3000 mesh SiC sandpaper, and then the surface of the magnesium alloy sample was polished with a W1 polishing paste with diamond content (metallographic polishing paste with a particle size of 1μm) in an anhydrous ethanol environment, and then ultrasonically cleaned with anhydrous ethanol for 1min, and finally placed in a drying oven for drying to obtain the pre-treated magnesium alloy.

[0027] Step 2. Fix the pretreated magnesium alloy to the processing platform and set the ultrasonic vibration parameters and laser parameters. The ultrasonic power is 60W and the ultrasonic vibration frequency is 20kHz; The laser parameters are as follows: power of 200 W, frequency of 1000 Hz, spot diameter of 50 μm, emission line width of 5 nm, scanning speed of 10 mm / s, spot pulse width of 200 μs, and wavelength of 1070 nm.

[0028] Step 3. Start the ultrasonic device, start the argon protection, start the laser, select the coaxial output of the laser and argon, and set the argon gas flow rate to 7L / min; During the ultrasonic-assisted laser treatment process, the laser head emits a laser beam that scans from left to right along the X direction. When reaching the set X-direction end point, the laser head stops emitting the laser beam and returns in the opposite direction (i.e., from right to left), forming a reciprocating scanning motion. After each scanning round trip, the laser beam moves in steps in the Y direction. In this embodiment, after each reciprocating scan, the laser steps 0.010 mm in the Y direction. The overlap rates in the X and Y directions are controlled to be 75.0%.

[0029] Step 4. After the ultrasonic-assisted laser modification is completed, the sample is taken out, ultrasonically cleaned with anhydrous ethanol for 1 min, and placed in a drying oven at 80° C. for 3 min to obtain a corrosion-resistant magnesium alloy, which is recorded as corrosion-resistant magnesium alloy I.

[0030] This example analyzes the microstructure characteristics of the corrosion-resistant magnesium alloy I. The details are as follows: The metallographic microscopic comparison of the magnesium alloy surface before and after ultrasonic-assisted laser modification is shown in Figure 2. Figure 1 As shown; Figure a) is a metallographic micrograph of the pretreated magnesium alloy surface; Figure b) is a metallographic micrograph of the corrosion-resistant magnesium alloy I surface. Figure 1a) It can be seen that the surface grains of the magnesium alloy before modification are clearly visible, and the average grain size is between 20μm and 40μm. Figure 1 b) It can be seen that compared with the situation before ultrasonic-assisted laser modification, the edge of the modified alloy molten pool has obvious splashing phenomenon, and fish-scale ripples appear on the surface due to ultrasonic vibration of the molten pool, which is mainly due to the cavitation effect and acoustic streaming effect caused by ultrasonic vibration. These effects will cause cavitation bubbles to form inside the molten pool, causing the molten pool to flow rapidly and splash.

[0031] The metallographic micrograph of the cross section of the corrosion-resistant magnesium alloy I is shown in Figure 2 As shown. Figure 2 It can be seen that the thickness of the remelted layer of the corrosion-resistant magnesium alloy I is 35.7 μm, and the forming effect and density of the remelted layer are good.

[0032] EBSD image of the cross section of corrosion-resistant magnesium alloy I Figure 3 As shown. Figure 3 It can be seen that the cross-section grains of the corrosion-resistant magnesium alloy I are Figure 1 a) has been greatly refined, and the grain size has been refined from 20μm~40μm to 1μm~2μm. About 97% of the remelted equiaxed grain size is concentrated in the range of 1μm~2μm, and the remelting effect is obvious. In addition, it can be found that the remelted layer has no obvious pores and no obvious abnormally grown grains, indicating that a good metallurgical bonding effect has been formed between the remelted layer and the matrix.

[0033] Example 2 This embodiment provides a method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification. Steps 1 and 4 in the method are the same as those in Example 1, except that the parameter settings in steps 2 to 3 are different. The specific method is as follows: Step 1. Prepare a pretreated magnesium alloy according to the method described in Example 1; Step 2. Fix the pretreated magnesium alloy to the processing platform and set the ultrasonic vibration parameters and laser parameters. The ultrasonic power is 100W and the ultrasonic vibration frequency is 22kHz; The laser parameters are as follows: power of 180 W, frequency of 1200 Hz, spot diameter of 28 μm, emission line width of 4 nm, scanning speed of 8 mm / s, spot pulse width of 200 μs, and wavelength of 1070 nm.

[0034] Step 3. Start the ultrasonic device, start the argon protection, start the laser, select the coaxial output of the laser and argon, and set the argon gas flow rate to 6L / min; During the ultrasonic-assisted laser treatment process, the laser head emits a laser beam that scans from left to right along the X direction. When reaching the set X-direction end point, the laser head stops emitting the laser beam and returns in the opposite direction (i.e., from right to left), forming a reciprocating scanning motion. After each scanning round trip, the laser beam moves in steps in the Y direction. In this embodiment, after each reciprocating scan, the laser steps 0.008 mm in the Y direction. The overlap rates in the X and Y directions are controlled to be 76.0%.

[0035] Step 4. According to the method described in Example 1, the sample after ultrasonic-assisted laser modification is ultrasonically cleaned and dried to obtain a corrosion-resistant magnesium alloy, which is recorded as corrosion-resistant magnesium alloy II.

[0036] This example analyzes the microstructure characteristics of the corrosion-resistant magnesium alloy II. The details are as follows: The metallographic micrograph of the surface of corrosion-resistant magnesium alloy II is as follows: Figure 4 As shown in the figure, the metallographic micrograph of the cross section of the corrosion-resistant magnesium alloy II is as follows Figure 5 As shown. Figure 4-5 It can be seen that the splash at the edge of the molten pool of the corrosion-resistant magnesium alloy II is more obvious, the thickness of its remelted layer is 34.9 μm, and the forming effect and density of the remelted layer are also good.

[0037] EBSD image of the cross section of corrosion-resistant magnesium alloy II Figure 6 As shown. Figure 6 It can be seen that compared with the unmodified state, the cross-sectional grains of the corrosion-resistant magnesium alloy II have been greatly refined, and the refined grains are basically equiaxed grains. The size of 96% of the remelted equiaxed grains is concentrated in the range of 1μm~2μm, with no obvious pores and no abnormally grown grains, indicating that a good remelting effect has been achieved. Compared with the corrosion-resistant magnesium alloy I, the remelting marks on the surface of the corrosion-resistant magnesium alloy II are more obvious, which may be mainly due to the fact that the ultrasonic wave is essentially a high-frequency mechanical vibration. The ultrasonic power and frequency of Example 2 increase, and the ultrasonic wave propagates in the liquid. The displacement amplitude of the particle is small, but the acceleration is large. The amplitude will be weakened during propagation, and part of the energy will be absorbed, resulting in a temperature increase and a thermal effect. At the same time, the heat transfer directionality brought by the acoustic streaming effect of the ultrasonic wave improves the transfer efficiency when the heat propagates downward, making the remelting marks more obvious.

[0038] Example 3 This embodiment provides a method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification. Steps 1 and 4 in the method are the same as those in Example 1, except that the parameter settings in steps 2 to 3 are different. The specific method is as follows: Step 1. Prepare a pretreated magnesium alloy according to the method described in Example 1; Step 2. Fix the pretreated magnesium alloy to the processing platform and set the ultrasonic vibration parameters and laser parameters. The ultrasonic power is 80W and the ultrasonic vibration frequency is 18kHz; The laser parameters are as follows: power of 200 W, frequency of 800 Hz, spot diameter of 40 μm, emission line width of 5.5 nm, scanning speed of 12 mm / s, spot pulse width of 200 μs, and wavelength of 1070 nm.

[0039] Step 3. Start the ultrasonic device, start the argon protection, start the laser, select the coaxial output of the laser and argon, and set the argon gas flow rate to 8L / min; During the ultrasonic-assisted laser treatment process, the laser head emits a laser beam that scans from left to right along the X direction. When reaching the set X-direction end point, the laser head stops emitting the laser beam and returns in the opposite direction (i.e., from right to left), forming a reciprocating scanning motion. After each scanning round trip, the laser beam moves in steps in the Y direction. In this embodiment, after each reciprocating scan, the laser steps 0.012 mm in the Y direction. The overlap rates in the X and Y directions are controlled to be 62.5%.

[0040] Step 4. According to the method described in Example 1, the sample after ultrasonic-assisted laser modification is ultrasonically cleaned and dried to obtain a corrosion-resistant magnesium alloy, which is recorded as corrosion-resistant magnesium alloy III.

[0041] The metallographic micrographs of the surface and cross section of the corrosion-resistant magnesium alloy III are shown in Figure 2. Figure 7 and Figure 8 As shown in the figure, the analysis shows that the overall remelting effect of the corrosion-resistant magnesium alloy III surface is good, the thickness of the remelting layer is 35.2μm, and the forming effect and density of the remelting layer are good. 97% of the remelted equiaxed grains in the remelting layer of the corrosion-resistant magnesium alloy III are concentrated in the range of 1μm~2μm, and there are no obvious pores and abnormally grown grains, and the remelting effect is good.

[0042] Comparative Example 1 This comparative example provides a method for laser modification of magnesium alloy, which is basically the same as the method provided in Example 1, except that ultrasonic assistance is not added, and only laser surface remelting treatment is performed. Therefore, ultrasonic parameters are not set in step 2, and the ultrasonic device is not started in step 3. The other steps and parameters are the same as those in Example 1 and will not be repeated here. Finally, laser modified magnesium alloy is obtained.

[0043] Furthermore, the present invention analyzes the microstructure characteristics of the laser-modified magnesium alloy. The details are as follows: Metallographic micrograph of laser modified magnesium alloy surface Fig. 9 As shown in the metallographic micrograph of the cross section of the laser modified magnesium alloy Fig.10 As shown. Figure 9-10 It can be seen that due to the lack of ultrasonic vibration assistance, the molten pool of the laser modified magnesium alloy has basically no spatter marks, and the thickness of the remelted layer is only 31.5μm.

[0044] EBSD image of the cross section of laser modified magnesium alloy Fig.11 As shown. Combined Fig.11 and Figure 1 It can be seen that compared with the unmodified state, the grains of the laser-modified magnesium alloy have been refined to a certain extent, but there are obviously abnormally grown grains. This may be because after the laser effect disappears, there will be a vertical temperature gradient in the molten pool. The grains in this direction will grow abnormally, and stress concentration will occur in this area, which may cause cracks on the surface of the remelted sample and fail to effectively resist corrosion.

[0045] Effect example In order to explore the influence of different modification conditions on the surface of magnesium alloy, the present invention tests the electrical corrosion resistance of pretreated magnesium alloy, corrosion-resistant magnesium alloy prepared in Examples 1-3 and laser-modified magnesium alloy prepared in Comparative Example 1 to explore its practical application effect. The specific test contents are as follows: 1. Electrochemical performance investigation The self-corrosion current density is an important parameter to measure the electrochemical reaction rate of materials in a corrosive environment. It reflects the current intensity generated by spontaneous corrosion reactions (such as oxidation reactions) on the surface of the material. The smaller the self-corrosion current density, the better the corrosion resistance of the material in a specific corrosive medium. On the contrary, the larger the self-corrosion current density, the faster the corrosion rate and the worse the corrosion resistance.

[0046] The pretreated magnesium alloy, the corrosion-resistant magnesium alloy prepared in Examples 1-3, and the laser-modified magnesium alloy prepared in Comparative Example 1 were placed in 1 mol / L NaCl solution for 30 minutes, and the self-corrosion current density of different products was investigated. The test results are shown in Table 1 below.

[0047] Table 1

[0048] It can be seen from the data in Table 1 that the corrosion current density of the magnesium alloy samples changed to varying degrees after different surface modification treatments. This shows that reasonable modification methods and appropriate parameters can change the corrosion resistance of magnesium alloys. Among them, the sample in Example 1 has the lowest self-corrosion current density and the best corrosion resistance; followed by the samples prepared in Examples 2-3. The self-corrosion current density of the laser-modified magnesium alloy obtained in Comparative Example 1 is lower than that of the magnesium alloy without laser modification, but significantly higher than that of corrosion-resistant magnesium alloy I ~ corrosion-resistant magnesium alloy III. Therefore, it can be seen that although the corrosion resistance of the samples that have only been treated with laser is worse than that of the samples that have not been laser modified, it is not as good as the samples that have been added with ultrasonic assistance during the modification process. The reason may be that the mechanical effect, cavitation effect and acoustic streaming effect brought about by ultrasonic assistance effectively promote the further refinement of the grains and make the remelted structure more dense and uniform.

[0049] 2. Corrosion resistance inspection The pretreated magnesium alloy, the corrosion-resistant magnesium alloy prepared in Examples 1-3, and the laser-modified magnesium alloy prepared in Comparative Example 1 were respectively exposed with a laser-treated surface of 1 cm×1 cm (the remaining surfaces were wrapped with acrylic glue insoluble in NaCl solution), and were respectively immersed in a 3.5% by mass NaCl solution for three days, and the total weight loss of different products under immersion was calculated to examine their corrosion resistance. The results of the investigation are shown in Table 2.

[0050] Table 2

[0051] It can be seen from the data in Table 2 that, compared with magnesium alloys that are only pretreated or laser modified, the corrosion-resistant magnesium alloys prepared in Examples 1-3 exhibit very good corrosion resistance.

[0052] In order to further explore the reasons for the improvement of corrosion resistance, the pretreated magnesium alloy, corrosion-resistant magnesium alloy I and laser-modified magnesium alloy after immersion for three days were photographed by scanning electron microscope to further analyze their microstructures. The scanning electron microscope images of the magnesium alloy surfaces treated with different treatments are shown in Figure 2. Fig.12 As shown, Figure a) represents the pretreated magnesium alloy, Figure b) represents the laser modified magnesium alloy, and Figure c) represents the corrosion-resistant magnesium alloy I.

[0053] Depend on Fig.12 It can be seen that the surface of the pretreated magnesium alloy has been completely corroded and severely damaged. The thin strip-shaped corrosion products (Mg(OH) 2 ) completely spread over the matrix; at the same time, cracking was found on the exposed matrix. This layer-by-layer cracking and shedding phenomenon makes the untreated AZ31B magnesium alloy extremely easy to fail. There are many corrosion products on the surface of laser-modified magnesium alloy, and the staggered distribution of large and small grains caused by laser remelting also aggravates the generation of dislocation accumulation, which in turn leads to stress concentration and cracking of the film layer. When the film layer is damaged, the corrosion products will invade the matrix, causing corrosion of the matrix, and then causing product failure. The overall surface of corrosion-resistant magnesium alloy I is less damaged, and some corrosion products appear, but there is no cracking phenomenon. The remelted layer plays a good protective role on the matrix.

[0054] From the above, it can be found that the surface of the magnesium alloy sample that was only pretreated was severely damaged during immersion corrosion. On this basis, laser melting was performed to improve the corrosion resistance, but due to the enrichment of large and small grains, it would cause obvious stress concentration, resulting in cracks in some areas of the surface. Ultrasonic assistance was further added on the basis of laser treatment. Since ultrasound causes high-frequency vibration of the sample, it can effectively solve the technical problem of enriched residual stress caused by coarse grains during the remelting process of magnesium alloys, and significantly improve the corrosion behavior of magnesium alloys. After ultrasonic-assisted laser modification, the cracking of the alloy surface was significantly reduced, the sample as a whole showed good corrosion behavior, and the corrosion resistance of the magnesium alloy was significantly improved.

[0055] In summary, the present invention uses ultrasonic-assisted laser surface melting in a protective gas flow to induce the formation of a micron-scale grain remelting layer with good forming effect and density and uniform grain size refinement on the surface of the magnesium alloy, wherein the thickness of the remelting layer is in the range of 34.9μm~35.7μm, and more than 96% of the remelted equiaxed grain sizes are concentrated in 1μm~2μm. The remelting layer can reduce the electrochemical activity of the magnesium alloy surface, including the self-corrosion current density, and significantly improve its corrosion resistance in a simulated seawater environment (NaCl solution with a mass fraction of 3.5%).

[0056] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent substitution or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification, characterized in that: The method comprises the following steps: Under protective gas, the surface of the magnesium alloy is subjected to ultrasonic-assisted laser treatment to obtain a corrosion-resistant magnesium alloy; the equiaxed grain size of the remelted layer is 1μm~2μm; Wherein, the power of the ultrasound is 60W~100W, and the power of the laser is 180W~200W.

2. The method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification according to claim 1, characterized in that: The thickness of the remelted layer is 34.9 μm to 35.7 μm.

3. The method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification according to claim 1, characterized in that: The ultrasonic vibration frequency is 18kHz~22kHz; and / or The protective gas and the laser are coaxially output; and / or The protective gas flow rate is 6L / min~8L / min; and / or The protective gas includes at least one of argon, neon or krypton.

4. The method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification according to claim 1, characterized in that: During the ultrasound-assisted laser treatment, the laser spot diameter is 28 μm to 50 μm; and / or The laser emission line width is 4nm~5.5nm; and / or The laser frequency is 800Hz~1200Hz; and / or The laser scanning speed is 8mm / s~12mm / s.

5. The method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification according to claim 1, characterized in that: During the ultrasound-assisted laser processing, the spot overlap rates of the laser in the X direction and the Y direction are the same.

6. The method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification according to claim 5, characterized in that: The light spot overlap rate is 46.0%-86.0%.

7. The method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification according to claim 1, characterized in that: During the ultrasound-assisted laser processing, the laser beam is set to be positively defocused.

8. The method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification according to claim 1, characterized in that: The magnesium alloy includes any one of AZ31B magnesium alloy, ZE41 magnesium alloy, AZ61 magnesium alloy or WE43 magnesium alloy.

9. The method for improving the corrosion resistance of magnesium alloy based on ultrasound-assisted laser modification according to claim 1, characterized in that: Before the ultrasonic-assisted laser treatment, the magnesium alloy is also subjected to pretreatment including grinding and polishing.

10. A corrosion-resistant magnesium alloy, characterized in that: The magnesium alloy is prepared by the method for improving the corrosion resistance of magnesium alloy by ultrasonic-assisted laser modification as described in any one of claims 1 to 9.

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