Composite welding method for improving corrosion resistance of magnesium alloy weld using carbon nanotubes

Abstract

This application belongs to the field of magnesium alloy welding technology, and relates to a composite welding method for improving the corrosion resistance of magnesium alloy welds using carbon nanotubes. The method includes: preparing a carbon nanotube dispersion using hydroxylated carbon nanotubes; coating a magnesium alloy plate with the carbon nanotube dispersion and air-drying it; butt-assembling and fixing the magnesium alloy plate coated with the carbon nanotube dispersion to another magnesium alloy plate not coated with the carbon nanotube dispersion; and welding the assembled pair of magnesium alloy plates in an inert gas environment using a laser-arc composite welding process. During welding: high-frequency oscillation of the laser beam is used to prevent the carbon nanotubes from agglomerating, ensuring their uniform dispersion; the carbon nanotubes inhibit the formation of coarse columnar crystals and dendrites during the solidification of the magnesium alloy molten pool, transforming the weld structure into fine equiaxed crystals. Furthermore, the carbon nanotubes pinnate at grain boundaries, hindering grain boundary migration and grain growth, and providing heterogeneous nucleation sites to promote the precipitation of precipitated phases; thereby improving the corrosion resistance of the magnesium alloy weld.

Description

Technical Field

[0001] This application belongs to the field of magnesium alloy welding technology, and more specifically, relates to a composite welding method that utilizes carbon nanotubes to improve the corrosion resistance of magnesium alloy welds. Background Technology

[0002] With the introduction of green policies such as energy conservation and emission reduction, the demand for lightweight materials in modern industry is constantly increasing. Magnesium alloys, as the lightest structural metals, are widely used in aerospace, automotive manufacturing, electronic equipment, and rail transportation due to their low density, high specific strength, and good vibration damping performance. Welding, as a key process in the industrial production of magnesium alloys, directly affects the reliability and durability of the overall structure. However, the physicochemical properties of magnesium alloys (such as low melting point, high coefficient of thermal expansion, and high chemical reactivity) pose significant challenges to their welding process.

[0003] Currently, magnesium alloy welding mainly employs traditional methods such as arc welding (e.g., TIG, MIG) and laser welding. Arc welding has a large heat input, easily leading to overheating and explosion of molten magnesium droplets, severe spattering, shallow penetration, coarse grains in the joint, and decreased mechanical properties. While high-energy laser welding has a low heat input, it easily causes the burning of alloying elements such as aluminum and zinc, resulting in weld surface depressions, porosity, and slag inclusions, severely affecting the weld quality. To balance efficiency and quality, laser-arc hybrid welding has emerged. This technology improves penetration and bridging ability through synergistic effects and improves weld formation through filler wire, but it still does not solve the core problem: the welding thermal cycle leads to significant grain coarsening in the weld zone, uneven distribution of precipitated phases, and the easy formation of oxide inclusions and microcracks in the heat-affected zone. These defects not only reduce joint strength but also provide penetration channels for corrosive media (such as chloride ions), accelerating localized electrochemical corrosion and significantly weakening corrosion resistance.

[0004] The corrosion resistance of magnesium alloy welds is mainly affected by the uniformity of the microstructure, the distribution of the second phase, and surface defects. Traditional methods for improving corrosion resistance (such as surface coatings, alloying modifications, and post-weld heat treatment) are effective for the base metal but difficult to apply to the weld area: surface treatment layers are prone to detachment at areas of microstructure inhomogeneity, the loss of alloying elements during welding is difficult to control, and post-weld heat treatment is costly and prone to deformation. In addition, current process parameter optimization relies heavily on experience and lacks precise control over the microstructure-property relationship, resulting in insufficient long-term service reliability of the weld, especially in marine environments, high humidity, or corrosive media, where the weld is prone to becoming a weak point in structural failure.

[0005] Therefore, there is an urgent need to develop a new method that is efficient, low-cost, and highly adaptable to the process to significantly improve the corrosion resistance of magnesium alloy welds, thereby expanding their application in key engineering fields. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this application is to provide a composite welding method that uses carbon nanotubes to improve the corrosion resistance of magnesium alloy welds, aiming to solve the problem of poor corrosion resistance caused by coarse grains and many defects in the welds produced by the existing magnesium alloy welding process.

[0007] To achieve the above objectives, a composite welding method for improving the corrosion resistance of magnesium alloy welds using carbon nanotubes includes:

[0008] S1 utilizes hydroxylated carbon nanotubes to prepare a carbon nanotube dispersion;

[0009] S2 involves coating the cleaned magnesium alloy plate with a carbon nanotube dispersion and then air-drying it.

[0010] S3 connects and fixes the magnesium alloy plate coated with carbon nanotube dispersion to another magnesium alloy plate that is not coated with carbon nanotube dispersion, with the two sides to be welded.

[0011] S4 employs a laser-arc hybrid welding process. During welding, the high-frequency oscillation of the laser beam prevents carbon nanotubes from agglomerating and ensures their uniform dispersion. During the solidification of the magnesium alloy molten pool, the carbon nanotubes inhibit the formation of coarse columnar crystals and dendrites, transforming the weld structure into fine equiaxed crystals. Furthermore, the carbon nanotubes pin to the grain boundaries, hindering grain boundary migration and grain growth, and providing heterogeneous nucleation sites to promote the precipitation of precipitated phases.

[0012] Furthermore, in step S1, the method for preparing the carbon nanotube dispersion includes:

[0013] S101 mixes carbon nanotubes with a solvent and then stirs the mixture to completely wet the carbon nanotubes;

[0014] S102 The solution obtained in step S101 is ultrasonically treated in a water bath environment of 4℃~10℃ for 1h~2h, and the cold water is changed every 10min before continuing the ultrasonic treatment.

[0015] S103 After centrifuging the solution obtained in step S102, the solution is filtered multiple times to obtain the carbon nanotube dispersion.

[0016] Furthermore, the centrifugation rate during the centrifugation process is 2000 r / min to 4000 r / min, and the centrifugation time is 30 min to 60 min.

[0017] Furthermore, the mass fraction of carbon nanotubes in the carbon nanotube dispersion is 1% to 5%.

[0018] Furthermore, the carbon nanotubes in the carbon nanotube dispersion have an average diameter of 40 nm to 60 nm and an average length of 5 μm to 10 μm.

[0019] Furthermore, in step S4, a welding mode is adopted in which the laser heat source is in front and the electric arc heat source is behind.

[0020] Furthermore, in step S4, the distance between the laser beam spot and the welding wire is 1mm~3mm, the defocusing amount is -5mm~5mm; the laser power is 2500W~4500W, the welding current is 90~120A, and the welding speed is 1.2~1.8m / min.

[0021] Furthermore, in step S4, the scanning amplitude of the laser beam is 0.5mm~1mm, and the scanning frequency is 100Hz~300Hz.

[0022] Furthermore, in step S4, a magnesium alloy welding wire is selected for arc welding, and the diameter of the magnesium alloy welding wire is 1.0 mm or 1.2 mm.

[0023] Furthermore, in step S4, the laser head and the welding torch adopt a side-axis composite mode, and the angle between the laser beam and the vertical direction is 0°~10°, while the angle between the welding torch and the surface to be welded is 55°~65°.

[0024] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art:

[0025] (1) The addition of carbon nanotubes in this application increases the amount of precipitates (such as Mg) in magnesium alloys. 17 Al 12 The content of Al8Mn5 is increased, and its more uniform distribution is promoted. These precipitated phases act as micro-couple cathodes during corrosion. Because their potential is higher than that of the magnesium matrix (anode), they can preferentially guide the corrosion current and slow down the uniform corrosion of the matrix. At the same time, the precipitated phases dissolve at the corrosion interface and release Al³⁺. + These ions react with oxygen in the medium to form a continuous and dense Al2O3 oxide film on the weld surface. As a physical barrier, the Al2O3 oxide film can effectively prevent further intrusion of corrosive media, thereby improving the corrosion resistance of magnesium alloy composite welds.

[0026] (2) This application employs a laser-arc hybrid welding process to improve the weldability of magnesium alloys. Specifically, it uses a welding mode where the laser comes first and the arc follows. The laser heat source forms the molten pool, and during the formation of the molten pool, magnesium, aluminum, and zinc elements in the magnesium-aluminum alloy are prone to volatilization, which can cause the weld surface to become concave. Therefore, the arc heat source is used to melt the welding wire to fill the concave area and better protect the molten pool. At the same time, the laser coming first can also stabilize the arc, resulting in better weld quality. The high-frequency vibration of the scanning laser beam also inhibits the agglomeration of carbon nanotubes, making them more homogeneous, and at the same time, it plays a role in eliminating porosity and refining grains.

[0027] (3) This application is simple to operate, low in cost, and can obtain high-quality welds without any post-weld treatment, and has the advantages of high efficiency, energy saving and environmental protection. Attached Figure Description

[0028] Figure 1 This is a schematic flowchart of the composite welding method for improving the corrosion resistance of magnesium alloy welds using carbon nanotubes, provided in Embodiment 1 of this application.

[0029] Figure 2 This is a schematic diagram of the microstructure of the weld provided in Embodiment 1 of this application;

[0030] Figure 3 This is a schematic diagram of the microstructure of the weld provided in Embodiment 2 of this application;

[0031] Figure 4 This is a schematic diagram of the microstructure of the weld provided in Comparative Example 1 of this application;

[0032] Figure 5 This is a schematic diagram of hydrogen evolution and weight loss rate of the weld provided in Comparative Example 1 of this application;

[0033] Figure 6 This is a schematic diagram of hydrogen evolution and weight loss rate of weld seam provided in Embodiment 1 of this application;

[0034] Figure 7 This is a schematic diagram of hydrogen evolution and weight loss rate of the weld provided in Embodiment 2 of this application;

[0035] Figure 8 This is a schematic diagram of the electrochemical polarization curve of the weld provided in Comparative Example 1 of this application;

[0036] Figure 9 This is a schematic diagram of the electrochemical polarization curve of the weld provided in Embodiment 1 of this application;

[0037] Figure 10 This is a schematic diagram of the electrochemical polarization curve of the weld provided in Embodiment 2 of this application. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0039] The embodiments of this application are described below with reference to the accompanying drawings.

[0040] Example 1

[0041] This embodiment provides a composite welding method that utilizes carbon nanotubes to improve the corrosion resistance of magnesium alloy welds, combined with... Figure 1As shown, the specific steps include:

[0042] Step S1: By stirring, sonicating, centrifuging and filtering, prepare a 1%~5% carbon nanotube dispersion using hydroxylated carbon nanotubes. The hydroxylated carbon nanotubes can better bind with the magnesium matrix.

[0043] Specifically, in this embodiment, a 3% carbon nanotube dispersion is prepared. First, 3 g of carbon nanotubes are mixed with 97 g of ethanol solvent (other organic solvents or water solvents can also be used), and stirred for 30 min to completely wet the carbon nanotubes. The average diameter of the carbon nanotubes in the dispersion is 40 nm to 60 nm, and the average length is 5 μm to 10 μm.

[0044] The dispersion obtained by stirring was then subjected to ultrasonic treatment. Because carbon nanotubes have high surface activity, excessively high ultrasonic treatment temperatures can easily lead to their aggregation, and alcohol and water are also prone to evaporation. Therefore, the dispersion was ultrasonicated in a cold water bath, with the cold water in the bath being replaced every 10 minutes to maintain the temperature between 4 and 10°C. Ultrasonication was then continued for a total time of 1 hour.

[0045] The ultrasonically treated dispersion was then centrifuged at a speed of 2000 r / min to 4000 r / min for 30 min to 60 min. Specifically, in this embodiment, the centrifugation speed was 2000 r / min and the centrifugation time was 30 min. After centrifugation, the dispersion was filtered multiple times using a 300-800 mesh filter cloth to finally obtain a carbon nanotube dispersion with a mass fraction of 3%.

[0046] Step S2: After grinding and cleaning the side of the magnesium alloy plate to be welded, the carbon nanotube dispersion prepared in step S1 is coated onto the side of the magnesium alloy plate.

[0047] Specifically, the magnesium alloy sheet is ground with an angle grinder to remove the oxide layer, and then cleaned with alcohol. After grinding and cleaning, the magnesium alloy sheet is fixed, and carbon nanotube dispersion is evenly coated on the side of the sheet using a dropper and then air-dried.

[0048] Step S3: After the carbon nanotube dispersion has dried, the magnesium alloy plate coated with carbon nanotubes is joined and assembled with another magnesium alloy plate that is not coated with carbon nanotubes.

[0049] Specifically, wait 30 minutes for the ethanol in the carbon nanotube dispersion to completely evaporate, and the carbon nanotubes will be uniformly attached to the sidewall of the magnesium alloy. Then, a magnesium alloy plate coated with carbon nanotubes is joined to a magnesium alloy plate without carbon nanotubes without leaving any gaps.

[0050] Step S4: Fix the butted plates onto the worktable using a fixture, set the welding parameters, and weld the two magnesium alloy plates using laser-arc hybrid welding. The laser beam heats the magnesium alloy to form a molten pool, while the high-frequency oscillation of the laser beam prevents the agglomeration of carbon nanotubes on the magnesium alloy plates, resulting in more uniform dispersion. Arc welding fills the weld depressions caused by the evaporation of magnesium, aluminum, and zinc elements during laser welding and maintains the stability of the molten pool. During welding, carbon nanotubes inhibit the formation of coarse columnar and dendritic crystals during the solidification of the magnesium alloy molten pool, transforming the weld structure into fine equiaxed crystals. Furthermore, the carbon nanotubes pinnate at grain boundaries, hindering grain boundary migration and grain growth, further refining the grains. In addition, carbon nanotubes can provide heterogeneous nucleation sites, promoting the precipitation of precipitates and improving the corrosion resistance of the weld. The aforementioned heterogeneous nucleation is a physical phenomenon where crystal nuclei attach to impurities or interface walls in the liquid metal during solidification.

[0051] Specifically, the two assembled magnesium alloy plates are fixed on the worktable using a clamp, and laser-arc hybrid welding is adopted, using a welding mode with the laser heat source in front and the arc heat source behind. The welding parameters are as follows: the distance between the laser beam spot and the welding wire is 1mm~3mm, the defocusing amount is -5mm~5mm; the laser power is 2500W~4500W, the welding current is 90A~120A, the welding speed is 1.2m / min~1.8m / min; and the angle between the laser beam and the vertical direction is 0°~10°, and the angle between the welding torch and the surface to be welded is 55°~65°.

[0052] In this embodiment, AZ31B magnesium alloy welding wire with a diameter of 1.2 mm is selected for arc welding. The specific overall welding parameters are as follows: laser power of 3000W, welding current of 100A, welding speed of 1.5m / min, defocusing amount of 0mm, wire spacing (i.e., the distance between the laser beam spot on the surface to be welded and the welding wire) of 2mm, shielding gas of high-purity argon, and argon flow rate of 20L / min. The laser head and welding torch adopt a side-axis composite mode, with the laser beam at an angle of 5° to the vertical direction and the welding torch at an angle of 60° to the surface to be welded. The introduced scanning laser beam path is circular (a linear path can also be used), with a scanning amplitude of 0.8mm and a scanning frequency of 200Hz.

[0053] like Figure 2 As shown, after introducing carbon nanotubes, the grain size of the magnesium alloy laser-arc composite weld obtained in Example 1 was significantly refined to 13.7 μm. 17 Al 12 The content of precipitates such as Al8Mn5 increased to 2.36%.

[0054] Example 2

[0055] The difference between this embodiment and embodiment 1 is that the welding parameters in step S4 are changed to a laser power of 3500W, a welding current of 110A, and a welding speed of 1.6m / min. All other steps and related parameters are the same as in embodiment 1.

[0056] like Figure 3 As shown, the magnesium alloy laser-arc composite weld obtained in this embodiment has a grain size of 17.8 μm and a precipitate content of 2.13%.

[0057] Comparative Example 1

[0058] This comparative example provides a method for laser-arc hybrid welding of AZ31B magnesium alloy. First, the magnesium alloy plates to be welded are ground to remove the surface oxide film, then cleaned with alcohol, and finally the two magnesium alloy plates are butt-jointed.

[0059] A laser-arc hybrid welding process was used to weld magnesium alloy plates, using AZ31B magnesium alloy welding wire with a diameter of 1.2 mm. The welding parameters were set as follows: laser power of 2500W-4500W, welding current of 90A-120A, welding speed of 1.2-1.8m / min, defocusing amount of 0mm, wire spacing of 2mm, and high-purity argon as the shielding gas with a flow rate of 20L / min.

[0060] The laser head and welding torch adopt a side-axis composite mode. The laser beam makes a 5° angle with the vertical direction, and the welding torch makes a 60° angle with the surface to be welded. The welding mode is laser in front and arc behind.

[0061] like Figure 4 As shown, the microstructure of the magnesium alloy laser-arc weld obtained in this comparative example consists of equiaxed grains with a grain size of 32.1 μm and a precipitate content of 1.14%.

[0062] The magnesium alloy laser-arc composite welds obtained in Examples 1, 2 and Comparative Example 1 were subjected to 72-hour immersion corrosion tests and electrochemical tests in 3.5% NaCl solution. The corresponding hydrogen evolution volume, sample weight loss and polarization curves were recorded, and the hydrogen evolution corrosion rate, weight loss rate, self-corrosion sites and self-corrosion current density were calculated.

[0063] like Figure 5 As shown, the average hydrogen evolution corrosion rate of the composite weld obtained in Comparative Example 1 was 14.38 ml / cm. 2 The rate of corrosion due to weight loss was 5.24 mg / cm³ per day. 2 ;like Figure 6 As shown, the average hydrogen evolution corrosion rate of the composite weld obtained in Example 1 was 9.62 ml / cm. 2The rate of corrosion due to weight loss was 3.46 mg / cm³ per day. 2 ;like Figure 7 As shown, the average hydrogen evolution corrosion rate of the composite weld obtained in Example 2 was 10.03 ml / cm. 2 The rate of corrosion due to weight loss was 3.65 mg / cm³ per day. 2 ;

[0064] like Figure 8 As shown, the self-corrosion potential of the composite weld obtained in Comparative Example 1 is -1.502 V, and the self-corrosion current density is 2.480 μA / cm². 2 ;like Figure 9 As shown, the self-corrosion potential of the composite weld obtained in Example 1 is -1.503 V, and the self-corrosion current density is 1.220 μA / cm². 2 ;like Figure 10 As shown, the self-corrosion potential of the composite weld obtained in Example 2 is -1.506 V, and the self-corrosion current density is 1.326 μA / cm². 2 .

[0065] The above comparison shows that Examples 1 and 2, by adding carbon nanotubes during the laser-arc hybrid welding process of magnesium alloys, effectively refined the weld grains and increased the Mg content. 17 Al 12 The content of precipitated phases is considered. Grain refinement further increases grain boundary density, and grain boundaries, as high-energy regions, can hinder the propagation path of corrosion cracks and promote the early formation of a protective oxide film, thereby preventing the intrusion of corrosive media. Weld precipitates have a higher potential and can protect the weld matrix during corrosion. Furthermore, the aforementioned precipitates dissolve during corrosion to generate a large amount of Al3+. + Ions form a uniform and dense Al2O3 protective film. The Al2O3 protective film acts as a physical barrier to effectively prevent further intrusion of corrosive media, thereby improving the corrosion resistance of magnesium alloy composite welds.

[0066] It should be understood that expressions such as “comprising” and “may include” used in this application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In this application, terms such as “comprising” and / or “having” are to be interpreted as indicating a particular characteristic, number, operation, constituent element, component, or combination thereof, but not to exclude the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0067] Furthermore, in this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A existing alone, A and B existing simultaneously, and B existing alone. The symbol " / " in this document indicates that the related objects are in an "or" relationship; for example, A / B means A or B.

[0068] The terms "first" and "second," etc., used in the specification and claims herein are used to distinguish different objects, not to describe a specific order of objects. For example, "first response message" and "second response message," etc., are used to distinguish different response messages, not to describe a specific order of response messages.

[0069] In the embodiments of this application, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or design. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0070] In the description of the embodiments of this application, unless otherwise stated, "multiple" means two or more, for example, multiple processing units means two or more processing units, multiple elements means two or more elements, etc.

[0071] In the description of the embodiments of this application, it should be noted that, unless otherwise explicitly specified and limited, the term "connection" should be interpreted broadly. For example, "connection" can be a detachable connection or a non-detachable connection; it can be a direct connection or an indirect connection through an intermediate medium. "Fixed connection" refers to a connection where the relative positional relationship remains unchanged after connection. "Rotary connection" refers to a connection where the components can rotate relative to each other after connection. "Sliding connection" refers to a connection where the components can slide relative to each other after connection. The directional terms mentioned in the embodiments of this application, such as "top," "bottom," "inner," "outer," "left," and "right," are only for reference to the directions in the accompanying drawings. Therefore, the directional terms used are for better and clearer explanation and understanding of the embodiments of this application, and are not intended to indicate or imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0072] Furthermore, the mathematical concepts mentioned in the embodiments of this application, such as symmetry, equality, parallelism, and perpendicularity, are limitations specific to the current technological level, rather than absolute and strict mathematical definitions. Slight deviations are permissible; approximations of symmetry, equality, parallelism, and perpendicularity are all acceptable. For example, "A and B are parallel" means that A and B are parallel or approximately parallel, and the angle between A and B can be between 0 and 10 degrees. "A and B are perpendicular" means that A and B are perpendicular or approximately perpendicular, and the angle between A and B can be between 80 and 100 degrees.

[0073] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A composite welding method for improving corrosion resistance of a magnesium alloy weld using carbon nanotubes, characterized by, include: S1 utilizes hydroxylated carbon nanotubes to prepare a carbon nanotube dispersion; S2 involves coating the cleaned magnesium alloy plate with a carbon nanotube dispersion and then air-drying it. S3 connects and fixes the magnesium alloy plate coated with carbon nanotube dispersion to another magnesium alloy plate that is not coated with carbon nanotube dispersion, with the two sides to be welded. S4 employs a laser-arc hybrid welding process. During welding, the high-frequency oscillation of the laser beam prevents the carbon nanotubes from agglomerating, ensuring their uniform dispersion. The carbon nanotubes inhibit the formation of columnar and dendritic crystals during the solidification of the magnesium alloy molten pool, transforming the weld microstructure into equiaxed crystals. Furthermore, the carbon nanotubes pinnify the grain boundaries, hindering grain boundary migration and grain growth. The carbon nanotubes also provide heterogeneous nucleation sites, promoting the precipitation of precipitated phases.

2. The composite welding method of claim 1, wherein, In step S1, the method for preparing the carbon nanotube dispersion includes: S101 mixes carbon nanotubes with a solvent and then stirs the mixture to completely wet the carbon nanotubes; S102 The solution obtained in step S101 is ultrasonically treated in a water bath environment of 4℃~10℃ for 1h~2h, and the cold water is changed every 10min before continuing the ultrasonic treatment. S103 After centrifuging the solution obtained in step S102, the solution is filtered multiple times to obtain the carbon nanotube dispersion.

3. The composite welding method of claim 2, wherein, The centrifugation rate during the centrifugation process is 2000 r / min to 4000 r / min, and the centrifugation time is 30 min to 60 min.

4. The composite welding method of claim 1, wherein, The carbon nanotube dispersion contains 1% to 5% carbon nanotubes by mass.

5. The composite welding method of claim 1, wherein, The carbon nanotubes in the carbon nanotube dispersion have an average diameter of 40 nm to 60 nm and an average length of 5 μm to 10 μm.

6. The composite welding method of claim 1, wherein, In step S4, a welding mode with the laser heat source in front and the electric arc heat source behind is adopted.

7. The composite welding method of claim 1, wherein, In step S4, the distance between the laser beam spot and the welding wire is 1mm~3mm, the defocusing amount is -5mm~5mm; the laser power is 2500W~4500W, the welding current is 90A~120A, and the welding speed is 1.2m / min~1.8m / min.

8. The composite welding method of claim 1, wherein, In step S4, the scanning amplitude of the laser beam is 0.5mm~1mm, and the scanning frequency is 100Hz~300Hz.

9. The composite welding method of claim 1, wherein, In step S4, a magnesium alloy welding wire is selected for arc welding, and the diameter of the magnesium alloy welding wire is 1.0 mm or 1.2 mm.

10. The composite welding method of claim 1, wherein, In step S4, the laser head and welding torch adopt a side-axis composite mode, and the angle between the laser beam and the vertical direction is 0°~10°, while the angle between the welding torch and the surface to be welded is 55°~65°.

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