Multi-directional steady magnetic field assisted coaxial wire feeding laser nitriding method and application thereof

Abstract

The application discloses a kind of multi-direction steady magnetic field auxiliary coaxial wire feeding laser in-situ nitriding method and application thereof, belong to laser additive manufacturing and surface strengthening technical field.The application uses multi-direction steady magnetic field as non-contact control field source, utilizes liquid metal itself heat flow active cutting multi-direction magnetic induction line, to induce spatial multidimensional Lorentz force in the molten pool;The Lorentz force is coupled with the depth of molten pool flow field, forms the three-dimensional global flow field through surface to bottom.The flow field effectively overcomes gas-liquid mass transfer resistance, promotes nitrogen element to deep diffusion reaction;Simultaneously rely on multidimensional fluid shear force to break the directional growth of high melting point TiN primary dendrite, and accelerate deep layer reaction pore floating escape.The application realizes the depth dispersion uniform distribution of hard phase in strengthening layer, significantly inhibits pore and microcrack defect, greatly improves the density, hardness uniformity and wear resistance of coating.

Description

Technical Field

[0001] This invention belongs to the field of laser additive manufacturing and surface strengthening technology, and particularly relates to a multi-directional constant magnetic field-assisted coaxial wire feeding laser in-situ nitriding method and its application. Background Technology

[0002] Titanium alloys occupy a core position in aerospace, marine engineering, and high-end equipment manufacturing due to their excellent specific strength, high-temperature resistance, and corrosion resistance. However, titanium alloys inherently possess defects such as low surface hardness and poor wear resistance. Under harsh service conditions involving high-frequency friction or high contact stress, they are highly susceptible to severe surface damage or even failure, which greatly limits their further application in critical moving and load-bearing components (such as compressor blades, landing gear, and high-pressure valves). Conventional surface treatment technologies often suffer from long processing cycles, thin reinforcing layers, or weak interfacial adhesion, making it difficult to meet the actual requirements of heavy-duty operating conditions.

[0003] To address the aforementioned issues, the industry has proposed coaxial wire feeding laser surface modification technology. This technology leverages the high energy density of lasers to rapidly form a molten pool on the material surface. This not only completely solves the problems of powder splashing, low material utilization, and environmental pollution easily caused by traditional coaxial powder feeding processes, but also significantly improves the stability of the processing. In particular, the introduction of nitrogen for in-situ laser nitriding induces a gas-liquid-solid multiphase metallurgical reaction in the fed titanium alloy wire within the molten pool, generating in-situ a high-hardness titanium nitride (TiN) hard phase with excellent metallurgical bonding to the matrix. This has become an important pathway to improve the wear resistance of titanium alloy surfaces. Nevertheless, in the actual processing of laser in-situ nitriding, due to the extremely fast solidification rate of the molten pool and the extremely large temperature gradient, the high-melting-point TiN generated in situ is very likely to grow into coarse dendrites or local aggregation along a specific direction. At the same time, the difference in thermophysical properties between the high-hardness phase and the matrix can easily cause the accumulation of residual tensile stress under extremely rapid cooling, which not only weakens the macroscopic mechanical properties of the material, but also significantly increases the risk of microcracks and porosity defects initiating inside the composite coating.

[0004] To address the aforementioned defects such as coarse texture and cracking, existing technologies have explored introducing auxiliary energy fields into the laser processing to improve coating quality. For example, some studies have used ultrasonic vibration-assisted laser cladding, attempting to refine grains through ultrasonic cavitation. However, ultrasonic impact, as a mechanical contact-based auxiliary method, exhibits significant limitations in practical applications: its energy transfer highly depends on the rigid follow-up contact between the amplitude transformer and the workpiece, making it difficult to adapt to the all-position processing requirements of complex curved surface components, and the high-temperature service environment easily leads to performance degradation of the contact tool. Some studies have also attempted to introduce a unidirectional steady magnetic field for non-contact intervention. Although this overcomes the limitations of mechanical contact, a unidirectional steady magnetic field can only induce a single-direction Lorentz force within the molten pool, resulting in a relatively weak effect on the molten pool. It is difficult to generate sufficiently strong spatial shear force to completely break coarse TiN primary dendrites, and its ability to control the dispersed distribution of deep hard phases is very limited.

[0005] To further improve the stirring effect, existing technologies have proposed a laser cladding technique using an alternating magnetic field-assisted method. This technique aims to utilize the induced electric field of the alternating magnetic field to drive periodic electromagnetic convection in the molten pool, thereby improving grain refinement. However, for in-situ chemical metallurgical systems under rapid thermal cycling, this technique is limited by its physical nature and exhibits some inherent defects: First, the alternating magnetic field relies on a high-frequency induced electric field to drive the molten pool flow, resulting in an unavoidable electromagnetic skin effect. The electromagnetic energy decreases exponentially with the depth of the molten pool, leading to a severe stirring blind zone in the lower part of the molten pool. Second, the existence of the stirring blind zone makes the TiN hard phase generated by density differences prone to gravitational agglomeration and edge enrichment, resulting in large fluctuations in the macroscopic hardness of the coating and significant anisotropy in wear resistance. Third, due to… Limited by the two-dimensional vibration of the surface, the high-viscosity fluid in the deep layer of the molten pool cannot effectively convect, making it difficult for in-situ reaction bubbles to overcome capillary resistance and float to the surface. This easily induces fatal defects such as micropores, root cracks, and lack of fusion at the interface at the bottom. Fourth, the periodic electromagnetic oscillations caused by the alternating magnetic field lead to violent fluctuations in the liquid surface and increased metal splashing, which seriously undermines the stability of the optical filament coupling. Fifth, the mass transfer process of nitrogen atoms breaking through the liquid surface is severely restricted, and the nitriding reaction can only occur in the shallow layer of the molten pool, resulting in a limited depth of the strengthening layer and a sharp decrease in TiN content along the depth direction.

[0006] In summary, existing ultrasonic-assisted techniques are limited by mechanical contact and adaptability to complex working conditions; conventional unidirectional steady magnetic fields are limited by insufficient electromagnetic shear strength; and the widely studied alternating magnetic field technology suffers from inherent bottlenecks such as the skin effect, which cannot be eliminated due to physical limitations. How to overcome the physical defects of existing magnetic field technologies without introducing traditional mechanical contact, construct a homogeneous stirring flow field without dead zones throughout the entire molten pool, pulverize TiN primary dendrites, actively pump out deep reactive pores, and achieve a full-domain dispersed distribution of the hard phase, is a pressing technical challenge in the field of laser surface engineering. Summary of the Invention

[0007] The purpose of this invention is to provide a multi-directional steady magnetic field-assisted coaxial wire-feeding laser in-situ nitriding method. Its core mechanism lies in utilizing high-energy lasers and multi-directional steady magnetic fields for multi-energy field spatial coupling, simultaneously inducing metallurgical reactions and a three-dimensional global flow field within a liquid molten pool protected by nitrogen-containing reactive gas. The introduced nitrogen gas reacts in-situ with the molten titanium alloy wire in the molten pool to generate a hard titanium nitride (TiN) phase. Through a non-contact method, the high-speed flow driven by the thermal properties of the liquid metal actively cuts the multi-directional steady magnetic field lines, inducing a three-dimensional multi-directional Lorentz force within the molten pool. Through the aforementioned multi-energy field coupling mechanism, this invention achieves a triple regulatory effect: First, by interrupting the directional growth of coarse dendrites in the primary TiN hard phase through a three-dimensional global flow field penetrating the entire molten pool, it induces the formation of fine particles that are dispersed and homogeneously distributed throughout the coating thickness, significantly improving the hardness uniformity and wear resistance of the modified layer; second, by accelerating the three-dimensional convection and global mass transfer of the fluid inside the molten pool, it effectively suppresses the segregation and local enrichment of alloying elements and promotes the efficient escape of reaction bubbles at the bottom of the molten pool, eliminating porosity, root cracks, and incomplete fusion defects within the composite coating from the root; third, it avoids the inherent periodic electromagnetic vibration and skin effect of alternating magnetic fields, significantly improving the surface processing quality of the coating while maintaining extremely stable molten pool formation. This invention overcomes the limitations of traditional single steady magnetic fields with weak electromagnetic shear force, the need for mechanical contact with ultrasonic assistance, and the existence of deep stirring blind zones in alternating magnetic fields, achieving the integrated preparation of a composite reinforced layer with high hardness, high homogeneity, no defects, and excellent forming quality on the metal surface.

[0008] Meanwhile, the purpose of this invention is to provide a key load-bearing component prepared by the above-mentioned multi-directional constant magnetic field-assisted coaxial wire feeding laser in-situ nitriding method and its application.

[0009] To achieve the above objectives, the present invention provides the following technical solution: A multi-directional steady magnetic field-assisted coaxial wire feeding laser in-situ nitriding method includes the following steps: Step 1: Clean the surface of the metal substrate (preferably a titanium alloy substrate) using grinding and polishing equipment, and then fix the metal substrate horizontally on the processing platform.

[0010] Step two: Adjust the spatial relationship between the laser coaxial wire feeding device and the multi-directional stable magnetic field device. The multi-directional stable magnetic field device uses a high-strength permanent magnet array, which is precisely arranged around and directly below the metal substrate. The magnetic induction intensity of the molten pool area is controlled by precisely adjusting the spatial distance (15mm-25mm) between each permanent magnet generating unit (i.e., the permanent magnet, grade N48H, manufactured by Dongguan Juxin Magnetic Technology Co., Ltd., with a magnetic strength range of 0.15T-0.35T) and the laser processing area.

[0011] Step 3: Place the entire metal substrate inside the atmosphere protection chamber and introduce high-purity argon gas through the protective gas inlet. Use the argon gas to displace the air, and expel the air from the chamber through the pre-reserved gap on the upper side of the atmosphere protection chamber. Subsequently, simultaneously introduce a reaction gas containing nitrogen. The reaction gas is pure nitrogen or a mixture of nitrogen and argon, with the volume fraction of nitrogen in the mixture controlled between 5% and 30%, and the remainder being argon.

[0012] Step four: Start the laser coaxial wire feeding equipment and control the laser head to perform cladding processing along the preset scanning path, so that the synchronously fed titanium alloy welding wire (the diameter of the welding wire is in the range of 0.8-1.6mm) and the metal matrix are locally and rapidly melted to form a liquid molten pool; at the same time, the liquid molten pool is stirred in real time by a pre-arranged multi-directional stable magnetic field device, so that the metal droplets, matrix material and nitrogen phase undergo a full in-situ metallurgical nitriding reaction.

[0013] The laser coaxial wire feeding processing parameters are: 2000-4000W, scanning speed 0.2-1.0m / min, and wire feeding speed 0.5-1.5m / min.

[0014] Step 5: Based on the preset surface nitriding strengthening area, Step 4 is executed cyclically using a single-layer multi-pass overlapping strategy, with the overlap rate between adjacent passes controlled at 30%-50%. After the overall composite coating is completed, the mixed protective gas in the protective chamber is kept circulating until the overall workpiece temperature drops below 200℃. Subsequent overlapping weld passes locally temper and soften the previous weld pass, while the electromagnetic plasticity induced by the multi-directional steady magnetic field continuously suppresses stress concentration and defect initiation in the newly formed molten pool and heat-affected zone. Through the continuous application mode of "in-situ reaction - multi-directional steady magnetic field global homogenization," the huge residual thermal stress generated during the solidification of the high-melting-point hard phase is offset, eliminating microcracks, bottom porosity, and incomplete fusion defects within the coating, effectively improving the comprehensive mechanical properties of the composite coating on the metal surface.

[0015] Furthermore, in step one, the cleaning process involves sequentially grinding and polishing the substrate surface with 80#-800# sandpaper, followed by ultrasonic cleaning with anhydrous ethanol for 10-15 minutes to thoroughly remove surface oil, oxide scale, and dust.

[0016] Furthermore, in step two, the multi-directional steady magnetic field device specifically includes two sets of horizontal magnetic field generating units and one set of vertical magnetic field generating units; the two sets of horizontal generating units are arranged on the left and right sides and front and back sides of the processing area with opposite magnetic poles, respectively, and the one set of vertical generating units is arranged directly below the substrate; the central geometric axis of each generating unit is three-dimensionally orthogonally aligned with the central axis of the laser beam in space, and the magnetic induction intensity of the three directional steady magnetic fields at the center of the liquid molten pool is maintained at about 0.3T (preferably 0.3T-0.35T), ensuring that the generated composite magnetic field region accurately covers and completely penetrates the surface of the liquid molten pool and its solidification heat-affected zone.

[0017] Specifically, the multi-directional stable permanent magnet device consists of five high-strength permanent magnet generating poles: the first generating pole is arranged directly below the worktable to generate vertically upward stable magnetic field lines; the second and third generating poles are arranged on the left and right sides of the processing area with opposite magnetic poles facing each other to generate horizontal stable magnetic field lines; the fourth and fifth generating poles are arranged on the front and rear sides of the processing area with opposite magnetic poles facing each other to generate horizontal vertical stable magnetic field lines.

[0018] Furthermore, in step three, high-purity argon gas is continuously introduced during the exhaust stage until the oxygen concentration in the chamber drops below 200 ppm as monitored by an oxygen concentration analyzer before the laser processing experiment is carried out. During the entire nitriding process, a mixed gas is continuously introduced to maintain a positive pressure environment, and the total flow rate of the mixed gas or pure nitrogen is controlled at 15-25 L / min.

[0019] The beneficial effects of adopting the above-mentioned further technical solutions are: ensuring that the harsh processing environment with extremely low oxygen is maintained throughout the process, preventing severe oxygen absorption embrittlement and oxidation discoloration of titanium alloys during high-temperature nitriding and cooling, and ensuring the purity of the in-situ nitriding reaction and the final service quality of the composite coating.

[0020] Furthermore, in step four, the laser power, scanning speed, and wire feeding speed are matched to ensure that the linear energy density per unit length is maintained within a reasonable range.

[0021] The beneficial effects of adopting the above-mentioned further technical solutions are: ensuring that the titanium alloy welding wire is in a semi-molten or just-molten liquid bridge transition state when it comes into contact with the molten pool, which avoids excessive burning of alloy elements and coarsening of the heat-affected zone due to excessive heat input, and also prevents incomplete fusion defects caused by insufficient heat input.

[0022] Furthermore, in step four, the in-situ spatial integration of the laser coaxial wire feeding system and the multi-directional steady magnetic field system ensures the synchronous operation of laser thermal input and magnetic field momentum input, thereby achieving efficient initiation and full-domain physical control of the metallurgical reaction in the molten pool.

[0023] Furthermore, in step five, adjacent cladding channels are arranged with equidistant overlapping paths, and the center-to-center distance between adjacent cladding channels is 50%-70% of the width of a single cladding channel (i.e., an overlap rate of 30%-50%).

[0024] The beneficial effects of adopting the above-mentioned further technical solutions are: ensuring the macroscopic thickness uniformity of the large-area composite coating, avoiding the occurrence of overlapping groove defects or heat accumulation and secondary coarsening of the structure due to excessive overlap rate.

[0025] Furthermore, in step five, a multi-directional steady magnetic field is continuously applied throughout the entire process of multi-layer and multi-channel overlapping, and each cladding channel uses the same laser power, wire feeding speed, and permanent magnet spatial distance parameters.

[0026] The beneficial effects of adopting the above-mentioned further technical solutions are: maintaining the stability of the three-dimensional global flow field topology inside the liquid molten pool, ensuring the high uniformity of the overall microstructure of the composite coating, thereby eliminating the anisotropy of mechanical properties.

[0027] The present invention discloses a TiN-reinforced composite coating prepared by a multi-directional steady magnetic field-assisted coaxial wire-feeding laser in-situ nitriding method. The average surface microhardness of the composite coating is [580-725] HV; its wear volume is [0.2315-0.4825] mm. 3 .

[0028] The multi-directional steady magnetic field-assisted coaxial wire-feeding laser in-situ nitriding method and the coating prepared by this invention are applicable to critical load-bearing components facing harsh service environments in the fields of aerospace, marine engineering, or high-end equipment manufacturing. Specifically, they can be used for surface strengthening and repair of aero-engine compressor blades, aircraft landing gear actuators, pressure hull connectors of deep-sea submersibles, and wear-resistant liners of mining machinery.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: Compared to traditional ultrasonic-assisted techniques and alternating magnetic field techniques, this invention overcomes the defects of mechanical interference and electromagnetic oscillation. Addressing the issues of mechanical contact interference, high-temperature energy attenuation, and insufficient adaptability to complex components inherent in traditional ultrasonic-assisted techniques, and the severe electromagnetic vibrations on the liquid surface caused by alternating magnetic fields, this invention employs a multi-directional stable magnetic field composed of permanent magnets as a non-contact auxiliary control method. This solution eliminates the need for additional mechanical servo devices, and the stable magnetic field does not generate periodic electromagnetic oscillations or additional Joule heating. This not only maintains an extremely stable flow field intervention effect during the strengthening process of complex curved surfaces and large areas, but also significantly reduces metal spatter in the molten pool, greatly improving the surface smoothness and forming quality of the coating, and significantly promoting the industrial application of automated laser strengthening processing.

[0030] Compared to simple thermal convection and conventional alternating magnetic fields, this invention overcomes the mass transfer limit and the deep stirring blind zone. Addressing the issues of traditional laser nitriding relying solely on natural thermal convection leading to hindered element diffusion, and the quasi-two-dimensional surface stirring blind zone and tendency for TiN hard phase agglomeration caused by the electromagnetic skin effect in alternating magnetic fields, this invention utilizes a metal fluid to cut multi-directional spatial orthogonal magnetic field lines, inducing a three-dimensional global flow field penetrating from the surface to the bottom and from the center to the edge within the molten pool. This three-dimensional flow field physically intervenes in the gas-liquid transport and liquid-solid reaction of nitrogen, breaking the boundary layer effect and gravitational stratification. By using multi-dimensional spatial shear force to break coarse TiN dendrites, it achieves equiaxed refinement and uniform dispersion of the hard phase throughout the entire coating thickness direction.

[0031] Compared to conventional laser cladding, this invention eliminates reaction porosity and microcrack defects. Addressing the issue of bottom-entry porosity, root crack initiation, and incomplete fusion in titanium alloy cladding and nitriding processes, the multi-directional steady magnetic field of this invention possesses global penetration characteristics, enabling uniform distribution throughout the entire depth of the molten pool. The induced three-dimensional global flow field can lift tiny reaction bubbles and impurities adhering to the bottom of the molten pool to the surface; simultaneously, this mechanism significantly reduces the temperature gradient within the molten pool, and, in conjunction with the high-temperature tempering generated by multiple overlapping processes, significantly reduces localized residual thermal stress concentration. Through coupled control, defects within the composite coating are eliminated, improving the quality and stability of the interfacial metallurgical bonding.

[0032] Compared to traditional powder feeding processes, this invention achieves high-quality and high-efficiency integrated preparation: It replaces traditional powder feeding with a coaxial laser wire feeding method, effectively improving the utilization rate of titanium alloy materials and avoiding powder scattering and optical path contamination. Combined with an in-situ nitrogen feeding mechanism within a sealed atmosphere protection chamber, the laser micrometallurgical melting and gas-liquid phase nitriding processes are synchronized, eliminating the need for any costly nitride powder pre-placement or surface pre-coating treatment. Furthermore, a multi-directional stable magnetic field is constructed using high-strength permanent magnets, eliminating the need for complex water-cooled power supplies and high-frequency excitation equipment. Precise topological control of the three-dimensional flow field can be achieved simply by adjusting the spatial physical distance between the permanent magnet poles, resulting in a simplified process and extremely low equipment modification costs.

[0033] This invention discloses a multi-directional steady magnetic field-assisted coaxial wire feeding laser in-situ nitriding method and its application, belonging to the field of laser surface strengthening and additive manufacturing technology. The method includes the following steps: cleaning the surface of a titanium alloy substrate; adjusting the spatial position of the multi-directional permanent magnet device and the laser coaxial wire feeding device; synchronously conveying nitrogen-containing reactive gas under a low-oxygen protective atmosphere; performing cladding processing using laser coaxial wire feeding, and simultaneously applying steady magnetic fields in multiple directions, including horizontal left-right, horizontal front-back, and vertical upward; and completing the composite coating preparation using a multi-layer overlapping method. This invention utilizes the coupling effect of the multi-directional steady magnetic field and the liquid molten pool to induce a three-dimensional flow field penetrating the entire domain within the molten pool under liquid conditions, achieving active control over nitrogen element transport and the in-situ reaction process. Simultaneously, this invention improves the solidification behavior of the molten pool through continuous multi-directional magnetic field intervention, effectively suppressing deep porosity and crack defects within the composite coating, significantly improving the coating's hardness uniformity and overall wear resistance, and achieving integrated preparation of a high-quality strengthening layer on the titanium alloy surface. This invention is applicable to the surface strengthening of key load-bearing components of titanium alloys in aerospace, marine engineering, and high-end equipment fields.

[0034] This invention employs a multi-directional steady magnetic field as a contactless control field source, utilizing the heat flow of the liquid metal itself to actively cut multi-directional magnetic field lines, thereby inducing a spatial multidimensional Lorentz force within the molten pool. This Lorentz force is deeply coupled with the molten pool flow field, forming a three-dimensional global flow field penetrating from the surface to the bottom. This flow field effectively overcomes the gas-liquid mass transfer resistance, promoting the diffusion reaction of nitrogen into deeper layers; simultaneously, it relies on multi-dimensional fluid shear force to interrupt the directional growth of primary dendrites of high-melting-point TiN and accelerate the upward escape of deep reaction pores. This invention achieves a deep, uniformly dispersed distribution of the hard phase within the reinforcing layer, significantly suppressing porosity and microcrack defects, and greatly improving the coating's density, hardness uniformity, and wear resistance. Attached Figure Description

[0035] The technical solution and experimental results of the present invention will be further described in detail below through equipment and microscopic tissue photographs.

[0036] Figure 1 This is a schematic diagram of a multi-directional stable magnetic field-assisted coaxial wire feeding laser nitriding method according to the present invention; wherein: 1-robotic arm; 2-laser coaxial wire feeding device; 3-atmosphere protection chamber; 4-permanent magnet; 5-protective gas inlet; 6-clamping worktable; 7-protective gas outlet; 8-ring laser; 9-wire feeding tube; 10-welding wire; 11-molten pool; 12-titanium alloy substrate; Figure 2 The metallographic structures of the coatings under different cladding processes in Example 1 are as follows: (a) Single steady magnetic field-laser coaxial wire feeding cladding; (b) Multi-directional steady magnetic field-laser coaxial wire feeding cladding; Figure 3 The microstructure of the coating under different cladding processes in Example 1 is as follows: (a) Single steady magnetic field-laser coaxial wire feeding cladding; (b) Multi-directional steady magnetic field-laser coaxial wire feeding cladding; Figure 4 This is a comparison of the wear volume of samples treated with different cladding processes. Detailed Implementation

[0037] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Example 1

[0038] In this embodiment, the titanium alloy substrate used is TC4 titanium alloy (Ti-6Al-4V), with the following chemical composition by mass percentage: Fe 0.30%, C 0.08%, O 0.20%, N 0.05%, H 0.015%, Al 6.04%, V 4.01%, and the balance being Ti and unavoidable impurities. The wire feeding material used in this embodiment is TC4 titanium alloy welding wire that matches the base material, with the same chemical composition. The applicable wire diameter range is 0.8mm-1.6mm, and in this embodiment, a wire diameter of 1.2mm is preferred.

[0039] It should be noted that the specific titanium alloy grades and composition ratios listed in this embodiment are only used to verify the feasibility and strengthening effect of the present invention and do not constitute a limitation on the scope of protection of the present invention. In practical applications, commercially available α-type titanium alloys, β-type titanium alloys, or α+β-type titanium alloys can be used as the base material according to different service conditions, and matching titanium alloy welding wires can be selected. As long as they have basic cladding applicability, the method of the present invention can be used, and will not be elaborated further here.

[0040] This embodiment employs a multi-directional stable magnetic field-assisted coaxial wire feeding laser in-situ nitriding method, as follows: Figure 1 The device shown includes: 1-robotic arm; 2-laser coaxial wire feeding device; 3-atmosphere protection chamber; 4-permanent magnet; 5-protective gas inlet; 6-clamp worktable; 7-protective gas outlet; 8-ring laser; 9-wire feeding tube; 10-welding wire; 11-molten pool; 12-titanium alloy substrate. The laser coaxial wire feeding device 2 is installed at the end of the robotic arm 1, and the laser processing trajectory is controlled by the robotic arm 1. The multi-directional stable magnetic field device uses a high-strength permanent magnet array (permanent magnet 4) arranged around and directly below the metal substrate (titanium alloy substrate 12) to apply a spatially orthogonal multi-directional stable magnetic field in the liquid molten pool 11 area. The titanium alloy substrate 12 is fixed on the fixture worktable 6 and placed inside the atmosphere protection chamber 3. The protective gas inlet 5 is used to deliver high-purity protective gas into the atmosphere protection chamber 3. The protective gas outlet 7 is used to discharge residual air inside. The ring laser 8 acts on the surface of the titanium alloy substrate 12 and the synchronously fed welding wire 10, causing it to melt rapidly to form a liquid molten pool 11, and a composite strengthening layer is formed after the molten pool solidifies. The wire feeding tube 9 is used to feed the welding wire 10.

[0041] This embodiment provides a multi-directional steady magnetic field-assisted coaxial wire feeding laser in-situ nitriding method, which specifically includes the following steps: Step 1: Select a TC4 titanium alloy plate with dimensions of 100mm×80mm×10mm as the substrate material. Use 80#-800# sandpaper to grind the oxide layer on the surface of the area to be processed and polish it to a roughness Ra≤3.2μm. Then, use anhydrous ethanol for ultrasonic cleaning for 12 minutes to thoroughly remove surface oil and impurities.

[0042] Step 2: Based on the geometric dimensions of the coating, the initial working coordinate system and laser scanning path of the robotic arm 1 are set by the teach pendant, so that the laser coaxial wire feeding device 2 is perpendicular to the surface of the titanium alloy substrate 12; at the same time, the spatial distance (about 20mm) of the permanent magnets 4 around and at the bottom is precisely adjusted so that the multi-directional stable magnetic field area completely penetrates the entire liquid molten pool 11.

[0043] Step 3: Given the extremely high chemical reactivity of titanium alloys, to prevent abnormal oxidation of the processing area at high temperatures, the titanium alloy substrate 12 is placed in an atmosphere protection chamber 3, and high-purity argon gas with a purity of 99.9% is introduced through the protective gas inlet 5. Utilizing the physical property that argon gas has a higher density than air, a bottom-inlet and top-outlet method is adopted, with residual air discharged through the protective gas outlet 7 reserved at the top of the chamber. A gas concentration monitoring and analysis instrument is installed in the atmosphere protection chamber 3 to collect atmospheric data in real time; when the oxygen concentration in the chamber drops below 200 ppm, it is considered to have formed a stable extremely low-oxygen protective atmosphere, and laser processing can be performed. Throughout the processing, the gas flow rate in the protection chamber is maintained, and a mixed reaction gas containing nitrogen is delivered to the surface of the molten pool through the coaxial gas delivery channel built into the laser coaxial wire feeder 2. The nitrogen gas accounts for 10% of the total gas volume, and the total flow rate of the mixed gas is controlled at 25 L / min to achieve synergistic protection of the molten pool and promote efficient in-situ nitriding reaction.

[0044] Step four: Activate the laser coaxial wire feeding device 2, causing the ring laser 8 to move along a preset path and simultaneously feed in the TC4 titanium alloy welding wire 10, causing the welding wire to rapidly melt with the substrate surface to form a liquid molten pool 11. Simultaneously, using pre-arranged permanent magnets 4, a multi-directional stable magnetic field is applied throughout the entire processing. The high-speed flow driven by the thermal properties of the liquid metal actively cuts the multi-directional orthogonal magnetic field lines, inducing a strong three-dimensional multi-directional Lorentz force within the molten pool. This multi-directional Lorentz force is deeply coupled with the molten pool flow field, inducing the formation of a three-dimensional global flow field penetrating from the surface to the bottom within the liquid molten pool. This forcibly breaks the gas-liquid mass transfer resistance, promoting uniform diffusion of nitrogen within the molten pool and in-situ metallurgical reactions.

[0045] The laser power was set to 2500W, the scanning speed to 0.5m / min, and the wire feeding speed to 1.2m / min. By adjusting the distance of the permanent magnet, the magnetic induction intensity of the three-directional constant magnetic field at the center of the liquid molten pool was maintained at around 0.3T.

[0046] Step 5: Based on the preset surface strengthening area, Step 4 is executed cyclically using a single-layer multi-pass overlapping strategy. Adjacent cladding passes are arranged at equal intervals, with an overlap rate controlled at 40%. After the overall processing of the composite strengthening layer is completed, the circulation of mixed protective gas in the atmosphere protection chamber 3 is continuously maintained until the overall temperature of the titanium alloy substrate 12 drops below 200°C to prevent oxygen absorption embrittlement and abnormal oxidation discoloration of the coating surface during the high-temperature cooling stage. In the subsequent multi-pass overlapping process, the multi-directional stable magnetic field generated by the permanent magnet 4 continuously acts on the newly formed liquid molten pool region. Through three-dimensional global stirring and multi-dimensional hydrodynamic shearing, the primary TiN dendrites are broken, completely eliminating the deep stirring blind zone of the traditional magnetic field. At the same time, the strong eddy pumping effect accelerates the escape of reaction bubbles at the bottom of the molten pool, and the synergistic local tempering effect reduces the concentration of residual thermal stress, effectively suppressing the formation of defects such as porosity, root cracks, and interface incomplete fusion.

[0047] The composite reinforcement layer on the titanium alloy surface obtained in this embodiment has an extremely uniform and dense microstructure, forming an excellent defect-free metallurgical bonding interface between the coating and the substrate. Microstructural observation revealed the formation of fine and completely dispersed equiaxed TiN reinforcing phase particles within the coating, completely eliminating the coarse oriented dendrite network and deep nitrogen-depleted regions. The average microhardness of the composite reinforcement layer obtained in this embodiment reaches 680 HV, approximately 2.3 times higher than that of the TC4 titanium alloy substrate, with minimal hardness fluctuation and a wear volume of only 0.2563 mm². 3 The wear rate was reduced by about 80% compared to the untreated substrate, exhibiting extremely excellent anti-adhesive wear performance and anti-stripping ability, indicating that the multi-energy field coupling method of the present invention can achieve the integrated preparation of titanium alloy surfaces with dense and defect-free surfaces and extremely high wear resistance.

[0048] The method described in this embodiment can be widely applied to in-situ surface strengthening and long-term protection of key titanium alloy components in aerospace, marine engineering and high wear-resistant equipment fields, including aero-engine blades, compressor discs, aircraft landing gear actuators, wear-resistant components of ships and key friction components of deep-sea equipment.

[0049] The key load-bearing component obtained by the method of this embodiment has a nitrided composite reinforcement layer on its surface.

[0050] The application of the key load-bearing components of this invention in the fields of aerospace, marine engineering, or high-end equipment manufacturing.

[0051] To further verify the significant progress of the present invention, the following detailed comparative analysis of the differences in microstructure and macroscopic mechanical properties between Embodiment 1 and Comparative Example 1, in conjunction with the accompanying drawings, is presented.

[0052] like Figure 2 As shown, (a) and (b) are the metallographic images of the single-direction steady magnetic field-assisted laser coaxial wire feeding coating and the multi-directional steady magnetic field-assisted coating of this embodiment, respectively. A comparison reveals that when a single-direction steady magnetic field is applied, the induced Lorentz force has a single direction, resulting in limited intensity within the molten pool, and significant coarse columnar crystals remain in the lower part of the coating. In contrast, in this embodiment, the original coarse columnar crystals in the coating are replaced by a large number of fine and uniform equiaxed crystals, exhibiting a high degree of uniformity and density across the entire microstructure. This demonstrates that the three-dimensional global flow field induced by the multi-directional steady magnetic field breaks the convection dead zone of a conventional single magnetic field, promotes the fracture of primary dendrites through fluid shear force, and increases the non-spontaneous nucleation rate. From a solidification kinetics perspective, this blocks the directional epitaxial growth of columnar crystals, achieving refinement and homogenization of the coating microstructure.

[0053] like Figure 3As shown, (a) and (b) are the microstructure diagrams of the unidirectional steady magnetic field-assisted laser coaxial wire feeding coating and the multidirectional steady magnetic field-assisted coating of this embodiment, respectively. By comparison, it can be found that under the unidirectional steady magnetic field, some coarse needle-like and dendritic TiN hard phases still exist inside the coating, and obvious long-sized lamellar agglomerates are formed in local areas, with strong orientation. However, after applying multidirectional steady magnetic field coupling assistance, the internal structure of the coating is deeply refined. The original coarse needle-like and dendritic TiN is replaced by a large number of fine and dispersed granular structures, the size of the reinforcing phase is drastically reduced, macroscopic component segregation is eliminated, and the overall distribution presents an extremely ideal dispersed and homogeneous state. This indicates that this embodiment utilizes the three-dimensional flow field and asymmetric spatial shear force excited inside the molten pool by the multidirectional steady magnetic field, which not only overcomes the gas-liquid surface mass transfer resistance and promotes an extremely uniform mixing reaction between the high-temperature titanium liquid and nitrogen, but also breaks the coarse primary dendrites of the high-melting-point TiN phase. The refined TiN reinforcing phase is extremely uniformly dispersed within the titanium alloy matrix, which not only significantly increases the overall macroscopic hardness of the coating, but also releases local residual thermal stress concentration, thereby significantly improving the stability of the reinforcing layer during service.

[0054] like Figure 4 The figure shows a comparison of the dry sliding friction wear volume between the unidirectional steady magnetic field assisted coating and the multidirectional steady magnetic field assisted coating of this embodiment. The untreated TC4 titanium alloy substrate (i.e., the TC4 titanium alloy (Ti-6Al-4V) substrate) has extremely poor resistance to adhesive wear, with a wear volume as high as 1.224 mm. 3 The comparative sample subjected to laser in-situ nitriding assisted by a unidirectional constant magnetic field generated some nitrogen-containing hard phases, but due to microstructure segregation and local dendrite coarsening, its wear volume only decreased to 0.7325 mm. 3 In this embodiment, the composite reinforcement layer assisted by a multi-directional steady magnetic field exhibits a wear volume of only 0.2563 mm. 3 This indicates that, with the assistance of a multi-directional steady magnetic field, the wear volume in this embodiment is further reduced by 65% ​​compared to that of a single-directional steady magnetic field, and by 79.1% compared to the untreated TC4 substrate. This significant effect confirms the improvement of the coating's wear resistance by the multi-directional steady magnetic field, while meeting the stringent service requirements of high-end critical load-bearing components. Comparative Example 1

[0055] The difference between this comparative example and Example 1 is that only a single horizontal, stable magnetic field is applied during the laser coaxial wire feeding nitriding process, including the following steps: Step 1: Select a TC4 titanium alloy plate with dimensions of 100mm×80mm×10mm as the substrate material. Use 80#-800# sandpaper to grind the oxide layer on the surface of the area to be processed and polish it to a roughness Ra≤3.2μm. Then, use anhydrous ethanol for ultrasonic cleaning for 12 minutes to thoroughly remove surface oil and impurities.

[0056] Step 2: Set the initial working coordinate system and laser scanning path of the robotic arm using the teach pendant, so that the laser coaxial wire feeding device remains perpendicular to the surface of the titanium alloy substrate. In this comparative example, only a set of high-strength permanent magnet generating units are placed on the left and right sides of the processing area to generate a single horizontal stable magnetic field line in the liquid molten pool area. No magnetic field generating devices in other directions are set.

[0057] Step 3: Given the extremely high chemical reactivity of titanium alloys, to prevent abnormal oxidation of the processing area at high temperatures, the titanium alloy test plate is placed in an atmosphere protection chamber, and high-purity argon gas (99.9% purity) is introduced through the protective gas inlet. Utilizing the physical property that argon gas is denser than air, a bottom-inlet and top-outlet method is adopted, with residual air discharged through the protective gas outlet reserved at the top of the chamber. A gas concentration monitoring and analysis instrument is installed in the atmosphere protection chamber to collect atmospheric data in real time; when the oxygen concentration in the chamber drops below 200 ppm, it is considered that a stable, extremely low-oxygen protective atmosphere has been formed, and laser processing can proceed. Throughout the processing, the gas flow rate in the protective chamber is maintained, and a mixed reaction gas containing nitrogen (a mixture of nitrogen and argon) is delivered to the surface of the molten pool through the coaxial gas delivery channel built into the laser coaxial wire feeding device. The nitrogen gas accounts for 10% of the total gas flow, and the total flow rate of the mixed gas is controlled at 25 L / min to achieve synergistic protection and effective nitriding of the molten pool.

[0058] Step four: Activate the coaxial laser wire feeding device, moving the ring laser along a preset path while simultaneously feeding in TC4 titanium alloy welding wire. This allows the welding wire to rapidly melt with the substrate surface, forming a liquid molten pool, achieving in-situ nitriding and forming a composite reinforcement layer. Simultaneously, a preset permanent magnet device applies a stable magnetic field in a single horizontal direction only in the molten pool area. To ensure the rigor of the controlled variables, the laser power is set to 2500W, the scanning speed to 0.5m / min, and the wire feeding speed to 1.2m / min; the magnetic induction intensity of the stable magnetic field in the single horizontal direction at the center of the molten pool is maintained at approximately 0.3T.

[0059] Step five: Based on the preset surface strengthening area, Step four is executed cyclically using a single-layer, multi-pass overlapping strategy. Adjacent cladding passes are arranged at equal intervals, with an overlap rate controlled at 40%. After the overall processing of the composite strengthening layer is completed, the circulation of mixed protective gas in the atmosphere protection chamber is continuously maintained until the overall temperature of the titanium alloy substrate drops below 200°C. Through atmosphere isolation measures, oxygen absorption embrittlement and abnormal oxidation discoloration of the titanium alloy joints during the high-temperature cooling stage are prevented, thereby ensuring the service performance of the strengthening layer.

[0060] The difference between Comparative Example 1 and Example 1 lies only in the application of a single horizontal, stable magnetic field. This results in limited planar convection in the left-right direction within the molten pool, leaving a large area of ​​blind stirring in the front-back direction. This leads to the formation of coarse, oriented columnar crystal structures at the front and back edges of the coating. Simultaneously, the lack of active Lorentz force in the vertical direction results in insufficient flow intensity at the bottom of the molten pool, preventing the effective breaking of the bottom stagnant layer and easily leaving behind porosity and microcracks. Nitrogen within the molten pool can only achieve limited mixing in the left-right direction, with extremely poor diffusion in the front-back direction. This leads to severe enrichment of the TiN reinforcing phase in the front and back edge regions, with a TiN content deviation of ±19% between the front and back regions, and the easy formation of coarse needle-like and lath-like TiN phases. Due to the inhomogeneous structure and internal defects, the average microhardness of the coating is approximately 420 HV, and its wear volume is 0.7325 mm². 3 Meanwhile, the large, brittle nitride aggregates are prone to microcracks that propagate rapidly during friction and wear, significantly reducing the coating's crack resistance and service stability. Comparative Example 2

[0061] The only difference between this comparative example and Example 1 is that only a single, vertically upward steady magnetic field is applied during the laser coaxial wire feeding nitriding process; steady magnetic fields in the horizontal front-back and horizontal left-right directions are not applied. The process includes the following steps: Step 1: Select a TC4 titanium alloy plate with dimensions of 100mm×80mm×10mm as the base material. Use 80#-800# sandpaper to grind and polish the oxide layer on the surface of the area to be processed. Then, use anhydrous ethanol for ultrasonic cleaning for 12 minutes to thoroughly remove surface oil, oxide scale and impurities.

[0062] Step 2: Based on the geometric dimensions of the coating, set the initial working coordinate system and laser scanning path of the robotic arm using a teach pendant, so that the laser coaxial wire feeding device remains perpendicular to the surface of the metal substrate; at the same time, adjust the position of the magnetic field generating device, and arrange the annular permanent magnet only directly below the non-magnetic clamping table, so that the central axis of the single vertically upward stable magnetic field passes through the geometric center of the liquid molten pool, ensuring that the magnetic field action area covers the liquid molten pool.

[0063] Step 3: Given the extremely high chemical reactivity of titanium alloys, in order to prevent oxidation, nitriding, and hydrogen embrittlement reactions in the welding area at high temperatures, this comparative example employs inert gas protection. The metal substrate is placed in an inert gas protection chamber, and high-purity argon gas (99.99% purity) is introduced through the protective gas inlet. Utilizing the physical property that argon gas has a higher density than air, a bottom-inlet and top-outlet method is adopted, with residual air discharged through a protective gas outlet reserved at the top of the chamber. An oxygen concentration monitoring and analysis instrument is installed in the inert gas protection chamber to collect atmospheric data in real time; when the oxygen concentration in the chamber drops below 200 ppm, a stable inert protective atmosphere is formed, and cladding can proceed. During the cladding process, the protective gas flow rate in the inert gas protection chamber is maintained at 40 L / min, and pure nitrogen gas is supplied to the molten pool and weld surface through the auxiliary gas delivery channel built into the laser coaxial wire feeder, with the nitrogen flow rate controlled at 15 L / min, to achieve synergistic protection of the cladding layer and effective in-situ nitriding.

[0064] Step four: Activate the laser coaxial wire feeding device, moving the laser beam along a preset path and simultaneously feeding in a titanium alloy welding wire. This causes the welding wire to rapidly melt with the substrate surface, forming a liquid molten pool. Simultaneously, a stable, vertically upward magnetic field with a magnetic induction intensity of 0.3T is applied to the molten pool area using a ring-shaped permanent magnet below the worktable. The molten pool flow cuts the magnetic field lines, generating a unidirectional Lorentz force that induces vertical convection within the molten pool. The laser power is 2500W, the scanning speed is 0.5m / min, and the wire feeding speed is 1.2m / min.

[0065] Step five: Based on the preset surface strengthening area, a single-layer, multi-pass overlapping strategy is used to iteratively execute step four, with the overlap rate of adjacent cladding passes controlled at 40%. After the overall composite coating is completed, the protective atmosphere within the inert gas chamber is continuously maintained until the overall temperature of the metal substrate drops below 200°C. This atmosphere isolation measure prevents oxygen embrittlement and excessive nitriding reactions in the titanium alloy joint at high temperatures, thereby ensuring the service performance of the welded joint.

[0066] The difference between Comparative Example 2 and Example 1 lies only in the application of a single, vertically upward, steady magnetic field. This results in limited vertical convection within the molten pool, creating a large area of ​​permanent flow dead zones across the entire horizontal plane. Consequently, the coating exhibits a coarse, horizontally columnar crystalline structure growing along the heat flow direction. Simultaneously, the lack of active Lorentz force in the horizontal direction leads to almost zero horizontal flow intensity in the molten pool, preventing effective breaking of the horizontal boundary layer and resulting in residual pores and unfused defects at the pool edge. Nitrogen within the molten pool can only achieve limited vertical mixing, exhibiting extremely poor horizontal diffusion. This results in significant stratification of the TiN reinforcing phase, with severe deposition at the bottom and the formation of coarse needle-like and lath-like TiN phases. Due to the inhomogeneous structure and internal defects, the coating's average microhardness is approximately 435 HV, and its wear volume is 0.6247 mm². 3 Meanwhile, the large, brittle nitride aggregates are prone to microcracks that propagate rapidly during friction and wear, significantly reducing the coating's crack resistance and service stability. Comparative Example 3

[0067] The only difference between this comparative example and Example 1 is that: during the laser coaxial wire feeding nitriding process, only a single horizontal front-to-back steady magnetic field is applied, and no horizontal left-to-right or vertical upward steady magnetic fields are applied. The process includes the following steps: Step 1: Select a TC4 titanium alloy plate with dimensions of 100mm×80mm×10mm as the base material. Use 80#-800# sandpaper to grind and polish the oxide layer on the surface of the area to be processed. Then, use anhydrous ethanol for ultrasonic cleaning for 12 minutes to thoroughly remove surface oil, oxide scale and impurities.

[0068] Step 2: Based on the geometric dimensions of the coating, the initial working coordinate system and laser scanning path of the robotic arm are set using a teach pendant, so that the laser coaxial wire feeding device remains perpendicular to the surface of the metal substrate; at the same time, the position of the magnetic field generating device is adjusted, and strip permanent magnets are symmetrically arranged only on the front and rear sides of the metal substrate, so that the central axis of the single horizontal front-back direction stable magnetic field passes through the geometric center of the liquid molten pool, ensuring that the magnetic field action area covers the liquid molten pool.

[0069] Step 3: Given the extremely high chemical reactivity of titanium alloys, in order to prevent oxidation, nitriding, and hydrogen embrittlement reactions in the welding area at high temperatures, this comparative example employs inert gas protection. The metal substrate is placed in an inert gas protection chamber, and high-purity argon gas (99.99% purity) is introduced through the protective gas inlet. Utilizing the physical property that argon gas has a higher density than air, a bottom-inlet and top-outlet method is adopted, with residual air discharged through a protective gas outlet reserved at the top of the chamber. An oxygen concentration monitoring and analysis instrument is installed in the inert gas protection chamber to collect atmospheric data in real time; when the oxygen concentration in the chamber drops below 200 ppm, a stable inert protective atmosphere is formed, and cladding can proceed. During the cladding process, the protective gas flow rate in the inert gas protection chamber is maintained at 40 L / min, and pure nitrogen gas is supplied to the molten pool and weld surface through the auxiliary gas delivery channel built into the laser coaxial wire feeder, with the nitrogen flow rate controlled at 15 L / min, to achieve synergistic protection of the cladding layer and effective in-situ nitriding.

[0070] Step four: Activate the laser coaxial wire feeding device, moving the laser beam along a preset path and simultaneously feeding in a titanium alloy welding wire. This causes the welding wire to rapidly melt with the substrate surface, forming a liquid molten pool. Simultaneously, a stable, horizontal magnetic field with a magnetic induction intensity of 0.3T is applied to the molten pool region using strip-shaped permanent magnets on both sides. The molten pool flow cuts the magnetic field lines, generating a unidirectional Lorentz force that induces planar convection in the front-to-back direction within the molten pool. The laser power is 2500W, the scanning speed is 0.5m / min, and the wire feeding speed is 1.2m / min.

[0071] Step five: Based on the preset surface strengthening area, a single-layer, multi-pass overlapping strategy is used to iteratively execute step four, with the overlap rate of adjacent cladding passes controlled at 40%. After the overall composite coating is completed, the protective atmosphere within the inert gas chamber is continuously maintained until the overall temperature of the metal substrate drops below 200°C. This atmosphere isolation measure prevents oxygen embrittlement and excessive nitriding reactions in the titanium alloy joint at high temperatures, thereby ensuring the service performance of the welded joint.

[0072] The difference between Comparative Example 3 and Example 1 lies only in the application of a single horizontal, stable magnetic field in the front-to-back direction. This results in limited planar convection in the front-to-back direction within the molten pool, while large permanent flow dead zones exist in the left-to-right direction. This leads to the formation of coarse, oriented columnar crystal structures at the left and right edges of the coating. Simultaneously, the lack of active Lorentz force in the vertical direction results in insufficient flow intensity at the bottom of the molten pool, preventing the effective breaking of the bottom stagnant layer and easily leaving behind porosity and microcrack defects. Nitrogen within the molten pool can only achieve limited mixing in the front-to-back direction, with extremely poor diffusion in the left-to-right direction. This leads to severe enrichment of the TiN reinforcing phase in the left and right edge regions, forming coarse needle-like and lath-like TiN phases. Due to the inhomogeneous structure and internal defects, the average microhardness of the coating is approximately 441 HV, and its wear volume is 0.6083 mm². 3 Meanwhile, the large, brittle nitride aggregates are prone to microcracks that propagate rapidly during friction and wear, significantly reducing the coating's crack resistance and service stability. Comparative Example 4

[0073] The only difference between this comparative example and Example 1 is that only two orthogonal steady magnetic fields in the horizontal left-right direction and the vertical upward direction are applied during the laser coaxial wire feeding nitriding process, and no steady magnetic field in the horizontal front-back direction is applied. The process includes the following steps: Step 1: Select a TC4 titanium alloy plate with dimensions of 100mm×80mm×10mm as the base material. Use 80#-800# sandpaper to grind and polish the oxide layer on the surface of the area to be processed. Then, use anhydrous ethanol for ultrasonic cleaning for 12 minutes to thoroughly remove surface oil, oxide scale and impurities.

[0074] Step two: Based on the geometric dimensions of the coating, the initial working coordinate system and laser scanning path of the robotic arm are set using a teach pendant, so that the laser coaxial wire feeding device remains perpendicular to the surface of the metal substrate; at the same time, the position of the magnetic field generating device is adjusted, with a ring-shaped permanent magnet arranged directly below the non-magnetic fixture worktable, and strip-shaped permanent magnets symmetrically arranged on the left and right sides of the metal substrate, so that the central axes of the stable magnetic field in the horizontal left-right direction and the vertical upward direction intersect at the geometric center of the liquid molten pool, ensuring that the magnetic field action area covers the liquid molten pool.

[0075] Step 3: Given the extremely high chemical reactivity of titanium alloys, in order to prevent oxidation, nitriding, and hydrogen embrittlement reactions in the welding area at high temperatures, this comparative example employs inert gas protection. The metal substrate is placed in an inert gas protection chamber, and high-purity argon gas (99.99% purity) is introduced through the protective gas inlet. Utilizing the physical property that argon gas has a higher density than air, a bottom-inlet and top-outlet method is adopted, with residual air discharged through a protective gas outlet reserved at the top of the chamber. An oxygen concentration monitoring and analysis instrument is installed in the inert gas protection chamber to collect atmospheric data in real time; when the oxygen concentration in the chamber drops below 200 ppm, a stable inert protective atmosphere is formed, and cladding can proceed. During the cladding process, the protective gas flow rate in the inert gas protection chamber is maintained at 40 L / min, and pure nitrogen gas is supplied to the molten pool and weld surface through the auxiliary gas delivery channel built into the laser coaxial wire feeder, with the nitrogen flow rate controlled at 15 L / min, to achieve synergistic protection of the cladding layer and effective in-situ nitriding.

[0076] Step four: Activate the laser coaxial wire feeding device to move the laser beam along a preset path and simultaneously feed in a titanium alloy welding wire, causing the welding wire to rapidly melt with the substrate surface to form a liquid molten pool. Simultaneously, two orthogonal stable magnetic fields—one horizontally (left-right) and one vertically (upward)—are applied to the molten pool area using an annular permanent magnet below the worktable and strip permanent magnets on both sides. The magnetic induction intensity of each magnetic field at the center of the molten pool is 0.3T. The flow of the liquid molten pool cuts the magnetic field lines, generating Lorentz forces in two orthogonal directions, inducing two-dimensional convection within the vertical-left-right plane inside the molten pool. The laser power is 2500W, the scanning speed is 0.5m / min, and the wire feeding speed is 1.2m / min.

[0077] Step five: Based on the preset surface strengthening area, a single-layer, multi-pass overlapping strategy is used to iteratively execute step four, with the overlap rate of adjacent cladding passes controlled at 40%. After the overall composite coating is completed, the protective atmosphere within the inert gas chamber is continuously maintained until the overall temperature of the metal substrate drops below 200°C. This atmosphere isolation measure prevents oxygen embrittlement and excessive nitriding reactions in the titanium alloy joint at high temperatures, thereby ensuring the service performance of the welded joint.

[0078] The difference between Comparative Example 4 and Example 1 lies only in the application of two orthogonal steady magnetic fields in the horizontal left-right direction and the vertical upward direction. This results in two-dimensional convection only within the vertical-left-right plane inside the molten pool, with permanent flow dead zones in the front-back direction. Consequently, coarse oriented columnar crystal structures still form at the front and back edges of the coating. Although the active Lorentz force in the vertical direction significantly enhances the bottom flow and greatly reduces bottom porosity defects, the flow intensity in the front-back direction is insufficient, making it easy for microcracks and unfused defects to remain at the front and back edges. Nitrogen elements inside the molten pool can only mix within the vertical-left-right plane, with limited diffusion capacity in the front-back direction. This leads to significant enrichment of the TiN reinforcing phase in the front and back edge regions, and a small amount of coarse needle-like TiN phase still exists in some local areas. Although the uniformity of the microstructure and the level of defect control are improved compared to the unidirectional magnetic field, they are still significantly insufficient. The average microhardness of the coating is approximately 450 HV, and its wear volume is 0.5936 mm. 3 Meanwhile, the coarse columnar crystals and brittle nitride accumulation areas at the front and rear edges will still become the source of crack initiation and propagation, and the crack resistance and service stability of the coating still need to be further improved. Comparative Example 5

[0079] The difference between this comparative example and Example 1 is that no constant magnetic field was applied to assist in the in-situ nitriding process using laser coaxial wire feeding; only the conventional in-situ nitriding process using laser coaxial wire feeding was employed, specifically including the following steps: Step 1: Select a TC4 titanium alloy plate with dimensions of 100mm×80mm×10mm as the substrate material. Use 80#-800# sandpaper to grind the oxide layer on the surface of the area to be processed and polish it to a roughness Ra≤3.2μm. Then, use anhydrous ethanol for ultrasonic cleaning for 12 minutes to thoroughly remove surface oil and impurities.

[0080] Step 2: Set the initial working coordinate system and laser scanning path of the robotic arm using the teach pendant, so that the laser coaxial wire feeding device remains perpendicular to the surface of the titanium alloy substrate; no multi-directional or unidirectional steady magnetic field generating device is set in this comparative example.

[0081] Step 3: Given the extremely high chemical reactivity of titanium alloys, to prevent abnormal oxidation of the processing area at high temperatures, the titanium alloy test plate is placed in a protective atmosphere chamber, and high-purity argon gas (99.9% purity) is introduced through the protective gas inlet. Utilizing the physical property that argon gas is denser than air, a bottom-inlet and top-outlet method is adopted, with residual air discharged through the protective gas outlet at the top of the chamber. A gas concentration monitoring and analysis instrument is installed in the protective atmosphere chamber to collect atmospheric data in real time; when the oxygen concentration in the chamber drops below 200 ppm, a stable extremely low-oxygen protective atmosphere is considered to have been formed, and laser processing can proceed. Throughout the processing, the gas flow rate in the protective chamber is maintained, and a mixed reaction gas containing nitrogen is supplied to the surface of the molten pool through the coaxial gas delivery channel built into the laser coaxial wire feeding device. The nitrogen gas component accounts for 10%, and the total flow rate of the mixed gas is controlled at 25 L / min to achieve synergistic protection and effective nitriding of the cladding layer.

[0082] Step four: Activate the coaxial laser wire feeding device, moving the ring laser along a preset path while simultaneously feeding in TC4 titanium alloy welding wire. This allows the welding wire to rapidly melt with the substrate surface, forming a liquid molten pool, achieving in-situ nitriding and forming a composite reinforcement layer. To ensure the rigor of controlled variables, the laser processing parameters are kept completely consistent with those in Example 1: laser power is set to 2500W, scanning speed is 0.5m / min, and wire feeding speed is 1.2m / min. Throughout this processing, no external magnetic field is applied to the liquid molten pool.

[0083] Step five: Based on the preset surface strengthening area, Step four is executed cyclically using a single-layer, multi-pass overlapping strategy. Adjacent cladding passes are arranged at equal intervals, with an overlap rate controlled at 40%. After the overall processing of the composite strengthening layer is completed, the circulation of mixed protective gas in the atmosphere protection chamber is continuously maintained until the overall temperature of the titanium alloy substrate drops below 200°C. Through atmosphere isolation measures, oxygen absorption embrittlement and abnormal oxidation discoloration of the titanium alloy joints during the high-temperature cooling stage are prevented, thereby ensuring the service performance of the strengthening layer.

[0084] The only difference between Comparative Example 5 and Example 1 is that no steady magnetic field was applied to physically intervene in the molten pool. Without an external electromagnetic Lorentz force, the flow within the molten pool relies solely on natural thermocapillary convection (Marangoni convection) and surface tension. This condition severely restricts the mass transfer and diffusion of nitrogen into the deeper parts of the pool, easily leading to severe macroscopic compositional segregation with a nitrogen-rich surface and a nitrogen-poor depth. Simultaneously, the high-melting-point TiN hard phase, upon precipitation, not only forms coarse columnar crystals but also interweaves to form a continuous, coarse, brittle dendritic network in localized areas. Due to the large size and extremely uneven distribution of the internal reinforcing phase, the average microhardness of the coating is low (approximately 322 HV), and its wear volume reaches as high as 1.098 mm. 3Because of the presence of numerous continuous brittle nitride accumulation zones within the structure, microcracks are easily generated and large-area brittle spalling occurs under periodic frictional shear stress, resulting in the overall service stability of the composite reinforcement layer being significantly inferior to that of the multi-directional steady magnetic field assisted embodiment of the present invention. Example 2

[0085] The only difference between this embodiment and Embodiment 1 is that the laser power is set to 2000W in step four. The wire feeding material used in this embodiment is TC4 titanium alloy welding wire, which matches the base material, and its chemical composition is consistent with the aforementioned base material. Preferably, the welding wire diameter is 0.8mm. This embodiment provides a multi-directional stable magnetic field-assisted coaxial wire feeding laser in-situ nitriding method, comprising the following steps: Step 1: Select TC4 titanium alloy sheet as the substrate material, and use sandpaper to sand and polish the oxide layer on the surface of the area to be processed. Then, use anhydrous ethanol for ultrasonic cleaning for 10 minutes to thoroughly remove surface oil and impurities.

[0086] Step 2: Based on the geometry of the coating, the initial working coordinate system and laser scanning path of the robotic arm are set using a teach pendant, so that the laser coaxial wire feeding device remains perpendicular to the surface of the titanium alloy substrate; at the same time, the spatial distance (approximately 15mm) of the high-strength permanent magnet array around and at the bottom is precisely adjusted so that the multi-directional stable magnetic field area completely penetrates the entire liquid molten pool.

[0087] Step 3: Place the assembled titanium alloy test plate in an atmosphere protection chamber and fill it with high-purity argon gas through the protective gas inlet until the oxygen concentration in the chamber drops below 200 ppm to form a stable ultra-low oxygen protective atmosphere. During the cladding process, pure nitrogen gas is delivered to the surface of the molten pool through the coaxial gas delivery channel built into the laser coaxial wire feeding device. The total gas flow rate is controlled at 20 L / min to achieve synergistic protection and effective nitriding of the cladding layer.

[0088] Step four: Activate the laser coaxial wire feeding device to move the ring laser along a preset path and simultaneously feed in the TC4 titanium alloy welding wire, causing the welding wire to rapidly melt with the substrate surface to form a liquid molten pool. Simultaneously, a multi-directional stable magnetic field is applied throughout the entire processing using a pre-arranged array of high-strength permanent magnets.

[0089] The laser power is 2000W, the scanning speed is 0.2m / min, and the wire feeding speed is 0.5m / min. By adjusting the spatial distance, the magnetic induction intensity of the three-directional constant magnetic field at the center of the liquid molten pool is maintained at about 0.35T.

[0090] Step 5: Based on the preset surface strengthening area, Step 4 is executed cyclically using a single-layer multi-pass overlapping strategy, with adjacent cladding passes arranged at equal intervals and an overlap rate controlled at 30%. After the overall composite coating is completed, the mixed protective gas in the atmosphere protection chamber is continuously maintained until the overall temperature of the titanium alloy substrate drops below 200°C.

[0091] The coating obtained in this embodiment has a well-formed surface with no obvious pores or cracks. Due to the reduction of laser power to 2000W, the overall heat input of the molten pool is relatively reduced, resulting in a slight decrease in the maximum temperature and thermodynamic driving force of the in-situ nitriding reaction. This leads to a slight reduction in the volume fraction of the TiN reinforcing phase generated within the coating compared to Example 1. The three-dimensional global flow field induced by the multi-directional steady magnetic field still breaks up the TiN particles and maintains an extremely excellent dispersed distribution. The composite reinforced layer obtained in this embodiment has an average microhardness of approximately 625 HV and a wear volume of 0.3842 mm. 3 Although the improvement in macroscopic wear resistance is slightly less than that of the optimal parameters in Example 1, its wear resistance and microhardness still show a significant leap forward compared to the untreated TC4 matrix, fully meeting the requirements of conventional industrial-grade wear protection. Example 3

[0092] The only difference between this embodiment and Embodiment 1 is that, in step four, the laser power is set to 4000W. The wire feeding material used in this embodiment is TC4 titanium alloy welding wire, which matches the base material, and its chemical composition is consistent with the aforementioned base material. Preferably, the welding wire diameter in this embodiment is 1.6mm.

[0093] This embodiment provides a multi-directional steady magnetic field-assisted coaxial wire feeding laser in-situ nitriding method, which includes the following steps: Step 1: Select TC4 titanium alloy sheet as the substrate material, and successively use sandpaper to grind the oxide layer on the surface of the area to be processed and polish it. Then, use anhydrous ethanol for ultrasonic cleaning for 15 minutes to thoroughly remove surface oil and impurities.

[0094] Step 2: Based on the geometry of the coating, the initial working coordinate system and laser scanning path of the robotic arm are set using a teach pendant, so that the laser coaxial wire feeding device remains perpendicular to the surface of the titanium alloy substrate; at the same time, the spatial distance (approximately 25mm) of the high-strength permanent magnet array around and at the bottom is precisely adjusted so that the multi-directional stable magnetic field area completely penetrates the entire liquid molten pool.

[0095] Step 3: Place the assembled titanium alloy test plate in an atmosphere protection chamber and fill it with high-purity argon gas through the protective gas inlet until the oxygen concentration in the chamber drops below 200 ppm to form a stable ultra-low oxygen protective atmosphere. During the cladding process, a mixed reaction gas containing nitrogen is delivered to the surface of the molten pool through the coaxial gas delivery channel built into the laser coaxial wire feeding device. The nitrogen gas fraction is 10%, and the total flow rate of the mixed gas is controlled at 15 L / min to achieve synergistic protection and effective nitriding of the cladding layer.

[0096] Step four: Activate the laser coaxial wire feeding device to move the ring laser along a preset path and simultaneously feed in the TC4 titanium alloy welding wire, causing the welding wire to rapidly melt with the substrate surface to form a liquid molten pool. Simultaneously, a multi-directional stable magnetic field is applied throughout the entire processing using a pre-arranged array of high-strength permanent magnets.

[0097] The laser power is 4000W, the scanning speed is 1.0m / min, and the wire feeding speed is 1.5m / min. By adjusting the spatial distance, the magnetic induction intensity of the three-directional constant magnetic field at the center of the liquid molten pool is maintained at about 0.3T.

[0098] Step 5: Based on the preset surface strengthening area, Step 4 is executed cyclically using a single-layer multi-pass overlapping strategy, with adjacent cladding passes arranged at equal intervals and the overlap rate controlled at 50%. After the overall processing of the composite coating is completed, the circulation of mixed protective gas in the atmosphere protection chamber is continuously maintained until the overall temperature of the titanium alloy substrate drops below 200°C.

[0099] The coating obtained in this embodiment has a well-formed surface and no obvious macroscopic cracks or porosity defects. The laser power was significantly increased to 4000W, resulting in a substantial increase in the overall heat input to the molten pool and a longer duration of the liquid molten pool. This, on the one hand, intensified the in-situ nitriding reaction, leading to an increase in the total amount of TiN reinforcing phase generated; on the other hand, the excessive heat input caused a decrease in the cooling rate of the molten pool and a reduction in undercooling, resulting in some thermal coarsening of the TiN particles and titanium alloy matrix grains compared to Example 1. The composite reinforced layer obtained in this embodiment has an average microhardness of approximately 645 HV and a wear volume of 0.3156 mm. 3 Although the slight grain coarsening resulted in slightly inferior overall wear resistance compared to the optimal parameters of Example 1, it still showed a significant improvement over the untreated TC4 matrix. This fully verifies that the multi-directional steady magnetic field coupling mechanism of the present invention still possesses excellent microstructure homogenization and crack resistance control capabilities under the harsh process boundaries of high heat input and high residual stress tendency. Example 4

[0100] The only difference between this embodiment and Embodiment 1 is that, in step three, the nitrogen content in the nitrogen-containing mixed reaction gas is set to 5%. The wire feeding material used in this embodiment is a TC4 titanium alloy welding wire that matches the base material, and its chemical composition is consistent with the aforementioned base material. Preferably, the welding wire diameter in this embodiment is 1.0 mm.

[0101] This embodiment provides a multi-directional steady magnetic field-assisted coaxial wire feeding laser in-situ nitriding method, which includes the following steps: Step 1: Select TC4 titanium alloy sheet as the substrate material, and successively use sandpaper to grind the oxide layer on the surface of the area to be processed and polish it. Then, use anhydrous ethanol for ultrasonic cleaning for 11 minutes to thoroughly remove surface oil and impurities.

[0102] Step 2: Based on the geometry of the coating, the initial working coordinate system and laser scanning path of the robotic arm are set using a teach pendant, so that the laser coaxial wire feeding device remains perpendicular to the surface of the titanium alloy substrate; at the same time, the spatial distance (approximately 25mm) of the high-strength permanent magnet array around and at the bottom is precisely adjusted so that the multi-directional stable magnetic field area completely penetrates the entire liquid molten pool.

[0103] Step 3: Place the assembled titanium alloy test plate in an atmosphere protection chamber and fill it with high-purity argon gas through the protective gas inlet until the oxygen concentration in the chamber drops below 200 ppm to form a stable ultra-low oxygen protective atmosphere. During the cladding process, a mixed reaction gas containing nitrogen is delivered to the surface of the molten pool through the coaxial gas delivery channel built into the laser coaxial wire feeding device. The nitrogen gas fraction is 5%, and the total flow rate of the mixed gas is controlled at 25 L / min to achieve synergistic protection and effective nitriding of the cladding layer.

[0104] Step four: Activate the laser coaxial wire feeding device to move the ring laser along a preset path and simultaneously feed in the TC4 titanium alloy welding wire, causing the welding wire to rapidly melt with the substrate surface to form a liquid molten pool. Simultaneously, a multi-directional stable magnetic field is applied throughout the entire processing using a pre-arranged array of high-strength permanent magnets.

[0105] The laser power is 2500W, the scanning speed is 0.5m / min, and the wire feeding speed is 1.2m / min. By adjusting the spatial distance, the magnetic induction intensity of the three-directional constant magnetic field at the center of the liquid molten pool is maintained at about 0.3T.

[0106] Step 5: Based on the preset surface strengthening area, Step 4 is executed cyclically using a single-layer multi-pass overlapping strategy, with adjacent cladding passes arranged at equal intervals and an overlap rate controlled at 40%. After the overall composite coating is completed, the mixed protective gas in the atmosphere protection chamber is continuously maintained until the overall temperature of the titanium alloy substrate drops below 200°C.

[0107] The coating obtained in this embodiment has a well-formed surface and no obvious pores or cracks. Because the nitrogen concentration in the mixed gas is reduced to 5%, the total amount of active nitrogen atoms consumed by the in-situ metallurgical reaction decreases, resulting in a significant decrease in the volume fraction of the TiN reinforcing phase generated inside the coating compared to Example 1. The composite reinforced layer obtained in this embodiment has an average microhardness of approximately 580 HV and a wear volume of 0.4825 mm. 3 Although the reduced total amount of hard phase resulted in lower macroscopic wear resistance than the optimal parameters of Example 1, its overall mechanical properties were still significantly improved compared to the untreated TC4 matrix, and it effectively avoided the local nitrogen-depleted zones that are prone to occur under low nitrogen concentrations. This fully verifies that the multi-directional steady magnetic field coupling mechanism of the present invention can still ensure the perfect homogeneous distribution of the reinforcing phase and the dense formation of the composite reinforcing layer under low-concentration reactive atmosphere boundaries. Example 5

[0108] The only difference between this embodiment and Embodiment 1 is that, in step three, the nitrogen content in the nitrogen-containing mixed reaction gas is set to 30%. The wire feeding material used in this embodiment is a TC4 titanium alloy welding wire that matches the base material, and its chemical composition is consistent with the aforementioned base material. Preferably, the welding wire diameter in this embodiment is 1.5 mm.

[0109] This embodiment provides a multi-directional steady magnetic field-assisted coaxial wire feeding laser in-situ nitriding method, which includes the following steps: Step 1: Select TC4 titanium alloy sheet as the substrate material, and use sandpaper to sand and polish the oxide layer on the surface of the area to be processed. Then, use anhydrous ethanol for ultrasonic cleaning for 13 minutes to thoroughly remove surface oil and impurities.

[0110] Step 2: Based on the geometry of the coating, the initial working coordinate system and laser scanning path of the robotic arm are set using a teach pendant, so that the laser coaxial wire feeding device remains perpendicular to the surface of the titanium alloy substrate; at the same time, the spatial distance of the high-strength permanent magnet array around and at the bottom is precisely adjusted so that the multi-directional stable magnetic field area completely penetrates the entire liquid molten pool.

[0111] Step 3: Place the assembled titanium alloy test plate in an atmosphere protection chamber and fill it with high-purity argon gas through the protective gas inlet until the oxygen concentration in the chamber drops below 200 ppm to form a stable ultra-low oxygen protective atmosphere. During the cladding process, a mixed reaction gas containing nitrogen is delivered to the surface of the molten pool through the coaxial gas delivery channel built into the laser coaxial wire feeding device. The nitrogen gas fraction is 30%, and the total flow rate of the mixed gas is controlled at 25 L / min to achieve synergistic protection and effective nitriding of the cladding layer.

[0112] Step four: Activate the laser coaxial wire feeding device to move the ring laser along a preset path and simultaneously feed in the TC4 titanium alloy welding wire, causing the welding wire to rapidly melt with the substrate surface to form a liquid molten pool. Simultaneously, a multi-directional stable magnetic field is applied throughout the entire processing using a pre-arranged array of high-strength permanent magnets.

[0113] The laser power is 2500W, the scanning speed is 0.5m / min, and the wire feeding speed is 1.2m / min. By adjusting the spatial distance, the magnetic induction intensity of the three-directional constant magnetic field at the center of the liquid molten pool is maintained at about 0.3T.

[0114] Step 5: Based on the preset surface strengthening area, Step 4 is executed cyclically using a single-layer multi-pass overlapping strategy, with adjacent cladding passes arranged at equal intervals and an overlap rate controlled at 40%. After the overall composite coating is completed, the mixed protective gas in the atmosphere protection chamber is continuously maintained until the overall temperature of the titanium alloy substrate drops below 200°C.

[0115] The coating obtained in this embodiment exhibits excellent surface formation, thanks to the powerful physical intervention of the multi-directional steady magnetic field throughout the entire process, with no obvious macroscopic cracks or porosity defects appearing internally. Due to the significant increase in nitrogen concentration in the mixed reaction gas to 30%, the nitrogen supersaturation inside the molten pool increased dramatically, resulting in a significant increase in the volume fraction of the in-situ generated TiN reinforcing phase. The three-dimensional global flow field induced by the multi-directional steady magnetic field not only broke up a large number of densely packed primary TiN dendrites but also disrupted the spontaneous aggregation tendency of the hard phase. The average microhardness of the composite reinforced layer obtained in this embodiment increased significantly to approximately 725 HV, and its wear volume decreased to 0.2315 mm. 3 Data shows that even under extremely high nitrogen concentration process boundaries, the multi-energy field coupling mechanism of this invention can still perfectly suppress cracking and segregation defects, achieving a balance between ultra-high hardness and excellent density of the composite reinforcement layer.

[0116] In the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof. However, this disclosure should not be construed as reflecting an intention that the claimed invention requires more features than expressly recited in each claim. Rather, as reflected in the claims, the inventive aspect lies in fewer than all the features of the foregoing disclosed embodiments. Therefore, the claims, following the detailed description, are hereby expressly incorporated into that detailed description, wherein each claim itself is a separate embodiment of the invention.

[0117] Although the invention has been described with reference to a limited number of embodiments, those skilled in the art will understand from the foregoing description that other embodiments are conceivable within the scope of the invention described herein. Furthermore, it should be noted that the language used in this specification has been chosen primarily for readability and instructional purposes, and not for the purpose of interpreting or limiting the subject matter of the invention. Therefore, many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims. The disclosure of the invention is illustrative and not restrictive, and the scope of the invention is defined by the appended claims.

[0118] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A multi-directional steady magnetic field-assisted coaxial wire feeding laser in-situ nitriding method, characterized in that, Includes the following steps: Step 1: Clean the surface of the metal substrate and fix it on the processing platform, ensuring the processing surface of the metal substrate remains horizontal. Step 2: Adjust the spatial position of the laser coaxial wire feeding device and the multi-directional stabilizing permanent magnet device according to the preset processing area of ​​the metal substrate, and adjust the spatial distance between the multi-directional stabilizing permanent magnet device and the processing area. Step 3: Place the metal substrate in an atmosphere protection chamber, reducing the oxygen concentration in the chamber to below 200 ppm; and simultaneously introduce a reaction gas containing nitrogen. Step 4: Start the laser coaxial wire feeding device to perform cladding along the preset scanning path. In the entire processing, a multi-directional stable permanent magnet device is used to simultaneously apply a stable magnetic field in three spatially orthogonal directions: horizontal left and right, horizontal front and back, and vertical upward. The processing parameters are: laser power 2000-4000W, scanning speed 0.2-1.0m / min, and wire feeding speed 0.5-1.5m / min. In step five, a single-layer multi-pass overlapping method is used to complete the preparation of the composite reinforcement layer, with an overlap rate of 30%-50% between adjacent passes. After processing, the flow of reaction gas containing nitrogen is maintained until the overall temperature of the workpiece drops below 200℃.

2. The method according to claim 1, characterized in that, The metal matrix includes a titanium alloy matrix, and the chemical composition of the welding wire is consistent with that of the metal matrix. The diameter of the welding wire is 0.8mm-1.6mm.

3. The method according to claim 1, characterized in that, In step three, the reaction gas containing nitrogen includes pure nitrogen or a mixture of nitrogen and argon, with the volume fraction of nitrogen in the mixture controlled at 5%-30%; the total flow rate of pure nitrogen or the mixture is controlled at 15-25 L / min.

4. The method according to claim 1, characterized in that, In step one, the cleaning process involves using grinding and polishing equipment to treat the surface in sequence, so that the surface roughness Ra of the area to be processed is ≤3.2μm, and then using anhydrous ethanol or acetone for ultrasonic cleaning to thoroughly remove surface oil, oxide scale and dust.

5. The method according to claim 1, characterized in that, In step two, the multi-directional stable permanent magnet device consists of five high-strength permanent magnet generating poles: the first generating pole is arranged directly below the processing platform to generate vertically upward stable magnetic field lines; the second and third generating poles are arranged on the left and right sides of the processing area with opposite magnetic poles facing each other to generate horizontal stable magnetic field lines; the fourth and fifth generating poles are arranged on the front and rear sides of the processing area with opposite magnetic poles facing each other to generate horizontally vertical stable magnetic field lines.

6. The method according to claim 5, characterized in that, The magnetic intensity of high-strength permanent magnets ranges from 0.15T to 0.35T.

7. The method according to claim 5, characterized in that, In step two, the spatial distance between each high-strength permanent magnet and the processing area is 15-25mm.

8. The key load-bearing component obtained by the method according to any one of claims 1-7, characterized in that, The surface of the key load-bearing components is a nitrided composite reinforcement layer with an average microhardness of 580-725 HV and a wear volume of 0.2315-0.4825 mm3.

9. The key load-bearing component according to claim 8, characterized in that, This key load-bearing component can be used in aero-engine compressor blades, aircraft landing gear actuators, pressure hull connectors for deep-sea submersibles, and wear-resistant liners for mining machinery.

10. The application of the key load-bearing component according to claim 8 in aerospace or marine engineering.

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