High-pressure multi-atmosphere assisted SLM gradient material preparation method and aluminum-lithium alloy coating

Abstract

The present invention provides a high-pressure multi-atmosphere assisted SLM gradient material preparation method, and an aluminum-lithium alloy coating. The method comprises the following steps: injecting a first mixed gas into a working chamber; by means of laser irradiation, causing a molten material in a micro-molten pool to react with a carbon-containing active gas, to form a first gradient aluminum-lithium alloy coating doped with carbides; injecting a second mixed gas into the working chamber; by means of laser irradiation, causing the molten material in the micro-molten pool to react with a carbon- and nitrogen-containing active gas, to form a second gradient aluminum-lithium alloy coating doped with carbonitrides; injecting a third mixed gas into the working chamber; by means of laser irradiation, causing the molten material in the micro-molten pool to react with a nitrogen-containing active gas, to form a third gradient aluminum-lithium alloy coating doped with nitrides. The present invention improves the surface forming quality of aluminum-lithium alloy materials, while achieving the overall material performance of being hard on the outside and tough on the inside.

Description

A high-pressure multi-atmosphere assisted SLM gradient material preparation method and aluminum-lithium alloy coating Technical Field

[0001] The present invention relates to the technical field of selective laser melting, and in particular to a high-pressure multi-atmosphere assisted SLM gradient material preparation method and an aluminum-lithium alloy coating. Background Art

[0002] In the field of aerospace, the third generation of new aluminum-lithium alloys has been widely used in aircraft skins, long stringers and floor beams due to their low density, high elastic modulus, good corrosion resistance and low anisotropy. However, with the improvement of the application technology of aluminum-lithium alloys, their application scenarios have gradually developed towards large and complex main load-bearing structures. The manufacture of load-bearing structures mostly adopts forming methods such as casting, forging, rolling and extrusion, and then splices them by riveting and welding. This processing method is currently gradually unable to adapt to the development needs of the complex structure and functional integration of aerospace engineering parts. In addition, in small-batch production, the preparation cost and production cycle also increase significantly. Therefore, there is an urgent need to develop a low-cost and short-cycle integrated preparation technology for aluminum-lithium alloy components.

[0003] Additive manufacturing technology has become a research hotspot in the overall preparation technology of metal materials due to its outstanding advantages in the manufacture of complex structural parts (integrated design and manufacturing, high processing precision, short manufacturing cycle, and excellent physical and chemical properties of products). Currently, the main additive manufacturing technologies for metal materials include laser, electron beam, and arc additive manufacturing. Among them, electron beam and arc additive manufacturing technologies are very prone to causing serious heat accumulation in parts. Excessive heat input leads to coarse metal structure and an increase in brittle phases, which in turn directly affects the metallurgical bonding strength, stacking dimensional accuracy, and surface quality of the parts. Therefore, laser additive manufacturing technology, with its characteristics of combining precise forming and high-performance forming, has become the most reliable and feasible method for metal additive manufacturing.

[0004] Selective Laser Melting (SLM) prepares parts by layering powder and selectively heating different areas with a laser beam, which can achieve low-cost, short-process manufacturing of complex components. Parts formed using SLM technology usually have low surface roughness, high dimensional accuracy, and can meet the requirements of smoothness without subsequent treatment or with only simple surface processing. However, compared with aluminum alloys with similar compositions, the addition of Li element further increases the thermal expansion coefficient and molten pool solidification temperature range of aluminum-lithium alloys, and significantly increases the intergranular low-melting eutectics, resulting in aluminum-lithium alloys with significantly higher thermal crack sensitivity than conventional aluminum alloys. Under the action of high-energy laser beams, their thermal cracking tendency is further increased, resulting in a significant decrease in the forming quality and mechanical properties of aluminum-lithium alloy parts prepared by SLM.

[0005] To reduce the tendency to thermal cracking, some researchers have attempted to use low-speed, high-energy-density, preheating, short-segment scanning, increased support, and other process methods, or trace element alloying methods. Studies have found that relying solely on process adjustments is difficult to completely avoid crack defects. While alloying methods are more effective, promoting the transformation of coating grain types to equiaxed grains, narrowing the melt pool solidification temperature range, and effectively refining the grain structure, the distribution and solubility of added trace elements in the coating are difficult to control. Trace element particles are prone to clustering and segregation at grain boundaries, which in turn affects the internal stress distribution and mechanical properties of the material. Current processes are unable to simultaneously meet the forming and performance requirements of SLM-formed specimens, hindering the development and application of aluminum-lithium alloy additive manufacturing components in actual production.

[0006] Summary of the Invention

[0007] In response to the shortcomings of the prior art, the present invention provides a high-pressure multi-atmosphere assisted SLM gradient material preparation method and aluminum-lithium alloy coating. Based on the existing SLM forming aluminum-lithium alloy coating process, a pulsed laser is selected as the laser heat source to melt the powder and the substrate. In the preparation process of the first gradient aluminum-lithium alloy coating, the CO2 in the active gas rapidly dissociates and forms a plasma under the action of laser irradiation. As the laser energy is continuously absorbed, the plasma continuously decomposes active carbon atoms. Under the high pressure and the expansion shock wave of the plasma, the carbon atoms fully react with the molten material in the molten pool to form carbides, thereby achieving carbide doping of the aluminum-lithium alloy coating and increasing the carbon content in the coating. This can effectively improve the hardness and wear resistance of the surface of the first gradient aluminum-lithium alloy coating. In the preparation process of the third gradient aluminum-lithium alloy coating using N2-doped active gas, the laser irradiates the aluminum-lithium alloy powder to form a high-temperature liquid micro-melting pool. The N2 decomposes near the high-temperature molten pool and then penetrates into the molten pool through the gas-liquid interface and undergoes migration and dissolution, achieving nitride doping in the coating and significantly improving the hardness and mechanical properties of the third gradient aluminum-lithium alloy coating. A second gradient aluminum-lithium alloy coating is prepared using a carbon- and nitrogen-containing active atmosphere. The second gradient aluminum-lithium alloy coating serves as a transition layer. While achieving the doping of two reinforcing particles in the transition coating, it ensures a gradient and continuous transition of the alloy structure, effectively slowing down the sudden change in the coating microstructure and reducing the accumulation of internal residual stress and thermal stress. The interlayer bonding force of the aluminum-lithium alloy gradient material is effectively strengthened. During the multi-atmosphere assisted SLM forming process, the powder laid by the porous powder sprayer is uniform and loose, increasing the contact area between the aluminum-lithium alloy powder and the active gas. Some of the active gas stored in the gaps between the powders dissociates directly into active atoms within the molten pool under the action of the high-temperature laser, effectively promoting the in-situ synthesis reaction in the molten pool, increasing the carbon and nitrogen content in the gradient material, and further improving the mechanical properties of the material. In addition, the high-pressure environment can reduce the critical nucleation free energy of the aluminum-lithium alloy, increase the critical nucleation density, and thus increase the nucleation rate, refine the grain structure and inhibit the cracking tendency of the coating. It can also effectively reduce the asymmetric force on the surface molecules of the high-temperature melt, increase the reaction efficiency of the active gas molecules and the molten material, improve the flatness of the formed surface, and promote the in-situ synthesis and doping of carbides and nitrides in the coating. At the same time, as the pressure gradient decreases during the SLM forming process, the grain size inside the coating shows an increasing evolution trend, especially during the preparation of the first gradient aluminum-lithium alloy coating, where the pressure is the highest, the coating grain size is significantly reduced, and the proportion of high-angle grain boundaries increases accordingly. The expansion of microcracks inside the coating requires more energy, and the strength and toughness of the first gradient aluminum-lithium alloy coating are effectively improved. Combined with the transition layer with extremely high hardness due to the doping of nitride hard phase particles and the third gradient coating, the prepared aluminum-lithium alloy gradient material exhibits the characteristics of being hard on the outside and tough on the inside.Therefore, the present invention prepares SLM aluminum-lithium alloy gradient materials through a gradient high-pressure environment and doping with multiple active atmospheres containing carbon and nitrogen, thereby improving the surface forming quality of the aluminum-lithium alloy material while achieving the overall performance requirements of the material being hard on the outside and tough on the inside.

[0008] The present invention achieves the above technical objectives through the following technical means.

[0009] A high-pressure multi-atmosphere assisted SLM gradient material preparation method comprises the following steps:

[0010] A first mixed gas is continuously injected into a sealed working box to maintain a high-pressure environment in the working box; the first mixed gas is a mixture of an inert gas and a carbon-containing active gas; an aluminum-lithium alloy powder laid on the surface of a substrate is irradiated with a laser to form a micro-melting pool, and the molten material in the micro-melting pool reacts with the carbon-containing active gas to form a carbide-doped first gradient aluminum-lithium alloy coating;

[0011] Expelling the first mixed gas from the sealed working box; continuously injecting a second mixed gas into the sealed working box to maintain a high-pressure environment in the working box; the second mixed gas is a mixture of an inert gas and a carbon- and nitrogen-containing active gas; laser irradiating the aluminum-lithium alloy powder laid on the surface of the first gradient aluminum-lithium alloy coating to form a micro-melting pool, so that the molten material in the micro-melting pool reacts with the carbon- and nitrogen-containing active gas to form a carbonitride-doped second gradient aluminum-lithium alloy coating;

[0012] The second mixed gas in the sealed working box is removed; a third mixed gas is continuously injected into the sealed working box to maintain a high-pressure environment in the working box; the third mixed gas is a mixture of an inert gas and a nitrogen-containing active gas; the aluminum-lithium alloy powder laid on the surface of the second gradient aluminum-lithium alloy coating is irradiated by laser to form a micro-melting pool, and the molten material in the micro-melting pool reacts with the nitrogen-containing active gas to form a nitride-doped third gradient aluminum-lithium alloy coating.

[0013] Furthermore, the carbon-containing active gas in the first mixed gas is CO2 gas, and the volume of the CO2 gas in the first mixed gas accounts for 10% to 20%; the carbon-nitrogen active gases in the second mixed gas are CO2 gas and N2 gas, and the volume of the CO2 gas in the second mixed gas accounts for 10% to 20%; the volume of the N2 gas in the second mixed gas accounts for 20% to 30%; the nitrogen-containing active gas in the third mixed gas is N2 gas, and the volume of the N2 gas in the third mixed gas accounts for 20% to 30%.

[0014] Furthermore, the pressure of the high-pressure environment generated by the first mixed gas is P1, the pressure of the high-pressure environment generated by the second mixed gas is P2, and the pressure of the high-pressure environment generated by the third mixed gas is P3, and P1>P2>P3.

[0015] Furthermore, the pressure P1 of the high-pressure environment generated by the first mixed gas is 3.5-5.0 MPa; the pressure P2 of the high-pressure environment generated by the second mixed gas is 2.0-2.5 MPa; and the pressure P3 of the high-pressure environment generated by the third mixed gas is 1.0-1.5 MPa.

[0016] Furthermore, the flow rate of the active gas is maintained at 20 L / min to 50 L / min during the SLM forming process, so as to keep the concentration of the active gas within a set range during the reaction of the molten material.

[0017] Furthermore, a porous powder sprayer is used to lay evenly and loosely the aluminum-lithium alloy powder so that part of the active gas is stored inside the powder.

[0018] Furthermore, the thickness of the first gradient aluminum-lithium alloy coating and the third gradient aluminum-lithium alloy coating are both 0.8-1.0 mm, and the thickness of the second gradient aluminum-lithium alloy coating is 0.6-0.8 mm.

[0019] Furthermore, the surface of the substrate is treated to increase the surface roughness so as to improve the bonding strength between the formed aluminum-lithium alloy coating and the substrate; and the treated surface is ultrasonically cleaned and dried using anhydrous ethanol.

[0020] Furthermore, the laser energy is a pulsed laser, and the parameters of the pulsed laser beam are: wavelength 1075nm, pulse energy 1J~30J, pulse period 80ms, pulse duration 20~60ms, duty cycle 50%, spot diameter ≤50μm; laser power is 80~100W, laser beam scanning speed is 100mm / s, and scanning spacing is 100~120μm.

[0021] An aluminum-lithium alloy coating is formed on the surface of a substrate using the high-pressure multi-atmosphere assisted SLM gradient material preparation method.

[0022] The beneficial effects of the present invention are:

[0023] 1. The high-pressure, multi-atmosphere-assisted SLM gradient material preparation method described in the present invention uses a pulsed laser as the irradiation heat source. Compared with the traditional continuous laser mode, the pulsed laser can precisely adjust the distribution of laser energy in the time domain by regulating the pulse parameters, thereby directly affecting the microstructure of the formed part. During the SLM forming process, the pulsed laser allows the molten pool to undergo periodic cooling and solidification, thereby enhancing the stirring effect on the liquid molten pool. The stirring effect not only reduces the dendrite spacing and allows the molten pool elements to diffuse more evenly and fully, but also increases the recrystallization ratio of the molten pool and reduces the anisotropy of the grains. In addition, under the action of the pulsed laser, the matrix temperature is reduced and the cooling rate of the molten pool is increased, so that the heat accumulation of the aluminum-lithium alloy as a whole is less, and the residual stress of the material is significantly reduced.

[0024] 2. In the high-pressure multi-atmosphere assisted SLM gradient material preparation method described in the present invention, the first mixed gas includes Ar and 10% to 20% CO2 active gas by volume. Ar and 10% to 20% CO2 active gas by volume are used as the atmosphere doping in the first gradient aluminum-lithium alloy coating preparation process. Under the action of high-power density intense pulse laser irradiation, the CO2 in the active gas dissociates under high temperature and high pressure environment and forms plasma above the molten pool. In the plasma, CO2 dissociates into CO and O, and continues to absorb laser energy, causing CO to further decompose into C and O. Since the solubility of active carbon atoms in high-temperature liquid metal is significantly higher than that of oxygen atoms, under the high-pressure auxiliary environment and the expansion shock wave of the plasma itself, the active carbon atoms in the dissociated material are stirred and modulated with the molten material inside the molten pool to form carbide particles in situ and evenly dispersed in the coating. The carbon content near the surface of the aluminum-lithium alloy coating increases, effectively improving the hardness and wear resistance of the first gradient aluminum-lithium alloy coating; at the same time, CO2, as a high thermal conductivity gas, can effectively increase the overall thermal conductivity of the molten alloy powder and increase the cooling rate of the molten pool, thereby further strengthening the microstructure and mechanical properties of the aluminum-lithium alloy coating.

[0025] 3. The high-pressure, multi-atmosphere-assisted SLM gradient material fabrication method of the present invention comprises a third mixed gas comprising Ar and a reactive gas (20% to 30% by volume) of N2. This mixed gas serves as the doping atmosphere during the fabrication of the third gradient aluminum-lithium alloy coating. The N2 in the reactive gas dissociates into reactive nitrogen atoms under high-power laser irradiation. Similarly, under the synergistic effects of the high-pressure environment and the plasma expansion shock wave, nitrogen atoms above the high-temperature molten pool penetrate into the molten pool and stir and modulate with the molten material, achieving nitride doping in the third gradient coating. Because nitride particles have significantly higher hardness than carbides, the hardness and mechanical properties of the third gradient aluminum-lithium alloy coating are significantly improved compared to the first gradient aluminum-lithium alloy coating. Furthermore, the high-pressure environment within the workbox increases the nitrogen partial pressure during the SLM melting and solidification processes. This prevents the solute nitrogen atoms in the molten pool from reaching saturation, increasing the nitrogen content diffusing into the high-temperature molten pool during the forming process and effectively increasing the nitriding capacity of the coating. Furthermore, the increased surface pressure of the molten pool, along with the high-pressure environment, effectively suppresses nitrogen escape. Therefore, the third gradient aluminum-lithium alloy coating prepared under high pressure and nitrogen-containing active gas assistance has higher surface hardness and mechanical stability.

[0026] 4. In the high-pressure, multi-atmosphere-assisted SLM gradient material preparation method described in the present invention, the third mixed gas comprises Ar + (10% to 20%) CO2 + (20% to 30%) N2 active gas. Ar + (10% to 20%) CO2 + (20% to 30%) N2 active gas is used as the atmosphere doping in the transition coating preparation process. CO2 and N2 simultaneously dissociate into active carbon atoms and nitrogen atoms under laser irradiation. The active atoms react with the molten material in the laser micro-melting pool, achieving simultaneous doping of carbides and nitrides in the transition coating. It should be noted that since the ionization energy of carbon (11.26 eV) is lower than that of nitrogen (14.35 eV), the carbon atoms in the transition layer react more easily with the molten material in the molten pool than nitrogen. As a result, under the assistance of the same N2 concentration atmosphere, the number of nitride inclusions in the transition layer is less than that of the third gradient aluminum-lithium alloy coating. The overall hardness of the coating is lower than that of the third gradient aluminum-lithium alloy coating, while its brittleness is also effectively alleviated. Furthermore, under the action of gradient pressure, the microstructural refinement within the transition layer lies between that of the first and third gradient aluminum-lithium alloy coatings, effectively mitigating sudden changes in the material's microstructure. Consequently, the presence of the transition layer results in a continuous transition between the mechanical properties and microstructure of the aluminum-lithium alloy, reducing the accumulation of thermal and residual stresses within the material, significantly enhancing the bonding strength between the gradient coatings, and further extending the service life of the SLM gradient material.

[0027] 5. The high-pressure multi-atmosphere assisted SLM gradient material preparation method described in the present invention lays a uniform and loose aluminum-lithium alloy powder on a substrate through a porous powder sprayer. The loose powder layer structure allows part of the active gas to be stored inside the powder. Under the action of a high-temperature laser, active atoms are directly dissociated inside the molten pool, which increases the concentration of active atoms dissolved in the molten pool and effectively promotes the reaction efficiency between the molten material and the active gas. The doping content of carbides and nitrides in the gradient material is increased, and the mechanical properties of the aluminum-lithium alloy material are further improved.

[0028] 6. The high-pressure multi-atmosphere assisted SLM gradient material preparation method described in the present invention prepares SLM specimens through a high-pressure assisted process. The physical effect of pressure provides nucleation work for the molten metal, prompting more atomic groups to participate in crystallization nucleation, significantly improving the nucleation rate of the molten pool, and thus achieving the effect of grain refinement. In addition, the high-pressure environment affects the surface tension between the metal melt and the gas, reducing the asymmetric force on the molecules on the surface of the high-temperature melt, increasing the carbon and nitrogen partial pressures at the gas-liquid interface on the surface of the molten pool, and increasing the absorption concentration of active carbon atoms and nitrogen atoms in the molten pool. While improving the smoothness of the formed surface, it also effectively promotes the in-situ synthesis and doping of carbides and nitrides in the aluminum-lithium alloy coating. At the same time, the enhanced stirring effect of the pulsed laser makes these inclusion particles more evenly distributed in the coating, thereby further improving the mechanical properties of the aluminum-lithium alloy.

[0029] 7. The high-pressure, multi-atmosphere assisted SLM gradient material preparation method described in the present invention uses a gradient pressure environment to assist in the formation of SLM gradient materials, resulting in a trend of increasing grain size from the inside out. The grain size of the first gradient coating is the smallest, and the number of fine grains per unit volume is significantly greater than that of the transition layer and the third gradient layer. Therefore, when plastic deformation occurs, due to the large number of grains with favorable orientations, the deformation energy is evenly distributed across each grain, reducing the deformation unevenness of fine-grained metals and making the internal stress distribution of the coating more uniform. In addition, the smaller the grains, the easier it is for grain boundary slip to occur. Due to the mobility of grain boundaries, the stress concentration generated by dislocations near the grain boundaries is easily relaxed, resulting in a more dispersed location of stress concentration, which is conducive to continued plastic deformation of the coating and significantly improving the plasticity and ductility of the first gradient coating. The grain refinement effect of the transition layer and the third gradient coating gradually weakens as the pressure decreases during the preparation process, and the strength and toughness of the coating decreases. However, due to the doping of high-hardness nitrides, the surface hardness and wear resistance of the transition layer and the third gradient coating are greatly improved, making the prepared aluminum-lithium alloy gradient material have the performance characteristics of being hard on the outside and tough on the inside.

[0030] 8. The high-pressure multi-atmosphere assisted SLM gradient material preparation method described in the present invention has all process parameters including pulse parameters, laser parameters, high pressure and atmosphere parameters, which can be controlled through the algorithm. It has the advantages of simple operation, low cost and high efficiency. It is easy to realize industrial application and is aimed at the service environment of aviation aluminum-lithium alloys, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. The drawings described below are some embodiments of the present invention. For ordinary technicians in this field, it is obvious that other drawings can be obtained based on these drawings without paying any creative work.

[0032] FIG1 is a schematic diagram showing the principle of the method for high-pressure multi-atmosphere assisted SLM gradient materials according to the present invention.

[0033] FIG2 is a diagram showing the cross-sectional morphology and carbon and nitrogen element contents of the gradient material prepared in Example 1 of the present invention.

[0034] FIG3 shows the average microhardness of each gradient coating cross section of the sample prepared in Example 1 of the present invention.

[0035] FIG4 is a comparison diagram of the surface microhardness of the gradient materials prepared in various embodiments of the present invention.

[0036] In the figure: 1-working box; 2-substrate; 3-porous powder sprayer; 4-pressure pump; 5-air suction pump; 6-gas flow meter; 7-pressure gauge. DETAILED DESCRIPTION

[0037] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0038] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "axial", "radial", "vertical", "horizontal", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "multiple" means two or more, unless otherwise clearly and specifically defined.

[0039] In the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," "connect," "fixed," etc. should be understood broadly. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediary; or internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0040] As shown in FIG1 , the high-pressure multi-atmosphere assisted SLM gradient material preparation method of the present invention comprises the following steps:

[0041] Before the SLM forming operation, the work box is cleaned of impurities and powders, and the surface of the substrate 2 is roughly ground to improve the bonding strength between the formed aluminum-lithium alloy coating and the substrate 2; the treated surface is ultrasonically cleaned with anhydrous ethanol and dried;

[0042] After injecting argon into the working box 1 for a period of time, a first mixed gas is continuously injected, wherein the first mixed gas is a mixture of argon and CO2 gas; the volume proportion of CO2 gas is 10% to 20%; the gas flow meter 6 at the inlet and outlet of the working box monitors that all active gases maintain a flow rate of 20L / min to 50L / min during the SLM process; a pressure pump 4 is used to maintain the high-pressure environment in the working box 1, and a pressure gauge 7 is set in the working box 1 to monitor and ensure that the pressure is in the range of 3.5 to 5.0MPa;

[0043] A loose aluminum-lithium alloy powder having a powder layer thickness of 40 to 50 μm and a diameter range of 20 to 63 μm is laid on the substrate using a porous powder sprayer; the aluminum-lithium alloy powder laid on the surface of the substrate 2 is irradiated using appropriate pulsed laser parameters and process paths to form a micro-melting pool, so that the molten material in the micro-melting pool reacts with the carbon-containing active gas to form a carbide-doped first gradient aluminum-lithium alloy coating; in addition, the grain structure of the coating is significantly refined under the action of high pressure, thereby achieving a synergistic enhancement of the hardness and toughness of the formed first gradient aluminum-lithium alloy coating; the aluminum-lithium alloy powder laying and irradiation operations are repeated until the thickness of the first gradient aluminum-lithium alloy coating reaches 0.8 to 1.0 mm;

[0044] After the first gradient aluminum-lithium alloy coating is completed, argon gas is injected into the working box 1 to exhaust the first mixed gas; then the second mixed gas is continuously injected into the closed working box 1, and the second mixed gas is a mixture of Ar gas and CO2+N2 active gas; the volume of the CO2 gas in the second mixed gas accounts for 10% to 20%; the volume of the N2 gas in the second mixed gas accounts for 20% to 30%; a pressure pump 4 is used to maintain the high-pressure environment in the working box 1, and a pressure gauge 7 is set in the working box 1 to monitor and ensure that the pressure is in the range of 2.0 to 2.5 MPa; when the pulse laser parameters and the process path remain unchanged, the aluminum-lithium alloy powder laid on the surface of the first gradient aluminum-lithium alloy coating is irradiated by laser to form a micro-melting pool, so that the molten material in the micro-melting pool reacts with the carbon- and nitrogen-containing active gas to form a second gradient aluminum-lithium alloy coating doped with carbonitride; the second gradient aluminum-lithium alloy coating is the transition layer, and the powder laying and irradiation operations are repeated until the thickness of the transition layer is 0.6 to 0.8 mm;

[0045] After the second gradient aluminum-lithium alloy coating is completed, argon is injected into the work box 1 to exhaust the second mixed gas. A third mixed gas is then continuously injected into the sealed work box 1 to maintain a high-pressure environment. The third mixed gas is a mixture of Ar and N2 reactive gas, with the N2 gas accounting for 20% to 30% by volume. A pressure pump 4 is used to maintain the high-pressure environment within the work box 1, and a pressure gauge 7 is installed inside the work box 1 to monitor and ensure the pressure remains within the range of 1.0 to 1.5 MPa. While maintaining the same pulsed laser parameters and process path, the aluminum-lithium alloy powder laid on the surface of the second gradient aluminum-lithium alloy coating is laser-irradiated to form a micro-melt pool. The molten material in the micro-melt pool reacts with the nitrogen-containing reactive gas to form a nitride-doped third gradient aluminum-lithium alloy coating. The powder laying and irradiation operations are repeated until the thickness of the third gradient aluminum-lithium alloy coating reaches 0.8 to 1 mm.

[0046] The high-pressure multi-atmosphere assisted SLM gradient material preparation method described in the present invention continuously injects carbon-containing and nitrogen-containing active gases into the working box 1 and maintains a gradient high-pressure environment. A pulsed laser is selected as a radiation heat source to perform SLM forming preparation on the aluminum-lithium alloy powder. During the forming process, the molten material in the molten pool reacts with the carbon-containing and nitrogen-containing active gases to complete the doping of different strengthening phases. At the same time, the microstructure of the gradient coating shows a transitional evolution trend under the action of different pressures. The prepared aluminum-lithium alloy gradient material achieves the performance requirements of being hard on the outside and tough on the inside.Carbon-containing active gas is used as the atmosphere doping in the SLM process of the first gradient aluminum-lithium alloy coating. Under the action of laser irradiation, active carbon atoms are dissociated and plasma is formed above the molten pool. Under the high pressure and the expansion shock wave of the plasma, the carbon atoms are stirred and modulated with the molten material in the molten pool, which greatly increases the carbon content of the surface layer of the aluminum-lithium alloy, thereby enhancing the wear resistance and mechanical stability of the first gradient aluminum-lithium alloy coating; nitrogen-containing active gas is used as the atmosphere doping in the preparation process of the third gradient aluminum-lithium alloy coating. The active nitrogen atoms separated under the action of the laser penetrate into the high-temperature molten pool through the gas-liquid interface and migrate and dissolve, realizing the coating. The doping of high-hardness nitride particles in the layer further improves the surface hardness and wear resistance of the third gradient aluminum-lithium alloy coating compared to the first gradient coating doped with only carbides; the introduction of carbon- and nitrogen-containing active gases to prepare the second gradient aluminum-lithium alloy coating, the active gases simultaneously dissociate into carbon atoms and nitrogen atoms under the action of laser irradiation, the active atoms react with the high-temperature molten pool to achieve the simultaneous doping of carbides and nitrides, which is beneficial to the gradient continuous transition change of the mechanical properties of the aluminum-lithium alloy gradient material and effectively enhances the interlayer bonding strength of the gradient material; a porous powder sprayer is used to lay uniform and loosely structured aluminum-lithium alloy powder, and part of the active gas penetrates into the loose Inside the loose powder layer, active atoms are directly dissociated under the action of laser irradiation and melted in the high-temperature molten pool. The carbon content and nitrogen content in the molten pool increase, and the mechanical properties of the aluminum-lithium alloy material are further improved; SLM forming preparation under a high-pressure environment is conducive to the increase of the nucleation rate in the molten pool, making the grain structure of the aluminum-lithium alloy coating more refined. The high-pressure auxiliary process also increases the absorption concentration of carbon atoms and nitrogen atoms by the molten metal in the molten pool, and promotes the in-situ synthesis and doping of carbides and nitrides in the aluminum-lithium alloy gradient material, thereby improving the molding quality of the prepared material and further improving the hardness and mechanical properties of the gradient material; through the gradient High pressure further assists SLM forming, so that the grain size inside the material shows an increasing evolution trend as the pressure of the working environment decreases. Therefore, the grain size of the first gradient coating is the smallest and the coating exhibits the highest strength and toughness. Combined with the transition layer doped with high-hardness nitride particles and the third gradient aluminum-lithium alloy coating, the gradient high-pressure assisted process makes the prepared SLM gradient material have the performance characteristics of hard outside and tough inside; using pulsed laser for SLM forming preparation, the stirring effect in the molten pool is enhanced, making the element distribution more uniform and effectively increasing the cooling rate of the molten pool, reducing the heat accumulation of the material, and thereby effectively reducing the residual stress of the prepared material.

[0047] In order to make the purpose, technical solutions and advantages of the present invention clearer, 2195 aviation aluminum-lithium alloy is selected as the research object and the present invention is described in detail in combination with specific embodiments.

[0048] Example 1:

[0049] The high-pressure multi-atmosphere assisted SLM gradient material preparation method described in Example 1 includes the following steps:

[0050] S01: Before the SLM forming operation, the work box 1 is cleaned of impurities and powders, and the 2195 aluminum-lithium alloy substrate 2 is sanded using 800# and 1000# grit sandpaper to improve the bonding strength between the formed aluminum-lithium alloy coating and the substrate 2; then the treated surface is ultrasonically cleaned with anhydrous ethanol and dried;

[0051] S02: After injecting argon gas into the working box 1 for 15 minutes, Ar + 10% CO2 active gas is introduced;

[0052] S03: The flow rate of the active gas is monitored by the gas flow meter 6 at the inlet and outlet of the working box 1. During the SLM process, all active gases maintain a flow rate of 50 L / min;

[0053] S04: Use the pressure pump 4 to maintain the high pressure environment in the working box. Set the pressure gauge 7 in the working box 1 to monitor and ensure that the pressure during the SLM process is maintained at 3.5 MPa;

[0054] S05: laying a loose aluminum-lithium alloy powder with a thickness of 50 μm and a diameter of 50 μm on the substrate 2 through a porous powder sprayer 3;

[0055] S06: An IPG-YLS-5000 fiber laser was selected, with the light output mode modulated to pulse mode. The specific processing parameters of the laser were: wavelength 1075nm, pulse energy 20J, pulse period 80ms, pulse duration 40ms, duty cycle 50%, and spot diameter 50μm. The specific process parameters of the SLM forming process were: laser power 90W, scanning speed 100mm / s, scanning pitch 110μm, and the scanning path of the forming process was an interlayer orthogonal strategy. The aluminum-lithium alloy powder laying operation was repeated until the thickness of the first gradient aluminum-lithium alloy coating reached 0.8mm.

[0056] S07: After the first gradient aluminum-lithium alloy coating is prepared, argon is injected into the working box for 15 minutes to exhaust the first mixed gas, and then Ar+10% CO2+20% N2 is introduced, and a high pressure environment of 2.0 MPa is maintained through the pressure gauge 7;

[0057] S08: With the pulse laser parameters and process path remaining unchanged, the thickness of the second gradient aluminum-lithium alloy coating (and transition layer) prepared by repeated powder laying work is 0.6 mm;

[0058] S09: Inject argon gas into the working box 1 for 15 minutes to exhaust the second mixed gas, then introduce Ar + 20% N2, and maintain a high pressure environment of 1.0 MPa through the pressure gauge 7;

[0059] S10: Under the same process conditions, the powder layers were laid and melted multiple times, resulting in a third gradient aluminum-lithium alloy coating with a final thickness of 0.8 mm. Based on these processing parameters, SLM operations were performed in a work chamber with a gradient high pressure and various reactive atmospheres, completing the preparation of a defect-free aluminum-lithium alloy gradient material with excellent wear resistance.

[0060] The cross-sectional morphology of the aluminum-lithium alloy prepared in Example 1 is shown in Figure 2. Compared with the traditional SLM formed specimen, the microstructure of the gradient material prepared in Example 1 shows a trend of gradual refinement from the surface to the inside. The first gradient coating prepared under the highest pressure environment has the finest grain structure and the highest coating toughness. In addition, the EDS scanning results of points A and B show that the hard phase particles in the gradient material are doped with 12.68 at.% C and 25.36 at.% N, respectively. It was confirmed that carbon-containing and nitrogen-containing active gases achieved the doping of carbides and nitrides in SLM materials; Figure 3 shows the average microhardness of the cross-section of each gradient coating layer of the sample prepared in Example 1. The results show that the microhardness increases from the first gradient layer to the third gradient layer. Combined with the decreasing trend of toughness from the inside to the outside of the gradient material, the SLM aluminum-lithium alloy gradient material prepared in Example 1 achieves the performance requirements of hard outside and tough inside; in addition, as shown in Figure 4, under the synergistic effect of high-pressure assistance and in-situ synthesis of active gases, the carbon and nitrogen elements inside the aluminum-lithium alloy material increase, greatly improving the surface hardness of the material. The microhardness value reaches 152.5HV, which is 35.5% higher than the average hardness value of 112.7HV on the surface of traditional SLM. This is of great significance for low-cost and efficient improvement of the wear resistance of SLM aluminum-lithium alloy materials.

[0061] Example 2

[0062] On the basis of Example 1, the active gas Ar+15% CO2 in the preparation process of the first gradient aluminum-lithium alloy coating in Example 2 is maintained at a pressure of 5.0 MPa; the active gas Ar+15% CO2+25% N2 used in the preparation of the transition layer is maintained at a pressure of 2.5 MPa; the active gas Ar+25% N2 in the preparation process of the third gradient aluminum-lithium alloy coating is maintained at a pressure of 1.5 MPa; the flow rate of all active gases is 35 L / min.

[0063] Compared with the material prepared in Example 1, the overall microstructure of the gradient material obtained in Example 2 is more refined, and the C content and N content of the two doped hard phase particles are increased to 16.92at.% and 28.2at.%, respectively. Example 2 adopts a higher pressure environment, a lower gas flow rate and a higher proportion of CO2 and N2 doped active gases to assist SLM forming of the specimen. The reaction efficiency of the molten material and the active gas is improved, the size and doping amount of carbides and nitrides in the material are increased, and the strengthening effect of the aluminum-lithium alloy material is effectively improved. As shown in Figure 4, the surface microhardness value of the aluminum-lithium alloy material prepared in this embodiment is 164.7HV, which is 46.1% higher than the average hardness value of 112.7HV of the surface of the specimen prepared by traditional SLM, and 8% higher than the average hardness of 152.5HV of the surface of the specimen after treatment in Example 1. Appropriately increasing the pressure, reducing the active gas flow rate and increasing the proportion of CO2 and N2 in the gas make the stirring reaction between the molten material in the molten pool and the dissociated active atoms more sufficient, and the hardness and mechanical stability of the surface of the aluminum-lithium alloy material are further improved.

[0064] Example 3

[0065] On the basis of Example 1, the active gas Ar+20% CO2 in the preparation process of the first gradient aluminum-lithium alloy coating in Example 3 was maintained at a pressure of 5.0 MPa, and the thickness of the first gradient aluminum-lithium alloy coating was 1.0 mm; the active gas Ar+20% CO2+30% N2 used in the preparation of the transition layer was maintained at a pressure of 2.5 MPa, and the thickness of the transition coating was 0.8 mm; the active gas Ar+30% N2 in the preparation process of the third gradient aluminum-lithium alloy coating was maintained at a pressure of 1.5 MPa, and the final preparation thickness of the third gradient coating was 1.0 mm; the flow rate of all active gases was 20 L / min; and the laser power was set to 1000 W.

[0066] Due to the increase in laser power and the thickness of each layer of gradient coating, the sample processed in Example 3 is doped with more carbide and nitride particles with larger sizes. The C content of the carbide particles is 17.18at.%, and the N content of the nitride particles is 29.7at.%. However, due to the accumulated heat influence inside the material, the microstructure of the sample prepared in Example 3 does not show obvious refinement effect compared with Example 1 even under increased pressure. The surface microhardness value of the aluminum-lithium alloy material prepared in this example is 160.2HV, which is 42.1% higher than the average hardness of 112.7HV of the traditional SLM surface. It can be seen that the material prepared in Example 3 also has high wear resistance and mechanical stability.

[0067] Table 1 shows the characterization results of samples treated with different processes of the present invention

[0068] An aluminum-lithium alloy coating is formed on the surface of a substrate 2 using the high-pressure multi-atmosphere assisted SLM gradient material preparation method of the present invention. The substrate 2 here can be the skin of an aircraft.

[0069] It should be understood that although this specification is described according to various embodiments, not every embodiment contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0070] The series of detailed descriptions listed above are only specific descriptions of feasible embodiments of the present invention. They are not intended to limit the scope of protection of the present invention. Any equivalent embodiments or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing high-pressure multi-atmosphere assisted SLM gradient materials, characterized in that, The method includes the following steps: Continuously inject a first mixed gas into the sealed working chamber (1) to maintain a high-pressure environment in the working chamber (1); the first mixed gas is a mixed gas of an inert gas and a carbon-containing active gas; irradiate the aluminum-lithium alloy powder laid on the surface of the substrate (2) with a laser to form a micro-melting pool, and react the molten material in the micro-melting pool with the carbon-containing active gas to form a carbide-doped first-gradient aluminum-lithium alloy coating; Exhaust the first mixed gas in the sealed working chamber (1); continuously inject a second mixed gas into the sealed working chamber (1) to maintain a high-pressure environment in the working chamber (1); the second mixed gas is a mixed gas of an inert gas and a carbon-nitrogen-containing active gas; irradiate the aluminum-lithium alloy powder laid on the surface of the first-gradient aluminum-lithium alloy coating with a laser to form a micro-melting pool, and react the molten material in the micro-melting pool with the carbon-nitrogen-containing active gas to form a carbonitride-doped second-gradient aluminum-lithium alloy coating; Exhaust the second mixed gas in the sealed working chamber (1); continuously inject a third mixed gas into the sealed working chamber (1) to maintain a high-pressure environment in the working chamber (1); the third mixed gas is a mixed gas of an inert gas and a nitrogen-containing active gas; irradiate the aluminum-lithium alloy powder laid on the surface of the second-gradient aluminum-lithium alloy coating with a laser to form a micro-melting pool, and react the molten material in the micro-melting pool with the nitrogen-containing active gas to form a nitride-doped third-gradient aluminum-lithium alloy coating.

2. The high-pressure multi-atmosphere assisted SLM gradient material preparation method according to claim 1, characterized in that The carbon-containing active gas in the first mixed gas is CO2 gas, and the volume ratio of the CO2 gas in the first mixed gas is 10% - 20%; the carbon-nitrogen-containing active gas in the second mixed gas is CO2 gas and N2 gas, the volume ratio of the CO2 gas in the second mixed gas is 10% - 20%; the volume ratio of the N2 gas in the second mixed gas is 20% - 30%; the nitrogen-containing active gas in the third mixed gas is N2 gas, and the volume ratio of the N2 gas in the third mixed gas is 20% - 30%.

3. The high-pressure multi-atmosphere assisted SLM gradient material preparation method according to claim 1, wherein The pressure P1 of the high-pressure environment generated by the first mixed gas, the pressure P2 of the high-pressure environment generated by the second mixed gas, the pressure P3 of the high-pressure environment generated by the third mixed gas, P1 > P2 > P3.

4. The method for preparing a high-pressure multi-atmosphere assisted SLM gradient material according to claim 3, characterized in that, The pressure P1 of the high-pressure environment generated by the first mixed gas is 3.5 - 5.0 MPa; the pressure P2 of the high-pressure environment generated by the second mixed gas is 2.0 - 2.5 MPa; the pressure P3 of the high-pressure environment generated by the third mixed gas is 1.0 - 1.5 MPa.

5. The high-pressure multi-atmosphere assisted SLM gradient material preparation method according to claim 1, wherein The active gas maintains a flow rate of 20 L / min - 50 L / min during the SLM forming process to make the concentration of the active gas in the molten material reaction process within a set range.

6. The high-pressure multi-atmosphere assisted SLM gradient material preparation method according to claim 1, characterized in that, Lay the uniform and loose aluminum-lithium alloy powder through a porous powder sprayer to store part of the active gas inside the powder.

7. The high-pressure multi-atmosphere assisted SLM gradient material preparation method according to claim 5, characterized in that The thicknesses of both the first-gradient aluminum-lithium alloy coating and the third-gradient aluminum-lithium alloy coating are 0.8 - 1.0 mm, and the thickness of the second-gradient aluminum-lithium alloy coating is 0.6 - 0.8 mm.

8. The high-pressure multi-atmosphere assisted SLM gradient material preparation method according to claim 1, characterized in that, The surface of the substrate (2) is treated to increase its surface roughness, which is used to improve the bonding strength between the formed aluminum-lithium alloy coating and the substrate (2); the treated surface is ultrasonically cleaned with absolute ethanol and then dried.

9. The high-pressure multi-atmosphere assisted SLM gradient material preparation method according to claim 1, characterized in that, The laser energy is pulsed laser, and the parameters of the pulsed laser beam are as follows: wavelength 1075 nm, pulse energy 1 J - 30 J, pulse period 80 ms, pulse duration 20 - 60 ms, duty cycle 50%, spot diameter ≤ 50 μm; the laser power is 80 - 100 W, the scanning speed of the laser beam is 100 mm / s, and the scanning pitch is 100 - 120 μm.

10. An aluminum-lithium alloy coating, characterized in that, An aluminum-lithium alloy coating is formed on the surface of the substrate (2) by using the high-pressure multi-atmosphere assisted SLM gradient material preparation method according to any one of claims 1 - 9.

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