Method for preparing gradient composite material through laser-assisted cold spraying

Abstract

The invention relates to a method for preparing a gradient composite material through laser-assisted cold spraying, which comprises a cold spraying step and a laser in-situ heating spraying step, the laser in-situ heating spraying step is continuously repeated, the gradient distribution of ceramic powder in the composite material is accurately regulated and controlled, and meanwhile, the laser power is gradually regulated along with the layer-by-layer advancing of spraying, so that the gradient distribution of the ceramic powder in the composite material is controlled. The preparation of the composite material with the controllable ceramic content gradient is realized. The method disclosed by the invention is simple and convenient to operate and low in cost, can meet the requirements of industrial production on large-batch high-performance gradient composite materials, and has a wide application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical fields of material surface engineering and additive manufacturing. Background Art

[0002] Composite materials are composed of two or more materials with different physical and chemical properties combined in a specific manner, proportion and distribution, aiming to exert the excellent characteristics of each component material, so as to achieve comprehensive properties that cannot be achieved by a single material. Traditional composite materials are usually called "homogeneous composite materials", in which the reinforcing phase is uniformly distributed in the matrix phase, and the structure and properties of each part of the material are consistent, showing isotropic characteristics. However, with the continuous progress of science and technology and the acceleration of the industrialization process, the performance requirements that materials need to meet in extreme service environments are becoming more and more stringent. The limitations of homogeneous composite materials, such as weak plastic toughness and impact resistance, make it difficult to meet the actual working conditions under certain harsh conditions. As a new type of composite material, gradient composite materials can provide more excellent service performance in extreme environments and meet the requirements of special working conditions by presenting gradient characteristics in composition, structure and mechanical properties.

[0003] At present, the traditional preparation methods of gradient composite materials mainly include powder metallurgy method, thermal spraying method, laser cladding method, self-propagating high-temperature combustion method and centrifugal casting method. However, these methods usually need to be processed in a high-temperature environment, which easily leads to adverse effects such as oxidation, phase transformation and element burning loss of the materials, thus significantly reducing the mechanical properties of gradient composite materials.

[0004] Cold spraying, as a new solid-state forming technology based on the principle of aerodynamics, can effectively overcome the above problems. During the cold spraying process, micron-sized powder particles are accelerated by a high-pressure gas stream and impact the surface of the substrate at an extremely high speed through a Laval nozzle, causing strong plastic deformation of the metal particles. When the speed of the particles exceeds the critical value, they can be effectively deposited on the surface of the substrate to form a coating or deposit. Since the temperature of cold spraying is always lower than the melting point of the powder, problems such as pores, segregation and phase transformation that may occur during the melting-solidification process can be avoided, thereby improving the mechanical properties of the materials.

[0005] In recent years, cold spraying has shown significant advantages in the preparation of gradient composites. There are mainly two common preparation methods: The first method is to use a single powder feeder to cold spray mixed powders with different proportions. After the first batch of powder spraying is completed, the old powder in the powder feeder needs to be thoroughly cleaned, and then different second batches of powder are loaded. Subsequently, spraying is carried out again, and this process is repeated. Although this method has a low cost, its workflow is cumbersome and the efficiency is low, making it difficult to meet the requirements of large-scale industrial production; The second method is to use two or more powder feeders to work simultaneously. The powder feeding rate of the main powder feeder remains unchanged, while the powder feeding rate of the non-main powder feeder is adjusted during the spraying process. By controlling the powder ratio, the preparation of gradient composites is achieved. This method is relatively easy to operate, but due to the need for multiple powder feeders and the high cost of powder feeders, the manufacturing cost increases significantly. In addition, when the rotation speed of the non-main powder feeder is low, the prepared gradient composites may have problems of uneven interlayer transition, which will have an adverse impact on the application performance of the materials in harsh environments. Therefore, a new method with simple operation and low cost is needed to prepare high-performance gradient composites. Summary of the Invention

[0006] The purpose of the present invention is to avoid the deficiencies of the prior art and provide a method for preparing high-performance gradient composites with simple operation and low cost. Its principle is to in-situ heat the deposited composite coating by adjusting the laser power, so that the deposited metal powders are thermally softened to different degrees, to change the deposition efficiency of the ceramic powders in the mixed powders to be deposited, and at the same time, it is also beneficial to increase the interfacial bonding strength between the deformed powders, thereby preparing high-performance gradient composites with different ceramic contents.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is: A method for preparing gradient composites by laser-assisted cold spraying, comprising the following steps: Cold spraying deposition step: Use high-pressure gas with a pressure of 1.0 - 10.0 MPa and a temperature not lower than 350 °C to spray the mixed powders provided by the powder feeder onto the surface of the substrate through a Laval nozzle. The spraying moving speed of the Laval nozzle is 10 - 2000 mm / s, and multi-pass spraying is carried out until a first composite coating with a thickness of 0.5 - 1.5 mm is deposited on the surface of the substrate; Among them, the mixed powders are mixed powders of metal powders and hard ceramic powders. The volume proportion of the ceramic powders in the mixed powders is 10% - 90%, and at the same time, the temperature of the high-pressure gas is not higher than the melting point of the metal powders; Laser in-situ heating deposition step: Control the coincidence of the laser spot of the laser and the spraying spot of the Laval nozzle on the substrate, and ensure that the spraying spot is within the laser spot. Start the multi-pass laser in-situ heating spraying of the first composite coating with a laser spot synchronized spraying at a laser power of 0.5 - 5 kW; until a second composite coating with a thickness of 0.5 - 1.5 mm is deposited on the surface of the first composite coating; Among them, the laser in-situ heating is used to soften the flattened metal powder deposited on the surface of the first composite coating, and enhance the deposition efficiency and embedding ability of the ceramic powder on the first composite coating, thereby increasing the content of the ceramic powder in the second composite coating; Finally, continuously repeat the laser in-situ heating deposition step until a gradient composite material with a thickness of 4 - 10 mm is obtained, and the laser power increased or decreased in each repetition of the laser in-situ heating deposition step is 0.1 - 2 Kw. The gradient composite material is a composite material formed with different ceramic deposition efficiencies along the spraying thickness direction.

[0008] Furthermore, the outlet diameter of the Laval nozzle does not exceed the maximum laser spot diameter of the laser, and both the Laval nozzle and the laser are installed on the robotic arm, so that the angle between the axis of the Laval nozzle and the axis of the laser is 10 - 60°; The spot diameter of the laser spot is 2 - 10 mm, and the spot diameter of the spraying spot is 3 - 9 mm.

[0009] Furthermore, the Laval nozzle is made of stainless steel or ceramic. The axis of the Laval nozzle is perpendicular to the axis of the substrate. In cold spraying, the distance between the outlet of the Laval nozzle and the substrate is 10 - 30 cm; And the upstream length of the Laval nozzle is 10 - 40 mm, the downstream length is 80 - 300 mm, the throat diameter is 2 - 3 mm, and the outlet diameter is 5 - 9 mm.

[0010] Furthermore, the mixed powder is obtained by mechanically mixing the metal powder and the ceramic powder in a ball mill for 30 - 50 minutes before the cold spraying step.

[0011] Furthermore, the metal powder is Al or Cu or Ti or Mg or Fe or Ni or any one of aluminum alloy, copper alloy, titanium alloy, magnesium alloy, steel, nickel alloy; The ceramic powder is SiC or TiN or Al 2 O 3 or WC; The particle size range of the mixed powder is 1 - 100 μm, and the shape is spherical or irregular.

[0012] Further, the high-pressure gas source is nitrogen or helium or compressed air or argon or a mixed gas of any two or more of nitrogen, helium, compressed air, and argon.

[0013] Further, the substrate is a flat substrate with a thickness of 1-6 mm; Before cold spraying, the surface is sandblasted with quartz sand of 150-450 mesh, and then cleaned and dried with acetone.

[0014] Further, the rotation speed of the powder feeder is 0.1-10 rpm.

[0015] Further, it further includes a powder feeder for providing mixed powder, a heater for heating high-pressure gas, and a pressure control device for controlling the pressure of high-pressure gas; A part of the high-pressure gas is sequentially connected to the inlet of the first pressure control device and the Laval nozzle through a pipeline, and the powder feeder is arranged on the pipeline between the pressure control device and the Laval nozzle; at the same time, another part of the high-pressure gas is sequentially connected to the inlet of the second pressure control device and the Laval nozzle through a pipeline, and the heater is arranged on the pipeline between the second pressure control device and the Laval nozzle; the mixed powder provided by the powder feeder is mixed with the high-pressure gas heated by the heater upstream of the Laval nozzle and preheated.

[0016] Further, it further includes an infrared thermometer for monitoring the temperature of the composite coating deposited on the substrate and the temperature of the spraying point; Among them, the measurement range of the infrared thermometer is 0-1100 °C.

[0017] Compared with the prior art, the present invention has the following advantages: The new method for preparing gradient composite materials by laser-assisted cold spraying proposed by the present invention significantly reduces the high cost brought by using two or more powder feeders, and at the same time avoids the cumbersome operation caused by the frequent replacement of different component powders by a single powder feeder.

[0018] By regulating the laser power to in-situ heat the deposited composite material, the metal powder therein undergoes different degrees of thermal softening, which can effectively change the deposition efficiency of the ceramic phase in the mixed powder to be deposited, thereby realizing the preparation of gradient composite materials with controllable ceramic content between layers.

[0019] The method of the present invention has low cost and simple operation, and can prepare gradient composite materials with high density. In addition, this method can operate stably for a long time and meet the needs of industrialized and large-scale production of gradient composite materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is the working principle diagram of the present invention; Figure 2 is the schematic diagram of the gradient composite material structure of the present invention.

[0021] In the figure: 1 - high - pressure gas source, 21 - first pressure - control device, 22 - second pressure - control device, 3 - heater, 4 - powder feeder, 5 - robotic arm, 6 - Laval nozzle, 7 - laser, 8 - infrared thermometer, 9 - substrate, 10 - metal powder, 11 - ceramic powder, 12 - first composite coating, 13 - second composite coating, 14 - third composite coating. Specific embodiments

[0022] The principles and features of the present invention will be described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0023] The object of the present invention is to overcome the deficiencies of the prior art and provide a method for preparing gradient composite materials by laser - assisted cold spraying, which can significantly reduce the high cost brought by using two or more powder feeders, and at the same time avoid the cumbersome operation caused by frequently replacing different - component powders when using a single powder feeder, and realize the preparation of high - performance gradient composite materials.

[0024] To achieve the above object, the present invention provides the following specific embodiments: Example 1: As Figure 2 shown, a method for preparing gradient composite materials by laser - assisted cold spraying includes the following steps: Step 1. Cold - spraying deposition: Use high - pressure gas 1 with a pressure of 1.0 - 10.0 MPa and a temperature not lower than 350 °C to spray the mixed powder provided by the powder feeder 4 onto the surface of the substrate 9 through the Laval nozzle 6. The spraying moving speed of the Laval nozzle 6 is 10 - 2000 mm / s, and multiple - pass spraying is carried out until a first composite coating 12 with a thickness of 0.5 - 1.5 mm is deposited on the surface of the substrate 9; Among them, the mixed powder is a mixed powder of metal powder 10 and hard ceramic powder 11, and is the mixed powder mechanically mixed in a ball mill for 30 - 50 minutes before the cold - spraying step. The volume ratio of the ceramic powder 11 in the mixed powder is 10% - 90%, and at the same time, the temperature of the high - pressure gas 1 is not higher than the melting point of the metal powder 10; Step 2. Laser in - situ heating deposition step: Control the laser spot of the laser 7 and the spraying spot of the Laval nozzle 6 to coincide on the substrate 9, and the spraying spot is within the laser spot. Start multi - pass laser in - situ heating spraying of the first composite coating 12 by synchronously spraying with a laser spot with a laser power of 0.5 - 5 kW; until a second composite coating 13 with a thickness of 0.5 - 1.5 mm is deposited on the surface of the first composite coating 12; Among them, laser in-situ heating is used to soften the metal powder 10 flattened on the surface of the first composite coating 12, and enhance the deposition efficiency and embedding ability of the ceramic powder 11 on the first composite coating 12, thereby increasing the content of the ceramic powder 11 in the second composite coating 13; Step 3: Continue with the laser in-situ heating and deposition step: Still control the coincidence of the light spot of the laser 7 and the spraying spot of the Laval nozzle 6 on the substrate 9, and the spraying spot is within the light spot. Start multi-pass laser in-situ heating spraying on the first composite coating 12 by synchronously spraying with the laser light spot; until a third composite coating 14 with a thickness of 0.5 - 1.5 mm is deposited on the surface of the first composite coating 12; At this time, the laser power is increased or decreased by 0.1 - 2 Kw based on 0.5 - 5 kW; Step 4: Continuously repeat the laser in-situ heating and deposition step until a gradient composite material with a thickness of 4 - 10 mm is obtained. At this time, the gradient composite material is a composite material formed with different ceramic deposition efficiencies along the spraying thickness direction.

[0025] In the step, the used metal powder 10 is any one of Al, Cu, Ti, Mg, Fe, Ni or aluminum alloy, copper alloy, titanium alloy, magnesium alloy, steel, nickel alloy; The ceramic powder 11 is SiC, TiN, Al 2 O 3 or WC; The particle size range of the mixed powder is 1 - 100 μm, and the shape is spherical or irregular.

[0026] The high-pressure gas source 1 is nitrogen, helium, compressed air, argon or a mixed gas of any two or more of nitrogen, helium, compressed air, and argon.

[0027] Example 2: The same as Example 1, except that: the outlet diameter of the Laval nozzle 6 does not exceed the maximum light spot diameter of the laser 7, and both the Laval nozzle 6 and the laser 7 are installed on the robotic arm 5, so that the angle between the axes of the Laval nozzle 6 and the laser 7 is 10 - 60°; The spot diameter of the light spot is 2 - 10 mm, and the spot diameter of the spraying spot is 3 - 9 mm.

[0028] The Laval nozzle 6 is a stainless steel or ceramic nozzle. The axis of the Laval nozzle 6 is perpendicular to the axis of the substrate 9. In cold spraying, the distance between the outlet of the Laval nozzle 6 and the substrate 9 is 10 - 30 cm; And the upstream length of the Laval nozzle 6 is 10 - 40 mm, the downstream length is 80 - 300 mm, the throat diameter is 2 - 3 mm, and the outlet diameter is 5 - 9 mm.

[0029] The substrate 9 is a flat substrate with a thickness of 1 to 6 mm; before cold spraying, the surface is sandblasted with quartz sand of 150 to 450 mesh, and then cleaned and dried with acetone.

[0030] The rotation speed of the powder feeder 4 is 0.1 to 10 rpm.

[0031] Example 3: As Figure 1 shown, it is the same as Example 1, except that it further includes a powder feeder 4 for providing mixed powder, a heater 3 for heating the high-pressure gas 1, and a pressure control device 2 for controlling the pressure of the high-pressure gas 1; Part of the high-pressure gas 1 is connected in sequence through a pipeline to the inlet of the first pressure control device 21 and the Laval nozzle 6, and the powder feeder 4 is arranged on the pipeline between the pressure control device 21 and the Laval nozzle 6; at the same time, another part of the high-pressure gas 1 is connected in sequence through a pipeline to the inlet of the second pressure control device 22 and the Laval nozzle 6, and the heater 3 is arranged on the pipeline between the second pressure control device 22 and the Laval nozzle 6; the mixed powder provided by the powder feeder 4 is mixed with the high-pressure gas 1 heated by the heater 3 upstream of the Laval nozzle 6 and the mixed powder is preheated.

[0032] It further includes an infrared thermometer 8 for monitoring the temperature of the composite coating deposited on the substrate 9 and the temperature of the spraying point; the measurement range of the infrared thermometer 8 is 0 to 1100 °C.

[0033] As Figure 1 、 Figure 2 shown, in order to further illustrate the technical solution and effect of the present invention, the following specific examples are provided: Specific Example 1: The target is to prepare an aluminum-based gradient composite material with a thickness of 5 mm, and the ceramic content of the composite material gradually increases from the inside to the outside.

[0034] As Figure 1 shown, first, the Laval nozzle 6 and the laser 7 are fixed on the robotic arm 5, the included angle between the laser and the Laval nozzle is set to 30°, the upstream length of the Laval nozzle is 30 mm, the downstream length is 160 mm, the throat diameter is 2.7 mm, the outlet diameter is 8 mm, and the material of the Laval nozzle is ceramic.

[0035] The powder selects spherical 2024 aluminum alloy 10 with a particle size of 15 to 53 μm and Al 2 O 3 ceramic 11, and is proportioned according to the ceramic volume ratio of 30% in the mixed powder, put into a ball mill and mixed for 30 minutes, and then put into the powder feeder 4.

[0036] The substrate is made of 2024 aluminum alloy 9, and the surface is sandblasted with 300-mesh quartz sand, then cleaned and dried with acetone. The substrate 9 is perpendicular to the axis of the Laval nozzle, and the distance is set to 30 mm. The center of the spraying point of the Laval nozzle coincides with the center of the laser spot on the substrate, and the diameter of the laser spot is set to 9 mm.

[0037] Nitrogen 1 is used as the high-pressure gas source, the spraying temperature is set to 450 °C, the spraying pressure is set to 3.5 MPa, and the rotation speed of the powder feeder 4 is set to 3 rpm.

[0038] At this time, the method for preparing the gradient composite material by laser-assisted cold spraying is as follows: First, spray on the substrate without enabling the laser 7. The moving speed of the robotic arm 5 is set to 200 mm / s, so that the thickness of the composite material obtained in this step reaches 1.5 mm, and the first composite coating 12 is obtained.

[0039] Then, during the spraying process, turn on the laser 7, and the power is set to 1 kW to in-situ heat the deposited composite material, so that the 2024 aluminum alloy in the deposited composite material is softened to a certain extent, and then the deposition efficiency of the subsequent Al 2 O 3 ceramics is improved, and the thickness of the deposited composite material reaches 3 mm, and the second composite coating 13 is obtained.

[0040] Subsequently, continue to increase the laser power, set it to 3 kW, and in-situ heat the deposited composite material to further improve the softening degree of the 2024 aluminum alloy in the deposited composite material, so as to further improve the deposition efficiency of the subsequent Al 2 O 3 ceramics to achieve the purpose of different Al 2 O 3 ceramic contents in the deposited composite material. The thickness of the deposited composite material reaches 5 mm, and the third composite coating 14 is obtained.

[0041] Through the above steps, a gradient composite material with gradually increasing Al 2 O 3 ceramic content from the inside to the outside is obtained.

[0042] It should be noted that the present invention can not only prepare a gradient composite material with gradually increasing ceramic content from the inside to the outside, but also prepare a gradient composite material with gradually decreasing ceramic content from the inside to the outside and a gradient composite material with first increasing and then decreasing ceramic content, corresponding to specific examples 2 and 3 respectively.

[0043] Specific example 2: The goal is to prepare a 6-mm-thick aluminum-based gradient composite material, and the ceramic content of the composite material gradually decreases from the inside to the outside.

[0044] First, fix the Laval nozzle 6 and the laser 7 on the robotic arm 5. The angle between the laser and the Laval nozzle is set to 30°. The upstream length of the Laval nozzle is 30 mm, the downstream length is 160 mm, the throat diameter is 2.7 mm, the outlet diameter is 8 mm, and the material of the Laval nozzle is ceramic.

[0045] The powder selected is spherical 7075 aluminum alloy 10 and SiC ceramic 11 with a particle size of 15 - 53 μm. Mix them according to a ceramic volume ratio of 35% in the mixed powder, put them into a ball mill and mix for 30 minutes, and then put them into the powder feeder 4.

[0046] The substrate selected is 7075 aluminum alloy 9. Sandblast the surface with 300 - mesh quartz sand, clean it with acetone and dry it. The substrate is perpendicular to the axis of the Laval nozzle, and the distance is set to 30 mm. The center of the spraying point of the Laval nozzle coincides with the center of the laser spot on the substrate, and the diameter of the laser spot is set to 9 mm.

[0047] Use nitrogen 1 as the high - pressure gas source, set the spraying temperature to 450 °C, the spraying pressure to 3.5 MPa, and the rotation speed of the powder feeder to 3 rpm.

[0048] At this time, the method for preparing gradient composite materials by laser - assisted cold spraying is as follows: Enable the laser 7 while starting spraying. The power of the laser is 5 kW, and the moving speed of the robotic arm 5 is set to 200 mm / s. In this step, the laser in - situ heats the substrate and the subsequently deposited composite materials, improving the deposition ability of the SiC ceramic powder in the mixed powder. The thickness of the composite material obtained in this step reaches 2.5 mm, that is, the first composite coating 12 is obtained.

[0049] Then, during the spraying process, reduce the power of the laser to 3 kW. Although it also in - situ heats the already deposited composite materials, the softening degree of the 7075 aluminum alloy in the already deposited composite materials in this step is reduced compared to the previous step, thereby reducing the deposition efficiency of the SiC ceramic powder in the mixed powder compared to the previous step, and making the thickness of the already deposited composite materials reach 4 mm, that is, the second composite coating 13 is obtained.

[0050] Subsequently, continue to reduce the laser power to 1 kW, further reducing the deposition efficiency of the SiC ceramic powder in this step. As spraying progresses, make the thickness of the already deposited composite materials reach 5 mm, that is, the third composite coating 14 is obtained. Finally, turn off the laser, and spraying continues until the thickness of the already deposited composite materials reaches 6 mm, that is, the fourth composite coating is obtained. Through the above steps, a gradient composite material with a gradually decreasing SiC ceramic content from the inside to the outside can be obtained.

[0051] Specific example 3: The goal is to prepare an aluminum-based gradient composite material with a thickness of 6 mm, and the ceramic content of the composite material first increases and then decreases from the inside to the outside.

[0052] First, fix the Laval nozzle 6 and the laser 7 on the robotic arm 5. The included angle between the laser and the Laval nozzle is set to 30°. The upstream length of the Laval nozzle is 30 mm, the downstream length is 160 mm, the throat diameter is 2.7 mm, the outlet diameter is 8 mm, and the material of the Laval nozzle is ceramic.

[0053] Select spherical 6061 aluminum alloy 10 with a particle size of 15 - 53 μm and WC ceramic 11 as the powder. Mix them according to a ceramic volume ratio of 40% in the mixed powder, put them into a ball mill and mix for 30 minutes, and then put them into the powder feeder 4.

[0054] Select 6061 aluminum alloy 9 as the substrate. Sandblast the surface with 300 - mesh quartz sand, clean and dry it with acetone. The substrate is perpendicular to the axis of the Laval nozzle, and the distance is set to 30 mm. The center of the spraying point of the Laval nozzle coincides with the center of the laser spot on the substrate, and the diameter of the laser spot is set to 9 mm.

[0055] Use nitrogen 1 as the high-pressure gas source, set the spraying temperature to 450 °C, the spraying pressure to 3.5 MPa, and the rotation speed of the powder feeder 4 to 3 rpm.

[0056] At this time, the method for preparing the gradient composite material by laser-assisted cold spraying is as follows: First, spray on the substrate without turning on the laser. Set the moving speed of the robotic arm 5 to 200 mm / s to make the thickness of the composite material obtained in this step reach 1.5 mm, that is, obtain the first composite coating 12.

[0057] Then, during the spraying process, turn on the laser 7, set the power to 2 kW, and in-situ heat the deposited composite material to soften the 6061 aluminum alloy in the deposited composite material to a certain extent, thereby improving the deposition efficiency of the subsequent WC ceramic to be deposited, and making the thickness of the deposited composite material reach 3 mm, that is, obtain the second composite coating 13.

[0058] Subsequently, continue to increase the laser power, set it to 4 kW, and in-situ heat the deposited composite material to further improve the softening degree of the 6061 aluminum alloy in the deposited composite material, so as to further improve the deposition efficiency of the subsequent WC ceramic to be deposited, in order to achieve the purpose of having a different WC ceramic content from that in the deposited composite material, and make the thickness of the deposited composite material reach 5 mm, that is, obtain the third composite coating 14.

[0059] Finally, turn off the laser, and the spraying continues until the thickness of the deposited composite material reaches 6 mm. Through the above steps, a gradient composite material with the WC ceramic content increasing first and then decreasing from the inside to the outside can be obtained.

[0060] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a gradient composite material by laser-assisted cold spraying, characterized in that: The following steps are involved: Cold spray deposition steps: Using high-pressure gas (1) with a pressure of 1.0 to 10.0 MPa and a temperature of not less than 350° C., the mixed powder provided by the powder feeder (4) is sprayed onto the surface of the substrate (9) through a Laval nozzle (6), the spraying movement speed of the Laval nozzle (6) being 10 to 2000 mm / s, and multiple spraying passes are performed until a first composite coating (12) with a thickness of 0.5 to 1.5 mm is deposited on the surface of the substrate (9); The mixed powder is a mixed powder of metal powder (10) and hard ceramic powder (11), the volume proportion of the ceramic powder (11) in the mixed powder is 10% to 90%, and the temperature of the high-pressure gas (1) is not higher than the melting point of the metal powder (10); Laser in-situ heating deposition steps: Controlling the light spot of the laser (7) and the spraying spot of the Laval nozzle (6) to overlap on the substrate (9), and the spraying spot is within the light spot, and starting to spray the first composite coating (12) with laser light spot synchronously with a laser power of 0.5 to 5 kW to perform multiple passes of laser in-situ heating spraying; until a second composite coating (13) with a thickness of 0.5 to 1.5 mm is deposited on the surface of the first composite coating (12); The laser in-situ heating is used to soften the flat metal powder (10) deposited on the surface of the first composite coating (12), and to enhance the deposition efficiency and embedding ability of the ceramic powder (11) on the first composite coating (12), thereby increasing the content of the ceramic powder (11) in the second composite coating (13); Finally, the laser in-situ heating deposition step is repeated until a gradient composite material with a thickness of 4 to 10 mm is obtained, and the laser power increased or decreased by 0.1 to 2 Kw each time the laser in-situ heating deposition step is repeated. The gradient composite material is a composite material formed with different ceramic deposition efficiencies along the spraying thickness direction.

2. The method for preparing a gradient composite material by laser-assisted cold spraying according to claim 1, characterized in that: The outlet diameter of the Laval nozzle (6) does not exceed the maximum spot diameter of the laser (7), and the Laval nozzle (6) and the laser (7) are both mounted on the robot arm (5) so that the angle between the axis of the Laval nozzle (6) and the axis of the laser (7) is 10 to 60 degrees; The spot diameter of the light spot is 2~10mm, and the spot diameter of the spray spot is 3~9mm.

3. The method for preparing a gradient composite material by laser-assisted cold spraying according to claim 1, characterized in that: The Laval nozzle (6) is a stainless steel or ceramic nozzle, the axis of the Laval nozzle (6) is perpendicular to the axis of the substrate (9), and during cold spraying, the distance between the outlet of the Laval nozzle (6) and the substrate (9) is 10 to 30 cm; The Laval nozzle (6) has an upstream length of 10-40 mm, a downstream length of 80-300 mm, a throat diameter of 2-3 mm, and an outlet diameter of 5-9 mm.

4. The method for preparing a gradient composite material by laser-assisted cold spraying according to claim 1, characterized in that: The mixed powder is obtained by mechanically mixing the metal powder (10) and the ceramic powder (11) in a ball mill for 30 to 50 minutes before the cold spraying step.

5. The method for preparing gradient composite materials by laser-assisted cold spraying according to claim 1, characterized in that: The metal powder (10) is Al, Cu, Ti, Mg, Fe, Ni, or any one of aluminum alloy, copper alloy, titanium alloy, magnesium alloy, steel, and nickel alloy; The ceramic powder (11) is SiC or TiN or Al2O3 or WC; The particle size of the mixed powder ranges from 1 to 100 μm, and the shape is spherical or irregular.

6. The method for preparing gradient composite materials by laser-assisted cold spraying according to claim 1, characterized in that: The high-pressure gas source (1) is nitrogen or helium or compressed air or argon or a mixture of any two or more of nitrogen, helium, compressed air and argon.

7. The method for preparing gradient composite materials by laser-assisted cold spraying according to claim 1, characterized in that: The substrate (9) is a flat plate substrate with a thickness of 1 to 6 mm; Before cold spraying, the surface was sandblasted with 150-450 mesh quartz sand and then cleaned and dried with acetone.

8. The method for preparing gradient composite materials by laser-assisted cold spraying according to claim 1, characterized in that: The rotation speed of the powder feeder (4) is 0.1-10 rpm.

9. The method for preparing a gradient composite material by laser-assisted cold spraying according to any one of claims 1 to 8, characterized in that: It also includes a powder feeder (4) for providing mixed powder, a heater (3) for heating the high-pressure gas (1), and a pressure control device (2) for controlling the pressure of the high-pressure gas (1); A portion of the high-pressure gas (1) is connected to the first pressure control device (21) and the inlet of the Laval nozzle (6) in sequence through a pipeline, and the powder feeder (4) is arranged on the pipeline between the pressure control device (21) and the Laval nozzle (6); at the same time, another portion of the high-pressure gas (1) is connected to the second pressure control device (22) and the inlet of the Laval nozzle (6) in sequence through a pipeline, and the heater (3) is arranged on the pipeline between the second pressure control device (22) and the Laval nozzle (6); the mixed powder provided by the powder feeder (4) is mixed with the high-pressure gas (1) heated by the heater (3) upstream of the Laval nozzle (6) to preheat the mixed powder.

10. The method for preparing a gradient composite material by laser-assisted cold spraying according to any one of claims 1 to 8, characterized in that: Also included is an infrared thermometer (8) for monitoring the temperature of the composite coating deposited on the substrate (9) and the temperature of the spraying point; The measuring range of the infrared thermometer (8) is 0-1100°C.

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