A method for forming a coating on the inner surface of a cylinder head metal mold and the cylinder head metal mold.

Abstract

This invention relates to the field of cylinder head technology for vehicle engines, and provides a method for forming a coating on the inner surface of a cylinder head metal mold and a cylinder head metal mold. The method of this invention forms a coating on the inner surface of the cylinder head metal mold, improving the service life of the mold coating and reducing casting defects. The method includes the following steps: (1) pre-treating the mold by heating and removing the oxide layer from the inner surface of the mold; (2) spraying a first liquid onto the area to be coated on the inner surface of the mold to form a first coating film, wherein the first liquid is a first paint diluted with water; (3) spraying a second liquid onto the surface of the first coating film to form a second coating film, wherein the second liquid is a second paint diluted with water; (4) sintering the mold to form the coating on the inner surface of the mold.

Description

Technical Field

[0001] This invention relates to the field of cylinder head technology for vehicle engines, and more specifically to a method for forming a coating on the inner surface of a cylinder head metal mold and a cylinder head metal mold. Background Technology

[0002] The cylinder head is a crucial component of a vehicle engine; its quality directly impacts the engine's lifespan. Commonly used cylinder heads are made of aluminum alloy and are typically formed using metal molds through low-pressure (anti-gravity) casting. To improve mold lifespan, facilitate cylinder head demolding, and reduce casting defects during the forming process, a coating is usually applied to the inner surface of the metal mold. This coating provides insulation, facilitates demolding, and improves the wettability of molten aluminum, thus reducing casting defects during cylinder head forming. Currently, the coating layer on the cylinder head metal mold has low bonding strength with the cylinder block, is prone to peeling and wear during use, has a short coating lifespan, and easily leads to casting defects such as under-casting and cold shuts.

[0003] Chinese invention patent CN103658562B discloses a "method for coating spraying on cylinder head metal casting mold and a stop block used therein". This patent describes a method for coating the cylinder head metal casting mold by using a stop block to cover the combustion chamber, effectively improving the uniformity of the coating thickness in the combustion chamber cavity during coating spraying, thus achieving a more stable combustion chamber shape and size. Summary of the Invention

[0004] This invention provides a method for forming a coating on the inner surface of a cylinder head metal mold and a cylinder head metal mold. Using the method of this invention to form a coating on the inner surface of the cylinder head metal mold can improve the wear resistance of the coating and the bonding strength between the coating and the mold, thereby improving the service life of the mold coating and reducing casting defects such as under-casting and cold shuts that may occur during cylinder head production.

[0005] To achieve its objective, the present invention provides the following technical solution:

[0006] This invention provides a method for forming a coating on the inner surface of a cylinder head metal mold, comprising the following steps:

[0007] (1) The mold is preheated and the inner surface of the mold is treated to remove the oxide layer;

[0008] (2) Spray the first liquid material onto the area to be coated on the inner surface of the mold to form a first coating film. The first liquid material is a first coating diluted with water. The first coating comprises the following components by mass percentage: 20-30% silica powder, 10-20% alumina powder, 1-7% magnesium oxide powder, 35%-45% binder, 2-8% titanium oxide powder, and 5%-25% water.

[0009] (3) A second liquid is sprayed onto the surface of the first coating film to form a second coating film. The second liquid is a second coating diluted with water. The second coating comprises the following components by mass percentage: 20-30% silica powder, 10-20% alumina powder, 1-7% magnesium oxide powder, 35%-45% binder, 2-8% titanium oxide powder, and 5%-25% water. Preferably, the water dilution ratio of the first coating in step (2) is greater than the water dilution ratio of the second coating in step (3).

[0010] (4) The mold is sintered to form the coating on the inner surface of the mold.

[0011] A second aspect of the present invention provides a cylinder head metal mold, wherein a coating is formed on the inner surface of the mold, the coating being formed using the method described above.

[0012] The technical solution provided by this invention has the following beneficial effects:

[0013] The method of the present invention forms a coating on the inner surface of the cylinder head metal mold. The resulting coating has good bonding strength with the mold and good wear resistance. It can improve the service life of the coating and reduce casting defects such as undercasting and cold shut in the cylinder head production process. Detailed Implementation

[0014] To facilitate understanding of the present invention, the following description, in conjunction with embodiments, will further illustrate the invention. It should be understood that the following embodiments are merely for a better understanding of the invention and do not imply that the invention is limited to these embodiments.

[0015] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The term "and / or" as used herein may include any and all combinations of one or more of the associated listed items. The terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0016] Where specific experimental steps or conditions are not specified in the examples, the corresponding conventional experimental steps or conditions in this technical field can be followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0017] This invention provides a method for forming a coating on the inner surface of a cylinder head metal mold, mainly comprising the following steps:

[0018] (1) The mold is preheated and the inner surface of the mold is treated to remove the oxide layer;

[0019] (2) Spray the first liquid material onto the area to be coated on the inner surface of the mold to form a first coating film. The first liquid material is a first coating diluted with water. The first coating comprises the following components by mass percentage: 20-30% silica powder, 10-20% alumina powder, 1-7% magnesium oxide powder, 35%-45% binder, 2-8% titanium oxide powder, and 5%-25% water.

[0020] (3) A second liquid is sprayed onto the surface of the first coating film to form a second coating film. The second liquid is a second coating diluted with water. The second coating comprises the following components by mass percentage: 20-30% silica powder, 10-20% alumina powder, 1-7% magnesium oxide powder, 35%-45% binder, 2-8% titanium oxide powder, and 5%-25% water. Preferably, the proportion of the first coating diluted with water in step (2) is greater than the proportion of the second coating diluted with water in step (3).

[0021] (4) The mold is sintered to form the coating on the inner surface of the mold.

[0022] In this invention, a first coating composed of specific components is used as a primer, and a second coating composed of specific components is applied to the surface of the primer. Following the construction steps of this invention, a coating layer is formed on the inner surface of the cylinder head metal mold. The resulting coating layer adheres tightly to the inner surface of the cylinder head metal mold, is not prone to peeling, and exhibits good wear resistance. This, in turn, helps improve the service life of the mold coating and reduces defects such as undercasting and cold shut. Specifically, the silica, alumina, and magnesium oxide in the first and second coatings, when combined in specific amounts, form a bridge-like structure of magnesium aluminum silicate during sintering. The resulting coating layer is tightly and firmly bonded, effectively improving the overall integrity of the coating. The addition of a specific amount of titanium oxide to the coating, in conjunction with other components, forms hard particles in the coating layer, improving the wear resistance of the coating and extending its service life.

[0023] In a preferred embodiment, in step (2), the mass ratio of the first coating to the water used for dilution is 1:1.8-2.5, for example, 1:1.8, 1:1.9, 1:2.0, 1:2.2, 1:2.5, etc.; and in step (3), the mass ratio of the second coating to the water used for dilution is 1:0.8-1.3, for example, 1:0.8, 1:1.0, 1:1.3, etc. The inventors have discovered that the first coating, obtained by appropriately diluting it in the above-mentioned specific ratio, can bond well with the inner surface of the mold and fully wet the inner surface of the mold, which facilitates the formation of an adhesive layer. Due to the heating pretreatment and oxide layer removal treatment in step (1), although the first coating is relatively thin, its moisture can dissipate quickly, and the first coating can firmly bond with the inner surface of the mold to form a base coating. The second coating, obtained by diluting it in the above-mentioned specific ratio, is a relatively thick second coating, which can improve the adhesion between the first coating film formed with the first coating and prevent the second coating film formed after spraying from flowing. It can form a denser top coating on the base coating, and the second coating has relatively less moisture, so the moisture is less likely to generate more voids during evaporation, resulting in a coating with better strength. Overall, by diluting the first and second coatings at the preferred specific dilution ratios, the base coating can be tightly bonded to the inner surface of the mold, and the top coating can be firmly bonded to the base coating. Ultimately, a coating with high bonding strength and excellent wear resistance is formed on the inner surface of the mold, which is beneficial to significantly improve the service life of the coating and significantly reduce casting defects such as undercasting and cold shut that exist in cylinder head metal molds during cylinder head production.

[0024] In some embodiments, the mass percentage of silica powder in the first coating is, for example, 20%, 23%, 25%, 28%, 30%, etc.; the mass percentage of alumina powder is, for example, 10%, 13%, 15%, 18%, 20%, etc.; the mass percentage of magnesium oxide powder is, for example, 1%, 3%, 5%, 7%, etc.; the mass percentage of binder is, for example, 35%, 37%, 40%, 42%, 45%, etc.; and the mass percentage of titanium oxide powder is, for example, 2%, 3%, 6%, 8%, etc. In some embodiments, the mass percentage of silica powder in the second coating is, for example, 20%, 23%, 25%, 28%, 30%, etc.; the mass percentage of alumina powder is, for example, 10%, 13%, 15%, 18%, 20%, etc.; the mass percentage of magnesium oxide powder is, for example, 1%, 3%, 5%, 7%, etc.; the mass percentage of binder is, for example, 35%, 37%, 40%, 42%, 45%, etc.; and the mass percentage of titanium oxide powder is, for example, 2%, 3%, 6%, 8%, etc. The mass percentages of each component in the first coating and the second coating may be the same or different.

[0025] In some specific embodiments, when diluting the first or second coating, the first or second coating is first diluted with water and stirred (e.g., stirred for 5-10 minutes) to disperse the coating evenly, and then filtered with a metal mesh to filter out larger foreign particles in the first or second liquid. This allows the first or second liquid to be sprayed more evenly.

[0026] Preferably, the first coating further includes 0.1-0.25% iron powder by mass, such as 0.1%, 0.13%, 0.15%, 0.2%, 0.25%, etc. The inventors have found that adding the above-mentioned amount of iron powder to the first coating used for direct contact with the inner surface of the mold makes it easier for the first liquid material to diffuse and fuse during the sintering process and form a metallurgical bond with the inner surface of the mold. This is beneficial to further improve the adhesion between the coating and the inner surface of the mold, improve the bonding strength of the coating, and thus further improve the service life of the mold coating and further reduce casting defects.

[0027] Optionally, the second coating also includes 0.1-0.25% by weight of iron powder, such as 0.1%, 0.13%, 0.15%, 0.2%, 0.25%, etc.

[0028] Preferably, the second coating further includes 3-5% by mass of chromium oxide powder, such as 3%, 4%, 5%, etc. The inventors have found that adding the above-mentioned amount of chromium oxide powder to the second coating, in combination with other components, can form hard spots during the sintering process, which is beneficial to further improve the hardness and wear resistance of the coating, thereby further improving the service life of the mold coating and further reducing casting defects.

[0029] Optionally, the first coating may further include 3-5% by weight of chromium oxide powder, such as 3%, 4%, 5%, etc.

[0030] In the text, "optionally" means either present or absent, for example, the optional component may or may not be added.

[0031] In a preferred embodiment, in step (2), the thickness of the first coating is 100μm-300μm, such as 100μm, 110μm, 150μm, 200μm, 250μm, 300μm, etc.; in step (3), the thickness of the second coating is 200μm-600μm, such as 200μm, 300μm, 400μm, 500μm, 600μm, etc.; preferably, the thickness of the first coating is less than the thickness of the second coating. Using the preferred thickness combination is beneficial for further improving the bonding strength of the coating on the inner surface of the mold, and also for further improving the wear resistance and service life of the coating. If the first and / or second coating is too thin, the coating may be easily worn, while if the first and / or second coating is too thick, the coating may easily peel off.

[0032] Furthermore, the binder in the first or second coating can be one or more of mineral oil, water glass, etc., which can improve the viscosity and adhesion of the coating. During spraying, it is beneficial for the coating to adhere to the mold surface, reduce or avoid coating flow, and improve the spraying quality.

[0033] Preferably, in step (1), the mold is heated to 330-360℃ and kept at that temperature for 2-4 hours; then cooled to 230-250℃ for the oxide layer removal treatment. The inventors have found that preheating the mold to 330-360℃ first, allowing the moisture in the metal mold to evaporate and keeping it dry, followed by cooling to 230-250℃ for surface oxide layer removal, can improve the bonding strength between the subsequent coating and the inner surface of the mold, thus improving the coating's adhesion. The inventors have also found that controlling the rust removal temperature between 230-250℃ prevents excessively high temperatures, resulting in a blue oxide layer forming on the surface after rust removal.

[0034] Preferably, in step (2), the spraying temperature is controlled at 190℃-230℃, and the spraying pressure is controlled at 0.2Kg / cm. 2 -0.6Kg / cm 2 Preferably, the spraying includes the following operations: first, spot spraying is performed on the inner surface of the mold, with each spraying time being 0.2-0.8 seconds; after spot spraying, a coating of 0.2 kg / cm² is applied. 2 -0.45Kg / cm 2 The surface of the mold is swept and sprayed under the spraying pressure, and the moving speed of the spray gun is controlled at 8-14CM / S;

[0035] Preferably, in step (3), the spraying temperature is controlled at 190℃-230℃, and the spraying pressure is controlled at 0.2Kg / cm. 2 -0.6Kg / cm 2Preferably, the spraying includes the following operations: first, spot spraying is performed on the inner surface of the mold, with each spraying time being 0.2-0.8 seconds; after spot spraying, a coating of 0.2 kg / cm² is applied. 2 -0.45Kg / cm 2 The surface of the mold is swept and sprayed under the spraying pressure, and the moving speed of the spray gun is controlled at 8-14CM / S;

[0036] The inventors have discovered that using the aforementioned preferred spraying temperature, spraying pressure, and spraying time when spraying the first and second liquid materials facilitates good adhesion and wear resistance in the resulting coating layer. It also provides good heat insulation and gap-filling effects, increases the bonding strength and wear resistance of the coating layer, and ultimately improves its service life. In this invention, the first and second liquid materials are sprayed in a point-spraying motion followed by a sweeping spray. The point-spraying increases the instantaneous spray pressure, enhancing coating adhesion; the subsequent sweeping spray allows for continuous coating application, improving the overall integrity of the coating adhering to the mold surface.

[0037] Preferably, when spraying is performed in step (2) or step (3), the nozzle of the spray gun forms an angle of 80-100° with the surface of the mold. The inventors have found that using the preferred spray angle can reduce or avoid paint rebound and splashing, thereby improving the spraying quality.

[0038] Preferably, during spraying in step (2), the distance between the nozzle of the spray gun and the inner surface of the mold is controlled at 20-30 cm; during spraying in step (3), the distance between the nozzle of the spray gun and the inner surface of the mold is controlled at 35-40 cm. The inventors have found that controlling the distance between the nozzle and the mold surface in steps (2) and (3) within the above range is beneficial for the sprayed paint to adhere to the mold surface, preventing the distance from being too close or too far, thus reducing the spraying quality.

[0039] Preferably, in step (4), the sintering conditions include: a sintering temperature of 450-600℃ and a sintering time ≥2h, such as 2h, 3h, 4h, etc. Sintering under the preferred sintering conditions allows the silica in the coating to form a bridging structure of magnesium aluminum silicate with alumina and magnesium oxide, which can further improve the tightness and firmness of the coating layer connection, and help reduce energy consumption and production costs. Using the preferred sintering temperature, compared with a lower sintering temperature, can effectively reduce the possibility of coating peeling. In some preferred embodiments, the heating furnace used for sintering is first heated to 195-205℃ and held for 0.5-1h, and then heated to the sintering temperature at a heating rate of 8-12℃ / min, which is conducive to the volatilization of moisture and harmful substances in the coating, and helps to further improve the sintering strength of the coating and further improve the service life of the coating layer.

[0040] Preferably, before spraying in step (2), the area on the inner surface of the mold that does not need to be sprayed is covered; further, the area that does not need to be sprayed includes the vent plug of the mold, which can reduce the amount of paint sprayed into the vent plug during the spraying process, thus preventing vent plug blockage. Preferably, the covering component used to cover the vent plug is provided with a magnet, and the covering component can be attracted to the vent plug by the magnet. Using magnet attraction facilitates the installation and removal of the covering component; the specific shape of the covering component is not particularly limited, and any shape that can be adapted to the vent plug and has the function of covering the vent plug is acceptable.

[0041] More preferably, after completing step (3) and before sintering in step (4), the covering component on the vent plug is removed, and the first liquid is sprayed onto the surface of the vent plug to form a third coating film. Preferably, the thickness of the third coating film is 10-20 μm, such as 10 μm, 13 μm, 15 μm, 17 μm, 20 μm, etc. Spraying the above-mentioned preferred third coating film onto the surface of the vent plug can prevent the vent plug from sticking to aluminum while ensuring the vent plug's ventilation (e.g., 0.03 MPa), facilitating demolding, and preventing vent plug blockage.

[0042] The present invention also provides a cylinder head metal mold, wherein a coating is formed on the inner surface of the mold, the coating being formed using the method described above.

[0043] The present invention will be further illustrated by the following examples.

[0044] Example 1

[0045] The steps for forming a coating on the inner surface of the cylinder head metal mold include:

[0046] (1) Place the cylinder head metal mold (made of mold steel) in a heating furnace, heat it to 330°C, keep it at that temperature for 3 hours, and then cool it down to between 230-250°C. Use an electric brush to remove the oxide layer from the inner surface of the mold.

[0047] (2) Cover the area of ​​the inner surface of the mold that does not need to be sprayed after step (1) with a cover plate, and cover the surface of the vent plug with a cover part (embedded with a magnet).

[0048] A first coating liquid with a thickness of 130 μm is sprayed onto the area to be coated. The first coating liquid is prepared as follows: the first coating material is diluted with water (the mass ratio of the first coating material to the diluting water is 1:2), mixed, stirred evenly, and after stirring for 6 minutes, the mixture is filtered through a metal mesh to obtain the first coating liquid; wherein the components of the first coating material are: 21 wt% silica powder, 18 wt% alumina powder, 2 wt% magnesium oxide powder, 40 wt% binder (water glass), 7 wt% titanium oxide powder, and the balance being water;

[0049] When spraying the first liquid coating, the spraying process conditions include: controlling the spraying temperature between 190℃ and 230℃, adding the first liquid coating into the spray gun, and controlling the spraying pressure at 0.2 kg / cm². 2 -0.6Kg / cm 2 First, spot spray the inner surface of the mold, with each blasting session lasting 0.2-0.8 seconds. After spot spraying, control the spraying pressure at 0.2 kg / cm². 2 -0.45Kg / cm 2 During the process, the mold surface is swept and sprayed, with the spray gun moving speed controlled between 8-14 cm / s. When spraying, the nozzle of the spray gun is perpendicular to the surface of the mold, and the distance between the nozzle and the inner surface of the mold is controlled between 20-30 cm.

[0050] (3) A second coating liquid is sprayed onto the surface of the first coating film to form a second coating film with a thickness of 480 μm. The second coating liquid is prepared as follows: the second coating is diluted with water (the mass ratio of the second coating to the diluting water is 1:1), mixed, stirred evenly, and then the mixture is filtered through a metal mesh to obtain the second coating liquid; the components of the second coating are: 21 wt% silica powder, 19 wt% alumina powder, 2 wt% magnesium oxide powder, 40 wt% binder, 8 wt% titanium oxide powder, and the balance being water;

[0051] When spraying the second liquid, the spraying process conditions include: controlling the spraying temperature between 190℃ and 230℃, adding the second liquid into the spray gun, and controlling the spraying pressure at 0.2 kg / cm². 2 -0.6Kg / cm 2 First, spot spray the inner surface of the mold, with each blasting session lasting 0.2-0.8 seconds. After spot spraying, control the spraying pressure at 0.2 kg / cm². 2 -0.45Kg / cm 2 Between spraying and spraying, the surface of the mold is swept and sprayed, and the moving speed of the spray gun is controlled between 8-14CM / S; during spraying, the nozzle of the spray gun is perpendicular to the surface of the mold, and the distance between the nozzle of the spray gun and the inner surface of the mold is controlled between 35-40cm.

[0052] (4) Remove the cover plate on the mold and the covering parts on the vent plug, and spray the first liquid in step (2) onto the surface of the vent plug to form a third coating film with a thickness of 15um; the spraying process in this step is the same as in step (2), and will not be repeated here.

[0053] (5) Place the mold in a heating furnace, heat the furnace to 200°C, hold for 0.6 hours, and then heat to 500°C at a heating rate of 10°C / min for 3 hours to form the coating on the inner surface of the mold.

[0054] Experimental results:

[0055] A dense coating was formed on the inner surface of the mold. Compared with the mold originally used in the workshop, the mold obtained in this embodiment was used for cylinder head casting, and the life of the mold coating was increased from more than 700 mold cycles to more than 900 mold cycles. The rates of undercasting and cold shut defects were reduced by about 2.2% and 1.23% respectively compared with the mold originally used.

[0056] Example 2

[0057] The procedure is the same as in Example 1, except that:

[0058] In step (2), the first coating consists of: 21 wt% silica powder, 18 wt% alumina powder, 2 wt% magnesium oxide powder, 40 wt% binder (water glass), 7 wt% titanium oxide powder, 0.15 wt% iron powder, and the remainder water.

[0059] The second coating in step (3) consists of: 21 wt% silica powder, 19 wt% alumina powder, 2 wt% magnesium oxide powder, 40 wt% binder, 8 wt% titanium oxide powder, 4 wt% chromium oxide powder, and the remainder water.

[0060] Experimental results:

[0061] Compared with the mold in Example 1, the coating life increased from more than 900 mold cycles to more than 1,000 mold cycles; the undercasting and cold shut defects also decreased by about 0.15% and 0.3%, respectively.

[0062] Example 3

[0063] The procedure is the same as in Example 1, except that:

[0064] In step (2), the components of the first coating are: 30wt% silica powder, 11wt% alumina powder, 7wt% magnesium oxide powder, 38wt% binder (water glass), 2wt% titanium oxide powder and the balance water; the thickness of the first coating is 170μm; the mass ratio of the first coating to the dilution water is 1:2.3.

[0065] In step (3), the second coating consists of: 29 wt% silica powder, 12 wt% alumina powder, 7 wt% magnesium oxide powder, 39 wt% binder, 3 wt% titanium oxide powder, and the remainder water; the thickness of the second coating is 440 μm. The mass ratio of the second coating to the diluting water is 1:0.8.

[0066] Experimental results: A dense coating was formed on the inner surface of the mold; compared with the mold originally used in the workshop, the life of the coating of the mold obtained in this embodiment was increased from more than 700 mold cycles to more than 900 mold cycles; the undercasting and cold shut defects were reduced by about 2.1% and 1.2% respectively compared with the mold originally used.

[0067] Example 4

[0068] The procedure is the same as in Example 1, except that:

[0069] In step (2), the components of the first coating are: 25wt% silica powder, 15wt% alumina powder, 4wt% magnesium oxide powder, 40wt% binder (water glass), 5wt% titanium oxide powder and the balance water; the thickness of the first coating is 150μm; the mass ratio of the first coating to the diluting water is 1:2.

[0070] In step (3), the second coating consists of: 25 wt% silica powder, 15 wt% alumina powder, 5 wt% magnesium oxide powder, 40 wt% binder, 4 wt% titanium oxide powder, and the remainder water; the thickness of the second coating is 460 μm. The mass ratio of the second coating to the diluting water is 1:1.

[0071] Experimental results: A dense coating was formed on the inner surface of the mold; compared with the mold originally used in the workshop, the mold obtained in this embodiment was used for cylinder head casting, and the life of the mold coating was increased from more than 700 mold cycles to more than 900 mold cycles; the undercasting and cold shut defects were reduced by about 2.4% and 1.26% respectively compared with the mold originally used.

[0072] Comparative Example 1

[0073] The procedure is the same as in Example 1, except that the first coating in step (2) is not applied, and the second coating in step (3) is applied directly. All other operations are the same as in Example 1.

[0074] Experimental results: It was found that the service life of the mold coating obtained in the comparative example was significantly reduced compared with that of Example 1, with the service life of the mold coating significantly reduced to more than 300 mold cycles; and the rate of undercasting and cold shut defects was significantly increased when using the mold of Comparative Example 1 for cylinder head casting.

[0075] It is readily understood that the above embodiments are merely illustrative examples for clear explanation and do not imply that the invention is limited thereto. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for forming a coating on the inner surface of a cylinder head metal mold, characterized in that, Includes the following steps: (1) The mold is preheated and the inner surface of the mold is treated to remove the oxide layer; (2) A first liquid is sprayed onto the inner surface of the mold to form a first coating film. The first liquid is a first paint diluted with water. The mass ratio of the first paint to the water used for dilution is 1:1.8-2.

5. The first paint includes the following components in the following mass percentages: 20-30% silica powder, 10-20% alumina powder, 1-7% magnesium oxide powder, 35%-45% binder, 2-8% titanium oxide powder, 0.1-0.25% iron powder, and 5%-25% water. (3) A second liquid is sprayed onto the surface of the first coating film to form a second coating film. The second liquid is a second coating material diluted with water. The mass ratio of the second coating material to the water used for dilution is 1:0.8-1.

3. The second coating material includes the following components in the following mass percentages: 20-30% silica powder, 10-20% alumina powder, 1-7% magnesium oxide powder, 35%-45% binder, 2-8% titanium oxide powder, 3-5% chromium oxide powder, and 5%-25% water. (4) The mold is sintered to form the coating on the inner surface of the mold.

2. The method according to claim 1, characterized in that, The second coating also includes 0.1-0.25% iron powder by mass.

3. The method according to any one of claims 1-2, characterized in that, The first coating also includes 3-5% chromium oxide powder by weight.

4. The method according to any one of claims 1-2, characterized in that, In step (2), the thickness of the first coating film is 100μm-300μm; In step (3), the thickness of the second coating is 200μm-600μm.

5. The method according to claim 4, characterized in that, The thickness of the first coating is less than the thickness of the second coating.

6. The method according to any one of claims 1-2, characterized in that, The binder in the first coating or the second coating is one or more of mineral oil and water glass.

7. The method according to any one of claims 1-2, characterized in that, In step (1), the mold is heated to 330-360℃ and kept at that temperature for 2-4 hours; then the temperature is lowered to 230-250℃ to perform the deoxidation treatment.

8. The method according to any one of claims 1-2, characterized in that, In step (2), the spraying temperature is controlled at 190℃-230℃, and the spraying pressure is controlled at 0.2Kg / cm. 2 -0.6Kg / cm 2 ; And / or, in step (3), the spraying temperature is controlled at 190℃-230℃, and the spraying pressure is controlled at 0.2Kg / cm. 2 -0.6Kg / cm 2 .

9. The method according to claim 8, characterized in that, In step (2), the spraying includes the following operations: first, spot spraying is performed on the inner surface of the mold, with each spraying time being 0.2-0.8 seconds; after spot spraying is completed, a coating of 0.2 kg / cm² is applied. 2 -0.45Kg / cm 2 The surface of the mold is swept and sprayed under the spraying pressure, and the moving speed of the spray gun is controlled at 8-14CM / S; And / or, in step (3), the spraying includes the following operations: first, spot spraying is performed on the inner surface of the mold, with each spraying time being 0.2-0.8 seconds; after spot spraying is completed, a 0.2 kg / cm² spraying solution is applied. 2 -0.45Kg / cm 2 The surface of the mold is swept and sprayed under the spraying pressure, and the moving speed of the spray gun is controlled at 8-14CM / S.

10. The method according to claim 8, characterized in that, When spraying is performed in step (2) or step (3), the nozzle of the spray gun forms an angle of 80-100° with the surface of the mold; When spraying in step (2), the distance between the nozzle of the spray gun and the inner surface of the mold is controlled at 20-30cm; When spraying in step (3), the distance between the nozzle of the spray gun and the inner surface of the mold is controlled at 35-40cm.

11. The method according to any one of claims 1-2, characterized in that, In step (4), the sintering conditions include: sintering temperature of 450-600℃ and sintering time of ≥2h.

12. The method according to claim 11, characterized in that, In step (4), the heating furnace used for sintering is first heated to 195-205℃ and held for 0.5-1h, and then heated to the sintering temperature at a heating rate of 8-12℃ / min.

13. The method according to any one of claims 1-2, characterized in that, Before performing the spraying in step (2), cover the areas of the inner surface of the mold that do not need to be sprayed.

14. The method according to claim 13, characterized in that, The area that does not require painting includes the vent plug of the mold; a covering component for covering the vent plug is provided with a magnet, and the covering component can be attracted to the vent plug by the magnet.

15. The method according to claim 14, characterized in that, After completing step (3), before sintering in step (4), remove the covering part on the exhaust plug and spray the first liquid onto the surface of the exhaust plug to form a third coating.

16. The method according to claim 15, characterized in that, The thickness of the third coating is 10-20 μm.

17. A cylinder head metal mold, characterized in that, The inner surface of the mold is coated with a coating formed by the method described in any one of claims 1-16.