A coating for aluminum-lithium alloy sand casting and its preparation method

Abstract

This invention discloses a coating for sand casting of aluminum-lithium alloys and its preparation method. The coating comprises a surface layer in contact with the sand mold and a top layer in contact with the melt. The surface layer material is an alcohol-based zirconium oxide coating containing graphene, and the top layer coating is an alcohol-based boride coating containing titanium diboride and zirconium diboride. This invention effectively improves the coating density and heat transfer rate by doping graphene with metal boride nanoparticles. Using graphene, ceramic materials, and metal borides as aggregates and dopants effectively suppresses the burn-off of light elements such as Li, reducing subcutaneous pinholes and porosity defects in the casting. Using zirconium oxide powder as aggregate increases nucleation sites, effectively improving the quality of non-ferrous metal castings.

Description

Technical Field

[0001] This invention belongs to the field of casting and casting auxiliary materials, specifically relating to a coating for aluminum-lithium alloy sand casting and its preparation method. Background Technology

[0002] Aluminum-lithium alloys possess numerous advantages, including low density, high specific strength and stiffness, and good corrosion resistance, leading to their widespread application in aerospace, transportation, and other fields. Among these, cast aluminum-lithium alloys, with their even lower density, excellent process flow properties, and load-bearing capacity, show promising prospects for large and complex structural components such as torpedo casings, engine casings, and cylinder blocks. Sand casting is a common method for producing large and complex castings, but the high reactivity of lithium, which readily reacts violently with binders in the sand mold, results in numerous defects such as porosity and oxide inclusions within the melt, limiting the application of cast aluminum-lithium alloys. To improve the quality of sand castings, a casting coating is typically applied to the working surface of the mold in the aluminum alloy sand casting industry. This coating primarily blocks direct contact between the molten metal and the sand mold surface, eliminating physical and chemical reactions between the molten metal and the sand mold surface.

[0003] Currently, commonly used coatings for aluminum alloys are prone to reacting with aluminum-lithium alloys. The studies by An Geying et al. of Harbin Institute of Technology ("Interaction between Aluminum-Lithium Alloys and Mold Surfaces") and Jiang Wenming et al. of Huazhong University of Science and Technology ("Investigation on Characteristic and Formation Mechanism of Porosity Defects of Al-Li Alloys Prepared by Sand Casting") investigated the effects of different sand molds (phenolic resin, furan resin, etc.) and different coatings (zirconia, graphite, etc.) on defects and microstructure of aluminum-lithium alloys prepared by sand casting. The results showed that the sources of defects in the sand casting process of aluminum-lithium alloys mainly fall into four categories: (1) Due to the high reactivity of Li, it reacts with water vapor, oxygen, nitrogen, carbon dioxide, etc. in the air, easily absorbing gas and generating subcutaneous pores; (2) The binders in molding sands such as ordinary clay sand, water glass sand, and resin sand react with the melt, increasing lithium burn-off. Simultaneously, impurities such as pores and lithium-containing compounds generated by the reaction exist inside the melt, reducing the mechanical properties of the aluminum-lithium alloy. (3) When the molding sand binder is heated, it is easy to generate nitrogen-containing and oxygen-containing gases, which react with Li and other substances in the melt, resulting in casting defects such as inclusions. (4) The poor permeability of coatings such as graphite and zirconium oxide causes gases to accumulate on the surface of the molding sand, creating negative pressure that enters the interior of the ingot and affects the surface quality and metallurgical quality of the casting.

[0004] A limited number of studies have been conducted on coating modification to address the challenges in sand casting of aluminum-lithium alloys. Patent and literature searches revealed that CN114985672 discloses the use of lithium silicate as a binder for sand casting of aluminum-lithium alloys. This method effectively inhibits the reaction at the interface between the aluminum-lithium alloy and the sand casting by improving the binder, thus improving the ingot quality. However, lithium silicate is insoluble in alcohols and organic solvents, but generally soluble in water or alkaline solutions. Alcohol-based coatings prepared with this method tend to settle, and water-based coatings increase the reactivity of the aluminum-lithium alloy. CN111085658 discloses a multi-layer composite mold coating for sand casting of aluminum-lithium alloys and its coating method. This method uses a composite coating of graphite, low-concentration hexachloroethane, and high-concentration hexachloroethane to coat the sand mold surface, avoiding direct contact between the molten metal and the sand mold, inhibiting interfacial reactions, and simultaneously refining the melt and improving the surface quality of the casting. However, deviations in hexachloroethane content can cause a gas pressure difference on the surface of the metal ingot, preventing gas from escaping from the sand mold and allowing it to enter the surface of the metal ingot, thus damaging the ingot quality.

[0005] Therefore, to solve the above problems, it is necessary to develop an alcohol-based coating suitable for sand casting that can effectively inhibit interfacial reactions, prevent sand adhesion to ingots, and improve surface quality. At the same time, the coating must ensure good air permeability, anti-sand adhesion, adhesion, and resistance to metal erosion. Summary of the Invention

[0006] To address the problems in aluminum-lithium alloy sand casting, effectively reduce casting defects such as sand adhesion and inclusion, block direct contact between molten metal and the sand mold surface, and inhibit physical or chemical reactions between the molten metal and the sand mold surface, this invention develops a coating for aluminum-lithium alloy sand casting that inhibits interfacial reactions, along with its preparation method. This coating effectively inhibits the reaction between the aluminum-lithium alloy and the sand mold, while also exhibiting good permeability, anti-sand adhesion, and anti-metal erosion properties.

[0007] To achieve the above effects, this invention provides an aluminum-lithium alloy sand casting alloy coating and its preparation method. This method differs significantly from existing aluminum-lithium alloy coating methods, which cannot guarantee the surface quality of the ingot. This method combines a composite coating with a coating process, utilizing the stability of materials such as SiC and BN in molten aluminum-lithium alloy, and employing high-temperature resistant, corrosion-resistant aggregates and binders to provide a durable coating for aluminum-lithium alloy sand casting. This coating exhibits excellent isolation, fluidity, resistance to metal erosion, permeability, and anti-sand adhesion properties, while effectively inhibiting the reaction between the aluminum-lithium alloy and the sand mold.

[0008] To achieve the above objectives, the present invention is implemented through the following techniques:

[0009] This invention relates to a coating for aluminum-lithium alloy sand casting. The coating consists of two layers, including a surface layer in contact with the sand mold and a top layer in contact with the melt. The surface layer material is an alcohol-based zirconium oxide coating containing graphene, and the top layer coating is an alcohol-based boride coating containing titanium diboride and zirconium diboride.

[0010] Preferably, the alcohol-based zirconia coating aggregate is composed of the following weight percentages: graphene 5-8%, mullite 10-16%, kyanite 5-15%, silicon dioxide 10-15%, and the balance being zirconia.

[0011] Preferably, the alcohol-based boride coating aggregate is formulated as follows by mass percentage: 10-27% titanium diboride, 8-28% zirconium diboride, 8-16% talc, 10-16% silicon nitride powder, and 12-18% boron nitride powder.

[0012] Preferably, the aggregate in the alcohol-based zirconium oxide coating accounts for 27-46% by mass; the aggregate in the alcohol-based boride coating accounts for 45-57% by mass.

[0013] Preferably, the alcohol-based zirconium oxide coating components further include polyvinyl butyral, sodium carboxymethyl cellulose, lithium-based bentonite, sodium-based bentonite, isopropanol, silicate resin, ammonium zirconium carbonate, modified rosin, polyethylene glycol, preservative, and ethanol.

[0014] Furthermore, the alcohol-based zirconium oxide coating is obtained by the following preparation method:

[0015] (1) Put the refractory aggregates mullite, kyanite, zirconium oxide, graphene and silicon dioxide into a dry powder mixer and mix them evenly for 20 to 40 minutes.

[0016] (2) Add the mixed dry powder to a wet ball mill, add polyvinyl butyral, sodium carboxymethyl cellulose, lithium-based bentonite, and sodium-based bentonite, and ball mill for 3-5 hours at a speed of 500-1000 r / min.

[0017] (3) Add the mixed slurry to a high-speed mixer, add silica resin, ammonium zirconium carbonate and modified rosin, and stir at 800-2000 r / min for 30-60 min. Then add isopropanol, polyethylene glycol and ethanol and stir at 500-100 r / min for 5-10 min. Add the preservative to the prepared coating and stir at 200-600 r / min for 5-10 min.

[0018] Preferably, the alcohol-based boron compound coating group further includes lithium-based bentonite, organoclay, calcium carbonate, cryolite, nano-titanium oxide powder, carbon fiber powder, stabilizer, phenolic resin, polyvinyl alcohol or polyvinyl butyral, methanol, and ethanol.

[0019] Furthermore, the alcohol-based boride coating is obtained by the following preparation method:

[0020] (1) Place silicon nitride powder, zirconium oxide powder, boron nitride powder, carbon fiber powder and ethanol into a wet ball mill and ball mill for 3-5 hours at a speed of 200-290 r / min to prepare slurry A for later use.

[0021] (2) Add titanium diboride and zirconium diboride to acetone and sonicate and disperse them. Then, dry them in a vacuum drying oven and place them in methanol for later use.

[0022] (3) Lithium-based bentonite, organic clay, cryolite, calcium carbonate, nano titanium dioxide powder, phenolic resin, polyvinyl alcohol or polyvinyl butyral are added to methanol solution and ultrasonically vibrated for 30-50 min to disperse them evenly in the solution. Then, they are placed in a wet ball mill and ball-milled for 1-2 h at a speed of 170-230 r / min to prepare slurry B for later use.

[0023] (4) Add ethanol to the slurry prepared in steps (1), (2), and (3), put it into a mill, stir it evenly, and then add stabilizer and talc to prepare coating.

[0024] This invention also relates to a method of applying (coating method) the composite coating for aluminum-lithium alloy sand casting, comprising the following steps:

[0025] S1. Stir the alcohol-based zirconia coating for 3-5 minutes, and apply the zirconia coating to the surface of the sand casting mold by brushing. The coating thickness should be ≤0.2mm. After multiple brushings, ignite the surface coating and calcify to obtain the alcohol-based surface coating. After drying for 5-15 minutes, polish the surface until the thickness is uniform.

[0026] S2. Stir the alcohol-based boride coating for 3-5 minutes, and then spray it evenly onto the surface of the sand mold at a distance of 8-20 cm from the surface. After drying for 3-6 minutes, ignite and burn to dry to obtain the alcohol-based expanded coating.

[0027] Furthermore, the thickness of the alcohol-based zirconia coating is less than 0.1 mm, the thickness of the alcohol-based boride coating is less than 0.15 mm, and the total coating thickness should be ≤0.2 mm. To ensure the permeability of the casting during solidification and to suppress the reaction between the aluminum-lithium alloy and the sand mold, the total coating thickness should be as thin as possible. Moreover, to avoid reactions caused by excessively thin coating during the filling process, the coating thickness should be maintained at a certain level. Specifically, to prevent coating detachment during pouring and to ensure the permeability of sand casting, the alcohol-based zirconia coating thickness should be ≤0.1 mm; to increase the nucleation sites during solidification and improve the surface quality of the casting, the alcohol-based boride coating thickness should not exceed 0.15 mm.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) This invention provides a composite coating for sand casting and its preparation method. The coating is effectively improved by doping with graphene and metal boride nanoparticles, which increases the density of the coating and the heat transfer rate. The use of graphene, ceramic materials, and metal boride as aggregates and dopants can effectively suppress the burning loss of light elements such as Li and reduce the defects of subcutaneous pinholes and pores in castings. The use of zirconium oxide powder as aggregates increases the nucleation sites and improves the quality of non-ferrous metal castings to a limited extent.

[0030] (2) By coating the surface of the sand mold with zirconia coating containing graphene and alcohol-based boride coating containing titanium diboride and zirconium diboride, the surface finish of the casting is high. The combined use of the two reduces the hydrogen content and effectively controls the chemical reaction between the melt and the mold interface. The coating has the advantages of good air permeability and thermal stability, which reduces the tendency of aluminum-lithium alloy casting to have porosity and microcracks and improves the defects of traditional alcohol-based coatings. Attached Figure Description

[0031] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0032] Figure 1 These are schematic diagrams showing the dimensions of the Y-shaped sand casting molds used in Examples 1-2 and Comparative Examples 1-4.

[0033] Figure 2 This is a schematic diagram of the X-ray dimensions of the casting sand mold for the embodiments; where A represents Embodiment 1 and B represents Embodiment 2.

[0034] Figure 3 This is a schematic diagram of the X-ray dimensions of the sand mold used for comparative example 1. Detailed Implementation

[0035] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0036] This invention relates to a method for preparing a coating for aluminum-lithium alloy sand casting. Except for differences in composition, the coating method and coating thickness remain consistent.

[0037] Alcohol-based zirconia coatings are obtained through the following preparation method:

[0038] (1) Put the refractory aggregates mullite, kyanite, zirconium oxide, graphene and silicon dioxide into a dry powder mixer and mix them evenly for 20 minutes.

[0039] (2) Add the mixed dry powder to a wet ball mill, add polyvinyl butyral, sodium carboxymethyl cellulose, lithium-based bentonite, and sodium-based bentonite, and mill for 3 hours at a speed of 600 r / min.

[0040] (3) Add the mixed slurry to a high-speed mixer, add silicate resin, ammonium zirconium carbonate, and modified rosin, and stir at 1200 r / min for 60 min. Then add isopropanol, polyethylene glycol, and ethanol and stir at 600 r / min for 5 min. Add the preservative to the prepared coating and stir at 300 r / min for 6 min.

[0041] Alcohol-based boride coatings are obtained through the following preparation method:

[0042] S1. Weigh out silicon nitride powder, zirconium oxide powder, boron nitride powder, carbon fiber powder and ethanol and put them into a wet ball mill and mix for 20-40 minutes. Set aside for later use.

[0043] S2, titanium diboride, and zirconium diboride boride particles were added to acetone and ultrasonically dispersed, then dried under vacuum in a vacuum drying oven and placed in methanol for later use.

[0044] S3. Lithium-based bentonite, organic clay, cryolite, calcium carbonate, nano titanium dioxide powder, phenolic resin, and polyvinyl alcohol are added to a methanol solution and ultrasonically vibrated for 30 minutes to disperse them evenly in the solution. Then, the mixture is placed in a wet ball mill and ball-milled for 1 hour at a speed of 180 r / min to prepare a slurry for later use.

[0045] S4. Add the slurry prepared in S1 and S2 to the slurry prepared in S3, add ethanol and put it into a ball mill. After stirring evenly, add stabilizer and talc to prepare coating.

[0046] The coating method of this invention is as follows:

[0047] (1) Stir the alcohol-based zirconia coating for 5 minutes, and apply the zirconia coating to the surface of the sand casting mold by brushing. The coating thickness should be ≤0.2mm. After multiple brushings, ignite the surface coating and burn it to obtain the alcohol-based surface coating. After drying for 10 minutes, polish the surface until the thickness is uniform.

[0048] (2) Stir the alcohol-based boride coating for 3 minutes, and spray it evenly on the surface of the sand mold at a distance of 15 cm from the surface. After drying for 3 minutes, ignite and burn to dry to obtain the alcohol-based expanded coating.

[0049] Example 1

[0050] The aggregate for alcohol-based zirconia coatings consists of 5% graphene (particle size 200 mesh, purity ≥99.6%), 63% zirconia (ZrO2 particle size ≤40μm, purity ≥99.3%), 10% silica (SiO2 particle size ≤60μm, purity ≥99.9%), 12% mullite, and 10% kyanite.

[0051] Alcohol-based zirconia coatings were prepared using alcohol-based zirconia coating aggregates. The coating formulation consisted of the following raw materials in weight percentages: aggregates 45%, polyvinyl butyral 0.6%, sodium carboxymethyl cellulose 1.2%, lithium-based bentonite 1.2%, sodium-based bentonite 1.5%, isopropanol 3.5%, silicate resin 0.5%, ammonium zirconium carbonate 0.5%, modified rosin 0.5%, polyethylene glycol 2%, preservative (Nordes EPW) 0.5%, and the balance being ethanol.

[0052] The refractory aggregate for boron nitride coating consists of 15% titanium diboride, 20% zirconium diboride, 25% silicon nitride powder (SiC, particle size ≤23μm, purity ≥99.6%), 16% boron nitride powder (BN, particle size ≤74μm, purity ≥99.9%), and the remainder is talc powder.

[0053] Alcohol-based boron nitride coatings were prepared using refractory aggregates. The formulation of the alcohol-based cast aluminum-lithium coating consisted of the following raw materials in weight percentages: 45% refractory aggregates, 4% lithium-based bentonite, 2% organic clay, 4% calcium carbonate, 3% cryolite, 15% nano-titanium oxide powder, 8% carbon fiber powder, 0.5% stabilizer (Dispersogen SP Plus, Clariant), 0.5% phenolic resin, 2% polyvinyl alcohol, and the balance being methanol and ethanol.

[0054] Prepare the coating according to the aforementioned method. Figure 1 The diagram shows the casting mold. The aforementioned method was used to coat and apply the coating to the mold surface before pouring. The casting surface was then cleaned, and it was found to have good surface quality with no quality issues. Figure 2 (A) is an X-ray schematic diagram of the sand casting process. Figure 2 As can be seen from (A), there are no macroscopic pores in the part where the casting contacts the surface of the mold.

[0055] Example 2

[0056] The aggregate for alcohol-based zirconia coatings consists of 7% graphene (particle size 200 mesh, purity ≥99.6%), 56% zirconia (ZrO2 particle size ≤40μm, purity ≥99.3%), 12% silica (SiO2 particle size ≤60μm, purity ≥99.9%), 15% mullite, and 10% kyanite.

[0057] Alcohol-based zirconia coatings were prepared using alcohol-based zirconia coating aggregates. The coating formulation consisted of the following raw materials in weight percentages: 35% aggregate, 0.8% polyvinyl butyral, 1.2% sodium carboxymethyl cellulose, 1.5% lithium-based bentonite, 1% sodium-based bentonite, 3% isopropanol, 0.5% silicate resin, 0.5% ammonium zirconium carbonate, 0.5% modified rosin, 1% polyethylene glycol, 0.5% preservative (Nordes EPW), and the balance being ethanol.

[0058] The refractory aggregate consists of 10% titanium diboride, 35% zirconium diboride, 20% silicon nitride powder (SiC, particle size ≤23μm, purity ≥99.6%), 20% boron nitride powder (BN, particle size ≤74μm, purity ≥99.9%), and the remainder is talc powder.

[0059] Alcohol-based coatings are prepared using refractory aggregates. The formulation of alcohol-based cast aluminum-lithium coatings consists of the following raw materials in weight percentages: 55% refractory aggregates, 4% lithium-based bentonite, 1% organic clay, 1.5% calcium carbonate, 4% cryolite, 12% nano-titanium oxide powder, 2% carbon fiber powder, and 0.6% stabilizer (Dispersogen SP Plus, Clariant), 1% phenolic resin, 1% polyvinyl alcohol, with the balance being methanol and ethanol.

[0060] Prepare the coating according to the aforementioned method. Figure 1 The diagram shows the casting mold. The aforementioned method was used to coat and apply the coating to the mold surface before pouring. The casting surface was then cleaned, and the surface quality was found to be good. Figure 2 (B) is an X-ray schematic diagram of the sand mold used for casting. Figure 2 As can be seen from (B), there are no macroscopic pores in the part where the casting contacts the surface of the mold.

[0061] Example 3

[0062] The alcohol-based zirconia coating aggregate consists of 8% graphene (particle size 200 mesh, purity ≥99.6%), 55% zirconia (ZrO2 particle size ≤40μm, purity ≥99.3%), 15% silica (SiO2 particle size ≤60μm, purity ≥99.9%), 10% mullite, and 12% kyanite.

[0063] Alcohol-based zirconia coatings are prepared using alcohol-based zirconia coating aggregates. The formulation consists of the following raw materials in weight percentages: 27% aggregate, 0.6% polyvinyl butyral, 1% sodium carboxymethyl cellulose, 1.3% lithium bentonite, 1.6% sodium bentonite, 1.7% isopropanol, 0.8% silicate resin, 0.6% ammonium zirconium carbonate, 0.7% modified rosin, 1.5% polyethylene glycol, 0.5% preservative (Nordes EPW), and the balance being ethanol.

[0064] The refractory aggregate consists of 35% titanium diboride, 15% zirconium diboride, 12% silicon nitride powder (SiC, particle size ≤23μm, purity ≥99.6%), 16% boron nitride powder (BN, particle size ≤74μm, purity ≥99.9%), and the remainder is talc powder. Alcohol-based refractory coatings are prepared using the refractory aggregate.

[0065] The alcohol-based coating formulation consists of the following raw materials in weight percentages: 50% refractory aggregate, 2% lithium-based bentonite, 0.8% organic clay, 1.2% calcium carbonate, 3.4% cryolite, 12% nano titanium dioxide powder, 6% carbon fiber powder, and 0.5% stabilizer (Dispersogen SP Plus, Clariant), 1% phenolic resin, 1% polyvinyl alcohol, with the balance being methanol and ethanol.

[0066] Prepare the coating according to the aforementioned method. Figure 1 The diagram shows the dimensions of the sand casting. The aforementioned method was used to coat and apply the coating to the surface of the mold before casting. The surface of the casting was cleaned, and the surface quality was found to be good. X-ray imaging showed that there were no macroscopic pores in the part of the casting that was in contact with the surface of the mold.

[0067] Comparative Example 1

[0068] The coating used is a zircon powder alcohol-based coating, a sand casting coating commonly used for aluminum alloys. The raw materials are: 0.3% oxidized rosin resin and 0.8% thermoplastic phenolic resin by mass percentage. The weight ratio of the components in the coating is: 72% zircon powder, 0.6% oxidized rosin resin, 0.8% thermoplastic phenolic resin, 1.6% suspending agent, 0.9% additives, and the remainder isopropanol.

[0069] Stir the coating for 5 minutes, then apply it to the surface of the sand mold by brushing. The coating thickness should be ≤0.2mm. After multiple brushings, ignite the surface coating and burn it to obtain an alcohol-based surface coating. After drying for 10 minutes, polish the surface until the thickness is uniform. Then, spray the coating evenly onto the surface of the sand mold at a distance of 15cm from the surface and move at a uniform speed. After drying for 3 minutes, ignite it and burn it to dry to obtain the coating. Figure 1 The diagram shows the dimensions of the sand casting. The coating was applied to the surface of the mold before pouring. The surface of the casting was cleaned, and it was found that there were a large number of pores on the surface of the casting, the surface quality was poor, and there was a large area of ​​sand adhering, which was difficult to clean. X-ray imaging showed that there were a large number of macroscopic pores in the part of the casting that was in contact with the surface of the mold.

[0070] Comparative Example 2

[0071] The coating used is a zirconium oxide coating, a sand casting coating commonly used for aluminum alloys. The mass percentage of the raw materials is as follows: lithium-based bentonite 32%, titanium dioxide 3%, sodium dodecyl sulfate 5%, expandable graphite 38%, quartz powder 6%, hexadecyltrimethylammonium chloride 0.3%, litchi leaf 10%, linseed oil 1%, stearamide 4%, hexamethylenetetramine 5%, ethanol 16%, and the remainder isopropanol.

[0072] Stir the coating for 5 minutes, then apply it to the surface of the sand mold by brushing. The coating thickness should be ≤0.2mm. After multiple brushings, ignite the surface coating and burn it to obtain an alcohol-based surface coating. After drying for 10 minutes, polish the surface until the thickness is uniform. Then, spray the coating evenly onto the surface of the sand mold at a distance of 15cm from the surface and move at a uniform speed. After drying for 3 minutes, ignite it and burn it to dry to obtain the coating. Figure 1 The diagram shows the dimensions of the sand casting. The coating was applied to the surface of the mold before pouring. The surface of the casting was cleaned and found to be of good quality. However, X-ray imaging revealed a large number of macroscopic pores in the area where the casting and the mold surface were in contact.

[0073] Comparative Example 3

[0074] This comparative example is basically the same as Example 1, except that:

[0075] The refractory aggregate for boron nitride coatings consists of 25% silicon nitride powder (SiC, particle size ≤23μm, purity ≥99.6%), 51% boron nitride powder (BN, particle size ≤74μm, purity ≥99.9%), and the remainder is talc powder.

[0076] The coating was prepared according to the method in Example 1. Figure 1 The diagram shows the dimensions of the sand casting. The aforementioned method was used to coat and apply the coating to the surface of the mold before pouring. The surface of the casting was cleaned, and it was found that the casting was stuck with sand. The quality of the casting was poor after cleaning. X-ray imaging revealed a large number of porosity defects on the inner and outer surfaces of the casting.

[0077] Comparative Example 4

[0078] This comparative example is basically the same as Example 1, except that:

[0079] The refractory aggregate for boron nitride coating consists of 35% titanium diboride, 25% silicon nitride powder (SiC, particle size ≤23μm, purity ≥99.6%), 16% boron nitride powder (BN, particle size ≤74μm, purity ≥99.9%), and the remainder is talc powder.

[0080] The coating was prepared according to the method in Example 1. Figure 1The diagram shows the dimensions of the sand casting. The aforementioned method was used to coat and apply the coating to the surface of the mold before pouring. The surface of the casting was cleaned, and it was found that the casting was stuck with sand. The quality of the casting was poor after cleaning. X-ray imaging revealed a large number of porosity defects on the surface of the casting.

[0081] Comparative Example 5

[0082] This comparative example is basically the same as Example 1, except that:

[0083] The refractory aggregate for boron nitride coating consists of 35% zirconium diboride, 25% silicon nitride powder (SiC, particle size ≤23μm, purity ≥99.6%), 16% boron nitride powder (BN, particle size ≤74μm, purity ≥99.9%), and the remainder is talc powder.

[0084] The coating was prepared according to the method in Example 1. Figure 1 The diagram shows the dimensions of the sand casting. The aforementioned method was used to coat and apply the coating to the surface of the mold before pouring. The surface of the casting was cleaned, and it was found that the casting was stuck with sand. The quality of the casting was poor after cleaning. X-ray imaging revealed a large number of porosity defects inside the casting.

[0085] Comparative Example 6

[0086] This comparative example is basically the same as Example 1, except that:

[0087] The alcohol-based zirconia coating aggregate consists of 68% zirconia (ZrO2 particle size ≤ 40 μm, purity ≥ 99.3%), 10% silica (SiO2 particle size ≤ 60 μm, purity ≥ 99.9%), 12% mullite, and 10% kyanite.

[0088] The coating was prepared according to the method in Example 1. Figure 1 The diagram shows the dimensions of the sand casting. The aforementioned method was used to coat and apply the coating to the surface of the mold before pouring. The surface of the casting was cleaned, and it was found that the casting was stuck with sand. The quality of the casting was poor after cleaning. X-ray imaging revealed a large number of porosity defects on the inner and outer surfaces of the casting.

[0089] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A coating for aluminum-lithium alloy sand casting, characterized in that, The coating comprises a surface layer in contact with the sand mold and a top layer in contact with the melt. The surface layer material is an alcohol-based zirconium oxide coating containing graphene, and the top layer coating is an alcohol-based boride coating containing titanium diboride and zirconium diboride. The alcohol-based zirconia coating aggregate is composed of the following raw materials in the following mass percentages: graphene 5-8%, mullite 10-16%, kyanite 5-15%, silicon dioxide 10-15%, and the balance being zirconia. The alcohol-based boride coating aggregate is composed of the following raw materials in the following mass percentages: titanium diboride 10-36%, zirconium diboride 8-35%, talc powder 8-26%, silicon nitride powder 10-25%, and boron nitride powder 12-25%.

2. The coating for aluminum-lithium alloy sand casting according to claim 1, characterized in that, The aggregate in the alcohol-based zirconium oxide coating accounts for 27-46% of the total mass; the aggregate in the alcohol-based boride coating accounts for 45-57% of the total mass.

3. The coating for aluminum-lithium alloy sand casting according to claim 1, characterized in that, The components of the alcohol-based zirconia coating also include polyvinyl butyral, sodium carboxymethyl cellulose, lithium-based bentonite, sodium-based bentonite, isopropanol, silicate resin, ammonium zirconium carbonate, modified rosin, polyethylene glycol, preservatives, and ethanol.

4. The coating for aluminum-lithium alloy sand casting according to claim 3, characterized in that, The alcohol-based zirconium oxide coating is obtained by the following preparation method: (1) Put the refractory aggregates mullite, kyanite, zirconium oxide, graphene and silicon dioxide into a dry powder mixer and mix them evenly for 20~40 minutes; (2) Add the mixed dry powder to a wet ball mill, add polyvinyl butyral, sodium carboxymethyl cellulose, lithium-based bentonite, and sodium-based bentonite, and mill for 3-5 hours at a speed of 500-1000 r / min. (3) Add the mixed slurry to a high-speed mixer, add silica resin, ammonium zirconium carbonate and modified rosin, and stir at 800~2000r / min for 30~60min. Then add isopropanol, polyethylene glycol and ethanol and stir at 500~100r / min for 5~10min. Add the preservative to the prepared coating and stir at 200~600r / min for 5~10min.

5. The coating for aluminum-lithium alloy sand casting according to claim 1, characterized in that, The alcohol-based boron compound coating components also include lithium-based bentonite, organoclay, calcium carbonate, cryolite, nano-titanium oxide powder, carbon fiber powder, stabilizer, phenolic resin, polyvinyl alcohol or polyvinyl butyral, methanol, and ethanol.

6. A method of using the coating for aluminum-lithium alloy sand casting according to claim 1, characterized in that, The method includes the following steps: S1. Stir the alcohol-based zirconia coating for 3-5 minutes, and apply the zirconia coating to the surface of the sand casting mold by brushing. The coating thickness is ≤0.2mm. After multiple brushings, ignite the surface coating and calcine to obtain the alcohol-based zirconia coating. After drying for 5-15 minutes, polish the surface until the thickness is uniform. S2. Stir the alcohol-based boride coating for 3-5 minutes, and then spray it evenly onto the surface of the sand mold at a distance of 8-20 cm from the surface. After drying for 3-6 minutes, ignite and burn to dry to obtain the alcohol-based boride coating.

7. The method of using the coating for aluminum-lithium alloy sand casting according to claim 6, characterized in that, The thickness of the alcohol-based zirconium oxide coating is less than 0.1 mm, the thickness of the alcohol-based boride coating is less than 0.15 mm, and the thickness of the coating should be ≤0.2 mm.

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