A universal coating for cast iron lost foam and full mold casting and its application

Abstract

The application relates to the technical field of special functional coating materials for lost foam casting, and particularly discloses a cast iron lost foam and universal coating for full mold casting and application thereof. The cast iron lost foam and universal coating for full mold casting comprises quartz powder, calcined bauxite, modified fly ash microbeads, microcrystalline graphite and an additive; the modified fly ash microbeads comprise fly ash and a composite coating layer coated on the surface of the fly ash, and the composite coating layer comprises aluminum oxide nanosheets and magnesium ion doped amorphous substances. Through functional design of key components, the coating can dynamically adjust its performance according to the temperature and chemical environment of the contact interface, can meet the high air permeability requirement of the lost foam process, can take into account the differentiated requirements of different cast iron materials on high-temperature strength, permeability resistance and interface purity, and can be compatible with both conventional negative pressure and non-negative pressure conditions, so that the universal coating is realized, and production management and supply chain processes are greatly simplified.

Description

Technical Field

[0001] This invention relates to the field of special functional coating materials for lost foam casting, and in particular to a general-purpose coating for lost foam and solid casting of cast iron and its application. Background Technology

[0002] Lost foam casting is increasingly widely used in the cast iron industry due to its significant advantages in near-net-shape forming of complex structural parts. However, this process places extremely stringent requirements on the coating, and the performance requirements of different cast iron materials are inherently contradictory, making the development of general-purpose coatings a major challenge.

[0003] Material differences can lead to conflicting performance requirements. For example, gray cast iron (HT) has a relatively low pouring temperature, but produces a lot of liquid residue from foam pyrolysis, requiring coatings with extremely high permeability for rapid venting and to prevent wrinkling and carbon buildup defects. Gray cast iron coatings often require the addition of a significant amount of graphite to improve lubrication and anti-sand adhesion properties. In contrast, ductile iron (QT) has a high pouring temperature, and its solidification is accompanied by graphitization expansion, which exerts enormous pressure on the coating, requiring coatings with excellent high-temperature strength and crush resistance. Simultaneously, to prevent spheroidization decay, ductile iron coatings must absolutely block the migration of harmful elements such as sulfur and phosphorus, demanding extremely high purity of the coating components. Traditional graphite-containing coatings designed for gray cast iron are prone to causing graphite floating problems in high-grade ductile iron parts. These differences force foundries to maintain different dedicated coatings for HT and QT production lines, resulting in complex formula management, high inventory costs, cumbersome process changeovers, and the risk of material mixing.

[0004] Furthermore, regardless of the cast iron material, permeability must be a core requirement for lost foam coatings to ensure the smooth discharge of foam vaporization products, a prerequisite for obtaining a sound casting. However, high permeability often contradicts the coating's high-temperature strength and density. For example, lost foam casting requires negative pressure conditions. Because of the lack of vacuum suction to assist in the discharge of the large amount of gas generated by foam pyrolysis, the coating must maintain high and stable permeability (to cope with the instantaneous high pressure of gas bursts), while possessing extremely high room-temperature strength (to resist the pressure during molding) and high-temperature strength (to resist the higher hydrostatic pressure of molten metal and the graphite expansion pressure during solidification), and excellent dynamic resistance to molten metal penetration (to compensate for the potential penetration risk due to high permeability). Most existing general-purpose coatings are designed for negative pressure conditions, and their balance of permeability, strength, and penetration resistance cannot meet the requirements of negative pressure-free processes, leading to complex defects in castings such as porosity, cold shuts, and sand adhesion.

[0005] To reduce costs, the industry has tried introducing large amounts of cheap solid waste such as fly ash into coatings. However, fly ash is chemically inert at high temperatures, has weak adhesion to the coating substrate, and its composition fluctuates greatly. Not only does it fail to contribute to active impermeability, but it may also introduce impurities, damaging the overall strength and stability of the coating, and in particular, it cannot meet the high requirements for interface purity of ductile iron parts.

[0006] Therefore, there is an urgent need to develop a universal coating for lost foam casting and solid casting of cast iron, so that the coating can not only construct ultra-high and stable venting channels to cope with the venting challenge under negative pressure, but also form a stronger and more stable chemical barrier in situ at the interface under the action of high-temperature molten iron of different cast iron materials. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a universal coating for lost foam casting and solid casting of cast iron, and its application. Through the synergistic effect of modified fly ash microspheres and other raw material components, a high-performance, low-cost, and widely adaptable universal lost foam casting coating for cast iron that is compatible with both conventional negative pressure and non-negative pressure working conditions has been developed, which is of great significance for promoting the simplification and upgrading of casting processes.

[0008] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows:

[0009] In a first aspect, the present invention provides a universal coating for lost foam casting and solid casting of cast iron, comprising the following raw materials in parts by weight: 40 to 50 parts of quartz powder, 20 to 30 parts of calcined bauxite, 15 to 25 parts of modified fly ash microspheres, 8 to 12 parts of microcrystalline graphite, and 3.6 to 6 parts of additives.

[0010] The modified fly ash microspheres include fly ash and a composite coating layer covering the surface of the fly ash. The composite coating layer includes alumina nanosheets and magnesium ion-doped amorphous material.

[0011] Compared to existing technologies, the universal coating for lost foam casting and solid casting provided by this invention uses modified fly ash microspheres. These microspheres form a core of fly ash with a glassy and crystalline structure, and an outer shell of alumina nanosheets and magnesium ion-doped amorphous materials. They possess a dual function of "medium-temperature activation-high-temperature catalysis," providing a material basis for intelligent self-adaptation of the coating. The introduction of modified fly ash microspheres enables the coating to simultaneously meet ultra-high exhaust requirements and ultra-strong interface protection under harsh conditions of no negative pressure through adaptive reactions. This is the fundamental reason for the universality of lost foam casting coatings across materials (HT / QT) and processes (negative pressure / no negative pressure).

[0012] When the coating is heated to 800℃~1100℃, the alumina nanosheets on the surface of the modified fly ash microspheres become highly efficient active interfaces due to their high surface energy and abundant lattice defects. They can undergo solid-state mass transfer reactions with the surrounding quartz powder and calcined bauxite, promoting the early formation of local eutectic phases (such as amorphous aluminosilicate transition phases, see Equation 1 for the reaction equation). This reaction significantly reduces the starting temperature for complete sintering of the coating substrate, allowing a preliminary dense "pre-sintered layer" to form on the inner surface of the coating before the molten iron arrives, thus blocking some of the large pores in advance and laying the foundation for resisting the hydrostatic pressure of the molten iron.

[0013] γ-Al₂O₃ (nanosheets) + SiO₂ → amorphous aluminosilicate transition phase

[0014] Formula 1

[0015] When high-temperature molten iron (especially cast iron containing C and Si, >1300℃) comes into contact with or approaches modified fly ash microspheres, magnesium ions (Mg²⁺) in the amorphous material on the surface of the microspheres are doped with magnesium ions. + At high temperatures, it can act as a catalyst, promoting short-range ordered deposition and reconstruction of carbon atoms in molten iron or microcrystalline graphite in coatings on the surface of modified fly ash microspheres, forming a denser, more crystalline pyrolytic carbon / graphite thin layer (i.e., a dense carbon layer), thus creating a physical barrier. Simultaneously, nano-alumina (from alumina nanosheets), magnesium components (from magnesium ion-doped amorphous materials, molten iron, or coatings), and penetrating molten iron oxide (FeO) and silicon react instantaneously to generate a high-melting-point composite spinel or olivine phase (reaction equations are shown in Equations 2-3). These in-situ generated high-melting-point ceramic phases, together with the catalytically formed dense carbon layer, constitute a "double dynamic barrier," effectively blocking further penetration of molten iron and fundamentally resolving the potential penetration contradictions caused by high permeability.

[0016] MgO + Al₂O₃ + FeO → (Mg, Fe)Al₂O₄ (spinel solid solution, melting point > 1700℃)

[0017] Formula 2

[0018] 2MgO + SiO2 → Mg2SiO4 (magnesium olivine, melting point 1890℃)

[0019] Formula 3

[0020] Preferably, the particle size of the quartz powder is 180 mesh to 220 mesh.

[0021] Preferably, the calcined bauxite contains ≥85% Al2O3 and has a particle size of 300-350 mesh.

[0022] Quartz powder, primarily composed of SiO2, has a high melting point (approximately 1713℃), providing fundamental refractory support and excellent thermal stability for lost foam coatings. Calcined bauxite possesses even higher refractoriness (melting point ≥2050℃) and superior high-temperature compressive strength, effectively resisting the scouring of molten iron and the pressure of solidification expansion. The synergistic combination of quartz powder and calcined bauxite forms a composite system of "quartz skeleton-high alumina reinforcement," providing not only a high-refractory base covering the entire casting temperature range of cast iron (1350℃~1500℃), but also a dense and stable aggregate support structure constructed through a reasonable particle size distribution, laying a solid foundation for the coating's high-temperature strength and thermal shock resistance.

[0023] Preferably, the preparation method of the modified fly ash microspheres includes the following steps:

[0024] S1. Immerse fly ash in an acid solution and perform acid washing to obtain pretreated fly ash;

[0025] Aluminum alkoxides are added to water to undergo a hydrolysis reaction, yielding aluminum sol; a magnesium salt solution is added to the aluminum sol and mixed thoroughly to obtain magnesium ion-containing aluminum sol.

[0026] S2. The pretreated fly ash is impregnated in the magnesium ion-containing aluminum sol, and then rotary evaporated to obtain gel-coated fly ash.

[0027] S3. The gel-coated fly ash is calcined to obtain the modified fly ash microspheres.

[0028] The method for preparing modified fly ash microspheres provided by this invention involves first acid washing the fly ash to remove free active impurities such as CaO and Fe2O3 from its surface, thereby improving the chemical stability of the fly ash microsphere surface; then, using a "sol-gel encapsulation-calcination" process, the pretreated fly ash is immersed in a magnesium-containing aluminum sol, and the sol is uniformly encapsulated on the surface of the fly ash microspheres by rotary evaporation, and then calcination is used to grow in situ a composite coating layer of defect-rich γ / θ-Al2O3 nanosheets and magnesium-doped amorphous material (MgAl2O4 precursor) on the surface of the fly ash microspheres (mainly mullite and quartz glass phases).

[0029] Preferably, in S1, the fly ash contains ≥60wt% cenospheres and the fly ash has a particle size of 30μm~150μm.

[0030] More preferably, in S1, the acid solution comprises a 5wt% to 10wt% hydrochloric acid solution.

[0031] More preferably, in S1, the mass-to-volume ratio of the fly ash to the acid solution is 1 g:(3~5) mL.

[0032] More preferably, in S1, the pickling temperature is 60℃~80℃, and the pickling time is 2h~4h.

[0033] For example, in S1, after pickling, the process also includes: washing with water until neutral, drying, and obtaining pretreated fly ash. The drying temperature is 100℃~120℃, and drying is carried out until constant weight.

[0034] More preferably, in S1, the aluminum alkoxide includes aluminum isopropoxide.

[0035] More preferably, in S1, the concentration of aluminum ions after the aluminum alkoxide is added to water is 0.5M~1M.

[0036] More preferably, in S1, the temperature of the hydrolysis reaction is 70℃~85℃, and the time of the hydrolysis reaction is 2h~4h.

[0037] More preferably, in S1, the magnesium salt solution comprises a 1M~2M aqueous solution of magnesium nitrate.

[0038] More preferably, in S1, the molar ratio of aluminum ions in the aluminum sol to magnesium ions in the magnesium salt solution is (8~12):1.

[0039] This invention effectively ensures the magnesium ion doping level by controlling the molar amounts of aluminum and magnesium ions, thereby guaranteeing the high-temperature catalytic activity of the modified fly ash microspheres. Extensive testing revealed that if the magnesium ion content is too low, the catalytic effect of the modified fly ash microspheres is insufficient; conversely, if the magnesium ion content is too high, excessive magnesium oxide will form, adversely affecting the adhesion of the coating.

[0040] For example, in S1, the aluminum sol and magnesium salt solution are mixed at room temperature for 1 to 2 hours.

[0041] More preferably, in S2, the mass-to-volume ratio of the pretreated fly ash and the magnesium ion-containing aluminum sol is 1g:(10~20)mL.

[0042] More preferably, in S2, the immersion temperature is 40℃~60℃, and the immersion time is 30min~60min.

[0043] For example, in S2, impregnation can be performed under ultrasonic conditions to ensure that the pretreated fly ash surface is adequately wetted.

[0044] More preferably, in S2, the temperature of the rotary evaporation is 50℃~70℃, the rotation speed of the rotary evaporation is 20rpm~40rpm, and the time of the rotary evaporation is 1.5h~3.5h.

[0045] By controlling the conditions of rotary evaporation, this invention can further control the evaporation rate of magnesium ion-containing aluminum sol, so that it can be completely gelled and coated on the surface of fly ash microspheres, forming a gel coating layer of uniform thickness.

[0046] In a further preferred embodiment, S2, after the rotary evaporation is completed, the process further includes drying at 80℃~100℃ for 4h~6h to obtain the gel-coated fly ash.

[0047] This invention uses drying to remove physically adsorbed water from gel-coated fly ash, creating favorable conditions for subsequent calcination.

[0048] More preferably, in step S3, the calcination step specifically includes: pre-calcining the gel-coated fly ash at a rate of 2℃ / min~5℃ / min to 300℃~400℃; then calcining at a rate of 3℃ / min~8℃ / min to 650℃~750℃; and cooling the mixture in the furnace to below 200℃ to obtain the modified fly ash microspheres.

[0049] This invention employs a segmented calcination method. First, pre-calcination thoroughly decomposes organic matter and nitrates, followed by calcination at a specific temperature to form interwoven alumina nanosheets and magnesium ion-doped amorphous materials. Extensive testing revealed that calcination temperatures below 650℃ result in insufficient crystallinity, while temperatures above 750℃ easily lead to coarsening of the nanosheets during sintering. Excessive heating rates can cause coating cracking, while excessively slow rates reduce production efficiency.

[0050] More preferably, in S3, the pre-calcination time is 1h to 2h.

[0051] More preferably, in S3, the calcination time is 1h to 2h.

[0052] Preferably, the thickness of the composite coating layer is 50nm~200nm.

[0053] Preferably, the particle size of the modified fly ash microspheres is 200 mesh to 400 mesh.

[0054] Preferably, the particle size of the microcrystalline graphite is 180 mesh to 220 mesh.

[0055] As a lubricating and conductive functional phase, microcrystalline graphite plays the following roles in coatings: (1) The layered structure of graphite gives the coating excellent lubricity, which can significantly reduce the friction between the coating and the mold at high temperatures, ensuring smooth demolding and surface integrity of the casting; (2) The inherent iron-repellent properties of graphite can effectively reduce the wettability of molten iron on the coating, forming a physical isolation layer at the interface, thereby significantly inhibiting the capillary penetration of molten iron into the pores of the coating, fundamentally preventing mechanical sand adhesion defects; (3) As a high thermal conductivity material, graphite can balance the heat conduction inside the coating, promote the stable discharge of pyrolysis gas from the foam mold, and avoid local thermal stress concentration. Especially in the negative pressure-free process, this characteristic is crucial for ensuring the integrity of the filling and the surface quality of the casting.

[0056] Preferably, the additives include: 2 to 3 parts of silica sol, 0.5 to 1 part of polyvinyl butyral, 1 to 1.5 parts of lithium-based montmorillonite, 0.1 to 0.3 parts of water-reducing agent, and 0.05 to 0.1 parts of defoamer.

[0057] More preferably, the SiO2 content in the silica sol is 28%~32%.

[0058] This invention uses a composite binder of silica sol and polyvinyl butyral (PVB). The reducing atmosphere generated by the high-temperature pyrolysis of PVB helps to protect microcrystalline graphite from oxidation.

[0059] More preferably, the water-reducing agent is a polycarboxylate water-reducing agent.

[0060] More preferably, the defoamer is an organosilicone defoamer.

[0061] Preferably, the raw materials of the general-purpose coating for lost foam casting and solid casting also include water, and the viscosity of the general-purpose coating for lost foam casting and solid casting is 25s~40s at 25℃±2℃ in a Coating-4 cup.

[0062] Secondly, the present invention provides a method for preparing the aforementioned universal coating for lost foam casting and solid casting of cast iron, comprising the following steps:

[0063] Quartz powder, calcined bauxite, modified fly ash microspheres, microcrystalline graphite and lithium-based montmorillonite are mixed to obtain a dry mixture;

[0064] Add silica sol, polyvinyl butyral, water-reducing agent and defoamer to water, then add the dry mixture, mix evenly, adjust the viscosity to obtain a general coating for lost foam casting and solid casting of cast iron.

[0065] Thirdly, the present invention provides the application of the aforementioned general-purpose coating for lost foam casting and solid casting in the casting of gray cast iron or ductile iron.

[0066] The universal coating for lost foam casting and solid casting provided by this invention is stably applicable to various cast iron materials such as HT150~HT350 and QT400~QT700. Through precise and controllable fine-tuning, a single formula system achieves high-quality compatibility with a wide range of cast iron materials, greatly simplifying the production management process.

[0067] The present invention has the following beneficial effects:

[0068] The universal coating for lost foam casting and solid casting provided by this invention is stably applicable to the production of castings made of different materials such as gray cast iron (HT) and ductile iron (QT), breaking through the limitations of traditional coatings that are formulated according to material. Through the functional design of key components (especially modified fly ash microspheres), the coating can dynamically adjust its performance according to the temperature and chemical environment of the contact interface. While meeting the high permeability requirements of the lost foam casting process, it also takes into account the different requirements of different cast iron materials for high-temperature strength, impermeability, and interface purity. It can also be compatible with both conventional negative pressure and non-negative pressure working conditions, thus achieving "one agent for all", which greatly simplifies production management and supply chain processes.

[0069] Modified fly ash microspheres possess a dual function of "medium-temperature activation and high-temperature catalysis": In the initial stage of casting (800℃~1100℃), they provide active sites, promoting matrix pre-sintering and enabling the coating to densify itself before the molten iron arrives; in the high-temperature zone at the molten iron front (>1300℃), they act as a high-temperature in-situ barrier catalyst, catalyzing the generation of locally high-melting-point compounds to construct a dynamic impermeability barrier. The high-temperature flexural strength at 1300℃ is ≥12MPa, sufficient to resist the higher hydrostatic pressure of molten metal and the solidification expansion force of cast iron during negative pressure casting. Modified fly ash microspheres upgrade the traditional quartz-bauxite-graphite system into a coating with "intelligent response" characteristics, effectively resolving the contradiction between versatility, impermeability, and economy.

[0070] This invention utilizes a precise particle size distribution of quartz powder, calcined bauxite, modified fly ash microspheres, and microcrystalline graphite to ensure high bulk density (low porosity) while retaining the functional micropores / channels provided by the modified fly ash microspheres and microcrystalline graphite. This constructs a hierarchical air-permeable network combining macroscopic skeletal pores and functional micropores within the coating. This design ensures that the coating's air permeability can be stably maintained at ≥90 (permeability is measured in centimeters) at room temperature under no negative pressure conditions. 4 / gram·minute (generally not specified, but considered a dimensionless value) provides a basic guarantee for the smooth discharge of foam pyrolysis gases. Detailed Implementation

[0071] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0072] In this embodiment of the invention, the average particle size of the quartz powder is 200 mesh; the Al2O3 content in the calcined bauxite is 87.5%, and the average particle size is 325 mesh; the average particle size of the microcrystalline graphite is 200 mesh; the cenosphere content in the fly ash is 65.4 wt%, and the particle size of the fly ash is 50 μm to 120 μm. The SiO2 content in the silica sol is 30%; the water-reducing agent is a polycarboxylate water-reducing agent, and the defoamer is a BYK series defoamer. All other materials not specifically mentioned are commercially available products.

[0073] To illustrate the applicability of the universal coating for lost foam casting and solid casting provided by this invention, examples are given in the embodiments of this invention, such as the production of ductile iron reducer housings (material QT600-3) using lost foam casting and the production of gray iron small machine tool parts (material HT250) using solid casting.

[0074] Example 1

[0075] This embodiment provides a universal coating for lost foam casting and solid casting of cast iron, comprising the following raw materials in parts by weight: 50 parts quartz powder, 22 parts calcined bauxite, 15 parts modified fly ash microspheres, 12 parts microcrystalline graphite, 2 parts silica sol, 1 part PVB, 1.5 parts lithium-based montmorillonite, 0.2 parts water-reducing agent, and 0.1 parts defoamer.

[0076] The preparation method of the above-mentioned modified fly ash microspheres includes the following steps:

[0077] S1. Immerse fly ash in a 10wt% hydrochloric acid solution, with a fly ash to acid solution mass-volume ratio of 1g:3mL. Acid wash at 60℃ for 4h, wash with water until neutral, and dry at 115℃ to obtain pretreated fly ash.

[0078] Aluminum isopropoxide was added to deionized water (aluminum ion concentration of 0.5M) and hydrolyzed at 82℃ for 2.5h to obtain aluminum sol; 2M magnesium nitrate aqueous solution was added to the aluminum sol, with a molar ratio of aluminum ions to magnesium ions of 12:1, and stirred at room temperature for 2h to obtain magnesium ion-containing aluminum sol.

[0079] S2. The pretreated fly ash was impregnated in magnesium ion-containing aluminum sol with a mass-volume ratio of 1g:10mL. After sonication at 40℃ for 55min, it was rotary evaporated at 55℃ and 40rpm for 2h and dried at 80℃ for 6h to obtain gel-coated fly ash.

[0080] S3. The gel-coated fly ash is heated to 300℃ at a rate of 5℃ / min and pre-calcined for 2 hours; then heated to 660℃ at a rate of 8℃ / min and calcined for 2 hours. It is then cooled to 180℃ in the furnace, removed from the furnace and air-cooled to room temperature, and ground to obtain modified fly ash microspheres with a particle size of 250-350 mesh.

[0081] Modified fly ash microspheres include fly ash and a composite coating layer on the surface of the fly ash. The composite coating layer includes alumina nanosheets and magnesium ion-doped amorphous materials, and the thickness of the composite coating layer is 70nm~130nm.

[0082] The preparation method of the above-mentioned general-purpose coating for lost foam casting and solid casting of cast iron includes the following steps:

[0083] S100. Weigh each raw material according to the design ratio, and mix quartz powder, calcined bauxite, modified fly ash microspheres, microcrystalline graphite and lithium-based montmorillonite to obtain a dry mixture.

[0084] S200: Add silica sol, polyvinyl butyral, water-reducing agent and defoamer to water, then add dry mix and mix evenly to obtain a general-purpose coating for lost foam casting and solid casting of cast iron.

[0085] The method of using the universal coating for lost foam casting and solid casting of cast iron in this embodiment to produce ductile iron reducer housings via lost foam casting includes the following steps:

[0086] The viscosity of S300, a general-purpose coating for lost foam casting and solid casting of cast iron, is adjusted to 28s (coating cup -4 at 25℃). Three coats are applied, each thoroughly dried, with a coating thickness of 1.2mm. The mixture is then embedded in a 20 / 40 quartz sand box and molten steel is poured at -0.06MPa and 1500℃ to obtain the reducer housing casting.

[0087] Casting results: The casting was smooth with no backflow, the mold was completely filled, there was no cold shut, the coating was sintered evenly and had good peelability, the casting surface had no wrinkles caused by graphite floating, there was no mechanical or chemical sand adhesion, and the surface roughness Ra after sand cleaning was ≤12.5μm.

[0088] The method of using the universal coating for lost foam casting and solid casting of cast iron in the production of small gray iron machine tool parts by solid casting process according to this embodiment includes the following steps:

[0089] The viscosity of S400, a general-purpose coating for lost foam casting and solid casting of cast iron, is adjusted to 33s (Coating-4 cup at 25℃). Three coats are applied, each thoroughly dried, with a coating thickness of 1.5mm. Furan resin sand molding is used, and molten steel is poured at 1400℃ under no negative pressure to obtain small machine tool castings.

[0090] Casting results: The casting was completely filled, with no cold shuts or porosity defects, and no adhering sand on the surface. After sand removal, the surface roughness Ra ≤ 12.5 μm. The production process was stable, verifying the reliability and versatility of the coating under negative pressure conditions.

[0091] Example 2

[0092] This embodiment provides a universal coating for lost foam casting and solid casting of cast iron, comprising the following raw materials in parts by weight: 45 parts quartz powder, 25 parts calcined bauxite, 20 parts modified fly ash microspheres, 10 parts microcrystalline graphite, 2.5 parts silica sol, 0.8 parts PVB, 1.2 parts lithium-based montmorillonite, 0.2 parts water-reducing agent, and 0.08 parts defoamer.

[0093] The preparation method of the above-mentioned modified fly ash microspheres includes the following steps:

[0094] S1. Immerse fly ash in an 8wt% hydrochloric acid solution, with a fly ash to acid solution mass-volume ratio of 1g:4mL, acid wash at 70℃ for 3h, wash with water until neutral, and dry at 110℃ to obtain pretreated fly ash.

[0095] Aluminum isopropoxide was added to deionized water (aluminum ion concentration of 0.7M) and hydrolyzed at 78℃ for 3 hours to obtain aluminum sol; 1.5M magnesium nitrate aqueous solution was added to the aluminum sol, with a molar ratio of aluminum ions to magnesium ions of 10:1, and stirred at room temperature for 1.5 hours to obtain magnesium ion-containing aluminum sol.

[0096] S2. The pretreated fly ash was impregnated in magnesium ion-containing aluminum sol with a mass-volume ratio of 1g:14mL. After sonication at 50℃ for 45min, it was rotary evaporated at 60℃ and 30rpm for 2.5h and dried at 90℃ for 5h to obtain gel-coated fly ash.

[0097] S3. The gel-coated fly ash is heated to 350℃ at a rate of 4℃ / min and pre-calcined for 1.5h; then heated to 700℃ at a rate of 5℃ / min and calcined for 1.5h. It is then cooled to 185℃ in the furnace, removed from the furnace and air-cooled to room temperature, and ground to obtain modified fly ash microspheres with a particle size of 300-400 mesh.

[0098] Modified fly ash microspheres include fly ash and a composite coating layer on the surface of the fly ash. The composite coating layer includes alumina nanosheets and magnesium ion-doped amorphous materials, and the thickness of the composite coating layer is 100nm~150nm.

[0099] The preparation method of the above-mentioned general coating for lost foam casting and solid casting of cast iron is the same as that in Example 1, and will not be repeated here.

[0100] The method of using the universal coating for lost foam casting and solid casting of cast iron in this embodiment to produce ductile iron reducer housings via lost foam casting includes the following steps:

[0101] The viscosity of S300, a general-purpose coating for lost foam casting and solid casting of cast iron, is adjusted to 29s (coating cup -4 at 25℃). Three coats are applied, each thoroughly dried, with a coating thickness of 1.22mm. The coating is then embedded in a 20 / 40 quartz sand box and molten steel is poured at -0.065MPa and 1490℃ to obtain the reducer housing casting.

[0102] Casting results: The casting was complete with no cold shuts. The coating was sintered evenly and had good peelability. There were no wrinkles caused by graphite floating on the surface of the casting. There was no mechanical or chemical sand adhesion. The surface roughness Ra after sand removal was ≤12.5μm.

[0103] The method of using the universal coating for lost foam casting and solid casting of cast iron in the production of small gray iron machine tool parts by solid casting process according to this embodiment includes the following steps:

[0104] The viscosity of S400, a general-purpose coating for lost foam casting and solid casting of cast iron, is adjusted to 35s (Coating-4 cup at 25℃). Three coats are applied, each thoroughly dried, with a coating thickness of 1.6mm. Furan resin sand molding is used, and molten steel is poured at 1400℃ under no negative pressure to obtain small machine tool castings.

[0105] Casting results: The casting was completely filled, with no cold shuts or porosity defects, and no adhering sand on the surface. After sand removal, the surface roughness Ra ≤ 12.5 μm. The production process was stable, verifying the reliability and versatility of the coating under negative pressure conditions.

[0106] Example 3

[0107] This embodiment provides a universal coating for lost foam casting and solid casting of cast iron, comprising the following raw materials in parts by weight: 40 parts quartz powder, 28 parts calcined bauxite, 25 parts modified fly ash microspheres, 8 parts microcrystalline graphite, 3 parts silica sol, 0.5 parts PVB, 1 part lithium-based montmorillonite, 0.1 parts water-reducing agent, and 0.05 parts defoamer.

[0108] The preparation method of the above-mentioned modified fly ash microspheres includes the following steps:

[0109] S1. Immerse fly ash in a 5wt% hydrochloric acid solution, with a fly ash to acid solution mass-volume ratio of 1g:5mL, acid wash at 80℃ for 2h, wash with water until neutral, and dry at 105℃ to obtain pretreated fly ash.

[0110] Aluminum isopropoxide was added to deionized water (aluminum ion concentration of 1M) and hydrolyzed at 73℃ for 4 hours to obtain aluminum sol; 1M magnesium nitrate aqueous solution was added to the aluminum sol, with a molar ratio of aluminum ions to magnesium ions of 8:1, and stirred at room temperature for 1.2 hours to obtain magnesium ion-containing aluminum sol.

[0111] S2. The pretreated fly ash was impregnated in magnesium ion-containing aluminum sol with a mass-volume ratio of 1g:17mL. After sonication at 60℃ for 35min, it was rotary evaporated at 65℃ and 20rpm for 3.0h and dried at 100℃ for 4h to obtain gel-coated fly ash.

[0112] S3. The gel-coated fly ash is heated to 400℃ at a rate of 2℃ / min and pre-calcined for 1h; then heated to 740℃ at a rate of 3℃ / min and calcined for 1.2h. It is then cooled to 188℃ in the furnace, removed from the furnace and air-cooled to room temperature, and ground to obtain modified fly ash microspheres with a particle size of 200-300 mesh.

[0113] Modified fly ash microspheres include fly ash and a composite coating layer on the surface of the fly ash. The composite coating layer includes alumina nanosheets and magnesium ion-doped amorphous materials, and the thickness of the composite coating layer is 120nm~180nm.

[0114] The preparation method of the above-mentioned general coating for lost foam casting and solid casting of cast iron is the same as that in Example 1, and will not be repeated here.

[0115] The method of using the universal coating for lost foam casting and solid casting of cast iron in this embodiment to produce ductile iron reducer housings via lost foam casting includes the following steps:

[0116] The viscosity of S300, a general-purpose coating for lost foam casting and solid casting of cast iron, is adjusted to 27s (coating cup -4 at 25℃). Three coats are applied, each thoroughly dried, with a coating thickness of 1.21mm. The coating is then embedded in a 20 / 40 quartz sand box and molten steel is poured at -0.058MPa and 1480℃ to obtain the reducer housing casting.

[0117] Casting results: The casting was complete with no cold shuts. The coating was sintered evenly and had good peelability. There were no wrinkles caused by graphite floating on the surface of the casting. There was no mechanical or chemical sand adhesion. The surface roughness Ra after sand removal was ≤12.5μm.

[0118] The method of using the universal coating for lost foam casting and solid casting of cast iron in the production of small gray iron machine tool parts by solid casting process according to this embodiment includes the following steps:

[0119] The viscosity of S400, a general-purpose coating for lost foam casting and solid casting of cast iron, is adjusted to 37s (Coating-4 cup at 25℃). It is applied in 3 coats, each coat is thoroughly dried, and the coating thickness is 1.59mm. Furan resin sand molding is used, and molten steel is poured at 1400℃ under no negative pressure to obtain small machine tool castings.

[0120] Casting results: The casting was completely filled, with no cold shuts or porosity defects, and no adhering sand on the surface. After sand removal, the surface roughness Ra ≤ 12.5 μm. The production process was stable, verifying the reliability and versatility of the coating under negative pressure conditions.

[0121] Comparative Example 1

[0122] This comparative example provides a lost foam casting coating for cast iron, similar to Example 2, except that the modified fly ash microspheres are replaced with an equal mass of fly ash. The remaining raw material components and proportions are the same as in Example 2 and will not be repeated.

[0123] The preparation method of the above-mentioned lost foam casting coating for cast iron is similar to that in Example 1, except that the modified fly ash microspheres are replaced with fly ash in S100. The remaining operation steps are the same as in Example 1 and will not be repeated.

[0124] The steps and conditions for using the cast iron lost foam casting coating of this comparative example to produce ductile iron reducer housings by lost foam casting process are the same as in Example 2, and will not be repeated here.

[0125] The casting and inspection results are as follows: When using the coating of this comparative example for lost foam casting, the process stability decreased and the quality of the castings deteriorated significantly. Slight backflow or abnormal flue gas was observed during casting; defects such as localized sand adhesion, wrinkling, and cold shuts appeared on the casting surface; the coating sintered unevenly and was difficult to peel off; after sand removal, the surface roughness Ra value of the casting increased to over 20 μm. This indicates that, due to the absence or alteration of the key component (modified fly ash microspheres) and optimized proportions of this invention, the coating of this comparative example lacks high-temperature strength, impermeability, and interfacial stability, and cannot meet the production requirements of high-quality ductile iron lost foam castings.

[0126] The steps and conditions for using the lost foam casting coating for cast iron in this comparative example to produce small gray iron machine tool parts by lost foam casting process are the same as in Example 2, and will not be repeated here.

[0127] The casting and inspection results are as follows: In negative pressure-free casting, the performance of the coating in this comparative example is severely inadequate. Poor mold filling during casting easily leads to cold shuts and porosity in the castings; large areas of sand adhere to the casting surface, making sand removal difficult; localized crushing or peeling of the coating occurs. The casting surface is rough and the precision is substandard. This indicates that coatings lacking the key component of this invention (modified fly ash microspheres) cannot meet the requirements for high-temperature strength and impermeability in negative pressure-free casting.

[0128] Comparative Example 2

[0129] This comparative example provides a lost foam casting coating for cast iron, similar to Example 2, except that the preparation method of the modified fly ash microspheres is different (in S2, pretreated fly ash is directly impregnated in aluminum sol); the modified fly ash microspheres include fly ash and a composite coating layer covering the surface of the fly ash, the composite coating layer including alumina nanosheets and magnesium-free doped amorphous material, and the thickness of the composite coating layer is 100nm~150nm. The remaining raw material components and proportions are the same as in Example 2, and will not be repeated.

[0130] The preparation method of the modified fly ash microspheres in this comparative example includes the following steps:

[0131] S1. Immerse fly ash in an 8wt% hydrochloric acid solution, with a fly ash to acid solution mass-volume ratio of 1g:4mL, acid wash at 70℃ for 3h, wash with water until neutral, and dry at 110℃ to obtain pretreated fly ash.

[0132] Aluminum isopropoxide was added to deionized water (aluminum ion concentration of 0.7M) and hydrolyzed at 78°C for 3 hours to obtain aluminum sol.

[0133] S2. The pretreated fly ash was impregnated in aluminum sol with a mass-to-volume ratio of 1g:14mL. After sonication at 50℃ for 45min, it was rotary evaporated at 60℃ and 30rpm for 2.5h and dried at 90℃ for 5h to obtain gel-coated fly ash.

[0134] S3. The gel-coated fly ash is heated to 350℃ at a rate of 4℃ / min and pre-calcined for 1.5h; then heated to 700℃ at a rate of 5℃ / min and calcined for 1.5h. It is then cooled to 185℃ in the furnace, removed from the furnace and air-cooled to room temperature, and ground to obtain modified fly ash microspheres with a particle size of 300-400 mesh.

[0135] The preparation method of the above-mentioned lost foam casting coating for cast iron is the same as that in Example 1, and will not be repeated here.

[0136] The steps and conditions for using the cast iron lost foam casting coating of this comparative example to produce ductile iron reducer housings by lost foam casting process are the same as in Example 2, and will not be repeated here.

[0137] The casting and inspection results are as follows: When using this comparative coating for lost foam casting, there was no significant sand adhesion on the casting surface, but coating crushing or cracking occurred at hot spots and thick walls. The coating's high-temperature strength was insufficient, making it difficult to effectively resist the graphitization expansion pressure during the solidification of ductile iron. The casting at these corresponding locations carries the risk of dimensional expansion or surface unevenness, affecting dimensional accuracy and assembly. The results indicate that the modified fly ash microspheres lacking magnesium ion doping have insufficient ability to catalyze the formation of a reinforcing phase at high temperatures, leading to a decrease in the coating's high-temperature mechanical properties and failing to meet the stringent requirements for coating crush resistance in high-grade ductile iron parts.

[0138] The steps and conditions for using the lost foam casting coating for cast iron in this comparative example to produce small gray iron machine tool parts by lost foam casting process are the same as in Example 2, and will not be repeated here.

[0139] The casting and inspection results are as follows: In negative pressure-free casting, the tendency for backflow during pouring increases, and the number of cold shuts and porosity defects in the castings increases. Furthermore, due to insufficient high-temperature strength of the coating, more widespread crushing or cracking occurs under the hydrostatic pressure of molten metal, leading to severe sand adhesion. The results indicate that modified fly ash microspheres lacking magnesium ion doping cannot effectively catalyze the formation of a high-temperature strengthening phase, resulting in insufficient overall high-temperature strength and thermal shock resistance of the coating, making it unsuitable for stable use under negative pressure conditions.

[0140] Comparative Example 3

[0141] This comparative example provides a lost foam casting coating for cast iron, similar to Example 2, except that the modified fly ash microspheres are replaced with 10 parts of quartz powder and 10 parts of calcined bauxite. Specifically, the lost foam casting coating for cast iron comprises the following raw materials in parts by weight: 55 parts of quartz powder, 35 parts of calcined bauxite, 10 parts of microcrystalline graphite, 2.5 parts of silica sol, 0.8 parts of PVB, 1.2 parts of lithium-based montmorillonite, 0.2 parts of water-reducing agent, and 0.08 parts of defoamer.

[0142] The preparation method of the above-mentioned lost foam casting coating for cast iron is similar to that in Example 1, except that modified fly ash microspheres are not added to S100. The remaining operation steps are the same as in Example 1 and will not be repeated.

[0143] The steps and conditions for using the cast iron lost foam casting coating of this comparative example to produce ductile iron reducer housings by lost foam casting process are the same as in Example 2, and will not be repeated here.

[0144] The casting and inspection results are as follows: When casting with this comparative example coating, the casting exhibited widespread and severe sand adhesion defects (including mechanical and chemical sand adhesion), making sand removal extremely difficult and resulting in a rough casting surface. The coating's high-temperature strength was significantly insufficient, and sand erosion and localized spalling occurred during casting. The results indicate that, lacking modified fly ash microspheres, the traditional quartz-bauxite-graphite system cannot balance the contradiction between high permeability and high strength / impermeability, making it unsuitable for lost foam casting processes that require high overall coating performance.

[0145] The steps and conditions for using the lost foam casting coating for cast iron in this comparative example to produce small gray iron machine tool parts by lost foam casting process are the same as in Example 2, and will not be repeated here.

[0146] The casting and inspection results are as follows: In negative pressure-free casting, due to the severely insufficient high-temperature strength and impermeability of the coating, the casting process is extremely unstable, often accompanied by violent backflow, and the cavity cannot be effectively maintained, resulting in incomplete filling of the casting or the formation of severe cold shuts and inclusions, making it impossible to obtain a complete casting. The results show that without modified fly ash microspheres, the traditional aggregate system cannot form an effective impermeability barrier and a stable skeleton at high temperatures, and its comprehensive performance is far from meeting the stringent requirements of cast iron standardization and negative pressure-free casting.

[0147] Comparative Example 4

[0148] This comparative example provides a lost foam casting coating for cast iron, similar to Example 2, except that the proportions of the raw materials—quartz powder, calcined bauxite, and microcrystalline graphite—are different. Specifically, the lost foam casting coating for cast iron comprises the following raw materials in parts by weight: 30 parts quartz powder, 35 parts calcined bauxite, 20 parts modified fly ash microspheres, 15 parts microcrystalline graphite, 2.5 parts silica sol, 0.8 parts PVB, 1.2 parts lithium-based montmorillonite, 0.2 parts water-reducing agent, and 0.08 parts defoamer.

[0149] The preparation method of the modified fly ash microspheres is the same as that in Example 1, and will not be repeated here.

[0150] The preparation method of the above-mentioned lost foam casting coating for cast iron is the same as that in Example 1, and will not be repeated here.

[0151] The steps and conditions for using the cast iron lost foam casting coating of this comparative example to produce ductile iron reducer housings by lost foam casting process are the same as in Example 2, and will not be repeated here.

[0152] The casting and inspection results are as follows: After casting with the coating of this comparative example, obvious black wrinkles and graphite floating defects appeared on the surface of the casting, with local carbon accumulation, seriously affecting the surface quality. The coating strength at room temperature is acceptable, but at high temperatures, the presence of excessive graphite may exacerbate oxidation and gas generation, inducing subcutaneous porosity on the surface of the casting. In addition, the imbalance of aggregate proportions led to a decrease in coating bulk density, resulting in uneven impermeability in some areas, with spot or local sand adhesion. The results show that the introduction of excessive microcrystalline graphite not only failed to improve impermeability but also disrupted the reasonable gradation and high-temperature stability of the aggregate system, causing new casting defects and failing to meet the production requirements of high surface quality cast iron parts.

[0153] The steps and conditions for using the lost foam casting coating for cast iron in this comparative example to produce small gray iron machine tool parts by lost foam casting process are the same as in Example 2, and will not be repeated here.

[0154] The casting and inspection results are as follows: When using this comparative coating in production, black wrinkles and carbon deposits caused by graphite floating are commonly found on the surface of the castings, severely affecting the appearance and subsequent processing. In lost foam casting, the coating's tendency to generate gas at high temperatures increases, potentially inducing subcutaneous porosity in the castings. Under the negative pressure-free conditions of lost foam casting, the overall high-temperature strength and density of the coating decrease due to aggregate imbalance, resulting in insufficient resistance to hydrostatic pressure of the molten metal. Sand adhesion defects in the castings (especially at hot spots) are more severe and penetrating. The results indicate that excessive microcrystalline graphite disrupts the rational gradation and high-temperature stability of the aggregate system, not only introducing new surface defects but also impairing the overall high-temperature performance of the coating, failing to meet the production requirements of general-purpose and high-quality cast iron parts.

[0155] Verification test

[0156] The casting coatings provided in Examples 1-3 and Comparative Examples 1-4 were used as test samples for corresponding tests, and the results are shown in Table 1.

[0157] (1) Air permeability test

[0158] Tools: NBD-M1700-22TI box furnace, ZTY intelligent air permeability tester, 101-2BS electric constant temperature drying oven, stainless steel ring with inner diameter Φ50mm and thickness 1.80mm, stainless steel ring fixing base plate, stainless steel pressure ring, high temperature sleeve device.

[0159] Adjust the coating sample to the appropriate viscosity, spread it evenly inside three stainless steel rings, and apply three coats. Each coat should be thoroughly dried, with a coating thickness of 1.80 mm ± 0.04 mm. After drying, demold the sample, ensuring both sides are smooth, without bumps or cracks. If there are gaps between the stainless steel rings and the coating, fill them with water glass, making the gap as small as possible, ensuring a tight seal. Allow the sample to stand at room temperature for at least 10 minutes before testing.

[0160] Connect the uncoated side (bottom) of the sample to the sleeve, place a stainless steel pressure ring on the coated side, and fix the sample on the high-temperature sleeve with stainless steel screws and nuts. Place it in the NBD-M1700-22TI box furnace. Fix the other side of the sleeve to the ZTY intelligent air permeability tester and adjust the instrument position to ensure the device is sealed.

[0161] Calibrate the ZTY intelligent air permeability tester and set the heating curve for the NBD-M1700-22TI box furnace. Start heating from room temperature, increasing the temperature by 10℃ per minute. Perform a room temperature air permeability test before heating, then measure at 150℃, and from 600℃ onwards, test the sample air permeability every 10℃. The test ends when the sample breaks. If the sample does not break, heat to 850℃ to end the test. The air permeability at 800℃ is the standard. If the sample breaks or warps and leaks air before 800℃, a new sample must be prepared and tested.

[0162] (2) Testing of flexural strength at room temperature and high temperature

[0163] Instruments and tools: paint mixer, standard sample mold (width 22.36mm±0.2mm, thickness 11.18mm±0.2mm, length 70mm), electric heating forced-air constant temperature drying oven, 1700℃ box-type high temperature electric furnace, XQY-Ⅱ intelligent sand strength tester, vernier calipers, long-handled pliers, high temperature resistant 77 ceramic boat or 60×30 square boat.

[0164] Adjust the coating sample to the appropriate viscosity, spread it evenly in the three cavities of the mold, apply three coats, and dry each layer thoroughly. The coating thickness should be slightly higher than the upper surface of the mold. After the sample is dried, demold it. Each group consists of three samples.

[0165] Place the sample upright on a ceramic boat resting flat on the bottom plate of the high-temperature furnace (to facilitate heating of the lower side of the sample), and close the furnace door after adding a door plug. Set the heating curve for the NBD-M1700-22TI box furnace, starting from room temperature and increasing the temperature by 10°C per minute, holding at 1300°C for 1 hour (if not starting from room temperature, the holding time needs to be extended to 2 hours to ensure that the temperature of the center of the sample is consistent with the surface temperature). Start the XQY-Ⅱ intelligent sand strength tester 10 minutes before the end of the holding time, and take a sample within 30 seconds to test the room temperature flexural strength of the sample.

[0166] (3) Heat exposure crack resistance test

[0167] Instruments and tools: long-handled pliers, NBD-M1700-22TI box furnace, 101-2BS electric thermostatic drying oven, refractory bricks, vernier calipers, hot melt glue gun, 80mm×20mm×120mm white mold, 40mm×20mm×120mm white mold, S-shaped hook, wallpaper knife.

[0168] Immerse the white mold in each coating to a depth of approximately 100 mm. Repeat the immersion and coating process three times, ensuring each layer is thoroughly dried. The coating thickness should be 1.50 mm to 1.80 mm. Raise the oven temperature to 1200℃, place the test sample inside, heat and hold for 3 minutes, then immediately remove from the oven and observe the crack condition of the mold shell with a magnifying glass and take photographs.

[0169] Criteria for evaluating heat exposure crack resistance:

[0170] Grade 1: The surface is smooth and without cracks, or has only very fine cracks;

[0171] Level 2: The surface has dendritic or net-like fine cracks, and the crack width is less than 0.5 mm;

[0172] Level 3: The surface has dendritic or network cracks, the crack width is less than 1 mm, the cracks are relatively deep, and there are no through coarse cracks in the transverse (and horizontal circumferential direction) or longitudinal direction.

[0173] Level 4: The surface has dendritic or network cracks, the crack width is greater than 1mm, and there are coarse through cracks in the transverse and longitudinal directions.

[0174] Table 1. Performance test results of casting coatings in the examples and comparative examples.

[0175]

[0176] The test results above show that the universal coating for lost foam casting and solid casting of cast iron according to the present invention can be compatible with both conventional negative pressure and negative pressure-free lost foam casting processes, solving the long-standing problem of relying on special coatings for negative pressure-free processes. The coating exhibits the synergistic advantages of high permeability (≥90, high temperature permeability ≥110), high high temperature bending strength (≥13MPa), and extremely low penetration depth (≤0.5mm), ensuring the filling integrity and surface finish of castings without vacuum assistance.

[0177] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A universal coating for lost foam casting and solid casting of cast iron, characterized in that, The raw materials include the following parts by weight: 40-50 parts quartz powder, 20-30 parts calcined bauxite, 15-25 parts modified fly ash microspheres, 8-12 parts microcrystalline graphite, and 3.6-6 parts additives. The modified fly ash microspheres include fly ash and a composite coating layer covering the surface of the fly ash. The composite coating layer includes alumina nanosheets and magnesium ion-doped amorphous materials. The fly ash contains ≥60wt% cenospheres and has a particle size of 30μm~150μm. The thickness of the composite coating layer is 50nm~200nm. The preparation method of the modified fly ash microspheres includes the following steps: S1. Immerse fly ash in an acid solution and perform acid washing to obtain pretreated fly ash; Aluminum alkoxides are added to water to carry out a hydrolysis reaction to obtain aluminum sol; magnesium salt solution is added to the aluminum sol and mixed evenly to obtain magnesium ion-containing aluminum sol. The acid solution comprises a 5wt%~10wt% hydrochloric acid solution; the mass-to-volume ratio of the fly ash to the acid solution is 1g:(3~5)mL; the aluminum alkoxide comprises aluminum isopropoxide; the concentration of aluminum ions after adding the aluminum alkoxide to water is 0.5M~1M; the magnesium salt solution comprises a 1M~2M aqueous solution of magnesium nitrate; the molar ratio of aluminum ions in the aluminum sol to magnesium ions in the magnesium salt solution is (8~12):1; S2. The pretreated fly ash is impregnated in the magnesium-containing aluminum sol, wherein the mass-to-volume ratio of the pretreated fly ash to the magnesium-containing aluminum sol is 1g:(10~20)mL; then rotary evaporated to obtain gel-coated fly ash. S3. The gel-coated fly ash is calcined to obtain the modified fly ash microspheres.

2. The universal coating for lost foam casting and solid casting of cast iron as described in claim 1, characterized in that, In S1, the pickling temperature is 60℃~80℃, and the pickling time is 2h~4h; In S1, the temperature of the hydrolysis reaction is 70℃~85℃, and the time of the hydrolysis reaction is 2h~4h; In S2, the immersion temperature is 40℃~60℃, and the immersion time is 30min~60min; In S2, the temperature of the rotary evaporation is 50℃~70℃, the rotation speed of the rotary evaporation is 20rpm~40rpm, and the time of the rotary evaporation is 1.5h~3.5h.

3. The universal coating for lost foam casting and solid casting of cast iron as described in claim 1, characterized in that, In S3, the calcination step specifically includes: pre-calcining the gel-coated fly ash at a rate of 2℃ / min to 5℃ / min to 300℃ to 400℃; then calcining the fly ash at a rate of 3℃ / min to 8℃ / min to 650℃ to 750℃; and cooling the fly ash to below 200℃ in the furnace to obtain the modified fly ash microspheres.

4. The universal coating for lost foam casting and solid casting of cast iron as described in claim 3, characterized in that, In S3, the pre-calcination time is 1h to 2h; the calcination time is 1h to 2h.

5. The universal coating for lost foam casting and solid casting of cast iron as described in claim 1, characterized in that, The particle size of the quartz powder is 180 mesh to 220 mesh; The calcined bauxite contains ≥85% Al2O3 and has a particle size of 300-350 mesh. The modified fly ash microspheres have a particle size of 200 mesh to 400 mesh; The microcrystalline graphite has a particle size of 180 mesh to 220 mesh.

6. The universal coating for lost foam casting and solid casting of cast iron as described in claim 1, characterized in that, The additives include: 2 to 3 parts silica sol, 0.5 to 1 part polyvinyl butyral, 1 to 1.5 parts lithium-based montmorillonite, 0.1 to 0.3 parts water-reducing agent, and 0.05 to 0.1 parts defoamer; The raw materials of the general-purpose coating for lost foam casting and solid casting also include water. The viscosity of the general-purpose coating for lost foam casting and solid casting is 25s~40s at 25℃±2℃ in a Coating-4 cup.

7. The application of the general-purpose coating for lost foam casting and solid casting of cast iron as described in any one of claims 1 to 6 in the casting of gray cast iron or ductile cast iron.

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