Alumina foamed ceramic for casting and filtering molten aluminum

Abstract

The invention discloses aluminum oxide foamed ceramic for casting and filtering molten aluminum and a preparation method of the aluminum oxide foamed ceramic, and belongs to the technical field of aluminum oxide ceramics. The method comprises the following steps: firstly, carrying out plasma activation on polyurethane foam, then immersing the polyurethane foam into a composite modifier system consisting of a main modifier, an auxiliary modifier and a catalyst, and sequentially carrying out first-stage grafting and second-stage cross-linking reaction to introduce carboxyl-hydroxyl composite functional groups and a heterocyclic cross-linking structure to the surface of the foam, so as to obtain a modified precursor; uniformly coating the precursor with aluminum oxide ceramic slurry by adopting a vacuum-pressure alternate impregnation method, and finally drying and sintering at high temperature to obtain a finished product. By constructing a strong interface bonding layer, the adhesion uniformity and firmness of ceramic slurry on an organic precursor are remarkably improved, the aluminum oxide foamed ceramic which is compact in framework, few in defect, high in mechanical strength, excellent in filtering performance and resistant to molten aluminum erosion is obtained, and the aluminum oxide foamed ceramic is suitable for molten liquid purification of high-end aluminum castings.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of alumina ceramics, and particularly relates to an alumina foam ceramic for casting and filtering of aluminum melt. BACKGROUND

[0002] Aluminum and aluminum alloy have been widely used in aerospace, rail transportation, automobile manufacturing, electronic appliances, building decoration and other core industrial fields due to their low density, high specific strength, excellent electrical and thermal conductivity, good corrosion resistance and other outstanding advantages. In the production and processing of aluminum and aluminum alloy products, melt casting is a key process for forming blanks or finished products, and the core requirement is to obtain castings with uniform composition, high purity and no obvious defects. However, during the smelting, transportation and casting of aluminum melt, various harmful impurities are easily introduced, which becomes the core bottleneck restricting the quality of castings.

[0003] As a key purification element in the aluminum melt casting process, the foam ceramic filter becomes a key technical means to solve the above problems through its unique porous structure and ceramic material properties. Foam ceramic can deeply purify the melt and remove harmful inclusions: the impurities in the aluminum melt mainly include non-metallic inclusions and gas inclusions. These impurities can seriously damage the continuity and density of the castings, resulting in pores, pinholes, inclusion defects in the castings, and thus significantly reducing the mechanical properties, processing performance and corrosion resistance of the castings, and even causing early failure of the castings during service. The labyrinth filter channel formed by the three-dimensional network structure of the foam ceramic filter can efficiently intercept non-metallic inclusions through multiple mechanisms such as mechanical interception, adsorption capture and gravity sedimentation, while promoting the aggregation and floating of bubbles in the melt, greatly reducing the gas content of the melt, and is an indispensable key equipment for improving the purity of aluminum melt. During the casting process of aluminum melt, if the flow rate is too fast or the flow field is turbulent, it will suck air to form secondary oxidation, and at the same time, it will impact the mold cavity to cause the mold wall to fall off, introducing impurities again. The porous structure of the foam ceramic has excellent flow resistance regulation ability, which can buffer the impact kinetic energy of the melt, convert turbulent flow into smooth laminar flow, make the melt fill the cavity uniformly and smoothly, reduce the risk of air entrainment and secondary oxidation, and avoid pollution caused by cavity flushing, providing protection for the forming quality of the castings. After the aluminum melt is filtered by the foam ceramic, the impurity content is significantly reduced, and the flow field is stable, so that the crystal nuclei of the castings are uniformly distributed during the solidification process, the grains are refined, and the micro defects such as composition segregation, shrinkage cavity and shrinkage porosity are reduced. The final castings have improved organization density, and not only the mechanical properties and corrosion resistance are significantly improved, but also the tool wear in the subsequent processing process is greatly reduced, and the processing efficiency is improved.

[0004] The preparation of foam ceramics by precursor is the most mainstream and common core technology route in the field of foam ceramics, which has high technical maturity and strong process scalability, and has been widely applied to the industrial production of various foam ceramics of alumina, zirconia, silicon carbide and the like. Among them, polyurethane foam (PU) is the most commonly used precursor of alumina foam ceramics for aluminum melt filtration, because its three-dimensional network structure is regular, the pore size distribution is uniform, the price is low, and it is easy to be decomposed by high temperature sintering, which perfectly matches the needs of aluminum melt filtration scene. However, the surface of polyurethane foam is hydrophobic, and it lacks functional groups that can form strong interaction with alumina ceramic particles, resulting in weak interfacial adhesion between ceramic slurry and precursor, which only relies on physical adsorption or weak hydrogen bond interaction. This makes the slurry prone to agglomeration, local leakage, uneven coating thickness and other problems during coating process, and some micropore channels are even not covered by the slurry, resulting in hollow skeleton or broken skeleton after subsequent sintering. Therefore, it is of great industrial application value to develop a foam ceramic preparation process which can strengthen the interfacial interaction between organic precursor and ceramic slurry, reduce skeleton defects and improve comprehensive performance. SUMMARY

[0005] In view of the above, in order to solve the key problems of poor interfacial compatibility, weak adhesion, insufficient coating uniformity, many ceramic skeleton defects, poor mechanical properties and corrosion resistance of traditional polyurethane foam precursor and alumina ceramic slurry, the present application prepares an alumina foam ceramic for aluminum melt casting and filtration, which has regular pore structure, dense skeleton, firm interfacial adhesion and excellent comprehensive performance, and meets the stringent requirements of high-end aluminum and aluminum alloy castings for melt purification efficiency and filter service life.

[0006] In order to achieve the above purpose, the following technical scheme is adopted: the present application provides an alumina foam ceramic for aluminum melt casting and filtration, comprising the following steps: (1) Pretreatment of polyurethane foam: the polyurethane foam is sequentially subjected to plasma activation, anhydrous ethanol ultrasonic cleaning and vacuum drying to obtain surface-activated polyurethane foam; (2) Composite organic modification treatment: the activated polyurethane foam is immersed in a composite modifier system, and sequentially subjected to first stage grafting reaction and second stage crosslinking reaction; the composite modifier system is composed of main modifier, auxiliary modifier and catalyst in a mass ratio of 10:3-5:0.5-1; the main modifier is obtained by compounding 2-glycidyl furanformate and 3-pyridine acrylic acid in a mass ratio of 2-3:1; the auxiliary modifier is 1,3-bis((trihydroxymethyl)methylamino)propane; the catalyst is a compound of dibutyltin dilaurate and p-toluenesulfonic acid in a mass ratio of 1:1; (3) Modification post-treatment: the foam after reaction is sequentially subjected to Soxhlet extraction with acetone to obtain purified modified polyurethane foam precursor; (4) Preparation of alumina ceramic slurry: alumina ceramic particles, dispersant, binder, coupling agent and water are mixed, and a stable alumina ceramic slurry is prepared after high-shear emulsification; (5) Dip coating: the purified modified polyurethane foam precursor is immersed in the alumina ceramic slurry by vacuum-pressure alternating immersion method to obtain a uniformly coated foam blank; (6) Drying and sintering: the foam blank is dried and sintered at high temperature to form a three-dimensional network structure of porous alumina foam ceramic.

[0007] Further, in step (1), the power of the plasma activation is 30-60 W, the treatment time is 5-15 min, and the atmosphere is argon / oxygen mixed gas with a volume ratio of 1:2-3.

[0008] Further, in step (2), the total mass concentration of the composite modifier system is 8%-15%, and the mass concentration of the main modifier accounts for 60%-70% of the total concentration of the composite modifier system.

[0009] Further, in step (2), the first stage grafting reaction conditions are: temperature 50-70℃, reaction time 2-4h, and the pH value of the system is controlled at 4.5-6.0; the second stage crosslinking reaction conditions are: heating to 75-90℃, reaction time 1.5-3h, and nitrogen protection is applied to form a heterocyclic crosslinked functional surface.

[0010] Further, in step (4), the coupling agent is at least one of γ-(2,3-epoxypropoxy) propyl trimethoxysilane and γ-methacryloyloxypropyl trimethoxysilane, and the addition amount is 0.3%-1.2% of the mass of the alumina ceramic particles; Further, in step (4), the particle size of the alumina ceramic particles is 1-30μm, and the mass fraction in the ceramic slurry is 45%-65%; Further, in step (4), the dispersant is at least one of poly(methacrylic acid ammonium salt), naphthalene sulfonate sodium formaldehyde condensate, and sodium lignosulfonate, and the addition amount is 0.5%-3% of the mass of the alumina ceramic particles.

[0011] Further, in step (4), the binder is a compound of waterborne polyurethane and sodium alginate with a mass ratio of 2-4:1, and the addition amount is 2%-8% of the mass of the alumina ceramic particles.

[0012] Further, in step (5), the slurry temperature of vacuum-pressure alternating immersion is controlled at 30-40℃, and hot air blowing is used after each immersion for 1-2min at a temperature of 60℃ and a wind speed of 1-2m / s before the next immersion.

[0013] Further, the high-temperature sintering stage in step (6) is at 1000-1200℃ for 1-3h.

[0014] The beneficial effects of the present application are: The present application successfully constructs an active interface layer rich in carboxyl-hydroxyl functional groups and heterocyclic crosslinking structure on the surface of polyurethane foam through plasma activation pretreatment and two-stage composite modification process. The interface layer not only forms stable Al-O-C chemical bonds through carboxyl and hydroxyl on the surface of alumina particles, but also produces strong hydrogen bonds through hydroxyl, and physically adsorbs ceramic particles by means of the π-π stacking effect of the heterocyclic structure. Through the synergy of chemical bonds, hydrogen bonds and physical adsorption, an interface bonding network stronger than traditional single physical impregnation or simple functional group modification is formed, ensuring that the ceramic slurry can be uniformly and firmly attached to the surface of the precursor three-dimensional skeleton, including the internal micropores, so as to form a continuous, dense and significantly reduced defect ceramic skeleton in the subsequent sintering process, and finally improve the mechanical strength of the foam ceramic.

[0015] The first-stage grafting reaction firmly grafts the carboxyl-containing heterocyclic main modifier under mild conditions; the second-stage crosslinking reaction forms a stable network structure by crosslinking the grafted chains through the auxiliary modifier under the protection of nitrogen by heating, prevents the loss of functional groups in the subsequent process, and adds a specific proportion of silane coupling agent in the slurry, which can not only be combined with the surface of alumina particles, but also interact with the functional groups on the surface of the modified polyurethane through its organic end, further bridging and strengthening the interface between the ceramic phase and the organic precursor. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 SEM images of the alumina foam ceramics prepared for the examples and comparative examples of the present application.

[0017] The accompanying drawings are included to provide a further understanding of the present application, and constitute a part of the specification, illustrate the present application together with the embodiments thereof, and explain the present application, but do not constitute a limitation of the present application. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, and not all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present application. The preferred materials and methods are described herein, although any method and material similar or equivalent to those described herein can be used. The materials, methods, and examples are illustrative only and not intended to be limiting.

[0020] The experimental methods in the following examples are routine unless otherwise stated. The experimental materials used in the following examples are commercially available unless otherwise stated. Example

[0021] An alumina foam ceramic for casting and filtering of molten aluminum, a preparation method thereof comprising the following steps: (1) Pretreatment of polyurethane foam: the polyurethane foam was sequentially subjected to plasma activation treatment for 5 min (at a power of 30 W, in an argon / oxygen mixed gas atmosphere at a volume ratio of 1:2), ultrasonic cleaning with absolute ethanol, and vacuum drying to obtain surface-activated polyurethane foam; (2) Composite organic modification treatment: the activated polyurethane foam was immersed in a composite modifier system with a total mass concentration of 8%, and sequentially subjected to first-stage grafting reaction and second-stage crosslinking reaction; the composite modifier system was composed of a main modifier, an auxiliary modifier, and a catalyst at a mass ratio of 10:3:0.5, wherein the mass concentration of the main modifier accounted for 60% of the total concentration of the composite modifier system; the main modifier was a compound of 2-furan carboxylic acid glycidyl ester and 3-pyridine acrylic acid at a mass ratio of 2:1; the auxiliary modifier was 1,3-bis((trihydroxymethyl)methylamino)propane; and the catalyst was a compound of dibutyltin dilaurate and p-toluenesulfonic acid at a mass ratio of 1:1; The first-stage grafting reaction was carried out at a temperature of 50°C for 2 h, and the pH value of the system was controlled at 4.5; the second-stage crosslinking reaction was carried out at a temperature of 75°C for 1.5 h under nitrogen protection, to obtain a modified polyurethane foam precursor; (3) Post-treatment of the modified foam: the foam after the reaction of step (2) was sequentially subjected to Soxhlet extraction with acetone, to obtain a purified modified polyurethane foam precursor; (4) Preparation of alumina ceramic slurry: alumina ceramic particles with a particle size of 1 μm, a dispersant, a binder, a coupling agent, and water were mixed and subjected to high-shear emulsification to prepare a stable alumina ceramic slurry; the mass fraction of the alumina ceramic particles in the ceramic slurry was 45%; the coupling agent was γ-(2,3-epoxypropoxy)propyltrimethoxysilane, and the addition amount was 0.3% of the mass of the alumina ceramic particles; the dispersant was poly(methacrylic acid ammonium salt), and the addition amount was 0.5% of the mass of the alumina ceramic particles; and the binder was a compound of waterborne polyurethane and sodium alginate at a mass ratio of 2:1, and the addition amount was 2% of the mass of the alumina ceramic particles; (5) Dip-coating: the purified modified polyurethane foam precursor is dipped into the alumina ceramic slurry by vacuum-pressure alternating dipping method, the slurry temperature is controlled at 30℃, after each dipping, hot air with a temperature of 60℃ and a wind speed of 1m / s is used to blow for 1min, then the next dipping is carried out, thus a foam blank with uniform coating is obtained; (6) Drying and sintering: the foam blank is dried and sintered at a high temperature, and is kept at 1000℃ for 1h, thus a porous alumina foam ceramic with three-dimensional network structure is formed. Embodiment

[0022] An alumina foam ceramic for aluminum melt casting and filtering, and a preparation method thereof, the preparation method comprising the following steps: (1) Pretreatment of polyurethane foam: the polyurethane foam is sequentially subjected to plasma activation treatment for 15min (the atmosphere is argon / oxygen mixed gas with a volume ratio of 1:3), ultrasonic cleaning with anhydrous ethanol, and vacuum drying, thus obtaining surface-activated polyurethane foam; (2) Composite organic modification: the activated polyurethane foam is dipped into a composite modifier system with a total mass concentration of 15%, and is sequentially subjected to first-stage grafting reaction and second-stage crosslinking reaction; the composite modifier system is composed of main modifier, auxiliary modifier, and catalyst with a mass ratio of 10:5:1, wherein the mass concentration of the main modifier accounts for 70% of the total concentration of the composite modifier system; the main modifier is a compound of 2-furan carboxylic acid glycidyl ester and 3-pyridine acrylic acid with a mass ratio of 3:1; the auxiliary modifier is 1,3-bis((trihydroxymethyl)methylamino)propane; and the catalyst is a compound of dibutyltin dilaurate and p-toluenesulfonic acid with a mass ratio of 1:1; The first-stage grafting reaction conditions are as follows: temperature 70℃, reaction time 4h, and pH value of the system controlled at 6.0; the second-stage crosslinking reaction conditions are as follows: temperature raised to 90℃, reaction time 3h, and nitrogen protection, thus obtaining modified polyurethane foam precursor; (3) Post-treatment of the modified polyurethane foam: the foam after the reaction in step (2) is sequentially subjected to Soxhlet extraction with acetone, thus obtaining purified modified polyurethane foam precursor; (4) Preparation of alumina ceramic slurry: alumina ceramic particles with a particle size of 30μm, dispersant, binder, coupling agent, and water are mixed and emulsified by high shear to prepare stable alumina ceramic slurry; the mass fraction of the alumina ceramic particles in the ceramic slurry is 65%; the coupling agent is γ-methacryloyloxypropyltrimethoxysilane, and the addition amount is 1.2% of the mass of the alumina ceramic particles; the dispersant is sodium lignosulfonate, and the addition amount is 3% of the mass of the alumina ceramic particles; and the binder is a compound of waterborne polyurethane and sodium alginate with a mass ratio of 4:1, and the addition amount is 8% of the mass of the alumina ceramic particles; (5) Dip coating: the purified modified polyurethane foam precursor is immersed in the alumina ceramic slurry by vacuum-pressure alternating immersion method, the slurry temperature is controlled at 40℃, after each immersion, hot air with a temperature of 60℃ and a wind speed of 2m / s is used to blow for 2min, then the next immersion is carried out, and a foam blank with uniform coating is obtained; (6) Drying and sintering: the foam blank is subjected to drying treatment and high-temperature sintering, and is kept at 1200℃ for 3h, and a three-dimensional network structure porous alumina foam ceramic is formed. Embodiment

[0023] An alumina foam ceramic for aluminum melt casting and filtering, a preparation method thereof comprises the following steps: (1) Pretreatment of polyurethane foam: the polyurethane foam is sequentially subjected to plasma activation treatment for 10min (the atmosphere is argon / oxygen mixed gas with a volume ratio of 1:2.5), ultrasonic cleaning with anhydrous ethanol, and vacuum drying, and a surface-activated polyurethane foam is obtained; (2) Composite organic modification treatment: the activated polyurethane foam is immersed in a composite modifier system with a total mass concentration of 12%, and is sequentially subjected to first-stage grafting reaction and second-stage crosslinking reaction; the composite modifier system is composed of a main modifier, an auxiliary modifier, and a catalyst with a mass ratio of 10:4:0.8, wherein the mass concentration of the main modifier accounts for 65% of the total concentration of the composite modifier system; the main modifier is obtained by compounding 2-furanformic acid glycidyl ester and 3-pyridine acrylic acid with a mass ratio of 2.5:1; the auxiliary modifier is 1,3-bis((trihydroxymethyl)methylamino)propane; and the catalyst is a compound of dibutyltin dilaurate and p-toluenesulfonic acid with a mass ratio of 1:1; The first-stage grafting reaction conditions are as follows: the temperature is 60℃, the reaction time is 3h, and the pH value of the system is controlled at 5.2; the second-stage crosslinking reaction conditions are as follows: the temperature is increased to 82℃, the reaction time is 2h, and nitrogen is introduced for protection, and a modified polyurethane foam precursor is obtained; (3) Post-treatment of the modified polyurethane foam: the foam after the reaction in step (2) is sequentially subjected to Soxhlet extraction with acetone, and a purified modified polyurethane foam precursor is obtained; (4) Preparation of alumina ceramic slurry: alumina ceramic particles with a particle size of 15 μm, dispersant, binder, coupling agent and water are mixed, and a stable alumina ceramic slurry is prepared after high-shear emulsification; the mass fraction of the alumina ceramic particles in the ceramic slurry is 55%; the coupling agent is composed of γ-(2,3-epoxypropoxy) propyl trimethoxysilane and γ-methacryloyloxypropyl trimethoxysilane at a mass ratio of 1:1, and the addition amount is 0.8% of the mass of the alumina ceramic particles; the dispersant is composed of poly-methacrylic acid ammonium salt, naphthalene sulfonate sodium formaldehyde condensate and sodium lignosulfonate at a mass ratio of 2:1:1, and the addition amount is 1.8% of the mass of the alumina ceramic particles; the binder is a compound of water-based polyurethane and sodium alginate at a mass ratio of 3:1, and the addition amount is 5% of the mass of the alumina ceramic particles; (5) Dip coating: the purified modified polyurethane foam precursor is immersed in the alumina ceramic slurry by using a vacuum-pressure alternating immersion method, the slurry temperature is controlled at 35℃, after each immersion, hot air with a temperature of 60℃ and a wind speed of 1.5 m / s is used for blowing for 1.5 min, and then the next immersion is performed, to obtain a foam blank with uniform coating; (6) Drying and sintering: the foam blank is subjected to drying treatment and high-temperature sintering, and is kept at 1100℃ for 2 h, to form a three-dimensional network structure of porous alumina foam ceramic.

[0024] Comparative Example 1: This comparative example only performs the steps (1) plasma activation and cleaning and drying of Example 3, and then directly performs the steps (4)-(6) of slurry preparation, dip coating and sintering. That is, the polyurethane foam which is only plasma-activated but not chemically modified is used as the precursor.

[0025] Comparative Example 2: This comparative example only performs the first-stage grafting reaction under the same pretreatment and composite modifier system conditions of Example 3, without performing the second-stage temperature rising crosslinking reaction, and then directly performs the subsequent steps.

[0026] Comparative Example 3: This comparative example replaces the composite modifier system with an equal mass concentration of a single acrylic acid (providing carboxyl) aqueous solution, uses the same pretreatment, reaction conditions and subsequent steps, and the rest is the same as Example 3.

[0027] Result analysis The microstructure of the samples of each example and comparative example of the application is characterized by SEM images, and the results are shown in Figure 1 wherein (a), (b), (c) and (d) correspond to Example 3, Comparative Example 1, Comparative Example 2 and Comparative Example 3, respectively; as shown in Figure 1 Example 3 shows a more complex and complete porous structure than the comparative examples.

[0028] The compression strength (MPa) and bending strength (MPa) of each group of example and comparative sample were tested by a universal material testing machine; each group of example and comparative sample was tested on the aluminum melt with temperature of 700-750℃, the mass concentration of solid particles in filtrate and raw material liquid was calculated by laser particle size analysis to characterize the filtration efficiency of the foamed ceramic, and the strength retention rate after the aluminum melt erosion after 10 cycles of filtration was tested. The above test results are shown in Table 1.

[0029] Table 1 Comparison of performance test results of each group of sample As shown in Table 1, the porosity of the examples is higher than that of all comparative examples, the average pore size is concentrated in 500-510 μm, the compression strength and bending strength of each example are 3-4 times of that of comparative example 1, far exceeding comparative examples 2 and 3, the filtration efficiency of the examples is all > 99.0%, while that of comparative example 1 is only 95.0%, that of comparative example 2 is 97.5%, and that of comparative example 3 is 98.0%, the strength retention rate after 10 cycles of filtration is much higher than that of comparative example 1 (70%), comparative example 2 (80%) and comparative example 3 (85%). Therefore, the present application can effectively solve the problems of poor interface compatibility of the precursor and the slurry and uneven coating in the traditional process, and finally make the foamed ceramic have the comprehensive advantages of regular pore structure, excellent mechanical properties, high filtration efficiency and strong resistance to aluminum melt erosion, which is significantly better than the comparative example scheme without chemical modification, lack of cross-linking reaction and single modifier, and can meet the strict requirements of high-end aluminum and aluminum alloy castings on melt purification efficiency and filter service life.

[0030] Although the embodiments of the present application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made thereto without departing from the principles and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

[0031] The above describes the present application and its embodiments, which is not limited, and the drawings only show one of the embodiments of the present application, and the actual application is not limited thereto. In general, if the ordinary skilled in the art is inspired, without departing from the purpose of the present application, without creative design, similar ways and embodiments of the technical solution should belong to the protection scope of the present application.

Claims

1. An alumina ceramic foam for casting and filtering of molten aluminum, characterized by: The method comprises the following steps: (1) Pretreatment of polyurethane foam: the polyurethane foam is sequentially subjected to plasma activation, ultrasonic cleaning with anhydrous ethanol and vacuum drying to obtain surface-activated polyurethane foam; (2) Composite organic modification treatment: the activated polyurethane foam is immersed in a composite modifier system, and sequentially subjected to first-stage grafting reaction and second-stage crosslinking reaction; the composite modifier system is composed of a main modifier, an auxiliary modifier and a catalyst at a mass ratio of 10:3-5:0.5-1; the main modifier is a compound of 2-furan carboxylic acid glycidyl ester and 3-pyridine acrylic acid at a mass ratio of 2-3:1; the auxiliary modifier is 1,3-bis((trihydroxymethyl)methylamino)propane; and the catalyst is a compound of dibutyltin dilaurate and p-toluenesulfonic acid at a mass ratio of 1:1; (3) Post-treatment of the modified foam: the foam after the reaction is sequentially subjected to Soxhlet extraction with acetone to obtain purified modified polyurethane foam precursor; (4) Preparation of alumina ceramic slurry: alumina ceramic particles, a dispersing agent, a binder, a coupling agent and water are mixed and subjected to high-shear emulsification to prepare stable alumina ceramic slurry; (5) Impregnation coating: the purified modified polyurethane foam precursor is immersed in the alumina ceramic slurry by using a vacuum-pressure alternating impregnation method to obtain foam blank with uniform coating; (6) Drying and sintering: the foam blank is subjected to drying treatment and high-temperature sintering to form three-dimensional network structured porous alumina foam ceramic.

2. The aluminum melt pouring filtration alumina ceramic foam according to claim 1, characterized in that: In the step (1), the power of plasma activation is 30-60 W, the treatment time is 5-15 min, and the atmosphere is argon / oxygen mixed gas at a volume ratio of 1:2-3.

3. The alumina foam ceramic for casting and filtering aluminum molten metal according to claim 2, characterized in that: In the step (2), the total mass concentration of the composite modifier system is 8%-15%, and the mass concentration of the main modifier accounts for 60%-70% of the total concentration of the composite modifier system.

4. The aluminum melt casting filtration alumina ceramic foam according to claim 3, characterized in that: In the step (2), the first-stage grafting reaction is carried out at a temperature of 50-70 ℃ for 2-4 h, and the pH value of the system is controlled at 4.5-6.0; the second-stage crosslinking reaction is carried out at a temperature of 75-90 ℃ for 1.5-3 h under nitrogen protection to form heterocyclic crosslinked functional groups on the surface.

5. The aluminum melt casting filtration alumina ceramic foam according to claim 4, characterized by: In the step (4), the coupling agent is at least one of γ-(2,3-epoxypropoxy)propyl trimethoxysilane and γ-methacryloyloxypropyl trimethoxysilane, and the addition amount is 0.3%-1.2% of the mass of the alumina ceramic particles.

6. The aluminum melt pouring filtration alumina ceramic foam according to claim 5, characterized in that: In the step (4), the particle size of the alumina ceramic particles is 1-30 μm, and the mass fraction in the ceramic slurry is 45%-65%.

7. The aluminum melt pouring filtration alumina ceramic foam according to claim 6, characterized in that: In the step (4), the dispersing agent is at least one of poly(methacrylic acid ammonium salt), naphthalene sulfonate sodium formaldehyde condensate and sodium lignosulfonate, and the addition amount is 0.5%-3% of the mass of the alumina ceramic particles.

8. The aluminum melt pouring filtration alumina ceramic foam according to claim 7, characterized in that: In the step (4), the binder is a compound of water-based polyurethane and sodium alginate at a mass ratio of 2-4:1, and the addition amount is 2%-8% of the mass of the alumina ceramic particles.

9. The aluminum melt pouring filtration alumina ceramic foam according to claim 8, characterized in that: The temperature of the slurry in the step (5) is controlled at 30-40℃, and hot air is blown for 1-2 min after each impregnation, with a temperature of 60℃ and a wind speed of 1-2 m / s, and then the next impregnation is performed.

10. The aluminum melt pouring filtration alumina ceramic foam according to claim 9, characterized in that: In the step (6), the high-temperature sintering stage is performed at 1000-1200℃ for 1-3 h.

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