Preparation method of precision casting surface layer high-temperature-resistant material for replacing zircon sand

Abstract

The application relates to the technical field of refractory materials, in particular to a preparation method of a precision casting surface layer high-temperature-resistant material for replacing zircon sand. The method comprises the following steps: firstly, alkali treatment and acid neutralization treatment are conducted on mullite powder to obtain hydroxylated mullite powder; secondly, surface adsorption is conducted by using a branched polyethyleneimine aqueous solution, so that aluminum phosphate formed by aluminum nitrate nonahydrate and ammonium dihydrogen phosphate is deposited on the surface of the mullite; then, a composite precursor is formed through lanthanum ion pre-adsorption, segmented phosphate dropwise addition, yttrium citric acid complex aqueous solution insertion and magnesium source graded deposition; finally, the surface layer high-temperature-resistant material is obtained through washing, drying and segmented heat treatment. The application takes mullite as the main body, can reduce the dependence on zircon sand and zircon powder, and can improve the metal liquid reactivity, high-temperature volume stability and casting surface forming quality of the precision casting surface layer.

Description

Technical Field

[0001] This invention relates to the field of refractory materials technology, specifically to a method for preparing a high-temperature resistant material for precision casting surface layers that can replace zirconium sand. Background Technology

[0002] Precision casting, also known as investment casting, is suitable for forming complex thin-walled castings made of stainless steel, heat-resistant steel, and nickel-based high-temperature alloys. The ceramic mold shell surface layer directly contacts the high-temperature molten metal. The chemical stability, thermal stability, and surface forming ability of its refractory powder and sand-spreading materials affect whether the casting will suffer from sand adhesion, sand inclusion, surface roughness, reaction layer thickening, and dimensional deviations. Therefore, the surface layer material typically requires a combination of high refractoriness, low high-temperature reactivity, and good thermal shock resistance.

[0003] The existing surface layer mostly uses zircon sand and zircon powder, which have high refractoriness, low thermal expansion, and weak reactivity with various molten metals. However, zircon materials are greatly affected by the source, grade, price, and impurity content. Some raw materials also involve the control of radioactive impurities. Long-term use will increase the raw material costs and supply fluctuation risks for precision casting enterprises.

[0004] Mullite possesses high refractoriness, good high-temperature strength, and stable sourcing, and its cost is generally lower than that of zircon sand, making it a potential substitute. However, the surface of ordinary mullite particles contains aluminum-oxygen sites, silicon-oxygen sites, and local glassy phase regions. Under the interfacial environment of molten metal scouring and oxide involvement, it is prone to wetting, adhesion, or reaction layer thickening, leading to difficulties in delamination and a decline in the surface quality of castings.

[0005] Meanwhile, under the thermal shock of shell baking and casting, mullite particles are prone to local sintering neck growth in the particle contact area, causing uneven surface shrinkage. If continuous dense coating is used to reduce reactivity, it may cause the slurry viscosity to increase, the air permeability to decrease, and the thermal expansion to be mismatched. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a method for preparing a high-temperature resistant material for precision casting surface layer that can replace zircon sand, so as to solve the problems of large cost and supply fluctuations of existing zircon sand surface layer, and the easy interface reaction and high-temperature shrinkage of ordinary mullite when used in complex thin-walled precision castings, which leads to sand adhesion, difficulty in shell removal and unstable surface forming quality.

[0007] To achieve the above objectives, the present invention provides a method for preparing a high-temperature resistant surface material for precision casting as an alternative to zirconium sand, comprising the following steps: (1) Alkali treatment and acid neutralization treatment of mullite powder to obtain hydroxylated mullite powder; (2) The hydroxylated mullite powder is dispersed in branched polyethyleneimine for adsorption treatment to obtain branched polyethyleneimine modified mullite wet powder. (3) Add aluminum nitrate nonahydrate and ammonium dihydrogen phosphate to the branched polyethyleneimine modified mullite wet powder to deposit aluminum phosphate on the surface of the mullite powder. After drying and pre-calcination, aluminum phosphate modified mullite powder is obtained. (4) The aluminum phosphate modified mullite powder is dispersed in deionized water and then lanthanum nitrate hexahydrate is added for lanthanum ion pre-adsorption. Then, the first stage of phosphate aqueous solution is added dropwise for lanthanum phosphate nucleation. Yttrium-citric acid complex aqueous solution is added, and then the second stage of phosphate aqueous solution is added dropwise for aging. Subsequently, the system is adjusted to weak alkalinity, and magnesium-citric acid complex aqueous solution is added first, followed by magnesium acetate tetrahydrate for deposition to obtain a composite precursor suspension. The composite precursor suspension is filtered, washed, dried, and subjected to segmented heat treatment to obtain a high-temperature resistant material for precision casting surface layer that can replace zirconium sand. Based on 1000 parts by weight of mullite powder, the amounts of branched polyethyleneimine, aluminum nitrate nonahydrate, ammonium dihydrogen phosphate, lanthanum nitrate hexahydrate, first-stage phosphate aqueous solution, yttrium-citric acid complex aqueous solution, second-stage phosphate aqueous solution, magnesium-citric acid complex aqueous solution, and magnesium acetate tetrahydrate are 4-8 parts, 28-34 parts, 9-11 parts, 50-60 parts, 600 parts, 300 parts, 400 parts, 300 parts, and 2-4 parts, respectively.

[0008] Preferably, the mullite powder in step (1) is fused white mullite powder, wherein the particle size of the fused white mullite powder is 500 mesh, the median particle size is 20-30 μm, the mass fraction of alumina is 70%-77%, the mass fraction of silica is 22%-29%, the mass fraction of iron oxide is not higher than 0.20%, the mass fraction of sodium oxide is not higher than 0.40%, and the bulk density is not lower than 2.90 g / cm³. 3 Its fire resistance is not less than 1850℃.

[0009] Preferably, the alkali treatment in step (1) includes adding mullite powder to an aqueous sodium hydroxide solution and stirring at 65-75°C for 30-50 minutes.

[0010] Preferably, the acid neutralization treatment in step (1) includes adding the alkali-treated mullite wet powder to a dilute nitric acid aqueous solution and stirring at 25-30°C for 15-25 minutes.

[0011] Preferably, the sodium hydroxide aqueous solution in step (1) has a mass fraction of 3%.

[0012] Preferably, the branched polyethyleneimine in step (2) exists in the form of an aqueous solution with a mass fraction of 50%.

[0013] Preferably, the adsorption treatment conditions in step (2) are 25-30℃ and stirring for 20-40 min.

[0014] Preferably, after the aluminum phosphate deposition in step (3), the system needs to be heated to 75-85°C for aging for 70-110 minutes.

[0015] Preferably, the drying in step (3) is maintained at 100-120°C for 2-4 hours.

[0016] Preferably, the pre-firing in step (3) includes placing the product in an air atmosphere and heating it to 430-470°C at a rate of 1.5-2.5°C / min and holding it at that temperature for 0.5-1.5 hours.

[0017] Preferably, based on a weight of 600 parts, the first phosphate aqueous solution in step (4) contains 8-11 parts of ammonium dihydrogen phosphate, and the remainder is deionized water.

[0018] Preferably, based on 300 parts by weight, the yttrium-citric acid complex aqueous solution in step (4) contains 3-5g of yttrium nitrate hexahydrate and 2-4g of citric acid monohydrate, with the remainder being deionized water.

[0019] Preferably, based on a weight of 400 parts, the second phosphate aqueous solution in step (4) contains 4-7 parts of ammonium dihydrogen phosphate, and the remainder is deionized water.

[0020] Preferably, based on 300 parts by weight, the magnesium-citric acid complex aqueous solution in step (4) contains 4-6g magnesium acetate tetrahydrate and 3-5g citric acid monohydrate, with the remainder being deionized water.

[0021] Preferably, the aluminum nitrate nonahydrate, ammonium dihydrogen phosphate, and magnesium acetate tetrahydrate all participate in the reaction in the form of aqueous solutions.

[0022] Preferably, the washing in step (4) is performed using deionized water to remove unadsorbed nitrate, ammonium and free salts.

[0023] Preferably, the drying in step (4) is maintained at 100-120°C for 5-8 hours.

[0024] Preferably, the segmented heat treatment in step (4) includes: placing the composite precursor powder in an air atmosphere, heating it to 320-380℃ at 1.5-2.5℃ / min and holding it at that temperature for 0.8-1.5h; then heating it to 680-730℃ at 1.5-2.5℃ / min and holding it at that temperature for 1.5-2.5h; then heating it to 1160-1220℃ at 2.5-3.5℃ / min and holding it at that temperature for 1.5-2.5h.

[0025] The beneficial effects of this invention are: This invention performs surface hydroxylation treatment on fused white mullite powder and utilizes branched polyethyleneimine aqueous solution to form temporary cation adsorption regions, so that aluminum phosphate is preferentially fixed on the active region of mullite particle surface; after pre-calcination, aluminum phosphate anchoring points are retained, providing a basis for the subsequent directional formation of lanthanum phosphate.

[0026] This invention utilizes lanthanum ion pre-adsorption, first-stage phosphate nucleation, yttrium citric acid complex aqueous solution insertion, and second-stage phosphate locking to form a discontinuous barrier around the aluminum phosphate anchoring point of lanthanum phosphate. This reduces direct contact between the molten metal and the aluminum oxide and silicon oxide sites on the mullite surface, while also taking into account the adaptability of the slurry for construction.

[0027] This invention allows the yttrium source to first enter the edge of lanthanum phosphate and the silicon-oxygen window, and then the magnesium source to enter the aluminum-oxygen window and the outer edge of aluminum phosphate. It also uses a slow-release magnesium citric acid complex aqueous solution and a fast-release magnesium acetate aqueous solution to be added in stages, thereby stabilizing the barrier edge and particle contact area and reducing the tendency of local sintering shrinkage.

[0028] This invention uses mullite as the main material to form a high-temperature resistant surface layer. While reducing the dependence on zircon sand and zircon powder, it improves the stability of the surface layer interface, which is beneficial to improving the shell removal quality, surface forming quality and high-temperature stability of thin-walled complex castings of stainless steel, heat-resistant steel and nickel-based high-temperature alloys.

[0029] Compared to existing methods that directly use zircon sand, zircon powder, or ordinary mullite surface layer, this invention does not simply replace the refractory aggregate, but instead constructs an anchoring, barrier, and pinning progressive structure on the surface of mullite particles, making it more suitable for use as a surface shell for complex thin-walled precision castings. This can reduce raw material cost fluctuations and improve interfacial reactions and surface shrinkage during high-temperature casting. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0031] Raw material source and model parameters Fused white mullite: 80-120 mesh standard fused white mullite, with an alumina mass fraction of 70% to 77%, a silica mass fraction of 22% to 29%, an iron oxide mass fraction not exceeding 0.20%, a sodium oxide mass fraction not exceeding 0.40%, and a bulk density not less than 2.90 g / cm³. 3 Its fire resistance is not less than 1850℃; Branched polyethyleneimine aqueous solution: average weight-average molecular weight of 1800, mass fraction of 50%.

[0032] Example 1: A method for preparing a high-temperature resistant surface material for precision casting as an alternative to zircon sand, the specific steps of which are as follows: S1: 100g of sodium hydroxide was added to deionized water and stirred until completely dissolved. The solution was then cooled to 30°C to form a 3% sodium hydroxide aqueous solution. 1000g of fused white mullite powder was added to the sodium hydroxide aqueous solution and stirred at 70°C and 300r / min for 40min. After filtration, the solution was washed three times with 2000g of deionized water. Separately, 50g of nitric acid was slowly added to 2450g of deionized water to form a dilute nitric acid aqueous solution. The washed mullite wet powder was added to the dilute nitric acid aqueous solution and stirred at 30°C for 20min. After filtration, the solution was washed three times with 2000g of deionized water until the pH of the final wash solution was 6 to 8. The solution was then dried at 110°C for 4h to obtain hydroxylated mullite powder. S2: 10g of nitric acid was slowly added to deionized water to form a 1% (w / w) dilute nitric acid aqueous solution for later use. Separately, 6g of branched polyethyleneimine aqueous solution was added to 1994g of deionized water and stirred at 25℃ for 20min to achieve complete dispersion. The pH of the system was then adjusted to 6.0 to 6.5 with dilute nitric acid aqueous solution. All the hydroxylated mullite powder obtained in S1 was added to the branched polyethyleneimine aqueous solution and stirred at 25℃ and 300r / min for 30min. After filtration, the mixture was not subjected to vigorous water washing, but was allowed to drain naturally for 10min to obtain branched polyethyleneimine modified mullite wet powder. S3: All the mullite wet powder obtained from S2 was added to 1800g of deionized water and stirred at 50℃ and 300r / min for 20min. 30g of aluminum nitrate nonahydrate was added and stirring was continued for 20min. Then, 10g of ammonium dihydrogen phosphate was dissolved in 500g of deionized water to obtain an ammonium dihydrogen phosphate aqueous solution. This ammonium dihydrogen phosphate aqueous solution was added dropwise to the above suspension over 60min. During the dropwise addition, the pH was maintained at 3.4 to 3.8 with ammonia water. After the dropwise addition was completed, the temperature was raised to 80℃ and aged for 90min. After filtration, it was quickly washed once with 1000g of deionized water, dried at 110℃ for 3h, and then placed in an air atmosphere and heated to 450℃ at 2℃ / min and kept at that temperature for 1h to obtain aluminum phosphate modified mullite powder. S4: All the powder obtained from S3 was added to 2500g of deionized water and stirred at 50℃ and 400r / min for 30min. 55g of lanthanum nitrate hexahydrate was added and stirring was continued for 45min. The pH was adjusted to 4.8 to 5.2 with ammonia. Then, 10g of ammonium dihydrogen phosphate was dissolved in deionized water to obtain 600g of the first-stage phosphate aqueous solution. The first-stage phosphate aqueous solution was added dropwise to the suspension at a rate of 5g / min. During the addition, the pH was maintained at 4.6 to 4.9 with ammonia. After the addition was completed, stirring was continued for 40min. S5: Separately, 4g of yttrium nitrate hexahydrate and 3g of citric acid monohydrate were added to deionized water, stirred until clear, and the pH was adjusted to 4.6 to 4.8 with ammonia water to obtain 300g of yttrium-citric acid complex aqueous solution. This yttrium-citric acid complex aqueous solution was added to the suspension obtained in S4 within 20 min, and the mixture was stirred at 50°C for 30 min. Subsequently, 5g of ammonium dihydrogen phosphate was dissolved in deionized water to obtain 400g of the second-stage phosphate aqueous solution. This second-stage phosphate aqueous solution was added dropwise to the suspension at a rate of 3g / min, and the pH was controlled at 4.6 to 5.0 during the addition. After the addition was completed, the temperature was raised to 70°C and aged for 2 h. S6: The suspension obtained from S5 was cooled to 50°C, and ammonia was added to raise the pH of the system from 4.6-5.0 to 8.4-8.8. Separately, 5g of magnesium acetate tetrahydrate and 4g of citric acid monohydrate were added to deionized water, stirred until clear, and the pH was adjusted to 6.5-6.8 with ammonia to obtain 300g of magnesium-citric acid complex aqueous solution. This magnesium-citric acid complex aqueous solution was added to the above suspension within 30 min, and the pH of the system was maintained at 8.4-8.8 with ammonia, and stirring was continued for 25 min. Subsequently, 3g of magnesium acetate tetrahydrate was dissolved in 200g of deionized water to obtain magnesium acetate aqueous solution. This magnesium acetate aqueous solution was added to the suspension within 10 min, and the pH was maintained at 8.4-8.8 with ammonia, and stirring was continued for 40 min. S7: The suspension obtained from S6 was filtered, and the filter cake was quickly washed once with 500g of deionized water to remove unadsorbed nitrate, ammonium and free salts. The filter cake was dried at 110℃ for 6h to obtain composite precursor powder. The composite precursor powder was placed in an air atmosphere and heated to 350℃ at 2℃ / min and held for 1h to remove citrate, acetate and residual organic groups. Then it was heated to 700℃ at 2℃ / min and held for 2h. Then it was heated to 1200℃ at 3℃ / min and held for 2h. Then it was cooled to room temperature in the furnace. After cooling, it was slightly depolymerized and passed through a 325 mesh sieve to obtain a high-temperature resistant material for precision casting surface layer to replace zircon sand.

[0033] The difference between Example 2 and Example 1 is as follows: In S2, 4g of branched polyethyleneimine aqueous solution and 1996g of deionized water are added; in S3, 28g of aluminum nitrate nonahydrate is added, and 9g of ammonium dihydrogen phosphate is dissolved in 450g of deionized water, with the ammonium dihydrogen phosphate aqueous solution added dropwise over 45min, followed by aging at 75°C for 70min after the addition is complete; in S4, 50g of lanthanum nitrate hexahydrate is added, and 8g of ammonium dihydrogen phosphate is dissolved in 550g of deionized water, with the solution added dropwise at a rate of 4g / min; in S5, 3g of yttrium nitrate hexahydrate is added. The mixture consisted of 2g of magnesium acetate tetrahydrate and 2g of citric acid monohydrate. 4g of ammonium dihydrogen phosphate was dissolved in 350g of deionized water and added dropwise at a rate of 2g / min. After the addition was complete, the mixture was aged at 65°C for 1.5h. In S6, the pH of the system was raised to 8.2, and a magnesium-citric acid complex aqueous solution prepared from 4g of magnesium acetate tetrahydrate and 3g of citric acid monohydrate was added, followed by a magnesium acetate aqueous solution prepared from 2g of magnesium acetate tetrahydrate. In S7, the third stage of heat treatment was carried out at 1160°C for 1.5h. The remaining conditions were the same as in Example 1.

[0034] The difference between Example 3 and Example 1 is as follows: In S2, 8g of branched polyethyleneimine aqueous solution and 1992g of deionized water were added; in S3, 34g of aluminum nitrate nonahydrate was added, and 11g of ammonium dihydrogen phosphate was dissolved in 550g of deionized water, with the ammonium dihydrogen phosphate aqueous solution added dropwise over 75min, followed by aging at 85°C for 110min after the addition was complete; in S4, 60g of lanthanum nitrate hexahydrate was added, and 11g of ammonium dihydrogen phosphate was dissolved in 650g of deionized water, with the solution added dropwise at a rate of 6g / min; in S5, 5g of yttrium nitrate was added. The hexahydrate and 4g citric acid monohydrate were added, and 7g ammonium dihydrogen phosphate was dissolved in 450g deionized water and added dropwise at a rate of 4g / min. After the addition was completed, the mixture was aged at 75°C for 2.5h. In S6, the pH of the system was raised to 8.9, and a magnesium citric acid complex aqueous solution prepared by 6g magnesium acetate tetrahydrate and 5g citric acid monohydrate was added, followed by a magnesium acetate aqueous solution prepared by 4g magnesium acetate tetrahydrate. In S7, the third stage of heat treatment was carried out at 1220°C for 2.5h. The remaining conditions were the same as in Example 1.

[0035] The difference between Example 4 and Example 1 is as follows: In step one, the sodium hydroxide aqueous solution treatment temperature is 65℃, the stirring time is 50 min, and the stirring speed is 250 r / min; in step S3, 32 g of aluminum nitrate nonahydrate is added, and 10 g of ammonium dihydrogen phosphate is dissolved in 520 g of deionized water. The ammonium dihydrogen phosphate aqueous solution is added dropwise over 70 min, and after the addition is complete, it is aged at 80℃ for 100 min; in step S4, 55 g of lanthanum nitrate hexahydrate is added, and the first stage of phosphate aqueous solution is added dropwise at a rate of 4 g / min, and after the addition is complete, the process continues. Stir for 50 min; in S5, the yttrium citric acid complex aqueous solution is added within 25 min, and then stirred at 55°C for 40 min; in S6, the magnesium citric acid complex aqueous solution is added within 35 min, and then stirred for another 35 min; the magnesium acetate aqueous solution is added within 12 min, and then stirred for another 50 min; in S7, the first heat treatment temperature is 380°C, the second heat treatment temperature is 730°C, the third heat treatment temperature is 1180°C, and the third heat treatment time is 2 h; the remaining conditions are the same as in Example 1.

[0036] The difference between Example 5 and Example 1 is as follows: In step 1, the sodium hydroxide aqueous solution treatment temperature is 75℃, the stirring time is 30 min, and the stirring speed is 350 r / min; in S2, the adsorption time of the branched polyethyleneimine aqueous solution is 40 min; in S3, 29 g of aluminum nitrate nonahydrate is added, and 9 g of ammonium dihydrogen phosphate is dissolved in 500 g of deionized water, with the ammonium dihydrogen phosphate aqueous solution added dropwise over 55 min, followed by aging at 78℃ for 80 min after the addition is complete; in S4, 58 g of lanthanum nitrate hexahydrate is added, and 10 g of ammonium dihydrogen phosphate is dissolved in 600 g of deionized water. In S5, 4g of yttrium nitrate hexahydrate and 4g of citric acid monohydrate were added, and 6g of ammonium dihydrogen phosphate was dissolved in 400g of deionized water and added dropwise at a rate of 3g / min. In S6, the pH of the system was raised to 8.6, and a magnesium-citric acid complex aqueous solution prepared by 5g of magnesium acetate tetrahydrate and 5g of citric acid monohydrate was added, followed by a magnesium acetate aqueous solution prepared by 3g of magnesium acetate tetrahydrate. In S7, the third stage of heat treatment was carried out at a temperature of 1210℃ for 2 hours. The remaining conditions were the same as in Example 1.

[0037] The difference between Comparative Example 1 and Example 1 is that: in S2, instead of adding 6g of branched polyethyleneimine aqueous solution, 6g of deionized water is added to make up the liquid volume; in S3, 30g of aluminum nitrate nonahydrate and 10g of ammonium dihydrogen phosphate are added as in Example 1; the other conditions are the same as in Example 1.

[0038] The difference between Comparative Example 2 and Example 1 is that: in S3, 30g of aluminum nitrate nonahydrate and 10g of ammonium dihydrogen phosphate are not added. Instead, all the mullite wet powder obtained in S2 is added to 1800g of deionized water, stirred at 50°C and 300r / min for 20min, and then directly filtered, dried, and pre-calcined at 450°C as in Example 1. The other conditions are the same as in Example 1.

[0039] The difference between Comparative Example 3 and Example 1 is that: in S4, lanthanum ion pre-adsorption and the first stage of phosphate addition are not performed; in S5, yttrium citric acid complexation insertion and the second stage of phosphate locking are not performed. Instead, 55g of lanthanum nitrate hexahydrate, 15g of ammonium dihydrogen phosphate, 4g of yttrium nitrate hexahydrate and 3g of citric acid monohydrate are reacted in another container according to the pH, temperature and time corresponding to Example 1. After filtration and drying, the resulting lanthanum-containing phosphate and yttrium-containing precursor solids are mechanically mixed with the mullite powder obtained in S3; the remaining conditions are the same as in Example 1.

[0040] The difference between Comparative Example 4 and Example 1 is that: in S5, instead of adding 4g of yttrium nitrate hexahydrate and 3g of citric acid monohydrate, 307g of deionized water was added and the pH was adjusted, the second phosphate aqueous solution was added, and aging was carried out in the same manner as in Example 1; the other conditions were the same as in Example 1.

[0041] The difference between Comparative Example 5 and Example 1 is that, in S5 and S6, the yttrium source is not added first and the magnesium source is added later. Instead, after the first stage of phosphate addition is completed in S4 and stirring is continued for 40 minutes, the yttrium citric acid complex aqueous solution, magnesium citric acid complex aqueous solution, and magnesium acetate aqueous solution from Example 1 are simultaneously added to the same suspension. The pH of the system is then adjusted to 8.4 to 8.8 with ammonia water, and stirring is continued for 70 minutes. After that, the filtration, drying, and segmented heat treatment are carried out as in Example 1. The remaining conditions are the same as in Example 1.

[0042] The difference between Comparative Example 6 and Example 1 is that: in S6, instead of using the two-stage addition method of magnesium citric acid complex aqueous solution and magnesium acetate aqueous solution, 8g of magnesium acetate tetrahydrate was dissolved in 500g of deionized water at once to obtain magnesium acetate aqueous solution, which was added to the suspension obtained in S5 within 10min. During and after the addition, the pH was maintained at 8.4 to 8.8, and stirring was continued for 65min; the other conditions were the same as in Example 1.

[0043] The difference between Comparative Example 7 and Example 1 is that: in S6, 5g magnesium acetate tetrahydrate, 4g citric acid monohydrate and 3g magnesium acetate tetrahydrate were not added, and the pH of the system was increased from 4.6 to 5.0 to 8.4 to 8.8 and then stirred for 65 minutes; the other conditions were the same as in Example 1.

[0044] Performance testing Examples 1 to 3, Comparative Examples 1 to 7, and the obtained powders were prepared into slurries according to the surface slurry preparation methods of the corresponding examples or comparative examples. The surface slurry preparation method was as follows: 1000g of the high-temperature resistant surface material obtained in this invention was taken, and 400g of silica sol, 40g of deionized water, 1.5g of wetting agent, and 1.0g of defoamer were added. The mixture was stirred at 400r / min for 40min and filtered through a 100-mesh sieve to obtain the surface slurry. The specific gravity of the surface slurry at 25℃ was controlled to be 1.80g / cm³. 3 The flow time of the No. 4 Zein cup is 15s; the wax mold is immersed in the surface slurry for 10s, then removed and drained for 60s, covered with 80-120 mesh electrofused white mullite sand, and dried at 25℃ and 60% relative humidity for 12h. For samples used in the refractory material performance testing, each group of surface slurry was poured into a polytetrafluoroethylene mold to form long strip or cylindrical specimens. After standing at room temperature for 24 hours, the specimens were demolded, dried at 110℃ for 6 hours, and then calcined at 1000℃ for 2 hours. For samples used in investment casting performance testing, ceramic shells were prepared using wax patterns of the same size and the same backing process. After dewaxing, the shells were calcined at 1000℃ for 2 hours. For each test item, three parallel specimens were prepared for each group of samples, and the test results were taken as the arithmetic mean.

[0045] Test for permanent linear change rate under heating: The test was conducted in accordance with GB / T 5988-2022 "Test Method for Permanent Linear Change of Refractory Materials under Heating". The slurries corresponding to Examples 1 to 5 and Comparative Examples 1 to 7 were prepared into strips of 25mm × 25mm × 150mm. After standing at room temperature for 24 hours, drying at 110℃ for 6 hours, and pre-calcining at 1000℃ for 2 hours, the initial length was measured. The samples were then placed in a high-temperature furnace, heated to 1550℃ at a rate of 5℃ / min, held at that temperature for 30 minutes, and then cooled to room temperature with the furnace. The length change after cooling was measured, and the rate of permanent linear change under heating was calculated.

[0046] Thermal expansion performance test: The test was conducted in accordance with GB / T 7320-2018 "Test Method for Thermal Expansion of Refractory Materials". Cylindrical specimens with a diameter of 6 mm and a length of 50 mm were prepared from the slurries corresponding to Examples 1 to 5 and Comparative Examples 1 to 7. After drying at 110℃ for 6 h and calcining at 1000℃ for 2 h, the specimens were tested using a top-rod thermal expansion apparatus. The temperature was increased from room temperature to 1200℃ at a rate of 5℃ / min, and the average linear expansion coefficient in the 1000-1200℃ range was recorded.

[0047] Thermal shock resistance and strength retention after thermal shock testing: Thermal shock resistance treatment was performed according to GB / T 30873-2014 "Test Method for Thermal Shock Resistance of Refractory Materials", and the room temperature flexural strength before and after thermal shock was determined according to GB / T 3001-2017 "Test Method for Flexural Strength of Refractory Materials at Room Temperature". The slurries corresponding to Examples 1 to 5 and Comparative Examples 1 to 7 were prepared into 25mm×25mm×150mm strip specimens. After drying at 110℃ for 6 hours and calcining at 1000℃ for 2 hours, the room temperature flexural strength of the un-thermally shocked specimens was tested first. Another batch of specimens was kept at 1100℃ for 30 minutes and then rapidly cooled in water at 20℃. After completing 10 thermal shock cycles, the room temperature flexural strength was tested again, and the strength retention rate was calculated.

[0048] High-temperature bending strength test of investment casting shells: The test was conducted according to JB / T 13412-2018 "Test Method for Bending Strength of Investment Casting Shells". Strip-shaped shell specimens were prepared using the same surface slurry and backing process as in Examples 1 to 5, and Comparative Examples 1 to 7. The specimen dimensions were 100mm × 20mm × 6mm. After dewaxing and calcination at 1000℃ for 2 hours, the specimens were placed in a high-temperature bending test apparatus. The temperature was increased to 1200℃ at a rate of 5℃ / min and held for 30 minutes. A three-point bending test was then performed at a loading rate of 0.5mm / min. The maximum load was recorded, and the high-temperature bending strength was calculated.

[0049] High-temperature air permeability test of investment casting shells: The test was conducted according to JB / T 4153-2025 "Test Method for High-Temperature Air Permeability of Investment Casting Shells". Cylindrical shell samples were prepared using the same surface layer slurry and back layer process as in Examples 1 to 5, Comparative Examples 1 to 7. The inner diameter of the samples was 30 mm, the wall thickness was 6 mm, and the height was 40 mm. After dewaxing and calcination at 1000℃ for 2 h, the samples were installed in the high-temperature air permeability testing device. The test temperature was 1000℃, and the temperature was stabilized for 20 min. The test pressure difference was 980 Pa. The air flow rate through the shell per unit time was recorded, and the high-temperature air permeability was calculated based on the sample wall thickness and inner surface area.

[0050] Observation of casting surface roughness and sand adhesion: The surface roughness grade of the castings was evaluated according to GB / T 15056-2017 "Method for Evaluating Surface Roughness of Castings" and GB / T 6060.1-2018 "Comparative Specimens for Surface Roughness Part 1: Casting Surfaces", and the contour method measurement was performed according to GB / T 10610-2009 "Rules and Methods for Evaluating Surface Structure by Surface Structure Profile Method in Geometric Specifications for Products (GPS)". 316L stainless steel plate samples were cast using the shells of Examples 1 to 5 and Comparative Examples 1 to 7, respectively, at a casting temperature of 1550℃. The sample size was 30mm × 30mm × 3mm. After shell removal, no sandblasting or polishing was performed; only surface dust was removed with a nylon brush. Surface roughness was measured at 5 locations in the central area of ​​each sample, with a cutoff wavelength of 2.5mm and an evaluation length of 12.5mm. The presence of continuous sand adhesion, localized sand adhesion, or surface pits was recorded.

[0051] Table 1 Performance Test Results

[0052] Data Analysis: Table 1 shows that in Comparative Example 1, without the addition of branched polyethyleneimine aqueous solution, although aluminum nitrate nonahydrate and ammonium dihydrogen phosphate were still added, the interfacial reaction layer thickened and the strength retention rate after thermal shock decreased, indicating that without temporary cation patches, aluminum phosphate is difficult to form effective anchoring points on the mullite surface. In Comparative Example 2, after further removing aluminum nitrate nonahydrate and ammonium dihydrogen phosphate, the interfacial reaction, high-temperature shrinkage, and sand adhesion became more obvious, indicating that the island-like aluminum phosphate is not ordinary phosphate filling, but rather the reaction basis for the subsequent confined formation of lanthanum phosphate. In Comparative Example 3, which adopted an externally generated and mechanically mixed method, although lanthanum phosphate-related crystal phases still existed, the resistance to metal melt reactivity and the surface quality of the casting were lower than those of the examples, indicating that lanthanum phosphate must be generated in situ on the mullite surface to play a barrier role. In Comparative Example 4, after removing the yttrium citric acid complex aqueous solution, the thermal shock stability and surface quality decreased, indicating that the yttrium-containing edge sites can stabilize the lanthanum phosphate barrier edge. Comparative Example 5 added yttrium and magnesium sources simultaneously, Comparative Example 6 added magnesium source all at once, and Comparative Example 7 removed magnesium source. None of them achieved the comprehensive effect of Example 1. This shows that there is a synergistic effect between the first entry of yttrium source and the subsequent entry of magnesium source, as well as the graded deposition of slow-release magnesium source and fast-release magnesium source, which can simultaneously suppress interfacial reaction and high-temperature sintering shrinkage.

[0053] In summary, the embodiments, without using zirconium sand or zirconium powder, still maintain a thin interfacial reaction layer, a low permanent linear shrinkage rate after heating, and a good strength retention rate after thermal shock. This indicates that the material is suitable for the surface shell of thin-walled precision castings of stainless steel, heat-resistant steel, and nickel-based high-temperature alloys, which can reduce the dependence on zirconium materials and improve the surface forming quality of castings.

[0054] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing a high-temperature resistant surface material for precision casting as an alternative to zircon sand, characterized in that, Includes the following steps: (1) Alkali treatment and acid neutralization treatment of mullite powder to obtain hydroxylated mullite powder; (2) The hydroxylated mullite powder is dispersed in branched polyethyleneimine for adsorption treatment to obtain branched polyethyleneimine modified mullite wet powder. (3) Add aluminum nitrate nonahydrate and ammonium dihydrogen phosphate to the branched polyethyleneimine modified mullite wet powder, and obtain aluminum phosphate modified mullite powder after drying and pre-calcination. (4) After dispersing the aluminum phosphate modified mullite powder, lanthanum nitrate hexahydrate, first-stage phosphate aqueous solution, yttrium-citric acid complex aqueous solution, second-stage phosphate aqueous solution, magnesium-citric acid complex aqueous solution, and magnesium acetate tetrahydrate are added sequentially to obtain a composite precursor suspension; the composite precursor suspension is filtered, washed, dried, and subjected to segmented heat treatment to obtain a high-temperature resistant material for precision casting surface layer that can replace zircon sand; Based on 1000 parts by weight of mullite powder, the amounts of branched polyethyleneimine, aluminum nitrate nonahydrate, ammonium dihydrogen phosphate, lanthanum nitrate hexahydrate, first-stage phosphate aqueous solution, yttrium-citric acid complex aqueous solution, second-stage phosphate aqueous solution, magnesium-citric acid complex aqueous solution, and magnesium acetate tetrahydrate are 4-8 parts, 28-34 parts, 9-11 parts, 50-60 parts, 600 parts, 300 parts, 400 parts, 300 parts, and 2-4 parts, respectively.

2. The preparation method according to claim 1, characterized in that, The mullite powder mentioned in step (1) is fused white mullite powder, wherein the particle size of the fused white mullite powder is 500 mesh, the median particle size is 20-30 μm, the mass fraction of alumina is 70%-77%, the mass fraction of silica is 22%-29%, the mass fraction of iron oxide is not higher than 0.20%, the mass fraction of sodium oxide is not higher than 0.40%, and the bulk density is not lower than 2.90 g / cm³. 3 Its fire resistance is not less than 1850℃.

3. The preparation method according to claim 1, characterized in that, The alkali treatment in step (1) includes adding mullite powder to an aqueous sodium hydroxide solution and stirring; the acid neutralization treatment includes adding the alkali-treated wet mullite powder to a dilute nitric acid aqueous solution and stirring.

4. The preparation method according to claim 1, characterized in that, The adsorption treatment conditions in step (2) are 25-30℃ and stirring for 20-40 min.

5. The preparation method according to claim 1, characterized in that, After the aluminum phosphate deposition in step (3), the system needs to be heated to 75-85℃ for aging for 70-110 minutes.

6. The preparation method according to claim 1, characterized in that, The pre-firing in step (3) includes placing the product in an air atmosphere and heating it to 430-470℃ at a rate of 1.5-2.5℃ / min and holding it at that temperature for 0.5-1.5h.

7. The preparation method according to claim 1, characterized in that, Based on a weight of 600 parts, the first phosphate aqueous solution in step (4) contains 8-11 parts of ammonium dihydrogen phosphate, with the remainder being deionized water; based on a weight of 300 parts, the yttrium-citric acid complex aqueous solution contains 3-5g of yttrium nitrate hexahydrate and 2-4g of citric acid monohydrate, with the remainder being deionized water; based on a weight of 400 parts, the second phosphate aqueous solution contains 4-7 parts of ammonium dihydrogen phosphate, with the remainder being deionized water; based on a weight of 300 parts, the magnesium-citric acid complex aqueous solution contains 4-6g of magnesium acetate tetrahydrate and 3-5g of citric acid monohydrate, with the remainder being deionized water.

8. The preparation method according to claim 1, characterized in that, The drying process in step (4) is carried out at 100-120℃ for 5-8 hours.

9. The preparation method according to claim 1, characterized in that, The segmented heat treatment in step (4) includes: placing the composite precursor powder in an air atmosphere, heating it to 320-380℃ at 1.5-2.5℃ / min and holding it at that temperature for 0.8-1.5h; then heating it to 680-730℃ at 1.5-2.5℃ / min and holding it at that temperature for 1.5-2.5h; then heating it to 1160-1220℃ at 2.5-3.5℃ / min and holding it at that temperature for 1.5-2.5h.

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