Heat insulation coating for large complex thin-wall aluminum alloy casting and preparation and use methods thereof

Abstract

The invention discloses a heat insulation coating for a large complex thin-wall aluminum alloy casting as well as a preparation method and a use method of the heat insulation coating. Belongs to the field of casting coatings. The method aims at solving the problem that in the solidification process of the large complex thin-wall aluminum alloy casting, due to the too high cooling speed, the mold filling capacity is weak. The invention relates to a water-based paint which is prepared from the following raw materials in percentage by mass: 15%-30% of refractory aggregate, 6%-10% of phenolic resin, 1.5%-3% of polyvinyl butyral (PVB), 0.5%-1.5% of polyethylene alkyl alcohol ether (OP-10), 0.5%-1.0% of n-butyl alcohol and the balance of absolute ethyl alcohol. By adjusting the proportion of the refractory aggregate, firstly, the suspension property and the coating property of the coating are directly improved, and the surface smoothness of the coating is improved; meanwhile, the mechanical strength and the heat insulation performance of the coating are synergistically improved; through the improvements, the mold filling capacity of the alloy liquid is finally enhanced, so that the casting defects of large complex thin-wall aluminum alloy castings are reduced.

Description

Technical Field

[0001] This invention belongs to the field of coatings for casting, specifically, it relates to a heat insulation coating for large, complex, thin-walled aluminum alloy castings, its preparation method, and its application method. Background Technology

[0002] Aluminum alloys, due to their low density and excellent high-temperature strength, have become the preferred alloy material for large castings in the aerospace field. These large castings are typically thin-walled parts with complex geometries, and the ratio of the filling area to the wall thickness is greater than 5000. This causes the aluminum alloy melt to cool down too quickly during the filling process, resulting in extremely poor filling capacity and various problems such as severe microstructure segregation and incomplete filling. Applying coatings to the inner surface of the mold can effectively improve the filling capacity, but existing casting coatings lack sufficient thermal insulation capabilities and cannot meet the requirements of large, complex, thin-walled castings. Therefore, developing a thermal insulation coating suitable for large, complex, thin-walled aluminum alloy castings has become an urgent problem to be solved.

[0003] Currently, there is no dedicated coating for large, complex, thin-walled aluminum alloy castings. The coatings used have low suspension properties, making application difficult and resulting in poor adhesion. During the aluminum alloy filling process, the coating is prone to peeling and flaking. Furthermore, the coating's poor thermal insulation properties reduce the alloy's filling capacity, leading to defects such as incomplete filling in large, complex, thin-walled aluminum alloy castings. Summary of the Invention

[0004] To address the current lack of dedicated coatings for large, complex, thin-walled aluminum alloy castings, this invention aims to provide a thermal insulation coating for such castings, along with its preparation and application methods. This improves the alloy's filling capacity and solves the problem of weak filling ability caused by excessively rapid cooling during solidification. By adjusting the proportion of refractory aggregates, this invention directly improves the coating's suspension and coatability, and enhances the surface finish. Simultaneously, it synergistically improves the coating's mechanical strength and thermal insulation properties. These improvements ultimately enhance the filling capacity of the molten alloy, thereby reducing casting defects in large, complex, thin-walled aluminum alloy castings.

[0005] To address the aforementioned technical problems, the present invention adopts the following technical solution: The purpose of this invention is to provide a thermal insulation coating for large, complex, thin-walled aluminum alloy castings, which, by weight percentage, is composed of the following raw materials: 15%~30% refractory aggregate, 6%~10% phenolic resin, 1.5%~3% polyvinyl butyral (PVB), 0.5%~1.5% polyvinyl alkyl alcohol ether (OP-10), 0.5%~1.0% n-butanol, and the balance being anhydrous ethanol.

[0006] The refractory aggregates are zirconium oxide, magnesium oxide, silicon oxide and iron oxide. Further specifying, the mass ratio of zirconium oxide, magnesium oxide, silicon oxide and iron oxide is 1∶(1-2)∶(1-2)∶(1-5).

[0007] Further specifying, the particle size of zirconium oxide is 5μm~15μm, the particle size of magnesium oxide is 5μm~15μm, the particle size of silicon oxide is 5μm~15μm, and the particle size of iron oxide is 5μm~15μm.

[0008] Another objective of this invention is to provide a method for preparing the above-mentioned thermal insulation coating for large, complex, thin-walled aluminum alloy castings.

[0009] A method for preparing a thermal insulation coating for large, complex, thin-walled aluminum alloy castings includes the following steps: Step 1: Ball mill zirconium oxide, magnesium oxide, silicon oxide and iron oxide separately, and mix them evenly in proportion to obtain refractory aggregate; Step 2: Add phenolic resin, PVB, OP-10, and n-butanol to anhydrous ethanol and stir until homogeneous. Then add the refractory aggregate from Step 1 and continue stirring until homogeneous to obtain the coating.

[0010] Another objective of this invention is to provide a method for applying the above-mentioned thermal insulation coating for large, complex, thin-walled aluminum alloy castings, comprising the following steps: drying a phenolic resin sand mold at 100°C for 24 hours. After drying, the surface of the sand mold is sanded with 1000-grit sandpaper, and then a blower is used to remove surface sand, ensuring the surface of the sand mold is smooth and clean. The prepared coating is stirred evenly, and then a 5cm wool brush is used to apply the coating to the surface of the sand mold. The coating is applied in two coats. The first coat is applied in the order of "edges first, then flat surfaces; vertical surfaces first, then horizontal surfaces." To avoid excessive coating and dripping, the brush is only dipped to about 1 / 3 of its capacity; the brushing pressure is even, and the brush is held at a 45° angle to the surface of the sand mold to ensure even coating without any missed areas or gaps. The coating is then allowed to air dry. The coating is sanded with 2000-grit sandpaper and the surface is cleaned with a blower. The second coat follows the "horizontal brushing + finishing" process, with less paint on the brush and the brushing direction as horizontal as possible to reduce cross-brush marks. After the final coat, gently "finish" (lightly sweep along the coating surface) with a clean brush to improve the smoothness and gloss of the coating, and then ignite to allow the paint to air dry.

[0011] Before using the coating of this invention, it should be mixed according to step 2.

[0012] To improve the filling capacity of large, complex, thin-walled aluminum alloy castings, the casting coating must possess two key properties: excellent thermal insulation to reduce heat loss during molten metal filling; and high surface smoothness to reduce flow resistance during filling. This invention uses thermally insulating materials such as zirconium oxide, magnesium oxide, silicon oxide, and iron oxide as the main components, fundamentally ensuring the coating's thermal insulation performance and facilitating molten metal flow. Phenolic resin provides the basic viscosity and adhesion, supplemented with polyvinyl butyral (PVB) to enhance suspension, coating flexibility, and adhesion, while also mitigating the brittleness of the phenolic resin. The addition of emulsifier OP-10 reduces surface tension to prevent aggregate agglomeration, improves dispersion uniformity, and enhances the coating's wetting and penetration into the mold / core, reducing air bubbles. n-Butanol is used to adjust the polarity of the ethanol solvent, promoting resin dissolution and improving coating leveling, thus facilitating the formation of a smooth coating.

[0013] Compared with the prior art, the present invention has the following beneficial effects: Without increasing the total amount of aggregate, the mechanical strength and thermal insulation performance of the coating are balanced and synergistically improved. The coating strength is maintained below 0.5g when tested with a coating surface abrasion tester, and the conditional viscosity of the coating is reduced to below 17s when tested with a Coating-4 viscosity cup, thus meeting the requirements of aerospace equipment for lightweight, high-strength thermal insulation coatings.

[0014] The thermal insulation coating for large, complex, thin-walled aluminum alloy castings obtained by this invention utilizes different proportions of refractory aggregates to improve both the coating's coatability and surface finish, thus ensuring the surface quality of the castings. This makes this invention promising for applications in large, complex, thin-walled aluminum alloy castings in the aerospace field, where high surface quality is required.

[0015] In the thermal insulation coating for large, complex, thin-walled aluminum alloy castings obtained by this invention, the different proportions of refractory aggregates ensure that the coating maintains both the alloy filling capacity and strength, preventing the coating from peeling off during the alloy liquid filling process. This avoids the casting quality problems caused by coating peeling in the past and facilitates its large-scale application and promotion on large, complex, thin-walled aluminum alloy castings.

[0016] For a deeper understanding of the features and technical content of this invention, please refer to the accompanying detailed description and drawings. It should be noted that the drawings are provided for illustrative purposes only and are not intended to limit the scope of the invention. Attached Figure Description

[0017] Figure 1 These are the uncoated cooling curves and first derivative curves; Figure 2 These are the cooling curve and first derivative curve of coating #1; Figure 3 These are the cooling curve and first derivative curve of coating #2; Figure 4 These are the cooling curve and first derivative curve of coating #3; Figure 5 These are the cooling curve and first derivative curve of coating #3; Figure 6 These are flowability samples of characteristic thin-walled parts: (a) no coating; (b) 1# coating; (c) 2# coating; (d) 3# coating; (e) 3# coating. Detailed Implementation

[0018] The present invention will be described in detail below with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but should not be considered as limiting the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0019] Example 1 (Coating No.: 1#) This embodiment provides a thermal insulation coating for large, complex, thin-walled aluminum alloy castings. Its components, by mass percentage, are: refractory aggregate: 20%, phenolic resin: 8.5%, polyvinyl butyral (PVB): 2%, polyvinyl alkyl alcohol ether (OP-10): 0.7%, n-butanol: 0.8%, and the remainder being anhydrous ethanol. The refractory aggregate consists of zirconium oxide, magnesium oxide, silicon oxide, and iron oxide, with a mass ratio of 1:1:1:1. It is prepared according to the following steps: Step 1: Place zirconium oxide, magnesium oxide, silicon oxide, and iron oxide into a ball mill and grind them separately until the particle size of zirconium oxide is 6μm, the particle size of magnesium oxide is 8μm, the particle size of silicon oxide is 10μm, and the particle size of iron oxide is 12μm. Mix them evenly in proportion to obtain refractory aggregate. Step 2: Add phenolic resin, PVB, OP-10, and n-butanol to anhydrous ethanol and stir until homogeneous (takes about 0.5 hours). Then add the refractory aggregate from Step 1 and continue stirring until homogeneous to obtain the coating.

[0020] The coating strength in this embodiment is 0.4g, and the conditional viscosity is 16.8s.

[0021] Example 2 (Coating No.: 2#) The difference between this embodiment and Embodiment 1 is that the mass ratio of zirconium oxide, magnesium oxide, silicon oxide, and iron oxide is 1:2:1:1. The other steps and parameters are the same as in Embodiment 1.

[0022] The coating strength in this embodiment is 0.5g, and the conditional viscosity is 15.1s.

[0023] Example 3 (Coating No.: 3#) The difference between this embodiment and Embodiment 1 is that the mass ratio of zirconium oxide, magnesium oxide, silicon oxide, and iron oxide is 1:1:1:2. All other steps and parameters are the same as in Embodiment 1.

[0024] The coating strength in this embodiment is 0.3g, and the conditional viscosity is 15.1s.

[0025] Example 4 (Coating No.: 4#) The difference between this embodiment and Embodiment 1 is that the mass ratio of zirconium oxide, magnesium oxide, silicon oxide, and iron oxide is 1:2:2:5. All other steps and parameters are the same as in Embodiment 1.

[0026] The coating strength in this embodiment is 0.5g, and the conditional viscosity is 14.8s.

[0027] The following experiments were used to verify the effectiveness of the invention. Testing methods and properties of coatings 1. Density Using a clean, dry 100 ml graduated cylinder, first weigh the empty cylinder using a balance. Then, keeping the cylinder vertical, slowly pour the paint to be tested into the cylinder, minimizing air bubble formation and avoiding liquid adhesion to the cylinder walls. Stop pouring when the cylinder reaches the 100 ml mark, and then measure the total mass of the cylinder and the paint. Calculate the paint density using Formula 1. (Formula 1) In the formula, The density of the coating (g / cm³) 3 ); m 2 represents the total mass (g) of the paint and the measuring cylinder; m 1 represents the mass of the graduated cylinder (g); V is Volume of paint (ml).

[0028] 2. Suspension Slowly pour the coating into a Φ25 mm × 100 ml graduated cylinder with a stopper until it reaches 100 ml. Let it stand for a period of time and record the volume of the supernatant as V. The coating suspension rate is calculated according to Formula 2.

[0029] (Formula 2) In the formula, C is the suspension rate of the coating (%); V is the volume of the clear liquid on the upper layer of the coating in the graduated cylinder (ml).

[0030] 3. Leveling properties Place a 30cm x 30cm glass plate flat on a table. Use a stand to fix the Cotton-4 viscosity cup 20cm directly above the glass plate. Block the bottom opening of the cup. Fill the viscosity cup with the paint to be tested, then open the bottom opening and let the paint flow out naturally. After the paint on the glass plate stops flowing, measure the diameter of the leveling circle formed by the paint on the glass plate from 5 different directions. Repeat the above process 3 times and take the arithmetic mean of 15 sets of data as the leveling circle diameter of the paint to be tested.

[0031] 4. Strength The surface strength of the coating was measured using a coating surface strength tester. The coating to be tested was evenly applied to a cylindrical sand sample (Φ50mm×50mm), with a coating thickness controlled between 0.2mm and 0.4mm. The sand sample was placed in a drying oven at a preset temperature of 250℃ for drying. After drying, the sand sample was removed and fixed onto the strength tester. A 200g weight was attached externally during the test, and the rotation was set to 64 revolutions. The instrument was started to measure the strength of the coating by weighing the mass of the coating that detached; a smaller mass of detached coating indicates higher coating strength.

[0032] 5. Drying and crack resistance A cylindrical sand sample (Φ50 mm × 50 mm) coated with the paint was placed in an electrically heated constant-temperature drying oven and dried for 60 minutes. After cooling, the surface condition of the coating was observed, and the presence of cracks was assessed. High-quality paint should have excellent crack-resistant drying performance, and the surface of the dried coating should be smooth and free of obvious cracks. The evaluation criteria for drying crack resistance are the same as those for heat exposure crack resistance.

[0033] 6. Heat exposure crack resistance After drying a Φ50 mm × 50 mm cylindrical sand sample coated with the coating, it was placed in a muffle furnace at 1200 ℃ for 2 minutes. The presence, size, and number of cracks in the coating under high temperature were observed, as well as any peeling between the coating and the substrate. According to the JB / T 9226-2008 standard "Coatings for Sand Casting," as shown in Table 1, the coating crack condition was judged according to grades I to IV.

[0034] Table 1 Evaluation Indicators for Heat Exposure and Crack Resistance Grade

[0035] 7. Conditional viscosity and brush index Place the Tu-4 viscosity cup on the workbench and slowly pour the paint into the cup until it is completely filled. Scrape off any excess paint from the cup opening to make the surface smooth. Place a beaker under the Tu-4 viscosity cup and open the outlet at the bottom of the Tu-4 viscosity cup to allow the paint to flow downwards. Simultaneously start the timer. When the flow of the paint begins to break, immediately stop the timer and record the time (in seconds). Perform three tests and take the average of the three results as the conditional viscosity of the paint to be tested.

[0036] The brush index of a coating reflects its covering power; a higher brush index indicates stronger coverage. After thoroughly mixing the coating, it was allowed to stand for 30 minutes. Then, using an NDJ-9S rotational viscometer, the apparent viscosities η6 and η2 of the coating were measured at rotation speeds of 6 and 60 r / min for 30 seconds at these different speeds. 60 At this point, the coating index M is calculated according to formula 3.

[0037] (Formula 3) In the formula, M is the brush index, and η6 is the viscosity (Pa·s) of the coating at a rotation speed of 6 r / min for 30 s; η 60 The viscosity (Pa·s) of the coating after rotating at 60 r / min for 30 s is given.

[0038] 8. Coating performance The four coatings were tested according to the above testing method, and the results are shown in Table 2.

[0039] Table 2 Performance of Coatings

[0040] III. Application of Coatings in Thin-Walled Parts Thin-walled parts are prone to incomplete filling due to their large-area thin walls. The Froude-Fr criterion (ratio of inertial force to gravity) and the Weber-We criterion (ratio of inertial force to surface tension) are factors affecting filling capacity. When the casting wall thickness is large, the Fr criterion is the primary factor; for thin-walled parts, the We criterion is the primary factor. The wall thickness corresponding to We / Fr = 1 is called the critical wall thickness, δ. c Calculate according to formula 4.

[0041] (Formula 4) In the formula, σ is the surface tension of the alloy, ρ is the density of the alloy, and g is the acceleration due to gravity.

[0042] The experimental alloy used was ZL114A. At a casting temperature of 720℃, σ was 0.9 N / m, g was 9.8 m / s, and ρ was 2400 kg / m. 3According to Formula 4, the critical wall thickness of ZL114A alloy at a casting temperature of 720°C is 6.2 mm. Therefore, when the wall thickness of the casting is less than 6.2 mm, it reflects the structural characteristics of a thin-walled casting. To test the thermal insulation performance of the coating, this paper uses a characteristic thin-walled part with a wall thickness of 3 mm. A type K thermocouple was used to measure the temperature. The thermocouple was protected by a ceramic tube. A compensating wire was used to connect the type K thermocouple to a digital temperature measuring instrument. The digital temperature measuring instrument recorded the temperature of the alloy at different times during solidification, with a measurement step of 0.5 s. A computer equipped with auxiliary software was connected to the digital temperature measuring instrument to control the instrument and store the temperature data.

[0043] The effect of coating on the filling characteristics of thin-walled parts of ZL114A alloy is as follows: Figure 1-5 As shown in the figure, the setting time is only 2.9 seconds without coating, while the setting times for coatings #1, #2, #3, and #4 are 4.5 seconds, 6.5 seconds, 8.0 seconds, and 3.9 seconds, respectively. A longer setting time indicates stronger filling capacity and better thermal insulation performance. In conclusion, coating effectively improves filling capacity, and coating #3 exhibits the best thermal insulation performance.

[0044] Figure 6 The flowability test specimens for the ZL114A filling-feature thin-walled component are shown in the figure. Without coating, the flowability specimen length is 6.5 mm. The lengths of the flowability specimens coated with coatings #1, #2, #3, and #4 are 21.5 mm, 26.2 mm, 27.4 mm, and 11.8 mm, respectively. A longer flowability specimen indicates stronger filling capacity and better thermal insulation performance. In conclusion, coating effectively improves filling capacity, and coating #3 exhibits the best thermal insulation performance.

[0045] The specific embodiments of the present invention have been described in detail above. It should be noted that the present invention is not limited to the specific embodiments described above. Various modifications or alterations can be made by those skilled in the art without departing from the scope of protection defined by the claims, and all such modifications or alterations fall within the scope of the present invention.

Claims

1. An insulating coating for large complex thin-walled aluminum alloy castings, characterized by, In mass percentage, is composed of the following raw materials: 15%~30% refractory aggregate, 6%~10% phenolic resin, 1.5%~3% polyvinyl butyral (PVB), 0.5%~1.5% polyvinyl alkyl alcohol ether (OP-10), 0.5%~1.0% n-butanol, and the balance of anhydrous ethanol; The refractory aggregate is zirconium oxide, magnesium oxide, silicon oxide and iron oxide.

2. The coating of claim 1, wherein, In mass percentage, is composed of the following raw materials: 20% refractory aggregate, 8.5% phenolic resin, 2% PVB, 0.7% OP-10, 0.8% n-butanol, and the balance of anhydrous ethanol.

3. The coating of claim 1 wherein, The mass ratio of zirconium oxide, magnesium oxide, silicon oxide and iron oxide is 1: (1-2): (1-2): (1-5).

4. The coating of claim 1 wherein, The mass ratio of zirconium oxide, magnesium oxide, silicon oxide and iron oxide is 1:1:1:

1.

5. The coating of claim 1 wherein, The mass ratio of zirconium oxide, magnesium oxide, silicon oxide and iron oxide is 1:2:1:

1.

6. The method of claim 4, wherein, The mass ratio of zirconium oxide, magnesium oxide, silicon oxide and iron oxide is 1:1:1:

2.

7. The coating of claim 1 wherein, The mass ratio of zirconium oxide, magnesium oxide, silicon oxide and iron oxide is 1:2:2:

5.

8. The coating of claim 1 wherein, The particle size of zirconium oxide is 5μm~15μm, the particle size of magnesium oxide is 5μm~15μm, the particle size of silicon oxide is 5μm~15μm, and the particle size of iron oxide is 5μm~15μm.

9. A process for the preparation of the coating according to any one of claims 1 to 8, characterized in that, Comprising the following steps: Step 1, ball mill zirconium oxide, magnesium oxide, silicon oxide and iron oxide respectively, mix uniformly according to the proportion, and obtain the refractory aggregate; Step 2, add phenolic resin, PVB, OP-10 and n-butanol into anhydrous ethanol, stir until uniform, then add the refractory aggregate of step 1, continue to stir until uniform, and obtain the coating.

10. Use of the coating according to any one of claims 1 to 8, characterized in that Comprising the following steps: Step 1) polish the surface of the mold using 1000 mesh sandpaper, and remove the surface dust using an air blower, so that the surface of the mold is smooth and clean; Step 2) after the prepared coating is stirred uniformly, a wool brush with a bristle width of 5 cm is used for coating, a total of two times; during the first coating, the amount of coating dipped is controlled to be 1 / 3 of the capacity of the wool brush, the brush is kept at an angle of 45° with the surface of the mold during brushing, uniform force is applied, and the order of "first edge then plane, first vertical direction then horizontal direction" is followed; after the first coating is completed, the coating is self-dried by ignition, then 2000 mesh sandpaper is used to polish the coating, and the air blower is used to clean the surface of the coating; during the second coating, the amount of coating dipped is reduced, horizontal brushing is mainly used to avoid multiple cross brush marks, and after brushing is completed, clean brushes are used to sweep along the surface of the coating to collect light; finally, the coating is self-dried by ignition.

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