A ceramic shell back coating powder suitable for large superalloy castings and a method of making the same

Abstract

This invention discloses a ceramic shell back coating powder suitable for large high-temperature alloy castings and its preparation method. The ceramic shell back coating powder comprises a mixed powder, organic fibers, and a thixotropic agent. By mass percentage, the mixed powder comprises 70-85% fused silica powder, 10-20% spherical cristobalite powder, and 5-10% silica. The organic fibers account for 1-1.5% of the total mass of the mixed powder, and the thixotropic agent accounts for 0.2-0.5% of the total mass of the mixed powder. This invention provides a ceramic shell back coating powder suitable for large high-temperature alloy castings and its preparation method. The ceramic shell back coating powder has good suspension and thixotropic properties, excellent coating performance, and the ceramic shells prepared using it have high strength, do not crack, have good air permeability and collapsibility, and result in high casting quality and yield.

Description

Technical Field

[0001] This invention relates to the field of investment casting technology, and in particular to a ceramic shell back coating powder suitable for large high-temperature alloy castings and its preparation method. Background Technology

[0002] Compared to smaller castings, large high-temperature alloy castings place higher demands on the performance of ceramic mold shells. The strength, permeability, and collapsibility of the mold shell significantly affect the quality and yield of large castings. Poor mold shell collapsibility and difficulty in shell removal can be fatal for large castings.

[0003] Coatings are essential in the preparation of shell molds and form the basis for shell molding. Coatings are made by mixing binders such as silica sol and refractory powders in a certain powder-liquid ratio. The composition, particle size, and crystal phase of the refractory powder, which is the main shell-forming material in the coating, have a significant impact on the viscosity, suspension, thixotropy, and coating properties of the coating. These process properties largely determine the performance of the shell mold.

[0004] Currently, there are no refractory powders specifically designed for use in the shell coating of large high-temperature alloy castings in China, such as powders for coatings of large castings like aircraft engine casings. This causes many inconveniences in actual production. One prominent issue is that the preparation of ceramic shells often requires adjustments to the process as problems arise. For example, if the shell strength is insufficient, the number of coating layers and the shell thickness are increased; if the coating viscosity is too high or too low, the powder-to-liquid ratio is adjusted. Without guaranteeing the performance of the ceramic shell, the quality of the casting cannot be guaranteed. Summary of the Invention

[0005] In view of this, the present invention discloses a ceramic shell back coating powder suitable for large high-temperature alloy castings and its preparation method, so as to overcome the defect that the materials in the prior art are not suitable for the preparation of large high-temperature alloy castings.

[0006] First, the present invention provides a ceramic shell back coating powder suitable for large high-temperature alloy castings, comprising a mixed powder, organic fibers, and a thixotropic agent. By mass percentage, the mixed powder comprises 70-85% fused silica powder, 10-20% spherical cristobalite powder, and 5-10% silica. The organic fibers account for 1-1.5% of the total mass of the mixed powder, and the thixotropic agent accounts for 0.2-0.5% of the total mass of the mixed powder.

[0007] Preferably, the fused silica powder comprises, by mass percentage: 10-15% particles with a particle size of 30-50 mesh and 60-70% particles with a D50 of 45-50 micrometers.

[0008] Preferably, the spherical cristobalite has a particle size D50 of 12–15 micrometers.

[0009] Preferably, the particle size D50 of the silica is 0.2 to 0.3 micrometers.

[0010] Preferably, the organic fiber has a length of 4-6 mm and a diameter of 15-17 μm.

[0011] Preferably, the thixotropic agent has a particle size of 325 mesh.

[0012] Preferably, the fused silica powder contains ≥99.7% silica; the spherical cristobalite powder contains ≥99.5% silica; and the white carbon black contains ≥99.8% silica.

[0013] Secondly, a method for preparing ceramic shell back coating powder suitable for large high-temperature alloy castings includes the following steps:

[0014] 1) Pretreatment of organic fibers;

[0015] 2) Immerse the pretreated organic fibers in a silane coupling agent ethanol solution, disperse them by ultrasonic vibration, remove the fibers, and place them on the surface of filter paper to evaporate and dry naturally.

[0016] 3) Weigh out the spherical cristobalite powder, white carbon black, and thixotropic agent according to the preparation ratio and mix them to obtain a preliminary mixture;

[0017] 4) Add the organic fibers obtained in step 2) to the preliminary mixture to obtain the mixture;

[0018] 5) Weigh the fused silica powder according to the preparation ratio and add it to the mixture obtained in step 4) to finally obtain the ceramic shell back coating powder.

[0019] Preferably, step 1) pretreatment of organic fibers includes: immersing organic fibers with a solution mass percentage of 20-30% into a silane coupling agent ethanol solution with a solution mass percentage of 2%, and dispersing them by ultrasonic oscillation for 15 minutes.

[0020] This invention provides a ceramic shell back coating powder suitable for large high-temperature alloy castings and its preparation method. The ceramic shell back coating powder has good suspension and thixotropic properties, excellent coating performance, and the ceramic shells prepared using it have high strength, do not crack, have good air permeability and collapsibility, and the castings have high quality and yield.

[0021] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit the disclosure of the present invention. Detailed Implementation

[0022] The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of systems consistent with some aspects of the invention as detailed in the appended claims.

[0023] To address the problem that the existing technology lacks material powders specifically for large high-temperature alloy castings, resulting in unreliable performance in the preparation of large castings, this embodiment provides a ceramic mold shell backing coating powder suitable for large high-temperature alloy castings. The powder comprises a mixed powder, organic fibers, and a thixotropic agent. By mass percentage, the mixed powder includes 70-85% fused silica powder, 10-20% spherical cristobalite powder, and 5-10% silica. The organic fibers account for 1-1.5% of the total mass of the mixed powder, and the thixotropic agent accounts for 0.2-0.5% of the total mass of the mixed powder.

[0024] By applying the powder material provided in this embodiment, adding it to the binder silica sol at a certain powder-to-liquid ratio, and adding appropriate amounts of wetting agent, defoamer, etc., and after thorough stirring and dispersion, a shell-forming slurry with suitable viscosity, stable suspension, and good coating performance can be obtained.

[0025] The above-mentioned powder material incorporates spherical cristobalite powder. The addition of spherical cristobalite powder helps to improve the uniformity and flowability of the slurry. When using powder material, it can improve the high-temperature strength and porosity of ceramic shells. At the same time, during the cooling process, due to the crystal transformation, a network of microcracks is formed inside the shell, which improves the collapseability of the shell and makes it easier to remove the shell.

[0026] The introduction of white carbon black into the powder material improves the suspension performance of the coating, acts as a mineralizer in the coating, and improves the high-temperature strength of the shell.

[0027] The thixotropic agent introduced into the powder material gives the overall coating good thixotropic properties. That is, the viscosity of the coating is relatively low during stirring and impregnation, while the viscosity of the coating increases when it is in a static state after coating. This improves the uniformity of coating and reduces the occurrence of sagging.

[0028] The organic fibers introduced into the powder material can effectively improve the room temperature strength of the shell, prevent cracking, and increase the porosity of the shell after burn-off, which helps to vent air and improve the collapseability of the shell.

[0029] After the powder provided by this invention is formed into a shell, the organic fibers in it are burned off, and the SiO2 content is more than 99%, which ensures that the shell material has a low coefficient of thermal expansion and good thermal shock resistance.

[0030] The above-mentioned fused silica powder comprises, by mass percentage: 10-15% particles with a particle size of 30-50 mesh and 60-70% particles with a D50 of 45-50 micrometers.

[0031] The particle size D50 of the above-mentioned spherical cristobalite is 12-15 micrometers.

[0032] The particle size D50 of the aforementioned white carbon black is 0.2–0.3 micrometers.

[0033] The organic fibers mentioned above have a length of 4-6 mm and a diameter of 15-17 μm; the organic fibers can be selected from polypropylene fibers, nylon fibers, and polyester fibers.

[0034] The particle size of the above thixotropic agent is less than 320 mesh. When applying it, one of magnesium aluminum silicate or organobentonite can be selected.

[0035] Table 1: Specifications and proportions of each raw material in this embodiment (wt%)

[0036]

[0037] The silica content in the above-mentioned fused silica powder is ≥99.7%; the silica content in the spherical cristobalite powder is ≥99.5%; and the silica content in the white carbon black is ≥99.8%.

[0038] The chemical composition analysis of the raw materials used in this invention is shown in Table 2.

[0039] Table 2: Chemical composition analysis (wt%) of fused silica, cristobalite, and white carbon black:

[0040]

[0041]

[0042] On the other hand, this embodiment also provides a method for preparing ceramic shell back coating powder suitable for large high-temperature alloy castings, including the following steps:

[0043] 1) Pretreatment of organic fibers;

[0044] 2) Immerse the pretreated organic fibers in a silane coupling agent ethanol solution, disperse them by ultrasonic vibration, remove the fibers, and place them on the surface of filter paper to evaporate and dry naturally.

[0045] 3) Weigh out the spherical cristobalite powder, white carbon black, and thixotropic agent according to the preparation ratio and mix them to obtain a preliminary mixture;

[0046] When mixing, a high-speed mixer can be used for stirring and mixing for 5 to 10 minutes. After mixing, transfer the mixture to a mixer. A V-type mixer or a double cone mixer can be selected.

[0047] 4) Add the organic fibers obtained in step 2) to the preliminary mixture to obtain the mixture;

[0048] Specifically, organic fibers can be added to the above-mentioned mixer, and the mixing speed can be controlled for 5 to 10 minutes.

[0049] 5) Weigh the fused silica powder according to the preparation ratio and add it to the mixture obtained in step 4) to finally obtain the ceramic shell back coating powder.

[0050] Specifically, weigh the fused silica powder according to the preparation ratio, add it to the above-mentioned mixer, control the mixing speed, and mix for 20 to 30 minutes.

[0051] Further, step 1) pretreatment of organic fibers includes: immersing organic fibers with a solution mass percentage of 20-30% into a silane coupling agent ethanol solution with a solution mass percentage of 2%, and dispersing them by ultrasonic oscillation for 15 minutes.

[0052] In the powder provided by this embodiment, fused silica and spherical cristobalite constitute the main body of the refractory powder. The 30-50 mesh coarse particles in the fused silica can play an anchoring role during the coating process, which helps to connect and transition between adjacent shell layers. The finer fused silica and spherical cristobalite particles form a good gradation with the above-mentioned coarse quartz particles, making the shell structure dense and improving the shell strength. Spherical cristobalite is easily dispersed in the slurry, effectively improving the uniformity and flow properties of the slurry. As a high-melting-point phase, it accelerates the crystallization rate of quartz glass under high-temperature casting conditions and forms a cross-interlock with the quartz glass, thereby improving the high-temperature stability of the mold shell. Furthermore, because spherical cristobalite inhibits the liquid-phase sintering process, it helps maintain the size and number of pores formed by powder accumulation, preserving the permeability of the mold shell. Finally, during the cooling process after casting, β-cristobalite transforms into α-cristobalite at 180–270℃, resulting in approximately 3% volume shrinkage. This shrinkage forms a network of microcracks inside the mold shell. The increase in porosity and the formation of microcracks improve the collapsibility of the mold shell, facilitating demolding.

[0053] The silica in the formulation has good affinity with silica sol and is easy to disperse in the binder. In the coating prepared according to a certain powder-liquid ratio, the uniformly dispersed silica particles can be linked together by hydrogen bonds to form a network structure in the liquid, which helps the coating maintain good suspension for a long time. At the same time, silica has a large specific surface area and can act as a mineralizer during the shell sintering process.

[0054] The introduction of organic fibers has two advantages. First, it can form a three-dimensional random structure in the shell during the molding process, which can improve the room temperature strength of the shell and prevent the shell from cracking during the drying process. Second, the organic fibers burn off at high temperatures, leaving pores that improve the air permeability and collapse resistance of the shell.

[0055] Thixotropic agents can establish a three-dimensional, "card-like" grid structure in coating systems, dividing and locking the dispersions within the system. Under undisturbed conditions, the viscosity of the system increases over time until it stabilizes. However, when subjected to shear forces exceeding its yield value, this "card-like" structure is opened and disrupted, causing the coating viscosity to decrease. The introduction of thixotropic agents can effectively regulate the coating properties of coatings.

[0056] To better illustrate the technical means and product effects of the present invention, the preferred embodiments of the present invention will be described below.

[0057] Example 1

[0058] Raw material preparation ratio

[0059] The proportions of powder raw materials by mass percentage are shown in Table 3.

[0060] Table 3 Powder Raw Material Ratio (wt%)

[0061]

[0062] The proportions of nylon fiber and magnesium aluminum silicate are each a percentage of the total mass of fused silica, cristobalite, and white carbon black.

[0063] The chemical composition of the raw materials is shown in Table 2.

[0064] Powder preparation method

[0065] a. Organic fiber treatment

[0066] Prepare a 2% silane coupling agent ethanol solution, immerse 20% of the solution mass of nylon fiber into the prepared solution, and disperse it by ultrasonic vibration. After 15 minutes, remove the fiber and place it on the surface of filter paper to evaporate and dry naturally.

[0067] b. Powder preparation

[0068] First, weigh out the quartz, white carbon black and thixotropic agent according to the proportions in Table 3, mix them using a high-speed mixer for 5 minutes, and then transfer them to a V-type mixer.

[0069] Next, weigh the treated polypropylene fibers according to the proportions in Table 3, add them to the above-mentioned V-type mixer, adjust the speed of the V-type mixer to 60 rpm, and mix for 5 minutes.

[0070] Finally, weigh the fused silica powder according to the proportions in Table 3, add it to the V-type mixer, adjust the speed of the V-type mixer to 60 rpm, and mix for 20 minutes.

[0071] After mixing, the powder provided by this invention is obtained.

[0072] Example 2

[0073] The powder and preparation method provided by this invention are as follows:

[0074] Raw material preparation ratio

[0075] The proportions of powder raw materials by mass percentage are shown in Table 4.

[0076] Table 4. Powder Raw Material Ratio (wt%)

[0077]

[0078] The proportions of polypropylene fiber and magnesium aluminum silicate added are each percentage of the total mass of fused silica, cristobalite, and white carbon black.

[0079] Powder preparation method

[0080] a. Organic fiber treatment

[0081] Prepare a 2% silane coupling agent ethanol solution, immerse 20% polypropylene fiber (by weight of the solution) into the prepared solution, and disperse it by ultrasonic vibration. After 15 minutes, remove the fiber and place it on the surface of filter paper to evaporate and dry naturally.

[0082] b. Powder preparation

[0083] First, weigh out the quartz, white carbon black and thixotropic agent according to the proportions in Table 3, mix them using a high-speed mixer for 5 minutes, and then transfer them to a V-type mixer.

[0084] Next, weigh the treated polypropylene fibers according to the proportions in Table 3, add them to the above-mentioned V-type mixer, adjust the speed of the V-type mixer to 60 rpm, and mix for 5 minutes.

[0085] Finally, weigh the fused silica powder according to the proportions in Table 3, add it to the V-type mixer, adjust the speed of the V-type mixer to 60 rpm, and mix for 20 minutes.

[0086] After mixing, the powder provided by this invention is obtained.

[0087] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the claims.

Claims

1. A ceramic mold shell back coating powder suitable for large high-temperature alloy castings, characterized in that, The mixture comprises a mixed powder, organic fibers, and a thixotropic agent. By mass percentage, the mixed powder comprises 70-85% fused silica powder, 10-20% spherical cristobalite powder, and 5-10% silica. The organic fibers comprise 1-1.5% of the total mass of the mixed powder, and the thixotropic agent comprises 0.2-0.5% of the total mass of the mixed powder. The fused silica powder comprises, by weight percentage: 10-15% particles with a particle size of 30-50 mesh and 60-70% particles with a D50 of 45-50 micrometers; The spherical cristobalite has a particle size D50 of 12-15 micrometers; The particle size D50 of the precipitated silica is 0.2~0.3 micrometers.

2. The ceramic mold shell back coating powder suitable for large high-temperature alloy castings according to claim 1, characterized in that, The organic fibers have a length of 4-6 mm and a diameter of 15-17 μm.

3. The ceramic mold shell back coating powder suitable for large high-temperature alloy castings according to claim 1, characterized in that, The thixotropic agent has a particle size of 325 mesh.

4. The ceramic mold shell back coating powder suitable for large high-temperature alloy castings according to claim 1, characterized in that, The fused silica powder contains ≥99.7% silica; the spherical cristobalite powder contains ≥99.5% silica; and the white carbon black contains ≥99.8% silica.

5. The method for preparing ceramic shell back coating powder suitable for large high-temperature alloy castings as described in claim 1, characterized in that, Includes the following steps: 1) Pretreatment of organic fibers; 2) Immerse the pretreated organic fibers in a silane coupling agent ethanol solution, disperse them by ultrasonic vibration, remove the fibers, and place them on the surface of filter paper to evaporate and dry naturally. 3) Weigh out the spherical cristobalite powder, white carbon black, and thixotropic agent according to the preparation ratio and mix them to obtain a preliminary mixture; 4) Add the organic fibers obtained in step 2) to the preliminary mixture to obtain the mixture; 5) Weigh the fused silica powder according to the preparation ratio and add it to the mixture obtained in step 4) to finally obtain the ceramic shell back coating powder.

6. The method for preparing ceramic shell back coating powder suitable for large high-temperature alloy castings according to claim 5, characterized in that, Step 1) Pretreatment of organic fibers includes: immersing organic fibers with a solution mass percentage of 20-30% into a silane coupling agent ethanol solution with a solution mass percentage of 2%, and dispersing them by ultrasonic vibration for 15 minutes.

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