RU2531335C1 - Production of ceramic casting moulds - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to ceramic moulds. Refractory plies of ethyl silicate binder-based suspension is formed are the pattern. To make 5-9 of said plies, used is ethyl silicate binder with content of silica decreased to 8-10 wt % which contains surfactant in amount of 0.2 wt %. Pattern is cast, ceramic mould is dried, impregnated, dried and annealed. Ceramic mould is impregnated on outer side to dipping with 5-10% aqueous solutions of barium and calcium chlorides or aluminium sulphide for 5-10 s.

EFFECT: invention provides ceramic molds with optimal and stable indicators of durability while reducing consumption of ethyl silicate.

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Description

The invention relates to the field of foundry and may find application in the production of precision castings in vacuum by casting.

To obtain a wide range of castings with high demands on the quality of their surface, corundum sillimanite or electrocorundum forms are used, obtained by investment casting. Ethylsilicate-40 hydrolyzed with an organic solvent with 16 ÷ 20% of conventional silica is used as a binder. This binder solution makes it possible to obtain a suspension with high technological properties, and after hardening the formation of the ceramic shell on the model, it does not break when the model is melted with hot water and provides the required ceramic strength after firing. These qualities of ethyl silicate make it practically indispensable in obtaining molds for cast parts with high quality requirements. A decrease in the binder content leads to a deterioration in the quality of the facing surface of the ceramic mold and a decrease in bulk strength. Attempts to replace ethyl silicate with other binder solutions have failed, since the unity of requirements is not ensured: high-quality coating with a model suspension, obtaining satisfactory surface and bulk strength, and inertness to a metal melt containing titanium, aluminum, chromium, and other chemically active elements.

Therefore, in order to partially replace the expensive ethyl silicate, attempts were made to add to the suspension substances that increase the strength of ceramics after firing, to form the last four layers in the eight-layer form based on a liquid glass binder, or to impregnate the ceramic shell with strengthening solutions.

A known suspension with a reduced content in a hydrolyzed SiO ₂ solution of 8 ÷ 12% and an addition of aluminum powder to the filler to 4% wt., Which oxidizes during firing of ceramics and initiates the reaction sintering mechanism, compensating for the loss of strength due to a decrease in the content of the main binder component ( Solodyankin A.A., Kulakov B.A., Stadnichuk V.I. Alexandrov V.M. Low-silica ethyl silicate binder for ceramic molds // Foundry, 1988. No. 3. P. 19 ÷ 20). This suspension is used to reduce the chemical activity of the form to a highly active alloy. From it form the first two facing layers of the form. Aluminum powder has a high cost and is not completely oxidized during the firing of ceramic forms. When casting, the oxide shell may break and release residual aluminum with the formation of surface defects on the casting. It is necessary to bake the ceramic mold in a vacuum or introduce special additives to it that promote oxidation. This complicates the process. Aluminum powder has a high cost. Therefore, the manufacture of all refractory layers from this suspension is impractical.

A known method of manufacturing ceramic molds according to removable models, according to which the form after firing at 1150 ÷ 1350 ° C is impregnated with a solution of silicone varnish 15 ÷ 25% containing one fluoride from the group of metal aluminum, chromium, barium, calcium 1.5 ÷ 5% (first option) or dispersed aluminum 2 ÷ 10% (second option). Then a second firing is carried out (Pat. No. 2343038 Russia. Method for the manufacture of ceramic molds using removable models). Strength increased by 14 ÷ 16%. The disadvantages include the use as an impregnating material of a silica-forming substance, expensive fluorides and aluminum powder, as well as double firing of a ceramic form at elevated temperatures.

The introduction into the suspension of another organosilicon binder, for example oligoorganohydridesiloxane liquid, the structure of which can be reflected by the scheme [(C ₂ H ₅ ) ₃ -Si-O-] _n , where n = 9 ÷ 14 (AS No. 772672 USSR. Suspension for manufacturing of casting molds by investment casting), increases the content of silica in ceramics and its chemical activity in relation to metal melts filled in vacuum, which incorporate aluminum and titanium.

A known method is when the last refractory layers of the mold shell are formed on the basis of a suspension with a liquid glass binder (Pat. No. 2177856 USSR Method for the manufacture of cast ceramic molds obtained by investment casting). However, the shells on ethyl silicate during firing expand by 0.2%, and the shells on liquid glass are compressed by 0.3 ÷ 0.7%. Warping and cracking of the form occurs.

The addition of sodium fluoride to the facing layers leads to the appearance of the sodium silicate liquid phase, which slightly reduces cracking of the form during firing, but the liquid phase remains upon pouring the forms preheated to 900 ÷ 950 ° C, which leads to the chemical interaction of highly active alloyed alloys with it and the appearance defects on the cast surface. Impregnation of liquid glass layers with other solutions significantly complicates the process.

When pouring molds in vacuum, molds based on liquid glass are not used. In the work (Special casting methods: Handbook / Ed. By V.A. Efimov. M., Mashinostroenie, 1991, p.234) it is not recommended to use combined forms when receiving castings for important purposes.

The known composition of the solution used for hardening 4-layer ceramics. It consists of,% vol .: triethanolamine - 10 ÷ 30; 5% aqueous emulsion of polymethylsiloxane liquid - 15 ÷ 30; water - the rest (Pat. No. 944730 RF Solution for processing ceramic foundry molds). For the manufacture of molds, an aqueous ethyl silicate binder was used, and pulverized silica was used as a filler in the suspension. Polymethylsiloxane liquid contains bonds Si-CH ₂ -Si, stable to hydrolysis. The impregnation was carried out by immersion of a 4-layer coating until the model was removed into the solution for 3–6 s. Ceramics were fired after the model was removed at a temperature of 950 ° C. In the process of firing, the organic groups of polymethylsiloxane liquid burn out and new bonds are created - siloxane. The application of this method allowed to increase the strength of ceramics at a temperature of 950 ° C from 20.5 to 26.5 ÷ 34.8 kg / cm ² , i.e. by 29.2 ÷ 69.8%. Obtaining such high rates is due to the fact that the specific strength of thin samples is greater than the specific strength of multilayer samples, for example, eight-layer ones made using the same technology. The second reason is the complete impregnation of a thin section of ceramic with an impregnating solution. The impregnation time of one layer was 0.75 ÷ 1.5 s. It should be noted a low concentration of polymethylsiloxane liquid in solution - 0.33 ÷ 0.17%. The disadvantage of this method is the increase in the content of silica in ceramics after firing, which will reduce its chemical activity to alloyed alloys.

The cheapest way is to impregnate a multilayer form after removing the model and drying with aqueous solutions of inorganic salts. However, it does not find wide application due to the lack of optimal parameters of the impregnation process in accordance with the conditions of its use. When processing the form with aqueous solutions of salts, the following tasks were solved:

1. Obtaining a fine-grained alloy structure.

2. Increase the chemical inertness of the form to the alloy.

3. Increasing the strength of the mold.

The implementation of the first direction is associated with the use of cobalt, chromium, iron, and yttrium ions in an impregnating solution.

The second direction is associated with the formation of a salt film on the ceramic surface. A feature of the method is the impregnation of the mold after firing, followed by drying and pouring the alloy. After drying, the released salt fills the pores of the ceramic, but it does not sinter with the ceramic.

Thus, in order to increase the strength of the ceramic mold, it is necessary to calcine it after impregnation, which ensures sintering of the salt with binder silica crystallizing from the amorphous state in its pores.

A known method according to which the form is proposed to be impregnated with a solution containing the following ingredients,% wt .: 2% aqueous solution of hydrochloric acid is the basis; calcium chloride - 15 ÷ 35 (AS No. 460102 USSR Solution for processing ceramic molds). In the process of firing, the formation of heat-resistant calcium silicate compounds with less linear expansion. Strength increases and the quality of molds improves. However, the impregnation time and size of the increase in strength are not given. The salt concentration is characteristic of methods in which the impregnation is carried out to create a salt film.

Increasing the strength of the ceramic mold is achieved by impregnating it after drying with aqueous solutions of aluminum salts, followed by drying and firing. For example, it is known to use aluminum phosphate for these purposes (Waki Mamoto, Yamashito Akira. Hardening of molds by investment casting). Abstract Journal. Engineering Technology, 1975. No. 8. P.40 (Pat. No. 49-39729 Japan). After impregnation, the molds were dried and calcined at a temperature of 500 ° C. The low firing temperature indicates the pouring of light alloys into such forms.

In the work (Serebryakov S.P., Larionov A.Ya., Tselmovich V.A. and others. Improving the surface of sillimanite-corundum forms // Foundry, 1984. No. 8. P. 22) it is shown that after impregnation of 8- layered samples with a solution of aluminum sulfate, drying and firing for 1 hour at a temperature of 1000 ° C there was an increase in their strength. With an increase in the aluminum sulfate content in the solution to 5%, the bending strength increased from 2.1 to 2.4 MPa, and at 5–25% to 2.7 MPa, i.e. 28.6%. The disadvantages include the lack of data on the impregnation time, the low initial strength of the samples - 2.1 MPa, which may be due to the insufficient sintering time of ceramics, a high firing temperature (usually sillimanite-corundum forms are fired at a temperature of 900 ° ÷ 950 ° C) and the probability of the formation of a salt film on the working surface of the ceramic mold at a high salt concentration, for example, more than 15%. It should be noted that during the formation of a continuous film of aluminum sulfate on the ceramic working surface during the firing process, aluminum oxide will form, which will sinter at temperatures above 1000 ° C. Aluminum chloride should be excluded for impregnation due to the very low sublimation temperature of 180.2 ° C, which is even lower than the melting point of 192.4 ° C (Brief Handbook of Physicochemical Values. Ed. 8, revised / Ed. A. A. Ravdel and A.M. Ponomareva. - L.: Chemistry, 1983.- 232 s).

The closest technical solution to the proposed method is the impregnation of corundum sillimanite ceramics after melting the model and drying at 200 ° C by immersion in aqueous solutions of barium chloride and holding them for 5 s ([Stadnichuk V.I. Strengthening the strength of corundosillimanite ceramics by impregnation with hardening solutions // Refractories and technical ceramics, 2012. No. 9. S.20 ÷ 26.). The concentration of salts is in the range of 5 ÷ 10%. Barium chloride has a boiling point of more than 1860 ° C (Chemical properties of inorganic substances: Textbook for universities. 3rd ed., Rev. / Under the editorship of RA Lidin - M .: Chemistry, 2008, 318 s. .), which is much higher than the pouring temperature of the metal. Samples were fired for 5 hours at a temperature of 950 ° C. The ceramic bending strength σ _{and was} determined at a temperature of 900 ° C. The results showed that the strength of samples impregnated with a 5% solution of BaCl ₂ increased from 39.90 g / cm ² to 42.40 ÷ 45.96 kg / cm ² . Moreover, a lower value of strength - 42.40 kg / cm ² corresponded to a larger thickness of the samples compared to the base case. Samples impregnated with a 10% salt solution had a strength of 51.25 kg / cm ² . Thus, the value of hardening, defined as the difference between the values of strength after and before impregnation and referred to the value of the initial strength, was in the range 6.3–86.8%.

The presented results showed that the use of a low salt concentration allows the ceramic form to be sufficiently impregnated in a short time. This is due to the fact that aqueous solutions of barium chloride of 10% concentration have a density of only 1.083 g / cm ³ , which favors rapid impregnation.

It should be noted that for impregnation it is possible to use salts having boiling points above the temperatures of alloys poured into ceramic forms - 1520 ÷ 1650 ° C. In a number of cases, this condition is satisfied by calcium chloride having a boiling point of 1600 ° C (Chemical properties of inorganic substances: Textbook for universities. 3rd ed., Rev. / Under the editorship of R. Lidin - M .: Chemistry, 2008, 318 p.). When pouring molds in vacuum, the temperature of sublimation of salts decreases, which is dangerous in the case of the formation of a salt film on the surface of the ceramic. However, to solve the problem of increasing the strength of the form, salt solution enters the pores of the form without forming a film on the ceramic surface. This contributes to the short duration of the impregnation. According to the works (Polees ME Analytical chemistry. M: Medicine, 1981. 216 p. And Glinka NL General chemistry. L: Chemistry, 1988. 702 p.) A noticeable decomposition of calcium chloride in the presence of SiO ₂ starts at temperatures over 720 ° C, i.e. at the firing temperature of the forms, and is described by the reaction:

2CaCl ₂ + SiO ₂ + O ₂ = 2CaO · SiO ₂ + 2Cl ₂

Strong calcium silicate is formed, while the chemical activity of silicon in ceramics decreases, which increases its inertness to highly alloyed alloys, such as heat-resistant. The formation of barium silicates proceeds according to a similar scheme. The increase in strength of ceramic molds after impregnation exceeds the required values. This often causes cracks in the castings, therefore, to reduce the strength, it is possible to reduce the content of ethyl silicate in 5 ÷ 9 refractory layers of the mold.

The objective of the invention: the development of a method of manufacturing ceramic molds by investment casting with a reduced consumption of ethyl silicate binder.

The technical result of the invention is to obtain ceramic molds with optimal and stable strength indicators while reducing the consumption of ethyl silicate.

The method is implemented due to the fact that the manufacture of casting ceramic molds involves the preparation of a refractory slurry based on an ethyl silicate binder, layer-by-layer formation of refractory layers on the model, smelting of the model, drying of the ceramic mold, its impregnation by immersion with 5 ÷ 10% aqueous solutions of barium chloride , calcium or aluminum sulfate for 5 ÷ 10 s, drying and firing, in addition, for the formation of 5 ÷ 9 layers using ethyl silicate binder with reduced to 8 ÷ 10% wt. the content of conventional silica containing a surfactant in an amount of 0.2 wt.%, and the impregnation is carried out from the outside.

, где σ_{и ср} (n) и σ_{и ср} (2) - среднее значение удельной прочности вариантов n и второго, кг/с².From hydrolyzed ethyl silicate-40 with a conditional SiO ₂ content _of 17.6%, a distensillimanite suspension was prepared with a VZ-4 viscometer of 48–50 s for the first layer and 28–30 s for subsequent layers. A suspension was prepared at a stirrer rod rotation speed of 2800 rpm. To sprinkle the first suspension layer, electrocorundum grains of EB20 grade and subsequent layers of EB50 were used. The layers were dried first in air - 1 hour, then in a vacuum chamber with the addition of ammonia. The model was a flat tile 1 cm thick of a urea-based model mass (MON10K) with four recesses on each side that marked the boundaries of standard samples. To form the 5th and subsequent layers, a suspension was used on hydrolyzed ethyl silicate containing 10% silica. This solution was obtained by diluting the existing solution with alcohol. To increase the survivability of the suspension and the wet strength of the samples, 0.2% polyvinyl butyral was dissolved in a binder solution in accordance with the recommendations (Lost wax casting. Monograph. / Under the editorship of Ya.I. Shklennik and V.A. Ozerov.- M .: Engineering , 1984. P.196). After the formation of an eight-layer shell, the ninth fixing layer was applied from the suspension, dried, the model mass was dissolved from the ceramic shell in water, and the mold shell was dried at a temperature of 200 ° C. Then the shells were briefly immersed in an aqueous solution of salt so that it did not get into them from the inside. The shells were fired at a temperature of 950 ° C for five hours. Then the shells were broken, the samples were turned on the machine along the contour to the required dimensions and tested for bending according to the standard method (three points) (Lost wax casting. Monograph. / Edited by Ya.I. Shklennik and V.A. Ozerov.- M .: Engineering, 1984. S.200). The average value of the strength σ _{and avg} and the deviation from this value Δ were calculated: $$\Delta =\left[{\sigma }_{\mathrm{and}\mathrm{from}R}\left(n\right)-{\sigma }_{\mathrm{and}\mathrm{from}R}\left(2\right)\right]\cdot \mathrm{one\; hundred}\%/{\sigma }_{\mathrm{and}\mathrm{from}R}\left(\text{2}\right)$$

where σ _{and cf} (n) and σ _{and cf} (2) are the average value of the specific strength of the options n and the second, kg / s ² .

The test results are shown in table 1.

Table 1 Strength of ceramic samples σ _and and impregnation parameters No. The salt concentration,% wt. Impregnation time, s σ _and , kg / cm ² σ _{and avg} , kg / cm ² Δ,% one - - 34.20 ÷ 34.94 34.57 - 2 - - 25.31 ÷ 25.92 25.61 - 3 5 BaCl ₂ 5 26.42 ÷ 26.93 26.67 4.1 four 5 BaCl ₂ 10 27.03 ÷ 27.78 27.40 7.0 5 10 BaCl ₂ 5 28.23 ÷ 28.86 28.52 11.3 6 10 BaCl ₂ 10 34.48 ÷ 34.92 34.71 35.5 7 5 CaCl ₂ 5 26.94 ÷ 27.78 27.36 6.8 8 10 CaCl ₂ 10 35.16 ÷ 35.94 35.39 38,2 9 10 Al ₂ (SO ₄ ) ₃ 10 32.88 ÷ 33.51 33.79 31.9 Note: Var. No. 1 - serial technology; var. No. 2 ÷ 9 - experimental technology (10% SiO ₂ in HRE in a suspension of 5 ÷ 9 layers).

The presented results show that the impregnation of the eight-layer form with 5% aqueous solutions of salts makes it possible to increase its specific strength by 4.1–7.0% with a duration of impregnation of 5–10 s compared to the strength of the experimental form without impregnation — var. No. 2. With an increase in salt concentration to 10%, the specific strength of the mold increases by 11.3–38.2% (var. No. 2 and No. 5, 6, 8, 9). Impregnation of the mold for 10 s allows you to increase its strength by 35.5% or more (var. No. 6, 8.9).

Thus, the application of the proposed method of hardening ceramic molds with a reduced consumption of ethyl silicate allows to control their specific strength over a fairly wide range, which is very important in line production with a wide range of cast parts with different profiles. It is important that the salts released from the solution do not form a salt layer on the surface of the ceramic, since chlorides, especially calcium chloride, can be sublimated in a heated state in vacuum. At a high temperature of more than 800 ° C, sintering of chlorides with crystallizing silica of an ethyl silicate binder occurs in the pores of the form with the formation of strong calcium and barium silicates. The increase in the content of SiO ₂ in the hydrolyzed ethyl silicate used to prepare the suspension of the 5th and subsequent refractory layers automatically leads to an increase in mold strength. The reduction in the content of conventional silica in hydrolyzed ethyl silicate from 10 to 8% wt. it is possible to reduce the value of mold strength to that required for a particular mold profile. Further dilution of ethyl silicate leads to a sharp loss of survivability and sedimentation stability of the suspension. The minimum impregnation time must be applied when using carbamide-based models. After the wax model is smelted, an unmelted model composition remains on the inner surface of the mold, which makes it difficult for the aqueous medium to penetrate when impregnated from the outside of the mold.

Claims (1)

A method of manufacturing casting ceramic molds, including preparing a refractory slurry based on an ethyl silicate binder, layer-by-layer formation of refractory layers on a model, melting the model, drying the ceramic mold, impregnating it with 5-10% aqueous solutions of barium, calcium or aluminum sulfate by immersion for 5-10 s, drying and firing, characterized in that for the formation of layers 5 through 9 use an ethyl silicate binder with a reduced to 8-10 wt.% The content of conventional silica and the content of surfactant in an amount of 0.2 wt.%, and the impregnation of the forms is carried out from its outer side.