A high-temperature resistant composite ceramic mold shell for precision casting

Abstract

The present invention belongs to the field of casting technology in physical metallurgy, and relates to a high-temperature resistant composite ceramic mold shell for precision casting, belonging to the field of casting in physical metallurgy. The composite ceramic mold shell described in the present invention is composed of a yttrium oxide surface layer, a yttrium oxide + alumina + zirconia transition layer, and an alumina + zirconia backing layer. It is manufactured by the process of layer-by-layer coating + sand sprinkling + drying, dewaxing, and roasting. The temperature resistance of the mold shell can reach 1850 °C, which can meet the precision casting requirements of high-melting-point and highly reactive alloys such as titanium and its alloys, and zirconium alloys.

Description

Technical Field

[0001] The present invention relates to a high-temperature resistant composite ceramic mold shell for manufacturing precision castings, belonging to the field of casting in physical metallurgy. Background Art

[0002] Modern industry has increasingly higher requirements for the complexity and precision of components. The raw materials used have covered various materials such as steel, superalloys, titanium and its alloys, zirconium alloys, etc. The casting process has also developed from the original ordinary casting method to investment precision casting, including fine grain casting, directional solidification casting, single crystal casting, etc. For complex and precision components, the ceramic mold shell used in their precision casting is a crucial auxiliary material, and its main role is to ensure the external dimensions, external surface quality, and metallurgical quality of the components.

[0003] Common precision casting mold shell systems mainly include those based on the aluminosilicate mullite system and the alumina system. Their raw materials include fused alumina, silica, aluminosilicate mullite, zircon sand, and silica sol, etc. The service temperature is about 1600°C, which can basically meet the precision casting requirements of current steel and superalloy components. However, titanium and its alloys, zirconium alloys have higher melting points, and the temperature of the alloy melt during precision casting will be above 1650°C; at the same time, since these alloys contain a large amount of metals with very high high-temperature chemical activity such as titanium, niobium, zirconium, hafnium, etc., it is necessary to avoid or reduce the reaction between the high-temperature melt of the alloy and the inner surface (surface layer) of the mold shell to ensure the surface quality and metallurgical quality of the casting. Therefore, the mold shell surface needs to have high high-temperature chemical inertness. However, the above aluminosilicate mullite system mold shell and alumina system mold shell have insufficient temperature resistance and relatively high chemical activity, and are no longer suitable for the precision casting of such high-melting-point and high-activity alloys.

[0004] Currently, corresponding research has been carried out at home and abroad on the preparation methods of such ceramic shells. In these studies, high-melting-point oxides, nitrides, etc. are mainly used as refractory materials to improve the temperature resistance of the shell and reduce the interfacial reaction between the shell and the alloy melt. However, in the shells provided in these studies, the use of sols such as zirconium sol, silicon sol, ethyl silicate, zirconium acetate, and zirconium acetate amine in the middle layer of the shell will make the middle layer contain oxides such as zirconia and silica. Moreover, in some shells, alumina powder and zirconia powder are used in the middle layer. Due to the limitation of their chemical stability, these oxides of zirconia, silica, and alumina are not sufficient to effectively inhibit the interfacial reaction between the active elements in the high-temperature alloy melt and the middle layer, thus causing phenomena such as penetration, which affects the metallurgical quality of the workpiece. At the same time, in some studies, silica sol is used as the back-layer binder for the shell. After roasting and forming, substances with relatively low melting points such as silica and mullite will appear in the back layer, affecting the temperature resistance of the shell. In addition, although some studies have mentioned using a single yttrium sol + yttria as raw materials, its single mixing method will affect the impact resistance of the shell, and cracks often appear during the thermal shock process, resulting in the problem of leakage of the molten metal (commonly known as steel leakage), and the shape and size of the precision casting parts cannot be guaranteed. Summary of the Invention

[0005] The object of the present invention is: aiming at the deficiencies of the existing shell technology, to solve the problems of insufficient temperature resistance, high chemical activity of the middle layer, and poor thermal shock resistance of the existing shell, and to provide a high-temperature-resistant composite ceramic shell for precision casting with a service temperature up to 1850 °C and its preparation method to meet the precision casting requirements of high-melting-point and high-activity alloys such as titanium and its alloys and zirconium alloys.

[0006] To solve this technical problem, the technical solution of the present invention is:

[0007] A high-temperature-resistant composite ceramic shell for precision casting, which is composed of a yttria middle layer, a yttria + alumina + zirconia transition layer, and an alumina + zirconia back layer;

[0008] The middle-layer coating is made by adding yttria powder to yttrium sol as a binder, and the powder-liquid ratio of yttria powder to yttrium sol is (3 - 6) kg: 1 L;

[0009] The transition-layer coating is made by adding a mixture of alumina + zirconia powder to yttrium sol as a binder, and the powder-liquid ratio of the mixture of alumina + zirconia powder to yttrium sol in the coating is (4 - 6) kg: 1 L;

[0010] The back-layer coating is made by adding a mixture of alumina + zirconia powder to aluminum sol or zirconium sol as a binder, and the powder-liquid ratio of the mixture of alumina + zirconia powder to aluminum sol or zirconium sol is (3 - 6) kg: 1 L.

[0011] The surface layer is a single-layer structure; the transition layer is a single-layer or double-layer structure; the back layer is 4 - 6 layers.

[0012] It is manufactured by the process of layer-by-layer coating + sand spreading + drying, dewaxing, and roasting; for the last layer of the back layer, only the coating is applied without sand spreading; the maximum service temperature is 1850 °C.

[0013] The surface layer is spread with yttrium oxide sand; the transition layer is spread with yttrium oxide sand; the back layer is spread with a mixture of alumina sand and zirconia sand.

[0014] The mass fraction of Y2O3 in the yttrium sol is 12% - 16%.

[0015] The mass fraction of Al2O3 in the aluminum sol is 20% - 25%.

[0016] The mass fraction of ZrO2 in the zirconia sol is 13 - 18%.

[0017] The mass fraction of alumina in the alumina + zirconia mixed powder is 10 - 30%.

[0018] The preparation steps of the high-temperature resistant composite ceramic shell for precision casting are as follows:

[0019] 1) Preparation of the shell surface layer:

[0020] Using yttrium sol as the binder, adding yttrium oxide powder to make the surface layer coating, where the powder-liquid ratio of the coating is (3 - 6) kg: 1 L; evenly coat the surface layer coating on the surface of the cleaned wax mold, then evenly spread yttrium oxide sand on the wax mold surface coated with the surface layer coating, and dry the shell.

[0021] 2) Preparation of the shell transition layer

[0022] Using yttrium sol as the binder, adding alumina + zirconia mixed powder to make the transition layer coating, where the powder-liquid ratio of the coating is (4 - 6) kg: 1 L; evenly coat the transition layer coating on the wax mold surface that has been coated with the surface layer above, then evenly spread yttrium oxide sand on the wax mold surface, and dry the shell.

[0023] 3) Preparation of the shell back layer

[0024] Using aluminum sol or zirconia sol as the binder, adding alumina + zirconia mixed powder to make the back layer coating, where the powder-liquid ratio of the coating powder is (3 - 6) kg: 1 L; evenly coat the back layer coating on the wax mold surface that has been coated with the transition layer above, then evenly spread the mixture of alumina sand and zirconia sand on the wax mold surface, and dry the shell; the process of back layer coating + sand spreading + drying is repeated 4 - 6 times; for the last time, only the back layer coating is applied without sand spreading.

[0025] 4) Put the above-mentioned coated and dried mold shell into a steam dewaxing kettle for dewaxing.

[0026] 5) Bake the dewaxed mold shell in a high-temperature baking furnace in the atmospheric environment.

[0027] The beneficial effects of the present invention are as follows:

[0028] The high-temperature resistant composite ceramic mold shell for precision casting and its preparation method provided by the present invention have the following advantages:

[0029] 1) For traditional mullite-based mold shells, the raw material used is mullite which is aluminosilicate, and the binder is silica sol; similarly, alumina-based mold shells also usually use silica sol. The main constituent phases of these two types of mold shells after baking are silica and alumina. During the casting process, the high-temperature melt of titanium and its alloys will react violently with silica, resulting in a reaction layer on the surface where the casting contacts the mold shell, and even causing adhesion between the mold shell and the casting, seriously affecting the quality of the casting. At the same time, the melting point of silica is only about 1700 °C, which is also the reason why the service temperature of traditional mold shells does not exceed 1600 °C. In order to minimize the reaction between the mold shell and the casting to the greatest extent, based on the relevant physical and chemical data of the ease of formation of oxides, and combined with the service environment of the mold shell and the feasibility of preparation and production, the present invention selects yttrium sol, aluminum sol, zirconium sol, yttrium oxide, alumina, and zirconia as the raw materials for the mold shell, avoiding silica sol and silica, thereby avoiding the reaction layer formed between the titanium alloy melt and the mold shell due to the presence of silica and the pollution of the casting surface; in addition, the melting points of yttrium oxide, alumina, and zirconia formed after baking the mold shell of the present invention are all 2000 °C, ensuring the high-temperature strength of the mold shell at 1850 °C.

[0030] 2) The surface layer of the mold shell is prepared from yttrium sol and yttrium oxide powder / sand. After baking, a surface layer containing only yttrium oxide is obtained. Yttrium oxide is an oxide with a high melting point and high chemical stability, which can effectively avoid the reaction between the high-temperature melts of titanium and its alloys and zirconium alloys and the surface layer of the mold shell, ensuring the surface metallurgical quality of the parts.

[0031] 3) The mold shell has an intermediate layer. The intermediate layer is prepared from yttrium sol, a mixture of alumina + zirconia powder, and yttrium oxide sand. After baking, an intermediate layer with yttrium oxide + alumina + zirconia is obtained, thus realizing the transition between the yttrium oxide surface layer and the back layer, avoiding the peeling phenomenon of the yttrium oxide surface layer, and at the same time enhancing the interfacial bonding force and improving the thermal shock resistance of the mold shell.

[0032] 4) The shell back layer contains alumina and zirconia. Since the melting point of zirconia is much higher than that of alumina, it can effectively ensure the high-temperature strength of the shell. Therefore, the service temperature of this shell is much higher than that of traditional alumina-aluminum silicon mullite system shells and alumina system shells. However, to meet the strength and other properties of the shell at 1850 °C, the quality of alumina and [oxidant] is reasonably matched. Brief Description of the Drawings

[0033] To more clearly illustrate the technical solutions implemented in the present invention, the drawings to be used in the examples of the present invention will be briefly explained below.

[0034] Figure 1 is a schematic diagram of the prepared composite gradient ceramic shell;

[0035] 1 is the wax pattern, 2 is the shell surface layer, 3 is the shell transition layer, 4 is the shell back layer, and 5 is the last layer of the back layer. Detailed Embodiment

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0037] The features of each aspect of the embodiments of the present invention will be described in detail below. The schematic diagram of the composite gradient ceramic shell of the present invention is as Figure 1 shown, and its preparation method is as follows:

[0038] 1) Preparation of the shell surface layer 2:

[0039] Using yttrium sol as the binder, add yttrium oxide powder to make the surface layer coating. The powder-liquid ratio of the coating is shown in Table 1 below; evenly apply the surface layer coating on the surface of the cleaned wax pattern 1, then evenly sprinkle yttrium oxide sand on the wax pattern surface coated with the surface layer coating, and dry the shell.

[0040] 2) Preparation of the shell transition layer 3

[0041] Using yttrium sol as the binder, add alumina + zirconia mixed powder to make the transition layer coating. The powder-liquid ratio of the coating is shown in Table 1 below; evenly apply the transition layer coating on the surface of the wax pattern that has been coated with the surface layer above, then evenly sprinkle yttrium oxide sand on the surface of this wax pattern, and dry the shell.

[0042] 3) Preparation of the shell back layer 4

[0043] It should be noted that there seems to be a typo in the original text where it says "aluminum and oxidation" in . I translated it as "aluminum and [oxidant]" based on the context, but it might need to be corrected in the original.Using zirconium sol as a binder, add a mixed powder of alumina + zirconia to make the back layer coating. The powder-liquid ratio of the coating powder is shown in Table 1 below. Uniformly apply the back layer coating on the surface of the wax mold that has been coated with the transition layer above. Subsequently, uniformly sprinkle zircon sand on the surface of the wax mold, and dry the mold shell. The process of back layer coating + sand sprinkling + drying is repeated multiple times. Among them, only the back layer coating is applied for the last layer, and no sand is sprinkled.

[0044] 4) Put the above-coated and dried mold shell into a steam dewaxing kettle for dewaxing.

[0045] 5) Bake the dewaxed mold shell in a high-temperature baking furnace in the atmospheric environment.

[0046] Among them, the mass fraction of Y2O3 in the yttrium sol used for the surface layer and transition layer of the mold shell, the mass fraction of ZrO2 in the zircon sol used for the back layer, the mass fraction of Y2O3 in the yttrium oxide used, the mass fraction of alumina in the mixed powder of alumina + zirconia and the mixed sand of alumina + zirconia are shown in different examples in Table 1 below.

[0047] Use the ceramic mold shell prepared above for the directional solidification casting of zirconium alloy or titanium aluminum alloy specimens.

[0048] After directional solidification is completed, inspect the mold shell and the specimen.

[0049] The components, contents, and the final inspection results of the mold shell of each of the above examples are shown in Table 1 below.

[0050] It can be seen from the test results that during the casting process of both zirconium alloy or titanium aluminum alloy specimens, the mold shell did not show deformation or cracking phenomena, the surface of the specimen was smooth, there was no sticking shell phenomenon, and no obvious interfacial reaction occurred between the alloy and the mold shell. During precision casting, the temperature of the melt and the mold shell was above 1650 °C, and the highest could reach 1800 °C.

[0051] Table 1 Data table of each example

[0052]

Claims

1. A high-temperature resistant composite ceramic shell for precision casting, characterized in that: This shell mold consists of a yttrium oxide surface layer, a yttrium oxide + alumina + zirconia transition layer, and an alumina + zirconia backing layer; The surface layer coating is made by adding yttrium oxide powder to yttrium sol as a binder, and the powder-liquid ratio of yttrium oxide powder to yttrium sol is (3 - 6) kg: 1 L; The transition layer coating is made by adding a mixture of alumina + zirconia powder to yttrium sol as a binder, and the powder-liquid ratio of the alumina + zirconia powder mixture to yttrium sol in the coating is (4 - 6) kg: 1 L; The backing layer coating is made by adding a mixture of alumina + zirconia powder to aluminum sol or zirconium sol as a binder, and the powder-liquid ratio of the alumina + zirconia powder mixture to aluminum sol or zirconium sol is (3 - 6) kg: 1 L; The mass fraction of Y2O3 in the yttrium sol is 12% - 16%; The mass fraction of alumina in the alumina + zirconia powder mixture is 10 - 30%.

2. The high-temperature resistant composite ceramic shell for precision casting according to claim 1, characterized in that: The surface layer is a single-layer structure; the transition layer is a single-layer or double-layer structure; the backing layer is 4 - 6 layers.

3. The high-temperature resistant composite ceramic shell for precision casting according to claim 1, characterized in that: This shell mold is manufactured by the process of layer-by-layer coating + sanding + drying, dewaxing, and roasting; for the last layer of the backing layer, only the coating is applied without sanding.

4. The high-temperature resistant composite ceramic shell for precision casting according to claim 1, characterized in that: Yttrium oxide sand is sprinkled on the surface layer; yttrium oxide sand is sprinkled on the transition layer; a mixture of alumina sand + zirconia sand is sprinkled on the backing layer.

5. The high-temperature resistant composite ceramic shell for precision casting according to claim 1, characterized in that: The mass fraction of Al2O3 in the aluminum sol is 20% - 25%.

6. The high-temperature resistant composite ceramic shell for precision casting according to claim 1, characterized in that: The mass fraction of ZrO2 in the zirconium sol is 13 - 18%.

7. The high-temperature resistant composite ceramic shell for precision casting according to claim 3, characterized in that: The maximum service temperature of this shell mold is 1850°C.

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