RU2412019C1 - Method of producing ceramic shell moulds for investment casting - Google Patents

Abstract

FIELD: process engineering. ^ SUBSTANCE: proposed method comprises producing mould, layer-by-layer application of fire-proof suspension based on pulverised alundum and coating every layer by granular alundum. First two layers are applied using organoaluminium binder in ceramic suspension of general formula RO{[-Al(OR)-O-]x[-Al(OR*)-O-]y}zH, where z=3-100; x+y=1; R*/Al=0.050.95; R=CnH2n+1; n=14; R*=C(CH3)=CHC(O)CnH2n+1; C(CH3)-CHC(O)OCnH2n+1, while the following layers are produced using hydrolysed solution of ethyl-silicate. Ceramic mould layers are dried in chamber with moisture content of, at least 95% of first layer for 1 h, second layer for 3 h, the other layer for 4-6 hours with subsequent convective drying of each layer. Ceramic mould is incinerated at 1250-1350C for 4-6 hours. ^ EFFECT: higher heat resistance and strength. ^ 1 tbl, 2 ex

Description

The invention relates to the field of foundry and can be used for the manufacture of ceramic shell molds for investment casting in the production of critical castings from chemically active steels and alloys.

Currently, industrially produced and widely used binders are technical ethyl silicates and silicas, which form, after calcination, silicon dioxide, the presence of which in ceramic form is highly undesirable.

In the process of pouring and cooling the melt, the metal interacts intensively with the silicon dioxide of the ceramic shell, which leads to the formation of difficult to remove pitting defects on the casting surface, which are detected at various stages of the manufacturing process of parts, increase surface roughness and reduce the corrosion resistance of the metal.

The closest in technical essence to the proposed method is a method for manufacturing combined ceramic molds using lost-wax models, where a suspension of hydrolyzed ethyl silicate binder is used as a suspension for the first two layers, and a suspension with liquid glass is used for subsequent layers (US Pat. , 2007).

This technical solution has significant disadvantages:

- during the casting process, a physicochemical interaction of the shell-shaped material with molten metal occurs. Moreover, the depth of interaction, determined by the thickness of the metal oxide layer, depends on the composition of the heat-resistant alloy, the content of SiO ₂ in the shell form, its structure and can reach 200 microns;

- when applying a liquid-glass layer to ethyl silicate, the binder migrates through the pores of the ethyl silicate layers to the working surface of the mold, thereby reducing the surface quality of the castings due to the interaction of the alloy being filled with sodium oxide contained in the liquid glass;

- during heating, the liquid-glass layers undergo shrinkage, which causes stresses in the shell form, which can lead to cracks and softening of the forms.

The objective of the invention is to improve the quality of investment casting by reducing chemical interaction at the metal-mold interface, eliminating pitting defects in the production of castings from chemically active steels and alloys, increasing heat resistance, strength of the ceramic shell, improving knockability of castings.

This problem is solved in such a way that the method of manufacturing ceramic shell molds using investment casting in the production of especially critical castings from heat-resistant and refractory metals with equiaxial structure, including the manufacture of a model, layer-by-layer application of a refractory slurry based on powdered electrocorundum, followed by sprinkling of each layer with granular electrocorundum , drying the layers of the ceramic mold, removing the model and calcining the ceramic mold, characterized in that the ceramic shell f the yoke is made of the combined, first two layers applied using an organoaluminum binder in the ceramic slurry:

RO {[- Al (OR) -O-] _x [-Al (OR *) - O-] _y } _z H,

where z = 3 ÷ 100; x + y = 1; R * / Al = 0.05 ÷ 0.95; R = C _n H _{2n + 1} ; n = 1 ÷ 4;

R * = C (CH ₃ ) = CHC (O) C _n H _{2n + 1} ; C (CH ₃ ) = CHS (O) OC _n H _{2n + 1} ,

while drying the layers is carried out with exposure in a chamber with a humidity of at least 95% for the first layer for 1 hour, the second layer for 3 hours and subsequent convective drying of each layer for an hour, the subsequent layers being performed using a hydrolyzed solution of ethyl silicate as a binder, and the calcination of the combined ceramic forms is carried out at a temperature of 1250-1350 ° C for 4-6 hours.

In order to save organoaluminum binder, reduce casting costs, improve knockability, an organoaluminum binder is used as a binder for the first and second layers, and a hydrolyzed solution of ethyl silicate is used as a binder for the subsequent layers.

The preparation of a ceramic suspension on an organoaluminum binder is carried out according to the patent of the Russian Federation No. 2082535, B22C 1/06, 1/16, 1997.

The manufacture of ceramic shell molds is carried out by successively dipping the model in a refractory suspension on an organoaluminum binder for the first two face layers and on an ethyl silicate binder for subsequent layers and sprinkling it with refractory granular material based on electrocorundum.

The drying of the first two ceramic layers on an organoaluminum binder is carried out in two stages. The first stage is carried out in a chamber with high humidity (at least 95%): exposure of the first layer - up to 1 hour, the second - up to 3 hours. The second stage is convective drying until the moisture content equilibrium with the environment. Drying of the subsequent layers on an ethyl silicate binder is carried out according to conventional technology.

The excavation of models from shell forms is carried out in any known manner.

The calcination of ceramic molds should be carried out at a temperature of 1250-1350 ° C for 4-6 hours, which contributes to the burning out of the organic components present in the mold and imparting the required mechanical properties to the shell mold.

Methods of manufacturing ceramic shell molds for investment casting is illustrated by the following examples.

Example 1

The present invention was worked out in a foundry in the manufacture of ceramic molds for the production of castings of blades of a gas turbine engine.

An organoaluminum binder with a conditional Al ₂ O ₃ content _of 11% was used as a binder for the first two layers of the coating.

On the model block layer-by-layer was applied a ceramic suspension, including, wt.%:

organoaluminum binder 25 refractory filler 75

followed by sprinkling of each layer with granular electrocorundum.

After applying the layer, the ceramic mold was dried in a chamber with a humidity of 95%, followed by convective drying.

Subsequent layers were prepared using a suspension on an ethyl silicate binder with drying of each layer in a vacuum-ammonia chamber. After applying the last layer, the operation of removing the model composition in the boiler clave was followed.

The ceramic form was calcined at a temperature of 1280 ° C for 4 hours. The calcined forms were filled with ZhS6U alloy at a temperature of 1530 ± 10 ° С. The ceramic shell was removed by sandblasting.

Further, metallographic studies found that when using a combined mold, the interaction layer between the casting metal and the shell mold is ~ 7 times smaller than on blades made using serial technology and does not exceed 5 microns.

The studies of the X-ray spectrum showed that on the front surface of the combined form there is no silicon dioxide diffusing from the inner layers, and its content is equal to the amount that is present in the composition of the mixture of electrocorundum micropowders.

Studies of the microstructure of the material of the blade obtained by pouring into a combined shell mold showed that the dispersion of the dendritic structure is higher compared to control blades. And also another feature of the microstructure of this scapula is a decrease in the amount of (γ-γ ') - eutectic in the interaxial zones of dendrites.

Table 1 shows comparative data on the thickness of the burn-on layer between the casting metal and the combined shell form, as well as the ethyl silicate form.

Example 2

Ceramic molds were made according to the technology described in example 1. Annealed molds were poured with 08Kh14N7ML steel at 1600-1650 ° C.

As a result, the surface of castings manufactured using serial technology was coated with hard-to-remove pitting defects, which increase the surface roughness and reduce the corrosion resistance of the metal.

The resulting castings in combined form did not contain pitting defects on their surface.

As in example 1, the interaction layer between the casting metal and the shell mold is ~ 7 times smaller than the castings made using serial technology and does not exceed 5 microns.

Table 1. Comparative data on the thickness of the layer of burning between the casting metal and shell form Binder Metal to be poured Filling temperature, ° С The thickness of the metal oxide layer, microns Hydrolyzed Ethyl Silicate Solution (GRETS-40) ZhS6U 1530 21.5 Combined form: 1 ÷ 2 layer - aluminum-organic binder 3 ÷ 7 layers - GRETS-40 2-5 Combined form: 1 ÷ 2 layer - aluminum-organic binder 3 ÷ 8 layers - GRETS-40 08X14H7ML 1600-1650 4-5

Claims (1)

A method of manufacturing ceramic shell molds according to investment casting in the production of especially critical castings from heat-resistant and refractory metals with equiaxial structure, including the manufacture of a model, layer-by-layer application of a refractory slurry based on dust-like electrocorundum, subsequent sprinkling of each layer with granular electrocorundum, drying of layers of the ceramic form, removal models and calcination of a ceramic mold, characterized in that the ceramic shell mold is made combined, the first two layers are applied using an aluminum-organic binder in a ceramic suspension:
RO {[- Al (OR) -O-] _x [-Al (OR *) - O-] _y } _z H,
where z = 3-100; x + y = 1; R * / Al = 0.05 ÷ 0.95; R = C _n H _{2n + 1} ; n = 1 ÷ 4;
R * = C (CH ₃ ) = CHC (O) C _n H _{2n + 1} ; С (СН ₃ ) = СНС (O) OC _n H _{2n + 1} , while drying the first two layers is carried out in a chamber with a humidity of at least 95% with an exposure for the first layer of 1 hour, the second layer of 3 hours and subsequent convective drying each layer, subsequent layers are applied using ethyl silicate as a binder hydrolyzed solution, and the calcination of the combined ceramic forms is carried out at a temperature of 1250-1350 ° C for 4-6 hours.