GB1577836A

GB1577836A - Calcia ceramic shell mould system

Info

Publication number: GB1577836A
Application number: GB1553477A
Authority: GB
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1976-04-22
Filing date: 1977-04-14
Publication date: 1980-10-29
Also published as: FR2348772A1; JPS5373419A; SE7704162L; CA1080428A; DE2716342A1; BE853798A

Description

(54) CALCIA MODIFIED CERAMIC SHELL MOULD SYSTEM (71) We, UNITED TECHNOLOGIES CORPORATION a Corporation organized and existing under the laws of the State of Delaware, United States of America, having a place of business at l, Financial Plaza, Hartford, Connecticut, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to the field of ceramic shell moulds which comprise a ceramic aggregate bonded together with colloidal silica. Such moulds are used in the production of precision castings. The particular mould system described is particularly applicable to the production of directionally solidified superalloy articles.

The use of shell moulds in precision casting is a process known in the art. In this process a meltable pattern for example of wax having a configuration which generally corresponds with that of the desired final product is coated with a ceramic mix which is then dried. After drying, the mould assembly is heated to remove the wax pattern. The coating process involved alternate repeated steps. In the first step the wax pattern is dipped in a slurry composed of fine ceramic particles in a binder, for example particulate zircon and colloidal silica. The second step is to apply coarser dry ceramic particles (termed stucco) to the wet slurry coat. After a drying step, these steps are repeated until the desired thickness is attained. Typical shell thicknesses are of the order of 2.94 cm.

Prior to casting, the shell moulds are heated to remove the wax pattern and are usually also fired at a high temperature to promote bonding.

Despite the general acceptance of this process in the high precision casting industry, problems still exist. Frequently it is desired to produce a cast part which needs no further production steps, one which is suited in size and surface finish for its end use. Despite the general strength of a ceramic shell mould, creep and distortion during casting is a problem and these phenomena make accurate dimensional control difficult. Likewise, there is always some reaction between the metal and the mould, and this is reflected in an undesirably rough casting surface or in important alloying elements being depleted from the surface due to interaction with the mould material.

The problems of dimensional control and surface finish are aggravated in the casting of the complex nickel and cobalt base superalloys, many of which contain reactive elements such as Hf, Ti, Zr, Al and Ta.

Directional solidification is a technique used to produce an oriented cast grain structure in articles, and is described in our U.S. Patent 3,260,505. The solidification process is controlled and the liquid-solid interface is constrained to move along a particular axis, and the long axes of the elongated grains are found to be generally parallel to this axis. Solidification rates must be slow if the desired grain structure is to be obtained and the total solidification time for a superalloy article such as a gas turbine blade may be several hours. This extended high temperature exposure aggravates both the dimensional and surface finish problems.

No references which disclose modification of the silica bonding agent to overcome these problems are known. U.S. Patent 2,883,723 discloses the addition of a wide variety of chemical compounds to improve the shakeout properties of silicate bonded cores which are hardened with carbon dioxide.

According to this invention there is provided a high strength creep resistant shell mould which is substantially non-reactive with molten superalloys, the mould being formed by: a) providing a meltable pattern, b) dipping the pattern in a slurry of refractory material and binder, c) applying a stucco of dry ceramic particles to the wet slurry coated surface, d) drying the stuccoed slurry, e) repeating steps 6,c, and d a plurality of times, until a required mould thickness is obtained, and f) heating the pattern/mould assembly to remove the meltable pattern, wherein the slurry comprises:-(i) from 15 to 30 weight per cent of a binder based on an aqueous mix which contains colloidal silica; (ii) an amount of CaO equivalent to that derived from 0.26 to 1.4 weight per cent of CaCO3; and iii) the balance alumina or zircon, and g) firing the mould at a temperature at which the CaC03 decomposes to form CaO, which is then available to react with the silica in the binder.

It is preferred that the calcium carbonate be added in amounts of from 5.8 to 15.45 weight percent of the colloidal silica present in the binder. After decomposition of the calcium carbonate by heating, the binder will contain from 3 to 8 weight percent of CaO.

The invention will now be described by way of example, with reference to the drawing, in which: Figure 1 shows a superalloy part cast in a calcia modified alumina shell mould made according to the present invention; Figure 2 shows a superalloy part cast in a shell mould having a calcia modified alumina face coat and a zircon backing; and Figure 3 shows a superalloy part cast in a prior art shell mould having a calcia-free alumina face coat and a calcia-free zircon backing.

The invention involves the discovery that if from 3 to 8 and preferably 5 weight per cent of the colloidal silica binder is replaced by CaO (calcium oxide or calcia), an improved mould material results. The mould materials of the invention have been developed and tested for the investment casting of the superalloys, and are especially suited for production of directionally solidified superalloy articles.

The binder conventionally used to produce shell moulds for superalloys is based on an aqueous mix which contains 30 weight per cent colloidal silica, although other colloidal silica concentrations may be used. The colloidal silica remains in suspension and thoroughly mixed as a result of interparticle electric charges which prevent the particles from settling out. Consequently all shell mould slurry compositions must be carefully formulated to avoid disrupting the colloidal silica particle equilibria. Such disruption can occur if the pH of the colloidal silica mix is changed.

Although the mechanism by which the present invention works is not completely understood, it is believed to involve the formation of a high viscosity glass-like phase during the firing/bonding step before casting which provides a smooth nonreactive surface and which also promotes interparticle bonding by liquid phase sintering for improved strength.

Ceramic phase diagrams indicated that such a glass phase might form if calcium oxide were added to the silica binding agent. However, calcium oxide ionizes in water and this ionization would disrupt the colloidal silica, causing the silica to gell and harden. It was then determined that a calcium compound which did not ionize significantly in water and which transformed to calcium oxide during the firing step, might be employed.

The best calcium compound appeared to be calcium carbonate (CaC03). Upon heating at elevated temperatures the calcium carbonate reacts as shown below.

CaC03eCaO + CO2 CO2 is a gas which leaves the permeable mould and causes no problems in the casting process.

Thus the shell mould binder includes colloidal silica and CaC03 which will transform to CaO upon heating. The CaC03 is preferably present in an amount which will form an amount of CaO equal to 3 to 8 weight percent of the silica binder. Approximately 1.93 parts by weight of CaC03 will yield 1 part by weight of CaO, and from 5.8 to 15.45 weight percent CaC03 will decompose to 3 to 8 weight percent CaO. The latter is preferably of high purity, containing less than about 50 parts per million of alkali metal impurities, since such impurities tend to cause the formation of undesirable glassy phases.

This modified silica binder will work with several particulate refractory materials including zircon and alumina, but alumina is the preferred ceramic aggregate. It is believed that where alumina is utilized as the refractory aggregate, a Ca0-Si02-A1203 glass-like phase occurs. Where alumina is used as the binder slurry aggregate, up to about 30% of alumina-silica compounds such as kyanite and mullite may also be present. It has been experimentally determined that the SiO2 +Ca0 binder in combination with an alumina refractory yields a mould stronger than one formed, for example using a zircon refractory material, in strength and creep resistance. However, very often the increase in strength obtained with the zircon system may be sufficient; if improved surface finish is the major objective, only the silica binder in the first slurry coat need be modified with CaO.

Figures 1, 2 and 3 are macrophotographs of nickel base superalloy blades. The blades are all of the same alloy, which contained 10% Cr, 15% Co, 4.5% Ta, 5.5% Al, 3.0% Mo, .17% C, 1% V, 2% Hf, balance mainly nickel. This alloy Tablet Slurry Components Weight Percent Stuccos Coats 1 2,3,4 Etc. 1 2 3,4 Etc.

24.44% Ludox HS 30* 25.74% Ludox HS 30* Fused Alumina Tubular Alumina Tubular Alumina 150-2l2um 322-644um 644-1400 m 0.66% CaC03 0.73% CaC03 52.73% -45 m 29.41%-45 m Tubular Alumina Tubular Alumina 5.54% -39m 3.68% -150pm Fused Alumina Tubular Alumina 6.34%-150 m 18.38%-250 m Tubular Alumina Tubular Alumina 2.38% -150 m 7.35% -644pm Kyanite Tabular Alumina 7.92% A-2 Unground 2.21% -150pm Alumina Kyanite 12.50% - 322 m Mullite *Ludox is a trademark of the DuPont Corporation.

Ludox HS 30 is a 30 weight percent aqueous suspension of colloidal silica.

Table II Slurry Components (Weight percent) Stuccos Coat 1 2 3,4,5 etc. 1 2 3,4,5 etc.

24.44% Ludox HS30 2i% Ludox HS30 22.2% Ludox HS 30 75-180 m 250-600um 25-150 m Zircon Sand Calcined Zircon 0.66% CaC03 23.5%-106 M 77.8% China Clay Aggregate Zircon Zircon Aggregate 52.73%- m 53.2%-45 m 25-150 m Alumina Zircon 5.54%-38 m 2.3% Cobalt Fused Alumina Aluminate 6.34%-150 m Alumina 2.38%-150 m Kyanite 7.92% Unground Alumina Table III Slurry Parameters For Calcium Oxide Modified Alumina Shell Slurry Viscosity and Specific Gravity Ranges:: Slurry Viscosity Specific Gravity (&num;4 Zahn cup) 1 (Prime) 12-20 Seconds 2.4 - 2.6 2,3,4 etc. 8-12 Seconds 2.35-2.5 (Backups) Table IV Slurry Composition Limits Weight Percent Colloidal silica 15-30 (30% aqueous mix) Clacium Carbonate* .26-1.4 -150pm Ceramic balance Aggregate, Alumina or Zircon Wetting Agent 0-1% *containing less than about 50 ppm of alkali metal impurities.

was cast at a temperature of 1538"C into shell moulds which had been preheated to 14820C, and directionally solidified. Solidification was complete in approximately 1 hour.

The blade shown in Figure 1 was cast in a CaO modified alumina shell according to the preferred embodiment of this invention. The details of the mould preparation and composition are given in Table I. The blade shown in Figure 2 was cast in a shell mould which had a CaO modified alumina first coat (prime coat) followed with silica bonded zircon based slurry coats and zircon stuccos. The details of this shell are given in Table II. The blade shown in Figure 3 was made in a mould which had a non-Ca0 modified alumina base coat and a zircon backing, the mould details would be identical to those set forth in Table II except that no calcium carbonate was added to the first slurry coat. These moulds were fired at 9820C prior to casting to convert the CaC03 (where present) to CaO.

Comparing Figure 2 and Figure 3 it can be seen that a small addition of CaO to the first shell coat makes a dramatic difference in the amount of mould-metal reaction which occurs. The blade in Figure 3 shows a significant amount of mould-metal reaction product on its surface, but the blade in Figure 2 shows virtually none.

Careful measurement of the blades in Figure 1 and Figure 2 showed that the Figure 1 blade had undergone significantly less distortion than the Figure 2 blade and this indicates that the CaO modified alumina shell is stronger, at casting temperatures, than the zircon shell.

The preferred physical parameters of the slurries of the present invention are listed in Table III and the broad composition limits of the slurry coats are given in Table IV.

WHAT WE CLAIM IS: 1. A high strength creep resistant shell mould which is substantially non-reactive with molten superalloys, the mould being formed by: a) providing a meltable pattern b) dipping the pattern in a slurry of refrac tory material and binder, c) applying a stucco of dry ceramic particles to the wet slurry coated surface, d) drying the stuccoed slurry, e) repeating steps b,c, and d a plurality of times, until a required mould thickness is obtained, and f) heating the pattern/mould assembly to remove the meltable pattern wherein the slurry comprises:-(i) from 15 to 30 weight per cent of a binder based on an aqueous mix which contains colloidal silica; (ii) an amount of CaO equivalent to that derived from 0.26 to 1.4 weight per cent of CaCO; and iii) the balance alumina or zircon, and g) firing the mould at a temperature at which the CaC03 decomposes to form CaO, which is then available to react with the silica in the binder.

2. A shell mould according to claim 1 wherein the aqueous mix contains 30 weight per cent of colloidal silica.

3. A shell mould according to claim 1 or claim 2 wherein the slurry comprises up to 0.2 weight per cent of a wetting agent.

4. A shell mould according to any preceding claim wherein the alumina includes up to 30 weight per cent of alumina-silica compounds.

5. A shell mould according to any preceding claim, wherein the stucco comprises substantially alumina.

6. A shell mould according to any of claims 1 to 4, wherein the stucco comprises substantially zircon.

7. A method for improving the surface finish of investment cast superalloy and ferrous articles, acast in ceramic shell mould according to any preceding claim, which incorporate an SiO2 binding agent, which are produced by placing multiple ceramic slurry coats and stuccos on a meltable pattern and which are fired at an elevated temperature prior to use, characterized by comprising: adding CaC03 to the silica-based binding agent in at least the first ceramic coat the CaC03 transforming to CaO upon exposure to the mould firing temperature, in an amount sufficient to produce a CaO:SiO2 ratio of substantially 1:20

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Table III Slurry Parameters For Calcium Oxide Modified Alumina Shell Slurry Viscosity and Specific Gravity Ranges: Slurry Viscosity Specific Gravity (&num;4 Zahn cup) 1 (Prime) 12-20 Seconds 2.4 - 2.6 2,3,4 etc. 8-12 Seconds 2.35-2.5 (Backups) Table IV Slurry Composition Limits Weight Percent Colloidal silica 15-30 (30% aqueous mix) Clacium Carbonate* .26-1.4 -150pm Ceramic balance Aggregate, Alumina or Zircon Wetting Agent 0-1% *containing less than about 50 ppm of alkali metal impurities. was cast at a temperature of 1538"C into shell moulds which had been preheated to 14820C, and directionally solidified. Solidification was complete in approximately 1 hour. The blade shown in Figure 1 was cast in a CaO modified alumina shell according to the preferred embodiment of this invention. The details of the mould preparation and composition are given in Table I. The blade shown in Figure 2 was cast in a shell mould which had a CaO modified alumina first coat (prime coat) followed with silica bonded zircon based slurry coats and zircon stuccos. The details of this shell are given in Table II. The blade shown in Figure 3 was made in a mould which had a non-Ca0 modified alumina base coat and a zircon backing, the mould details would be identical to those set forth in Table II except that no calcium carbonate was added to the first slurry coat. These moulds were fired at 9820C prior to casting to convert the CaC03 (where present) to CaO. Comparing Figure 2 and Figure 3 it can be seen that a small addition of CaO to the first shell coat makes a dramatic difference in the amount of mould-metal reaction which occurs. The blade in Figure 3 shows a significant amount of mould-metal reaction product on its surface, but the blade in Figure 2 shows virtually none. Careful measurement of the blades in Figure

1 and Figure 2 showed that the Figure 1 blade had undergone significantly less distortion than the Figure 2 blade and this indicates that the CaO modified alumina shell is stronger, at casting temperatures, than the zircon shell.

upon firing.

8. A method according to claim 7, wherein the binder also contains a refractory aggregate based on zircon.

9. A high strength creep-resistant shell mould according to claim 1, which is substantially non-reactive with molten superalloys as herein described with reference to Figures 1 and 2 of the drawing.

10. A method for improving the surface finish or investment cast superalloy articles as herein described with reference to Figures 1 and 2 of the drawing.