GB2538268A

GB2538268A - Shell mould production

Info

Publication number: GB2538268A
Application number: GB1508133.4A
Authority: GB
Inventors: A Withey Paul; Blackburn Stuart
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2015-05-13
Filing date: 2015-05-13
Publication date: 2016-11-16
Also published as: GB201508133D0

Abstract

A method of producing a shell mould from a stucco coating with a fast gellation and drying step is provided. The method includes: dipping a preformed expendable pattern into an aqueous-based slurry of refractory particles, forming a slurry layer on the pattern; depositing refractory particles on the slurry layer; and drying the slurry layer. The dipping, depositing and drying steps are repeated as needed to build up a stucco coating on the pattern from the slurry layer(s) and deposited refractory particles. At least 30% by mass of the deposited refractory particles are particles of a first component which is highly water retaining gamma and/or eta alumina, the second component can be alpha alumina. The alumina layer can be applied to the dry slurry layer using a rainfall sander or fluidised bed and the expendable pattern can be removed and the shell mould optionally fired which can form alpha alumina.

Description

SHELL MOULD PRODUCTION

Field of the Invention

The present invention relates to an improved method for producing a shell mould, e.g. for use in investment casting.

Background of the Invention

Investment casting involves the production of metal castings using a ceramic shell mould, which is in turn produced using an expendable pattern of the component to be cast.

More particularly, the ceramic shell is formed by dipping the pattern into a slurry of refractory particles to form a slurry layer on the pattern, depositing refractory particles on the slurry layer, and drying the slurry coat. The dipping, depositing and drying steps are repeated as needed to build up a stucco coating of desired thickness. It is common to use alpha alumina (aluminium oxide) as the deposited stucco material. This is due to its abundance, suitability as a refractory material and relatively low cost.

The industrial use of investment casting places great demands and requirements on the ceramic shell. This shell has to coat the pattern, harden or cure, allow the pattern to be removed, and then contain the solidifying metal during casting. The slurry of refractory particles used to form the shell typically consists of ceramic particles and a liquid binder. In the early 1990s the industry moved to aqueous-based ceramic slurries away from alcohol-based slurries to meet environmental targets. This changed the ammonia driven chemical set of the liquid slurries to an evaporation driven one. The drying of aqueous-based shells is now a key process step with inter-coat drying times typically of at least 2 hours and final drying typically taking at least 16 hours. As it is desirable to keep the pattern at the same temperature throughout the process, heating the shell to accelerate drying is generally not a viable option. Many different approaches have been taken to reduce drying time, including large fans, low power infra-red lamps, reduced humidity, and inclusion of water-absorbing polymer with the refractory particles. A difficulty with this latter approach, however, is that the polymer addition is difficult to control in a production environment. In particular, the equipment used to deposit refractory particles, such as a fluidised bed or a rainfall sander, is an abrasive environment which tends to wear off polymer coatings. Alternatively, if the polymer is mixed with the refractory particles in the deposition equipment, density differences between polymer and refractory particles can lead to segregation and a variable product.

Thus it would be desirable to provide an improved method for forming a shell mould. Summary of the Invention According to a first aspect of the invention there is provided a method of producing a shell mould from a stucco coating including: dipping a preformed expendable pattern into an aqueous-based slurry of refractory particles to form a slurry layer on the pattern; depositing refractory particles on the slurry layer; drying the slurry layer; and repeating the dipping, depositing and drying as needed to build up a stucco coating on the pattern from the slurry layer(s) and deposited refractory particles; wherein at least 30% (and preferably at least 50%) by mass of the deposited refractory particles are particles of a first component which is gamma and/or eta alumina.

Alpha alumina is conventionally used to form shell moulds, but typically has a relatively low absorptivity for absorption of water. In contrast, gamma and eta alumina typically have relatively high absorptivity for absorption of water. As a higher absorptivity allows more water to be absorbed into a given particle, advantageously, by introducing gamma and/or eta alumina into the deposited stucco material, more water can be absorbed by the refractory particles, helping to increase the speed at which the slurry is prepared.

The first component (gamma and/or eta alumina) particles may have an absorptivity of at least 0.6 g/g for absorption of water, and preferably at least 0.7 or at least 0.8 g/g.

100% of the deposited refractory particles can be the first component. However, another option is to include a fraction of, e.g. cheaper, second component particles. In particular, the deposited refractory particles can be a mixture of first component particles and second component particles. For example, the deposited refractory particles may include particles of a second component which is alpha alumina. An advantage of using a mixture of alpha alumina particles and gamma and/or eta alumina particles is that these alumina phases generally have quite similar densities, and thus there is little tendency for the particles to separate out during the deposition stage, which is typically performed using a rainfall sander or a fluidised bed.

However, the use of a first component having a high absorptivity for absorption of water is not limited to alumina systems. Thus, according to a second aspect of the invention there is provided a method of producing a shell mould from a stucco coating including: dipping a preformed expendable pattern into an aqueous-based slurry of refractory particles to form a slurry layer on the pattern; depositing refractory particles on the slurry layer; drying the slurry layer; and repeating the dipping, depositing and drying as needed to build up a stucco coating on the pattern from the slurry layer(s) and deposited refractory particles; wherein the deposited refractory particles are a mixture of first component particles and second component particles, the first component particles having an absorptivity of at least 0.6 g/g for absorption of water (and preferably at least 0.7 or at least 0.8 g/g), and the second component particles having an absorptivity of at most 0.3 g/g for absorption of water.

The first component particles may form at least 30% or at least 50% by mass of the mixture.

According to a third aspect of the invention there is provided a process for casting a component including: performing the method of the first or second aspect to produce a shell mould, the pattern being a pattern for the component; and using the shell mould to investment cast the component.

Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention.

The method may further include removing the expendable pattern from the dried stucco coating. The expendable pattern may be formed from wax, and in this case the wax pattern may conveniently be removed from the dried stucco coating by heating.

The method may further include firing the dried stucco coating after the expendable pattern has been removed. The firing can have an effect of converting the first component into the second component, e.g. gamma and/or eta alumina into alpha alumina.

Absorptivity values for absorption of water by a given type of particle can be determined by weighing a suitable moulded sample of the particles, placing the sample in water, waiting for e.g. two minutes at which point bubbles stop being produced, removing the sample from the water, removing surface water from the sample, and then re-weighing the sample.

Detailed Description and Further Optional Features of the Invention In an investment casting process, a wax pattern of an article, or component, is produced, e.g. by injecting wax into an accurately formed die. The wax pattern is a replica of the article to be produced. Usually a number of wax patterns are assembled together on a wax gating tree to form a cluster or wax mould assembly. The wax mould assembly is immersed in aqueous-based ceramic slurry. Strengthening refractory particles are deposited on the ceramic slurry covered wax mould assembly and the refractory particles bond to the slurry coating to produce a ceramic layer on the wax mould assembly. The slurry is dried and the process generally repeated several times to produce a stucco coating from many ceramic layers which can have a total thickness of about 1/4 inch (6 mm) to 1/2 inch (12 mm) on the wax mould assembly. The wax is then melted out leaving a ceramic shell mould having an internal cavity identical in shape to that of the original wax mould assembly. This ceramic shell mould is also called an investment casting mould. The mould is fired at a high temperature to remove all traces of residual wax, and cure the ceramic shell mould. The ceramic shell mould is then transferred to a casting furnace, which may be operated at either vacuum conditions or at atmospheric conditions. A charge of molten metal is poured into the ceramic shell mould and the mould is allowed to cool to room temperature, after which the ceramic shell mould is removed leaving the cast article or articles. The ceramic shell mould may be cooled by applying a temperature gradient across the ceramic shell mould to directionally solidify the metal in order to produce columnar grains, or single crystals in the finished article or articles.

The refractory particles can be deposited using a rainfall sander or a fluidised bed. The deposited refractory particles may be a mixture of first component particles and second component particles. The first component particles being for example gamma and/or eta alumina particles and the second component particles being for example alpha alumina particles.

Particulate alpha alumina typically has an absorptivity of up to about 0.3 g/g for absorption of water while particulate gamma alumina typically has an absorptivity of at least about 0.6 g/g for absorption of water and particulate eta alumina typically has an a higher absorptivity than particulate gamma alumina. Thus the addition of high absorptivity gamma and/or eta alumina into the stucco material can increase the speed of gellation of the wet slurry coat, thereby reducing drying times and leading to a faster pace through the shell room in investment foundries. However, advantageously, the chemical set process in the shell mould is unaffected. Further, the chemistry of the mould is unaffected and, other than the addition of gamma and/or eta alumina, conventional equipment and processes can be used.

The above absorptivity values were tested for alpha and gamma alumina by peptizing boehmite (Pural SBTM) with nitric acid and then destabilising with ammonia to produce a thick paste. The paste was then put into sample moulds of approximately 50mm x 25mm x 10mm and left to dry. When dry, the samples were taken out of the moulds and exposed to heating schedules of 4°C per minute up to either 500°C or 700°C (to produce gamma phase samples) or 1525°C (to produce alpha phase samples) with dwells of 2 hours at the given temperature. After cooling the samples were weighed before placing them in water, waiting for 2 minutes at which point bubbles stopped being produced and then removing the samples from the water. The surface water was removed from the samples by lightly shaking the samples until their surfaces were no longer shiny from the surface water. The samples were then reweighed. For gamma alumina fired at 500°C, an absorptivity of 0.85 g/g was obtained. For gamma alumina fired at 700°C, an absorptivity of 0.63 g/g was obtained. For alpha alumina fired at 1525°C, an absorptivity of 0.29 g/g was obtained.

The proportion of gamma and/or eta alumina to alpha alumina can vary, increasing the proportion of gamma and/or eta alumina speeding up drying, but increasing cost. In general, however, the gamma and/or eta alumina is at least 30% by mass of the deposited refractory particles. Indeed, it is possible to eliminate the alpha alumina entirely so that the deposited refractory particles are 100% gamma and/or eta alumina.

The firing of the dried stucco coating after the wax pattern has been removed is preferably performed at a temperature above 1150°C, which converts the gamma and/or eta alumina to alpha alumina. The firing also drives off the absorbed water. As a result, the fired shell can be largely indistinguishable from an alumina shell conventionally produced without the gamma and/or eta alumina particles.

Advantageously, gamma and eta alumina can have a similar density to alpha alumina. This facilitates addition and control, and ultimately improves shell mould performance. For example, the gamma and/or eta alumina only needs to be a weighed and added to the alpha alumina shortly before depositing on the slurry layer. Further, in the rainfall sander or fluidised bed, the gamma and/or eta alumina do not tend to separate out from the gamma alumina.

The approach described above can also be adopted for other ceramic systems. For example, for investment casting shell moulds in which aluminosilicate is the stuccoing refractory particle, gamma and/or eta alumina can be added to the aluminosilicate particles. For shell moulds in which silica is the stuccoing refractory particle, silica gel and/or other porous silica materials can be added to the silica particles.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Claims

CLAIMS1. A method of producing a shell mould from a stucco coating including: dipping a preformed expendable pattern into an aqueous-based slurry of refractory particles to form a slurry layer on the pattern; depositing refractory particles on the slurry layer; drying the slurry layer; and repeating the dipping, depositing and drying as needed to build up a stucco coating on the pattern from the slurry layer(s) and deposited refractory particles; wherein at least 30% by mass of the deposited refractory particles are particles of a first component which is gamma and/or eta alumina.
2. A method according to claim 1, wherein the first component particles have an absorptivity of at least 0.6 g/g for absorption of water.
3. A method according to claim 1 or 2, wherein the deposited refractory particles further include particles of a second component which is alpha alumina.
4. A method of producing a shell mould from a stucco coating including: dipping a preformed expendable pattern into an aqueous-based slurry of refractory particles to form a slurry layer on the pattern; depositing refractory particles on the slurry layer; drying the slurry layer; and repeating the dipping, depositing and drying as needed to build up a stucco coating on the pattern from the slurry layer(s) and deposited refractory particles; wherein the deposited refractory particles are a mixture of first component particles and second component particles, the first component particles having an absorptivity of at least 0.6 g/g for absorption of water, and the second component particles having an absorptivity of at most 0.3 g/g for absorption of water.
5. A method according to claim 4, wherein first component particles form at least 30% by mass of the mixture.
6. A method according to any one of the previous claims further including: removing the expendable pattern from the dried stucco coating.
7. A method according to claim 6 further including: firing the dried stucco coating after the expendable pattern has been removed.
8. A method according to claim 7, wherein the firing converts the first component into the second component.
9. A process for casting a component including: performing the method of any one of the previous claims to produce a shell mould, the pattern being a pattern for the component; and using the shell mould to investment cast the component.