This invention relates to the removal of refractory material from components. The invention finds particular application in removing refractory cores from cast components such as blades for use in gas turbine engines, the cores defining, for example, openings such as cavities or passages required for cooling purposes.

Typically in the casting of such blades, a core defining the cooling passages is inserted into a mould, molten blade material is introduced into the mould, the blade is solidified and the core is removed from within the blade.

Fused silica is most commonly used as the core material because of its good chemical removability.

However, considerable problems occur with this material due to bowing and distortion- of the core, which problems are due to the relatively poor refractory properties of the material. In particular, directional solidification techniques (which are necessary or desirable in many applications to produce high strength, long life blades) may impose excessively severe conditions for fused silica to be used as the core material. Hence, in such applications the use of fused silica as the core material precludes the use of directional solidification techniques and results in blades being relatively weak and having a relatively short life.

It has long been recognised that other materials might be used as core materials and considerable effort has been expended in looking for materials, other than fused silica, of high strength and high refractoriness which can be easily removed. High temperature fired, recrystallised alumina has the required properties of high strength and high refractoriness but, until this invention, such alumina has been considered generally unsuitable as a core material because of the difficulty of removing the material at practically useful rates. Indeed much effort has gone into devising structural forms of alumina which present an increased surface area to a dissolving agent and so dissolve more quickly. An example of such a structural form of fired alumina is disclosed in U.S. Patent No. Ij. _f l81j. _t 885.

The inventors have made the surprising discovery that high-temperature fired, re-crystallised alumina can, in fact, be readily removed from components at a practically useful rate. -The method of the invention has also been found to be applicable to removing other refractory material such as magnesia, steatite and spinel, which were previously thought to be generally unsuitable as core materials because of the dif iculties of removing the materials at practically useful rates. It is believed that the method of the invention may also be applicable to the removing of other refractory materials which were previously considered unsuitable as blade core materials and which have not-yet been tried in the present invention.

SUBSTITUTE SHEET

According to a first aspect of the invention a method of removing refractory material from a component comprises contacting the material with a reduced concentration aqueous solution of dissolving agent at an elevated temperature and an elevated pressure.

The present invention is thus distinguished from previous attempts to remove refractory oxide materials such as alumina from components since whereas these previous attempts have sought to dissolve the material directly, the present invention first reacts a chemically reactive agent with the refractory material to convert it to a substance which is more easily removable and then removes this substance.

According to a second aspect of the invention a method of casting a component having an opening therein comprises the steps of:

inserting into a mould refractory material defining the opening;

introducing into the mould molten component material;

solidifying the component and dissolving the refractory material by a method according to the first aspect of the invention.

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One method of casting blades having internally cooling passages for use in a gas turbine engine will now be described, by way of example only.

Into a blade mould of known type is inserted a core of pure substantially 100% dense recrystallised alumina. The alumina is of tubular, preferably extruded, form and is shaped to de ine the cooling passages required in the blade to be case in the mould. In its simplest form the core may comprise one or more straight tubular strips of alumina, but the exact arrangement and shape will depend on the particular cooling requirements of the blade to be cast.

Molten blade material of the desired type, e.g. an alloy sold by INCO Ltd., ^' under the trade name IN100, is then introduced into the mould. The blade is then allowed to solidify. In order to avoid imperfections in the structure of the blade and so to improve the strength of the blade, the solidi ication of the blade may be directionally controlled. Such directional solidification techniques are well known in the art and will not be further described herein.

When the solidification is complete, the cast blade is removed from the mould and the alumina core is removed from within the blade by immersing the blade containing the core in an aqueous solution made up of potassium hydroxide (approximately 90% /V) and water (approximately 10% V/V) at a temperature of approximately 350 C and at atmospheric pressure.

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With this method it has been found possible to remove from blades of IN100 material, in approximately seventeen hours, tubed cores of pure, substantially 100% dense recrystallised alumina of some seven inches long, with external diameter approximately 0.08 inches and internal diameter approximately O.Ol}. inches.

In the method, it is thought that nascent hydrogen is generated, at the temperature used, from the hydrogen provided in the solution by the water. This nascent hydrogen is highly reactive and is thought to react with the largely inert alumina to reduce it to alumin¬ ium hydroxide. The aluminium hydroxide then dissolves in the potassium hydroxide in the solution. It has been found that the temperature used in the method is not critical, decreased temperature resulting in slower alumina removal and increased temperature resulting quicker alumina removal, but that if too great a temper¬ ature is used considerable chemical attack of the blade by the nascent hydrogen can occur.

Other methods of removing similar size tubular cores of recrystallised alumina from blades cast in IN100 material which also proved successful are described hereafter.

Alumina cored blades of IN100 material immersed in a solution of sodium hydroxide (approximately 80% W/V) and water (approximately 20% W/V) at a temperature of approximately 220°C and at atmospheric pressure were successfully de-cored in approximately 20 hours.

Alumina cored blades of IN100 material immersed in a solution of potassium hydroxide (approximately 65%W/V)

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and water (approximately 35% W/V) at a temperature of approximately 200 C and at atmospheric pressure were successfully de-cored in approximately 20 hours.

Alumina cored blades of IN100 material immersed in a solution of potassium hydroxide (approximately 65%W/V) and water (approximately 35% W/V) at a temperature of approximately 370°C and at a pressure of approximately 3 atmospheres were successfully de-cored in approximately 17 hours. It will be appreciated that the increased pressure allows an increased temperature to be used with¬ out the solution boiling away. Such a method of re¬ moving alumina cores is conveniently carried out in an autoclave.

Alumina cored blades of IN100 material immersed in a solution of potassium hydroxide (approximately 60%W/V) and water (approximately 1^0 W/V) at a temperature of approximately 350°C and at a pressure of approximately 100 atmospheres were successfully de-cored in approxi¬ mately 20 hours. Such a method of removing alumina cores is conveniently carried out in an autoclave.

Alumina cored blades of IN100 material immersed in a solution of sodium hydroxide (approximately 2050 W/V) and water (approximately 80% W/V) at a pressure of appro¬ ximately 5 atmospheres and at a temperature repeatedly increased from approximately 150°C to 157°C , i.e. from just below to just above the boiling point of the solu- tion, to boil the solution repeatedly were successfully de-cored in approximately 20 hours. It will be under¬ stood that in this method physical activity in the solution due to

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boiling improves removal of the core while maintaining substantially the same composition of the solution. Such a method of removing alumina cores is also conveniently carried out in an autoclave.

Alumina cored blades of IN100 material immersed in a solution of potassium hydroxide (approximately 65% W/V lithium hydroxide (approximately 15% W/V) and water (approximately 20% W/V) at a temperature of approximatel 350 C and at atmospheric pressure were successfully de-cored in approximately 17 hours.

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Similar favourable results have been obtained, using the above methods of removal, in removing from blades of IN100 material cores of high-temperature fired magnesia, steatite, spinel and unillite, these materials having previously been considered unsuitable as blade core materials.

It is believed that the invention may also be applicable to the removal from components of other refractory materials which were previously considered unsuitable as blade core materials and which have not yet been tried in the present invention.

It will be appreciated that although in the above- described examples of methods of removing recrystallised alumina cores from blades nascent hydrogen is believed to be generated from water, other hydrogen containing compounds may alternatively be used, e.g. sodium hydride or potassium hydride.

It will also be appreciated that the inventive principle demonstrated in the above examples of chemically converting the refractory material to a more easily removable substance and subsequently removing this substance may alternatively be employed utilising other chemically reactive converting agents and methods of removal.