GB2032310A - Composite casting - Google Patents

Abstract

A method of making a composite casting such as a gas turbine engine blade comprises providing a mould having at least part of an interior surface made of the ceramic material, casting metal into the mould and removing the mould from the casting so as to leave the ceramic material in situ on the casting. The mould can be made using a lost wax process, the ceramic material which becomes part of the casting being applied in the form of a slurry as the first layers of the investment.

Description

SPECIFICATION Method of manufacturing a cast metallic object This invention relates to a method of manufacturing a cast metallic object.

There has recently been considerably interest in the application of ceramic coatings to metallic work-pieces. These ceramic coatings provide a thermal barrier which reduces the rate of heat transfer from hot ambient conditions to the work-piece and may therefore be of considerable value in situations where work-pieces have to withstand very hot conditions. Atypical example of such an environment is in the turbine and combustion chamber area of a gas turbine engine.

Previously it has been the practice to apply ceramic coatings by plasma spraying or other simi lartechniques. It has been difficult and expensive to effect coating in this way so that the coating will be resistant to all the conditions of thermal shock etc.

which may be experienced by the work-piece. In an effort two provide a different method of coating which may have advantages in cheapness, adherence of a coating and resistance to thermal shockthe method of the present invention has been devised.

According to the present invention a method of making a cast metallic object at least part of a surface of which is coated with a ceramic material comprises the steps of making a mould in which the object is to be cast, the mould having at least part of an interior surface made of said ceramic material, casing metal into the mould to produce the object, and removing said mould from the cast object so as to leave said ceramic material in situ on the object.

In one embodiment the mould is made by coating the-ceramic material on a duplicate of the object made in a disposable material, investing the coated pattern to produce a shell mould, and removing the disposable material from the shell mould.

In a second embodiment the ceramic coating is pre-formed and the mould is made so as to incorporate the pre-formed coating. Thus the pre-formed coating may be held in a die into which is injected the disposable material to produce a pattern of the object, the pattern being used to produce a shell mould.

As an alternative to this latter procedure the pre-formed ceramic coating may be used to form part of the ceramic core used to define an internal casting or coating in the cast object.

In any case, it may be advantageous to provide an intermediate weak or release coating between the ceramic material and the remainder of the mould so that the mould may be easily removed from the object and its ceramic coating.

It may be desirable to form the ceramic coating from a material which is insoluble in a reagent which will dissolve the material of the remainder of the mould.

Suitable materials for the ceramic coating include zirconia, alumina, magnesia and silicone nitride.

The invention will now be particularly described merely by way of example with reference to the accompanying drawings in which: Figures 1 to 4 illustrate diagrammatically subsequent stages in the carrying out of a method in accordance with the invention, and Figures 5 and 6 illustrate a modified method in accordance with the invention.

In Figure 1 there is shown a two-part die comprising the parts 10 and 11 which together define a die cavity 12. The cavity 12 is arranged to be of the same shape as that of the desired final object; in this particular instance the cavity has the shape of a blade blank for a gas turbine engine. A sprue passage 13 is provided through which a disposable material in fluid form may be injected to fill the cavity 12. Suitable materials will be well known to those skilled in the art and one common material would be a hard wax. This wax would be injected while melted and allowed to solidify in the cavity 12.

When the material has hardened the die halves 10 and 11 separated to leave a wax pattern 14 which is shown in Figure 2. The wax pattern is then provided with a coating 15 of the ceramic material which is required to form the final coating on the metallic object which is being cast. In the present instance the coating 15 comprises zirconia stabilised with a small portion of yttria. The coating is applied in the form of a slurry; thus the wax pattern 14 may be dipped in the slurry and allowed to dry, the process being repeated as many times as is necessary to build up the required thickness of coating. It should be appreciated that this coating material is exactly the same as has been conventionally applied by flame or plasma spraying, however, it may be desirable to add other constituents which will improve the eventual bond between the ceramic and the cast metal.

When a sufficient thickness of the ceramic material 15 has been built up a release coating 16 is applied to the coating pattern. The release coating comprises a different ceramic material and it may either be applied in such a way as to be mechanically weak or it may be of a chemical composition which may easily be leached out. In the present instance using a zirconia based ceramic coating the release coating and subsequent coatings will be silica based so that they may be easily removed using a caustic alkaline solution.

The release coating 16 is applied by an exactly similar method to that described in relation to the ceramic coating 15, however, in order to avoid the overall shell being too weak the coating 16 is preferably made very thin.

After the coating 16 has dried the coated pattern is then invested in the usual way to produce a shell mould. As is well known to those skilled in the art this investment process may involve repeated dipping in ceramic slurries followed by stuccoing with various granular materials. In this way a relatively rigid shell 17 is produced containing the pattern 14.

The shell is then fired which removes the wax 14 and hardens the composite shell to leave it in a state suitable for receiving molten metal. Metal is melted and poured into the shell and is allowed to solidify.

Because the molten metal is in intimate contact with the layer of ceramic 15 it forms at least a good mechanical bond with this coating, and if any chemically reactive materials have been added to the coating a chemical reaction may be arranged to take place which provides even more firm adherence of the coating to the metal. When the metal has solidified it is then necessary to remove the shell mould 17 and the release coating 16 without damaging the coating 15.

There are various ways in which this might be done using mechanical or chemical processes. In the present instance the cast metal complete with the shell mould is immersed in a hot solution of caustic alkali which attacks the material of the shell mould and the release coat without attacking the coating 15. The presence of the relatively weak release coating is intended to allow a relatively clean separation of the shell mould material from the coating. In this way the metal object is produced with its ceramic coating comprising an integral whole and the resistance of the coating to external conditions is likely to be enhanced.

Figures 5 and 6 illustrate a modified method in accordance with the invention. In this case a twopiece die made of the separable pieces 20 and 21 defines a cavity 22 having the shape of the required ceramic coating on the object to be made. Into this cavity the ceramic material is injected in fluid form; thus it may for instance be injected in the form of a water based slurry, with a water-soluble resin added to cause the injected mass to harden off.

Once injected, the ceramic is allowed or caused to harden and the die pieces are separated so that the ceramic part 23 may be removed. This part may be hard enough to handle in this condition or it may require firing to harden it off completely. The part is then assembled into a die exactly similar to that of Figure 1 and comprising pieces 10 and 11. It will be seen that the part 23 is arranged to fit exactly in the die cavity 12 and that it has the same shape as would a partial coating of the pattern to be formed in the die.

Disposable material is next injected into the cavity 12 and the manufacturing process proceeds as described with reference to the previous method, except that since the ceramic part or coating is already present it is not necessary to coat the disposable piece as set out in relation to Figure 2.

However, it may still be necessary to provide the release coating as described with reference to Figure 3.

It should be noted that using methods analogous to either of the above techniques it will be possible to provide ceramic coatings or similar parts on internal surfaces of a metallic object. Thus these internal surfaces are generally formed by the use of a ceramic core around which the casting is made, and which is subsequently leached from the casting.

It would of course be possible to provide the ceramic core with a coating of a different type of ceramic which remains in situ inside the final casting to provide the coating.

Alternatively a ceramic piece could be made in a similar manner to that of Figure 6; this piece would then have further and different ceramic injected into or around it to provide the core.

Of course, the same considerations regarding the internal ceramic coating material not being leached out or removed with the core apply as in the case of the external coating and the ceramic mould. Again a release coat may be necessary between the ceramic coating material and the core itself.

It should be understood that there are a number of alternatives available for the material used for the coating, for the release material and for the material of the shell mould although it is likely that all of these will basically comprise ceramics, also there is no restriction on the metal used to form the object although it is clear that the provision of these ceramic coatings is only likely to be necessary where very high temperatures are involved and therefore the metal is likely to have to be resistant to high temperatures. Therefore the metal will typically comprise a nickel or cobalt based superalloy.

It should also be appreciated that this technique could be equally well used to coat portions of an object rather than the complete object described above.

Claims (6)

1. A method of making a cast metallic object at least a part of a surface of which is coated with a ceramic material comprises the steps of making a mould in which the object is to be cast, the mould having at least part of an interior surface made of said ceramic material, casting metal into the mould to produce the object, and removing said mould from the cast object so as to leave said ceramic material in situ on the object.

2. A method as claimed in claim 1 in which the mould is made by coating the ceramic material on a duplicate of the object made in a disposable material, investing the coated pattern to produce a shell mould, and removing the disposable material from the shell mould.

3. A method as claimed in claim 1 in which the ceramic coating is pre-formed and is incorporated into the mould.

4. A method as claimed in claim 3 in which the pre-formed coating is held in a die and disposable material is injected into the die to produce a pattern of the object, the pattern then being used to produce the shell mould.

5. A method as claimed in any of the preceding claims in which the ceramic material is selected from the group comprising zirconia, alumina, magnesia and silicon nitride.

6. A method of making a cast metallic object substantially as herein described and with reference to the accompanying drawing.