Often, conventional machine parts and the like are made by casting. This basically comprises making a mould which has a cavity conforming to the desired shape of the finished part, filling the cavity with a molten material - e.g. metal - which is then allowed to cool and solidify and removing the mould.

The need to remove the mould from the article after it has been cast limits the complexity of parts which can be made in this way. In some circumstances, investment casting can be used to produce parts with internal cavities but this method also has limitations and more complex parts such as cooled turbine blades can be difficult to produce. Some more complex parts can be produced by fixing sub-components together but this is not ideal as it compromises structural strength and is more time consuming.

Moreover it is acknowledged to be difficult to obtain an accurate reproduction of shape with standard casting techniques and to avoid unintended voids from forming where pockets of gas get trapped during the casting process. In many cases, it is also difficult to control the surface roughness of the finished article.

The present invention aims to provide an improvement over casting for at least some applications and when viewed from a first aspect the invention provides a method of manufacturing an article comprising:

providing a mould blank of a first material having a first melting point, said mould blank having an external shape matching a desired internal shape of said article;

cold-spraying a powdered material having a second melting point onto said mould blank to form a continuous layer over at least part of the mould blank; and heating the mould blank to a temperature between the first and second melting points such that said mould blank melts to leave the article. When viewed from a second aspect the invention provides an article made using the method set out above. Thus it will be seen by those skilled in the art that in accordance with the invention, rather than using a solid mould to shape a molten material, the article is built up on the outside of the mould using cold spraying before the mould is melted.

Cold spraying is an emerging industrial technique in which a powdered material - typically having particle size in the range of 10 to 50 microns - is directed at a target using a high pressure gas jet of gas. The gas accelerates the particles to a very high speed (typically 200 to 1000 metres per second) which causes the particles to plastically deform and bond with the target to form a continuous layer over it. In contrast with thermal spraying which uses heat to melt the coating material so that the material is molten as it travels towards the target, by using a fine powder of material in its solid phase, the material is subjected to much lower temperatures.

Methods in accordance with the invention allows for complex and intricate shapes to be produced since removal of the mould afterwards is quite simple. The use of cold spray means of course that the article can be made from a higher melting point material than the mould but can be formed without melting the mould. Another advantage that the Applicant has appreciated is that the cold spray process leaves the material with a residual compressive stress that is believed to improve fatigue life.

The article could be formed with a single material but this is not essential. For example in a set of embodiments the article is built up in layers of different materials. An important example of this is where an oxidation protective coating or potentially a thermal barrier coating is applied first to the mould blank, followed by a bulk structural material such as metal. This allows an effective coating to be provided on the inside of a complex shape, particularly one which has a re-entrant portion. Traditionally however it was difficult to apply a coating to the inside of a small hollow part since spraying the coating is a line-of-sight process. In accordance with embodiments of the invention however such coatings can easily be applied to the outside of the 'negative' mould blank. Of course coatings other than thermal barrier coatings or oxidation protective coatings can be applied in exactly the same way.

In another set of embodiments different bulk structural layers are applied. This could be advantageous for example to exploit the thermal gradient across the thickness of the wall of a part used in a high temperature application such as a gas turbine engine. The material or materials can be layered so that nearer to the interior, where the temperature is the highest, a higher specification alloy can be used; whereas towards the outer surface of the part, where temperatures are lower, a lower specification (and hence lower cost) alloy can be used.

Of course with cold-spraying it is not necessary to build up discrete layers of different materials. In some embodiments, the different layers comprise a plurality of materials in different proportions. By changing the relative proportions of different powdered materials in the spray mix as the process continues, the material can change gradually throughout its thickness or at least over part of it. This can give a significant advantage over traditional coatings in that it is less prone to thermomechanical fatigue, spallation and delamination with increasing depth of coating than other forms of thermal sprayed coatings.

The material from which the article is made could be anything which is susceptible to cold-spraying. This requires at least one ductile component to allow for the necessary plastic deformation on impact. A non-exhaustive list of examples includes: metals such as aluminium, zinc, copper, nickel, chrome, titanium, silver, tantalum, zirconium, tungsten etc; alloys such as steel, aluminium alloys, nickel alloys, and indeed any alloys including one or more of the above metals; and composites such as copper-tungsten, aluminium-silicon carbide, aluminium- aluminium oxide. The mould blank could be made of any suitable material having a suitable melting point. In a set of embodiments the mould blank comprises a low melting

temperature metal such as zinc, pewter or aluminium. In another set of

embodiments the mould blank comprises a thermo-plastic. The mould blank could be made by any suitable process such as machining, casting, three-dimensional printing etc. dependent on the material being used. Surface finishing could be used if necessary - e.g. to obtain a required surface roughness.

The method could be used to make a wide variety of different articles and the materials used will of course depend on the nature of the article and its intended use. A non-exhaustive list of examples includes: a turbine scroll, a compressor scroll, a pipe (with or without a regular profile), a diffuser, an engine casing, gun barrel etc. The method could be used to make a discrete article but it is also envisaged that it could be used to form indefinite-length pipes by incrementally adding mould segments.

In a set of embodiments the method comprises providing struts in the mould blank that are attached to the external profile by the coating process. In a set of embodiments the method comprises providing struts in the mould blank that are attached to the continuous layer by the cold-spraying process. This could avoid complicated welding processes. Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figs. 1 to 5 are schematic cross sections showing the stages of manufacturing an article in accordance with a method embodying the invention;

Fig. 6 is a perspective view of the article prior to removal of the mould;

Fig. 7 is a perspective view of the finished article; and Figs. 8 and 9 are perspective views of an article made in accordance with a second embodiment of the invention before and after removal of the mould respectively.

Turning first to Fig. 1 there may be seen a cross section mould blank 2. For the purposes of illustration this is shown as a simple circular cylinder but in practice this technique can be used to produce very complex shapes. It might be made of a low- melt metal such as zinc, pewter or aluminium. The mould blank could be cast, moulded, machined or produced in any other way appropriate to the shape and material. The next phase is illustrated in Fig. 2 in which cold-spraying is used to apply a protective coating 4 to the outside of the mould blank 2. In one illustrative example MCrAIY particles in the range of about 10 to 30 microns are projected onto the blank 2 at speeds of about 500 - 1000 metres per second. The impact causes the MCrAIY particles to deform and adhere to the blank 2 as they gradually build up the coating.

Cold-spraying is then used to apply a thicker bulk structural layer 6 as shown in Fig. 3. This might for example be a nickel alloy. After this a further outer coating 8 is applied. This could, for example, be an oxidation protective coating. It could also be cold-sprayed but this is not essential, it could be more conventionally thermally sprayed (where molten droplets of the material are sprayed rather than solid particles) or indeed applied by any other known technique. The resulting article is shown in Figs. 4 and 6. In order to complete the manufacturing process the low-melt mould blank 2 is removed by heating it and allowing it to drain away. Since the melting point of the blank 2 is lower than that of the spray-coated materials 4, 6, etc. these are left unaffected. The finished article 10 is therefore seen in Figs. 5 and 7. Although a simple cylinder is shown for the purposes of illustration it will be appreciated that articles with complex internal structures can be formed since they are built up from the inside out. This technique can therefore be used for example for complex machine parts such as turbine scrolls that are very difficult to internally coat using conventional techniques.

Figs. 8 and 9 show another possible embodiment which is used to produce an article with internal structure. Here the mould blank 2' is not simply a monolithic form but has longitudinally extending cross plates 12 embedded in it. These can be produced by any conventional process and then the mould blank 2' is over-moulded onto them so that the edges of the plates 12 are exposed but flush with the surface of the blank. The cross plates 12 are made of a material having a significantly higher melting point than the low-melt material of the mould blank 2'.

As before the article is built up with a protective coating 4', bulk layer 6' and outer layer 8'. As it is applied the protective coating 4' bonds to the exposed edges of the cross plates 12 to form an integral structure.

Again, as before, the mould blank 2' is then heated and melted away to leave the finished article 10' which has an encased internal structure that would be difficult to produce by conventional casting or machining.