GB2118208A - Method of making an alloy - Google Patents

Abstract

In a method of making an alloy in which superior cyclic properties may be achieved by avoiding the presence of oxide inclusions and of Titanium Carbo-Nitride stringers, the alloy constituents are first melted in a way which avoids contact with the refractory material from which the oxide inclusions normally derive, for example, by electron-beam melting in a water-cooled copper crucible in a vacuum or protective atmosphere. The thus formed alloy is then powdered, again in a refractoriless process, by melting the end of a spinning ingot, so that the physical separation of the powder particles prevents agglomeration of the stringers. The clean powder is finally consolidated. <IMAGE>

Description

SPECIFICATION Method of making an alloy This invention relates to a method of making an alloy, and to the alloy made thereby. It is applicable to the nickel-based superalloys, and enables the production of alloy with superior cyclic properties particularly for disc applications.

There are two common forms of inclusions in these alloys which can provide sites for crack initiation and which therefore weaken the alloy.

The first comprises oxide particles. These particles of alumina or magnesia or other refractory oxides are mainly introduced into the metal from the refractory lining of the crucibles used to hold the molten metal during processing. It is possible to remove the larger particles by filtering or by processes such as slag refinement, but it is difficult to remove the smaller particles once they are present.

Fortunately, disc materials produced by the conventional cast/forge route are quite tolerant of these smaller particles, at the lower stress level capability of such materials, and their presence has not been very deleterious. Material which has been made by a powder metallurgy route is normally of higher strength and is much less tolerant to this type of inclusion, at the higher operating stress levels, and it has been the presence of these inclusions which has led to the powder metallurgy materials being less advantageous than was hoped.

A second form of inclusion, which is more damaging to the conventionally produced materials, takes the form of stringers of compounds such as Titanium Carbo-Nitrides.

Super-alloys ordinarily contain Titanium and Carbon as constituents of the alloy, and it is almost impossible to prevent Nitrogen from dissolving into the alloy at some stage. These three elements react to form the Titanium Carbo Nitrides. When first formed in the liquid metal, these compounds comprise very small particles. If maintained as small isolated particles in the solid state they do not effect any significant weakening of the alloy.

However, during certain melting processes the Titanium Carbo-Nitrides tend to agglomerate to form much larger 'stringers'. In such a condition they form potential sites for crack initiation and growth.

It is therefore desirable to produce material without these stringers of Titanium Carbo Nitrides. We have appreciated that one method of avoiding not the production but the agglomeration of these compounds is to form the material by a powder route. The small size of the particles of power precludes the formation of stringers. At the same time, however, it is necessary to avoid the oxide inclusions which otherwise affect the properties of the powder material.

According to the present invention, therefore, a method of making a clean alloy comprises melting and mixing the alloy constituents in a vacuum or protective atmosphere in a manner such as not to expose them to contact with a refractory material from which oxides may contaminate the alloy, subsequently producing an alloy powder from said alloy in a powdering process in a vacuum or protective atmosphere which does not involve the alloy coming into contact with refractory material from which oxides may contaminate the alloy, and consolidating the alloy powder thus formed.

It may be desirable to form an ingot of the alloy between the initial melting and the powder production steps.

In a preferred embodiment the alloy constituents are melted in a crucible of e.g. copper using an electron beam melting process, and the alloy is powdered using a process in which the alloy is formed into an ingot which is spun rapidly about its axis while the end of the ingot is melted using an electric arc or plasma.

The invention will now be particularly described, merely by way of example, with reference to the accompanying drawings in which: Fig. 1 illustrates apparatus for carrying out the first stage of a process in accordance with the invention and, Fig. 2 illustrates apparatus for carrying out the second stage of a process in accordance with the invention.

In fig. 1 there is shown a copper crucible 10 containing at 11 the constituents of the alloy it is intended to make. These constituents may be in the form of elemental materials, or use may be made of master alloys in which some of the elemental constituents are pre-alloyed. For the case of nickel-based super-alloys the constituents will include for example all or some of the elements Nickel, Chromium, Cobalt, Tungsten, Aluminium, Titanium, Tantalum, Niobium, Hafnium and Carbon.

The crucible 10 is provided with conduit 1 2 through which water may flow to cool the crucible, and a plurality of electron beam guns 1 3 are arranged so that their beams may play on the material 11 to heat it. Controi means (not shown) for the guns cause the beams to move over the material 11 in such a way as to heat the material evenly.

In order to allow the guns to operate, and to avoid contamination of the metal, the whole assembly of crucible and guns is housed in an evacuated chamber 14. Means (not shown) are provided to enable the chamber to be evacuated at will and to allow access for the raw materials and finished alloy to be put into and removed from the chamber respectively.

In operation the raw materials 11 are loaded into the crucible 10 and the crucible water cooling is caused to operate. The chamber 14 is evacuated, and the guns 1 3 caused to operate, heating the constituents 11 and melting and mixing them.

Once the upper surface of the material is satisfactorily mixed and allowed to solidify, the charge of metal is then inverted in the crucible and the other half of the charge is melted and mixed.

In this way an ingot of alloy is produced.

It will be understood that at this stage the ingot of alloy will be free from oxide inclusions but may have Titanium Carbo-Nitride inclusions, albeit in the form of separate particles rather than stringers.

In order to adapt it for the next stage of the process the ingot is made in an elongated cylindrical form, and two such ingots are required for the powder manufacturing process illustrated in fig. 2. The two ingots 20 and 21 are mounted horizontally and coaxially in supporting devices 22 and 23 respectively so that their exposed ends face each other and are spaced apart by a predetermined spacing which may be set and maintained by axial motion of the supporting device 22. This axial motion is performed using a ram 24 operating on the spindle 25 extending from the supporting device 22.

The pair of ingots 20 and 21 are located within a vacuum chamber 26, again provided with the necessary access doors etc. Where the spindle 25 of the device 22 and the similar spindle 27 of the device 23 extend through the chamber wall they are carried in sealed bearings 28 and 29 which allow the spindles and thus the ingots to be rotated about their axes.

In order to cause this rotation, respective electric motors 30 and 31 are provided, operating through gear trains 32 and 33 to drive the ingots.

As so far described, the arrangement allows the ingots to be rapidly rotated about their axes in an evacuated chamber, and it is necessary to provide some means of melting the metal of the ingots in order that the molten particles will spin off and solidify as powder. In order to do this an electric arc is struck between the electrodes. A power supply 34 is connected to the spindles 25 and 27 by brushes at 35 and 36, and the output of the supply 34 and spacing between the ingots 20 and 21 is arranged to cause an arc to strike between the ingots and thus to melt portions of the end of either or both ingots, which effectively become electrodes.

The molten particles of alloy spin off by virtue of the rotation of the ingots, and rapidly solidify in the form of a powder. This powder is collected in a tray 37 for subsequent use. The process is continued until either the ingots are fully consumed or sufficient powder is produced.

Because the process described in relation to fig. 2 does not involve the metal coming into contact with refractory oxides, there should be no danger of the alloy becoming contaminated with oxide inclusions. Again, the physically small size of the powder particles prevents the agglomeration of the Titanium Carbo-Nitride particles, even though these may still be present in the form of individual particles. Any agglomerates existing in the ingots will be dispersed during the powdering process.

The clean alloy powder thus formed may then be compacted and formed into its final shape by hot isostatic pressure or another consolidation process. These final steps of the process are not illustrated since they are conventional and known to those skilled in the art, there being no need to treat the clean alloy material thus produced any differently from normal material. However, it will be understood that because of its lack of inclusions the clean material will have better properties than those of the normal, contaminated materials.

It will be appreciated that there are alternatives to the particular technique described above. Thus for instance a consumable vacuum arc process may be used to melt the initial alloy mixture.

Again, the particular alloy used is of course a matter of choice, although the invention applies particularly to nickel-based superalloys.

Claims (9)

1. A method of making a clean alloy comprising melting and mixing the alloy constituents in a vacuum or protective atmosphere in a manner such as not to expose them to contact with a refractory material from which oxide may contaminate the alloy, and subsequently producing an alloy powder from said alloy in a powdering process in a vacuum or protective atmosphere which does not involve the alloy coming into contact with refractory material from which oxides may contaminate the alloy, and consolidating the alloy powder thus formed.

2. A method as claimed in claim 1 and in which an ingot is formed from said alloy after said melting step and prior to said powder producing step.

3. A method as claimed in claim 1 and in which said alloy constituents are melted using an electron-beam melting process.

4. A method as claimed in claim 3 and in which said alloy constituents are melted in a copper crucible.

5. A method as claimed in claim 4 and in which said copper crucible is water-cooled.

6. A method as claimed in claim 2 and in which said alloy ingot is formed into a powder by spinning the ingot rapidly about its axis in an evacuated chamber while its end is melted using an electric arc.

7. A method as claimed in claim 6 and in which said arc is struck between two said ingots which are both spun.

8. A method as claimed in claim 2 and in which said ingot is formed into a powder by spinning the ingot rapidly about its axis while its end is melted by a plasma gun.

9. A method substantially as hereinbefore particularly described with reference to the accompanying drawings.