GB2257161A

GB2257161A - Process of forming a metal article.

Info

Publication number: GB2257161A
Application number: GB9113677A
Authority: GB
Inventors: Wilfred Hendrik Marie Alsem; Cornelis Rensen
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1991-06-25
Filing date: 1991-06-25
Publication date: 1993-01-06
Also published as: GB9113677D0

Abstract

Process of forming an article by subjecting a metal powder present under vacuum in a closed container to hot isostatic pressing in which the container comprises at least one local difference in wall thickness.

Description

PROCESS OF FORMING A METAL ARTICLE The invention relates to a process of forming a metal article by subjecting a metal powder present under vacuum in a closed container to hot isostatic pressing (HIP-ing).

Hot isostatic pressing (HIP-ing) is a processing technique in which high isostatic pressure can be applied to a powder present in a closed container at elevated temperature to produce a fully dense product. This process usually results in the manufacture of a fully dense body, although partially dense bodies also can be produced.

During processing, the compact is subjected to isostatic pressure using a fluid. This process is further described in more detail in "Metals Handbook", 9th Ed., 1984, American Society of Metals, Metals Park, Ohio, pp. 419-443, "Hot isostatic pressing of metal powders".

During HIP-ing of a powder in a closed container collapse of the container takes place as a result of the applied high gas pressure resulting in the compaction of the powder. This collapse is often uneven, resulting in a distortion of the container and of the ultimate article present in the container at the end of the HIP-ing.

It has now been found that this drawback of HIP-ing can be obviated by applying one or more local differences in wall thickness to the container to be used for HIP-ing.

The invention therefore relates to a process of forming an article by subjecting a metal powder present under vacuum in a closed container to hot isostatic pressing characterized in that the container comprises at least one local difference in wall thickness.

The HIP-ing is advantageously applied to powders comprising at least one metal or alloy. The powders can consist of one single metal or of a mixture of several metals. Other non-metal materials can be present. Metals which are preferably used in the HIP process are aluminium, iron, steel, nickel, titanium, copper, tantalum, molybdenum, tungsten and alloys thereof.

In order to produce very strong articles which are resistant to wear and tear a ceramic material is advantageously added to the metal powder. This ceramic material component can consist of a powder but even better results are obtained when the ceramic material comprises fibres. Ceramic materials which are preferably used in powders to be subjected to HIP-ing are boron carbide, silicon carbide and/or aluminium oxide. In the articles which are produced by subjecting a metal(s) and ceramic(s)-containing powder to HIP-ing the metal is preferably the continuous phase. So in these articles every ceramic particle or fibre is preferably fully surrounded by metal. Therefore the volume ratio between ceramic material and metal in the powder to be subjected to HIP-ing is advantageously not higher than 1.5.

The articles resulting from the HIP-process can have any shape, e.g. cylinders, turbine blades, propellers and disks.

As mentioned hereinbefore one or more local differences in wall thickness are applied to the container to be used for HIP in order to avoid distortion of the article produced. The local differences in wall thickness are preferably grooves. These grooves can run in any direction over the in- or outside surface of the container. Preferably the grooves are situated in planes which are perpendicular to the main axis of the container, as shown in case (a) of Figure 1. This is especially the case if one allows densification of the powder in the container to take place preferably in the direction of the main axis.

For the production of articles in which a radial shrinkage should take place it can be advantageous to use containers on which the grooves run parallel with the main axis of the container, as shown in case (b) of Figure 1. This is especially of advantage if the material in the container contains fibres parallel to the main axis. In some cases a combination of grooves situated in planes perpendicular to the main axis and grooves running parallel with the main axis of the container are particularly preferred, as shown in case (c) of Figure 1. An example of such a case is an article which has a columnar shape with rectangular cross-section. It is also possible to locate the grooves at an arbitrary inclination with respect to the main axis of the container, as is demonstrated in case (d) of Figure 1.

The grooves may be applied to the container by any means, machining and etching being preferred as they are simple and convenient techniques.

The grooves may have a rectangular shape or be rounded off.

The wall of the container should not be too thin because in that case cracks can occur in the container during HIP causing gas leakage into the container. This has to be avoided, because limited or no densification of the powder will take place and it can lead to dangerous situations, e.g. explosion of the container after depressurization.

On the other hand the container wall should not be too thick.

In that case too much material is wasted and unnecessary high pressures have to be applied. For these reasons it is preferred to use containers for HIP which have a wall thickness which allows enough difference in strength between the grooves and the areas where there are no grooves. For example a typical wall thickness for steel is 1.5-2 mm.

The container material is advantageously chemically compatible with the metal powder and does not suitably react therewith. The container material is preferably easy to machine, and easy to weld or solder.

The container suitably consists of a material which exhibits enough plasticity but which is not too weak or will even melt at the temperature at which the article is HIP'ed (- produced by HIP).

A steel container can be readily used for a great variation of metal articles. Also copper or aluminium or alloys thereof are suitable as container material for metal powders which are used for HIP-ing (- production by HIP) at temperatures below the temperature at which the container metals start to melt.

If the grooves are too broad and/or the wall thickness of the container at the place of a groove is too small, cracks can be generated in the container wall during HIP. These should be obviated as mentioned hereinbefore.

If on the other hand the grooves are too narrow and not deep enough, distortion of the container during HIP may occur leading to an article having an undesired shape. This situation is similar to the one in which no grooves are used at all.

The geometrical contraction and hence the relative shrinking which is required perpendicular to the grooves and the surface roughness of the densified article determine the ratio of the width of the grooves and the mutual distance between the grooves.

Therefore it is preferred to use grooves having a width in the range from 1-100 mm and having a depth such that at the place of the groove the wall thickness of the container is larger than 0.1 mm.

The mutual distance between the centre lines of two grooves is preferably in the range from 3 to 300 mm. At smaller distances the container may become too weak, at greater distances the presence of grooves may have too little effect. In some cases it may be advantageous to locate on one particular area of the container surface more grooves than on another area, e.g. with the purpose of compensating for temperature inhomogeneities or to control desired inhomogeneous densification(s).

The HIP process is usually carried out at a temperature in the range from 20 to 2000 "C and at a pressure in the range from 0.1 to 300 MPa.

Preferred temperature and pressure ranges are 200-1800 "C and 50-250 MPa respectively.

Advantageously the maximum temperature (K) during HIP-ing is 0.9 times the melting point (K) of the metal powder being processed.

As mentioned hereinbefore the HIP process is applied to powders which are present under vacuum in closed containers.

Preferably the vacuum in the closed container is in the range from 10 5 - 760 mmHg before the start of HIP-ing.

The invention also relates to articles which have been produced by HIP-ing using containers comprising one or more local differences in wall thickness as described hereinbefore.

The invention will now be further illustrated by means (Figure 2) of the drawings.

In figure 2A of the drawings a cylindrical metal container without grooves or any other type of local differences in wall thickness is schematically shown; This container is filled with metal powder and subjected to hot isostatic pressing. The result is shown in figure 2B: A highly distorted product.

In figure 2C a cylindrical metal container having grooves situated in planes which are perpendicular to the axis of the cylinder is schematically depicted. After this cylinder has been filled with metal powder it is also subjected to HIP-ing. From figure 1D it is clear that in this case the cylinder is not distorted during HIP-ing and a desired cylindrical metal article is obtained within the non-distorted cylindrical container.

Claims

1. Process of forming an article by subjecting a metal powder present under vacuum in a closed container to hot isostatic pressing characterized in that the container comprises at least one local difference in wall thickness.

2. Process as claimed in claim 1 in which the local differences in wall thickness are grooves.

3. Process as claimed in claim 2 in which the metal powder comprises at least one of the following metals: aluminium, iron, steel, nickel, titanium, copper, tantalum, molybdenum, tungsten and alloys thereof.

4. Process as claimed in any one of claims 1-3 in which the metal powder comprises at least one metal and at least one ceramic material.

5. Process as claimed in claim 4 in which the ceramic material consists of powder and/or fibres.

6. Process as claimed in claim 4 or 5 in which the ceramic material comprises boron carbide, silicon carbide and/or aluminium oxide.

7. Process as claimed in any one of claims 4-6 in which the volume ratio between ceramic material and metal is in the range from 0 to 1.5.

8. Process as claimed in any one of claims 1-7 in which the grooves are situated in planes which are perpendicular to the main axis of the container.

9 Process as claimed in any one of claims 1-8 in which the grooves run parallel with the main axis of the container.

10. Process as claimed in any one of claims 1-9 in which the grooves are located at an arbitrary inclination with respect to the main axis of the container.

11. Process as claimed in any one of claims 1-10 in which the grooves have been made by machining.

12. Process as claimed in any one of claims 1-11 in which the wall thickness of the original steel container is 1.5 to 2 mm.

13. Process as claimed in any one of claims 1-12 in which the width of the grooves is 1-100 mm.

14. Process as claimed in any one of claims 1-13 in which the mutual distance between the centre lines of two grooves is from 3 to 300 mm.

15. Process as claimed in any one of claims 1-143 in which the depth of the grooves is such that at the place of a groove the wall thickness of the container is larger than 0.1 mm.

16. Process as claimed in any one of claims 1-15 in which the container is made of metal.

17. Process as claimed in claim 16 in which the metal is chemically compatible with the metal powder.

18. Process as claimed in any one of claims 1-17 in which the container is made of steel, aluminium or copper, or alloys thereof.

19. Process as claimed in any one of claims 2-18 in which the maximum temperature (K) during the hot isostatic pressing is 0.9 times the melting point (K) of the metal powder.

20. Process as claimed in any one of claims 1-19 in which the -5 vacuum in the closed container is 10 - 760 mmHg before the start of the hot isostatic pressing.

21. Articles whenever produced with the use of a process as claimed in any one of claims 1-20.