GB1566858A - Isostatic pressing - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ISOSTATIC PRESSING (71) I, THE SECRETARY OF STATE FOR DEFENCE, Whitehall, London, SW1A 2BH, a British Corporation Sole, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to methods of isostatic pressing and has one application in pneumatic isostatic pressing of powders, especially metal powders.

The isostatic pressing of powders to form compact bodies is a well known technique.

For example the powder may be enclosed in a rubber bag which is immersed in the oil of a hydrostatic press. Instead of oil a gas may be used; this facilitates pressing at high temperatures, but under such conditions a rubber bag cannot be used and thin metal cans have been substituted which are stripped off after pressing. Such cans can be shaped to the desired form of the compacted body, at least for simpler (and preferably not re-entrant) shapes, allowance being made for the shrinkage of the powder on compaction; such shaping reduces the amount of subsequent machining required.

Hitherto such metal cans have been made from sheet metal with welded seams. It is found, however, that the deformation of the can on pressing tends to cause leakage at the welded seams. Since the material within is porous, such leakage destroys the pressure differential and prevents compaction.

The present invention provides an improved method in which the leakage problem is reduced or prevented, and which also enable cans of more complicated shape, eg re-entrant, to be more readily produced.

According to the present invention, in a method of isostatic pressing wherein a powder is enclosed in a metal can which is then subjected to isotatic pressure, the can is formed by the electrodeposition of metal on a mandrel, which mandrel is afterwards dissolved away to leave a hollow can.

The mandrel may be made of aluminium, which can be dissolved away with sodium hydroxide solution. The e lectrodeposited metal is suitably nickel, though other metals capable of withstanding the isotatic pressing temperature may be used.

The thus-formed can may include an opening for filling the can with the powder, the opening being thereafter closed, eg by welding. Such a weld need only be of a very small length relative to the length of the welding in a can made from welded sheetmetal, and presents a correspondingly small risk of leaking when the filled can is subjected to isostatic pressure. The opening may be constituted by a hollow neck and the mandrel may be shaped to form such a neck.

The opening may be sealed by a plug inserted in the neck and welded thereto.

Since in electrodeposition the metal is preferentially deposited on the edges and/or corners of the cathode, the mandrel can be shaped, if desired, to produce strengthening ribs thereon.

Although devised for use in, and of especial value in, high-temperature pneumatic isostatic pressing, the method may also be applicable to other isostatic pressing techniques including hydraulic isostatic pressing.

The invention also provides an isostatically pressed body when produced by a method as aforesaid.

To enable the nature of the present invention to be more readily understood, an embodiment will now be described with reference to the accompanying drawings, wherein Figure 1 is a vertical section of a can electrodeposited upon a mandrel and Figure 2 is an enlarged vertical section of the neck of the can of Figure 1 showing a method of sealing.

In the present example a hollow conical body is to be formed by the isostatic pressing of beryllium powder. In Figure 1 a mandrel 1 is formed of aluminium, eg by machining or casting, and includes a neckforming portion 2. The mandrel is connected as the cathode in a conventional nickel-plating bath and a coating 3 of nickel is deposited thereon. Conveniently the mandrel is suspended in the bath by the portion 2, which also acts as an electrical connection. After deposition, the coating and portion 2 are cut through, suitably where indicated by the arrow 4. The coated mandrel is then immersed in hot sodium hydroxide solution to dissolve-out the aluminium, leaving a hollow nickel can with a neck extending from its apex.

The can is then vibro-filled with beryllium powder 9 through the neck, which is thereafter sealed by a short stainless-steel plug 5 driven into the powder and welded to the neck at 6, as shown in Figure 2. Extending through plug 5 and sealed thereto by welding is a length of small-diameter stainlesssteel tube 7 which is used to out-gas the powder by heating the filled can to about 900"C for a few hours under vacuum. The gas is drawn off through tube 7, which is subsequently sealed by crimping at 8. A porous stainless-steel filter insert 10 prevents powder being withdrawn from the can during out-gassing.The out-gassed can is then isostatically pressed at 1050"C and 15,000 pounds/in2 (71050 kg/cm2) in an inert gas (eg He or Ar) to form the conical body, after which the can is stripped off and the body finished by machining.

It is advantageous to taper the plug 5 towards its inner end as shown to avoid a sharp discontinuity between the incompressible plug and the compressible powder which might fracture the can when the isostatic pressure is applied. In the described sealing method not only is the circumferential weld 6 relatively short but is is located where substantially no deformation will occur on isostatic pressing, thereby reducing the risk of leakage.

In the described example the mandrel is about 12 inches (30 cm) in diameter at the base, about 12 inches (30 cm) high, and about 1.5 inch (3.75cm) thick. The neckportion 2 is about 1.0 inch (2.5cm) in diameter. The coating 3 is about 40 thou (imam) thick, but this dimension is not critical. In dimensioning the mandrel account must naturally be taken of the shrinkage of the powder on compaction. For example in the present example the shrinkage is nearly 509.

With re-entrant bodies such as that shown, it is necessary to insert a solid support member (indicated at 5) within the re-entrant portion to resist collapse of the filled can during pressing.

The present method is applicable to powders of metals other than beryllium, and to powders of non-metals.

WHAT I C-LAIM IS: 1. A method of isostatic pressing wherein a powder is enclosed in a metal can which is then subjected to isostatic pressure, wherein the can is formed by the electrodeposition of metal on a mandrel, which mandrel is afterwards dissolved away to leave a hollow can.

2. A method as claimed in claim 1 wherein the mandrel is made of aluminium.

3. A method as claimed in claim 2 wherein the mandrel is contacted with sodium hydroxide solution to dissolve away the aluminium.

4. A method as claimed in any preceding claim wherein the electrodeposited metal is nickel.

5. A method as claimed in any preceding claim wherein the mandrel includes a portion shaped to form on electrodeposition a hollow neck which acts as an opening to the can.

6. A method as claimed in claim 5 wherein, after filling the can with powder, the neck is sealed by a metal plug inserted in the neck and sealed thereto.

7. A method as claimed in claim 6 wherein the metal plug is sealed to the neck by a circumferential weld at the outer end of the plug.

8. A method as claimed in claim 6 or claim 7 wherein the plug has sealed therethrough a small-diameter tube for outgassing the powder prior to isotatic.

9. A pressing method as claimed in claim 8 wherein after out-gassing, the tube is sealed by crimping.

10. A method of high-temperature pneumatic isostatic pressing of a metal powder as claimed in any preceding claim.

11. A method as claimed in claim 10 wherein the metal is beryllium.

12. A method of isostatic pressing as claimed in claim 1 and substantially as hereinbefore described.

13. An isostatically pressed body when produced by a method as claimed in any preceding claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. body is to be formed by the isostatic pressing of beryllium powder. In Figure 1 a mandrel 1 is formed of aluminium, eg by machining or casting, and includes a neckforming portion 2. The mandrel is connected as the cathode in a conventional nickel-plating bath and a coating 3 of nickel is deposited thereon. Conveniently the mandrel is suspended in the bath by the portion 2, which also acts as an electrical connection. After deposition, the coating and portion 2 are cut through, suitably where indicated by the arrow 4. The coated mandrel is then immersed in hot sodium hydroxide solution to dissolve-out the aluminium, leaving a hollow nickel can with a neck extending from its apex. The can is then vibro-filled with beryllium powder 9 through the neck, which is thereafter sealed by a short stainless-steel plug 5 driven into the powder and welded to the neck at 6, as shown in Figure 2. Extending through plug 5 and sealed thereto by welding is a length of small-diameter stainlesssteel tube 7 which is used to out-gas the powder by heating the filled can to about 900"C for a few hours under vacuum. The gas is drawn off through tube 7, which is subsequently sealed by crimping at 8. A porous stainless-steel filter insert 10 prevents powder being withdrawn from the can during out-gassing.The out-gassed can is then isostatically pressed at 1050"C and 15,000 pounds/in2 (71050 kg/cm2) in an inert gas (eg He or Ar) to form the conical body, after which the can is stripped off and the body finished by machining. It is advantageous to taper the plug 5 towards its inner end as shown to avoid a sharp discontinuity between the incompressible plug and the compressible powder which might fracture the can when the isostatic pressure is applied. In the described sealing method not only is the circumferential weld 6 relatively short but is is located where substantially no deformation will occur on isostatic pressing, thereby reducing the risk of leakage. In the described example the mandrel is about 12 inches (30 cm) in diameter at the base, about 12 inches (30 cm) high, and about 1.5 inch (3.75cm) thick. The neckportion 2 is about 1.0 inch (2.5cm) in diameter. The coating 3 is about 40 thou (imam) thick, but this dimension is not critical. In dimensioning the mandrel account must naturally be taken of the shrinkage of the powder on compaction. For example in the present example the shrinkage is nearly 509. With re-entrant bodies such as that shown, it is necessary to insert a solid support member (indicated at 5) within the re-entrant portion to resist collapse of the filled can during pressing. The present method is applicable to powders of metals other than beryllium, and to powders of non-metals. WHAT I C-LAIM IS:

1. A method of isostatic pressing wherein a powder is enclosed in a metal can which is then subjected to isostatic pressure, wherein the can is formed by the electrodeposition of metal on a mandrel, which mandrel is afterwards dissolved away to leave a hollow can.

2. A method as claimed in claim 1 wherein the mandrel is made of aluminium.

3. A method as claimed in claim 2 wherein the mandrel is contacted with sodium hydroxide solution to dissolve away the aluminium.

4. A method as claimed in any preceding claim wherein the electrodeposited metal is nickel.

5. A method as claimed in any preceding claim wherein the mandrel includes a portion shaped to form on electrodeposition a hollow neck which acts as an opening to the can.

6. A method as claimed in claim 5 wherein, after filling the can with powder, the neck is sealed by a metal plug inserted in the neck and sealed thereto.

7. A method as claimed in claim 6 wherein the metal plug is sealed to the neck by a circumferential weld at the outer end of the plug.

8. A method as claimed in claim 6 or claim 7 wherein the plug has sealed therethrough a small-diameter tube for outgassing the powder prior to isotatic.

9. A pressing method as claimed in claim 8 wherein after out-gassing, the tube is sealed by crimping.

10. A method of high-temperature pneumatic isostatic pressing of a metal powder as claimed in any preceding claim.

11. A method as claimed in claim 10 wherein the metal is beryllium.

12. A method of isostatic pressing as claimed in claim 1 and substantially as hereinbefore described.

13. An isostatically pressed body when produced by a method as claimed in any preceding claim.