Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2000-03-23
2002-05-07
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C419S028000, C419S029000, C419S038000
Reexamination Certificate
active
06383248
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the manufacture of precious metal artifacts, especially items of jewelry such as rings. The invention also relates to precious metal artefacts whenever manufactured by that process.
BACKGROUND TO THE INVENTION
A process currently in commercial use for the manufacture of wedding rings, which was developed by Karl Klink, comprises the following operations. Initially, long ingots of gold are produced in a continuous casting process, which ingots are then subdivided into slabs, typically about 20 kg in weight and about 1 inch (25.4 mm) in thickness. The surfaces of the slabs are machined in order to remove the roughness left by the casting process, after which the slabs are annealed, that is to say subjected to a heat treatment in order to homogenise the metal, following which the slabs are subjected to a rolling process in order to produce a thinner sheet. In practice, the slabs are subjected to a number of alternating rolling and annealing steps, typically five annealing steps with four rolling steps interspersed between them. When the desired thickness has been achieved, typically about 0.1 inch (2.5 mm), washers are stamped out of the sheet by means of a suitable press. The washers are then further annealed and then formed into hollow cones which, following a further annealing step, are subjected to a drawing operation to convert them into cylinders. Following a further annealing step, the cylinders are “repressed”, that is to say squashed to approximately the height of the intended rings. After yet another annealing step, the repressed cylinders are subjected to “ring rolling”, that is to say they are passed between rollers in order to produce semi-finished wedding rings, which are then ready for final finishing operations, such as polishing, diamond cutting and/or engraving.
The foregoing process suffers from a number of disadvantages. In particular, the numerous operations that are required result in a process that is lengthy and costly. In addition, a very considerable quantity of scrap is generated, especially during machining, trimming and the stamping-out of washers: as a result, the yield of finished product, namely the formed wedding rings, is low, typically at about 30% of the original melt weight.
U.S. Pat. No. 4,479,823 (Hohmann, assigned to Blendax-Werke R. Schneider GmbH & Co.), which corresponds to DE-C-3,240,256, discloses a process for the production of a master alloy powder useful for amalgamation with mercury to form a dental filling material, which process comprises atomising an alloy containing silver, tin and copper into a spherical powder, the pulverisation being carried out for example, by inert-gas pulverisation or by a high-pressure-water method. The resultant powder is dried and formed into a coherent shaped article: thus, square blocks may be formed by mechanical pressing or round rods may be formed isostatically. The shaped article is then sintered at an elevated temperature, in particular in the recrystallisation temperature range of 150-350° C. for about half-an-hour under a reducing atmosphere. The sintered article is then pulverised by machining, e.g. in a milling or turning operation, to form the master alloy powder.
DE-C-3,336,526 (Degussa A G) discloses sintered blanks for stampings made of various precious metals (in particular gold, silver, palladium, platinum and their alloys) for the manufacture of jewellery and such articles as coins, medals and plates, which blanks exhibit a porosity of 8 to 35% by volume. The blanks are said to exhibit excellent stampability, in that they may be fully minted after one or two stamping cycles. Thus, Example 6 of this German patent discloses a mixture of 95% by weight of a gold alloy powder and 5% by weight of glass powder produced by atomising, which mixture was pressed into a mould under a pressure of 2 kbar in order to produce a die piece having about 70% of the theoretical density. After one hour's sintering at 800° C., the porosity was about 20% by volume. The resultant circular blanks could be fully minted in a die-set in two cycles (compared with conventional blanks of the same carat value, for which at least 5-8 cycles would be necessary).
Japanese laid-open (Kokai) patent publication No. 64-65203 (Tokuriki Honten Co. Ltd.) discloses a silver/metal oxide rod useful in medium-load electrical contacts. Such a rod is produced by welding, under pressure and at a temperature of 600° C. or higher, a sheath of silver onto the outer surface of a silver/metal oxide material which contains from 5 to 30% by weight of oxide dispersed in the silver. The resultant composite is worked to a prescribed diameter, bringing the thickness of the silver sheath to between 0.02 mm and 0.3 mm. The oxides include the oxides of cadmium, tin, antimony, zinc or indium. In a working example, silver, cadmium and antimony were fused together and then pulverised by water atomisation. The powder was subjected to internal oxidation at 700° C. and the material, described as “moulded and sintered”, was then plugged into a tube made from silver and lids also made of silver were then welded on. The composite billet was heated to 850° C. and hot-extruded to form a rod with a diameter of 10 mm, after which a composite rod with a diameter of 3 mm was obtained by repeated annealing and drawing. In another working example, the silver tube and lids were replaced with a tube and lids made of silver to which 0.1% by weight of nickel had been added.
Aggie Bevnon, “Powder Metallurgy”,
Metalsmith
(Fall 1982) describes the production of artefacts made of heterogeneous mixtures of metals. The process comprised the charging to a die cavity of various powdered metals in layers, followed by tamping-down to eliminate air pockets and then compression under pressure to consolidate the powders in a green compact strong enough to be handled. Pressures of 30-40×10
3
psi (206.8 to 275.8 MPa) appear to have been preferred. The green compact was then sintered in a reducing atmosphere, the furnace being set at either 732° C. for a silver-monel mixture or 816° C. for a copper-monel mixture. Depending upon the intended design of the artefacts, the sintered compact was milled, rolled or cut. The article suggests that several experiments employing 14 carat milled gold and atomised 8 carat gold powder were carried out but indicates that the gold powders have been barely satisfactory: the milled gold particles were too large, in comparison to the other materials, which resulted in much shrinking during sintering, whereas in the case of the commercial atomised gold, the powder refused to bond, except when locked in place by the adjacent metal.
SUMMARY OF THE INVENTION
In one aspect of the present invention, there is provided a process for the manufacture of an artefact from a precious metal, which process comprises compacting a precious-metal powder into a shaped body, wherein at least 80% by weight of the powder has been produced by water atomization of a stream of molten precious metal; sintering the said body; and thereafter annealing the said body.
As discussed hereinafter, the term “annealing” is to be construed broadly to include not only full annealing but also partial annealing or any heating step in which stress induced in the body by any preceding step is at least partially relieved.
The expression “precious metal” herein applies not only to an elemental precious metal, such as gold, silver or platinum, in pure or substantially pure form, but also to an alloy that has a substantial content, e.g. at least 30% by weight, of an elemental precious metal or of a mixture of elemental precious metals.
In another aspect of the present invention, there is provided an artefact manufactured by a process according to the first aspect of the invention.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
In certain preferred embodiments the process comprises, between the steps of sintering the body obtained by compaction and the annealing of the body, the further step of modifying the shape and/or dime
Hurford Stephen Paul
Taylor Terence Ernest
Engelhard-CLAL UK Ltd.
Mai Ngoclan
Welsh & Katz Ltd.
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