Metal alloy manufacturing

Abstract

Silver alloys containing copper and germanium e.g. about 1 wt % Ge and of very low copper content e.g. about 0.8 wt % Cu can be precipitation hardened to 65 HV or above, whereas alloys of similar copper content and not containing germanium remain soft. In an embodiment, a silver alloy comprises 92.5-97 wt % Ag, 1-4.5 wt % Cu, 0.4-4 wt % Zn, 0.8-1.5 wt % Ge, 0 to 0.2 wt % Si, In or Sn and 0-0.2 wt % Mn, the balance being boron as grain refiner, incidental ingredients and impurities. The said alloy preferably comprises boron as grain refiner added as a boron hydride, e.g. sodium borohydride. A further group of alloys comprises a ternary alloy of silver, copper and germanium containing from more than 93.5 wt % to 95.5 wt % Ag, from 0.5 to 3 wt % Ge and the remainder, apart from incidental ingredients (if any), impurities and grain refiner, copper, the grain refiner being sodium borohydride or another boron hydride. Silicon-containing casting grain that gives rise to bright as-cast products is also disclosed. In a further embodiment, a zinc-containing silver alloy resistant to tarnish under severe conditions e.g. exposure to human sweat or French dressing comprises 1-5 wt % Zn, 0.7-3 wt % Cu, 0.1-3 wt % Ge, 0-0.3 wt % Mn, 0-0.25 wt % Si, B in an amount effective for grain refinement, up to 0.5 wt % incidental ingredients, the balance being Ag in an amount of 92.5-96 wt %, and impurities. A preferred manufacturing method giving an alloy with favourable physical properties involves melting together the ingredients, and incorporating boron by dispersing into molten silver alloy to foirn the whole or a precursor pait of said alloy a compound selecting fiom alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof The alloy is particularly suitable for castings which may be hardened in an oven e.g. at about 300° C. for 30-45 min.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part of U.S. patent application Ser. No. 11 / 628260 having a filing date of 12 Jan. 2006 and which is a 371 of PCT / GB2005 / 050074 filed 27 May 2005 (International Publication Number WO 2005 / 118903) which claims priority from UK Patent Application Number 0421172.8 filed 23 Sep. 2004 and UK Patent Application Number 0412256.0 filed 2 Jun. 2004. It is also a continuation-in-part of PCT / GB2006 / 050116 filed 19 May 2005 (International Publication Number WO 2006 / 123190) which claims priority from UK Patent Application Number 0523002.4 filed 11 Nov. 2005 and UK Patent Application Number 0510243.9 filed 20 May 2005.The disclosure of each application is hereby incorporated by reference in its entirety where appropriate for teachings of additional or alternative details, features and / or technical background, and priority is asserted from each.BACKGROUND TO THE INVENTION [0002] This invention relates to a metal a...

AI Technical Summary

Benefits of technology

[0028] We have now found that Ag—Cu—Ge alloy workpieces heated to an annealing temperature can be hardened by gradual cooling followed by mild reheating to effect precipitation hardening, and that products of useful hardness can be obtained. The use of reheating to e.g. 180-350° C., and preferably 250-300° C., to develop precipitation hardness is typical. Significantly it has been found that over-aging of Ag—Cu—Ge alloys during precipitation hardening does not cause a significant drop-off of the hardness achieved. The new method of processing workpieces is applicable, for example as part of soldering or annealing in a mesh belt conveyor furnace or in investment casting, eliminates quenching e.g. with water which as explained above is required for Ag—Cu Sterling silver, and which as explained above can give rise to distortion or damage to the product, and therefore can be used for nearly finished work. The process is applicable to alloys of the general kind disclosed in GB-B-2255348. It is also believed to be applicable to some or all of the alloys disclosed in U.S. Pat. No. 6,726,877, including those of relatively high germanium content and also those of lesser germanium content and relatively high zinc and silicon content.

[0031] providing a silver alloy containing silver in an amount of at least 77 wt %, copper and an amount of germanium that is at least 0.5 wt % and is effective to reduce tarnishing and / or firestain;

[0035] The above process is based on a surprising difference in properties between conventional Sterling silver alloys and other Ag—Cu binary alloys on the one hand and Ag—Cu—Ge alloys on the other hand, in which gradual cooling of the binary Sterling-type alloys results in coarse precipitates and only limited precipitation hardening, whereas gradual cooling of Ag—Cu—Ge alloys results in fine precipitates and useful precipitation hardening, particularly where the alloy contains an effective amount of grain refiner. Gradual cooling includes the avoidance of any abrupt cooling step as when an article is plunged into water or other cooling liquid, and normally implies that cooling to ambient temperatures takes more than 10 seconds, preferably more than 15 seconds. Control can be achieved during the mesh belt conveyor furnace treatment of workpieces to be brazed and / or annealed by gradual cooling as the workpiece is moved towards the discharge end of the furnace. Control can also be achieved during investment casting if the piece being cast is allowed to air-cool to ambient temperature, the rate of heat loss being moderated by the low conductivity investment material of the flask.

Problems solved by technology

However, the degree of copper solubility in the solid alloy depends on the heat treatment used, and the overall physical properties of the sterling can be materially affected, not only by heating the silver to different temperatures, but also by employing different cooling rates.

This type of structure is hard, but it is also difficult to work, and has a tendency to crack.

Furthermore, there are very few times in practical production that a silversmith can safely quench a piece of nearly finished work because of the risk of distortion of the article being made and / or damage to soldered joints.

It is too difficult for commercial or industrial production of articles of jewellery, silver plate, hollowware, and the like (see Fischer-Buhner, “An Update on Hardening of Sterling Silver Alloys by Heat Treatment”, Proceedings, Santa Fe Symposium on Jewellery Manufacturing Technology, 2003, 20-47 at p.

When the depth of the fire-stain exceeds about 0.025 mm (0.010 inches) the alloy is additionally prone to cracking and difficult to solder because an oxide surface is not wetted by solder so that a proper metallurgical bond is not formed.

In the experience of the present inventors, although tarnish resistance is exhibited to some extent, together with some firestain reduction on investment casting, firestain resistance on soldering or annealing is not obtained because of the copper content.

The alloys are stainless in ambient air during conventional production, transformation and finishing operations, are easily deformable when cold, easily brazed and do not give rise to significant shrinkage on casting.

Furthermore the development of tarnish is appreciably delayed by the addition of germanium, the surface turning slightly yellow rather than black and tarnish products being easily removed by ordinary tap water.

Experiments by the present applicants have not confirmed fire resistance of available embodiments of the alloy, especially during torch annealing.

A passive layer is formed by the germanium, which significantly slows the formation of silver and copper sulphides, the main cause of tarnish on conventional silver alloys.

However, there is no suggestion that precipitation hardening is appropriate for nearly finished work and that the problems of distortion and damage to soldered joints can be avoided.

WO2004 / 106567 discloses the desirability of reducing or avoiding the formation and / or melting of the above mentioned binary copper-germanium eutectic which melts at 554° C. During the production of e.g. 925 Argentium silver alloys, the formation of this phase can be avoided by careful control of the casting conditions since under equilibrium cooling conditions the crystallisation is complete at below 640° C. However, this binary phase can create problems during subsequent thermal treatment of the alloys, e.g. using brazing alloys which typically have melting points in the range 680-750° C. and torch annealing which typically involves heating a workpiece to a dull red heat at 700-750° C. On heating the workpiece to or beyond these temperatures incipient melting occurs with a small amount of material corresponding to this binary phase becoming molten while the bulk remains stable.

This contributes brittleness and e.g. as noted in GB-B-2255348 there is a tendency for the alloy to sag when heated for joining or annealing operations.

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