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 anneali...

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 th...

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