GB2414739A

GB2414739A - Process for making finished or semi-finished articles of silver alloy

Info

Publication number: GB2414739A
Application number: GB0412256A
Authority: GB
Inventors: Peter Gamon Johns
Original assignee: Middlesex Silver Co Ltd
Current assignee: Middlesex Silver Co Ltd
Priority date: 2004-06-02
Filing date: 2004-06-02
Publication date: 2005-12-07
Anticipated expiration: 2024-06-02
Also published as: GB0421172D0; GB0412256D0; EA200602286A1; GB2414739B; CN1961092A; CN100478485C

Abstract

A process is provided for making a finished or semi-finished article of silver alloy, said process comprising the steps of: ```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; ```making or processing the finished or semi-finished article of the alloy by heating at least to an annealing temperature; and ```gradually air cooling the article from the temperature at which it is made or processed to develop a Vickers hardness of more than 70.

Description

24 1 4739

PROCESS FOR MAKING FINISHED OR SEMI-FINISHED

ARTICLES OF SILVER ALLOY

FIELD OF THE INVENTION

The present invention relates to a process for making finished or semifinished articles of silver alloy and to articles made by the above process.

BACKGROUND TO THE INVENTION

Patent GB-B-2255348 (Rateau, Albert and Johns; Metaleurop Recherche) discloses a novel silver alloy that maintains the properties of hardness and lustre inherent in Ag-Cu alloys while reducing problems resulting from the tendency of the copper content to oxidise. The alloys are ternary Ag-Cu-Ge alloys containing at least 92.5 wt% ; - F Ag, 0.5-3 wt% Ge and the balance, apart from impurities, copper. 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. They also exhibit superior ductility and tensile strength. Germanium was stated to exert a protective function that was responsible for the advantageous combination of properties exhibited by the new alloys, and was in solid solution in both the silver and the copper phases. The microstructure of the alloy was said to be constituted by two phases, a solid solution of germanium and copper in silver surrounded by a filamentous solid solution of germanium and silver and copper. The germanium in the copper-rich phase was said to inhibit surface oxidation of that phase by forming a thin GeO and/or GeO2 protective coating which prevented the appearance of firestain during brazing and flame annealing. Furthermore the development of tarnish was appreciably delayed by the addition of germanium, the surface turned slightly yellow rather than black and tarnish products were easily removed by ordinary tap water. The alloy was said to be useful inter a/ia in jewellery.

Patents US-A-6168071 and EP-B-0729398 (Johns) disclosed a silver/germanium alloy which comprised a silver content of at least 77 wt % and a germanium content of between 0.4 and 7%, the remainder principally being copper apart from any impurities, which alloy contained elemental boron as a grain refiner at a concentration of greater than Oppm and less than 20ppm. The boron content of the alloy could be achieved by providing the boron in a master copper/boron alloy having 2 wt % elemental boron. It was reported that such low concentrations of boron surprisingly provided excellent grain refining in a silver/germanium alloy, imparting greater strength and ductility to the alloy compared with a silver/germanium alloy without boron. The boron in the alloy inhibited grain growth even at temperatures used in the jewellery trade for soldering, and samples of the alloy were reported to have resisted pitting even upon heating repeatedly to temperatures where in conventional alloys the copper/germanium eutectic in the alloy would melt. Strong and aesthetically pleasing joints between separate elements of the alloy could be obtained without using a filler material between the free surfaces of the two elements and a butt or lap joint could be formed by a diffusion process or resistance or laser welding techniques. Compared to a weld in Sterling silver, a weld in the above described alloy had a much smaller average grain size that improved the formability and ductility of the welds, and an 830 alloy had been welded by plasma welding and polished without the need for grinding.

Argentium (Trade Mark) sterling comprises Ag 92.5 wt % and Ge 1.2 wt %, the balance being copper and about 4 ppm boron as grain refiner. The Society of American Silversmiths maintains a website for commercial embodiments of the above-mentioned alloys known as Argentium (Trade Mark) at the web address http://www.silversmithing.com/largentium.htm. It discloses that Argentium Sterling is precipitation hardenable, that a doubling in final hardness can be achieved by heating at temperatures obtainable in a domestic oven e.g. 450 F (232 C) for about 2 hours or 570 F (299 C) for about 30 minutes and that the hard alloy can be softened by conventional annealing and then hardened again if required. Hardening is desirable in the case of Argentium sterling because when fully annealed and quenched, its hardness is below 80 Vickers which is less than is desirable for many end uses.

Precipitation hardening of conventional sterling silver can be achieved by (a) heating the alloy to or above775 C, (b) holding the alloy at that temperature for 15-30 minutes for annealing thereof (i.e. dissolving all the copper in the silver), (c) quenching rapidly in cold water, which prevents formation of Cu-rich coarse precipitates which are ineffective in bringing about hardening, (d) re-hardening the softened alloy by heating to e.g. 300 C. for 30-60 minutes resulting in the formation of very fine Cu-rich particles which are effective in bringing about hardening and (e) air cooling. The annealing temperatures involved are very high and are close to the onset of melting. Silversmiths therefore regard precipitation hardening of sterling silver as of metallurgical interest only.

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. 29.) and it is unnecessary because sterling silver as produced generally has hardness of 70 Vickers or above. Alloys of higher.Vickers hardness are obtained by work hardening rather than precipitation hardening. 2 ' Annealing of sheet is used for softening the sheet which has become work hardened during rolling from its as-cast thickness so that additional rolling operations can be carried out to achieve a required thickness without cracking of the sheet. By way of background, US-A-4810308 (Leach & Garner) discloses a hardenablc s.

silver alloy comprising not less than 90% silver; not less than 2.0% copper; and at least one metal selected from the group consisting of lithium, tin and antimony. The silver alloy can also contain up to 0.5% by weight of bismuth. Preferably, the metals comprising the alloy are combined and heated to a temperature not less than 1250-1400 F (676 760 C) e.g. for about 2 hours to anneal the alloy into a solid solution, a temperature of 1350 (732 C) being used in the Examples. The annealed alloy is then quickly cooled to ambient temperature by quenching. It can then be age hardened by reheating to 300-700 F (149-371 C) for a predetermined time followed by cooling of the age hardened alloy to ambient temperature. The age-hardened alloy demonstrates hardness substantially greater that that of traditional sterling silver, typically 100 HVN (dickers Hardness Number), and can being returned by elevated temperatures to a relatively soft state. The disclosure of US-A-4869757 (Leach & Garner) is similar. In both cases the disclosed annealing temperature is higher than that of Argentium, and neither reference discloses firestain or tarnish-resistant alloys. The inventor is not aware of the process disclosed in these patents being used for commercial production.

A silver alloy called Steralite is said to be covered by US-A-05817195 and 5882441 and to exhibit high tarnish and corrosion resistance. The alloy of US-A-5817195 (Davitz) contains 90-92.5 wt % Ag, 5.75-5.5 wt % Zn, 0.25 to less than I wt % Cu. 0.25 0.5 wt % Ni, 0.1 -0.25 wt % Si and 0.0-0.5 wt % In. The alloy of US-A- 5882441 (Davitz) contains 90-94 wt % Ag, 3.5-7.35 wt % Zn, 1-3 wt % Cu and 0.1-2.5 wt % Si. A similar high zinc low copper alloy is disclosed in US-A-4973446 (Bernhard) and is said to exhibit reduced firestain, reduced porosity and reduced grain scale. None of these references discusses annealing or precipitation hardenining.

SUMMARY OF THE INVENTION..

We have now found that Ag-Cu-Ge alloy workpieces heated to an annealing temperature are self-hardening on controlled air cooling, and that products of useful hardness can be obtained without the need for reheating to effect annealing and/or.

precipitation hardening. The use of reheating to e.g. 180-350 C, and preferably 250- A. . 300 C, to develop further hardness is, however, also possible according to the invention.

Significantly it has been found that over-aging of Ag-Cu-Ge alloys during precipitation hardening does not cause a significant crop-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, reduces the number of process steps required to produce articles of a required hardness and in particular eliminates quenching e.g. with water which as explained above is required for Ag-Cu Sterling silver.

The present invention provides a process for making a finished or semifinished article of silver alloy, said process comprising the steps of: 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; making or processing the finished or semi-finished article of the alloy by heating at least to an annealing temperature; and gradually air-cooling the article from the temperature at which it is made or processed to develop a Vickers hardness of more than 70.

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 little 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. Furthermore, the addition of germanium to sterling silver changes the thermal conductivity of the new alloy, compared to standard sterling silver. The International Annealed Copper Scale (IACS) is a measure of conductivity in metals. On this scale the value of copper is 100%, pure silver is 106%, and standard sterling silver 96%, while a sterling alloy containing 1.1% germanium has a conductivity of 56%. The significance of this is that the Argentium sterling and other germanium-containing silver alloys do not dissipate heat as quickly as standard sterling silver or their non- germanium-containing equivalents, a piece will take longer to cool, and precipitation hardening to a commercially useful level (preferably to 110 or above, more preferably to 115 or above) can take place during natural air cooling or during slow controlled air cooling.

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, the workpiece preferably spending at least 15 minutes and typically 30 minutes to I hour in the range 200-300 C where precipitation hardening proceeds rapidly. 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 f ask.

DESCRIPTION OF PREFERRED EMBODIMENTS

Alloys that may be used in the above process The alloys that may be treated according to the invention include an alloy of at least 77 wt% silver containing copper and an amount of germanium that is effective to reduce firestain and/or tarnishing. The inventor considers that 0.5 wt% Ge provides a lower limit and that in practice use of less than lwt% is undesirable, amounts of 1-1.5 wt% being preferred.

The ternary Ag-Cu-Ge alloys and quaternary Ag-Cu-Zn-Ge alloys that can suitably be treated by the method of the present invention are those having a silver content of preferably at least 80 wt%, and most preferably at least 92.5 wt %, up to a maximum of no more than 98 wt%, preferably no more than 97 wt%. The germanium content of the Ag-Cu-(Zn)- Ge alloys should be at least 0.5%, more preferably at least 1.1%, and most preferably at least 1.5%, by weight of the alloy, up to a maximum of preferably no more than 6.5%, more preferably no more than 4%. If desired, the germanium content may be substituted, in part, by one or more incidental ingredient elements selected from Al, Ba, Be, Cd, Co, Cr. Er, Ga, In, Mg, Mn, Ni, Pb, Pd. Pt. Si, Sn, Ti, V, Y. Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly adversely affected. The weight ratio of germanium to incidental ingredient elements may range from 100: 0 to 60: 40, preferably from 100: 0 to 80: 20. In current commercially available Ag-Cu-Ge alloys such as Argentium incidental ingredients are not added.

The remainder of the ternary Ag-Cu-Ge alloys, apart from impurities, incidental ingredients and any grain refiner, will be constituted by copper, which should be present in an amount of at least 0.5%, preferably at least 1%, more preferably at least 2%, and most preferably at least 4%, by weight of the alloy. For an '800 grade' ternary alloy, for example, a copper content of 18.5% is suitable.

The remainder of the quaternary Ag-Cu-Zn-Ge alloys, apart from impurities and any grain refiner, will be constituted by copper which should be present in an amount of at least 0.5%, preferably at least 1%, more preferably at least 2%, and most preferably at least 4%, by weight of the alloy, and zinc which should be present in a ratio, by weight, to the copper of no more than 1: 1. Therefore, zinc is optionally present in the silver-copper alloys in an amount of from O to 100 % by weight of the copper content. For an '800 grade' quaternary alloy, for example, a copper content of 10.5% and zinc content of 8% is

suitable.

In addition to silver, copper and germanium, and optionally zinc, the alloys preferably contain a grain refiner to inhibit grain growth during processing of the alloy.

Suitable grain refiners include boron, iridium, iron and nickel, with boron being particularly preferred. The grain refiner, preferably boron, may be present in the Ag-Cu- (Zn)-Ge alloys in the range from I ppm to 100 ppm, preferably from 2 ppm to 50 ppm, more preferably from 4 ppm to 20 ppm, by weight of the alloy.

In a preferred embodiment, the alloy is a ternary alloy consisting, apart from impurities and any grain refiner, of 80% to 96% silver, 0.1 % to 5% germanium and 1 % to 19.9% copper, by weight of the alloy. In a more preferred embodhnent, the alloy is a ternary alloy consisting, apart from impurities and grain refiner, of 92.5% to 98% silver, 0.3% to 3% germanium and 1% to 7.2% copper, by weight of the alloy, together with I ppm to 40 ppm boron as grain refiner. In a further preferred embodiment, the alloy is a ternary alloy consisting, apart from impurities and grain refiner, of 92.5% to 96% silver, 0. 5% to 2% germanium, and 1% to 7% copper, by weight of the alloy, together with 1 ppm to 40 ppm boron as grain refiner. A particularly preferred ternary alloy being marketed under the name Argentium comprises comprises 92.5-92.7 wt% Ag, 6.1-6.3 wt% Cu and about 1.2 wt% Ce.

Shaped or fabricated articles In one embodiment the article is a shaped or fabricated article e.g. of jewellery, woven mesh or chain, or of hollowware spun from sheet or tube and is treated by heating to a soldering or annealing temperature by passage through a continuous mesh belt conveyor brazing or annealing furnace. Such conveyors are available from e.g. Lindberg, of Watertown, Wl, USA and Dynalab of Rochester NY as mentioned above. Generally such articles will be a soldered or brazed assembly of two or more components.

When annealing, it is desirable that the furnace gas, although protective, should not deplete the surface layer of germanium, as this will reduce the tarnish resistance of the alloy and its resistance to firestain. Atmospheres may be of nitrogen, cracked ammonia (nitrogen and hydrogen) or hydrogen. The annealing temperature should preferably be within the range 620 - 650 C. It is desirable not to exceed a maximum temperature of 680 C. The annealing time for this temperature range is 30 to 45 minutes.

When brazing it should be noted that the addition of germanium lowers the melting temperature of the alloy by 59 F (15 C) relative to sterling silver. It is recommended that an "easy" or "extra easy" grade of solder should be used. The brazing temperature is preferably not more than 680 C, and preferably in the range 600-660 C. A low-melting solder (BAg-7) which may be used contains 56% silver, 22% copper, 17% zinc, and 5% tin. The BAg-7 solder (an international standard) melts at 1205 F (652 C) Solders containing germanium, which will give better tarnish protection are described in UK Patent Application 03 26927.1 filed 19 November 2003, the contents of which are incorporated by reference. A suitable solder which melts in the range 600-650 C comprises about 58 wt % Ag, 2 wt % Ge, 2.5 wt % Sn, 14.5 wt % Zn 0.1 wt % Si, 0.14 wt % B. and the balance Cu., a practically used variant of that solder having the analysis 58.15wt%Ag, 1.51 wt%Ge, 2.4 wt%Sn, 15.1 wt%Zn,0.07wt%Si, 0.14 wt%B, and the balance Cu.

Articles that are brazed by passage through a brazing furnace will, of course, have simultaneously been annealed. It has been found that precipitation hardening can develop without a quenching step by controlled gradual air-cooling in the downstream cooling region of the furnace. For this purpose, it is desirable that the material should spend at least about 10-15 minutes in the temperature range 200-300 C which is most favourable for precipitation hardening. Articles which have been brazed in a furnace in this way and gradually cooled have achieved hardness of 110115 Vickers.

Compared to what is required for sterling silver, it will be noted that what is necessary for Argentium sterling and other germanium-containing silver alloys involves a reduced number of processing steps with avoidance of quenching and reheating, so that precipitation hardening to a required hardness can be achieved following oven brazing of finished or semi-finished articles.

Investment cast articles Argentium casting grain is melted using traditional methods (solidus 766 C, 1iquidus 877 C) and is cast at a temperature of 950-980 C and at a flask temperature of not more than 676 C under a protective atmosphere or with a protective boric acid flux.

Flask temperatures during investment casting may be e.g. 500-700 C and it has been found that sound castings are relatively insensitive to flask temperature. The investment material which is of relatively low thermal conductivity provides for slow cooling of the cast pieces.

Investment casting with air-cooling for 15-20 minutes followed by quenching of the investment flask in water after 15-20 minutes gives a cast piece having a Vickers hardness of about 70 which is approximately the same hardness as sterling silver.

Surprisingly it has been found that a harder cast piece can be produced by allowing the flask to cool in air to room temperature, the piece when removed from the flask having a Vickers hardness of about 110. Most standard investment removers will successfully remove the investment powder, as will a pneumatic hammer whose vibration can break up the investment. A water-knife can also be used for removing the investment. The production by casting of pieces that combine this degree of hardness with firestain and tarnish resistance has not been reported.

Even more surprisingly, and contrary to experience with Sterling silver, where necessary, the hardness can be increased even further by precipitation hardening e.g. by placing the castings or the whole tree in an oven set to about 300 C for 45 minutes to give heat-treated castings of approaching 125 Vickers.

In particular, as explained by Fischer-Buhner (supra) at p. 41, with conventional sterling silver simple slow cooling of flasks after casting results in growth of coarse Cu- rich precipitates and eliminates the possibility of precipitation hardening during a subsequent aging treatment. Water quenching is required within a narrow and critical range of times after casting, typically 4 minutes after casting, the hardening effect being reduced both by quenching too soon and too late. In the case of pieces cast on a tree different cooling conditions at different places on the tree prior to quenching result in the individual cast pieces differing in their ability to become hardened during the subsequent precipitation hardening step. All these problems of additional processing steps and control difficulties are avoided by the use of Ag-Cu-Ge alloys as described herein.

e me,

Claims

1. A process for making a finished or semi-finished article of silver alloy, said process comprising the steps of: 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; making or processing the finished or semi- finished article of the alloy by heating at least to an annealing temperature; and gradually air cooling the article from the temperature at which it is made or processed to develop a Vickers hardness of more than 70. :.

2. The process of claim 1, wherein the article is formed of a ternary alloy of silver, : copper and germanium. 15, .

3. The process of claim 2, wherein the ternary alloy consists, apart from impurities, .

incidental ingredients and any grain refiner, of 80-96% silver, 0. 1-5% germanium and 1- . 19.9% copper, by weight of the alloy. ,

4. The process of claim 2, wherein the ternary alloy consists, apart from impurities, incidental ingredients and grain refiner, of 92.5-98% silver, 0.3-3% germanium, and 1 7.2% copper, by weight of the alloy, together with 1-40 ppm boron as grain refiner.

5. The process of claim 2, wherein the ternary alloy consists, apart from impurities, incidental ingredients and grain refiner, of 92.5-96% silver, 0.5-2% germanium, and 1-7% copper, by weight of the alloy, together with 1-40 ppm boron as grain refiner.

6. The process of claim 2, wherein the ternary alloy comprises 92.5-92.7 wt% Ag, 6.1-6.3 wt% Cu. about 1.2 wt% Ge and 1-20 ppm boron as grain refiner.

7. The process of any preceding claim, wherein annealing is during brazing the article in a furnace, and hardening is by subsequent air cooling.

8. The process of claim 7, wherein the alloy is annealed or brazed by heating in a furnace et 600-680 C.

9. The process of claim 7, wherein the alloy is annealed or brazed by heating in a furnace at 600-660 C.

10. The process of claim 7, 8 or 9, wherein the alloy is brazed using a solder which comprises 56% silver, 22% copper, 17% zinc, and 5% tin.

11. The process of claim 7, 8 or 9, wherein the alloy is brazed using a solder which comprises58wt%Ag,2wt%Ge, 2.5 wt%Sn, 14.5wt%ZnO.1 wt%Si,0. 14wt%B, .... :.

and the balance Cu. . A:

12. The process of any of claims 7-11, wherein annealing and/or brazing is carried out at a. temperature of from 600-650 C

13. The process of any of claims 1-6, wherein annealing is during investment casting, and hardening is by air-cooling the investment or allowing it to air cool.

14. The process of claim 13, wherein the article is of jewellery or giftware.