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WO2006123190A1 - Fabrication d'alliages argent-cuivre-germanium - Google Patents

Fabrication d'alliages argent-cuivre-germanium Download PDF

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Publication number
WO2006123190A1
WO2006123190A1 PCT/GB2006/050116 GB2006050116W WO2006123190A1 WO 2006123190 A1 WO2006123190 A1 WO 2006123190A1 GB 2006050116 W GB2006050116 W GB 2006050116W WO 2006123190 A1 WO2006123190 A1 WO 2006123190A1
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WO
WIPO (PCT)
Prior art keywords
alloy
boron
silver
copper
alloys
Prior art date
Application number
PCT/GB2006/050116
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English (en)
Inventor
Peter Gamon Johns
Original Assignee
Middlesex Silver Co. Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from GB0510243A external-priority patent/GB2426250A/en
Priority claimed from GB0523002A external-priority patent/GB0523002D0/en
Application filed by Middlesex Silver Co. Limited filed Critical Middlesex Silver Co. Limited
Priority to EP06744318A priority Critical patent/EP1888797A1/fr
Publication of WO2006123190A1 publication Critical patent/WO2006123190A1/fr
Priority to US13/036,288 priority patent/US9708691B2/en
Priority to US14/884,885 priority patent/US10323310B2/en

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C5/00—Alloys based on noble metals
    • C22C5/06—Alloys based on silver
    • C22C5/08—Alloys based on silver with copper as the next major constituent
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C1/00—Making non-ferrous alloys
    • C22C1/02—Making non-ferrous alloys by melting
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C1/00—Making non-ferrous alloys
    • C22C1/02—Making non-ferrous alloys by melting
    • C22C1/03—Making non-ferrous alloys by melting using master alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C1/00—Making non-ferrous alloys
    • C22C1/06—Making non-ferrous alloys with the use of special agents for refining or deoxidising
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C5/00—Alloys based on noble metals
    • C22C5/06—Alloys based on silver
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/14—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of noble metals or alloys based thereon

Definitions

  • This invention relates to a method of manufacturing precious metal alloy, and to precious metal products made of the above alloy.
  • Suwa Seikosha discloses a tarnish- resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10 wt% Sn, 0.5-12 wt% In, and 0.1-5 wt% Zn, the remainder being silver.
  • the Sn, In and Zn are alleged to be synergistic in their effects.
  • Small quantities of Ti, Zn, Be, Cr, Si, Al, Ge and Sb when used in addition to Sn, In and Zn are alleged to further increase resistance to tarnishing by sulphur-containing materials.
  • the alloy is said not to suffer from f ⁇ restain because of the absence of copper but this property is not confirmed by torch annealing experiments carried out by the present applicants and furthermore the alloy is soft.
  • US-A-4973446 (Bernhard et ah, United Precious Metal Refining) discloses a silver alloy composition of the Sn, In, Zn type that also contains copper and boron. It comprises 89-93.5 wt% Ag, 0.01-2 wt% Si, about 0.001-2 wt% B, about 0.5-5 wt% Zn, about 0.5-6 wt% Cu, about 0.25-2 wt% Sn, and about 0.01-1.25 wt% In. Silicon is added as a de-oxidant. Boron is added to reduce the surface tension of the molten alloy, and to allow it to blend homogeneously.
  • Zinc is added to reduce the melting point of the alloy, to add whiteness, to act as a copper substitute, to act as a deoxidant, and to improve fluidity of the alloy.
  • Copper is added as a conventional hardening agent for silver, as well as acting as the main carrying agent for the other materials.
  • Tin is added to improve tarnish resistance, and for its hardening effect.
  • Indium is added as a grain- refining agent, and to improve the wetability of the alloy.
  • Silver must be present in the necessary minimal percentage to qualify as either coin silver or sterling silver. In the experience of the present inventors, although tarnish resistance is exhibited to some extent, together with some f ⁇ restain reduction on investment casting, firestain resistance on soldering or annealing is not obtained because of the copper content.
  • US-A-5039479 (Bernhard, United Precious Metals) discloses a silver alloy composition allegedly exhibiting the desirable properties of reduced fire scale, reduced porosity, reduced grain size and reduced oxide formation when heated. It consists essentially of about 89-93.5 wt% Ag, about 0.02-2 wt% Si, about 0.001-2 wt% B, about
  • US-A-5817195 discloses an alloy alleged to be highly tarnish resistant, corrosion resistant and non-brittle, which comprises 90% to 92.5 wt% Ag, 0.25% to 0.5 wt% Ni, 0% to 0.5 wt% In, metal silicate consisting of 5.75% to 7.5 % zinc by weight of the silver coloured alloy, 0.25% to ⁇ 1 wt% copper by weight of the silver coloured alloy and 0.1% to 0.25 wt% silicon by weight of the silver coloured alloy.
  • US-A-5882441 discloses a silver coloured allegedly highly tarnish resistant, corrosion resistant and non-brittle free alloy consisting essentially of 90% to
  • a preferred alloy formula is 92.5 wt% Ag, 4.5 wt% Zn, 2.9 wt% Cu and 0.1 wt% silicate
  • US-A-6841012 discloses an allegedly tarnish-resistant silver alloy comprising at least 85 wt% Ag, the balance including zinc, copper, indium, tin and iron, see also WO 04/097056 (Croce, Steridyne Laboratories, Inc). Copper contents of up to 1.5 wt% are disclosed, e.g. an allegedly a tarnish resistant silver alloy including at least
  • US-A-2005/0186107 discloses a silver-coloured, tarnish- resistant, corrosion-resistant alloy. It includes 92.5-95 wt% silver combined with a master alloy of 24-34 wt% Zn, 60-74 wt% Cu, 0.5-1.8 wt% Si and 0.0-8.0% Sn or 0.0- 1.5 wt% In.
  • the alloy can be used for jewellery items, tableware, dental items or other items that should resist tarnishing or corrosion and which require a non-brittle alloy.
  • It consists essentially of the about 92.5-92.8 wt% Ag, about 2.0-3.0 wt% Cu, about 2.0-3.0 wt% Zn, about 0.03-0.05 wt% In, about 0.01-0.03 wt% Sn, about 0.20-0.50 wt% Cu/B alloy (2.0 wt% B, 98.0 wt% Cu) about 0.50-0.90 wt% Si/Cu alloy (10.0 wt% Si, 90.0 wt% Cu), and 0.01%-0.10 wt% Mn.
  • Patent GB-B-2255348 discloses a 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% 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 exerts a protective function that is 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 is 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 inhibits surface oxidation of that phase by forming a thin GeO and/or GeO 2 protective coating that prevents firestain during brazing and flame annealing.
  • 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.
  • the alloy is useful inter alia in jewellery and silversmithing.
  • US-A-6168071 (Johns) describes and claims inter alia a silver/germanium alloy having an Ag content of at least 77% by weight, a Ge content of between 0.5 and 3% by weight, the remainder being copper apart from any impurities, which alloy contains boron as a grain refiner at a concentration of up to about 20 parts per million. It further discloses providing the boron content by a master Cu/B alloy having a boron content of about 2 percent by weight. The boron in the copper/boron alloy is elemental boron. Providing the boron content within a Cu/B alloy is stated to overcome the problem of handling pure boron, which typically is a lightweight powder.
  • Such copper/boron master alloys are said to be commonly available, and for example Belmont Metals Inc offers on its website a copper-based master alloy containing 2 wt% boron with any of As, Be, Cd, Cr, Fe, Li, Mg, Ni, P, Si, Te, Ti, Zn or Zr.
  • US 6726877 discloses an allegedly fire scale resistant, work hardenable jewellery silver alloy composition
  • the alloy may also include rheology modifying and other additives to aid in improving the castability and/or wetting performance of the molten alloy. For example, about up to 3.5% by weight of a modifying additive selected from In, B or a mixture thereof may be added to the alloy to provide grain refinement and/or provide greater wettability of the molten alloy.
  • the copper content of a 925 alloy is recommended to be in the range of 2-3 wt%, the amount of zinc being 2-4 wt%, and silicon being added in proportion to the amount of zinc incorporated and being preferably 0.15-0.2 wt%.
  • the compositions may be formed by the addition of a master alloy to fine silver, the master alloy comprising e.g. 52.5 - 99.85 wt % Cu, 0.1 - 35 wt % Zn and 0.05 - 12.5 wt% Ge.
  • the present applicants have not confirmed fire resistance of available embodiments of the alloy, especially during torch annealing.
  • Argentium As used herein refers to these alloys.
  • the 925 grade Argentium alloy comprises 92.5 wt% (minimum) Ag, 1.1-1. 3 wt% Ge, 6 ppm B, the balance being copper and impurities.
  • the alloy shows excellent resistance to tarnishing even under very arduous conditions.
  • 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.
  • 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.
  • 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.
  • 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.
  • the invention of WO 2003/106567 therefore provides 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, impurities and grain refiner, copper.
  • a typical alloy that has been found to be suitable contains about 94.5 wt% Ag, about 4.3 wt% Cu and about 1.2 wt% Ge.
  • the weight ratio of Cu to Ge is about 3.6 : 1 whereas in the existing 925 grade Argentium the ratio can be from 5.8 : 1 (1.1 wt% Ge) to 4.8 : 1 (1.3 wt% Ge).
  • the ratio is preferably from 4: 1 to 3: 1, more preferably about 3.5 : 1.
  • 4: 1 the alloy is more likely to exhibit firestain, whereas below 3: 1 the high germanium content gives rise to formability problems.
  • preferred Ag contents ranged from about 94.0 to about 95.5 wt%, lower values being preferred for reducing the expense of the silver.
  • the invention provides a shaped object of Ag-Cu-Ge of a silver alloy including at least 92.5 wt% Ag, 0.5-2 wt% Cu and 0.1-3 wt% Ge together with boron as grain refiner, the object being precipitation hardened to at least 65 HV, e.g. to at least 70 HV and preferably to at least 75 HV.
  • the invention provides 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, an amount of germanium that is at least 0.5 wt% and is effective to reduce tarnishing and/or firestain and boron incorporated by dispersing throughout said alloy a compound selecting from the group consisting of alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof; making or processing the finished or semi- finished article of the alloy by heating at least to an annealing temperature; cooling the article gradually, without an abrupt cooling step, so that cooling to ambient temperature takes more than 10 seconds; and reheating the article to effect precipitation hardening thereof.
  • IACS International Annealed Copper Scale
  • 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 (e.g. to about Vickers hardness 70 or above, preferably to Vickers hardness 110 or above, more preferably to 115 or above) can take place during natural air cooling or during slow controlled air cooling.
  • a commercially useful level e.g. to about Vickers hardness 70 or above, preferably to Vickers hardness 110 or above, more preferably to 115 or above
  • the ability of the alloys of the Ag-Cu-Ge family to precipitation harden to useful values without the need for quenching is retained to copper contents as low as lwt% or even as low as 0.5 wt%, whereas other silver alloys become unacceptably soft at such low copper contents and cannot be hardened sufficiently by heat treatment.
  • the ability of the present silver alloys containing 0.5 wt% Cu or above e.g. 1 wt% copper or above and optionally zinc and/or palladium as well as germanium to precipitation harden makes it practical to reduce the copper content of the alloy. Even though an alloy of lower copper content may be relatively soft as cast, reheating at a low temperature e.g. 150°C or 200°-400°C e.g.
  • 300°C may bring the hardness up to the level of normal sterling silver or better. This is a significant advantage because from the standpoint of corrosion resistance the copper content is the most detrimental part of the alloy, but reduction of copper in a standard Sterling alloy gives rise to unacceptably low hardness. In the present alloys, if the copper content is reduced, the silver content may simply be increased or there may be incorporated zinc e.g. in an amount of 1-2 wt%. Other possibilities include increasing the germanium content or adding further zinc or another alloying element e.g. palladium .
  • Silver alloy of Ag 973 parts per thousand and containing about 1.0 wt% Ge, balance copper, has been successfully precipitation hardened by gradual air cooling from an annealing temperature, and it is believed that Ag-Cu-Ge alloys with silver content above this level are also precipitation hardenable. Significant hardness has been reported for air cooled/quenched Ag-Zn-Cu alloys containing as little as 0.8 wt% Cu.
  • annealing temperatures may be defined to be temperatures above 500°C, whereas precipitation hardening temperatures may be defined to be in the range 150°C - 400°C, the lower value of 150°C permitting embodiments of the alloys of the invention to be precipitation hardened in a domestic oven.
  • Silver alloys of the first group comprise 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 or impurities.
  • Alloys of this group also comprise 92.5 - 97 wt% Ag, 1-3 wt% Cu, 1-4 wt%
  • the alloys of this group may be provided as casting grain as aforesaid containing silicon in an amount effective to produce an as-cast silvery appearance and inhibit mould reactions in articles made by investment casting. Such reactions are generally nor detrimental to the properties of finished products, but require processing for their removal and can be disconcerting for those new to the use of the present alloys.
  • the alloys of this group therefore include silver alloy casting grain comprising 92.5 - 97 wt% Ag, 1-4.5 wt% Cu, 0.4-4 wt% Zn, 0.8-1.5 wt% Ge, 0.05-2 wt% Si, 0 to 0.2 wt% In or Sn and 0-0.2 wt% Mn, the balance being boron as grain refiner, incidental ingredients and impurities.
  • 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.
  • the invention therefore contemplates alloys containing Sn, In or Si in an amount effective to suppress zinc oxide formation during torch annealing. Surprisingly, the presence of zinc has not proved detrimental to the ability of Ag-Cu-Ge based silver alloys to precipitation harden using the methods described below. If desired palladium may be added in partial or total replacement of zinc in an amount of up to 3 wt%.
  • Boron may be added as described below. Grain refining Ag-Cu-Ge-based silver jewellery or silversmithing alloys using sodium borohydride can also improve hardness by about 10 HV. An optimum combination of Ag, Zn and sodium borohydride can produces a sterling silver alloy with improved annealed hardness and superior mechanical properties. Furthermore, sodium from e.g. sodium borohydride could make the alloy useful as an electrical contact material. Sodium at ppm levels has arc- quenching properties, as also does germanium.
  • the germanium or copper content may be substituted, in part, by one or more incidental ingredient elements selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected.
  • the weight ratio of germanium to incidental ingredient elements may range from 100: 0 to 80: 20, preferably from 100: 0 to 60: 40.
  • the term "incidental ingredients" permits the ingredient to have ancillary functionality within the alloy e.g. to improve colour or as- moulded appearance, and includes the previously mentioned metals or metalloids Si, Zn, Sn or In in amounts appropriate for "deox".
  • Silicon in particular, may be added to silver alloys for casting grain e.g. in an amount of up to 0.5 wt % more usually 0.1-0.2 wt%, and is conveniently provided in the form of a copper-silicon master alloy containing e.g. about 10 wt% Si.
  • a silver-copper-germanium ternary alloy it can provide bright investment castings immediately on removal from the mold. It may be added to casting grain e.g. before investment casting or it may be incorporated into the silver at the time of first melting to form an alloy.
  • a second group of silver alloys 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.
  • the alloys of this second group may be made by melting together the ingredients; and incorporating boron by dispersing into molten silver alloy to form the whole or a precursor part of said alloy a compound selecting from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof.
  • Shaped articles or casting grain of the alloys of this group are also within the invention.
  • a method of making a shaped article which comprises: providing a molten silver alloy comprising 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; casting an article in a mould with said molten silver alloy; allowing said cast article to cool; and reheating said article to effect hardening thereof.
  • Silver contents of this group of alloys may be from 92.5-96 wt%, preferably 92.5-95.5 wt%, and most preferably at or closely above 92.5 wt%. For present purposes, it is desirable to maximize the content of zinc which is inexpensive compared to silver.
  • Copper content is desirably in the range 0.7-3 wt%, preferably about 1-2 wt% and especially about 1.5 wt%.
  • a certain amount of copper is desirable for hardness and ability to precipitation harden, but the proportion of copper is relatively low in order to minimise tarnishing.
  • Zinc content may be in the range 1-4.5 wt% especially about 2-4 wt%, and should preferably be close to the upper limits of the above ranges.
  • zinc-containing alloys have in the past been relatively soft, and it is the ability of germanium-containing alloys to precipitation harden that has enabled useful hardnesses of the order of 100 HV to be achieved with relatively high zinc content alloys.
  • the very fine grain structure consequent on the addition of sodium borohydride or other decomposable boron compounds also contributes to the achievement of unexpectedly good hardness and/or other physical at high zinc content because of the excellent grain refinement that can be achieved.
  • One unexpected advantage is that the borohydride-treated alloys exhibit relatively high ductility compared to standard material.
  • Manganese has unexpectedly been found to improve tarnish resistance and is incorporated in amounts of e.g. 0.02-0.2 wt%, especially about 0.1-0.2 wt%.
  • An additional or alternative advantage of manganese is that it adds hardness in the annealed state after a slow cool.
  • silicon it is preferable to use silicon to give brighter castings. Silicon is also believed to inhibit the formation of zinc oxide. Amounts of silicon may be 0.04-0.25 wt% e.g. about 0.1 wt%.
  • the alloy may contain one or more incidental ingredients known per se in the production of silver alloys in amounts that are not detrimental to the mechanical strength, tarnish resistance and other properties of the material.
  • Cadmium may also be added in similar amounts although its use is presently not preferred.
  • Tin may be beneficial, typically in an amount of 0.5 wt%.
  • Indium may be added in small quantities e.g. as a grain refiner and to improve the wetability of the alloy.
  • Other possible incidental ingredient elements selected from Al, Ba, Be, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected.
  • Boron is incorporated into the present silver alloys as a grain refiner and may be incorporated as described below.
  • the invention also relates to Ag-Cu-Ge alloys of Ag content at least 92.5 wt% and Ge content 0.1-3 wt%, preferably 0.8-1.5 wt%, more preferably about 0.8-1 wt% that have copper contents below 2 wt %, e.g. below 1.7 wt%, e.g. below 1.5 wt % e.g. below 1 wt% down to e.g. 0.5 wt% (in some embodiments as low as 0.2 wt%), become precipitation hardened following cooling from a melting or annealing temperature by baking at e.g. 200°C-400°C and on baking can achieve a hardness of 65 HV or above e.g. 70 HV or above.
  • the invention also relates to shaped articles of the alloys mentioned above that have been precipitation hardened to 65 HV or above e.g. 70 HV or above.
  • the alloys of this group in typical embodiments have silver contents of 93-94 wt%.
  • the combined content of Cu and Ge ranges from 0.6 wt% to 5 wt% leaving at least 1 wt% and usually at least 3 wt%, more usually at least 4 wt% of other silver- compatible metals that make up the remainder of the alloy together with incidental ingredients (if any) and impurities. There are various possibilities for making up the balance of the alloy.
  • One possibility is to avoid other alloying ingredients, except incidental ingredients (if any), and increase the amount of silver e.g. up to 97 wt%. As previously explained, if the silver content rises above 96 wt%, then the problem of firestain reappears, but in some embodiments this disadvantage can be accepted. In other embodiments up to 4.5 wt% Zn e.g. 3-4 wt%, up to 2.5 wt% Sn e.g. about 1 wt% Sn or up to 3 wt% Pd may be incorporated into the alloy.
  • Manganese has been found to improve tarnish resistance and may be incorporated in amounts of e.g. 0.02-0.2 wt%, especially about 0.1-0.2 wt%. As previously explained, an additional or alternative advantage of manganese is that it adds hardness in the annealed state after a slow cool.
  • silicon For casting alloys, it is preferable to use silicon to give brighter castings. Silicon is also believed to inhibit the formation of zinc oxide. Amounts of silicon may be 0.04- 0.25 wt% e.g. about 0.1 wt%.
  • the alloy may contain up to 0.5 wt% of one or more incidental ingredients known per se in the production of silver alloys in amounts that are not detrimental to the mechanical strength, tarnish resistance and other properties of the material.
  • Cadmium may also be added although its use is presently not preferred.
  • Indium may be added in small quantities e.g. as a grain refiner and to improve the wetability of the alloy.
  • Other possible incidental ingredient elements selected from Al, Ba, Be, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly affected.
  • Boron may be incorporated into the alloy using the procedures indicated below. With the relatively low quantities of copper present in this group of alloys, it is considered particularly advantageous to incorporate the boron as a compound selecting from alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof.
  • a boron compound may be introduced into molten silver alloy in the gas phase, advantageously in admixture with a carrier gas which assists in creating a stirring action in the molten alloy and dispersing the boron content of the gas mixture into said alloy.
  • Suitable carrier gases include, for example, hydrogen, nitrogen and argon.
  • the gaseous boron compound and the carrier gas may be introduced from above into a vessel containing molten silver e.g. a crucible in a silver-melting furnace, a casting ladle or a tundish using a metallurgical lance which may be a elongated tubular body of refractory material e.g.
  • graphite or may be a metal tube clad in refractory material and is immersed at its lower end in the molten metal.
  • the lance is preferably of sufficient length to permit injection of the gaseous boron compound and carrier gas deep into the molten silver alloy.
  • the boron-containing gas may be introduced into the molten silver from the side or from below e.g. using a gas-permeable bubbling plug or a submerged injection nozzle.
  • a gas-permeable bubbling plug or a submerged injection nozzle.
  • the alloy to be heated is placed in a solid graphite crucible, protected by an inert gas atmosphere which may for example be oxygen- free nitrogen containing ⁇ 5 ppm oxygen and ⁇ 2 ppm moisture and is heated by electrical resistance heating using graphite blocks.
  • an inert gas atmosphere which may for example be oxygen- free nitrogen containing ⁇ 5 ppm oxygen and ⁇ 2 ppm moisture and is heated by electrical resistance heating using graphite blocks.
  • Such furnaces have a built-in facility for bubbling inert gas through the melt.
  • Compounds which may be introduced into molten silver or gold or alloys thereof in this way include boron trifluoride, diborane or trimethylboron which are available in pressurised cylinders diluted with hydrogen, argon, nitrogen or helium, diborane being preferred because apart from the boron, the only other element is introduced into the alloy is hydrogen.
  • a yet further possibility is to bubble carrier gas through the molten silver to effect stirring thereof and to add a solid boron compound e.g. NaBH 4 or NaBF 4 into the fluidized gas stream as a finely divided powder which forms an aerosol.
  • a boron compound may also be introduced into the molten silver or gold alloy in the liquid phase, either as such or in an inert organic solvent.
  • Compounds which may be introduced in this way include alkylboranes or alkoxy-alkyl boranes such as triethylborane, tripropylborane, tri- «-butylborane and methoxydiethylborane which for safe handling may be dissolved in hexane or THF.
  • the liquid boron compound may be filled and sealed into containers of silver or of copper foil resembling a capsule or sachet using known liquid/capsule or liquid/sachet filling machinery and using a protective atmosphere to give filled capsules sachets or other small containers typically of capacity 0.5-5 ml, more typically about 1-1.5 ml.
  • the capsules or sachets may be of a polymer e.g. polyethylene or polypropylene.
  • the filled capsules or sachets in appropriate number may then be plunged individually or as one or more groups into the molten silver or gold or alloy thereof.
  • a yet further possibility is to atomize the liquid boron-containing compound into a stream of carrier gas which is used to stir the molten silver as described above.
  • the droplets may take the form of an aerosol in the carrier gas stream, or they may become vaporised therein.
  • the boron compound is introduced into the molten silver alloy in the solid phase, e.g. using a solid borane e.g. decaborane B 10 H 14 (m.p. 100°C, b.p. 213°C).
  • the boron is preferably added in the form of either a boron containing metal hydride or a boron containing metal fluoride.
  • suitable metals include sodium, lithium, potassium, calcium, zinc and mixtures thereof.
  • sodium is the preferred metal.
  • sodium borohydride, NaBH 4 which has a molecular weight of 37.85 and contains 28.75% boron.
  • Boron can be added to the other molten components both on first melting and at intervals during casting to make up for boron loss if the alloy is held in the molten state for a period of time, as in a continuous casting process for grain. This facility is not available when using a copper/boron master alloy because adding boron changes the copper content and hence the overall proportions of the various constituents in the alloy.
  • the boron content of finished pieces may be closer to the 1-20 ppm characteristic of the prior art, but the ability to achieve relatively high initial boron concentrations means that improved consistency may be achieved during the manufacturing stages and in the final finished products.
  • higher boron content is desirable for master alloys which will be melted with precious metal to make casting grain and then further melted to form rod, wire, or investment casting.
  • Articles made e.g. by casting the present alloy may be hardened by heating in an oven e.g. at about 300°C for about 45 minutes.
  • protective agent there may be used a compound containing a long chain alkyl group and a -SH or -S-S- group, e.g. an alkanethiol, dialkyl sulfide or dialkyl disulfides in which the chain is preferably at least 10 carbon atoms long and may be C 12 -C 24 .
  • the -SH or - S-S- compounds that many be used include straight chain saturated aliphatic compounds containing 16-24 carbon atoms in the chain, for example cetyl mercaptan (hexadecyl mercaptan) and stearyl mercaptan (octadecyl mercaptan) and cetyl and stearyl thioglyco Hates whose formulae appear below.
  • Octadecyl mercaptan is a white to pale yellow waxy solid that is insoluble in water and that melts at 30 0 C.
  • Hexadecyl mercaptan is also a white or pale yellow waxy solid that melts at 15-16°C.
  • Example 1 How the invention may be put into effect will now be further described with reference to the following Examples: Example 1
  • 80 ppm are packaged or wrapped in pure silver foil of thickness e.g. about 0.15mm.
  • the foil wrapper holds the pellets of sodium borohydride in a single group and impedes individual pellets becoming separated and floating the surface of the melt.
  • the wrapped pellets are placed into the hollow cupped end of a graphite stirring rod and plunged beneath the surface of the melt which at this stage is covered with a ceramic fibre blanket to quench the resulting flame from decomposition of the borohydride.
  • the hydrogen burns off over a period of about 1-2 minutes with a stirring action being applied, after which evolution of hydrogen ceases and the boron content is substantially incorporated into the melt together with at least some of the sodium which is believed innocuous to properties of the resulting alloy.
  • the crucible pivots to permits the molten alloy to be poured into a tundish whose bottom is formed with fine holes.
  • the molten silver pours into the tundish and runs through the holes in streams which break into fine pellets which fall into a stirred bath of water and become solidified and cooled.
  • the cast pellets are removed from the bath and dried.
  • the resulting alloy granules are used in investment casting using traditional methods and using a calcium sulphate bonded investment, and are cast at a temperature of 950-980°C and at a flask temperature of not more than 676°C under a protective atmosphere.
  • 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- 25 minutes followed by quenching of the investment flask in water after 15-25 minutes gives a cast piece having an expected Vickers hardness of about 70, which is approximately the same hardness as sterling silver.
  • the resulting casting has a matt silvery finish when removed from the mold, and an even finer grain structure than when Cu/B master alloy is used, due e.g.
  • the alloy can be polished easily, is free from boron hard spots, and gives products that exhibit excellent tarnish and firestain resistance. Precipitation hardening to expected hardness values of e.g. about 110 Vickers can be achieved by subsequent torch annealing, quenching and reheating in an oven at about 300°C.
  • 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 an expected Vickers hardness of about 110 which is similar to the value that can be achieved by the torch anneal/quench/reheat method. Contrary to experience with Sterling silver, where necessary, the hardness can be increased even further by precipitation hardening e.g. by placing castings or a whole tree in an oven set to about 300°C for 20-45 minutes to give heat-treated castings of an expected hardness approaching 125 Vickers.
  • Alloys were prepared with the compositions and boron i contents indicated in Table I below using CuB master alloys the source of boron.
  • Zinc containing alloys according to the invention are prepared as set out in Table II below and their hardness is measured.
  • boron is added as CuB master alloy; a further improvement is obtained using lithium borohydride as described above.
  • Zinc and manganese containing alloys according to the invention are prepared as set out in Table III below and their hardness is measured.
  • Table III boron is added as CuB master alloy; a further improvement is obtained using lithium borohydride as described above.
  • Sample 5.4 contains Mn. and has an air cooled hardness of 76 HV.
  • Sample 5.8 does not contain Mn but has an increase in Zinc, otherwise it is the same composition. Its annealed air cooled hardness 64 HV.
  • a germanium- containing Silver alloy of high copper content with no added zinc or manganese showed improved resistance both to droplets of the artificial sweat and to immersion in the artificial sweat but were subject to some discoloration.
  • Other germanium-containing silver alloys with no zinc content or with added zinc below about 0.8 wt% showed a similar level of sweat resistance. Increase in zinc content permitted germanium content to be somewhat reduced without significant detriment to sweat resistance.
  • Alloys of high silver content, zinc content of 1 wt% or above and germanium content of lwt% or above showed good sweat resistance properties. The best sweat resistance amongst the alloys tested is obtained with an alloy containing a high level of zinc with low copper and relatively low germanium content. Addition of Mn is believed to significantly improve sweat resistance. Results are indicated in Table IV.
  • the casting grain of the first stage of Example 1 made using sodium borohydride as grain refiner is re-melted with about 2 wt% zinc in a continuous caster and cast to give strip of composition about 95.7 wt% Ag, 2 wt% Zn, 1 wt% Ge, balance copper, grain refiner and impurities.
  • the resulting alloy strip can be made into shaped articles by stamping, further hardens by oven treatment at less than 500° e.g. 300°C for 30-45 min to >about 100 HV, and exhibits resistance to tarnish by artificial sweat and by French dressing.
  • An alloy is made comprising Ag 93.20 wt%, Cu 1.456 wt%, Ge 1.00 wt%, Mn 0.20 wt%, Zn 3.00 wt%, Pd 1.00 wt% and CuB 0.144 wt%. It combines the properties of firestain resistance, high Vickers hardness and resistance to corrosion in a salt spray test.
  • An alloy is cast comprising Ag 93.8 wt%, Zn 4.0 wt%, Ge 1.0 wt%, Cu 0.82 wt%, Mn 0.2 wt% and Cu/B 0.18 wt% (Cu 98.2 wt%, B 1.8 wt%).
  • the above alloy has an annealed and air-cooled hardness of about 75 HV, similar to that of standard sterling silver.
  • An alloy developed at Sheffield Hallam University and being sold under the trade name Carrs Lustre Silver by Carrs of Sheffield and which contains 1.6 wt% copper when treated under the same conditions has an HV of 45.
  • the cast samples are air-cooled from annealing temperature for 3 minutes, after which they were water quenched. It gives good performance in sweat resistance and tarnish resistance tests.
  • Precipitation hardening air cooled
  • Precipitation hardening quenched
  • Precipitation hardening quenched

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Abstract

Des alliages d'argent contenant du cuivre et du germanium, par exemple environ 1 % en poids de Ge et une très faible teneur en cuivre, par exemple, 0,8 % en poids de Cu peuvent être durcis par précipitation jusqu'à 65 HV ou plus, alors que les alliages ayant une teneur similaire en cuivre mais ne contenant pas de germanium restent mous. Dans un mode de réalisation, cette invention concerne un alliage d'argent qui comprend 92,5 à 97 % en poids d'Ag, 1 à 4,5 % en poids de Cu, 0,4 à 4 % en poids de Zn, 0,8 à 1,5 % en poids de Ge, 0 à 0,2 % en poids de Si, In ou Sn et 0 à 0,2 % en poids de Mn, le reste étant constitué de bore sous la forme d'agent d'affinage du grain, d'ingrédients imprévus et d'impuretés. Cet alliage comprend de préférence du bore sous la forme d'agent d'affinage du grain ajouté sous la forme d'hydrure de bore, par exemple sous la forme de borohydrure de sodium. Un autre groupe d'alliages comprend un alliage ternaire d'argent, de cuivre et de germanium contenant plus de 93,5 % en poids à 95,5 % en poids d'Ag, 0,5 à 3 % en poids de Ge, le reste, à part les ingrédients imprévus (éventuels), les impuretés et l'agent d'affinage du grain, étant constitué de cuivre, l'agent d'affinage du grain étant constitué par du borohydrure de sodium ou par un autre hydrure de bore. Un grain de moulage contenant du silicium qui donne des produits bruts de coulée clairs est également décrit. Dans un autre mode de réalisation, cette invention concerne un alliage d'argent contenant du zinc résistant au ternissement dans des conditions sévères, telles que l'exposition à la transpiration humaine ou à la vinaigrette, cet alliage comprenant 1 à 5 % en poids de Zn, 0,7 à 3 % en poids de Cu, 0,1 à 3 % en poids de Ge, 0 à 0,3 % en poids de Mn, 0 à 0,25 % en poids de Si, du bore B en quantité suffisante pour l'affinage du grain, jusqu'à 0,5 % en poids d'ingrédients imprévus, le reste étant constitué d'Ag en une quantité comprise entre 92,5 et 96 % en poids, et d'impuretés. Un procédé de fabrication préféré donnant un alliage ayant des propriétés physiques favorables consiste à faire fondre ensemble ces ingrédients et à y incorporer du bore, en dispersant dans l'alliage d'argent en fusion, en vue de former l'alliage dans son entier ou une partie précurseur de cet alliage, un composé choisi parmi les composés de bore alkyle, des hydrures de bore, des halogénures de bore, des hydrures métalliques contenant du bore, des halogénures métalliques contenant du bore et des mélanges de ceux-ci. Cet alliage est particulièrement approprié pour des moulages qui peuvent être durcis dans un four par exemple à environ 300 °C pendant 30 à 45 minutes.
PCT/GB2006/050116 2004-06-02 2006-05-19 Fabrication d'alliages argent-cuivre-germanium WO2006123190A1 (fr)

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US13/036,288 US9708691B2 (en) 2004-06-02 2011-02-28 Process for investment casting and casting grain for use in the process
US14/884,885 US10323310B2 (en) 2004-06-02 2015-10-16 Process for making finished or semi-finished articles of silver alloy

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GB0510243A GB2426250A (en) 2005-05-20 2005-05-20 Silver alloys
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GB0523002.4 2005-11-11
GB0523002A GB0523002D0 (en) 2005-11-11 2005-11-11 Silver alloy

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US12/830,612 Continuation-In-Part US9222150B2 (en) 2004-06-02 2010-07-06 Process for making finished or semi-finished articles of silver alloy

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Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2434376A (en) * 2006-01-23 2007-07-25 Middlesex Silver Co Ltd Making boron containing gold alloys using a master alloy
ITVR20090169A1 (it) * 2009-10-15 2011-04-16 Legor Group S R L Lega d'argento antitarnish e lega madre per l'ottenimento della stessa
WO2014203007A1 (fr) * 2013-06-21 2014-12-24 Argentium International Limited Compositions d'alliage et d'argent et procédés
KR101741681B1 (ko) 2015-09-17 2017-06-30 삼덕금속(주) 내변색성 및 경도가 우수한 Ag-Cu계 합금 조성물 및 이의 제조방법
GB2576080A (en) * 2018-06-01 2020-02-05 Allied Gold Ltd Treatment of articles of silver alloy
DE102022109312A1 (de) 2021-05-05 2022-11-10 Frieder Weidhase Vorrichtung zum beschleunigten, miniaturisierten Kühlen und Heizen in der Medizintechnik
RU2821457C2 (ru) * 2022-04-20 2024-06-24 Александр Николаевич Савченко Состав для получения сплава на основе серебра плавлением
CN118996359A (zh) * 2024-10-23 2024-11-22 深圳众诚达应用材料股份有限公司 一种铽靶材及其制备方法和稀土永磁体、新能源汽车
US12214416B2 (en) 2021-07-13 2025-02-04 James Avery Craftsman, Inc. Laser weldable sterling alloy

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6726877B1 (en) * 1993-11-15 2004-04-27 Anthony Phillip Eccles Silver alloy compositions
WO2004106567A1 (fr) * 2003-06-03 2004-12-09 Middlesex Silver Co. Limited Alliage ternaire d'argent
WO2005118903A1 (fr) * 2004-06-02 2005-12-15 Middlesex Silver Co. Limited Procede pour produire des articles finis ou semi-finis a partir d'un alliage d'argent comprenant du cuivre et du germanium
WO2006032933A1 (fr) * 2004-09-23 2006-03-30 Middlesex Silver Co. Limited Alliage pilote de cuivre et de bore et son utilisation dans la fabrication des alliages d’argent et de cuivre

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6726877B1 (en) * 1993-11-15 2004-04-27 Anthony Phillip Eccles Silver alloy compositions
WO2004106567A1 (fr) * 2003-06-03 2004-12-09 Middlesex Silver Co. Limited Alliage ternaire d'argent
WO2005118903A1 (fr) * 2004-06-02 2005-12-15 Middlesex Silver Co. Limited Procede pour produire des articles finis ou semi-finis a partir d'un alliage d'argent comprenant du cuivre et du germanium
WO2006032933A1 (fr) * 2004-09-23 2006-03-30 Middlesex Silver Co. Limited Alliage pilote de cuivre et de bore et son utilisation dans la fabrication des alliages d’argent et de cuivre

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2434376A (en) * 2006-01-23 2007-07-25 Middlesex Silver Co Ltd Making boron containing gold alloys using a master alloy
ITVR20090169A1 (it) * 2009-10-15 2011-04-16 Legor Group S R L Lega d'argento antitarnish e lega madre per l'ottenimento della stessa
WO2014203007A1 (fr) * 2013-06-21 2014-12-24 Argentium International Limited Compositions d'alliage et d'argent et procédés
KR101741681B1 (ko) 2015-09-17 2017-06-30 삼덕금속(주) 내변색성 및 경도가 우수한 Ag-Cu계 합금 조성물 및 이의 제조방법
GB2576080A (en) * 2018-06-01 2020-02-05 Allied Gold Ltd Treatment of articles of silver alloy
DE102022109312A1 (de) 2021-05-05 2022-11-10 Frieder Weidhase Vorrichtung zum beschleunigten, miniaturisierten Kühlen und Heizen in der Medizintechnik
US12214416B2 (en) 2021-07-13 2025-02-04 James Avery Craftsman, Inc. Laser weldable sterling alloy
RU2821457C2 (ru) * 2022-04-20 2024-06-24 Александр Николаевич Савченко Состав для получения сплава на основе серебра плавлением
CN118996359A (zh) * 2024-10-23 2024-11-22 深圳众诚达应用材料股份有限公司 一种铽靶材及其制备方法和稀土永磁体、新能源汽车

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