GB2561376A - Silver alloys, investment casting using the alloys and casting grain - Google Patents

Abstract

A silver-based alloy which comprising (by weight): 92.5-96 % silver, 0.1-1.5 % germanium, 3-60 ppm iridium, 0-2 % zinc, 0-0.4 % tin, 0-0.3 % indium, 0-0.08 % silicon and 0-60 ppm boron, with the balance being copper and impurities. The alloy can be made into cast and rolled strip or casting grains which can be used for investment casting the alloy.

Description

(71) Applicant(s):

Argentium International Limited (Incorporated in the United Kingdom)

110 Elliott Court Coventry Business Park,

Herald Avenue, Coventry, West Midlands, CV5 6UB, United Kingdom (72) Inventor(s):

Peter Gamon Johns (56) Documents Cited:

GB 2414739 A US 20080078484 A1

CN 102690969 A US 20070039665 A1 (58) Field of Search:

INT CL A44C, A61K, C22C

Other: Online: EPODOC, WPI, PATENT FULLTEXT, INSPEC (74) Agent and/or Address for Service:

Lucas & Co

135 Westhall Road, WARLINGHAM, Surrey, CR6 9HJ, United Kingdom (54) Title of the Invention: Silver alloys, investment casting using the alloys and casting grain Abstract Title: A silver-based alloy comprising germanium and iridium (57) A silver-based alloy which comprising (by weight):

92.5-96 % silver, 0.1-1.5 % germanium, 3-60 ppm iridium, 0-2 % zinc, 0-0.4 % tin, 0-0.3 % indium, 0-0.08 % silicon and 0-60 ppm boron, with the balance being copper and impurities. The alloy can be made into cast and rolled strip or casting grains which can be used for investment casting the alloy.

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SILVER ALLOYS, INVESTMENT CASTING USING THE ALLOYS AND CASTING GRAIN

FIELD OF THE INVENTION

The present invention relates to Sterling and Britannia alloys within the Ag-CuGe alloy genus, to casting grain of those alloys and to their use in mill-type products e.g. of strip and wire and in lost wax investment casting.

BACKGROUND TO THE INVENTION

Users of Sterling silver AgCu binary alloys do not normally incorporate grain refiners into the binary alloy that they use. For continuous casting to form strip, grain refiners are not needed because the crystal structure of the alloy is broken down by mechanical deformation e.g. by the rolling operations used to form strip. For lost wax investment casting, e.g. of rings it has become industry practice to silver plate articles after recovery from the investment, which conceals inter alia crystal structure.

GB 2255348 (Rateau et al) disclosed the genus of ternary alloys of silver, copper and germanium containing on a weight basis > 92.5wt% Ag, 0.5 to 3% Ge and the remainder, apart from impurities, Cu, preferred alloys containing 92.5 wt% Ag, 1.5-3 wt% Ge and 4.5-6 wt% Cu.

However, simple mechanical deformation does not suffice to break down the grain structure of AgCuGe alloys. For that reason, US 6168071 (Johns) disclosed a silver/copper/germanium alloy having > 77% Ag, 0.4 - 7% wt% Ge, 0.9 - 20 ppm B as a grain refiner, balance copper. It was found that, remarkably, such low concentrations of boron provided excellent grain refining in a silver/germanium alloy, Indeed, a concentration of < 10 ppm and as low as 2 or even 0.9 ppm was effective for this purpose, imparting greater strength and ductility to the alloy compared with a silver/germanium alloy without a boron content and permitting strong and aesthetically pleasing joints to be obtained. AgCuGe alloys of the kind disclosed by Johns and containing boron as grain refiner have since been sold under the trade name Argentium for a range of mill products including sheet, tube, strip and wire and also for small to high volume casting applications.

Investment casting of AgCuGe alloys has given rise to a range of unexpected and difficult problems. For investment casting of AgCu alloys, germanium was described in a 2006 publication by Progold entitled Evaluation of Hot Tearing in 925% Silver Alloys as “a true ‘poison’ for silver alloys”, increasing hot tearing susceptibility.

US 2011/139318 (Johns, P.G., Argentium International) published 16 June 2011, the contents of which are incorporated herein by reference in their entirety, discloses that in an Ag, Cu, Ge alloy containing boron as grain refiner, investment castings of a clean bright silvery appearance and/or free from cracking defects are obtained by incorporation of silicon, in some embodiments in the absence of added zinc. Amongst the references listed in that application is Jorg Fischer-Buhner, Silver casting revisited: the alloy perspective, The Santa-Fe Symposium 2010, the contents of which are incorporated herein by reference. In the BACKGROUND section of that specification the historical development of significant silver alloys for investment casting and other purposes and relevant patent specifications are discussed.

Silicon had been known to give rise to hot cracking and shrinkage porosity problems in investment casting as is apparent from the following quotation form the Fischer-Buhner paper:

“Especially for alloys with a broad melting range, like all 925 silver alloys, ‘hot cracking’ or ‘hot tearing’ can be a problem. Hot cracking mainly occurs when mechanical stress is acting on the metal during the final stages of solidification, hence when there is only a small amount of liquid metal left 25 between the growing grains. The thermal shrinkage of the solidifying metal coupled with the thermal expansion of the investment material (heating up when in contact with the hot metal) exerts local stresses and tears the metal apart. Hot cracking typically occurs on ring shanks opposite to the feed-sprue attachment point and is a well-known problem for coarse-grained standard sterling silver alloys...

The somewhat increased risk for hot cracking of silicon-containing alloys if compared to silicon-free alloys can be theoretically understood. Silicon tends to segregate to grain boundary areas during solidification where it eventually forms low melting phases. This broadens the melting range, from a width of typically AT~120°C (250 F) for silicon-free alloys to AT~150-170°C (300-340 F) for medium-to-high silicon levels. This also increases the solidification time: For example, an item that would need 1.5 min for completion of solidification if cast in a silicon-free alloy at a flask temperature of 500°C (930 F), needs around 2.5 min if cast in an alloy with medium-to-high silicon content...

Alloys with very broad melting ranges are more susceptible to shrinkage porosity, since it becomes increasingly difficult to feed liquid metal through the growing interdendritic network. This is confirmed by our investigations, where we have seen in reproducible way (for identical items and process parameters) a somewhat larger amount of shrinkage porosity for silicon-containing alloys (increased width of melting range) if compared to silicon-free alloys ...”

The Fischer-Buhner paper concerning deox alloys should not be interpreted as disclosing the use of silicon as an individual element in conventional Sterling or

Britannia alloys. The author discloses in relation to zinc that together with silicon it serves as a deoxidant. As is apparent from the table below which is reproduced from his paper, Si-containing deox alloys ail contain large amounts of zinc. If UPM and other manufacturers had been able to obtain bright castings with less zinc or without zinc, they would have done so because zinc (b.p. 907°C) is volatile at silver casting temperatures (~1000°C), reduces hardness and gives rise to gas porosity and shrinkage porosity.

Category	Alloy code	Silicon	Zinc	Comment
High Si- content	Arg-Deox	++++	+++	Highest fluidity, firestain and oxidation resistance and reduction of tarnish rate
Low to	SF928CHA	+++	+++	Medium-to-high firestain and
medium	AG113MA	++	+++	oxidation resistance, reliability and
Si-content	AG 114MA	+	++	user-friendliness
Si-free	S925PHA S925PTA	- no - - no -	+++ +	Most easy-to-cast and forgiving, universal usage, high productivity

Alloys of the AgCuGe family sold under the trade name Argentium were initially supplied to small craft jewellers who fabricate from sheet and wire. They lead in this field because of Argentium’s unique ability to prevent firscale when heated with a torch for annealing, fusing and soldering. Firescale is labour intensive to remove by hand. The individual artist craftsmen lack the equipment to remove it the way a larger workshop would do using cyanide solutions. Artist craftsmen generally do not use casting in their work because they seldom make multiples and the process is not compatible with a home or small workshop. If craft jewellers require casting they would go to specialist companies who generally do no other part of jewellery making than casting. The use of Argentium for making investment cast articles such as rings which is something of a separate trade was a more recent development.

Any serious deficiency in a silver alloy for investment casting will result in its rejection. The metal-mould reaction referred to in the above US patent specification represented a deficiency of this kind and indeed proved to be a barrier to acceptance of the original ternary AgCuGe alloys by investment casting manufacturers. Another potential deficiency, referred to in the examples forming part of that specification is the creation of cracks and voids during investment casting. A third potential deficiency that may result in rejection of a silver alloy for investment casting is embrittlement. In the context of investment casting, one difficulty which may arise from embrittlement of the cast alloy is that, for example, the claws of a ring may break during setting of a stone. A further difficulty which will be routinely encountered as a result of embrittlement is that a ring which is a product commonly made by investment casting will be prone to break during the stretching that jewellers carry out routinely in order for that ring to fit a finger of a purchaser, potentially resulting in rejection and return to the manufacturer.

Only if an alloy for investment casting is free from all these deficiencies will it become accepted within the investment casting portion of the trade.

Embodiments of the invention described in US 2011/193138 (Johns) related to a process for lost wax investment casting a germanium-containing silver alloy into a ring while avoiding reaction between germanium at the surface of the casting and sulphate of the investment giving rise to dark grey blemishes, said process consisting essentially of:

melting casting grain of a silver-copper germanium alloy comprising apart from impurities 93-95.5 wt% silver, 0.7-1.2 wt% germanium, 0.05-0.08 wt% silicon and 3-60 ppm boron as grain refiner, the balance copper, said alloy being free of added zinc;

pouring the molten alloy into a hydraulically set investment based on a gypsum 15 binder and containing a pattern for said ring;

allowing the investment and alloy to cool; and recovering said ring as a casting exhibiting tarnish and firestain resistance, having a clean silvery appearance when removed from the investment, substantially crack free and substantially free of shrinkage porosity.

Further embodiments provided a process for lost wax investment casting a germanium-containing silver alloy into a ring while avoiding reaction between germanium at the surface of the casting and sulphate of the investment giving rise to dark grey blemishes, said process consisting essentially of:

melting casting grain of a silver-copper germanium alloy comprising apart from impurities 95.5-96 wt% silver, 0.7-1.2 wt% germanium, 0.4-0.8 wt% zinc, 0.05-0.08 wt% silicon and 3-60 ppm boron as grain refiner, the balance copper;

pouring the molten alloy into a hydraulically set investment based on a gypsum binder and containing a pattern for said ring;

the investment and alloy to cool; and recovering said ring as a casting exhibiting tarnish and firestain resistance, having a clean silvery appearance when removed from the investment, substantially crack free and substantially free of shrinkage porosity.

It should be mentioned that the ability of Sterling or Britannia alloys containing both Ge and Si to give useful results in investment casting was surprising.

An alloy called Karatium Sterling is disclosed in US 2010/008818 (Thielemann) and may comprise 92.5-97.5 wt% Ag, 0.01-7.7 wt% Au, 0.01-7 wt% Cu, no more than 5 wt% Zn, no more than 1 wt% Si, no more than 0.5 wt% Ir, no more than 1 wt% Ge and no more than 0.1 wt% boron. An exemplified mill alloy comprises 2.5 wt% Au, 92.5 wt% Ag and 5 wt% Cu. A casting alloy is similar except that Cu is 4.9 wt% and the alloy contains 0.1 wt\% Ge. No detailed suggestions are given for incorporating iridium, an iridium-containing alloy is not exemplified and the use of either boron or iridium as grain refiner is neither disclosed nor suggested.

SUMMARY OF THE INVENTION

It is an object of the invention to provide AgCuGe alloys with alternative or improved grain refinement, the alloys being suitable for making into mill products including sheet, tube, strip and wire and also for small to high volume casting applications.

In one aspect, the invention provides an alloy comprising Ag 92.5 - 96 wt%, Ge 0.1 - 1.5 wt%, Ir 3 - 60 ppm, Zn 0-2 wt%, Sn 0 - 0.4 wt%, In 0 - 0.3 wt%, Si 0 - 0.08 wt%, B 0 - 60 ppm, the balance apart from impurities copper.

In embodiments of these alloys, Ag may be 93-96 wt% e.g. 93.5 wt%, Ge may be 0.2-1.5 wt%, e.g. 0.5-1.5 wt%, in further embodiments 0.7-1.2 wt%, B may be 3-60 ppm so that the alloy is grain refined by boron and iridium in combination, Ir may be 1040 ppm and B may also be 10-40 ppm.

The invention also provides a silver alloy useful for mill products comprising Ag 93-96 wt%, Ge 0.7-1.2 wt%, Ir 3-60 ppm, the balance apart from impurities copper. A further silver alloy useful for mill products comprises Ag 93-96 wt%, Ge 0.7-1.2 wt%, Ir 3-60 ppm, B 3-60 ppm, the balance apart from impurities copper.

Other embodiments of the invention provide further development of the alloys of US 2011/139318 to provide castings of further improved surface appearance and/or which can be formed in lost wax investment casting without embrittlement at temperatures similar to those used for casting standard Sterling silver.

Some embodiments provide a tarnish and firestain-resistant silver alloy comprising Ag 93-96 wt%, Ge 0.7-1.2 wt%, Si 0.06-0.08 wt%, B 3-60 ppm, Ir 3-60 ppm, optionally up to lwt% Zn when Ag > 95.5 wt%, the balance apart from impurities copper. Further embodiments of the alloy comprise Ag 93-96 wt%, Ge 0.7-1.2 wt%, Si 0.06-0.08 wt%, B 3-60 ppm, Ir 3-60 ppm, B 3-60 ppm, optionally up to lwt% Zn when Ag >95.5 wt%, the balance apart from impurities copper. The alloy may be provided as casting grain.

Also provided is the use in a tarnish and firestain-resistant silver alloy comprising Ag 93-96 wt%, Ge 0.7-1.2 wt%, Si 0.06-0.08 wt%, optionally up to lwt% Zn when Ag > 95.5 wt%, the balance apart from impurities copper of the combination of B 3-60 ppm and Ir 3-60 ppm to reduce or prevent the risk of surface pitting in lostwax investment casting.

Further provided is a process for lost wax investment casting a germaniumcontaining silver alloy into a ring or other article while avoiding reaction between germanium at the surface of the casting and sulphate of the investment giving rise to dark grey blemishes, and while reducing the risk of surface defects, said process consisting essentially of:

melting the casting grain described above;

pouring the molten alloy into a hydraulically set investment based on a gypsum binder;

allowing the investment and alloy to cool; and recovering said ring as a casting exhibiting tarnish and firestain resistance, having a clean silvery appearance when removed from the investment, substantially crack free and substantially free of shrinkage porosity or surface defects.

A further process is provided for lost wax investment casting a germaniumcontaining silver alloy into a ring or other article while avoiding reaction between germanium at the surface of the casting and sulphate of the investment giving rise to dark grey blemishes, said process consisting essentially of:

melting the casting grain described above e.g.at 1050-1150°C or above in vacuo or in an argon or other inert gas atmosphere e.g. for 5 minutes at 1200°C;

pouring the molten alloy into a hydraulically set investment based on a gypsum binder, the investment being at 550-600°C;

allowing the investment and alloy to cool; and recovering said ring as a casting exhibiting tarnish and firestain resistance, having a clean silvery appearance when removed from the investment, substantially crack free, substantially free of shrinkage porosity and having a reduced risk of cracking when subsequently deformed. In these processes boron and iridium may be present at 15-40 ppm and in similar amounts, for avoidance of embrittlement advantageously 3040 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

Tests for deformation cracking following investment casting are illustrated in the accompanying Figs 1-4, which show cast ring trees in which some of the rings have been tested by deformation and Fig. 5 which shows a ring tree cast at a flask temperature of 550°C.

DESCRIPTION OF PREFERRED EMBODIMENTS

Investment casting

The general procedure for making solid investment moulds in the jewellery industry in centrifugal or vacuum assisted lost wax investment casting involves attaching patterns having configurations of the desired metal castings to a runner system to form a set-up or tree. The patterns and runner system may be made of wax, plastics or other expendable material. To form the mould, the set-up or tree consisting of the pattern or patterns attached to the runner system are placed into a flask which is filled with a hydrauiicaiiy hardenable refractory investment slurry (e.g. an gypsum-based slurry) that is allowed to harden in the flask around the tree or set-up to form the mould. A typical tree diameter is about 50mm and when this is incorporated into an investment a typical investment diameter is about 100mm. After the investment slurry is hardened, the patterns are melted out of the mould by heating in an oven, furnace or autoclave.

The mould is then fired to an elevated temperature to remove water and burn-out any residual pattern material in the casting cavities.

Conventional investment formulations used for non-ferrous moulds are comprised of a binder and a refractory made up of a blend of fine and coarse particles. A typical refractory usually is wholly or at least in part silica, such as quartz, cristobalite or tridymite. Other refractories such as calcined mullite and pyrophyllite also can be used as part of the refractory. Gypsum powder (calcium sulfate hemihydrate) is almost universally used as a binder for moulds intended for casting gold, silver and other metals and alloys having relatively low melting points. After de-waxing, when the temperature of the flask rises above 100°C (212°F), free water evaporates and gypsum (CaSO4.2H2O) begins to lose its water of hydration. However the complete transformation of gypsum into the anhydrous form of calcium sulphate (anhydrite) occurs over a wide temperature range, through complex transformations of the crystal lattice. These transformations take place with a considerable volume contraction, which is particularly severe at 300-450°C (572-842°F). If gypsum alone were used to produce investment for lost wax casting, the moulds would crack in service and would also produce castings a great deal smaller than the original patterns. Silica is used to compensate for this gypsum shrinkage and to regulate the thermal expansion of the mould. Silica exists in several crystalline forms, and two of them are used in the production of investment powders. Quartz is the most readily available form and its conversion from a to b crystal forms is accompanied by an increase in volume at around 570°C (1058°F). Cristobalite is the other major constituent of investment powder and this form of silica also undergoes a significant increase in volume as it transforms from its a to b crystal structure at around 270°C (518°F). Thus, these two allotropic forms of silica are used to override the shrinkage effect of the gypsum binder, and it is understood from the trade literature that many commercially available moulding particles are based on cristobalite, silica and gypsum

Refractory moulding materials are mentioned in the patent literature. For example, a composition for making a refractory mould based on cristobalite, silica flour and gypsum is disclosed in US 3303030 (Preston). US-A-4106945 (Emdt) discloses that conventional non-ferrous investment formulations are comprised of a binder and a refractory made up of a blend of fine and coarse particles. The refractory usually is wholly or at least in part a silica, such as quartz, cristobalite or tridymite. Calcined fireclay also is often used as a part of the refractory. The binder is typically a fine gypsum powder (calcium sulphate hemihydrate). The binder and refractory, together with minor chemical additives to control setting or hardening characteristics, are dry blended to produce the investment. The dry investment is then prepared for use by mixing it with sufficient water to form a slurry which can be poured into the flask around the setup. Vacuuming of the slurry and vibration of the flask are frequently employed steps to eliminate air bubbles and facilitate filling of the flask. Pyrophyllite, a hydrous aluminium silicate, is present to prevent mould cracking, see also US-A-5310420 (Watts).

In practice manufacturers will use commercially available investment powders e.g. SRS Global available from Specialist Refractory Services Limited, Riddings, Derbyshire, UK or Gold Star XL, XXX, Gem Set or Omega+ available from Gold Star Powders of Newcastle-under Lyme, Staffordshire, UK or investment casting materials for jewellery casting available from Ransom & Rudolph of Maumee, Ohio, US.

Silver content

For mill products, silver content may be as described above.

Embodiments of the present alloy have silver contents complying with the

Sterling and Britannia standards and silver content can range from 92.5-96 wt%, with a preferred reference value of 93.5 wt% for Sterling-type alloys and a preferred reference value of 96 wt% for Britannia alloys. It will be appreciated that silver contents in commercial production may deviate from the reference values, with a greater margin of deviation being permitted on the high silver content side, so that permissible silver contents of a 935 alloy may be from 93.45 to 93.8 wt% and permissible silver contents for a 960 alloy may be from 95.95 to 96.4 wt%. For Sterling alloys, embodiments have reference silver contents of 93 - 95.5wt% e.g. about 93.5 wt% or above, the onset of reduction in copper elution compared to that with 925 alloys being believed to be in the range 93.0-93.5 wt% Ag.

A reason why it is feasible to reduce the copper content of the alloy to improve 5 physical properties and reduce copper elution compared to standard 925 Argentium alloys is because of the unique hardening properties of the AgCuGe system. Incorporating germanium improves as-cast hardness. Further hardening can occur either by slow cooling alone (e.g. when an investment flask is allowed to air cool to ambient or near-ambient temperatures) or by low temperature baking which is advantageous because quenching any red-hot silver alloy into cold water will always lead to cracking and solder joint failure. We have observed a surprising difference in properties between conventional sterling silver alloys and other silver alloys of the Ag-Cu family on the one hand and silver alloys of the Ag-Cu-Ge family on the other hand. Gradual cooling of e.g. the binary Sterling-type alloys results in coarse precipitates and little precipitation hardening, whereas gradual cooling of Ag-Cu-Ge alloys optionally containing incidental ingredients results in fine precipitates and useful precipitation hardening, especially in those embodiments where the silver alloy contains an effective amount of grain refiner e.g. boron.

Experimental evidence has shown that Ag-Cu-Ge alloys of Ag content 93.5 wt% and above become precipitation hardened following cooling from a melting or annealing temperature by baking at e.g. 200°C-400°C and that baking the alloy can achieve a hardness of 65 HV or above, preferably 70 HV or above and still more preferably 75 HV or above which is equal to or above the hardness of standard sterling silver used to make jewellery and other silverware. These advantageous properties are believed to be the result of the combination of Cu and Ge in the silver alloy and are independent of the presence and amounts of Zn or other incidental alloying ingredients. However the commercially available alloy made according to Eccles I does not exhibit these properties and can only be age hardened on heating to an annealing temperature and quenching.

Addition of germanium to sterling silver changes the thermal conductivity of the 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 65%. The significance 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 (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.

The benefit of not having to quench to achieve the hardening effect is a major advantage of the present silver alloys. There are very few times in practical production that a silversmith can safely quench a piece of nearly finished work. The risk of distortion and damage to soldered joints when quenching from a high temperature would make the process not commercially viable. In fact standard sterling can also be precipitation hardened but only with quenching from the annealing temperature and this is one reason why precipitation hardening is not used for sterling silver.

In order to distinguish the operations of annealing and precipitation hardening (which are regarded as distinct by silversmiths) 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.

Further embodiments of the present alloy are of Britannia silver which has a minimum silver content of 95.84 wt%, and will typically have a reference silver content of 96 wt%. Such alloys retain the ability to precipitation harden as described above. Silver contents in the range 96 - 97.2 wt% are also contemplated.

Germanium

In some alloys, Ge may be 0.1-1.5 wt%, e.g. 0.2-1.5 wt%, in further embodiments 0.5-1.5 wt%.

In the present alloys and process, amounts of germanium desirably fail within the range 0.7-1.2 wt%. In the inventor’s experience with the alloys within the AgCuGe family, a germanium content of 0.7 wt% provides a practical minimum for desirable firestain and tarnish resistance. Above 1.2 wt% there is no significant improvement in tarnish and firescale resistance, and instead the disadvantages of increased risk of embrittlement and unnecessary cost. For example, embodiments of the 935 alloy and 960 alloy may have a germanium content of 0.7 wt% although for improved hardening properties 0.8 or 0.9 wt% may desirable, and improved performance and tarnish resistance may be obtained e.g. in the 935 alloy at a germanium content of 1.0-1.2 wt% e.g. 1.1 wt%.

Silicon

Silicon is desirable for alloys intended for investment casting, but may be omitted from alloys intended for mill product production.

Both germanium and silicon are embrittling agents for silver alloys, since both of them can precipitate at grain boundaries either as intermetallics or in elemental form and the precipitated material is brittle. As explained in GB-A-2255348 germaniumcontaining alloys of Ge content <3wt% may escape embrittlement because germanium remains in solid solution as intermetallics in the silver and copper phases. However, that specification also discloses that silicon which is insoluble in silver and only slightly soluble in copper gives rise to alloys which are brittle to varying degrees, as also taught by Fischer-Buhner (above). In the alloys with which this invention is concerned both germanium and silicon are associated with the copper content of the alloys and form a secondary phase at the grain boundaries which may be a phase of predominantly CuGe-Si with some silver. The formation of this copper-germanium-silicon phase at the grain boundary would be expected on the basis of conventional teaching give a highly brittle alloy. In practice, in the embodiments specified herein, it does not. It was unexpected to be able to combine two elements known to give a brittle investment casting alloy in such a ratio as to give an alloy with embodiments having no brittleness problems, good flow and low porosity and no hot cracking

However, the amount of silicon added should be kept as low as possible since silicon is about 10 times as effective as germanium as an embrittling agent for silver, even in alloys containing relatively large amounts of copper. The preferred silicon content in the present alloys and process is 0.05 - 0.08 wt% with a reference value of 0.07 wt% (700 ppm). Although silicon contents up to 0.2 wt% are recommended in AgGe binary alloys and for some additional silver alloys the above limited range of silicon content in AgCuGeSi alloys of has proved to be a practical necessity. The reason for the lower limit is that below 0.05 wt% freedom from metal-mould reaction is not reliably obtained, inter alia because part of the silicon content of casting grain may be lost when that casting grain is melted and much of the cast metal has to be recycled, often repeatedly. For example, when casting using a tree, only that material which enters the patterns is used as product. The remaining poured metal which forms the sprue button, the main sprue, any auxiliary sprues and the feed sprues leading to the patterns has to be and in fact is recycled. In consequence at each investment casting operation, only about 50% of the metal poured into the investment is freshly melted. All of the remainder will have passed through at least one previous melting and pouring cycle and a portion of the remainder will have passed through several melting and pouring cycles, inevitably losing part of the silicon content at each cycle through unintended but in practice unavoidable contact with atmospheric oxygen. Above 0.08 wt% there is no additional benefit from additional silicon, and instead the positive disadvantage of increased risk of embrittlement, especially since casting conditions vary from one user to another. Furthermore, silicon present in the alloy is subject to unavoidable contact with oxygen during the casting process, is highly susceptible to attack by oxygen during melting, forming slag and unwanted crucible residue and also potentially forming a surface dross on the casting which has to be removed. The more silicon is present, the greater are these problems.

Boron

In some alloys where iridium is used as grain refiner, boron may be omitted e.g. for mill product alloys. In preferred alloys for both mill products and for investment casting, boron and iridium are used in combination.

The use of boron as grain refiner has been considered practical necessity when 30 investment casting silver having an appreciable content of germanium. It is advantageously introduced at the time of manufacture of casting grain which then has the boron content needed for grain refinement on re-melting and investment casting e.g. 3-60 ppm, typically 5-20 pp. especially about 10 ppm. The amount of boron added should be sufficient to bring about grain refinement but below levels at which boron hard spots appear, boron contents ranging from 3-60 ppm.

A conventional method of introducing boron into a precious metal alloy or master alloy is through the use of 98 wt% Cu, 2 wt% B master alloy. Many manufacturers have been able to use that alloy without difficulty but others have reported that it introduces hard spots into the products. These hard spots are believed to be non-equilibrium phase Q1B22 particles that form in copper saturated with boron when cooled from the liquid phase to the solid phase. The hard spots may not be detected until after the precious metal jewellery alloy is polished and inspected resulting in needless expense for the processing of ultimately unsatisfactory product. When using CuB master alloys, amounts of added boron in this invention may typically be 15-40 ppm e.g. 18 ppm - 36 ppm.

Other methods of introducing boron are disclosed in US 2011/193138, and the disclosure of these alternative methods is incorporated herein by reference.

Iridium

The present alloys may contain 3-60 ppm iridium, e.g. 10-40 ppm, more preferably 15-40 ppm. Incorporation may be by a master alloy containing 2 wt% Ir, balance copper. In embodiments the ppm of boron and iridium in the alloy are similar or the same.

Zinc and other incidental ingredients

Embodiments of the present alloys up to 95.5 wt% Ag for use in investment casting are free from added zinc or other added metals save copper, germanium, boron, iridium and silicon and have the advantage inter alia of simplicity of formulation and of production. At higher silver contents and at relatively low germanium contents, addition of zinc in Britannia embodiments may be desirable e.g. in amounts of 0.2-1 wt% e.g. about 0.4 wt%., although other Britannia embodiments may incorporate no zinc. Above wt% in some embodiments zinc becomes unacceptably volatile. Other incidental ingredients are set out in claim 1 of the accompanying claims.

Procedure

Silver for investment casting is commonly supplied in the form of casting grain,

e.g. smooth spheroidal grain of average diameter 2-4 mm.

Deoxidation of silver to form casting grain is desirable if easily oxidisable alloying ingredients such as germanium, silicon and boron are to be incorporated successfully and consistently into a silver alloy. The oxygen content of fine silver sold as bullion is not of technical importance and such metal which is typically used as the main constituent of casting grain often contains large quantities of dissolved oxygen and as previously explained the saturation solubility of oxygen in molten silver is about 0.3 wt%. The thermodynamics of oxidising constituents of casting grain used in the present method (calculated for 1000°C) is summarised in the following table:

Si + O2 = SiO2	AG° =	-907030 + 175.7T	=-731,330 kJ mol1 O2
4/2B + O2 = 2/3Β2θ3	AG° =	-827040 +147.9T	= -679,500 kJ mol1 O2
2Zn + O2 = 2ZnO	AG° =	-711120 + 214.IT	= -497,020 kJ mol1 O2
Ge + O2 = GeO2	AG° =	-577780 + 191.3T	= -386,480 kJ mol1 O2
4Cu + O2 = 2Cu2O	AG° =	-344180 + 147.2T	= -196,980 kJ mol1 O2
2Cu2O + O2 = 4CuO	AG° =	-290690+ 196.2T	= - 94,490 kJ mol1 O2
4Ag + O2 = 2Ag2O	AG° =	+61780 + 132T	= +70,220 kJ mol1 O2

The value for silver oxide is positive, indicating that silver oxide does not form 25 under casting conditions. The more negative the quoted values, the more likely that the reaction will proceed. Germanium is a deoxidant, zinc is a stronger deoxidant, and boron and silicon are even more strongly deoxidising and when present in silver are the most susceptible to attack by oxygen. It will be apparent that the molten silver content, if not carefully deoxidised, could easily convert the boron grain refiner added in ppm amounts to oxide and could also easily convert added silicon e.g. in an amount of

0.7wt% to oxide, and oxygen in the copper content could assist that process if assistance were needed.

For this reason, it is preferred to firstly add to the melting vessel e.g. a graphite or silica crucible the bulk of the silver and copper needed to form the alloy, to bring the constituents to a melting temperature e.g. about 1000°C and to deoxidise before adding further more oxygen-sensitive constituents.

Various ways of deoxidizing molten silver alloys are known. One possibility is to use a graphite cover and a hydrogen protective flame for an initial mixture of molten silver and copper, the graphite forming CO which reacts with oxygen in the molten metal, and optionally additionally with graphite stirring of the molten metal. Better results are obtainable by covering the silver with graphite powder of particle size >5mm. However, such measures may not be effective, especially if the furnace as a whole is open to ambient air and does not have provision for vacuum or a protective atmosphere and if protective conditions are not maintained during subsequent pouring and processing. In an embodiment silver and copper are melted together e.g. in a graphite crucible and held at a casting temperature of ~1000°C. A protective atmosphere e.g. of nitrogen or argon is provided above the melt and dissolved oxygen in the silver is removed by stirring the molten AgCu alloy with graphite rods. Melting in a closed furnace with a protective atmosphere or vacuum may give better deoxidation, the molten silver and copper being treated with a deoxidiser e.g. lithium metal red phosphorus or copper phosphorus. Lithium metal in small amounts is a known deoxidant for silver, and is volatile so that residual lithium in the silver alloy after deoxidation may be at the limits of detectability e.g. 2-3 ppm. Red phosphorus or copper phosphorus are alternatives and the reaction with dissolved oxygen can be mild, but if iron is present in the silver hard spots may form and the amount of residual phosphorus in the molten metal should be less than 30 ppm to avoid formation of copper phosphides.

The melt may then be reduced in temperature e.g. to about 825°C to prevent excessive reaction as germanium enters the surface of the molten silver, after which the germanium is added e.g. in the form of particles which are dropped into the molten alloy or by wrapping the germanium in a known weight of copper or silver foil and plunging the resulting packet to the bottom of the crucible.

Zinc is a deoxidant and may be added, when present in the alloy, before silicon, iridium and boron. Irrespective of the deoxidant used, it is desirable that levels of oxygen in the casting grain produced should be <40ppm, e.g. <30 ppm, more preferably <20ppm and if possible <10ppm.

When de-oxidation has been completed boron e.g. as Cu/B alloy or sodium borohydride and silicon in pure elemental form or as Cu/Si alloy and iridium as a Culr alloy may be added while maintaining the protective atmosphere, care being taken with addition of sodium borohydride (if used) because of the evolution of combustible hydrogen gas. The resulting alloy is poured under a protective atmosphere into a grain box or tundish and converted into casting grain. It will be appreciated that vacuum conditions may be employed as an alternative to a protective atmosphere. A minimum of delay between the end of deoxidation, the addition of silicon, iridium and boron and the casting into casting grain is desirable to minimise the risk of oxygen getting into the molten alloy and reacting with the boron and silicon constituents, resulting in an alloy with less than the intended amounts of these materials.

In a variation, the elemental silicon, Culr alloy or Cu/B alloy may be added to the molten metal in the grain box or tundish while maintaining the protective atmosphere.

Re-melting of casing grain for investment casting is also carried out in a vacuum or under a protective atmosphere: if needed silicon, iridium and boron can be added at this stage. Castings should be maintained in a protective atmosphere or vacuum for at least one minute before removal from the casting chamber, and allowed to stand, preferably in a protective atmosphere, for e.g. 20 minutes before quenching in water. Additional hardness may be obtained by allowing the flask to cool to room temperature before removing castings from the investment.

The invention is further illustrated in the following examples.

EXAMPLE 1

An embodiment of a 935alloy had 93.5 wt% Ag, 0.7 wt% Ge, 0.07 wt% (700 ppm) Si, 18 ppm B, 18 ppm Ir, the balance being copper, the boron being added as 2% CuB and the iridium being added as 2% Culr. A reference embodiment had the same constituents but no iridium.

The alloy was created by placing part of the the required weight of copper into a 10 crucible, melting the CuB into the copper at 1150-1200°C in vacuo or in an argon protective atmosphere, adding silicon as 10 wt% Si in Cu together with the balance of the copper, adding the required weight of elemental germanium and then adding the silver, the melt then being at 1150-1050°C and poured into a tundish.

In a first test, the alloy was used in an all graphite Rautomead continuous caster 15 to form strip which was rolled into sheet. The resulting sheet exhibited favourable properties, being resistant to sagging or formation of a surface gloss on torch annealing as a result of partial melting. Casting partially recycled alloy continued to give consistent results, whereas casting with alloy containing only boron as gran refiner gave rise to less satisfactory products as a result of boron loss.

In lost wax investment casting under x 50 microscopic examination the iridiumcontaining alloy exhibited a significantly reduced population of small pits and other surface defects.

The experiment was repeated with 36ppm B and 36 ppm Ir in the alloy to give castings of unusually good surface finish and freedom from microscopic pits.

EXAMPLE 2

Argentium Silver 935 Pro casts well if high flask temperatures are used, typically 650°C. The problem is that this temperature is well above those used to cast standard Sterling silver designs of a similar size and weight. Casters may not wish to keep changing furnace temperatures if they are casting in both standard sterling and Argentium which may may impact sales of Argentium.

The following trials conducted with alloy having reference values of Ag 93.50 wt %, Ge 0.70 wt %, Si 0.07 wt%, Cu and grain refiners balance show that (a) by incorporating iridium grain refiner in relatively high amounts, Argentium Silver can be cast at temperatures much closer to those used for standard Sterling and (b) increasing the level of boron only has a marginal effect/improvement. In these trials there were used for addition of grain refiner 2% boron in copper/boron alloy and 2% iridium in copper/iridium alloy. The test to prove an improvement is to bend a casting at the bottom and top of the tree. If cracking occurs when the casting is bent about 90° the alloy fails.

Two samples of the 935 Pro alloy were cast at: Flask temperature 575°C, Metal temperature 1050°C (both trees) with the results shown in Fig. 1 Left tree: grain refiner - Cu/B 0.09% (=18ppm B) Cu/Ir 0.09% (=18ppm Ir); Right tree: grain refiner - Cu/B

0.18% (=36ppm B) Cu/Ir 0.18% (=36ppm Ir). The top ring and lower ring on each tree have been bent after casting. The higher grain refiner in the second alloy results in no cracking when 935 Pro is cast at a lower than normal flask temperature. The first alloy alloy (containing half the amount of boron and iridium) cracks when cast below a flask temperature of 650°C with this pattern.

The second tree material was further tested at a metal temperature of 980°C instead of 1050°C with the results shown in Fig. 2 which show that lower metal temperature, lower flask temperature + higher grain refiner produces no cracking when bent. The advantages are that a 575°C flask temperature is much closer to the temperatures used for casting standard Sterling of similar design and weight. This would mean that a caster also working in standard Sterling would not have to alter temperatures in his furnaces and experience delays whilst temperatures stabilise.

A yet further test is shown in Fig. 3, casting being at Flask temperature 575°C, Metal temperature 1050°C. The alloy had grain refiner - Cu/B 0.18% (=36ppm B) Cu/Ir 0.09% (=18ppm Ir). Increasing copper/boron from 0.09% (18ppm boron) to

0.18% (36ppm boron) and leaving copper/iridium at 0.09% (18ppm iridium) resulted in only a minor improvement. The ring at top of tree cracked, whilst lower rings were sound.

By way of comparison, Fig. 4 shows an Argentium 935 Pro alloy containing 36 ppm B which is close to the highest level that can be added using CuB without risking hard spots, casting being at Flask temperature 575°C, Metal temperature 1050°C. It will be seen that both of the rings tested were fractured. This alloy would have probably produced sound castings using a higher flask temperature of 650°C.

The results of a more recent test on an Argentium 935 Pro Alloy 36ppm of both iridium and boron cast at a flask temperature of 550°C (which is identical to that commonly used for casting standard Sterling silver alloys) are shown in Fig 5. The casting has not been pickled in acid, it had only been power water washed, but nevertheless exhibited a bright appearance. Again, the cast rings did not exhibit embrittlement, as shown by distortion of rings at the upper and lower ends of the casting which did not exhibit brittle fracture.

Claims (23)

1. An alloy comprising Ag 92.5-96 wt%, Ge 0.1-1.5 wt%, Ir 3-60 ppm, Zn 0-2 wt%, Sn 0-0.4 wt%, In 0-0.3 wt%, Si 0-0.08 wt%, B 0-60 ppm, the balance apart from impurities copper.

2. The alloy of claim 1, wherein Ag is 93-96 wt%.

3. The alloy of claim 1 or 2, wherein Ge is 0.2-1.5 wt%.

4. The alloy of claim 3, wherein Ge is 0.5-1.5 wt%.

5. The alloy of any preceding claim, wherein B is 3-60 ppm.

6. The alloy of any preceding claim, wherein Ir is 10-40 ppm.

7. The alloy of any preceding claim, wherein B is 10-40 ppm.

8. A tarnish and firestain-resistant silver alloy comprising Ag 93-96 wt%, Ge 0.71.2 wt%, Si 0.06-0.08 wt%, B 3-60 ppm, Ir 3-60 ppm, optionally up to lwt% Zn when Ag >95.5 wt%, the balance apart from impurities copper.

9. The alloy of claim 8, wherein Ag is 93.5-95.5 wt%

10. The alloy of claim 9, wherein Ag is nominally 93.5 wt%.

11. The alloy of any of claims 8-10, wherein B and Ir are 10-40 ppm.

12. The alloy of any of claims 8-10, wherein B and Ir are 15-40 ppm.

13. Cast and roiled strip of the alloy of any preceding claim.

14. Casting grain of the alloy of any of claims 8-12.

15. Use in a tarnish and firestain-resistant silver alloy comprising Ag 93-96 wt%, Ge 0.7-1.2 wt%, Si 0.06-0.08 wt%, optionally up to lwt% Zn when Ag > 95.5 wt%, the balance apart from impurities copper of the combination of B 3-60 ppm and Ir 3-60 ppm to reduce or prevent the risk of surface pitting in lost-wax investment casting.

16. The use of claim 15, wherein B and Ir are 10-40 ppm.

17. The use of claim 16, wherein B and Ir are 15-40 ppm.

18. A process for lost wax investment casting a germanium-containing silver alloy into a ring or other article while avoiding reaction between germanium at the surface of the casting and sulphate of the investment giving rise to dark grey blemishes, and while reducing the risk of surface defects, said process consisting essentially of:

melting the casting grain of claim 14;

pouring the molten alloy into a hydraulically set investment based on a gypsum binder;

allowing the investment and alloy to cool; and recovering said ring as a casting exhibiting tarnish and firestain resistance, having a clean silvery appearance when removed from the investment, substantially crack free and substantially free of shrinkage porosity.

19. A process for lost wax investment casting a germanium-containing silver alloy into a ring or other article while avoiding reaction between germanium at the surface of the casting and sulphate of the investment giving rise to dark grey blemishes, said process consisting essentially of:

melting the casting grain of claim 14;

pouring the molten alloy into a hydraulically set investment based on a gypsum binder, the investment being at 500-600°C;

allowing the investment and alloy to cool; and recovering said ring as a casting exhibiting tarnish and firestain resistance, having a clean silvery appearance when removed from the investment, substantially crack free, substantially free of shrinkage porosity and having a reduced risk of cracking when subsequently deformed.

20. The process of claim 19, wherein the molten alloy is poured into the hydraulically set investment, the investment being at 500-600°C.

21. The process of claim 20, wherein boron and iridium are present at 15-40 ppm 10 and in similar amounts.

22. A silver alloy comprising Ag 93-96 wt%, Ge 0.7-1.2 wt%, Ir 3-60 ppm, 0-lwt% Zn when Ag > 95.5 wt%, the balance apart from impurities copper.

15

23. A tarnish and firestain-resistant silver alloy comprising Ag 93-96 wt%, Ge 0.ΤΙ.2 wt%, Si 0.06-0.08 wt%, Ir 3-60 ppm, optionally up to lwt% Zn when Ag > 95.5 wt%, the balance apart from impurities copper.

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Application No: Claims searched:

GB 1705894.2 1-23