|WO/2002/095082A||METHOD OF PRODUCING SILVER-COPPER ALLOYS|
|5039479||Silver alloy compositions, and master alloy compositions therefor|
|6168071||Method for joining materials together by a diffusion process using silver/germanium alloys and a silver/germanium alloy for use in the method|
|6726877||Silver alloy compositions|
This invention relates to a process for making silver alloys using the master metal compositions, and to the optional further treatment of the alloys to make shaped articles and/or to effect precipitation hardening thereof.
The process according to the invention provides copper-based master alloys for alloying with silver, said master alloys containing germanium, boron and optionally other alloying ingredients including silver and/or zinc and/or silicon and/or indium.
The process according to the invention further provides substantially pure copper or a copper alloy (e.g. a Cu-Ge or Cu-Zn-Ge or Cu-Ge-Si or Cu-Ge-Zn-Si alloy) containing up to 2 wt% boron introduced into the copper by means of a compound that is decomposable in situ in molten copper to form boron. Said compounds may be selected from the group consisting of alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof. Decomposition in situ is believed superior to current methods of making copper-boron master alloys by rapid melting together of copper and finely divided boron, which tends to give rise to boron hard spots. In some embodiments, usable master alloys are therefore obtainable which can impart greater boron content to the alloys in which they are incorporated while keeping development of hard spots to low acceptable levels. Boron contents of such alloys may be up to the 2 wt% level of currently available Cu-B alloys, or may be less where boron in the resulting precious metal alloy is being used as a grain refiner. Some embodiments provide Ag-Cu-Ge-B, Ag-Cu-B, Ag-Cu-B-Si or Ag-Cu-Ge-B-Si containing silver in an amount sufficient to facilitate melting or casting of the copper e.g. 1-30 wt% Ag, typically 1-25 wt% Ag and more typically 10-25 wt% Ag.
The invention according to the claims provides a master metal composition adapted for alloying with silver to give a silver alloy containing at least 77 wt % Ag and at least 0-5 wt% Ge, said master metal also comprising Cu and 0.001-0.5, typically 0.005-0.3 wt% boron together with any further ingredients for said alloy and any impurities.
The invention further provides a process for making a silver alloy containing at least 77 wt% Ag, 1-7.2 wt% Cu, at least 0.5 wt% Ge and 0.005-0.3 wt% B together with any further ingredients for said silver alloy and any impurities, comprising the step of melting together fine silver and the master metal composition as aforesaid.
The invention provides in a yet further embodiment a process for making a master alloy used in the manufacture of silver articles, which process comprises melting copper and optionally germanium or other alloying ingredients, and adding boron to the melt in the form of a compound selected from the group consisting of alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof. The present invention is applicable e.g. to the manufacture of master alloys e.g. Cu-Ge-B master alloys and Cu-B master alloys.
Use of a master alloy provides a number of technical benefits. Boron is a very light element that is easily lost in the melting process. If the boron level in the alloy is too high or the boron has not been dissolved properly the result is boron hard spots, which appear as drag marks in the surface of the silver when the piece is polished. However, more boron than is needed is routinely added to compensate for its loss during melting. What happens to the additional boron is at present unknown. One possibility is that it may react with oxygen present in the silver. Another possibility is that it may react with the material of a graphite crucible in which the alloy is typically melted. A third possibility is that it may diffuse towards the surface of the melt and become oxidized by any atmospheric oxygen present. However, the combination of germanium and boron in the master alloy is believed to exhibit a protective effect and germanium may protect boron in the same manner that it protects copper.
In some embodiments, order of addition of the alloying ingredients may be significant. It is difficult to add the germanium first to a copper alloy and then to add the boron. The problem is that when using copper boride as the source of boron a much higher temperature is required to dissolve the boron into the alloy and the germanium content of the alloy may therefore be put at risk by overheating. The present master alloy is therefore normally made by melting together the highest melting elements first and progressively working through the lower melting temperature elements. Alternatively, boron is added e.g. as a boron hydride or metal boron hydride, which decomposes in contact with the molten metal of the master alloy and disperses boron into the alloy with reduced opportunity for development of hard spots and the like.
The invention further provides a method for casting a master alloy as given in the claims.
The master alloy may comprise 80-95 wt % Cu (or of Cu together with further ingredients for said alloy as set out below) and 20-5 wt % Ge. A preferred class of such alloys comprises 80-86.7 wt % Cu (or of Cu together with further ingredients for said alloy) and 20-13.3 wt % Ge. A still more preferred class of alloys comprises 82-84.55 wt % Cu and any further ingredients for said alloy and 15.5-18 wt% Ge. Alloys with about 0.03 wt% B can give desirable boron contents in the silver alloys into which they are incorporated. A preferred class of the master alloys comprises only copper, germanium, boron and impurities.
The master alloy may provide the whole of the copper required for the silver alloy. Alternatively, the silver alloy may be made by melting together copper and a master alloy of the above defined genus.
The master alloy precursor to which boron is added is Cu-Ge, or Cu-Ge further comprising small amounts of casting adjuvants e.g. Si or Ag to facilitate casting and prevent development of surface cracking and porosity. The copper or alloy will normally be at a nominal temperature for casting or pouring e.g. about 1150-1200°C. The melting temperature influences the kinetics of boron evaporation which determines the final boron concentration in the cast master alloy. The selected temperature should be sufficiently above the liquidus temperature of the alloy to prevent freezing in a die during continuous casting or freezing in a grain box during grain making. While the alloys are readily cast at atmospheric pressures, higher or lower pressures should not affect the benefits of the invention, but will affect the kinetics of boron evaporation. Furthermore higher boron content is desirable for master alloys which may be melted with precious metal to make casting grain and then further melted to form rod, wire, or investment casting.
In an embodiment, sufficient boron is added to the master alloy so that an effective amount remains in the cast precious metal alloy or master alloy for effective grain refinement and deoxidation. Typically, the boron content is between 100 ppm and 1600 ppm for a master alloy, with a nominal boron content in the cast master alloy of about 250 ppm being more typical. Typically, from 0.0 1 % to 0.16% of boron added to the precursor alloy melt is effective.
Boron is incorporated into the present master alloys for use, in the eventual silver alloys, as an oxygen scavenger and/or as a grain refiner. It may be added as given in the claims, so it may be added e.g. to the molten master alloy e.g. Cu, Cu-Ge, Ag-Cu-Ge, Ag-Cu-Si or Ag-Cu-Ge-Si containing at least 50 wt% Cu and optionally containing incidental ingredients by bubbling a gaseous borane e.g. diborane into the master alloy in admixture with a non-reactive gas such as argon, by introducing into the master alloy a borane which is solid at ambient temperatures e.g. decaborane B10H14 (m.p 100°C, b.p. 213°C), or by adding an alkylated borane e.g. triethylborane or tri-n-butyl borane, although the latter reagents are spontaneously combustible and require care in handling. Preferably, however, the boron is added as a metal borohydride, e.g. a borohydride of an alkali metal, a pseudo-alkali metal or an alkaline earth metal, e.g. lithium borohydride. Sodium borohydride is especially preferred because it is widely available commercially and can be obtained in the form of relatively large pellets which are convenient to handle during precious metal melting operations.
As previously explained, the boron compound may be introduced into molten copper or copper alloy in the gas phase, advantageously in admixture with a carrier gas which assists in creating a stirring action in the molten copper or copper 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 copper or copper alloy e.g. a crucible in a copper-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 copper or alloy. The lance is preferably of sufficient length to permit injection of the gaseous boron compound and carrier gas deep into the molten copper or copper alloy. Alternatively the boron-containing gas may be introduced into the molten copper or copper alloy from the side or from below e.g. using a gas-permeable bubbling plug or a submerged injection nozzle. For example, Rautomead International of Dundee, Scotland manufacture horizontal continuous casting machines in the RMK series for the continuous casting of semi-finished products. The copper or alloy to be heated which may be 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 may be heated by electrical resistance heating using graphite blocks. Such furnaces have a built-in facility for bubbling inert gas through the melt.
Addition of small quantities of thermally decomposable boron-containing gas to the inert gas being bubbled through the melt readily provides from a desired few ppm to some hundreds or even thousands of ppm of boron into the molten metal or alloy. The introduction of the boron compound into the copper or copper alloy as a dilute gas stream over an period of time, the carrier gas of the gas stream serving to stir the molten copper or alloy, rather than in one or more relatively large quantities is believed to be favourable from the standpoint of avoiding development in the metal or alloy of boron hard spots, with the result that the resulting boron-containing alloy can serve as a master alloy for precious metal alloy manufacture with reduced development of hard spots. Compounds which may be introduced into molten copper or alloy 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 copper or alloy thereof to effect stirring thereof and to add a solid boron compound e.g. NaBH4 or NaBF4 into the fluidized gas stream as a finely divided powder which forms an aerosol.
The boron compound may also be introduced into the molten copper or copper 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-n-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 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. As an alternative 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 copper 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 copper or copper alloy as described above. The droplets may take the form of an aerosol in the carrier gas stream, or they may become vaporised therein.
Also as previously explained, preferably the boron compound is introduced into the molten copper or copper alloy in the solid phase, e.g. using a solid borane e.g. decaborane B10H14 (m.p. 100°C, b.p. 213°C). However, the boron is preferably added in the form of either a boron containing metal hydride or a boron containing metal fluoride or other halide. When a boron-containing metal hydride is used, suitable metals include sodium, lithium, potassium, calcium, zinc and mixtures thereof. When a boron-containing metal fluoride is used, sodium is the preferred metal. Most preferred is sodium borohydride, NaBH4 which has a molecular weight of 37.85, contains 28.75% boron and can be obtained in the form of relatively large pellets which are convenient to handle during precious metal melting operations.
Boron may be lost as vapour from molten copper or copper alloy at elevated temperatures and it may be necessary to make sequential additions of boron to maintain an adequate concentration for grain refining. To enable better mixing into the copper or copper alloy, the boron compound may be wrapped in a thin copper foil or thin foil of an inert material (i.e. a material which decomposes in the molten silver substantially without residue), such as paper or plastics sheet. Preferred metal for the foil is copper, but silver may also be used since it assists casting properties. The foil preferably has a thickness of from about 0.01 mm to about 0.3 mm to enable the foil-wrapped boron compound to be well submerged in the molten copper or alloy before the foil melts through releasing the boron compound. Once released, the constituents of the boron compound combine with oxygen in the melt to effectively deoxidize the melt and the boron is believed to react (although the effectiveness of the invention does not depend on the accuracy of this theory) with some of the elements in the melt to form discrete insoluble particles dispersed throughout the base material which act as nucleation sites promoting the formation of fine grains that are uniform in size and resist growth.
When boron is added to molten metal e.g. as diborane, the compound decomposes to boron and hydrogen e.g.
B2H6→ 2B(s) + 3H2(g).
The hydrogen is effective to deoxidize the melt
When sodium borohydride is first added to the molten metal, the initial reaction is believed to be decomposition of the boron containing grain refiner.
After decomposition, the sodium, hydrogen and boron are all effective to deoxidize the melt as follows:
(2) Na(g)+0.5O2(g) →Na2O(s)
(3) H2(g)+0.5O2(g) →H2O(g)
(4) B(s)+0.5O2(g)+0.5H2(g) →HBO(g)
To achieve a uniform casting, the boron may be dispersed throughout the molten metal by stirring for in excess of 1 minute and typically for from 1-5 minutes. Stirring may be by any means which does not contaminate the molten metal such as with a graphite stirring rod.
The resulting master alloy is then cast by a method suitable for forming a desired product. One such useful product is casting grains. Casting grains are particles that are sold to jewellery manufacturers who then investment cast the grains of master alloy with grains of precious metal to form a desired article of jewellery. Subsequent to stirring, molten master alloy is poured into a grain box which is a container with openings in the bottom, through which the liquid metal flows to make the desired shape and size of grains. The grain box may be made from materials similar to a melting crucible, such as, but not limited to, graphite, clay/graphite, ceramic and silicon carbide. The molten master alloy is formed into discreet droplets in the grain box as it flows through the openings and is then solidified into roughly spherical particles in grain tank containing water into which the master alloy droplets fall and solidify. The master alloy casting grain is then removed from the grain tank and dried e.g. by centrifugal force and hot air. The resulting roughly spherical grains have a typical diameter of from about 0.1 mm to about 5 mm.
The present master alloys may be used to make alloys of silver.
The present master alloys may be used to make silver/germanium alloys 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 incidental ingredients and impurities, which alloy contains boron as a grain refiner. If desired, the copper content may also be substituted, in part, by one or more incidental ingredient elements selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Mn, Ni, Pb, Pd, Pt, Si, Sn, Ti, V, Y, Yb, Zn 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 be 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 amounts of the metals or metalloids Si, Zn, Sn or In appropriate for "deox".
The alloys that may be made include coinage grade, 800-grade (including 830 and 850 grades and the like) and standard Sterling silver and an alloy of silver containing an amount of germanium that is effective to reduce firestain and/or tarnishing. The ternary Ag-Cu-Ge alloys and quaternary Ag-Cu-Zn-Ge or Ag-Cu-Ge-Si alloys that can suitably be made by the method of the present invention are those having a silver content of at least 80%, and most preferably at least 92.5%, by weight of the alloy, up to a maximum of no more than 98%, preferably no more than 97%. The germanium content of the Ag-Cu-(Zn)-Ge or Ag-Cu-(Si)-Ge alloys should be at least 0.1 wt%, preferably at least 0.5 wt%, more preferably at least 1.1 wt% The germanium content is most preferably not more than 1.5%, by weight of the alloy, more preferably no more than 4 wt% up to a maximum of preferably no more than 6.5 wt%.
Silicon, in particular, may be added to silver alloys e.g. in an amount of up to 0.5 wt %, typically 0.5-3 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. When incorporated e.g. into casting grain of a silver-copper-germanium ternary alloy it can provide investment castings that appear bright immediately on removal from the mould. 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.
The remainder of ternary Ag-Cu-Ge alloys, apart from impurities, incidental ingredients and any grain refiner, will be constituted by copper, which should be present in an amount of at least 0.5%, preferably at least 1%, more preferably at least 2%, and most preferably at least 4%, by weight of the final alloy. For an '800 grade' ternary silver alloy, for example, a copper content of 18.5% is suitable. Appropriate levels of copper are incorporated into the master alloy, copper usually comprising at least 50 wt% of said master alloy.
The remainder of quaternary Ag-Cu-Zn-Ge alloys, apart from impurities and any grain refiner, will be constituted by copper which should be present in an amount of at least 0.5%, preferably at least 1%, more preferably at least 2%, and most preferably at least 4%, by weight of the alloy, and zinc which should be present in a ratio, by weight, to the copper of no more than 1:1. Therefore, zinc is optionally present in the silver-copper alloys in an amount of from 0 to 100 % by weight of the copper content. For an '800 grade' quaternary silver alloy, for example, a copper content of 10.5% and zinc content of 8% is suitable. Where present, zinc may be incorporated into the master alloy.
In addition to silver, copper and germanium, and optionally zinc, the silver alloys preferably contain a grain refiner to inhibit grain growth during processing of the alloy, and this grain refiner is added as part of the master alloy. Suitable grain refiners include boron, iridium, iron and nickel, with boron being particularly preferred. The grain refiner, preferably boron, may be present in the Ag-Cu-(Zn)-Ge or Ag-Cu-(Si)-Ge alloys in the range from 1 ppm to 100 ppm, preferably from 2 ppm to 50 ppm, more preferably from 4 ppm to 20 ppm, by weight of the alloy.
The silver alloy is a ternary alloy consisting, apart from impurities and any grain refiner, of 80% to 96% silver, 0.1 % to 5% germanium and 1 % to 19.9% copper, by weight of the silver alloy. The silver alloy may be a ternary alloy consisting, apart from impurities and grain refiner, of 92.5% to 98% silver, 0.3% to 3% germanium and 1% to 7.2% copper, by weight of the alloy, together with 1 ppm to 40 ppm boron as grain refiner. The silver alloy may also be a ternary alloy consisting, apart from impurities and grain refiner, of 92.5% to 96% silver, 0. 5% to 2% germanium, and 1% to 7% copper, by weight of the alloy, together with 1 ppm to 40 ppm boron as grain refiner. A silver alloy being marketed under the name ArgentiumTM comprises 92.7-93.2 wt% Ag, 6.1-6.3 wt% Cu and about 1.2 wt% Ge.
Particular known silver alloys that may benefit from incorporation of boron as Cu-Ge-B using the master alloys produced by the invention include the following:
Ag-Cu-Ge silver alloy workpieces and shaped articles made from the above master alloys and heated to an annealing temperature may be self-hardening on controlled air cooling, so that products of useful hardness can be obtained without the need for reheating to effect annealing and/or precipitation hardening. The use of reheating to e.g. 180-350°C, and preferably 250-300°C, to develop further hardness is, however, also possible according to the invention. Over-aging of Ag-Cu-Ge silver alloys during precipitation hardening does not cause a significant drop-off of the hardness achieved. Processing workpieces is possible, for example as part of soldering or annealing in a mesh belt conveyor furnace or in investment casting, reduces the number of process steps required to produce articles of a required hardness and in particular eliminates quenching e.g. with water which is required for Ag-Cu Sterling silver.
A surprising difference in properties exists between conventional Sterling silver alloys and other Ag-Cu binary alloys on the one hand and Ag-Cu-Ge silver alloys on the other hand, in which gradual cooling of the binary Sterling-type alloys results in coarse precipitates and little precipitation hardening, whereas gradual cooling of Ag-Cu-Ge alloys results in fine precipitates and useful precipitation hardening, particularly where the silver alloy contains an effective amount of grain refiner. Furthermore, the addition of germanium to sterling silver changes the thermal conductivity of the silver alloy, compared to standard sterling silver. The International Annealed Copper Scale (IACS) is a measure of conductivity in metals. On this scale the value of copper is 100%, pure silver is 106%, and standard sterling silver 96%, while a sterling alloy containing 1.1% germanium has a conductivity of 56%. The significance of this is that the Argentium sterling and other germanium-containing silver alloys do not dissipate heat as quickly as standard sterling silver or their non-germanium-containing equivalents, a piece will take longer to cool, and precipitation hardening to a commercially useful level (preferably to Vickers hardness 110 or above, more preferably to 115 or above) can take place during natural air cooling or during slow controlled air cooling.
Thus after the master metal defined above has been incorporated into a silver alloy starting with e.g. 999 or 9999 fine silver from a bullion manufacturer, the resulting alloy may be subjected to the further steps of annealing and/or brazing a shaped article of the alloy in a furnace, and hardening by subsequent air cooling. Thus the alloy may be annealed and/or brazed by heating in a furnace at 600-680°C, preferably 600-660°C and more preferably 600-650°C. The annealing may be during investment casting, and hardening may be by air-cooling the investment or allowing it to air cool. The final product may be an article of jewellery or giftware.
The ability of the present silver alloys to precipitation harden enables the copper content of the alloy to be reduced. Even though an alloy of lower copper content may be relatively soft as cast, reheating at a low temperature e.g. 200-300°C may bring the hardness up to the level of normal sterling silver or better. This is a significant advantage because the copper content is actually the most detrimental part of the alloy from the standpoint of corrosion resistance, but in a standard sterling alloy less copper means unacceptably low hardness. If the copper content is reduced, the silver content may simply be increased which is a preferred option. Other possibilities include increasing the germanium content or adding zinc or another alloying element. 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. The copper in a master alloy may be adjusted according to the silver content.
The benefit of not having to quench to achieve the hardening affect is a major advantage of silver alloys that can be made from the present master 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 on subsequent quenching and this is one reason why precipitation hardening is not used for sterling silver.
How the invention may be put into effect will now be further described, by way of illustration only, in the following Examples.
A master alloy is made by melting together 79 wt% Cu, 18 wt% Ge and 3 wt% of a CuB alloy containing 2 wt% boron. The Cu is melted together with the Cu/B master alloy. High temperatures can be used because there are no other elements to damage. The temperature is then lowered and the germanium is added just above the Ge melting point. Melting is therefore in descending order of melting temperatures i.e. copper/copper-boron master alloy/germanium. The resulting master alloy comprises, apart from impurities, and with a 50% boron loss on melting, about 82 wt% Cu, about 18 wt% Ge and about 0.03 wt% boron, together with any impurities.
There is then added 72g of the above master alloy and 928 g of 9999 purity fine silver which when melted together just above the melting point of the fine silver (e.g. at about 960-1200°C) with a 50% boron loss gives the desired silver/copper/germanium ternary alloy of composition about 92.8 wt% Ag, 5.90 wt% Cu, 1.30 wt% Ge and about 11 ppm boron. The master alloy is weighed and placed in a crucible for melting and the fine silver is weighed and placed in the crucible, which is then heated to melt the silver and the master alloy under a protective cover of natural gas to prevent unnecessary oxidation. Silver has a known affinity for oxygen, which affinity increases with temperature. When exposed to air, molten silver will absorb about twenty-two times its volume of oxygen. Like silver, copper also has a great affinity for oxygen, typically forming copper oxide. Thus, in forming or re-melting sterling silver and other silver-copper alloys, care must be taken to prevent oxidation. When the mixture becomes molten, it may be stirred e.g. with a carbon rod and poured through a tundish into water, so that the silver becomes solidified into shot-like granules or pellets of diameter about 3-6 mm which is the form in which sterling silver is typically sold.
The resulting alloy granules are used in investment casting using traditional methods and is cast at a temperature of 950-980°C and at a flask temperature of not more than 676°C under a protective atmosphere. 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 a Vickers hardness of about 70 which is approximately the same hardness as sterling silver. The products exhibit excellent tarnish and firestain resistance and have a fine grain structure due to their boron content. It has been found that a harder cast piece can be produced by allowing the flask to cool in air to room temperature, the piece when removed from the flask having a Vickers hardness of about 110. 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 approaching 125 Vickers. The germanium content is towards the upper limit of that presently considered desirable in a 0.925 type alloy.
As an alternative, the master alloy and fine silver in the form of granules can be mixed together in a crucible, and poured straight into the investment mould, giving similar results to those described above.
The fine silver granules and the master alloy of Example 1 in the proportions set out in that example are formed into sheet by continuously casting at 1150-1200°C. Pieces of the sheet are brazed together to form shaped articles by passage through a brazing furnace and are simultaneously annealed. Precipitation hardening develops without a quenching step by controlled gradual air-cooling in the downstream cooling region of the furnace. For this purpose, it is desirable that the material should spend at least about 8-30 minutes in the temperature range 200-300°C which is most favourable for precipitation hardening. Articles which have been brazed in a furnace in this way and gradually cooled can achieve hardness of 110-115 Vickers.
A second master alloy is made by melting together 81.5 wt% Cu, 15.5 wt% Ge and 3 wt% of a Cu/B alloy containing 2 wt% boron. The resulting master alloy comprises, apart from impurities, and with a 50% boron loss on melting about 84.5 wt% Cu, about 15.5 wt% Ge and about 0.03 wt% boron, together with any impurities.
There is then added 72g of the second master alloy and 928 g of 9999 silver which when melted together at about 960-1200°C with a 50% boron loss gives the desired silver/copper/germanium ternary alloy of composition about 92.8 wt % Ag, 6.08 wt % Cu, 1.12 wt % Ge and about 11 ppm boron. The subsequent performance of the alloy is similar to that of Example 1. The germanium content is towards the lower limit of that presently considered desirable in a 0.925 type alloy.
A master alloy is made by melting together copper and germanium in the proportions given in Example 1. The copper is melted by heating in a gas-fired furnace or an induction furnace to about 1150° under a charcoal melt cover which gives a reducing atmosphere. The germanium is added to the copper by wrapping pieces of the germanium in copper foil and plunging the wrapped germanium to the bottom of the melt using a graphite or plumbago stirring rod. When addition of the copper is complete the temperature is lowered to 1100°C, pellets of sodium borohydride to give 0.5 wt% boron are wrapped in copper foil and are plunged to the bottom of the melt using a graphite or plumbago stirring rod as described above. The sodium borohydride decomposes with evolution of hydrogen over a period of 1-2 minutes leaving boron and some sodium in the melt.
After boron addition, the crucible is pivoted to permit the molten alloy to be poured into a tundish whose bottom is formed with very fine holes. The molten alloy pours into the tundish and runs through the holes in fine 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 to give a master alloy as casting grain. The above master alloy can be used in the manufacture of Ag-Cu-Ge alloys containing boron as melt refiner e.g. using the procedures of the preceding Examples. Dispersion of boron into the master alloy using the borohydride is very effective, and the resulting silver alloys can contain up to 20 ppm boron, or if desired above 20 ppm boron without development of hard spots.
In particular, the procedure of the Example may be used to manufacture Ag-Cu-Ge casting grain for Sterling-type alloys containing about 40 ppm boron. Boron loss on re-melting reduces the boron content of the final cast alloy to 20 ppm or below, which is still an effective amount for grain refinement, and offers the possibility of producing cast, investment cast or other products having more consistent microstructure and properties.
The procedure of Example 4 is repeated except that prior to addition of the boron, silicon is added in an amount that will impart to the intended final alloy 0.05 - 0.2 wt% Si as incidental ingredient.
A master alloy is made by melting together 56 wt% Cu, 28 wt% Ag, 13 wt% Ge and 3 wt% of a Cu/B alloy containing 2 wt% boron. The Cu (m.p. 1085°C) is melted together with the Cu/B master alloy. High temperatures can be used because there are no other elements to damage. The temperature is then lowered and the silver (m.p. 962°C) is added followed by the germanium which is added just above the Ge melting point (m.p. 938°C). Melting is therefore in descending order of melting temperatures i.e. copper/copper-boron master alloy/silver/germanium. The resulting master alloy comprises about 0.03 wt% boron.
There is then added 100g of the above master alloy and 900 g of 9999 purity fine silver which when melted together just above the melting point of the fine silver (e.g. at about 960-1200°C) with a 50% boron loss gives the desired silver/copper/germanium ternary alloy of composition similar to that in Example 1. Addition of the master alloy to the fine silver is as described in Example 1, and it is formed as described in that example into alloy granules are used in investment casting as described in Example 1.
A master alloy is made by melting together 59 wt% Cu, 28 wt% Ag and 13 wt% Ge. Sodium borohydride is then introduced into the alloy as described in Example 4 to give a boron content of about 1000-1100 ppm. The master alloy is used to make a Sterling grade jewellery or silversmithing alloy as described in Example 7.
In a modification of the procedure in Example 7, the sodium borohydride is wrapped in silver foil and introduced into said master alloy.