Process for investment casting and casting grain for use in the process

Abstract

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.

Claims

1. A process for lost wax investment casting a germanium-containing silver alloy, 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 binder; allowing the investment and alloy to cool for at least one minute under a protective atmosphere, cooling the alloy in air, and subsequently quenching the alloy; and recovering 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.

2. The process of claim 1, wherein the casting comprises patterns attached to a tree.

3. The process of claim 2, wherein the alloy comprises 10 ppm boron.

4. The process of claim 1, wherein the oxygen content of the casting grain is <40 ppm.

5. The process of claim 1, wherein silver is about 93.5 wt %.

6. The process of claim 1, wherein germanium is 1.0-1.2 wt %.

7. The process of claim 1, wherein boron is present in the alloy in an amount of about 10 ppm.

8. The process of claim 1, further comprising reheating the casting at 150-400 C. to effect precipitation hardening thereof, reheating giving an increase of hardness of at least 15 HV.

9. The process of claim 1, wherein the alloy is made into a ring having claws, and further comprising the step of setting a stone into claws of the ring.

10. A process for lost wax investment casting a germanium-containing silver alloy, 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; allowing the investment and alloy to cool for at least one minute under a protective atmosphere, cooling the alloy in air, and subsequently quenching the alloy; and recovering 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.

11. The process of claim 10, wherein the casting comprises patterns attached to a tree.

12. The process of claim 10, wherein the alloy comprises 0.7 wt % germanium.

13. The process of claim 10, wherein the oxygen content of the casting grain is <40 ppm.

14. A process for lost wax investment casting a germanium-containing silver alloy into a ring, 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 binder and containing a pattern for said ring; allowing the investment and alloy to cool for at least one minute under a protective atmosphere, cooling the alloy in air, and subsequently quenching the alloy; 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.

15. The process of claim 14, wherein casting is at a mold temperature of 500-600 C. using molten silver at 900-1050 C.

16. The process of claim 15, wherein the oxygen content of the casting grain is <40 ppm.

17. A process for lost wax investment casting a germanium-containing silver alloy, 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; allowing the investment and alloy to cool for at least one minute under a protective atmosphere, cooling the alloy in air, and subsequently quenching the alloy; 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.

18. The process of claim 17, wherein casting is at a mold temperature of 500-600 C. using molten silver at 900-1050 C.

19. The process of claim 18, wherein the oxygen content of the casting grain is <40 ppm.

20. 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 binder and containing a pattern for said ring; allowing the investment and alloy to cool for at least one minute under a protective atmosphere, cooling the alloy in air, and subsequently quenching the alloy; 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.

21. The process of claim 20, wherein casting is at a mold temperature of 500-600 C. using molten silver at 900-1050 C.

22. The process of claim 20, wherein the oxygen content of the casting grain is <40 ppm.

23. 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; allowing the investment and alloy to cool for at least one minute under a protective atmosphere, cooling the alloy in air, and subsequently quenching the alloy; 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.

24. The process of claim 23, wherein casting is at a mold temperature of 500-600 C. using molten silver at 900-1050 C.

25. The process of claim 23, wherein the oxygen content of the casting grain is <40 ppm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Tests for cracking during investment casting are illustrated in the accompanying drawings, in which FIG. 1 is a diagram representing an alloy test casting for showing the performance of the alloy in investment casting of rings, and FIGS. 2-4 are micrographs showing sections of cast ring at position 7 in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

(2) Investment Casting

(3) 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 an hydraulically 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 50 mm and when this is incorporated into an investment a typical investment diameter is about 100 mm. 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. Casting is typically at a mould temperature of 500-600 C. using molten silver at 900-1000 C.

(4) 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, cristabolite 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 (CaSO.sub.4.2H.sub.2O) 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

(5) 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 U.S. Pat. No. 3,303,030 (Preston). U.S. Pat. No. 4,106,945 (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 fire-clay 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 set-up. 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 U.S. Pat. No. 5,310,420 (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, USA.

(6) Silver Content

(7) Embodiments of the present alloy have silver contents complying with the Sterling and Britannia standards.

(8) Sterling silver has a minimum silver content of 92.5 wt %. However, embodiments have silver contents of 93-95.5 wt % 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.

(9) A reason why it is feasible to reduce the copper content of the alloy to improve 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 AgCu family on the one hand and silver alloys of the AgCuGe 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 AgCuGe 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.

(10) Experimental evidence has shown that AgCuGe 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.

(11) 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.

(12) 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.

(13) 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.

(14) 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 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.

(15) Germanium

(16) Embodiments of the present alloy have germanium content of 0.5-3 wt %, in embodiments 0.5-1.5 wt % and in further embodiments 0.7-1.2 wt %. 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 %.

(17) Silicon

(18) Silicon may be added in amounts of e.g. 10 ppm up to 0.2 wt % and may be added as elemental silicon or as a CuSi alloy containing e.g. 10-30 wt % Si, in some embodiments 10 wt % Si or alternatively as a AgSi alloy.

(19) 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 germanium-containing alloys of Ge content <3 wt % 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-Bhner (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 CuGeSi 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

(20) 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. Amounts of silicon in embodiments of the alloy may be 0.01-0.1 wt % in embodiments 0.05-0.1 wt % e.g. 0.05-0.08 wt % with a reference value of 0.07 wt % (700 ppm). In embodiments the wt % silicon is 20% of the weight % of germanium, e.g. 10% of the weight of the germanium e.g. about 10% of the weight of the germanium. The upper limit for silicon in molten metal for the investment casting stage is, as noted above, 0.2 wt %, preferably <0.15 wt %. Bright castings can surprisingly be obtained with low amounts of incorporated silicon e.g. 75 ppm or above. Above 0.2 wt % Si the incidence of hot cracking/brittleness is greatly increased. The above maximum wt % of silicon selected on the grounds of embrittling properties greatly decreases the overall effectiveness of silicon as the primary deoxidant present in the metal (not only to you have the uptake of oxygen by the silver but you also have complete solubility of the oxygen in any copper present in the alloy). In addition, when combined with oxygen silicon forms silicon dioxide which forms insoluble hard ceramic particles which are deleterious to the overall quality of the alloy if not removed prior to casting as they would cause hard spots in the finished castings which would lead to drag marks on polishing.

(21) Boron

(22) The use of boron as grain refiner is a practical necessity when 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.

(23) 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 CuB.sub.22 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.

(24) A boron compound may be introduced into molten silver alloy in the gas phase, advantageously mixed 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 an 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. Alternatively 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.

(25) The alloy to be heated may be 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. 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 a desired few ppm or few tens of ppm boron content The introduction of the boron compound into the alloy as a dilute gas stream over an period of time, the carrier gas of the gas stream serving to stir the molten metal 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. Compounds which may be introduced into molten silver 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.sub.4 or NaBF.sub.4 into the fluidized gas stream as a finely divided powder which forms an aerosol.

(26) A boron compound may also be introduced into the molten silver 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 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 filled capsules or sachets in appropriate number may then be plunged individually or as one or more groups into the molten silver alloy. 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.

(27) Conveniently the boron compound is introduced into the molten silver alloy in the solid phase, e.g. using a solid borane e.g. decaborane B.sub.10H.sub.14 (m.p. 100 C., b.p. 213 C.). However, the boron is conveniently added in the form of either a boron containing metal hydride or a boron containing metal fluoride. 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, NaBH.sub.4 which has a molecular weight of 37.85 and contains 28.75% boron.

(28) 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.

(29) It has been found that when adding a borane or borohydride that more than 20 ppm can be incorporated into a silver alloy without the development of boron hard spots. This is advantageous because boron is rapidly lost from molten silver: according to one experiment the content of boron in molten silver decays with a half-life of about 2 minutes. The mechanism for this decay is not clear, but it may be an oxidative process. It is therefore desirable to incorporate more than 20 ppm boron into an alloy as first cast i.e. before investment casting or before rolling into strip, and amounts of e.g. up to 60 ppm may be incorporated. Thus there could be produced according to the present method silver casting grain containing about 40 ppm boron, although in another embodiment the casting grain may be nominally about 10 ppm boron. Owing to boron loss during subsequent re-melting and investment casting, the boron content of finished pieces may be closer to the 1-20 ppm 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. Although sodium is lost during casting, alloys to which boron is added as sodium borohydride may on analysis show some ppm of sodium e.g. >5 but <100 ppm.

(30) Incidental Ingredients

(31) Embodiments of the present alloys are free from added zinc or other added metals save copper, germanium, boron 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 other embodiments may be desirable e.g. in amounts of 0.2-1 wt % e.g. about 0.4 wt %. Above 1 wt % zinc becomes unacceptably volatile. Other metals may be added in small amounts e.g. up to 0.2 wt % provided that they do not interfere with the overall properties of the alloy, and such metals include e.g. gallium which in some embodiments may further decrease cracking defects. In embodiments small amounts of indium may also be present, so that a 960 alloy may comprise boron in ppm amounts as grain refiner, indium, gallium, zinc, silicon, germanium, copper and silver.

(32) Major alloying ingredients that may be used to replace copper in addition to zinc (e.g. in amounts of up to 1 wt % e.g. 0.5 wt %) are Au, Pd and Pt. Other alloying ingredients may be selected from selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Mn, Ni, Pb Si, Sn, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of providing firestain and tarnish resistance is not unduly adversely affected. The weight ratio of germanium to incidental ingredient elements may range from 100:0 to 60:40, preferably from 100:0 to 80:20. In some current commercially available AgCuGe alloys such as Argentium incidental ingredients are not added.

(33) Procedure

(34) Silver for investment casting is commonly supplied in the form of casting grain.

(35) 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:

(36) TABLE-US-00003 Si + O.sub.2 = SiO.sub.2 G = 907030 + 175.7T = 731,330 kJ mol.sup.1 O.sub.2 4/3B + O.sub.2 = B.sub.2O.sub.3 G = 827040 + 147.9T = 679,500 kJ mol.sup.1 O.sub.2 2Zn + O.sub.2 = 2ZnO G = 711120 + 214.1T = 497,020 kJ mol.sup.1 O.sub.2 Ge + O.sub.2 = GeO.sub.2 G = 577780 + 191.3T = 386,480 kJ mol.sup.1 O.sub.2 4Cu + O.sub.2 = 2Cu.sub.2O G = 344180 + 147.2T = 196,980 kJ mol.sup.1 O.sub.2 2Cu.sub.2O + O.sub.2 = 4CuO G = 290690 + 196.2T = 94,490 kJ mol.sup.1 O.sub.2 4Ag + O.sub.2 = 2Ag.sub.2O G = +61780 + 132T = +70,220 kJ mol.sup.1 O.sub.2

(37) The value for silver oxide is positive, indicating that silver oxide does not form 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 0.7 wt % to oxide, and oxygen in the copper content could assist that process if assistance were needed.

(38) 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.

(39) 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 >5 mm. 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 mid, 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.

(40) 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.

(41) Zinc is a deoxidant and may be added, when present in the alloy, before silicon and boron.

(42) Sodium borohydride used to add boron to the molten metal is a powerful deoxidant and may be used for that purpose in addition to addition of boron.

(43) Irrespective of the deoxidant used, it is desirable that levels of oxygen in the casting grain produced should be <40 ppm, e.g. <30 ppm, more preferably <20 ppm and if possible <10 ppm.

(44) 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 may be added while maintaining the protective atmosphere, care being taken with addition of sodium borohydride 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 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.

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

(46) Re-melting of casing grain for investment casting is also carried out in a vacuum or under a protective atmosphere: if needed silicon and boron can be added at this stage. Castings should be maintained in a protective atmosphere 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.

(47) The invention is further illustrated in the following examples.

Examples 1 and 2

(48) An embodiment of a 935 alloy (Example 1) has 93.5 wt % Ag, 1.1 wt % Ge, 700 ppm Si, 3-60 ppm e.g. 10 ppm B, the balance being copper. Hardness of the alloy on investment casting depends on the design of the article being cast and on the casting conditions. It is typically about 72 HV if the investment is cast at a temperature of about 950-1050 C. e.g. about 1000 C. into an investment at about 500-600 C. and allowed to cool for one minute in the flask chamber and about 30 minutes in air at which point it will have cooled to about 250 C., after which it is quenched in water. Subsequent heat treatment at about 300 C./2 hours can give a hardness of about 97 HV but for many applications may not be necessary as the as-cast hardness is similar to that of conventional Sterling silver.

(49) An embodiment of a 960 alloy (Example 2) has 96 wt % Ag, 0.4-0.8 e.g. 0.65 wt % zinc, 0.6-0.8 e.g. 0.7 wt % Ge, 500-800 e.g. 700 ppm silicon, 3-60 e.g. 10 ppm boron, balance copper. Hardness of the alloy on investment casting as described above depends on the design of the article being cast and on the casting conditions but with the casting/cooling/quench conditions described above is typically about 52 HV. Subsequent heat treatment at about 300 C./2 hours can give a hardness of about 67 HV which is similar to that of conventional Sterling silver as cast, the reduction in hardness compared to the 935 alloy being partly the result of the reduced copper content and partly the result of zinc in the alloy.

(50) Both of the above alloys exhibit bright stain-free castings following investment casting and are either substantially crack and void-free or are significantly lower in voids, see FIG. 2 which shows a standard Sterling test casting for a ring exhibiting gross porosity and FIGS. 3-4 which are micrographs of the illustrated alloys in the vicinity of position 7 in FIG. 1 where the body of the ring joins the sprue and which show little or no porosity. It will be appreciated since molten metal contracts on cooling, a sprue should solidify last to allow molten metal to be fed to the cooling casting, as the metal contracts on cooling and to minimise development of shrinkage porosity. Therefore the most sensitive area to display shrinkage porosity (or the potential for cracking due to hot cracking or hot tearing) is the area where the sprue and item to be cast join. This is why P7 was chosen, as the region at which there was the greatest possibility of shrinkage porosity being present.

(51) Castings in both the above alloys were bright and free from mould discoloration experienced with alloys not containing silicon.

Example 3

(52) A quaternary silver-copper-germanium alloy (Ag=94.7 wt %, Ge=1.2 wt %, Cu=3.9 wt % Si=0.2 wt % (added as a Cu/Si master alloy), is prepared by melting silver, copper, germanium and master alloy together in a crucible by means of a gas-fired furnace which becomes heated to a pour temperature of about 2000 F. (1093 C.). The melt is covered with graphite to protect it against atmospheric oxidation and in addition a hydrogen gas protective flame is provided. Stirring is by hand using graphite stirring rods. When the above ingredients have become liquid, pellets of sodium borohydride to give up to 100 ppm boron e.g. 80 ppm are packaged or wrapped in pure silver foil of thickness e.g. about 0.15 mm. 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.

(53) After boron addition, 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.

(54) 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. to the relatively high boron content permitted by the sodium borohydride and the energetic dispersion of the boron into the molten silver as the borohydride decomposition reaction proceeds. 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.

(55) However, 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.

Example 4

(56) A silver alloy is made by melting together 93.2 wt % fine silver casting grains, 1.3 wt % germanium in the form of small broken pieces, 0.2 wt % Si (added as a Cu/Si master alloy containing 10 wt % Si), the balance being copper granules. Melting is by means of an electric furnace which becomes heated to a pour temperature of about 1093 C. (2000 F.) having a melting crucible provided with ports for introduction of stirring gas, and the melt is protected by bubbling a stream of nitrogen gas through the melt to simultaneously effect stirring thereof, the nitrogen also providing a protective atmosphere.

(57) When the above ingredients have become liquid, small quantities of diborane are added to the nitrogen stream passing through the melt over a period of 1-5 minutes to give a total boron content in the melt of about 50 ppm. The melt is covered with a ceramic fibre blanket to quench any resulting flame from decomposition of the diborane. The hydrogen burns off almost immediately on contact with the molten metal with a stirring action from the nitrogen stream, after which evolution of hydrogen ceases and the boron content has been substantially incorporated into the melt. After boron addition, the molten alloy is poured into a tundish whose bottom is formed with fine holes. The molten silver runs through the holes in fine streams which break into pellets which fall into a stirred bath of water and become solidified and cooled. The cast pellets are removed from the bath and dried. Pellets are tested by investment casting using a calcium sulphate bonded investment. The resulting casting has a matt silvery finish when removed from the mould, a fine grain structure and can be polished easily. It is free from boron hard spots and is ductile.