ES2697223T3 - Alloy for casting lost wax - Google Patents

Abstract

A method for casting a germanium-containing silver alloy to the lost wax, said laundry comprising patterns attached to a tree, said method comprising: melting a silver-copper-germanium alloy comprising either (a) in addition to impurities the 93- 95.5% by weight of silver, 0.7-1.2% by weight of germanium, 0.05-0.08% by weight of silicon and boron in an amount effective to confer refinement of the grain, the rest copper, said zinc-free alloy being added or (b) 95.84-96% by weight of silver, 0.7-1.2% by weight of germanium, optionally 0.2-1.0% by weight of zinc, 0.05-0.08% by weight of silicon and boron in an amount effective to confer refinement of the grain, the remainder copper; pouring the molten alloy into a hydraulically set shell using gypsum binder, and allow the shell and alloy to cool; and recovering a laundry having a clean silver appearance when removed from the wrap, substantially free of cracks and substantially free of shrinkage porosity.

Description

DESCRIPTION

ALLOY TO GLAD TO THE LOST WAX

Field of the invention

The present invention relates to silver casting alloys, to a lost wax casting using the alloys and casting articles to the lost wax of the alloys.

BACKGROUND OF THE INVENTION

For a long time, it has been desired to produce castings of lost wax in silver with a bright and vivid color of the cast.

The lost-wax casting of sterling silver and conventional deoxy alloys is reviewed by Jorg Fischer-Bühner, Advances in the Prevention of Investment Casting Defects Assisted by Computer Simulation, Santa-Fe Symposium on the Manufacture of Jewelry, 2010, whose content is incorporated in this document as a reference.

Fischer-Buhner commented on various alloy components. Copper remains the main addition in variations of conventional sterling silver despite its many disadvantages. Accelerates surface discoloration. It reduces the melting point of silver and leads to a wide melting range, making the alloy intrinsically prone to hot cracking. It oxidizes easily, which leads to dark oxide layers on the surface of rough casting trees during cooling in air after pouring or during reheating, for example for welding. It also leads to internal or subsurface oxidation that may reveal itself as "fire stains" (gray, bluish or reddish areas) on finished surfaces. Zinc is used up to ~ 2.5% by weight. It reduces the surface tension of the melt, increases the fluidity and the filling of forms and reduces the roughness of the surface. Together with silicon, it helps prevent the development of dark copper oxide layers and fire stains. However, the high vapor pressure of zinc can lead to loss of Zn by evaporation depending on the melting conditions and zinc fumes. Silicon is used up to ~ 0.2% by weight. It has greater affinity for oxygen than silver, copper and zinc and, therefore, acts as a deoxidizer of the molten alloy, but depending on the equipment and the process conditions it can also lead to slag on the surface. It avoids the formation of dark copper oxide layers by preferential formation of live and white silicon oxide layers on raw cast trees. Like zinc, it increases fluidity and helps filling forms. It also extends the melting range and tends to segregate and form low melting point phases along the grain boundaries, which leads to an increased risk of hot cracking. If used in large quantities, silicon and zinc can reduce the rate of surface discoloration.

A vivid and bright coarse color of the casting tree is often a practical necessity, especially for companies that carry out pre-wear of gems. In such cases Fischer-Bühner currently considers that alloys with a medium to high silicon level are the only safe choice. Although dark copper oxide layers on raw casting tree surfaces obtained for silicon-free alloys can be removed by pickling, they are sometimes difficult to completely remove under the gems. A high level of silicon gives the color of the most vivid casting tree in all manufacturing conditions and the whitest metal color after finishing, making it particularly attractive for the pre-wear of gems. In addition, the greater fluidity of an alloy of this type allows lower temperatures of the mold box, which reduces the risk of damaging the gems.

Depending on the composition of the alloy, the liveliness of the gross casting trees also depends significantly on the cooling procedure of the mold boxes after pouring. A common conventional cooling procedure consists of removing the molding box from the mold box chamber ~ 1 min after pouring followed by cooling in air for another 10-20 min before extinguishing. For silicon-free alloys, the surface of the raw casting tree is then covered by a copper oxide layer from gray to dark depending on the temperature of the mold box. The oxidation can be drastically reduced if a molding box is kept for a prolonged period (for example 3-5 min) in the vacuum molding box or protective gas chamber which is then followed by removal of the molding box from the mold. machine and immediate extinction. In this case, only a slightly gray, sometimes yellowish color alteration is observed and internal oxidation (copper subsurface) in the alloy that eliminates fire stains for Si-free alloys and significantly improves the quality of the scrap is avoided. For alloys containing Si, a modification of the process of this type is not significant, since the liveliness of the raw casting tree is not affected by different cooling procedures of the mold box. However, a more protected cooling reduces the consumption of silicon and also improves the quality of the scrap.

Especially for alloys with a wide melting range, like all 925 silver alloys, "hot cracking" or "hot tearing" can be a problem. Hot cracking occurs mainly when mechanical tension acts on the metal during the final phases of solidification, therefore, when there is only a small amount of liquid metal remaining between the growing grains. The contraction The heat of the solidification metal coupled with the thermal expansion of the lost wax material (heating up when in contact with the hot metal) exerts local stresses and tears the metal. Fischer-Buhner explains that silicon-containing alloys are more prone to hot cracking than silicon-free alloys. The somewhat increased risk of hot cracking of silicon-containing alloys as compared to silicon-free alloys can theoretically be understood. Silicon tends to segregate in areas of grain boundaries during solidification where it eventually forms low melting phases. This extends the melting range, from an amplitude of typically ~ 120 ° C for silicon-free alloys to ~ 150-170 ° C for medium to high silicon levels and also increases the solidification time. For example, an article that would need 1.5 min for the completion of solidification if cast in a silicon-free alloy at a molding box temperature of 500 ° C needs about 2.5 min if it is cast in a alloy with a medium to high seat content. Therefore, the danger zone (temperature and time interval) during which hot cracking can occur is extended for silicon-containing alloys.

The so-called "des-ox" sterling silver alloys are available, among others, from United Precious Metal Refining, Inc. ("UPM"), which claims on its website that it has the only available sterilized silver grains with silicon available and that it is said to have the advantages of casting capacity, reduced porosity, absence of incrustations by fire and resistance to surface discoloration. Des-ox alloys are described in two US patents of UPM. US-A-4793446 (Bernhard I) discloses a silver alloy composition consisting essentially of the elements set forth in the table below. A typical composition comprises 92.5% by weight of silver, about 0.5% by weight of copper, about 4.25% by weight of zinc, about 0.48% by weight of tin, about 0%. , 02% by weight of indium, approximately 1.25% by weight of a boron-copper alloy containing 2% boron and 98% copper, and 1% of a silicon-copper alloy containing about 10% silicon and about 90% copper. Silver is present in the minimum percentage necessary to qualify as either silver for coins or sterling silver, as appropriate. Copper (2.625% by weight) is added as a conventional hardening agent for silver as well as the main carrier agent for the other materials. Zinc is added to reduce the melting point of the alloy, to add whiteness, to act as a copper substitute, as a deoxidizer and to improve the fluidity of the alloy. Tin is added to provide resistance to surface discoloration, and for its hardening effect. Indian is added as a grain refining agent and to improve the wettability of the alloy. Silicon (0.1% by weight) acts as a deoxidizer which reduces the porosity of the recolated alloy and has a slight hardening effect. Boron is added to reduce the surface tension of the molten alloy and to allow it to mix homogeneously.

US-A-5039479 (Bernhard II) discloses a master metal composition for making alloys of the above type, tin being apparently optional. An alloy used as a reference example in EP-B-0752014 (Eccles I) and said to be made according to Bernhard II consists of 92.5% by weight of silver, 3.29% by weight of copper , 3.75% by weight of zinc, 0.25% by weight of indium, 0.01% by weight of boron and 0.2% by weight of silicon; it is reasonable to conclude that this is an analysis of a commercial alloy of UPM.

As a result of discussions with Melvin Bernhard of UPM, Anthony Eccles of Apecs Investment Castings Pty Ltd developed the alloys disclosed in EP-B-0752014 (Eccles I) as set forth in the table below. It is said that zinc influences the color of the alloy and acts as a reducing agent for silver and copper oxides. It is said that silicon provides resistance to fire incrustations and to maintain good color. It is said that germanium provides resistance to scale by fire and hardening properties by work and indium and boron can be provided for the modification of rheology, reduction of surface tension and grain refinement. In Eccles I it is said that one disadvantage of the alloys in Bernhard I and Bernhard II is that they have poor hardening properties and do not achieve the mechanical strength of products worked in traditional sterling silver. Many finds of silver, for example, pearls, links, amulets, end bars and the like are made by casting the lost wax, and are not intended to be worked after they have been cast, so the reference to work hardening in Eccles I discourages the use of the alloys given to know to strain the lost wax. The same is true for many finished jewelry products made by casting lost wax, for example, rings and bracelets.

WO 96/22400 (Eccles II) discloses that in some desox alloys a high copper content is desired for the copper capacity to confer hardness, and it is intended to produce alloys with a high copper content which have reduced fire incrustations, porosity and formation of reduced oxides and reduced grain size in relation to conventional sterling silver. The disclosed solution will provide alloys having the general composition set forth in the table below, with optional constituents being in parentheses. It will be appreciated that the essential novelty with respect to Eccles I is the absence of zinc, although a high tin content is considered acceptable. The report explains that alloys with high copper content are inherently prone to fire incrustations and that creating a sterling silver with high copper content, free of scale by fire was unexpected. In particular, it was unexpectedly found that the choice of deoxidizing additive (silicon) provided the ease of a high copper content without significant fire incrustation production, while the more common aggressive deoxidizers such as zinc did not. Fire resistance was considered particularly important for hot working to confer hardness and the use of germanium as an alloying agent provided alloys that are both resistant to scale by Fire as hardenable by work and which were harder than the alloys of the prior art due to their high copper content. Rheology modification additives such as indium and boron are optional components but the ability of boron to act as a grain refiner was not indicated. The disclosed embodiments were alloys of Ag-Cu-Ge-Si and Ag-Cu-Ge-Si-In and there was no embodiment containing boron. However, Eccles II alloys were never developed to give a commercial product despite its seemingly desirable properties, and it is believed that a major motive is a propensity to develop cracks that would have prevented its use to strain the lost wax and for which none of Bernhard I, Bernhard II, Eccles I and Eccles II discloses a solution. In addition, there is no disclosure or suggestion that alloys should be used for casting lost wax and repeated mention of sheeting, rolling and work hardening discourage the use of these alloys for that purpose.

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GB-B-2255348 (Rateau, Albert and Johns, Metaleurop Recherche) discloses a silver alloy that maintains the inherent hardness and luster properties of Ag-Cu alloys while reducing problems resulting from the tendency of the content in copper to oxidize. The alloys are ternary Ag-Cu-Ge alloys containing at least 92.5% by weight of Ag, 0.5-3% by weight of Ge and the remainder, in addition to impurities, copper. The alloys are stainless in ambient air during conventional operations of production, transformation and finishing, are easily deformable when cold, are easily brazed and are said to not give rise to significant shrinkage in the casting. They also have superior ductility and tensile strength. Germanium exerts a protective function that is responsible for the advantageous combination of properties presented by the new alloys, and is in solid solution in both the silver and copper phases. It is said that the microstructure of the alloy is constituted by two phases, a solid solution of germanium and copper in silver surrounded by a solid filament solution of germanium and silver and copper that contains by itself a few dispersed intermetallic Cu-Ge phases. The germanium in the copper-rich phase inhibits the surface oxidation of that phase forming a protective coating of GeO and / or GeO ² thin that prevents staining by fire during the brass and annealing to the flame. In addition, the development of surface discoloration is appreciably delayed by the addition of germanium, the surface becoming slightly yellow instead of black and the surface discoloration products being easily removed by normal tap water. The alloy is useful, among others, in jewelry and silver crafts. Conventional grain refining agents were tested, with the specific materials evaluated or suggested gold, nickel, manganese or platinum. No loss of the lost wax of the alloy was reported.

US-A-6168071 (Johns) describes and claims, among others, a silver / germanium alloy having an Ag content of at least 77% by weight, a Ge content of between 0.5 and 3% by weight, the remainder being copper in addition to any impurity, alloy containing boron as refiner of the grain at a concentration of up to about 20 parts per million. Boron is provided as a copper-boron alloy, for example, which contains 2% by weight of boron and confers greater strength and ductility to the alloy and which allows strong and aesthetically pleasing joints to be obtained using resistance welding or by To be. It was explained that silver refinement alloys of the grain had proved difficult and that one of ordinary skill in the art would not have previously considered boron for this purpose, and that it is effective in inhibiting grain growth even at welding temperatures. Again, the cast wax lost from the alloy was not reported.

EP-B-1631692 discloses a ternary alloy resistant to surface discoloration and fire stains of silver, copper and germanium containing from more than 93.5% by weight to 95.5% by weight of Ag , from 0.5 to 3% by weight of Ge and the rest, in addition to secondary components (if any), impurities and grain refiner, copper. Casting is reported to the lost wax of the strip and the strip is said to be free of short hot defects (cracking). The appearance of the strip as a cast was not evaluated.

GB2426250 (Johns) discloses a silver alloy comprising 92.5-97% by weight of Ag, 1-4.5% by weight of Cu, 0.4-4% by weight Zn, 0.8-1.5% by weight Ge, from 0 to 0.2% by weight of Si, In or Sn and 0-0.2% by weight of Mn, the rest being boron as refiner of the grain, secondary components and impurities. Alternatively, silicon may be present from 0.05-2% by weight. Said alloy preferably comprises boron as a refiner of the added grain as copper boride or as copper hydride, for example sodium borohydride. In one example, an alloy comprising 94.7% by weight of Ag, 1.2% by weight of Ge, 3.9% by weight of Cu and 0.2% by weight of Si together with example 80 ppm of boron becomes agglomerates of castings that are re-melted and used to strain the lost wax using an envelope bound to calcium sulfate. The cast produced had a matte silver finish, could be easily polished and gave products that had excellent hardness and resistance to fire stains. Similar examples are found in GB-A-2438045 (Johns), US 2008/069722 (Johns) and US 2008/078484 (Johns).

Summary of the invention

AgCuGe alloys of the type described above have been useful in continuous casting and for the production of forged articles from the resulting sheet, strip, tube or the like, but their performance as casting alloys to the lost wax has been less good because to alteration of the color that arises during the casting to the lost wax. The articles of these alloys, when removed from the wrap, have exhibited a significant color alteration that has required prolonged processing for removal.

It has now been found that the addition of silicon to the alloys largely or completely prevents such color alteration and also cracking defects and does not result in cracking and other processing problems associated with conventional silicon-containing alloys when used in laundry. to the lost wax.

The invention provides a process for casting a germanium-containing silver alloy to the lost wax, said laundry comprising patterns attached to a shaft, said process being as defined in claim 1 of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate tests for determining cracking during lost-wax casting, in which Figure 1 is a diagram representing an alloy test casting to show the performance of the lost-wax casting of rings, and Figures 2-4 are micrographs showing sections of the casting ring in position 7 in Figure 1.

Description of preferred embodiments

Lost wax casting

The general procedure for making solid wrap molds in the jewelry industry involves joining patterns having configurations of the desired metal castings to a casting channel system to form an arrangement or "tree". The patterns and casting system can be made of wax, plastics or other fungible material. To form the mold, the arrangement or tree consisting of the pattern or patterns attached to the casting system are placed in a molding box that is filled with a refractory wrap suspension that is allowed to harden in the molding box around the tree or be arranged to form the mold. After the wrapping suspension hardens, the patterns melt out of the mold by heating in an oven, stove or autoclave. The mold is then burned to an elevated temperature to remove water and burn-off any residual pattern material in the pouring cavities.

Specific reactions take place with an envelope bound to calcium sulfate during burning. After removing the wax, when the temperature of the mold box rises above 100 ° C (212 ° F), free water evaporates and the gypsum (CaSO4.2H2O) begins to lose its water of hydration. However, the complete transformation of gypsum into the anhydrous form of calcium sulfate (anhydrite) occurs over a wide temperature range, through complex transformations of the crystal lattice. These transformations take place with a contraction of considerable volume, which is particularly intense at 300-450 ° C (572-842 ° F). If plaster were to be used only to produce a casket for lost wax casting, the molds would crack during service and also produce much smaller castings than the original patterns. Silica is used to compensate for this contraction of the gypsum and to regulate the thermal expansion of the mold. There is silica in various crystalline forms, and two of them are used in the production of wrapping powders. Quartz is the most readily available form and its conversion from crystalline forms to b is accompanied by an increase in volume at about 570 ° C (1058 ° F). The cristobalite is the other major constituent of the casing powder and this form of silica also experiences a significant increase in volume as it transforms from its crystalline structure to b up to around 270 ° C (518 ° F). Therefore, these two allotropic forms of silica are used to cancel out the contraction effect of the gypsum binder.

Conventional wrap formulations used for non-ferrous molds are composed of a binder and a refractory material consisting of a combination of fine and coarse particles. Typically, a typical refractory material is completely or at least partially silica, such as quartz, crystalbolite or tridymite. Other refractory materials such as calcined mullite and pyrophyllite can also be used as part of the refractory material. Gypsum powder (calcium sulfate hemihydrate) is almost universally used as a binder for molds intended for casting gold, silver and other metals and alloys having relatively low melting points. The state of the art prior to this invention is reviewed in the article by Jorg Fischer-Buhner commented previously. It will be appreciated that the conditions for casting lost wax differ significantly from the conditions for continuous casting, for example, for the manufacture of rods, bars, tubes, strips and sections where wrapping is not used and the laundry is quickly followed by extinction with water. An indication in a prior art reference that an alloy is suitable for casting should not be interpreted to mean that it is suitable for all forms of casting. In particular, as discussed above, the alloy can have a good performance in continuous casting but can have a poor performance in casting the lost wax, as indicated above.

Silver

The embodiments of the present alloy have a silver content that complies with the standards for sterling silver and Britannia.

Sterling silver has a minimum silver content of 92.5% by weight. However, the embodiments have a silver content of 93-95.5% by weight, for example about 93.5% by weight or more, it being believed that the appearance of reduction in copper elution compared with that of alloys 925 is in the range of 93.0-93.5% by weight of Ag.

One reason why it is feasible to reduce the copper content of the alloy to improve physical properties and reduce copper elution compared to conventional 925 silver alloys is due to the unique hardening properties of the AgCuGe system. Hardening can occur either by slow cooling alone or by cooking at low temperature, which is advantageous because the quenching of any silver alloy heated to red in cold water will always lead to cracking and failure of weld joints. A striking difference has been observed in the 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. The gradual cooling of, for example, binary-type sterile alloys results in coarse precipitates and little precipitation hardening, while the gradual cooling of Ag-Cu-Ge alloys optionally containing secondary components results in fine precipitates and hardening by useful precipitation, especially in those embodiments in which the silver alloy contains an effective amount of grain refiner, for example, boron.

Experimental tests have shown that Ag-Cu-Ge alloys of Ag content of 93.5% by weight and more are hardened by precipitation after cooling from a melting or annealing temperature by baking to for example 200 ° C-400 ° C and that firing the alloy can achieve a hardness of 65 or more, preferably 70 HV or more and still more preferably 75 HV or more, which is equal to or greater than the hardness of conventional sterling silver used to make jewelry and other articles silver. 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 secondary alloying components.

The addition of germanium to sterling silver changes the thermal conductivity of the alloy compared to conventional sterling silver. The International Annealed Cooper Scale (IACS) is a measure of conductivity in metals. On this scale the value of copper is 100%, pure silver is 106% and conventional sterling silver is 96%, while a sterling alloy containing 1.1% germanium has a conductivity of 65%. The importance is that sterling silver and other germanium-containing silver alloys do not dissipate heat as quickly as conventional sterling silver or its germanium-free equivalents, a piece will take longer to cool, and hardening by precipitation to a commercial level useful (for example up to about a Vickers hardness of 70 or more, preferably up to a Vickers hardness of 110 or more, more preferably up to 115 or more) may take place during cooling with natural air or during cooling with slow controlled air. The silver alloy of Ag 973 parts per thousand and containing about 1.0% by weight of Ge, the copper moiety, has been advantageously hardened by precipitation by gradual air cooling from an annealing temperature, and it is believed that the Ag-Cu-Ge alloys with a silver content above this level can also be hardened by precipitation, and this property is retained after incorporation of silicon.

The benefit of not having to extinguish to achieve the hardening effect is a major advantage of the present silver alloys. There are very few times in the practical production in which a silversmith can safely extinguish a practically finished work piece. The risk of distortion and damage to welded joints when extinguished from a high temperature would make the process commercially unviable. In fact, conventional sterling silver can also be hardened by precipitation but only with extinction from the annealing temperature and this is one reason why precipitation hardening is not used for sterling silver.

In order to distinguish annealing and precipitation hardening operations (which are considered different by silversmiths), annealing temperatures can be defined as temperatures above 500 ° C, while precipitation hardening temperatures can be defined as being in the range of 150 ° C - 400 ° C, the lower value of 150 ° C allowing embodiments of the alloys of the invention to be harden by precipitation in a domestic furnace. Annealing generally takes place over a relatively short period of time (approximately 45 minutes maximum, at temperatures above 500 ° C) and is designed to recrystallize the worked metal alloy structure to allow additional work operations to take place. The precipitation hardening takes place over longer times (2-3 hours at the lower temperature) and is designed to increase the hardness of the metal alloy by altering the solid solubility of a secondary phase present in the alloy.

Further embodiments of the present alloy are of Britania silver having a minimum silver content of 95.84% by weight, and will normally have a silver content of 96% by weight. Such alloys retain the ability to harden by precipitation as described above. A silver content in the range of 96-97.2% by weight is also contemplated.

Germanium

Embodiments of the present alloy have a germanium content of 0.7-1.2% by weight. The embodiments of alloy 935 and alloy 960 can have a germanium content of 0.7% by weight, but improved surface discoloration resistance and performance can be obtained for example in alloy 935 at a germanium content of 1, 0-1.2% by weight, for example 1.1% by weight.

Silicon

Silicon can be added in amounts of 0.05 - 0.08% by weight with a reference value of 0.07% by weight (700 ppm) and can be added as a CuSi alloy containing, for example, 10-30 % by weight of Si. In an alloy in which germanium is present, for example, in at least equal amounts it is fully compatible with germanium so that the two elements (which are both metalloids in group IV of the periodic table) form individual phase (s) (en) and the tendency of silicon to migrate to grain boundaries is reduced or eliminated. Accordingly, the advantages arising from the incorporation of silicon in deoxidation and formation of living castings can be obtained and the cracking and other problems associated with conventional silicon-containing silver alloys do not appear or are significantly alleviated. In embodiments, the wt% of silicon is <20% wt% of germanium, for example <10 wt% of germanium, for example, about 10% of the weight of germanium.

Boron

The use of boron as refiner of the grain is a practical necessity when casting the lost wax of silver having an appreciable germanium content. It is advantageously introduced at the time of manufacture of the casting grain which then has the boron content necessary for refining the grain after remelting and casting the lost wax.

A conventional method of introducing boron into a precious metal alloy or master alloy is through the use of the master alloy of 98% by weight of Cu, 2% by weight of B. Many manufacturers have been able to use that alloy without difficulty. but others have reported that it introduces hard points in the products. These hard points are believed to be particles of CuB ²² that are not in the equilibrium phase that are formed in copper saturated with boron when cooled from the liquid phase to the solid phase. The hard points may not be detected until the precious metal jewelry alloy has been polished and examined resulting in unnecessary costs for the processing of an ultimately unsatisfactory product.

A boron compound can be introduced into a molten silver alloy in the gas phase, advantageously mixed with a carrier gas, which aids in the creation of a stirring action in the molten alloy and disperses the boron content of the gas mixture. in said alloy. Suitable carrier gases include, for example, hydrogen, nitrogen and argon. The gaseous boron compound and the carrier gas can be introduced from above into a container containing molten silver, for example, a crucible in a silver melting furnace, a ladle or a tundish using a metallurgical lance which it may be an elongated tubular body of refractory material, for example, graphite or it may be a metal tube plated in refractory material and submerged at its lower end in the molten metal. The lance is preferably of sufficient length to allow injection of the gaseous boron and carrier gas compound deep into the interior of the molten silver alloy. Alternatively, the boron-containing gas can be introduced into the molten silver from the side or from below, for example, using a gas-permeable bubbling buffer or a submerged injection nozzle.

The alloy to be heated can be placed in a solid graphite crucible, protected by an inert gas atmosphere which can, for example, be oxygen-free nitrogen containing <5 ppm oxygen and <2 ppm moisture and heated by heating by electrical resistance using graphite blocks. Such furnaces have a built-in function for bubbling inert gas through the melt. The addition of small amounts of boron-containing gas that can be thermally decomposed to the inert gas that is bubbled through the melt easily provides a few desired ppm or a few tens of ppm of boron content. It is believed that the introduction of the boron compound into the alloy as a stream of gas diluted over a period of time, the gas carrying gas serving the gas stream to stir the alloy or molten metal, rather than into a more relatively large quantities, it is favorable from the point of view of avoiding the development of hard points in the metal or boron alloy. Compounds that can be introduced into silver or molten gold or alloys thereof in this manner include boron trifluoride, diborane or trimethylboro which are available in pressurized cylinders diluted with hydrogen, argon, nitrogen or helium, with diborane being preferred because, apart from boron, The only other element that is introduced into the alloy is hydrogen. Still a further possibility is to bubble carrier gas through the molten silver to effect agitation thereof and to add a solid boron compound, for example, NaBH ⁴ or NaBF ⁴ in the fluidized gas stream as a finely divided powder forming a aerosol.

A boron compound can also be introduced into the molten gold or silver alloy in the liquid phase, either as such or in an inert organic solvent. Compounds which can be introduced in this manner include alkylborane or alkoxyalkylborane such as triethylborane, tripropylborane, tri-n-butylborane and methoxydiethylborane which, for safe handling, can be dissolved in hexane or THF. The liquid boron compound can be introduced and sealed in silver or copper foil containers resembling a capsule or envelope using liquid / capsule filling machinery or liquid / sachet known and using a protective atmosphere to give filled capsules or sachets or other small containers normally with a capacity of 0.5-5 ml, more usually about 1-1.5 ml. Capsules or envelopes filled in appropriate numbers can then be dipped individually or as one or more groups in the molten silver alloy. Still a 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 can take the form of an aerosol in the carrier gas stream, or they can be vaporized therein.

Conveniently, the boron compound is introduced into the molten silver alloy in the solid phase, for example using a solid borane, for example decaborane B10H14 (mp 100 ° C, bp 213 ° C). However, boron is conveniently added in the form of either a metal hydride containing boron or a boron-containing metal fluoride. When a metal hydride containing boron is used, suitable metals include sodium, lithium, potassium, calcium, zinc and mixtures thereof. When a metal fluoride containing boron is used, sodium is the preferred metal. Most preferred is sodium borohydride, NaBH ⁴ which has a molecular weight of 37.85 and contains 28.75% boron.

Boron can be added to the other molten components both in the first casting and at intervals during casting to compensate for the loss of boron if the alloy is kept in the molten state for a period of time, such as in a continuous casting process for the grain . This function is not available when a copper / boron master alloy is used because adding boron changes the copper content and, therefore, the overall proportions of the various constituents in the alloy.

It has been found that, when a borane or borohydride is added, 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 the molten silver: according to one experiment, the boron content in the molten silver drops with a half-life of about 2 minutes. The mechanism for this decay is not clear, but it can be an oxidative process. Therefore, it is desirable to incorporate more than 20 ppm of boron into an alloy as the first cast, ie, before casting the lost wax or before laminating to a strip, and amounts of, for example, up to 50 ppm can be incorporated. , normally up to 80 ppm, and in some cases up to 800 or even 1000 ppm. Thus, a silver melt grain containing approximately 40 ppm boron could be produced according to the present method. Due to the loss of boron during subsequent remelting and casting to lost wax, the boron content of finished parts may be closer to 1-20 ppm of the prior art, but the ability to achieve relatively high initial boron concentrations it means that an improved consistency can be achieved during the manufacturing stages and in the final finished products. In addition, a higher boron content is desirable for master alloys that will melt with precious metals to make the casting grain and then melt further for casting the lost wax.

Secondary components

The embodiments of the present alloys are free of added zinc or other metals added except copper, germanium, boron and silicon and have the advantage, among others, of simplicity of formulation and production. At a higher silver content and a relatively low germanium content, the addition of zinc in other embodiments may be desirable, for example, in amounts of 0.2-1% by weight, for example about 0.4% by weight. weight. Other metals may be added in small amounts, for example up to 0.2% by weight as long as they do not interfere with the overall properties of the alloy, and such metals include, for example, gallium which in some embodiments may further decrease cracking defects. In the embodiments, small amounts of indium may also be present, so that an alloy 960 may comprise boron in amounts of ppm as grain refiner, indium, gallium, zinc, silicon, germanium, copper and silver.

Alloys by way of example

One embodiment of alloy 935 is ^{93, 5%} by weight of Ag, 1.1% by weight of Ge, Si 700 ppm, 3-60 ppm eg 10 B, being the remainder copper. One embodiment of an alloy 960 has 96% by weight of Ag, 0.65% by weight of zinc, 0.7% by weight of Ge, 700 ppm of silicon, 3-60 for example 10 ppm of boron, copper rest. The above alloys exhibit free stain castings after casting to the lost wax and are either substantially free of cracks and voids or have significantly fewer voids, see Figure 1 showing a conventional test cast for a ring having macroscopic porosity and Figures 2-7 which are micrographs of the illustrated alloys in the vicinity of the position 7 where the body of the ring is attached to the casting channel and showing little or no porosity. It will be appreciated, since the molten metal shrinks upon cooling, that a casting channel must ultimately solidify to allow a molten metal to be fed into the cooling melt, since the metal contracts as it cools and to minimize the development of the metal. porosity by contraction. Therefore, the area most sensitive to presenting contraction porosity (or the potential for cracking due to hot cracking or hot tearing) is the area where the pouring channel and the article to be cast are joined. This is why P7 was chosen as the region where contraction porosity was most likely to be present.

Claims (5)

A method for casting a silver alloy containing germanium into the lost wax, said laundry comprising patterns attached to a tree, said method comprising: melt a silver-copper-germanium alloy comprising either (a) in addition to impurities, 93-95.5% by weight of silver, 0.7-1.2% by weight of germanium, 0.05-0.08% by weight of silicon and boron in an amount effective to confer refinement of the grain, the remainder copper, said zinc-free alloy being added or (b) 95.84-96% by weight of silver, 0.7-1.2% by weight of germanium, optionally 0.2-1.0% by weight of zinc, 0.05-0 , 08% by weight of silicon and boron in an amount effective to confer refinement of the grain, the remainder copper; pouring the molten alloy into a hydraulically set shell using gypsum binder, and allow the shell and alloy to cool; Y recovering a laundry having a clean silver appearance when removed from the wrap, substantially free of cracks and substantially free of shrinkage porosity. 2. The process according to claim 1, wherein the silver is approximately 93.5% by weight. 3. The process according to claim 2, wherein the germanium is 1.0-1.2% by weight. 4. The process according to claim 1, wherein the alloy comprises 95.84-96% by weight of silver and 0.7% by weight of germanium. The method according to any preceding claim, wherein the cast is a ring.