Patent Application
20060260778
Not Applicable.
Not Applicable.
1. Field of the Invention
This invention relates to a process to manufacture boron containing precious metal alloys and master alloys. More particularly a solid compound that is either a boron containing metal hydride, and preferably a solid tetrahydroborate, or a boron containing metal fluoride is dispersed throughout a molten precious metal alloy or master alloy.
2. Description of the Related Art
Precious metal jewelry alloys are frequently worked into complex ornamental shapes. To withstand extensive working without fracture, the precious metal alloys require high ductility and high strength. High ductility and high strength are facilitated by an alloy having a low oxygen content and a fine grain structure.
Boron is known to both deoxidize and refine the grain of precious metal alloys. When boron scavenges oxygen from a melt and other oxides in the melt, it cleanses surfaces of the metal. U.S. Pat. No. 5,384,089 to Diamond discloses the use of boron as a deoxidizer for gold-base alloys. This patent discloses that boron causes hard spots. U.S. Pat. No. 6,168,071 to Johns discloses a diffusion bondable silver-copper-germanium alloy that may contain up to 20 parts per million of boron as a grain refiner. The boron is disclosed as added as a component of a copper-2%, by weight, boron master alloy. Both U.S. Pat. No. 5,384,089 and U.S. Pat. No. 6,168,071 are incorporated by reference herein in their entireties. Throughout this patent application, all percentages are weight percent, unless otherwise specified.
A conventional method of introducing boron into a precious metal alloy or master alloy is through the use of the 98% copper-2% boron master alloy. However, the use of such a master alloy frequently introduces hard spots into the products. These hard spots are believed to be non-equilibrium phase CuB22 particles that form in copper saturated with boron when cooled from the liquid phase to the solid phase. Hard spots can also form with other metal-boride compounds such as iron borides (for example Fe5B2 and FeB2). The hard spots are frequently not detected until after the precious metal jewelry alloy is polished and inspected resulting in needless expense for the processing of ultimately unsatisfactory product.
Copper—2% boron master alloys are frequently contaminated with silicon. The silicon contamination may lead to brittleness, as a result of the formation of brittle intermetallic compounds, oxides and low melting eutectics.
A further disadvantage with the use of a copper—2% boron master alloy is that the high mass percent of copper may not be desired for the alloy product.
There remains, therefore, a need for a more effective way to introduce boron as a grain refiner and oxygen/oxide scavenger into a precious metal melt.
In accordance with the invention, there is provided a method to produce a precious metal alloy or master alloy. This method includes the steps of (a) forming a molten precursor alloy of the precious metal alloy or master alloy, (b) disbursing a boron containing compound throughout the molten precursor alloy, and (c) solidifying the boron containing precursor alloy.
It is feature of the invention that the boron containing compound is either a boron containing metal hydride or a boron containing metal fluoride. When a boron containing metal hydride, the metal may be sodium, lithium, potassium, calcium, zinc and mixtures thereof. When a boron containing metal fluoride, the metal is sodium. Most preferred as a boron containing compound is sodium borohydride.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference numbers and designations in the various drawings indicate like elements.
The following definitions are used throughout this patent application:
Master Alloy—Constituents of a precious metal alloy omitting the predominant precious metal. For example, a yellow 10, 14 or 18 karat alloy may contain both silver and gold, only the gold would be omitted in the master alloy. Silver would be present. For a sterling silver alloy, the silver would be omitted and there would be no precious metal constituent present. Master alloys are usually sent to an end user who adds the required amount of precious metal.
Precious Metal Alloy—An alloy having a desired composition for jewelry applications. The alloy includes required amounts of gold, silver, palladium and/or platinum.
Precursor Alloy—A composition slightly off specification for a desired master alloy or precious metal alloy. The addition of a metal foil containing boron compound places the composition on specification. If the boron compound is not wrapped in metal foil, for example wrapped in paper or not wrapped, the precursor alloy composition is on specification for the desired master alloy or precious metal alloy.
The process of the invention is useful to add boron to precious metal alloys and to master alloys with a minimal formation of hard spots. Exemplary of the precious metal alloys are sterling silver alloys and silver alloys containing in excess of 75% silver with the balance being alloying elements, including, but not limited to, copper and zinc, and inevitable impurities. Silver alloys and sterling silver alloys having between 80% and 97% silver are most benefited by the process of the invention.
The process is also useful for gold jewelry alloys having at least 33% (8 karat) of gold with the balance being alloying elements including, but not limited to silver, nickel, copper and zinc as well as inevitable impurities. Most benefited by the process of the invention are those gold alloys having between 37.5% and 77% gold.
The alloy is melted 10 in a suitable crucible. For silver alloys, one suitable crucible is formed from clay-graphite and for gold alloys one suitable crucible is ceramic. The metals are heated to a temperature effective to fully liquify and flow the mixture, typically in the range of from 1950° F. to 2300° F., with a nominal temperature on the order of 2150° F. The melting temperature influences the kinetics of boron evaporation which determines the final boron concentration in the cast precious metal alloy or master alloy. The selected temperature should be sufficiently above the liquidous 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.
To reduce the formation of an oxide slag, the molten precursor alloy should be covered to isolate the metal surface from oxygen. Suitable gas covers include, but are not limited to, a carbon monoxide flame, forming gas flame, argon, nitrogen, hydrogen flame and natural gas flame. Suitable powdered solid covers include, but are not limited to, borax, boric acid, graphite and charcoal.
Once the precursor melt is at the desired molten temperature, a boron containing compound is added 12 to the precursor melt. The boron is added in the form of either a boron containing metal hydride, and preferably as a solid tetrahydroborate, or a boron containing metal fluoride. When a boron containing metal hydride, suitable metals include sodium, lithium, potassium, calcium, zinc and mixtures thereof. When a boron containing metal fluoride, sodium is the preferred metal. Most preferred is sodium borohydride, NaBH4, also referred to as sodium tetrahydroborate. Sodium borohydride has a molecular weight of 37.85 and contains 28.75% boron.
Sufficient boron is added so that an effective amount remains in the cast precious metal alloy or master alloy for effective grain refinement and deoxidation. Between 1 ppm and 1600 ppm boron remaining is effective. Preferably, the boron content is between 100 ppm and 1600 ppm for a master alloy and between 1 ppm and 1000 ppm for a silver- or gold-base precious metal alloy. A nominal boron content in the cast precious metal alloy or master alloy of about 250 ppm is most preferred. Typically, from 0.001% to 0.16% of boron added to the precursor alloy melt is effective.
Boron reacts to form a gas that evaporates at elevated temperatures and it may be necessary to make sequential additions of boron as described hereinbelow to maintain an adequate concentration for grain refining. To enable better mixing into the precursor alloy, the boron compound may be wrapped in a thin metallic foil. The foil may be any constituent of the master melt or an inert material, such as paper, and is preferably a ductile metal that may be formed into a relatively thin foil. Preferred metals for the foil include silver, copper and gold. The foil has a thickness of from about 0.01 millimeter to about 0.3 millimeter to enable the foil wrapped boron compound to be well submerged in the master melt before the foil melts through releasing the boron compound. Once released, the constituents of the boron compound combine with oxygen in the precursor melt to effectively deoxidize the melt and the boron reacts 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 first added to the master melt, the initial reaction is believed to be decomposition of the boron containing grain refiner.
NaBH4(s)→Na(g)+B(s)+2H2(g) (1)
After decomposition, the sodium, hydrogen and boron are all effective to deoxidize the melt as follows:
Na(g)+0.5O2(g)→Na2O(s) (2)
H2(g)+0.5O2(g)→H2O(g) (3)
B(s)+0.5O2(g)+0.5H2(g)→HBO(g) (4)
To achieve a uniform casting, the boron is dispersed throughout the precursor melt by stirring 14. Preferably, the boron is stirred 14 for in excess of 1 minute and typically for from 1-5 minutes. Stirring may be by any means which does not contaminate the precursor melt such as with a graphite stirring rod.
The molten precious metal alloy or master alloy is then cast 16 by a method suitable for forming a desired end product.
One such useful end product is casting grains. Casting grains are roughly spherical particles which are sold to jewelry manufacturers who then investment cast to form a desired article of jewelry. Subsequent to stirring 14, molten precious metal alloy is poured into a grain box 18,
The particles are then removed from the grain tank 20 and dried 22 by centrifugal force and hot air. The roughly spherical grains have a typical diameter of from about 0.1 millimeter to about 5 mm.
In accordance with a second embodiment of the invention, continuous casting may be used to form wrought mill products such as sheet, tubing and wire that is later made into finished products such as jewelry. The stirred boron containing molten metal alloy is transferred to a die 24,
For a well stirred batch process, as illustrated in the combination of
CB=CB,0exp(−kρt/m) (5)
Where CB=the present boron content in ppm.
CB,0=the initial boron content in ppm.
k=a rate constant dependent on alloy composition, temperature, gas cover and melt cover expressed in units of inch3/minute.
ρ=the density of the alloy in troy ounces/inch3.
t=time in minutes.
m=melt weight in troy ounces.
Equation (5) predicts that the rate of boron evaporation is faster for smaller melt sizes and this was observed in practice.
For a continuous casting process as illustrated in the combination of
CB=CB,0(m0/(m0−F0t))exp−(kρ/F0) (6)
Where CB, CB0, t, k and ρ were previously defined.
m0=the initial mass of alloy in a crucible at time=0.
F0=the casting rate in moles/minute.
The time, t, is reset to zero and the initial alloy mass m0, is recalculated after each incremental addition of boron.
The above-described invention is better understood by the examples which follow:
Two hundred troy ounces of a sterling silver precursor melt were melted in a clay-graphite crucible. The precursor melt had a nominal composition of 93% Silver and 7% copper, germanium or zinc. The precursor melt constituents were mixed together and heated under a carbon monoxide flame and covered with a one inch thick layer of borax salt. When the precursor melt temperature reached the flow temperature, 0.0125% boron was added as NaBH4. The boron compound was wrapped in 0.15 mm silver foil for introduction to the master melt. Sufficient power was provided to maintain the temperature of the molten precious metal alloy at the flow temperature. The molten precious metal alloy was then stirred with a graphite stirring rod for 3.7 minutes and poured into a grain box. The molten precious metal alloy was protected by a reducing atmosphere during pouring at the flow temperature. After about 0.25 minutes, all of the molten precious metal alloy was converted into casting grains.
The casting grains were assayed and found to have 13.8 ppm boron. The grains were mounted, polished and etched for examination of grain structure and hard spots. The resulting grain structure was fine and contained no boron hard spots. The material was not brittle when reduced 75% by thickness in a rolling mill. Investment cast rings formed from the casting grains contained no fire scale or hard spots. The rings were stretched 3.25 sizes without annealing before failure.
Table 1 illustrates that the process of the invention is effective to add boron to a sterling silver precursor alloy and that the melt cover appears to have more of an effect on the boron content in the precious metal alloy than does the pour gas. In no instance were hard spots detected on the cast grains.
4500 troy ounces of a sterling silver precursor melt were melted in a clay-graphite crucible. The master melt had a nominal composition of 93% silver and 7% copper, germanium or zinc. The precursor melt constituents were mixed together and heated under a natural gas flame and covered with a layer of charcoal. When the precursor melt temperature reached the flow temperature, 0.0020% boron was added as NaBH4. The boron compound was wrapped in 0.15 mm silver foil for introduction to the precursor melt. Sufficient power was provided to maintain the temperature of the molten precious metal alloy at the flow temperature. The NaBH4 was added incrementally to maintain a good boron concentration by taking into account the change in melt weight and the evaporation of boron with time. A timer was used to add the boron as scheduled below. Each boron addition was placed inside a graphite plunger and mixed into the molten precious metal alloy at the times indicated in Table 2.
At transfer, to the continuous casting die, the molten precious metal alloy was heated to the transfer temperature and cast into a continuous two inch diameter cylindrical bar at the casting temperature under a natural gas flame and charcoal cover. Samples of the cast precious metal alloy were assayed for boron level. Table 3 summarizes the assay results.
125 troy ounces of a white gold precious metal alloy were melted in a ceramic crucible. The precious metal alloy melt had a nominal composition of 75% gold and the balance 6% nickel, 14% copper and 5% zinc. The precious metal alloy constituents were melted together and heated under a carbon monoxide flame to the flow temperature at which time 0.08% boron as NaBH4 was added. The boron compound was wrapped in a paper envelope for introduction to the melt. Sufficient power was provided to maintain the temperature of the molten precious metal alloy at the flow temperature. The molten precious metal alloy was stirred for 1.75 minutes subsequent to the boron addition and then poured into a grain box. The gas cover during pour into the grain box was a reducing atmosphere. After about 0.25 minutes, all of the molten precious metal alloy was converted into casting grain. Analysis of the casting grain showed a clean surface, no hard spots and a fine grain size.
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.
