US20130287622A1

US20130287622A1 - Metal alloy and jewelry articles formed therefrom

Info

Publication number: US20130287622A1
Application number: US13/870,610
Authority: US
Inventors: Asher C. Hoffman
Original assignee: Black and Blue
Current assignee: Black and Blue
Priority date: 2012-04-27
Filing date: 2013-04-25
Publication date: 2013-10-31

Abstract

An article is formed of a metal alloy that includes the following constituents: (a) cobalt in an amount of between about 50.0% to about 51.0% by weight of the article; (b) tungsten in an amount of between about 21.0% to about 22.0% by weight of the article; (c) chromium in an amount of about 22.0% by weight of the article; (d) nickel in an amount of between about 2.5% to about 3.0% by weight; and (e) molybdenum in an amount of about 5.0% by weight. The article can be in the form of a jewelry article, such as a ring, a bracelet, a necklace, or an earring and can be formed by a shell casting process.

Description

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Patent Application Ser. No. 61/639,671, filed Apr. 27, 2012, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to methods of making jewelry items such as finger rings, bracelets, necklaces, pendants, earrings, body jewelry and the like, and more particularly to a metal alloy, such as a tungsten cobalt alloy, that is particularly adapted to form the above jewelry articles by a casting process and more specifically, the present metal alloy composition is especially adapted to be machined after its cast and offers a number of other desirable and advantageous material properties.

BACKGROUND

Jewelry has for centuries been made of soft materials such as gold, silver, platinum and other soft materials, because such metals were malleable, castable, forgeable, moldable or otherwise formable. However, whereas such materials are relatively easy to mold, shape and polish, they are equally subject to wear, scratching and other damage detracting from their longevity appearance and value, i.e., wearing down of edges to a smooth and rounded state.
More recently, the industry has focused on other materials including tungsten, cemented carbide and high tech ceramics that are much harder than the previously mentioned precious metals, and once formed, have increased harness when used in a normal jewelry wearing environment. The problem with such materials is that because of their hardness, they are very difficult to shape, and once formed, require special machining and/or grinding tools to alter their configuration and appearance.
Many of these types of jewelry articles are in the form of tungsten carbide or plain cobalt metal jewelry. However, as discussed herein, there are a number of disadvantages with using these types of materials. The metal alloy of the present invention overcomes the deficiencies of these materials and provides a superior metal that is particularly adapted for use in making various jewelry articles.
With respect to using hard metals, including tungsten, to form jewelry articles, one of the most common manufacturing techniques is a metal injection molding (MIM) process. The metal injection molding (MIM) process is commonly used to precisely manufacture small metal components that exhibit complicated or unusual shapes. The basic MIM process involves: 1) the injection molding of a feedstock comprised of fine metal powders mixed with a polymeric binder, 2) a debinding step wherein the polymeric binder is removed from the component, and 3) a sintering step wherein the porosity of the component is reduced. However, MIM components are often considered to exhibit mechanical properties that are inferior to the properties that are attainable from components produced via machining operations. In the case of surgical devices, the MIM process is often considered an inferior method since these devices require excellent mechanical performance. The MIM process is further discussed in U.S. Pat. No. 4,113,480, the disclosure of which is incorporated herein by reference. The feedstock will vary depending upon the final articles that are to be produced and can include, in addition to the binders that serve to carry the metal powders into the mold, a percentage by weight of tungsten-carbide (WC) and a percentage by weight of cobalt (the percentages of each can vary widely and metallic binders other than cobalt (e.g. nickel) can be used, as well). In addition, other alloying metals or compounds can be added to the feedstock as additives (e.g. tantalum, tantalum-carbide, titanium-carbide, niobium-carbide, chromium-carbide, cobalt-nickel, nickel-tantalum, titanium-nitride, and diamond dust), which produce different chemical and physical properties in the resulting cemented carbide.
Conventionally, tungsten jewelry, including tungsten rings, is manufactured using the MIM process; however, as can be appreciated based on the above disclosure, the MIM process is expensive and is time consuming. As a result, there is a need for an improved tungsten alloy that is suitable for undergoing different, less expensive and less complex manufacturing techniques to product jewelry articles having superior properties.

SUMMARY

In accordance with one exemplary embodiment, an article is formed of a metal alloy that includes the following constituents: (a) cobalt in an amount of between about 20.0% to about 60.0% by weight of the article; and (b) tungsten in an amount of between about 20.0% to about 60.0% by weight of the article. Other constituents such as chromium, nickel, molybdenum and carbon can be used in forming the metal alloy of the present invention. The formed article is particularly suited for making jewelry articles.
In accordance with one exemplary embodiment, an article is formed of a metal alloy that includes the following constituents: (a) cobalt in an amount of between about 50.0% to about 51.0% by weight of the article; (b) tungsten in an amount of between about 21.0% to about 22.0% by weight of the article; (c) chromium in an amount of about 22.0% by weight of the article; (d) nickel in an amount of between about 2.5% to about 3.0% by weight; and (e) molybdenum in an amount of about 5.0% by weight. The article can be in the form of a jewelry article, such as a ring, a bracelet, a necklace, or an earring. In accordance with one exemplary embodiment, the article is formed by a casting process, such as a shell casting process.
In yet another exemplary embodiment, a method for forming a scratch resistant, machinable article of jewelry includes the steps of: (a) preparing a metal alloy that comprises: (1) cobalt in an amount of between about 50.0% to about 51.0% by weight of the article; (2) tungsten in an amount of between about 21.0% to about 22.0% by weight of the article; (3) chromium in an amount of about 22.0% by weight of the article; (4) nickel in an amount of between about 2.5% to about 3.0% by weight; and (5) molybdenum in an amount of about 5.0% by weight; (b) pouring the metal alloy in a melted state into a mold (shell); and (c) forming the jewelry article by means of a shell casting process, wherein the formed jewelry article is at least substantially scratch resistant, yet flexible, and is machinable and of such hardness that a stone can be set directly into the jewelry article.

BRIEF DESCRIPTION OF DRAWING FIGURES

FIG. 1 is a perspective view of a ring made with a tungsten carbide composition of the present invention and according to a shell casting process as described herein;

FIG. 2 is a perspective view of a jewelry article made with the tungsten carbide composition of the present invention;

FIG. 3 is a cross-sectional view showing shell components used in a shell casting process; and

FIG. 4 is a side perspective view showing a resulting blank formed with the casting shells of FIG. 3.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The present invention includes a composition and method for making composite articles that have desirable material characteristics including being scratch resistant (yet still flexible), machinable and can be manufactured using a casting process. Jewelry items such as finger rings, bracelets, necklaces, pendants, earrings, body jewelry, and the like, are one particular example of such articles. Medical, dental, and industrial devices or components are other examples of such articles.
More specifically and according to one embodiment, the present invention relates to a tungsten carbide composition that is particularly suitable for making jewelry articles and has unique properties that allow the composition to be cast (e.g., using a shell casting process as described herein), machined, and manipulated (e.g. set with stones), while still being at least substantially scratch resistant and heavy in weight (all of which are desirable properties). The tungsten carbide composition of the present invention can thus be used to form jewelry articles using techniques other than metal injection molding (MIM) techniques, while providing superior results. In other words, the tungsten carbide composition of the present invention allows tungsten jewelry articles, such as rings, to be manufactured using an alternative process other than MIM.
In accordance with one embodiment of the present invention, a metal alloy that is particularly suitable for use in making jewelry articles by shell casting includes the following constituents (% by weight):


	Cobalt (Co)	20.0 to 60.0%
	Tungsten (W)	20.0 to 60.0%

In accordance with one embodiment of the present invention, a metal alloy that is particularly suitable for use in making jewelry articles by shell casting includes the following constituents (% by weight):


	Cobalt (Co)	45.0 to 55.0%
	Tungsten (W)	20.0 to 25.0%

In accordance with another embodiment of the present invention, a metal alloy that is particularly suitable for use in making jewelry articles by shell casting includes the following constituents (% by weight):


	Cobalt (Co)	50.0 to 51.0%
	Tungsten (W)	21.0 to 22.0%
	Chromium (Cr)	22.0%
	Nickel (Ni)	2.5 to 3.0%
	Molybdenum (Mo)	5.0%

In accordance with the present invention, the metal alloy can include carbon as a constituent; however, when carbon is part of the metal alloy, the carbon is present in no more than 2.0% by weight.
The above metal alloy of the present invention also has the following properties. The metal alloy has a hardness value on the Rockwell scale of HRC39-43 and in particular, HRC40-42. As is known, the determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load, and then noting the depth of penetration, vis a vis, hardness value directly from a dial, in which a harder material gives a higher number.
The density of the metal alloy is about 10.2 g/cm and the melting point of the metal alloy is between about 1495° C. to about 1907° C. depending upon the specific formulation of the alloy.
It will be appreciated that there can be some variation in the weight percentages of the above constituents that form the metal alloy of the present invention while still maintain the advantageous properties and characteristics described herein.
In accordance with one embodiment, a three-dimensional article (a blank) that is formed with the metal alloy described above by means of a shell casting process (see blank 400 of FIGS. 3-4). As will be understood, the article (blank) is then further processed using conventional techniques, such as machining/polishing, to form an article of jewelry, such as a ring 100 shown in FIG. 1. As described herein, the article (blank) can be processed into any number of different types of jewelry articles and therefore, a ring is merely one exemplary jewelry article and is not limiting of the present invention. Other exemplary jewelry articles include but are not limited to bracelets, necklaces, pendants, earrings, etc. FIG. 2 shows another jewelry article 200 which can be formed using the alloy composition of the present invention and can be formed from a blank as discussed herein. FIG. 2 illustrates that multiple articles can be combined to form a complete jewelry article.
In addition, as described herein, the article can be further manipulated to produce the finished jewelry article. For example, the article can be set with stones or the like and can be easily machines (to allow intricate details to be formed) and also can be easily polished. All of these attributes are advantageous and desirable and are in contrast to the more limiting processing available with articles formed using a MIM process.
In accordance with the present invention, the article of jewelry can be formed using any number of different techniques; however, in accordance, with one preferred technique, the article of jewelry is manufactured using a casting process. More specifically, the article of jewelry is formed using a shell casting process as described below.
Investment casting is a traditional industrial process based on and also called lost-wax casting, which happens to be one of the oldest known metal-forming techniques. Casts can be made of a wax model itself (the direct method); or of a wax copy of a model that need not be of wax (the indirect method). The following steps are for the indirect process and are provided for illustrative purposes only and are not limiting of the present invention. A first step is to produce a master pattern (model). An artist creates an original pattern from wax, clay, wood, plastic, steel, or another material. A mold, known as the master die, is made of the model (master pattern). The model can be made from a low-melting-point, such as metal, steel, etc. Although called a wax pattern, pattern materials can also include plastic and other materials. In one process the wax is poured into the mold and swished around until an even coating covers the inner surface of the mold. This is repeated until the desired thickness is reached. Another method is filling the entire mold with molten wax, and let it cool, until a desired thickness has set on the surface of the mold. After this the rest of the wax is poured out again, the mold is turned upside down and the wax layer is left to cool and harden. If a core is required, there are generally two options: soluble wax or ceramic. Soluble wax cores are designed to melt out of the investment coating with the rest of the wax pattern, whereas ceramic cores remain part of the wax pattern and are removed after the workpiece is cast. The wax patterns can then be assembled by removing from the mold. Depending on the application multiple wax patterns may be created so that they can all be cast at once. In other applications, multiple different wax patterns can be created and then assembled into one complex pattern. In the first case the multiple patterns are attached to a wax sprue, with the result known as a pattern cluster, or tree; as many as several hundred patterns may be assembled into a tree. The wax patterns are attached to the sprue or each other by means of a heated metal tool. The wax pattern can also be chased, which means the parting line or flashing are rubbed out using the heated metal tool. Finally it is dressed, which means any other imperfections are addressed so that the wax now looks like the finished piece.
The ceramic mold, known as the investment, is produced by three repeating steps: coating, stuccoing, and hardening. The first step involves dipping the cluster into a slurry of fine refractory material and then letting any excess drain off, so a uniform surface is produced. This fine material is used first to give a smooth surface finish and reproduce fine details. In the second step, the cluster is stuccoed with a coarse ceramic particle, by dipping it into a fluidized bed, placing it in a rainfall-sander, or by applying by hand. Finally, the coating is allowed to harden. These steps are repeated until the investment is the required thickness. Common refractory materials used to create the investments are: silica, zircon, various aluminum silicates, and alumina. Silica is usually used in the fused silica form, but sometimes quartz is used because it is less expensive. Exemplary binders used to hold the refractory material in place include: ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying), sodium silicate, and a hybrid of these controlled for pH and viscosity. The investment is then allowed to completely dry. Drying can be enhanced by applying a vacuum or minimizing the environmental humidity. It is then turned upside-down and placed in a furnace or autoclave to melt out and/or vaporize the wax. Most shell failures occur at this point because the waxes used have a thermal expansion coefficient that is much greater than the investment material surrounding it, so as the wax is heated it expands and induces great stresses. In order to minimize these stresses the wax is heated as rapidly as possible so that the surface of the wax can melt into the surface of the investment or run out of the mold, which makes room for the rest of the wax to expand. In certain situations holes may be drilled into the mold beforehand to help reduce these stresses. Any wax that runs out of the mold is usually recovered and reused.
The mold is then subjected to a burnout to remove any moisture and residual wax, and to sinter the mold. Sometimes this heating is also used as the preheat, but other times the mold is allowed to cool so that it can be tested. If any cracks are found they can be repaired with ceramic slurry or special cements. The mold is preheated to allow the metal to stay liquid longer to fill any details and to increase dimensional accuracy, because the mold and casting cool together.
The investment mold is then placed cup-upwards into a tub filled with sand. The metal may be gravity poured, but if there are thin sections in the mold it may be filled by applying positive air pressure, vacuum cast, tilt cast, pressure assisted pouring, or centrifugal cast. The shell is hammered, media blasted, vibrated, waterjeted, or chemically dissolved (sometimes with liquid nitrogen) to release the casting. The sprue is cut off and recycled. The casting can then be cleaned up to remove signs of the casting process, usually by grinding or other machining process.
While suitable for many uses, the above described investment cast process is not particularly suited for use in the present invention and in particular to form article with the present metal alloy due to a number of reasons including the fact that the high melting temperatures of the material (metal alloy) prevent investment from being used. As a result, another casting process is required for use with the present metal alloy.
Thus, the above-described traditional casting process is modified for alternative-metal casting. In accordance with the present invention, the article is formed using a shell casting process (alternatively, a modified lost wax casting). Normally, the wax that is inside the special plaster (referred to as the investment as described above) dissolves in water at the end of the process, thereby freeing up the tree that is made of a metal, such as gold, platinum or silver. For alternative metal casting, the melting temperatures are too high for investment since, as described above, the melting point temperature for the metal alloy of the present invention is between about 1495° C. and about 1907° C.
In view of the foregoing high melting point temperatures, the wax trees are dipped in special types of plaster to build up layers. The pour step of the present process is aggressive. Instead of simply using water to release the tree from the plaster by dissolving the wax, the alternative process includes a hammer step in which the article is hammered and the plaster is shattered in order to free the casted article, such as article.
In particular, one exemplary shell process for use with metal alloy of the present invention in order to produce jewelry articles is described below. The shell casting process can be described as including the following steps:
1) production of wax/plastic “sub-masters”: these waxes are either 1) final designs that will go directly to finishing after casting, 2) ingots/blanks that are then machined after casting and then sent to finishing, or 3) final designs that will also get machined and then go to finishing.
2) addition of waxes to a tree: this tree is structured in the traditional way for casting.
3) layer by layer application of a ceramic slurry: a tree is dipped into this slurry once a day and allowed to dry (harden). Generally, 5 layers are applied. After 5 days the coated tree is ready.
4) burn out: the hardened ceramic shell is placed in an oven. The wax/plastic is burned out
5) molten alloy is prepared in accordance with the present invention: previously prepared metal mixture in melted.
6) pour: the ceramic shell is removed from the oven and the molten metal is poured in.
7) cooling: the filled ceramic shell is allowed to cool for a day.
8) release: the metal core is released from the ceramic shell by shattering the shell with heavy blows. Usually this is done manually with a hammer, but in some cases a pneumatic tool is used.
9) clipping: metal blanks, or final designs are clipped from the tree and the left over button of the pieces is ground off.
10) finishing 1: castings are sent to a tumbling process. This rough polishes and burnishes the metal.
11) finishing 2: if and machining is needed, combination with other metals, or inlay.
13) finishing 3: any stone setting that might be required.
14) finishing 4: polishing using a buffing wheel and various abrasive compounds of increasing fineness to bring about a bright luster.
15) finishing 5: wash-out/steam: pieces are submerged in an ultrasonic bath to release polishing debris/compound. then, they are steamed to blast off any residual material loosed by the bath.
16) finishing 6: any additional finish like satin, sandblast, beadblast, scratch, acid etch, photo etch, plating (ionic, DLC, etc. . . . ), and the like. This could also be the point where other components are laser welded to the main body of the piece, or laser engraving of any additional design details like the logo, or hallmark.
17) finishing 7: final steam clean.
18) Quality assurance (QA) inspection: pieces are inspected to ascertain if the final product is properly made.
19) repeat of any step needed to correct a flaw found in QA.
20) package and ship.
FIG. 3 shows an exemplary shell cast process. Element 300 represents a shell that has a wall 302 that defines a hollow interior 305 into which the molten alloy material is delivered (poured). It will be appreciated that the wall 300 can be formed of one or more layers of plaster that form the shell. The hollow portion 305 of the shell 302 defines a flow path for the molten material. Molten material is delivered into the hollow interior 305 and flows according to the arrows 310 to allow complete filling of the hollow interior 305. The shape of the resulting blank that is formed and in effect is in the shape of the hollow interior 305 of the shell 300. FIG. 4 shows a resulting blank 400 that is formed after the wall 302 (e.g., plaster layers) shown in FIG. 3 is removed. Element 410 represents a screw that is formed in the pour step (see FIG. 3) and this portion is removed, thereby forming a ring shaped blank 400 in this illustrated embodiment. As discussed above, the blank 400 is then processed (machined, etc.) and stones are optionally set, etc.
It will be understood that the preceding steps describe an exemplary shell casting process and are not limiting of the present invention since one or more the steps can be modified and/or eliminated.
In one embodiment, the casting process to form blank 400 (and ultimately article 100) is performed at a temperature about 2000° C. which is above the melting point for the metal alloy of the present invention. The casting process takes a number of days and in particular, can take a number of days to form the blank 400 (and ultimately article 100).
There are a number of advantages obtained by using the metal alloy of the present invention and in particular, why the tungsten cobalt based metal alloy of the present invention is superior to using conventional tungsten carbide or conventional cobalt for forming jewelry articles. First, the present metal alloy has improved appearance relative to conventional articles of tungsten carbide but with a lower cost due to the simplified manufacturing process and the use of the special alloy of the present invention. Second, the metal alloy is scratch resistant but still flexible, thereby making it less likely to shatter compared to tungsten carbide. Third, the metal alloy of the present invention is machinable. The metal alloy is soft enough that designs can be more elaborate and stones can be set directly in the metal. Fourth, consumers are already familiar with tungsten and cobalt metal in terms of jewelry and therefore, on a business side, using the metal alloy of the present invention will be familiar.
In one embodiment, the present invention is directed to providing a metal alloy that is castable (as opposed to being limited to being produced by a MIM process) and machinable due to the material properties of the metal alloy composition and provides other advantageous properties such as being at least substantially scratch resistant (yet retain some flexibility). This is in contrast to conventional metal alloys, such as the ones described herein.
While the invention has been described above and illustrated with reference to certain embodiment of the invention, it is to be understood that the invention is not so limited. Modifications and alterations will occur to others upon reading and understanding of the specification including the drawings.

Claims

What is claimed is:

1. An article formed of a metal alloy comprising:

cobalt in an amount of between about 50.0% to about 51.0% by weight of the article;

tungsten in an amount of between about 21.0% to about 22.0% by weight of the article;

chromium in an amount of about 22.0% by weight of the article;

nickel in an amount of between about 2.5% to about 3.0% by weight; and

molybdenum in an amount of about 5.0% by weight.

2. The article of claim 1, wherein the article comprises a jewelry article.

3. The article of claim 2, wherein the jewelry article is selected from the group consisting of a ring, a bracelet, a necklace, a pendant, and an earring.

4. The article of claim 1, wherein the article is formed by a shell casting process.

5. The article of claim 1, wherein the metal alloy has a melting point of between about 1495° C. and about 1910° C.

6. The article of claim 1, wherein the metal alloy has a hardness value on a Rockwell scale of between HRC40 and HRC42.

7. The article of claim 1, wherein the metal alloy has a density of about 10.2 g/cm.

8. The article of claim 1, wherein the metal alloy is shell castable and can be machined after being formed by a shell casting process.

9. A method forming a scratch resistant, machinable article of jewelry comprising the steps of:

preparing a metal alloy that comprises:

chromium in an amount of about 22.0% by weight of the article;

nickel in an amount of between about 2.5% to about 3.0% by weight; and

molybdenum in an amount of about 5.0% by weight;

pouring the metal alloy in a melted state into a mold; and

forming the jewelry article by means of a shell casting process, wherein the formed jewelry article is at least substantially scratch resistant, yet flexible, and is machinable and of such hardness that a stone can be set directly into the jewelry article.

10. A method forming a scratch resistant, machinable article of jewelry comprising the steps of:

preparing a metal alloy that at least includes:

cobalt in an amount of between about 50.0% to about 51.0% by weight of the article; and

pouring the metal alloy in a melted state into a mold; and

11. The method of claim 10, wherein the metal alloy includes carbon in an amount up to about 2% by weight of the article.

12. The method of claim 10, wherein the metal alloy further includes chromium, nickel and molybdenum.

13. A jewelry article formed of a metal alloy that undergoes a shell casting process to form a blank that is then processed to form the jewelry article, wherein the metal alloy includes cobalt present in an amount between 20.0% to about 60.0% by weight of the article and tungsten present in an amount between 20.0% to about 60.0% by weight of the article.

14. The jewelry article of claim 13, wherein the blank is processed by a machining process.

15. The jewelry article of claim 13, wherein the cobalt is present in an amount of between about 50.0% to about 51.0% by weight of the article; and the tungsten is present in an amount of between about 21.0% to about 22.0% by weight of the article.

16. A jewelry article formed of a metal alloy comprising:

cobalt in an amount of between about 20.0% to about 60.0% by weight of the article; and

tungsten in an amount of between about 20.0% to about 60.0% by weight of the article;

wherein the jewelry article is formed of a blank that is formed of the metal alloy by a shell casting process;

wherein the blank is machinable.