WO2012125516A2 - Alliages d'argent premier titre résistant au ternissement - Google Patents

Alliages d'argent premier titre résistant au ternissement Download PDF

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WO2012125516A2
WO2012125516A2 PCT/US2012/028676 US2012028676W WO2012125516A2 WO 2012125516 A2 WO2012125516 A2 WO 2012125516A2 US 2012028676 W US2012028676 W US 2012028676W WO 2012125516 A2 WO2012125516 A2 WO 2012125516A2
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silver
titanium
alloy
sterling
melting
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WO2012125516A3 (fr
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Kenneth FOGEL
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KF Licensing Inc
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KF Licensing Inc
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Priority to US14/004,620 priority Critical patent/US20140003992A1/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C5/00—Alloys based on noble metals
    • C22C5/06—Alloys based on silver
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps

Definitions

  • the present invention relates to sterling silver alloy compositions
  • the invention relates to alloy compositions alloying silver with titanium and optionally palladium or niobium, among other metals.
  • Silver metal is very ductile and malleable (being only slightly harder than gold) and is the most lustrous metal on Earth. Silver's brilliant white metallic luster can take a high degree of polish making silver highly desirable in the production of jewelry and tableware. Silver's unique properties are important in the decorative arts, coinage, industry and photography, for example.
  • Fine silver contains at least 99.5 percent pure silver. Fine silver is generally too soft for the production of large, functional objects.
  • Fine silver has good tarnish resistance.
  • the tarnish resistance of silver alloys increases as the percentage of fine silver increases, as pure silver is unreactive in clean air under normal conditions and unreactive with clean water. Because of its softness and malleability, however, fine silver is commonly combined with other metals to produce more durable products, thus increasing its susceptibility to tarnish.
  • the new material is referred to as an alloy.
  • Alloyed silver cannot be classified as sterling silver unless it consists of at least 92.5 percent fine silver, whereas the remaining 7.5 percent may be a combination of other elements in various proportions.
  • Sterling silver alloy is the material of choice where appearance is paramount and strength is important, such as in the manufacture of jewelry, coinage, and silverware.
  • Sterling silver sets the standard for high quality silver products. In addition to offering increased strength and durability, products made with sterling silver will not wear away, as silver plating can.
  • the most common sterling silver alloy consists of at least 92.5 percent silver and up to 7.5 percent copper. Adding copper to silver improves hardness and durability while maintaining the beautiful color of the pure silver, and is a well-known practice in the art of silver manufacture. The difference between the softness of fine silver and the hardness of copper-sterling silver alloy is such that the practice is widely, if not nearly ubiquitously, used in the vast majority of sterling silver production because of the great need in the industry for material that is harder than fine silver.
  • the present invention provides for a unique titanium-containing sterling silver alloy composition with a noteworthy combination of hardness and tarnish resistance.
  • the present invention delivers these benefits and improvements by providing a sterling silver alloy containing the following parts by weight: about 92.5 to about 99.5% silver, about 0.5 to about 7.0% titanium, about 0 to about 6.5%> palladium, and about 0 to about 7.0% niobium.
  • the percentage of silver may be varied depending upon the desired quality and/or desired properties of the alloy to be produced. That is, the proportions of the alloy components may be varied relative to each other and to the silver content depending upon the desired quality and/or desired properties of the alloy to be produced, keeping in mind that the maximum percent of non-silver elements cannot exceed 7.5 percent if the final composition is to maintain a sterling silver classification.
  • the present invention provides a sterling silver alloy composition containing from about 92.5 wt% to about 99.5 wt% silver and from about 0.5 wt%> to about 7.0% wt%> titanium. Depending on desired properties, the
  • composition of the silver alloy can be adjusted in order to have enhanced or balanced properties of hardness, color, firestain resistance and/or tarnish resistance in comparison with conventional sterling silver alloy having a composition of silver (92.5%) and copper (7.5%).
  • the titanium-containing sterling silver alloy can contain additional metal components selected from palladium, niobium, aluminum, germanium, boron, zinc, copper and zirconium.
  • the present invention provides a method of making a titanium-containing sterling silver alloy, including melting a mixture of silver and titanium containing 92.5-99.5 wt% of silver and 0.5-7.5 wt% titanium until the mixture is uniformly mixed, and transferring the melt mixture into a cast or mold.
  • the mixture can contain additional metal components selected from palladium, niobium, aluminum, germanium, boron, zinc, copper and zirconium.
  • the present invention provides an article of manufacture made from a titanium- containing sterling silver alloy described according to any of the embodiments disclosed herein. The article of manufacture includes, but is not limited to, jewelry, silverware, tubing, electrical contacts and dental or body implants.
  • FIG. 1 illustrates work hardenability and heat treatability of a sterling silver containing 1.9 wt% titanium in silver. Even as little as 2% titanium, which is on the borderline of solid solubility, shows signs of age hardening capability.
  • FIG. 2 shows the results of various ageing treatments after solution treating eight samples at 800 °C for two hours and quenching.
  • the hardness results are averages of three readings on each sample.
  • Silver has been an attractive and affordable material for jewelry and larger fine ware for centuries. However, it is not a perfect all-round material and there have been many attempts to enhance some properties or to overcome perceived disadvantages. Fine silver (in the sense of less than 0.1% impurities in otherwise pure silver) is soft and does not work harden steeply. It does have a very attractive color and retains that color in a dry clean atmosphere at room temperature for a useful time. Warmer, humid and particularly sulphurous atmospheres eventually cause even fine silver to tarnish. Cleaning away the tarnish is not difficult but gradually repeated cleaning of the soft surfaces leads to scratching and wear removal of some of the silver.
  • the simplest version of sterling silver is an addition of 7.5% copper. This confers steeper work hardenability, hardens the surface so that it takes and keeps a better polish and the body of the piece is mechanically stronger. Even after annealing to remove cold work hardening effects, the hardness of the alloy is considerably higher than hardness of fine silver. It is possible to heat-treat high copper sterling silver by solution treating for an hour or two at a high temperature, quenching and then ageing for a few hours at a lower temperature around 200-400 °C. This is rarely done because it takes extra time, interferes with the normal sequence of cold working and annealing and may melt existing soldered joints.
  • a 7.5% addition of copper changes the color of the alloy significantly. It is a widely accepted color in the trade but not as brilliantly white as fine silver; it shows very slightly more grey and pink color. High copper sterling silver changes color with time. It is said to tarnish. In fact, the copper appears to intensify the reactions that lead to the lighter tarnishing of fine silver mainly because copper sulphide forms more easily than silver sulphide and has a darker brown to black color.
  • a recent new approach to giving silver more resistance to tarnishing is to cover the surface with a very thin transparent film of oxide of one or more reactive metals such as aluminium, niobium, titanium, zirconium, a process called Atomic Layer Deposition (ALD).
  • ALD Atomic Layer Deposition
  • the silver alloy series described here provides an optimum balance of properties to suit individual manufacturers of silver articles.
  • the balance may be deliberately tilted in favour of higher hardness, better tarnish resistance, less risk of fire-stain.
  • the principle relies on the use of small amounts of reactive metals that diffuse to the surface and form a protective oxide/nitride film. Some of these metals also form a limited solid solution in silver and render it capable of solution treatment and age hardening.
  • compositions of two or three components in ranges as will be understood by a person of ordinary skill in the art, the total sum of the compositions cannot be higher than 100%.
  • a Ti-Ag alloy is defined to contain 92.5 to 99.5% of silver, 0.5 to 7.0% of titanium, and 0 to 7.0% niobium, it is understood that when the silver content is 95.0% and titanium is 5.0%, then the content of niobium will be 0%, but should not be interpreted otherwise.
  • % refers to weight percent.
  • tarnish-resistant refers to the ability of the titanium- containing sterling silver alloys to resist corrosion or discoloration processes, which should be interpreted to be inclusive of yet distinct from terms such as “tarnish- free” or “tarnish-proof in the absolute sense.
  • a "tarnish-resistant” product may contain imperceptible levels of tarn-ishing that only show up using sensitive reflectivity and color measuring instruments.
  • the present invention is a novel titanium sterling silver alloy composition that exhibits an exceptional combination of tarnish resistance and hardness.
  • the titanium sterling silver alloys, made according to the principles of this invention, are thus ideally suited for use as jewelry sterling silver.
  • the titanium sterling silver alloys made according to the teachings presented herein, contain a proportion of silver that meets sterling silver requirements, i.e., from about 92.5 to about 99.5% silver. Titanium is added in proportions of from about 0.5 to about 7.0% titanium.
  • the titanium content of the alloy has unexpectedly resulted in alloys having improved hardness and work hardening characteristics and tarnish resistance compared to conventional sterling silver alloys. Other forms of discoloration are prevented as well, such as fire staining. And for comparable percentage alloying elements by weight, the new alloys have steeper work hardening and age hardening characteristics than sterling.
  • the present invention provides a sterling silver alloy composition containing from about 92.5 wt% to about 99.5 wt% silver and from about 0.5 wt% to about 7.0%) wt%> titanium.
  • the compositions of the silver alloy can be adjusted to provide enhanced or balanced properties of hardness, color, firestain resistance and/or tarnish resistance in comparison with conventional sterling silver alloy having a composition of silver (92.5%>) and copper (7.5%>).
  • the silver content of the alloy is between about 95.0 and about 99.5%> by weight.
  • the silver content is between about 98.5 and about 99.5%.
  • the titanium content of the alloy is between about 4.0 and about 6.0%. In another embodiment the titanium content is about 5.0%. In another embodiment of this aspect, in particular when significantly enhanced hardness is desired, the sterling silver alloy contains the titanium between about 5.0 and about 7.0%.
  • the titanium-containing sterling silver alloy according to any of the embodiments described above, further containing additional metals selected from palladium, niobium, aluminum, germanium, boron, zinc, copper and zirconium.
  • additional metals selected from palladium, niobium, aluminum, germanium, boron, zinc, copper and zirconium.
  • the content of the additional metal or metals is in the range of about 0.1 to about 7.0% by weight, preferably about 0.1 to about 5.0% by weight, and more preferably 0.5 to 2.5%, when present.
  • the titanium-containing sterling silver alloy according to any of the embodiments described above further contains up to about 6.5% palladium, wherein the ternary alloy has improved tarnish resistance and hardness.
  • Palladium also contributes to tarnish resistance but produces a soft alloy.
  • a ternary combination of silver, titanium and palladium has an unexpected combination of hardness and tarnish resistance.
  • the palladium content of the alloy is less than 5.0%. In another embodiment the palladium content is less than 2.5%.
  • the titanium-containing sterling silver alloy according to any of the embodiments described above further contains up to about 7% niobium, wherein the ternary alloy has balanced tarnish resistance and hardness.
  • the niobium content is less than about 6.5%. Niobium processes similarly to titanium. In one embodiment, the niobium content of the alloy is less than 5.0%. In another embodiment, the niobium content is less than 1.5%.
  • the niobium and titanium containing silver sterling composition has improved resistance to deformation, including but not limited to scratches or dents.
  • the present invention provides a method of making a titanium-containing sterling silver alloy, including melting a mixture of silver and titanium containing 92.5-99.5 wt% of silver and 0.5-7.5 wt% titanium until the mixture is uniformly mixed, and transferring the melt mixture into a cast or mold.
  • the method of making a titanium-containing sterling silver alloy further includes hot-working, annealing and/or solution treatment.
  • the melting container is a crucible made of refractory material selected from graphite, zirconia, calcia, and yittria.
  • the crucible is made of stabilized zirconia.
  • the melting and casting steps are performed under an inert atmosphere to avoid reaction of titanium with materials it is in contact.
  • the inert gas is argon.
  • the argon is free of or with a low content of oxygen.
  • the melting step is conducted under a second argon fill after flushing with clean dry argon twice.
  • the melting step is conducted at a temperature from about 100 to about 700 °C higher than melting point of silver.
  • the melting and/or casting steps are conducted under vacuum.
  • the titanium used is granular titanium to avoid titanium segregation.
  • the charge is super-heated from about 150 to about 300 °C above the melting point of silver, and cooled down to the casting temperature while stirring.
  • the melting container is a crucible made from a material selected from zirconia and graphite.
  • the mixture further contains one or more metals selected from palladium, niobium, aluminum, germanium, boron, zinc, copper and zirconium.
  • an additional metal is palladium
  • the casting temperature is increased by an additional 0 to about 100 °C, wherein the palladium content is about 2.5% to about 7.0% by weight.
  • the titanium used is from scrap coated with a film of titanium compound, and the melting and casting temperatures are at least 700 °C above the melting and casting temperatures for silver, and melting and casting are conducted with extensive fluxing treatment to break down the film.
  • the alloy is cast and worked into a piece of jewelry, ingot, or billet, or the like.
  • the alloy is cast and worked into a fine sterling silver product selected from silverware, tubing, electrical contacts, dental implants, and body implants, or the like, by conventional means.
  • the mixture further contains up to about 6.5% palladium, wherein the ternary alloy has improved tarnish resistance and/or hardness.
  • the mixture further contains up to about 7% niobium, wherein the ternary alloy has improved tarnish resistance and/or hardness.
  • the present invention provides an article of manufacture made from a sterling silver alloy described according of any of the embodiments described above.
  • the article of manufacture according to this aspect includes, but is not limited to, jewelry, silverware, tubing, electrical contacts, and dental or body implants.
  • the preferred embodiments of the present invention exhibit the particularly useful advantage of combining hardness and tarnish-resistance in the as-cast state better than known sterling silver compositions.
  • Sterling silver alloys made according to the present invention also provide a major advantage in the hardness of the alloys, and the silver alloys described herein are resistant to deformation. This attribute is especially useful in the making of jewelry because jewelry made using the sterling silver alloys as taught herein will demonstrate a greater resistance to scratches and dents which permits the jewelry to maintain its attractiveness thereby increasing the value of the jewelry to its owners.
  • the titanium addition necessarily modifies the melting, casting, hot-working, annealing and solution treatment.
  • the titanium must be distributed as uniformly as possible at every stage to obtain the best properties.
  • One way to accomplish this is to use finely divided titanium for dissolving in the melt.
  • Casting the alloy of the present invention is preferably performed under an inert atmosphere. Titanium in gold or silver can react with air, nitrogen, and even graphite under certain conditions; even impurities in argon such as moisture are undesirable. For a brief period in other than a highly protective atmosphere, the alloy can act more like titanium than gold or silver.
  • the melting points of palladium (1552 °C) and titanium (1668 °C) are nearer to platinum casting practice than silver (961 °C). As the minority elements they are best dissolved or melted into the silver rather than melted outright.
  • the alloy is either melted and cast under good vacuum, or flushed with clean dry argon twice and melted under the second argon fill.
  • Coarse pieces of titanium should not be used.
  • the charge should be superheated from about 150 to about 300 °C above the melting point of silver, and cooled down to the casting temperature with good stirring action.
  • Melting in a graphite crucible is slow by radiation and conduction from the preheated graphite. Direct induction of the charge in a refractory crucible is faster and the stirring action is useful at melt-out.
  • the melt surface may be clear but there may be un-melted material at the bottom of the crucible, which should be avoided because it can act as a sieve, holding back more solids and slowing discharge into the mold.
  • the appropriate casting temperatures need not be much higher than for conventional sterling silver. Palladium does raise the necessary casting temperature by as much as an additional 40 °C for 2.5% Pd; 60 °C for 5%; 80 °C for 7.5% but fluidity should be good so the extra temperature may not be needed.
  • the final casting temperature however need not differ from normal sterling silver practice.
  • Direct induction heating i.e., not by radiation as in a graphite crucible
  • the stirring action is needed to dissolve the titanium.
  • a higher than normal flask temperature may be chosen. Melt on high power and superheat to about 1270°C with good stirring action.
  • the cool-down and casting temperature may be the same as would normally be used for fine silver.
  • the appropriate casting temperatures need not be much higher than for conventional sterling silver. Fluidity should be good.
  • the flask can be quenched into water but to avoid a slight interference film color appearing as the still hot flask is exposed to air again, allow the flask to cool to a lower temperature than for sterling silver.
  • Scrap particularly fines that have reacted with their previous environment may be coated with a hard tenacious refractory oxide/nitride film which effectively insulates the underlying metal from the bulk of the melt.
  • Melting as opposed to 'dissolving' the 'skin', requires a temperature at least 700°C above the likely casting temperature for silver or gold.
  • Melting 'dirty' Ti containing scrap requires more extensive fluxing treatment to break down the tenacious films. All special scrap can be kept separate from standard sterling; it may be more economical to refine back to silver if special scrap cannot be re-melted with like material.
  • the principles stated above apply whether making castings or, on a larger scale, making a hardener alloy as grain or casting small ingots for further working.
  • a 95/5%) Ag/Ti alloy is among the preferred.
  • the content of titanium may vary, but an optimum alloy for making jewelry castings is usually not to exceed 5%. Less than 5% can be easily made by re-melting 5% alloy with the appropriate fine silver shot or clippings.
  • Using 5% Ti for melting and making jewelry castings follows the same principles as outlined above but should not need such extensive stirring at high temperatures before casting. Argon atmosphere control will still be needed but otherwise the parameters are little different than sterling silver.
  • the T-Ag alloy contains at least 1% by weight of titanium to enhance tarnish-resistance, with an optimum around 5% Ti.
  • the titanium content can be up to 7.49 wt% with silver equal to or greater than 92.5 wt%, the alloy would still be described as sterling silver.
  • This application compares properties of titanium- silver alloys directly with standard sterling silvers.
  • the hardening capability of the Ti-Ag alloys containing only 1.9 wt% of titanium in silver as shown in FIG. 1 is already significant. It work hardens more steeply than fine silver and with only limited ageing capability increases the hardness by 50%. It handles well enough and does not suffer from firestain at all. Final cold working of the alloys after a solution treatment followed by ageing will make the cold work hardening and age hardening additive.
  • This titanium-containing alloy also enhances tarnish resistance. Whether it can be called “tarnish- free” depends on the definition of "tarnish” and how to distinguish “tarnish” from “firestain”. Conventionally, the general public applies the word
  • tarnish to any discoloration of silver, including silver plate, and well beyond to other metals and minerals.
  • the term “tarnish” refers to only that of silver, more specifically, discoloration of sterling silver after manufacture.
  • the term “tarnish- free” or the like is not to say the alloy will never “tarnish”.
  • Firestain in manufacture and tarnish in after life are different concepts, but they are connected. Firestain is caused by preferential oxidation of copper in sterling silver; and tarnish is accelerated by copper but not confined to copper compounds. In the alloy series of the present invention, the copper may be removed, but substituting reactive metals may
  • the alloys may contain palladium, which is quite different from a reactive metal additive, such as one or more of titanium, zirconium, niobium, germanium, boron, zinc, copper and aluminium, which can be up to a maximum of 7.5%, and preferably a maximum of 7%.
  • a reactive metal additive such as one or more of titanium, zirconium, niobium, germanium, boron, zinc, copper and aluminium, which can be up to a maximum of 7.5%, and preferably a maximum of 7%.
  • Palladium is the most reactive of the PGMs but nowhere near as reactive as the other non-PGMs listed. It is also regarded as a precious metal while the rest are non-precious metals. However, in the definition of sterling silver, it comes within the 7.5% maximum.
  • Other metals in the non-silver category up to 7.5% may also include cadmium, copper (the main one), nickel, tin and zinc.
  • the Ti-Ag alloy can be prepared using powder metallurgy techniques, although one downside may be the safety concern of handling titanium powder. They need to be blended mechanically, compacted, and hot extruded. The process produces a tube/rod that is subject to further sintering to complete the alloying process.
  • the Ti-Ag alloy billets can also be readily extruded, which can be examined and further processed to other sterling silver products.
  • an inert gas such as argon
  • a filter may be used to fit over the relief port so that any silver dust that forms is not lost.
  • titanium grains in granular form, about 1 mm grains
  • the titanium does not contain oxide or nitride, because they make the dissolving process more difficult.
  • Some aluminum and/or zirconium in 7.4% titanium/silver would be diluted by a factor of 13.5 and so probably acceptable (and may be potentially useful).
  • Other titanium sources include scrap from other commercial titanium alloys, for example, 2.5% Cu alloy (though preferably copper free for the present invention); 2%A1/2.5%V, 6%A1/4%V, and Al/Mn alloys.
  • the PLC on the vacuum furnace should be programmed to handle the backfill gas.
  • the furnace chamber requires at least some negative pressure, but it can run up to about 700 tor.
  • the backfill gas may cause a greater heat transfer to the chamber and the interior parts, so adequate cooling is needed, especially if the melt cycle becomes extended. To avoid losing silver vapor at the high temperature point in the melting cycle, full vacuum on the grounds is usually avoided.
  • the top temperature limit should be judged against crucible life rather than chemical reactions or fluidity.
  • Titanium does not raise the solidus significantly above pure silver melting point, and unlike conventional sterling silver with low superheat, fluidity is good.
  • Another important aspect of the present invention is the level of superheat, which needs to be regarded in two related but different ways.
  • a successful (a 26 g green powder compact) melt good lost wax castings were obtained by exceeding the melting point of titanium briefly with very good stirring, stopping power for a few seconds and pouring. Relative to pure silver this was a melt-out superheat of about 650 °C and a casting superheat of about 300-400 °C.
  • the superheat for castings may not be very important, but pouring into a billet mold would not normally require much superheat. However even with half vacuum the drop in temperature of the melt will probably be quicker than ten minutes.
  • a billet can be prepared by pouring the melt into a cylindrical mold, which will then be machined and extruded into tube.
  • the limits of solid solubility for Ti in Ag at 962 °C are about 2 wt% Ti, or about 5 atomic % (Murray et al, ASM, 1987; Murray et al, "The Ag-Ti (Silver- Titanium) System”; Bull. Alloy Phase Diag., Vol. 4(2), 1983), the 5 wt% or 7 wt% Ti compositions are well beyond the solid solubility limits.
  • the 2 nd phase is shown to be AgTi, which behaves like most intermetallic compounds, i.e. hard and non-ductile, which along with the tarnish resistance properties are among the important advantages of the present invention.
  • These materials have adequate age-hardening potential, but the volume bulk of the matrix will be pure silver. Age-hardening may or may not be needed.
  • the work hardening potential is steeper than pure silver, but not embrittling. A brittleness problem may be an issue if severe segregation of titanium occurs, but it can be avoided with good
  • Zirconia permits good induction penetration and stirring (unlike graphite based crucibles) and molten titanium must be the fastest way of getting a homogeneous melt.
  • a slight positive pressure of argon at 1650 °C raises no doubts at all, but at 500 torr the volatilization data versus atmospheric pressure need to be watched.
  • control of superheating is an important aspect of the process, which can be achieved by using reduced power to maintain the temperature for soak time. It will probably be necessary to also use partial power during the pour, as the "rate of pour" is critical toward getting a sound cylindrical casting. (If the pour is too fast, a substantial center pipe will form.) Use of a metal mold can speed solidification.
  • the alloy can be cast into investment moulds or billet forms.
  • pouring rate needs to be controlled. Solidification may be directional from the base rather than the sides. On balance, the lower the superheat on pouring, the less tendency for piping.
  • Scrap particularly fines that have reacted with their previous environment may be coated with a hard tenacious refractory oxide/nitride film, which effectively insulates the underlying metal from the bulk of the melt.
  • Melting as opposed to dissolving the skin, requires a temperature at least 700°C above the likely casting temperature for silver or gold. Melting titanium scrap coated with such a film requires more extensive fluxing treatment to break down the tenacious films. In some cases it may be more economical to refine back to silver if special scrap cannot be remelted with like material.
  • the alloys of the present invention can be cast into other products by conventional means, subject to the casting conditions discussed herein.
  • examples of such products include, but are not limited to, silverware, tubing, electrical contacts, dental or body implants, and the like. The following non-limiting examples provide further description of the invention.
  • the conventional additives to silver make sterling silver dissolve in molten silver and tend to lower rather than raise the melting point of the alloy. With the exception of aluminium, the melting point of silver has to be exceeded significant- ly if the reactive metals are to be uniformly mixed into the silver. It is preferable to exceed the melting point of the additive metal and apply stirring. Any of the reactive metals will oxidise before they dissolve in silver if air is the atmosphere above the furnace charge.
  • an initial master alloy of silver with up to 7.5% of the reactive metals the well-mixed charge is placed in a highly refractory crucible (usually in an electric induction furnace), the air is removed and an inert atmosphere (argon) is introduced. Once the reactive metal is dissolved, the temperature can be allowed to fall to about 1000 to 1100 °C before pouring/casting the melt. The remaining superheat is chosen depending on whether an ingot, shot or a fine detail casting is being produced. Subsequent remelting does not require conditions very different from conventional sterling silver apart from the need for an inert atmosphere at temperatures above about 500 °C.
  • An alternative route is by powder technology.
  • Thorough mixtures of reactive metal powders in silver powder may be evacuated of air, compacted under hydraulic pressure in steel dies or by hydrostatic pressure (HIP) and sintered under inert atmosphere at temperatures below the melting point of silver. This produces a workable compact without complete fusion, which may be forged, rolled or extruded. The product of that primary working process can then go on to other working processes, even tensile working processes such as wire drawing, sheet forming. Tungsten inert gas (TIG) welding and laser welding are preferable to torch soldering because the latter requires powerful fluxes to control the reactive metal oxidising preferentially at the joint interfaces. Inert gas furnace soldering is possible. Alloys made by the powder route including off-cuts and similar scrap can be fed back into the melting production route. Other forming methods are under development.
  • Tungsten inert gas (TIG) welding and laser welding are preferable to torch soldering because the latter requires powerful fluxes to control the reactive metal oxidising preferentially at the joint
  • the solid solubility of some of the reactive metals in silver at around 800-900 °C decreasing down to temperatures 200-400 °C introduces the possibility of precipitation or age hardening heat treatments. This may be particularly useful with castings where work hardening is limited but a harder surface is desirable for getting and retaining a good polish and resistance to wear.
  • a test melt is conducted using fine silver in a zirconia crucible to test the furnace procedures.
  • the chamber was charged, the doors sealed, pumped down, purged with dry nitrogen, pumped down again and the backfilled with Argon to 730 tor. Temperatures were monitored optically. Everything went well on the first melt until the superheat reached a point where there substantial silver volatilization began. It reached the point where the optical pyrometer was obscured and stopped working.
  • the melt was allowed to cool and poured the ingot. Upon opening the chamber, there was very noticeable coating of silver dust on most of the chamber interior and furnace parts. However, the fine dust wiped off easily with a cloth.
  • the zirconia crucible was clean, with no evidence of deterioration.
  • the test run generated an alloy ingot useful for testing.
  • the top 15% or so is rough and looks to contain some non-homogeneous material.
  • the central 75% looks as though it can be machined into a useful billet for extrusion. It is likely that the central portion of the ingot contains less than the target 7% Ti, though possibly homogeneous.
  • the inventors examined whether or not there is any "skin" on the metal surface after extrusion.
  • the billet was heated under dissociated ammonia atmosphere. The transfer from the furnace is done in air, but there was no detrimental surface effect on the tube.
  • the Ag-Ti was heated under Argon atmosphere, then transferred in air as usual. The transfer takes less than 10 seconds. However, the operator noticed a "white coating" on the surface of the billet as it came out of the furnace. The 6 mm wedding bands will be machined, so any skin would have been removed.
  • reactive metals suitable for the present invention also include, e.g., aluminium, germanium, niobium, and zirconium.
  • the present invention establishes a balance between several desirable properties and issues associated with them. For purpose of enhancing tarnish resistance, less or no germanium should be used.
  • An alloy tube is made by extrusion, which is used to make flat wedding bands by cold drawing and machining.
  • they can be plain flat bands, approximately 6 mm wide by 2 mm high in US Finger Size 10 (British T+ - U).
  • the XRF tests show Ti concentration between 2.5% and 3.5%. This is lower than the 7% target, because a part of Ti was not completely dissolved in the melt.
  • the furnace chamber is equipped with a relief valve which automatically opens to relieve any positive pressure higher than about 25 torr above ambient atmospheric. In light of this, a potential boil-off problem should be prevented.
  • This method involves alloy formulation by mechanical blending of two or more metallic constituents in specific ratios by weight.
  • the blended powder is progressively inserted into a metal "can" prepared from a compatible material; in this case pure silver.
  • the powder is mechanically compacted with an appropriate steel tool after each aliquot of powder.
  • the open end is closed with a plug of the same material as the can, and the plug is welded in place. Provision is made for placement of an evacuation tube by which the air is removed from the interior of the can by means of a suitable vacuum pump. Upon completing the evacuation, the tube is sealed.
  • the entire can is then heated to an appropriate temperature and hot extruded or forged using an appropriate forging press.
  • the simultaneous application of heat, pressure and plastic flow combine to densify the powder compact and begin the process of solid-state diffusion to form a true alloy. Additional subsequent application of mechanical deformation and heat may be required to complete the alloying process.
  • Samples of residue skulls, etc. can be analyzed by XRF.
  • the melting container and/or cast should be made of refractory materials not reactive with titanium. For example, zirconia is very stable but not as stable as titania. Under certain circumstances raw titanium might 'extract' oxygen from zirconia. These reactions are very tempera- ture dependent as well as relative concentrations and pressure.
  • a test melt of fine silver was used to test the furnace procedures.
  • the chamber was charged, the doors sealed, pumped down, purged with dry nitrogen, pumped down again and the backfilled with Argon to 730 tor. Temperatures were monitored optically. Everything went well on the first melt until the superheat reached a point where there substantial silver volatilization began. It reached the point where the optical pyrometer was obscured and stopped working.
  • the melt was allowed to cool, after which the ingot was poured.
  • the zirconia crucible was clean, with no evidence of deterioration.
  • ISO standard IS04538 1995 has been used in jewelry and watch industries, which is based on exposure to thioacetamide vapour at 75% humidity and ambient temperature.
  • Isomaki et al. Proc. 25th Santa Fe Symposium, 2011
  • a dip test in 2.5% potassium sulphide solution or using thioacetamide are more accelerated/aggressive.
  • the logical starting point for comparison is fine silver because different types of sterling silver tarnish at different rates in different atmospheres.
  • Argentium is probably the most easily available tarnish-resistant sterling silver at present. It is a copper bearing sterling silver containing 1% germanium developed as much for fire stain resistance as tarnish resistance (Johns, Firestain Resistant Silver Alloys, Proc. SFS, 1997). The germanium is believed to achieve its effect by forming preferentially a very thin but protective germanium oxide film that reduces fire scale and at the same time also confers better resistance to tarnish. Another variety is attributed to Eccles et al, who also included copper (Bright Silver-Effect of Trace Elements; Proc SFS, 2006).
  • Fine silver does not exhibit firestain. Repeated alternating cold work and annealing may cause grain growth and this may reticulate the surface so that polishing is more difficult and planar reflectivity is reduced but there is no reason for significant color change.
  • the worst instances of firescale in sterling silver usually occur when copper content is as high as 7.5%. Firestain is significantly reduced but not eliminated as the copper content is lowered towards 2% or less. Some other elements substituting for copper may decrease the firestain propensity. If copper is removed altogether, the substituting element may or may not cause its own color patchiness during polishing.
  • True (copper) firestain happens during annealing in air particularly when an oxidizing flame is used. Annealing in a protective atmosphere prevents copper oxidation.
  • the downsprue from a tree cast was successively rolled and annealed down to about 2 mm square section wire.
  • the cast rings were sound and had a good surface but the titanium averaged just less than 2%.
  • the square wire showed distinct signs of cold working to a hardness that showed promise in combining further careful bending with residual springiness. This is difficult to achieve in sterling silver and virtually impossible in fine silver.
  • Silver has the highest thermal diffusivity of all alloys and so general furnace or flame torch annealing rapidly removes cold work all over. Even soldering is likely to soften cold worked sterling silver in the joint zone.
  • An age hardenable silver alloy might allow hardening to a 'springiness level' if age hardening is applied after final shaping.
  • the circlet was made by a combination of round nose pliers bending and residual springiness.
  • the top joint was made by turning out the tabs, which required some effort, wiring them together after closing a gap of about 4 cm and slowly PUK welding down both sides of the tabs.
  • the argon arc left the joint clean and the heat did not "spread" much beyond the joint, so the circlet remained "springy".
  • the piece was not heat-treated.
  • FIG. 2 shows the results of various ageing treatments after solution treating eight samples for two hours at 800 °C and quenching. They cooled rapidly even before quenching, which is why short lengths of wire were used.
  • the hardness results in FIG. 2 are averages of three readings on each sample and there was considerable variation within each sample.
  • the solution-treated sample zero ageing time
  • the titanium variation should be below 2%.
  • FIG. 2 shows an ageing effect.
  • the apparent over-ageing effect at the longer times is likely to be due to a rogue result in a low titanium area.
  • one of the hardness results for 4 hours at 300 °C was 55.9 HV. Dropping that one result raises the average to 71.6 HV (see FIG. 1).
  • 2 cm x 2 cm coupons are prepared.
  • a 2-cm length of tube can also be cut longitudinally into two segments, flattened in a press and then rolled down to about 1 mm thick "strip", which would give enough area for 600 grit finish coupons.
  • Each ring was exposed to XRF at three points on each edge near the hardness impressions. At each point the XRF beam was first directed at the surface (sur) and again after scraping the surface (sub).
  • the silver results in Table 2 are by difference from 100% so the apparent scatter is caused mainly by the titanium segregation. The difference between surface and sub-surface for silver is not significant. The average silver content is 92.63, just above the sterling silver threshold.
  • the surface and sub-surface results for zirconium and copper are not significantly different and average 0.82 and 1.53% respectively.
  • the titanium results (1.51 to 8.24% in ring Al) indicate considerable segregation and an overall average of 4.62% titanium. There is no consistent pattern as to whether the titanium is higher at the surface or sub-surface. In both rings the average sub-surface titanium appears to be higher than the surface titanium but bearing in mind the large range, this may be a coincidence.
  • XRF does not 'notice' light elements so titanium oxide/ nitride/ carbide at the surface would lead to lower titanium content.
  • the base of the crucible was 12.5 mm curving up to a parallel- sided wall 11.8 mm thick. It was a biscuit deep cream/yellow color inner surface through fracture cross-section of the wall to external surface. Analysis results: 94.6% Zr0 2 , 2.7% MgO, 1.5% Si0 2 , 0.8% A1 2 0 3 , 0.1% Fe 2 0 3 , 0.1% Ti0 2 . This was a magnesia stabilized zirconia crucible. As expected, an X-ray fluorescence (XRF) scan (see later) gave only 100% Zr.
  • XRF X-ray fluorescence
  • the crucible not only showed reaction zones in the base and walls, but both base and walls were now thinner. This assumes that, as new, it was the same size as the 'as new' crucible described above.
  • the base was about 10.6 mm, the wall 8.5 to 9.4 mm thick.
  • the metallic grey inner surface also showed a rippled droplet effect indicating a liquid/solid reaction had taken place. This was not confined only to the thinnest part of the crucible wall but was most obvious in that area and evidently an important part of the erosion of the wall thickness.
  • Typical cross-sections obtained by breaking through a section of the wall showed a distinct zoning. Starting with a silvery light grey color at the inner working surface, there was a 2-4 mm of light grey, 2-3 mm dark grey. 2-3 mm brown grey, before the remaining un-reacted cream/yellow color resumed at the outer surface of the crucible. Near the outer centre of the base, now 10.4 mm thick, the brown-grey had just begun to show through the cream/yellow color.
  • Table 3 shows the results of the XRF analysis for five spots roughly 1 mm diameter.
  • zirconium metal appeared in the melt, it is not unreasonable to suppose titanium substituted for zirconium as a complex oxide in the reacted crucible inner layers. It is possible for silver to have penetrated as a very liquid metal near volatilisation but very unlikely as oxide. Ignoring all except the remaining main three elements and convert the Zr and Ti to their most stable oxides and counting the other oxides originally known to be part of the zirconia crucible as zirconia give results in Table 4, with the original trace elements left in for reference.
  • Table 4 shows the results of the XRF analysis converted to oxides.

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Abstract

L'invention concerne des compositions d'alliage d'argent premier titre et de titane qui montrent une résistance renforcée au ternissement tout en maintenant une dureté acceptable. Les applications et les procédés de fabrication de celles-ci sont divulgués.
PCT/US2012/028676 2011-03-11 2012-03-11 Alliages d'argent premier titre résistant au ternissement Ceased WO2012125516A2 (fr)

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CN103710565A (zh) * 2013-12-19 2014-04-09 北海鑫利坤金属材料科技开发有限公司 一种高硬度抗时效软化的银合金及其制备方法
US20140127075A1 (en) * 2012-11-06 2014-05-08 Richline Group, Inc. Reversibly age hardenable, palladium containing tarnish resistant sterling silver alloys
CN114250350A (zh) * 2021-12-28 2022-03-29 有研亿金新材料有限公司 Pt-10Ir-15Ag合金箔材加工方法
US11702723B2 (en) 2019-01-24 2023-07-18 Subodh Subas Pethe Silver alloy with improved mechanical properties

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CN109943745A (zh) * 2019-04-29 2019-06-28 邵明战 一种抗变色银合金及其制备方法
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US11702723B2 (en) 2019-01-24 2023-07-18 Subodh Subas Pethe Silver alloy with improved mechanical properties
CN114250350A (zh) * 2021-12-28 2022-03-29 有研亿金新材料有限公司 Pt-10Ir-15Ag合金箔材加工方法

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