Tarnish and sweat resistant low karat gold alloys

Abstract

This invention provides low karat, low silver, 6 kt gold-copper-zinc alloys with acceptable workability that can be processed into wire, tube, sheet stock, or cast. The alloys are annealed at 1200° F., rapidly cooled, and heat treated at about 600° to 800° F., which increases the hardness and durability in finished parts made from these alloys. The alloys include grain refiners. The alloys are resistant to oxidation from sweat and tarnishing. Additional fabrication operations can form jewelry items such as balls, chain, hoops and studs.

Claims

1. A castable 6 karat gold alloy comprising (w/w): Au 25%; Cu 45-60%; Zn 15-21%; at least one additional element selected from the group consisting of Al at up to 2%, Pd 4-6%, and Pt 4-6%; and Co exceeding 3% and not exceeding 4%.

2. The alloy of claim 1, wherein the alloy has CIELAB colors (L, a, b) in a range of L=72 to 84; a=−1.0 to +1.0; b=17.5 to 28.

3. The alloy of claim 1 wherein said at least one additional element is limited to Al.

4. The alloy of claim 3, wherein the alloy has CIELAB colors (L, a, b) in a range of L=72 to 84; a=−1.0 to +1.0; b=17.5 to 28.

5. The alloy of claim 1 wherein said at least one additional element is limited to Pd.

6. The alloy of claim 5, wherein the alloy has CIELAB colors (L, a, b) in a range of L=72 to 84; a=−1.0 to +1.0; b=17.5 to 28.

7. The alloy of claim 1 wherein said at least one additional element is limited to Pt.

8. The alloy of claim 7, wherein the alloy has CIELAB colors (L, a, b) in a range of L=72 to 84; a=−1.0 to +1.0; b=17.5 to 28.

9. The alloy of claim 1, further including at least one additional additive to increase hardness selected from the group consisting of B, Ru, and Ir.

10. The alloy of claim 9, wherein the alloy has CIELAB colors (L, a, b) in a range of L=72 to 84; a=−1.0 to +1.0; b=17.5 to 28.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1. Work-Hardening and annealing curves for selected 6 kt and 10 kt alloys. (FIG. 1A) Work-Hardening curves. (FIG. 1B) Annealing curves.

(2) FIG. 2. Backscatter Electron Image (FIG. 2A) and EDS Spectra (FIG. 2B) of RD 0106 after tarnish testing.

(3) FIG. 3: Backscatter Electron Image (FIG. 3A) and EDS Spectra (FIG. 32) of RD 0106 before tarnish testing.

(4) FIGS. 4A and 4C are electron micrographs of cast alloy G0026 (Example 1).

(5) FIGS. 4B and 4D are electron micrographs of cast alloy G0026 with grain refiners (Example 1).

DETAILED DESCRIPTION

(6) Table 1 lists some of the low karat alloys developed and the tarnish/sweat test results. Prior to tarnish and sweat testing, the alloys were solution-annealed at 1200° F. for 1 hr and then air cooled. Any thermal oxides formed during annealing were removed using a mass finishing process. All the compositions developed had a comparable tarnish resistance to Leach-Garners 10 kt yellow gold denoted LG-0120, a prior art conventional 10 kt gold alloy. The addition of Pd and Pt improved the sweat resistance when compared to LG-0120.

(7) TABLE-US-00001 TABLE 1 Tabulated Sweat and Tarnish Testing Results Alloy Tarnish Sweat Numbers Composition (% w/w) Resistance.sup.A Resistance.sup.B LG 0120 Au 41.7; Ag 11.2; Ag: Cu 40.5; Yes No Zn 6.5 (prior art) LG 0026 Au 25.0; Ag 7.9; Cu 57.8; =10 kt =10 kt Zn 9.4 RD 0100 Au 25.0; Cu 52.5; Zn 22.5 =10 kt =10 kt G1236 RD 0106 Au 25.0; Cu 50.8; Zn 17.7; =10 kt >10 kt G1244 Ru 0.5; Pd 6.0 RD 0107 Au 25.0; Cu 48.8; Zn 20.6; =10 kt >10 kt G1245, Ru 0.5; Pt 5.0 G1246 RD 0111 Au 25.0; Cu 48.6; Zn 20.8; =10 kt >10 kt G1249 Pt 5.1; B 0.5 RD 0112 Au 25.0; Cu 54.6; Zn 18.4; =10 kt >10 kt G1252 Al 2.0 RD 0114 Au 25.0; Ag 7.9; Cu 55.7; =10 kt =10 kt Zn 8.9; Co 2.0; B 0.5 RD 0115 Au 25.0; Cu 50.8; Zn 15.2; =10 kt >10 kt G1256 Pd 6.0; Co 2.5; B 0.5 .sup.ATarnish solution: Immersion in a 2% sulfurated potash (potassium sulfide) in deionized (DI) water solution at room temperature. .sup.BSweat solution: Immersion in a 0.5% sodium-chloride, 0.1% urea, and 0.1% lactic acid solution at room temperature.

(8) Evaluation of samples: tarnish and sweat test results for a preferred 6 kt embodiment (RD 0106) were compared to the conventional LG 1020 alloy in cast rings that were dipped into the tarnish or sweat test solutions described in Table 1. After 2 mins in the tarnish solution, the RD 0106 and LG 0120 samples were identical, and showed no discoloration. After 20 hrs. of exposure to the sweat test solution the 10 kt alloy developed a dark corrosion layer, however the RD 0106 sample was resistant to corrosion and did not form a dark layer.

(9) Table 2 compares the CIELAB colors of the inventive RD 0106 alloy compared with the prior art 10 kt LG 0120 alloy. The 6 and 10 kt colors are comparable although the 10 kt is slightly tinted red and the 6 kt is slightly tinted green. This is expected due to the high zinc content of the 6 kt alloy. After tarnish testing, both alloys darkened slightly (L decreased by about 13 points for each) but the colors were otherwise nearly indistinguishable between the two samples tested.

(10) TABLE-US-00002 TABLE 2 CIELAB Color scale results Material L a b 6 kt-RD 0106 79.5 −0.2 22.4 6k-RD 0106 - post tarnish 66.5 0.1 26.2 10 kt LG 0120 (prior art) 76.9 −0.4 24.6 10 kt-LG 0120 - post tarnish 64.0 −0.5 28.7 CIELAB colors: L is a scale of lightness from black (0) to white (100), a is a measure from green (−) to red (+), and b is a measure from blue (−) to yellow (+). CIELAB was designed so that the same amount of numerical change in these values corresponds to roughly the same amount of visually perceived change

(11) In an embodiment, the inventive alloys have CIELAB colors (L, a, b) in a range of L=72 to 84; a=−1.0 to +1.0; b=17.5 to 28 (without exposure to a tarnish solution).

(12) Table 3 compares the heat treatability of the 6 kt alloys with prior art 10 kt alloys. All alloys are annealed by exposure to 1000° F. to 1400° F. for 0.5 hours to 2.0 hours and rapidly cooled in air to room temperature. In an embodiment, the alloys are heat treated at 1200° F. for 1 hour and cooled to room temperature. Any thermal oxides formed during annealing were removed using a mass finishing process. The alloys are then heat treated, which for example can be performed in a furnace at 400° F. to 900° F. The workpiece is kept at this temperature for 0.5 to 3 hours and cooled to room temperature. In an embodiment, the workpiece is heat treated at 600° F. to 800° F. for one hour. This process of annealing followed by heat treatment will increase hardness and durability in finished parts made from these alloys.

(13) The preferred RD 0106 6 kt alloy had moderate to little age hardenability. The addition of Ru (RD 0106 vs. LG 0026) made the alloys heat treatable while Co and B (RD 0114 and RD 0115) improved the overall hardness of the alloy. Co provided better age hardenability than Ru by standard metallurgical practice. A Pd—Ru master alloy should be used as this improves the dispersion of the hardening element more evenly through the cast structure. This will have the effect of improving hardenability as well as providing an increased response to heat treatment.

(14) TABLE-US-00003 TABLE 3 Heat Treatability of 6 kt Alloys Annealed Aged Hardness Hardness Alloy (HRB) (HRB) Quenching conditions 10 kt LG 0120 78.0 87.5 1200° F., 1 hr air-cool .fwdarw. 600° F., 1 hr 10 kt LG 0026 54.0 56.5 1200° F., 1 hr air-cool .fwdarw. 600° F., 1 hr 6 kt RD 0106 63.5 71.0 1200° F., 1 hr air-cool .fwdarw. 700° F., 1 hr 6 kt RD 0114 68.0 77.5 1200° F., 1 hr air-cool .fwdarw. 800° F., 1 hr 6 kt RD 0115 73.5 81.0 1200° F., 1 hr air-cool .fwdarw. 800° F., 1 hr Experimental conditions: each sample is annealed by exposure to 1200° F. for 1 hour, then rapidly air cooled to room temperature. Any thermal oxides formed during annealing were removed using a mass finishing process. The annealed hardness was measured. Each sample was then heat treated for the described temperature (600°-800° F.) for one hour, and the aged hardness was measured.

(15) The 6 kt alloys described here are highly workable using rod-rolling, sheet rolling, swaging, and wire drawing. FIGS. 1A and 1B give example work-hardening and annealing curves of these alloys. All 6 kt alloys have an improved work hardening capacity (WH=HRB.sub.HARD:80% Reduction/HRB.sub.Annealed: 1200F,1 hr) over the LG 0120 10 kt alloy (WH of RD 0106 is 1.71; WH of LG 0120 is 1.49). Adding Al (RD 0112, WH=1.72) and Pt & B (RD 0111, WH=1.71) improved the work-hardening capacity of the alloys. Improved hard hardening capacities means heavier cumulative reductions can be taken during wire/sheet/tube and deep drawing without being susceptible to necking failure. This resistance to necking means thinner gauge wire, tube, and shells can be more easily produced. The annealing curves indicate that ductility can be recovered by heat-treating above 1200° F. for 1 hr. From the hard temper, the alloys can also be further age-hardened by subjecting the material to a heat-treatment at 600° F. for 1 hr.

(16) The improved tarnish resistance of the high zinc alloys can be attributed to a dealloying/gold-enrichment process. In the high zinc 6 kt alloys like RD 0106 the dealloying occurs during tarnish testing with Cu and Zn going into solution leaving behind a Au enriched, tarnish resistant layer. FIGS. 2 and 3 show electron backscatter images and Energy Dispersive Spectroscopy (EDS) spectra of tarnish tested and untested RD 0106. The chemistries of these samples are tabulated in Table 4. Clearly the surface after testing was enriched in Au after tarnish testing. This is not true for LG 0026, which showed no surface enrichment of gold. This phenomenon was observed previously in higher karat golds during exposure to corrosive media such as chloride solutions where the copper and zinc were leeched out leaving behind a gold-enriched surface (Gunnar Hultquist. Surface enrichment of low gold alloys. Gold Bulletin. June 1985, Volume 18, Issue 2, pp 53-57 https://doi.org/10.1007/BF03214686). Since hydrogen-sulfide (H.sub.2S) is formed in the tarnish test solution of potassium sulfide and DI water it is expected the Zn and Cu would react with the H.sub.2S leading to the formation of ZnS and CuS. However, based on our observations the CuS/ZnS precipitates do not form a stable, adherent film and get corrosively removed leading to Au-enrichment at the surface.

(17) TABLE-US-00004 TABLE 4 EDS Chemistry Results of 6 kt Tarnish Test Samples Experiment Au Cu Zn Ag Pd RD 0106 - Before Tarnish Test 26.45 51.22 16.78 0.00 5.13 RD 0106- After Tarnish Test 30.43 47.61 13.96 0.00 5.62 LG 0026- Before Tarnish Test 25.41 56.70 8.77 8.80 0.00 LG 0026- After Tarnish Test 25.01 55.18 8.88 9.96 0.00 All numbers are % w/w.

(18) The inventive alloys can be formed into jewelry or other articles by wrought or casting production methods.

Example 1. Casting Procedure in Neutec J-zP Casting Machine

(19) Grain size in gold alloys for jewelry manufacture is important because of its influence on a material's properties and behavior. A metal structure is a combination of three-dimensional crystals (grains) of varying sizes and shapes. Rolling and drawing elongates the grains and introduces stresses. Annealing relieves the stresses and recrystallizes the grains. Grain growth occurs when these thermo-mechanical processes are inadequately controlled. A material with “large” grain is generally softer and more ductile (though weaker) than the same material with smaller grain. Jewelry made from large grain material often exhibits an undesirable rough surface (orange peel). Supplying soft, ductile materials with fine grain is a challenge to the manufacturer.

(20) Grain (small round uniform pellets of solid alloy) was added to the crucible of the casting machine at room temperature. The crucible was then heated with argon purge of the crucible chamber at 5 L/min. The mold was preheated to 1300° F. and loaded into the chamber when the crucible temperature was 1620° F. The chamber temperature was increased to 1800° F. and the alloy was poured into the mold. The casting was quenched in water 2 mins after pouring.

(21) TABLE-US-00005 TABLE 5 Composition of alloys in casting experiment Alloy Number Composition (% w/w) G0026 Au 25.0; Ag 7.9; Cu 57.8; Zn 9.4 G0026 + Grain Au 24.92; Ag 7.9; Cu 57.52; Zn 9.38; B 0.05; Ir 0.05; refiners Si 0.18

(22) The cast structure of alloy G0026 without grain refiners added is columnar/dendritic, which is undesirable and can cause casting issues, such as porosity, cracks, or breaking in cast products. FIG. 4A is an electron micrograph of G0026, having columnar/dendritic morphology, and large porosities (100). FIG. 4C is another electron micrograph of alloy G0026 without grain refinement showing porosity that tends to be interconnected.

(23) Grain refinement, from adding grain refiner materials to the mixture, including silicon, iridium, or boron, produces a mixture of equiaxed/dendritic and equiaxed/co-cellular grains, which have superior casting properties. The addition of grain refiners causes the porosity to be more isolated. The grain refinement tends to break up large pockets of porosity that otherwise would form. FIG. 4B is an electron micrograph of G0026 with grain refiners added (Table 5), showing the appearance of equiaxed/dendritic and equiaxed/co-cellular grains. No large porosities are present, FIG. 4D is an alternative electron micrograph of alloy 00026 with grain refiners (Table 5) showing how the added grain refiners tend to break up the porosity compared to the sample (FIG. 4C) without grain refiners. The G0026 with grain refiners is a soft, ductile materials with fine grain suitable for fabrication into high quality jewelry items such as rings, balls, chain, hoops and studs.

(24) Casting without boron or silicon additives was discolored due to copper oxide formation during cooling after pouring. Boron/silicon additives appeared to produce a “bright” casting (i.e., higher L in the CIELAB color scale) by preventing thermal oxidation of copper. Close inspection using an eye-loop didn't reveal any tears or porosity.

(25) Conclusions: The combination of boron, iridium and silicon appeared to change the solidification structure from columnar/dendritic to equiaxed/dendritic and equiaxed/co-cellular. The grain refinement appears to break up or redistribute the micro porosity thereby producing a more sound casting. The grain refiners also produced a brighter casting by preventing the thermal oxidation of copper.