Method to increase elastic limit, tensile strength and flexibility of precious metals

Abstract

The present invention relates to an article of precious metal which exhibits shape retention and has a Vickers hardness ranging between 125 to 360 determined in accordance with ASTM E384-22, an average metal grain size number in the range of 8 to 10 determined in accordance with ASTM E112- 24, and an Elasticity Modulus in the range of 95-120 GPa determined in accordance with ASTM E1876-22 Further, the present invention also relates to a method for making the above article.

Description

202421091434TITLE OF THE INVENTIONMETHOD TO INCREASE ELASTIC LIMIT, TENSILE STRENGTH AND FLEXIBILITYOF PRECIOUS METALSTECHNICAL FIELD

[0001] The present invention relates to the field of jewellery -making and metalworking. In particular, the present invention relates to an article of precious metal exhibiting shape retention property and a method for making the article.BACKGROUND

[0002] Precious metals such as Gold, Silver, Platinum and Palladium, have been used since time immemorial to make jewellery and decorative artefacts. These metals are valued not only for their rarity and beauty but also for their malleability, which allowed smiths and artisans to shape and adorn them with detailed patterns, gemstones, and enamels.

[0003] Some common issues have always remained associated with the use of precious metals due to their high cost and density. Even the smaller items made from solid metals such as gold, silver and platinum are very expensive and have considerable weight. In recent years, materials like titanium, ceramic, and eco-friendly resins have been incorporated into jewellery, creating unique looks and appealing to environmentally conscious consumers. However, the appeal of traditional precious metals cannot be replaced due to their characteristic visual beauty and value.

[0004] One approach has been to develop hollow jewellery with thin sheets or tubing in an effort to produce a quality item at a lower cost. However, due to their inherent malleability, precious metals, whether manually processed or mechanically processed, often develop cracks after being worn for a long time, and the normal wearing of the jewellery is affected. Previous attempts to solve this problem, have used a former which may be flexible or rigid, such as a plastic support or another metal upon which the precious metal is overlaid. However, this is not ideal.

[0005] Another approach has been to carry out work hardening of the precious metal or alloy. Work hardening, also known as strain hardening, occurs when a metal is repeatedly bent, hammered, or deformed, causing changes in its internal structure. For precious metals like gold, silver, and platinum, this process can significantly affect their elasticity and malleability. Pure gold (24Kt) is highly malleable and does not work harden as quickly as other metals. However, in alloys (like 18Kt or 14Kt), work hardening is more noticeable. As it is repeatedly worked, gold becomes more brittle, especially in lower-purity alloys, and loses its ability to flex without cracking. Silver is relatively soft but can work harden quickly with bending or hammering, especially in pure (fine) or sterling forms. After work hardening, silver becomes stiffer, reducing its ability to return to its original form if bent202421091434 or flexed. This makes it more prone to breaking under stress. Platinum is a durable metal often alloyed with small amounts of other metals for jewellery, can also work harden. However, it hardens slower than gold or silver, so it remains more elastic for longer periods under stress. But over time, especially with repeated manipulation, platinum can also lose its elasticity, becoming stiffer and more resistant to bending without damage.

[0006] Work hardening is a consequence of plastic deformation, a permanent change in shape. This is distinct from elastic deformation, which is reversible. Most materials do not exhibit only one or the other, but rather a combination of the two. The following discussion mostly applies to metals, especially steels, which are well studied. Work hardening occurs most notably for ductile materials such as metals. Ductility is the ability of a material to undergo plastic deformations before fracture (for example, bending a steel rod until it finally breaks).

[0007] A material generally deforms elastically under the influence of small forces; the material returns quickly to its original shape when the deforming force is removed. This phenomenon is called elastic deformation. This behaviour in materials is described by Hooke's Law. Materials behave elastically until the deforming force increases beyond the elastic limit, which is also known as the yield stress. At that point, the material is permanently deformed and fails to return to its original shape when the force is removed. This phenomenon is called plastic deformation, i.e., where once a material is stretched beyond the elastic limit, it will remain deformed and won't return to its original state.

[0008] For production of items which have good hardness and scratch resistance, work hardening is a preferred method, however such items become brittle and prone to wear. This is due to the metal reaching its elastic limit due to repeated working. Annealing the item, where the metal is heated to a specific temperature and then allowed to cool slowly, rearranges the metal’s crystalline structure, reducing the dislocations and restoring elasticity and malleability. Annealing allows metals to be worked without risk of breaking; however, it returns the tensile strength and hardness of the metal to its normal limits, counteracting the benefits of work hardening.

[0009] Due to work hardening, certain designs in jewellery such as chains, clasps, and prongs might require periodic annealing during the crafting process to maintain flexibility. Harder metals (like certain gold alloys) are often used for settings that need to hold stones securely without bending, while softer, more malleable metals are reserved for parts of jewellery that benefit from flexibility.

[0010] Thus, there is a need for an article of precious metal which exhibits shape retention without compromising other physical properties (such as hardness and strength). Further, there is also a need for an improved method for making the article which imparts the aforesaid characteristics to the article and addresses at least the aforementioned problems.202421091434SUMMARY OF THE INVENTION[Oi l] An aspect of the present invention is directed towards an article of precious metal which exhibits shape retention and has a Vickers hardness ranging between 125 to 360 determined in accordance with ASTM E384-22, an average metal grain size number in the range of 8 to 10 determined in accordance with ASTM El 12-24, and an Elasticity Modulus in the range of 95 - 120 GPa determined in accordance with ASTM El 876-22.

[0012] In an embodiment, the article exhibits shape retention by regaining at least 99% of shape when subjected to compression testing in accordance with ASTM A 370-24.

[0013] In an embodiment, the precious metal is selected from the following: (i) a gold alloy comprising between 37% to 96% pure Au by mass, or (ii) a silver alloy comprising between 37% to 96% pure Ag by mass, or (iii) platinum alloy comprising between 37% to 96% pure Pt by mass.

[0014] In an embodiment the article is selected from bangle, bracelet, cuff, anklet, armlet, necklace, chain, earring, waistband, headband, ring, brooch, cufflink, and tie clip, eyewear, wrist wear, hair clips, money clips, bookmarks, pens, pencils, and components for purses, wallets, bags and personal wear items.

[0015] Another aspect of the present invention relates to a method for making an article of precious metal. The method comprising the following steps: (a) obtaining at least one precious metal strip having a Vickers hardness value in the range of 25 to 250 determined in accordance with ASTM E384-22; (b) subjecting the at least one precious metal strip to an extrusion die to obtain a raw article; (c) optionally soldering or brazing the raw article; and (d) finishing the raw article to obtain the article exhibiting shape retention and having a Vickers hardness ranging between 125 to 360 determined in accordance with ASTM E384-22, an average metal grain size number in the range of 8 to 10 determined in accordance with ASTM El 12-24, and an Elasticity Modulus in the range of 95 - 120 GPa determined in accordance with ASTM El 876-22.

[0016] In an embodiment, the at least one precious metal strip is obtained by subjecting an ingot or a bar of the precious metal to one or more cold rolling steps, the precious metal being annealed and / or cooled after each cold rolling step

[0017] In an embodiment, the cold rolling step is carried out at an ambient temperature ranging between 0°C to 50°C, by passing it one or more times through a series of rollers at temperatures between 0°C to 60°C.

[0018] In an embodiment, the at least one precious metal strip has a thickness ranging between 0.05 to 1.5mm.

[0019] In an embodiment, the raw article is in the form of a hollow tube.

[0020] In an embodiment, after soldering or brazing, the raw article is subjected to one or more cold rolling steps, each cold rolling step being optionally followed by annealing and / or cooling.202421091434

[0021] In an embodiment, the annealing is carried out at a temperature ranging between 400°C to 1200°C, and cooling is carried out to bring the temperature down to a range between 0°C to 50°C.BRIEF DESCRIPTION OF FIGURES

[0022] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.Figure 1(a) shows an optical microscopy at lOOx magnification observed in longitudinal direction for a 18Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 1(b) shows an optical microscopy at 500x magnification observed in longitudinal direction for a 18Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 1(c) shows an optical microscopy at lOOx magnification observed in transverse direction for a 18Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 1(d) shows an optical microscopy at 500x magnification observed in transverse direction for a 18Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 1(e) shows a metal grain number determination at 500x magnification for a 18Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 1(f) shows a Scanning Electron Microscopy at lOOOx magnification for a 18Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 1(g) shows a Scanning Electron Microscopy at 1500x magnification for a 18Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 2(a) shows an optical microscopy at lOOx magnification observed in longitudinal direction for a 22Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 2(b) shows an optical microscopy at 500x magnification observed in longitudinal direction for a 22Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 2(c) shows an optical microscopy at lOOx magnification observed in transverse direction for a 22Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 2(d) shows an optical microscopy at 500x magnification observed in transverse direction for a 22Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 2(e) shows a metal grain number determination of the sample at 500x magnification for a 22Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 2(f) shows a Scanning Electron Microscopy at lOOOx magnification for a 22Kt gold sample prepared in accordance with an embodiment of the present invention.Figure 2(g) shows a Scanning Electron Microscopy at 1500x magnification for a 22Kt gold202421091434 sample prepared in accordance with an embodiment of the present invention.Figure 3(a) shows an optical microscopy at lOOx magnification observed in longitudinal direction for a commercially sourced 22Kt gold sample.Figure 3(b) shows an optical microscopy at 500x magnification observed in longitudinal direction for a commercially sourced 22Kt gold sample.Figure 3(c) shows an optical microscopy at lOOx magnification observed in transverse direction for a commercially sourced 22Kt gold sample.Figure 3(d) shows an optical microscopy at 500x magnification observed in transverse direction for a commercially sourced 22Kt gold sample.Figure 3(e) shows a metal grain number determination of the sample at 500x magnification for a commercially sourced 22Kt gold sample.Figure 3(f) shows a Scanning Electron Microscopy at lOOOx magnification for a commercially sourced 22Kt gold sample.Figure 3(g) shows a Scanning Electron Microscopy at 1500x magnification for a commercially sourced 22Kt gold sample.DETAILED DESCRIPTION OF THE INVENTION

[0023] Various features and embodiments of the present invention here will be discernible from the following further description thereof, set out hereunder.

[0024] The following description illustrates embodiments of the present disclosure and ways in which they can be implemented. Although some modes of carrying out the present disclosure have been disclosed, those skilled in the art would recognize that other embodiments for carrying out or practicing the present disclosure are also possible.

[0025] The term “precious metal” as used herein refers to gold, silver, platinum or palladium, or alloys thereof. It would be known to persons skilled in the art that various alloys are possible with one or more of the above metals which have varying properties such as colour and hardness, based on the content of the precious metals and the selected alloying ingredients. For example, alloys of gold are defined in karats based on the percentage of pure (fine) gold, with 37.5% (9 karat), 58.3% (14 karat), 75% gold (18 karat) and 92% gold (22 karat) being widely used. Similar combinations exist incorporating more than one precious metal such as gold and silver.

[0026] The term “elasticity” as used herein refers to the property in metals that helps them resist deformation when a force is applied. Elasticity refers to a material's ability to return to its original shape after the applied force is removed. When a force is within the elastic limit of the metal, it can stretch or compress temporarily but will regain its original form once the force is released. However, if the force exceeds the metal's elastic limit, the metal enters a state of plastic deformation, where it202421091434 does not return to its original shape. The combination of elasticity and plasticity is what makes metals versatile in various structural applications.

[0027] The related term “tensile strength” refers to the maximum amount of stress a material can withstand before breaking when stretched or pulled. Tensile strength is a key material characteristic that's used to evaluate the mechanical properties of a material, product, or component. It is calculated as maximum load that a material can support without fracture when being stretched, divided by the original cross-sectional area of the material. Tensile strength is an inherent characteristic of a material that is dependent upon the composition and molecular structure of the material. The tensile strength of pure gold (24 karat) is approximately around 120 MPa, while that of pure silver is approximately 125 MPa.

[0028] The term “elastic modulus” as used herein is a measure of the stiffness of a material. It refers to an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. Various types of elastic moduli exist depending upon how the stress is applied and how the strain is measured. Young’s modulus (E) describes tensile and compressive elasticity, or the tendency of an object to deform along an axis when opposing forces are applied along that axis; it is defined as the ratio of tensile stress to tensile strain. It is often referred to simply as the elastic modulus. Other moduli describe deformation from different types of strains. The shear modulus or modulus of rigidity (G) describes an object's tendency to shear (the deformation of shape at constant volume) when acted upon by opposing forces; it is defined as shear stress over shear strain. The shear modulus is part of the derivation of viscosity. The bulk modulus (K) describes volumetric elasticity, or the tendency of an object to deform in all directions when uniformly loaded in all directions; it is defined as volumetric stress over volumetric strain and is the inverse of compressibility. The bulk modulus is an extension of Young's modulus to three dimensions.

[0029] “Work Hardening” or “Strain Hardening” process increases a material's ability to bear a load through plastic deformation. Work hardening strengthens the metal and increases the Elastic Modulus.

[0030] The term “casting” as used herein refers to melting a metal and pouring it into a mould. Casting tends to produce a coarse grain structure in the metal, with larger and irregularly shaped grains compared to metals that have been worked (e.g., forged or rolled). A coarse grain structure is more prone to fracture because the bonds between grains are weaker, leading to brittleness. Casted metal does not undergo work hardening, which is a process where metal is mechanically worked to increase its strength and toughness.

[0031] The term “annealing” refers to a heat treatment process that changes a material's physical and chemical properties by softening it and reducing internal stresses. During annealing, the material is heated above its recrystallization temperature but below its melting point. This gives the metal enough202421091434 energy to allow dislocations to be rectified and internal stresses to be relieved. The material is then cooled down.

[0032] Cast precious metals such as gold are used to form bullion stock or bars, which are worked by forging or rolling. Working the metal creates significant plastic deformation in the metal and changes the crystal structure, thereby increasing hardness. However, this also introduces significant internal stresses to the metal, making it brittle and prone to wearing down or cracking when exposed to long term stress from regular use. The present invention provides a balance between work hardening and annealing, thereby overcoming the challenges associated with conventional approaches.

[0033] Accordingly, an aspect of the present invention relates to an article of precious metal.

[0034] In an embodiment, the article exhibits shape retention and has a Vickers hardness (HV) ranging between 125 to 360 determined in accordance with ASTM E384-22. Vickers Hardness (HV), also referred to as microhardness is a measure of a material's resistance to indentation, determined by pressing a square-based diamond pyramid indenter into the material with a specific force and then measuring the resulting indentation's diagonals. The Vickers hardness number (HV) is calculated by dividing the applied load by the surface area of the indentation. This method is versatile, applicable to all metals, and useful for testing thin sections or fine structures because the hardness value is independent of the indenter size. Unless specified otherwise, all references to hardness or HV is to be understood as to refer to Vickers Hardness determined in accordance with ASTM E384-22.

[0035] In another embodiment, the article has an average metal grain size number in the range of 8 to 10 determined in accordance with ASTM El 12-24. Metal grain size is the average diameter of the individual crystals, or grains, within a metal or alloy expressed in millimetres. Grain size number is determined based on the number of grains per unit area. A higher number indicates smaller grain size and a finer microstructure. Unless specified otherwise, all references to grain size number are to be understood to mean ASTM grain size number determined in accordance with ASTM El 12-24.

[0036] In another embodiment, the article has an Elasticity Modulus in the range of 95-120 GPa determined in accordance with ASTM El 876-22. As described in earlier paragraphs, the elasticity modulus refers to a material’s resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. Unless specified otherwise, all references to elasticity modulus is to be understood to mean Young’s modulus determined in accordance with ASTM El 876-22.

[0037] In another embodiment, the article exhibits shape retention by regaining at least 99% of shape when subjected to compression testing in accordance with ASTM A 370-24. Shape retention is a function of elasticity, compressibility and tensile strength of a material enabling the article to maintain its geometric shape, resist collapse under load or impact, and exhibit controlled non-plastic deformation. Said otherwise, an article exhibiting shape retention properties is able to deform in a202421091434 controlled manner when under load and return to its original shape when the load is removed. This may be referred to as “memory” effect in the article whereby the article returns to its original shape when subjected to external force such as compression. The memory effect in the article has been demonstrated by compression testing determined in accordance with ASTM A 370-24.

[0038] However, the memory effect in the article of the present invention is distinct from and unrelated to shape memory alloys (SMAs) which require a phase transformation. Unless specified otherwise, compression and tensile strength determination is to be performed in accordance with ASTM A 370-24. While the procedure is generally preferred for testing steel and ferrous materials, the methodology can be utilised on precious metals without any modification. Since parameters such as tensile strength are much lower for precious metals as compared to steels, adequate care is taken to ensure accuracy of measurement in the lower ranges, i.e., tensile strength in the range of 200 - 1000 MPa as will be understood by persons skilled in the art.

[0039] In another embodiment, the precious metal is selected from the following: (i) a gold alloy comprising between 37% to 96% pure Au by mass, or (ii) a silver alloy comprising between 37% to 96% pure Ag by mass, or (iii) platinum alloy comprising between 37% to 96% pure Pt by mass. Different grades of precious metals are known to persons skilled in the art based on composition of the major element (Au, Ag or Pt) and variations in alloying elements. For instance, commonly used grades of gold include 22Kt (91.8% Au and remainder alloying elements), 18Kt (75% Au and remainder alloying elements), 14Kt (58.3% Au and remainder alloying elements) and 9Kt (37.5% Au and remainder alloying elements).

[0040] Similarly, commonly used grades of silver include 925 (92.5% Ag and remainder alloying elements), 900 (90% Ag and remainder alloying elements), 835 (83.5% Ag and remainder alloying elements) and 800 (80% Ag and remainder alloying elements).

[0041] Likewise, commonly used grades of platinum include Pt950 (95% Pt and remainder alloying elements), Pt900 (90% Pt and remainder alloying elements) and Pt850 (85% Pt and remainder alloying elements).

[0042] The composition of the alloying elements can be adjusted to influence properties such as colour, durability and tensile strength. For instance, 18Kt gold can be produced in multiple versions based on aesthetic demand such as yellow gold, rose gold, white gold etc. by changing the composition of the alloying elements, with high copper content providing rose gold whereas high silver and palladium providing white gold colour. All such modifications are known to persons skilled in the art and are within the purview of the present invention without deviating from the scope of the present invention.

[0043] The article of the present invention having the aforesaid characteristics enables it to be used in various applications. One such application where the memory effect of the article is highly202421091434 desirable is in jewellery and decorative articles. In an embodiment, the article of the present invention is selected from bangle, bracelet, cuff, anklet, armlet, necklace, chain, earring, waistband, headband, ring, brooch, cufflink, and tie clip, eyewear, wrist wear, hair clips, money clips, bookmarks, pens, pencils, and components for purses, wallets, bags and personal wear items.

[0044] Another aspect of the present invention relates to a method for making the article of precious metal, as described above. Accordingly, the embodiments pertaining to the article are applicable here as well.

[0045] In an embodiment, the method comprises the steps of obtaining at least one precious metal strip having a Vickers hardness value in the range of 25 to 250 determined in accordance with ASTM E384-22 followed by subjecting the at least one precious metal strip to an extrusion die to obtain a raw article. The method also comprises optionally soldering or brazing the raw article followed by finishing the raw article to obtain the article of the present invention. As discussed above, the article exhibits shape retention and has the Vickers hardness ranging between 125 to 360 determined in accordance with ASTM E384-22, average metal grain size number in the range of 8 to 10 determined in accordance with ASTM El 12-24, and Elasticity Modulus in the range of 95-120 GPa determined in accordance with ASTM El 876-22.

[0046] In another embodiment, the extrusion is performed by passing the at least one precious metal strip through an extrusion die using a drawing equipment, and is carried out at room temperature. In an embodiment, the temperature ranges between 0°C to 50°C.

[0047] In another embodiment, the at least one precious metal strip is obtained by subjecting an ingot or a bar of the precious metal to one or more cold rolling steps. Further, the precious metal is annealed and / or cooled after each cold rolling step.

[0048] In another embodiment, the cold rolling step is carried out at an ambient temperature ranging between 0°C to 50°C. Additionally, the cold rolling step is carried out one or more times, depending on the size and dimensions of the output desired. In this regard, a series of rollers at temperatures between 0°C to 60°C can be used for cold rolling. The series of rollers form part of a rolling machine which gradually reduces the thickness of the metal and form sheets or strips of varying thickness. Rolling and die extrusion are a type of work hardening which occurs at “cold” temperatures i.e., at room temperature, known as cold working. It has been observed in the present invention that cold working results in articles having shape retention properties, along with acceptable and / or enhanced physical properties of hardness, tensile strength and elasticity.

[0049] Before work hardening, as described above, the lattice of the precious metal exhibits a regular, nearly defect-free pattern (almost no dislocations). The defect-free lattice can be created or restored at any time by annealing. As the material is work hardened it becomes increasingly saturated with new dislocations, and more dislocations are prevented from nucleating (a resistance to dislocation-202421091434 formation develops). This resistance to dislocation-formation manifests itself as a resistance to plastic deformation; hence, the observed shape retention property.

[0050] Further, cold working generally results in a higher yield strength as a result of the increased number of dislocations and the Hall - Petch effect of the sub-grains, and a decrease in ductility. The effects of cold working may be reversed by annealing the precious metal at high temperatures where recovery and recrystallization reduce the dislocation density. The precious metal’s work hardenability can be predicted by analysing a stress-strain curve or studied in context by performing hardness tests before and after a process. This has been demonstrated by way of nonlimiting examples in the present invention.

[0051] Depending on the application, suitable dimensions and thickness of the precious metal strip can be obtained. In an embodiment, the at least one precious metal strip has a thickness ranging between 0.05mm to 1.5mm.

[0052] In an embodiment, the raw article is in the form of a hollow tube. Suitable size and dimensions as well as shape (e.g., D-tube, O-tube, etc.) may be selected depending on the desired application.

[0053] In an embodiment, after soldering or brazing, the raw article is subjected to one or more cold rolling steps, each cold rolling step being optionally followed by annealing and / or cooling.

[0054] In an embodiment, the annealing is carried out at a temperature ranging between 400°C to 1200°C, and cooling is carried out to bring the temperature down to a range between 0°C to 50°C. Further, annealing and / or cooling may be carried out more than once throughout the method of the present invention.

[0055] The annealing temperature is carefully controlled to remain higher than recrystallization temperature but lower that the melting point of the specific grade / alloy of the precious metal. For instance, annealing temperatures for commonly used materials like 18 karat gold is approximately 700 - 760°C, 22 karat gold is 550 - 600°C, fine silver is 200 - 420°C, Silver 925 is 650°C, Platinum 950 is 900°C, and the likes. Further, the annealing temperature may be selected depending on various parameters - for instance, specific grade / alloy of the precious metal, specific application of the article, and the type of equipment for carrying out annealing.

[0056] The annealing step is followed by the cooling step to bring the temperature down to a range between 0°C to 50°C. Suitable methods for cooling are generally known. For instance, cooling may be carried out by air cooling or quenching in water or oil. The cooling method and rate of cooling achieved by said method influences the grain structure of the metal.

[0057] In another embodiment, one or more metal strips of same or different precious metals or their alloys can optionally be layered together by brazing them with flux followed by rolling them together and annealing the laminated strip one or more times. This may include different alloys of same metal such as white gold and rose gold or different metals such as gold and silver. The layering process can202421091434 achieve a metal strip with different visual properties on different surfaces or a patterned appearance leading to varied jewellery and decorative articles with designer effects.

[0058] In yet another embodiment, the raw article is finished by suitable techniques. For instance, the raw article is subjected to buffing and / or polishing the surface to achieve the desired surface effect such as shine or textured appearance. Alternately, the raw article may be subjected to one or more of the following finishing techniques: antique finish, oxidation treatments, rhodium plating, rough finish, and laser finish Any additional components required for the jewellery or decorative article are attached during this finishing step, followed by one or more rounds of polishing to obtain the finished article of precious metal exhibiting shape retention, improved properties of hardness, elasticity, and tensile strength.

[0059] In still another embodiment, the additional components are selected based on the intended use of the article, such as pins for broaches, hooks for necklaces and earrings, mounting pins for gemstones, twist locks for cufflinks. The additional components may be one or more from the following categories - latches, pins, clasps, hooks, sheet metal cuttings, cast, studded / non-studded, die-struck, wired, machined, and handmade components. It is understood that the person skilled in the art such as a designer or jeweller will be able to select any number of the above components either alone or in combination to add to the article produced in accordance with an embodiment of the present invention to produce a functional article.

[0060] Advantageously, the present invention provides a precious metal article and a method for making the same, wherein the article has exceptional shape retention property, without compromising other physical properties such as tensile strength, elasticity, and hardness. The article of the present invention features the following key improvements over the existing state of the art, namely metal grain refinement or fine grain structure, higher hardness (HV), higher tensile and compressive strength, superior shape retention, and significantly higher residual stresses.EXAMPLES

[0061] The following examples are illustrative of the invention but not limitative of the scope thereof since numerous modifications and variations within the scope of the present invention will be apparent to those skilled in the art. Unless otherwise noted, all parts, percentages and ratios reported in the following examples are on a weight basis, and all materials used in the examples were obtained or are available from the precious metal suppliers.

[0062] Furthermore, following testing methodologies have been adopted: Vickers Hardness (HV) was determined in accordance with ASTM E384-22, metal grain size number was determined in accordance with ASTM El 12-24, Elasticity Modulus was determined in accordance with ASTM El 876-22, Tensile Strength and compression testing was performed in accordance with ASTM A202421091434370-24, Residual Stress analysis by XRD was performed in accordance with ASTM E 2860-20.Example 1 - Method for making an article (such as a bangle) in accordance with the present inventionExample 1 A - Making an article of 18 karat Gold

[0063] A metal bar of 18 karat gold composed of 75 wt.% Au & 25 wt.% Alloy (comprising 4 wt.% Ag, 93 wt.% Cu, 3 wt.% Zn along with other deoxidizers and grain refiners - wt.% based on the total weight of the composition) was taken having dimensions of 100mm x 7mm x 6mm (length x breadth x height). The metal bar was annealed at 760°C, cooled at room temperature / quenched while still pink for increased malleability. The metal bar underwent a cold rolling process where it was passed through a rolling mill which has an upper roll and a lower roll of same dimensions. The bar underwent through 6 reduction cycles through the rollers. At every cycle there was certain reduction in thickness (h). At a reduced thickness (h) of 3mm, the bar was annealed again at 760°C and cooled / quenched. The bar went through a total 5 cycles of reduction in the rollers and was brought down to 0.8mm thickness (h) strip. At that point the strip’s hardness was recorded at a range of 140 to 210 HV (hardness value), preferably 190 HV, determined according to the Vickers Hardness scale. The strip was again annealed at 760°C and cooled / quenched. Post annealing the hardness was recorded at 150HV. Another cycle of rolling produced a work hardened strip which of 0.4mm thickness with a 200HV. The strip has an elongated length of 1600mm and a width of 9.1mm. Based on the results presented in the Table 1 it can be seen that the hardness of samples increases from 140 HV in molten state, up to 190 HV after 6 reductions in rolling mills, with the total degree of deformation increasing up to 66.66%. After the first annealing hardness decreases to 170 HV and subsequently increases monotonically up to 190 HV with increasing degree of cold deformation to 93.33%, after the final reduction on the rolling mill.

[0064] Table 1 illustrates the ratio of elongation, hardness for the gold alloy Au75% / Cu23%.25 / Ag01% / ZnO.75%, depending on the degree of reduction.Table 1 :202421091434

[0065] A 200mm part of this strip was taken for evaluation. This part of the strip was annealed at 760°C and cooled / quenched and made into a D shaped tube by passing it through a draw die on a regular drawing bench apparatus. While the results have been showcased for the D-shaped tube, the properties are visible in other shaped tubes as well. The D tube was closed by a laser solder / brazing process and annealed at 760°C and cooled / quenched. The tube underwent the same process of rolling through the rolling mills for another 6 reductions. As results represent in Table 2, there was a total elongation of 30% in the D tube with 6 rolling reductions reaching up to 0.4mm where the HV increases consistently from 190 HV to 260 HV.

[0066] The tensile strength of gold after the above process was measured at 699MPa compared to 520MPa for standard 18kt gold.

[0067] Table 2 illustrates the ratio of elongation of the D tube depending on the degree of reduction and its final hardness.Table 2:

[0068] The final D tube was bent in a circular chape and the ends enclosed by soldering or capping to produce a raw article of bangle. The raw bangle was buffed and polished to a shine to obtain the finished bangle.Example IB - Making an article of 22 Karat Gold

[0069] In another exemplary embodiment, a metal bar of 22kt gold was taken composed of 92 wt.% Au and 8 wt.% alloying elements. The general composition of alloying elements was 15 wt.% Ag, 85 wt.% Cu along with deoxidizers and grain refiners. The dimensions of the bar were 100mm x 7mm x 6mm (LxBxH). The alloy composition or size or dimensions may differ, but it would not have an impact on the steps outlined herein.202421091434

[0070] The metal bar was annealed at 600°C, cooled at room temperature / quenched while still pink for increased malleability. The metal bar underwent cold rolling where it was passed through a rolling mill which has an upper roller and a lower roller of same dimensions. The bar went through 6 reduction cycles through the rollers. At every cycle there was certain reduction in thickness (h in Table 3). At a reduced thickness (h) of 3mm, the bar was annealed again at 600°C and cooled / quenched. The bar went through a total 5 cycles of reduction in the rollers and was brought down to 0.8mm thickness (h) strip. At this point the strip’s hardness was recorded at 90HV as seen in the table 3. The strip was again annealed at 600°C and cooled / quenched. Post annealing the hardness was recorded at 80HV. Another cycle of rolling produced a work hardened strip which was 0.4mm thick with a hardness of 120HV. The strip reached an elongated length of 1800mm and a width of 9.3mm. Based on the results presented in the table 3 it can be seen that the hardness of samples increases from 80 HV in molten state, up to 200 HV after 6 reductions in rolling mills, with the total degree of deformation increasing up to 70%. After the first annealing, hardness decreases to 80 HV and subsequently increases monotonically up to 90 HV with increasing degree of cold deformation to 85%, after the final reduction on the rolling mill.

[0071] Table 3 illustrates the ratio of elongation, hardness for the gold alloy Au 92% / Cu85 / Agl5%, depending on the degree of reduction.Table 3:

[0072] The process was continued further and a 200mm part of this strip was taken for evaluation. This part of the strip was annealed at 600°C and cooled / quenched and a made into an O shaped tube by passing it through a draw die on a regular drawing bench apparatus. The O tube was closed by a laser solder / brazing process and annealed at 600°C and cooled / quenched. The tube underwent the same process of rolling through the rolling mills for a further 6 reductions. As results represent in Table 4, there was a total elongation of 38% in the O tube with 6 rolling reductions reaching up to 0.4mm where the HV was increased consistently from 90 HV to around 200 HV.

[0073] Table 4 illustrates the ratio of elongation of the D tube depending on the degree of reduction202421091434 and its final hardness.Table 4:Example 1C - Making an article of Platinum 950

[0074] In another exemplary embodiment, a metal bar of 950 Platinum was taken having a composition which included 95 wt.% Pt and 5 wt.% alloying elements. The general composition of alloying elements was 2 wt.% In, 3 wt.% Ga along with other deoxidizers and grain refiners - the wt.% being based on the total weight of the composition. The dimensions of the bar were 100mm x 7mm x 6mm (LxBxH). The alloy composition or size or dimensions may differ, but it will not have an impact on the steps outlined hereinbelow.

[0075] The metal bar was annealed at 1000°C, cooled at room temperature / quenched while still pink for increased malleability. The metal bar underwent a cold rolling process where it was passed through a rolling mill which has an upper roller and a lower roller of same dimensions. The bar went through 6 reduction cycles through the rollers. At every cycle there was certain reduction in thickness (h in Table 5). At a reduced thickness (h) of 3mm, the bar was annealed again at 1000°C and cooled / quenched. The bar went through a total 5 cycles of reduction in the rollers and was brought down to 0.8mm thickness (h) strip. At this point the strip’s hardness was recorded at 200HV as seen in the table 5. The strip was again annealed at 1000°C and cooled / quenched. Post annealing the hardness was recorded at 190HV. Another cycle of rolling produces a work hardened strip which was 0.4mm with a 200HV. The strip reached an elongated length of 1600mm and a width of 9.1mm. Based on the results presented in the table 5 it can be seen that the hardness of samples increase from 200 HV in molten state, up to 300 HV after 6 reductions in rolling mills, with the total degree of deformation increasing up to 66.66%. After the first annealing hardness decreases to 190 HV and subsequently increases monotonically up to 220 HV with increasing degree of cold deformation to 90%, after the final reduction on the rolling mill.202421091434

[0076] Table 5 illustrates the ratio of elongation, hardness for the Platinum 950 alloy Pt 95% 2% In, 3% Ga, depending on the degree of reduction.Table 5:

[0077] The process was continued further and a 200mm part of this strip was taken for evaluation. This part of the strip was annealed at 1000°C and cooled / quenched and a made into a O shaped tube by passing it through a draw die on a regular drawing bench apparatus. The O tube was closed by a laser solder / brazing process and annealed at 1000°C and cooled / quenched. The tube underwent the same process of rolling through the rolling mills for another 6 reductions. As shown in Table 6, there was a total elongation of around 22.75% in the O tube with 6 rolling reductions reaching up to 0.4mm where the HV increases consistently from 200 HV to around 360 HV.

[0078] Table 6 illustrates the ratio of elongation of the O tube depending on the degree of reduction and its final hardness.Table 6:Example 2 - Testing of shape retention (memory effect) properties by compression testing202421091434

[0079] Example 2 A - An 18 Karat Gold article (bangle) was prepared in accordance with the method of Example 1A, having a thickness of 0.5mm and an outer diameter of 56 mm, and having composition of 75% Au, 23.25% Cu, 1% Ag and 0.75% Zn.

[0080] Example 2B - An 22Kt Gold article (bangle) was prepared in accordance with the method of Example IB, having a thickness of 0.5mm and an outer diameter of 64.37 mm, and having composition of 91.8% Au, 6% Cu, and 2.2% Ag.

[0081] Example 2C - A commercially available 18 Karat Gold bangle was sourced having a thickness of 0.5mm and an outer diameter of 66 mm, and having composition of 75% Au, 23.25% Cu, 1% Ag and 0.75% Zn. The commercially available bangle was not prepared in accordance with the method of the present invention. Commercial strips were prepared by rolling the metal bar through multiple cycles of reduction and intermittent annealing.

[0082] Compression testing was carried on the above bangles in accordance with ASTM A370-24 using a lateral compression machine (Universal Testing Machine (UTM)). The results are presented in Table 7 below.Table 7:083] As is evident from the results presented above, examples 2A and 2B exhibits excellent shape retention properties - showing the ability to return back to its original shape with no or negligible permanent deformation after load applied compression of over 40% of its original dimension, i.e., memory effect. Whereas example 2C shows high load applied compression of roughly 50% of its original dimension with high permanent deformation of over 17% of its original dimension. Further,202421091434 the sample of example 2C was unable to return to its original shape once the load is removed - i.e., absence of memory effect.Example 3 - Determination of Tensile Strength and Elasticity Modulus Example 3A - Tensile Strength

[0084] A sample of 22Kt gold, two samples of 18Kt gold with differing degrees of work hardening, and a sample of 14Kt gold in the form of strips were prepared in accordance with the method of Example 1A and IB, with thickness in the range of 0.35 mm to 0.5 mm. (Samples 3A, 3B, 3C and 3D respectively)

[0085] Commercially available sample of 22Kt strip was sourced having thickness of 0.35 mm(Samples 3E) - i.e. not prepared in accordance with the method of the present invention.

[0086] Tensile Strength was tested in accordance with ASTM A 370-24. The results are presented in Table 8 below.Table 8:202421091434087] As is evident from the results provided above, the samples 3 A to 3D showed marked increase in tensile strength and significant decrease in % elongation as compared to sample 3E.Experiment 3B - Elasticity Modulus

[0088] Sample 3B was taken for further testing to determine Young’s Modulus in accordance withASTM El 876-22 and the same was compared with the elasticity modulus for a commercially sourced 18Kt gold strip (Sample 3F). The results are provided in Table 9 below.Table 9:

[0089] As is evident from the results above, the 18Kt gold sample (Sample 3B) shows significant increase in Young’s Modulus (elasticity modulus) denoting improved elastic properties.Example 4 - Determination of Vickers Hardness (HV)

[0090] Samples of 22Kt and 18Kt gold were prepared in accordance with the method of Example 1 A and IB.

[0091] Samples of commercially available 22Kt and 18Kt gold were sourced - i.e. not prepared in accordance with the method of the present invention.

[0092] All samples were tested for Microhardness to determine Vickers Hardness in accordance with ASTM E384-22. The results are provided in Table 10 below.Table 10:202421091434

[0093] As can be seen from the above results, the samples made in accordance with the present invention showed significant increase in Microhardness over comparable grades of commercially available samples. For instance, the 22Kt gold sample made in accordance with the embodiments of the present invention showed greater HV than commercially available 22Kt and 18Kt gold samples, while the 18Kt gold sample made in accordance with the embodiments of the present invention showed significantly higher HV than commercial 18Kt sample at same load (50g each) and still showed comparable HV at double load (100g load for present invention vs 50g load for commercial sample). Example 5 - Microstructure Examination

[0094] Samples of 18Kt and 22Kt gold were prepared in accordance with the method of Example 1 A and IB. Reference is made to Figures 1(a) to 1(g) and Figures 2(a) to 2(g) in this regard.

[0095] Samples of commercially available 22Kt gold was sourced. Reference is made to Figures 3(a) to 3(g) in this regard.

[0096] All samples were evaluated for microstructure by performing Optical Microscopy at 1 lOx to500x and Scanning Electron Microscopy (SEM) at lOOOx and 1500x as per ASM handbook Vol 9 and the ASTM metal grain size was determined in accordance with ASTM El 12-24 using Aqua Regia as an etchant. The results are provided in Table 11 below.Table 11 :202421091434097] As can be seen from the above results, the samples produced in accordance with the embodiments of the present invention consistently show finer grains over their commercial counterpart, indicating successful grain refinement by cold working. Further, the commercial sample lacks the other attributes of the present invention - such as shape retention, hardness, and tensile strength.Example 6 - Residual Stress Analysis by X-Ray Diffraction

[0098] Samples of 18Kt and 14Kt gold were prepared in accordance with the method of Example 1 A and IB.

[0099] Commercial sample of 22Kt gold was sourced - i.e. not prepared in accordance with the method of the present invention.202421091434

[0100] All samples were analysed by X-Ray Diffraction (XRD) to measure residual stress in the metal sample in accordance with ASTM E 2860-20. The results are provided in Table 12 below.Table 12:0101] As is seen in the above data, the samples prepared in accordance with embodiments of the present invention retain significant internal stress indicating high degree of cold working induced internal deformation.

[0102] The foregoing examples of the invention have been set out merely to illustrate the invention and is not intended to be limiting. While the present invention has been described with respect to certain embodiments, it will be apparent to those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.

Claims

202421091434CLAIM:

1. An article of precious metal, wherein the article exhibits shape retention and has a Vickers hardness ranging between 125 to 360 determined in accordance with ASTM E384-22, an average metal grain size number in the range of 8 to 10 determined in accordance with ASTM El 12-24, and an Elasticity Modulus in the range of 95 - 120 GPa determined in accordance with ASTM El 876-22.

2. The article as claimed in claim 1, wherein the article exhibits shape retention by regaining at least 99% of shape when subjected to compression testing in accordance with ASTM A 370-24.

3. The article as claimed in claim 1, wherein the precious metal is selected from the following: (i) a gold alloy comprising between 37% to 96% pure Au by mass, or (ii) a silver alloy comprising between 37% to 96% pure Ag by mass, or (iii) platinum alloy comprising between 37% to 96% pure Pt by mass.

4. The article as claimed in claim 1, wherein the article is selected from bangle, bracelet, cuff, anklet, armlet, necklace, chain, earring, waistband, headband, ring, brooch, cufflink, and tie clip, eyewear, wrist wear, hair clips, money clips, bookmarks, pens, pencils, and components for purses, wallets, bags and personal wear items.

5. A method for making an article of precious metal, the method comprising:(a) obtaining at least one precious metal strip having a Vickers hardness value in the range of 25 to 250 determined in accordance with ASTM E384-22;(b) subjecting the at least one precious metal strip to an extrusion die to obtain a raw article;(c) optionally soldering or brazing the raw article; and(d) finishing the raw article to obtain the article exhibiting shape retention and having a Vickers hardness ranging between 125 to 360 determined in accordance with ASTM E384-22, an average metal grain size number in the range of 8 to 10 determined in accordance with ASTM El 12-24, and an Elasticity Modulus in the range of 95 - 120 GPa determined in accordance with ASTM El 876-22.

6. The method as claimed in claim 5, wherein the at least one precious metal strip is obtained by subjecting an ingot or a bar of the precious metal to one or more cold rolling steps, the precious metal being annealed and / or cooled after each cold rolling step.2024210914347. The method as claimed in claim 6, wherein the cold rolling step is carried out at an ambient temperature ranging between 0°C to 50°C, by passing it one or more times through a series of rollers at temperatures between 0°C to 60°C.

8. The method as claimed in claim 5, wherein the at least one precious metal strip has a thickness ranging between 0.05 to 1.5mm.

9. The method as claimed in claim 5, wherein the raw article is a hollow tube.

10. The method as claimed in claim 5, wherein after soldering or brazing, the raw article is subjected to one or more cold rolling steps, each cold rolling step being optionally followed by annealing and / or cooling.

11. The method as claimed in claim 5, wherein the annealing is carried out at a temperature ranging between 400°C to 1200°C, and cooling is carried out to bring the temperature down to a range between 0°C to 50°C.

Images