Process for manufacturing precious metal parts based on SPS sintering and precious metal part thus obtained

Abstract

The invention relates to a method for manufacturing a metallurgical part based on precious metal, characterized by the following steps: - Using a metallurgical material, powdered or solid, having a grain size of less than 400 micrometers and comprising at least 75% of precious metal, - reducing the size of the grains and / or crystallites of the metallurgical material so as to obtain aggregates with a characteristic size of less than 1000 micrometers, and an average crystallite size of less than 200 nanometers, - Sintering the reduced material using an SPS sintering process, so that the metallurgical part obtained has a Vickers hardness greater than 150 Hv. Abstract figure: Fig. 1

Description

Title of the invention: Method for manufacturing a precious metal part based on SPS sintering and precious metal part thus obtained TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the manufacture by sintering of parts made of precious metals, or noble metals, for example based on gold or silver, in particular the manufacture of parts having mechanical properties of particular hardness. STATE OF THE ART

[0002] Precious metals are used in particular in the fields of watchmaking and jewelry. Gold has the particularity of being one of the most malleable and ductile of known metals, both dense and soft.

[0003] Pure gold is 24 carats (999 thousandths) and is not used in jewelry due to its high capacity to deform. This is why it is used in alloy, mixed with other metals (copper, silver, palladium, rhodium or nickel). This allows for better mechanical resistance.

[0004] Similarly, pure silver is not used in jewelry. This is why it is used in alloy, mixed with copper for example according to the following mixture / composition: 92.5% fine silver and 7.5% copper in order to make the material harder.

[0005] It is thus desirable to propose a material having a hardness equivalent to or greater than those of the state of the art, and / or a hardness which is homogeneous in volume. Another aim of the invention is to limit the number of operations and / or treatments. THE INVENTION

[0006] To this end, and according to a first aspect, the invention proposes a method for manufacturing a metallurgical part based on precious metal characterized by the following steps - use a metallurgical material with a grain size of less than 400 pm (micrometers) and comprising at least 75% precious metal, - reducing the size of the grains and / or crystallites of the metallurgical material so as to obtain aggregates with a characteristic size of less than 1000 pm (micrometers), and an average crystallite size of less than 200 nm (nanometers), - sinter using an SPS sintering process the reduced material,

[0007] so that the metallurgical part obtained has a Vickers hardness greater than 150 Hv.

[0008] The part obtained according to the invention makes it possible to increase the hardness compared to the results of the prior art, while limiting the costs thanks in particular to the reduction the number of operations and / or treatments.

[0009] For the above and for the remainder of the description, the following terms are understood to mean:

[0010] - SPS sintering, acronym for “Spark Plasma Sintering”, a sintering process under pressure based on the densification of a powder sample by applying mechanical stress associated with the passage of a pulsed current to heat the sample; for example a sintering method related to hot isostatic pressing but using the Joule effect to heat the pre-compacted powder in a hollow cylindrical crucible between two graphite electrodes under an inert atmosphere or under vacuum, the whole being subjected to a pressure of several megapascals under the action of a hydraulic press. A direct or alternating current of several kiloamperes, pulsed or not, is applied between the electrodes with a voltage of a few volts. ;

[0011] - binder, any material making it possible to improve densification and / or the mechanical properties final canics giving mechanical cohesion to the final part, for example cobalt material or another sintering agent;

[0012] - grain size, or granulometry, or grain granulometry, the characterized size by the values dlO, d90, d50 in order to quantify the dispersion of this grain size distribution,

[0013] - crystallite size, each grain being able to have crystallites, the size is relating to coherent crystallographic domains and which is measured by techniques such as SEM, TEM, etc.;

[0014] - form factor, the ratio between two characteristic lengths, each length extending in a determined direction, said characteristic lengths having a non-zero angle with respect to each other, for example an angle of 90 degrees;

[0015] - atomization or atomizing, in particular concerning a powder, a method of transformation of a metal ingot into spherical powder by melting and projecting the metal drops under gas flow to make them spherical;

[0016] - spheroidization, or spheroidizing, in particular concerning a powder, a method of transforming an angular ground metal powder by fusion, most often assisted by plasma, to make it spherical;

[0017] - grinding or crushing, in particular concerning a powder, a method of trans forming by mechanical action, for example by beads, so as to reduce the size of the crystallites and / or the size of the grains of a powder;

[0018] - aggregates, the result of a reduction in grain size and / or scree size tallites, for example by grinding, which results in an agglomeration of small grains to form larger agglomerates, but each grain constituting the agglomerates has smaller crystallite sizes;

[0019] - hardness, the resistance of a material to be marked by another, we will use here the Vickers hardness.

[0020] Preferably, the metallurgical material is a metallurgical material powder.

[0021] Preferably, the precious metal is: - gold or a gold-based alloy, or - silver or a silver-based alloy.

[0022] According to one embodiment, the material comprises at least 90% precious metal.

[0023] According to the embodiments, the material further comprises copper, nickel, rhodium, palladium or silver, the silver being added in the case where the base precious metal is gold.

[0024] According to another embodiment, the metallurgical material is at least one plate element. The at least one plate element has a thickness greater than or equal to one millimeter.

[0025] According to alternative embodiments which may or may not be combined, the reduction in the size of the grains and / or crystallites of the powder comprises: - a step of atomizing the metallurgical material, and / or - a step of grinding the metallurgical material, so that the powder used has a particle size of less than 1000 micrometers.

[0026] Preferably, the method comprises a step of atomizing the powder used so that the grain size has a size less than or equal to 250 micrometers, preferably less than or equal to 150 micrometers, preferentially less than or equal to 100 micrometers.

[0027] According to other embodiment variants, which may or may not be combined, the reduction in the size of the grains and / or crystallites of the powder comprises: - a step of atomization of the metallurgical material, and / or - a step of grinding the metallurgical material, so that the size of the agglomerates is less than 1000 micrometers.

[0028] Preferably, and in the case of the combination of the two steps, the atomization step is carried out before the grinding step.

[0029] Grinding makes it possible to obtain a powder which will have “final” properties in terms of geometry, form factor, crystallite size. Grinding also makes it possible in certain cases to form active sites on the surface of the powder which promote and improve the sintering behavior.

[0030] Each grain type has a predetermined grain size, a predetermined crystallite size, and a predetermined shape factor.

[0031] Preferably, the metallurgical material has a particle size of less than 200 pm (micrometers).

[0032] Preferably, the powder is reduced so that: - the aggregates have a characteristic size of less than 200 micrometers, and / or - the average size of the crystallites is less than 100 nanometers.

[0033] The predetermined grain microstructure may have the following characteristics: - Particle size distribution: d50 being between 0.1 and 100 pm, - Crystallite size: 20 to 1000 nm, - Form factor: between 1 and 5 (spherical to angular, without being cylindrical).

[0034] One embodiment consists of using a monomodal particle size distribution before grinding of between 0.1 and 100 micrometers (pm).

[0035] According to another embodiment, the powders have a bimodal distribution before grinding with d50 values separated by a decade, typically 0.1 pm and I pm or I pm and l 0 pm or l 0 pm and l 0 0 pm. This bimodal distribution may be separated by 2 decades, typically 0.1 and l 0 pm or 1 and l 0 0 pm.

[0036] According to yet another embodiment, the distribution is trimodal with d50s separated by a decade, typically O.lpm, Ipm and lOpm. These examples are obviously non-limiting.

[0037] In one embodiment, the powder is used as is, raw from the supplier. For example, this powder may have a d50 value, in particular a grain diameter of less than 100 micrometers, preferably less than 50 micrometers, preferably less than 15 micrometers.

[0038] In a preferred embodiment, the powder is ground in order to refine the size of the crystallites (coherent crystallographic domains) which is different from the particle size distribution. Thus, after grinding, a reduction in the size of the crystallites is observed, but not necessarily a reduction in the size of the grains.

[0039] Preferably, the size of the crystallites is between 20 and 1000 nanometers (nm). Preferably, the size of the crystallites is between 20 and 100 nm. Finally, preferably, the size of the crystallites is between 20 and 50 nm. In one embodiment, it is possible to combine several crystallite sizes.

[0040] According to one embodiment, the manufacturing method comprises a step of adding at least one doping agent with the metallurgical material, before the sintering step.

[0041] Preferably, the at least one doping agent is or comprises boron nitride BN, titanium carbide TiC, tungsten carbide WC, silicon carbide SiC, niobium carbide NbC, boron carbide B4C, silicon nitride Si3N4, aluminum oxide A12O3, zirconium oxide ZrO2, yttrium oxide Y2O3 or a mixture thereof. Preferably, the at least one doping agent is or comprises the doped variants of the preceding elements.

[0042] According to a particular embodiment, the manufacturing method comprises only a step of atomization of the metallurgical material, and then the powder obtained, called intermediate powder, can be mixed, or not, with at least one doping agent.

[0043] According to another particular embodiment, the manufacturing method comprises only a step of grinding the metallurgical material, and then the powder obtained, called intermediate powder, can be mixed, or not, with at least one doping agent.

[0044] According to a first embodiment, the sintering step is carried out until a part of predetermined shape is obtained which is composed or made up of the sintered metallurgical material. Preferably, the part of predetermined shape is composed or made up solely of the sintered metallurgical material, the metallurgical material comprising one or more of the characteristics stated above.

[0045] According to a second embodiment, the sintering step is carried out until a part, called the starting part, is covered with a layer of the sintered metallurgical material so as to obtain a part of predetermined shape.

[0046] For example, the manufacturing method further comprises the following steps: - choose a room, called the starting room, - sinter the reduced powder on the starting part until it covers the said part in order to obtain the metallurgical part.

[0047] According to a first variant embodiment, the starting part is obtained by the sintering step according to the first embodiment.

[0048] According to a second variant embodiment, the starting part is composed of a metallurgical material which is not a precious metal, or which is not composed of a precious metal such as silver or gold.

[0049] Preferably, according to any embodiment, the manufacturing method comprises a step of adding at least one substrate metal powder with the metallurgical material, before the sintering step.

[0050] By substrate metal powder is meant any alloy, thermochemically compatible with the metallurgical powder resulting in high hardness metallic materials. For example, the substrate metal powder is 316L steel or nickel-free stainless steel.

[0051] Preferably, the manufacturing method further comprises a heat treatment step after the sintering step.

[0052] According to a second aspect, the invention proposes a metallurgical part based on precious metal obtained according to one or more of the characteristics of the manufacturing method of the first aspect.

[0053] In particular, the precious metal-based metallurgical part is obtained by SPS sintering of a powder of a metallurgical material characterized in that the powder has a grain size of less than 1000 micrometers and / or a crystallite size of less than 200 nanometers, so that said part obtained has a Vickers hardness greater than 150Hv.

[0054] Preferably, the reduction in grain size is obtained after the grinding step.

[0055] Preferably, the powder has an agglomerate size, after the grinding step, of less than 1000 micrometers.

[0056] The metallurgical part is for example a watch case, or a decorative part.

[0057] Description of the figure

[0058] [fig. 1] [fig.l] represents a flowchart presenting the different modes of rea lization of the manufacturing process for the specific case of gold.

[0059] With reference to [fig. 1], a method of manufacturing a metal part is provided during which:

[0060] - the powder of metallurgical material “Gold” can be only atomized or only crushed, see the first two lines,

[0061] - the powder of metallurgical material “Gold” can be atomized then ground, see the third line,

[0062] - the powder of metallic material “Gold” can be atomized and mixed with a addition element or doping agent, see fourth line,

[0063] - the powder of metallurgical material “Gold” can be ground and mixed with a addition element or doping agent, see fifth line,

[0064] - the powder of metallurgical material “Gold” can be atomized, then ground and mixed with an addition element or doping agent, see sixth line.

[0065] Obtaining this powder, called intermediate powder, is then sintered using the SPS sintering method, see “SPS sintering A”.

[0066] The hardness of the ex nihilo metallurgical part obtained or of the coating of the metallurgical part obtained is: - greater than 150Hv in the case of atomization alone or grinding alone, - greater than 250Hv in the case of atomization then grinding, - greater than 350Hv in other cases.

[0067] According to another embodiment, the intermediate powder can be deposited before or after a metal substrate powder so as to form a superposition of layers.

[0068] Then this superposition of layers is sintered using the SPS sintering method, see “SPS sintering B”, making it possible to obtain an ex nihilo metallurgical part.

[0069] According to an alternative embodiment, compared to the previous embodiment, it is possible to carry out the previous manufacturing method so as to form a coating, see “SPS C sintering”, on a metallurgical part obtained ex nihilo, after “SPS A sintering”.

[0070] According to another variant, the coating can be applied, see “SPS D sintering”, on a part, called the starting part, for example a steel, called 316L.

[0071] The coating may have a thickness greater than or equal to one millimeter.

[0072] The hardness of the part obtained is increased to reach a value between 150 and 250 HV, and up to 350 HV with doping agents.

[0073] For example, for a grain size of 5 micrometers and with the presence of doping agents A12O3 and Y2O3, the hardness of the part obtained is 270 Hv.

Claims

Claims

1. Method for manufacturing a metallurgical part based on precious metal characterized by the following steps: - Using a metallurgical material having a grain size of less than 400 micrometers and comprising at least 75% of precious metal, - reducing the size of the grains and / or crystallites of the metallurgical material so as to obtain aggregates with a characteristic size of less than 1000 micrometers, and an average crystallite size of less than 200 nanometers, - Sintering the reduced material using an SPS sintering process, so that the metallurgical part obtained has a Vickers hardness of greater than 150 Hv.

2. A manufacturing method according to the preceding claim, wherein the precious metal is gold or a gold-based alloy.

3. A manufacturing method according to claim 1, wherein the precious metal is silver or a silver-based alloy.

4. A manufacturing method according to one of the preceding claims, wherein the material comprises at least 90% precious metal.

5. Manufacturing method according to one of the preceding claims, in which the metallurgical material is a powder.

6. Manufacturing method according to one of the preceding claims, in which the metallurgical material is at least one plate element.

7. Manufacturing method according to one of the preceding claims, comprising a step of atomizing the metallurgical material so that it has a particle size of less than 1000 micrometers.

8. Manufacturing method according to one of the preceding claims, comprising a step of grinding the metallurgical material so that it has a particle size of less than 1000 micrometers.

9. Manufacturing method according to one of the preceding claims, in which the metallurgical material has a particle size of less than 200 pm (micrometers).

10. Manufacturing method according to one of the preceding claims, comprising a step of adding at least one doping agent with the metallurgical material, before the sintering step.

11. Manufacturing method according to the preceding claim, in which the at least one doping agent is boron nitride, titanium carbide, tungsten carbide, silicon carbide, niobium carbide, boron carbide, silicon nitride, aluminum oxide, zirconium oxide, yttrium oxide or a mixture thereof.

12. Manufacturing method according to one of the preceding claims, in which the sintering step is carried out until a part of predetermined shape composed of the sintered metallurgical material is obtained.

13. Manufacturing method according to one of claims 1 to 11, in which the sintering step is carried out until a part, called the starting part, is covered with a layer of the sintered metallurgical material so as to obtain a part of predetermined shape.

14. Manufacturing method according to the preceding claim, in which the starting part is obtained by the sintering step according to claim 19.

15. IZr. Manufacturing method according to claim 13, wherein the starting part is composed of a metallic material which is not a precious metal.

16. Manufacturing method according to one of the preceding claims, comprising a step of adding at least one substrate metal powder with the metallurgical material, before the sintering step.

17. Manufacturing method according to one of the preceding claims, comprising a heat treatment step after the sintering step.

18. Metallurgical part characterized in that it is obtained according to one of the preceding claims.

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