EP3121297B1 - Method for obtaining a trim component in platinum alloy - Google Patents

Description

Technical area

The present invention relates to a process for obtaining an ornamental component made of a platinum alloy containing at least 80% by weight of platinum, having a particularly high hardness and which can be machined by the usual methods. The component is intended for applications in watchmaking, jewelry, fine jewelry, eyewear, writing instruments, luxury accessories, and other decorative or functional applications where visual appearance is also important.

State of the art

In watchmaking, jewelry, eyewear and other applications, it is common to use alloys of precious metals, in particular platinum, since the properties of pure precious metal, such as platinum, are often not satisfactory, in particular in due to low hardness as well as medium gloss.

The document EP0621349 describes a process for hardening platinum on the surface with a surface layer containing boron. However, the method described only allows the treatment of a finished part, and tends to affect the aesthetic appearance of the latter (color and temperature). On the other hand, even hardened, the surface can be damaged. The restoration or refreshing of such a part requires a specific post-restoration process which can only be carried out under very specific conditions and is therefore not within the reach of restoration workshops equipped with conventional means.

The document GB2279967 provides a high purity platinum alloy containing cerium in small amounts. This alloy makes it possible to achieve hardnesses comparable, or even slightly higher, to that of a standard platinum alloy. Cerium being an element very sensitive to oxidation, the preparation of the alloy must be carried out under vacuum or under an inert gas, making the process complex. On the other hand, cerium has an average solid solubility in platinum, which also limits the homogeneity of the alloy obtained.

The document GB578956 provides a hardened platinum alloy by addition of oxides using a sintering process. The process is complex and depends on the size of the powders and the homogeneity of the compound to be obtained.

The document JP1988145759 offers a platinum alloy containing in particular iron, palladium and copper, and the hardening of which is obtained in part by an operation of plastic deformation to the shape of the part to be obtained and by a heat treatment operation after the setting steps in shape. This solution is therefore limited on the one hand by expensive matrix tools, and on the other hand by the heat treatment step which could promote recrystallization of the material, or even cause a surface texture to appear.

The document EP0947595 discloses a dispersion hardened platinum alloy. This alloy uses dispersion oxides in order to oxidize the non-noble metal. On the other hand, the dispersing elements not only tend to favor the wear of the tools, but they also make the polishing steps difficult.

Heat treatment hardening processes on finished platinum alloy components are also known. They have the disadvantage that the grain structure is changed, which affects the texture of polished surfaces.

Furthermore, severe plastic deformation processes applied to non-noble materials, such as aluminum, titanium or even copper, to increase their mechanical properties are known. The application of these processes to precious metals and alloys is however not known.

Finally, the document EP2705170A1 proposes a process for obtaining a very high hardness platinum alloy by means of an amorphous structure, namely a metallic glass. This high hardness, however, involves a very particular material and a shaping process and moreover require very specific tools and very high associated costs. The application of this technology to a wide range of products, very diverse in their geometries, is therefore not the most suitable from an industrial point of view.

Brief description of the figures

• la figure 1 reporte l'évolution de la limite d'élasticité et de la contrainte maximale pour un alliage comprenant au moins 80.0 % de platine en fonction de la température; et
• figure 2 reporte l'évolution de la dureté du même alliage en fonction de la température.

Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which:

• the figure 1 reports the evolution of the elastic limit and the maximum stress for an alloy comprising at least 80.0% of platinum as a function of the temperature; and
• figure 2 reports the evolution of the hardness of the same alloy as a function of temperature.

Brief summary of the invention

• fournir un matériau brut comprenant au moins 80.0 % massique de platine et ayant une première dureté;
• réaliser une étape d'écrouissage de l'alliage, de manière à obtenir un produit semi-fini ayant une seconde dureté, plus élevée que la première dureté; et
• réaliser une étape d'usinage du produit semi-fini pour obtenir le composant; l'étape d'ecrouissage comportant au moins un cycle de déformation plastique sévère réalisé à l'aide de l'une des méthodes suivantes, ou d'une combinaison de ces méthodes : l'extrusion angulaire à canaux égaux, le colaminage accumulatif, la torsion à haute pression, le matriçage rotationnel, l'extrusion-compression cyclique ou la corrugation et redressement répétés de manière à obtenir un produit semi-fini dans lequel la structure cristallographique de l'alliage est transformée en une structure à grain ultrafins, c'est-à-dire inférieure à 1um, de préférence inférieure à 500 nm

The invention relates to a method for obtaining an ornamental component comprising at least 80% by weight of platinum, the method comprising:

• providing a raw material comprising at least 80.0 wt% platinum and having a first hardness;
• carrying out a step of hardening the alloy, so as to obtain a semi-finished product having a second hardness, higher than the first hardness; and
• performing a step of machining the semi-finished product to obtain the component; the hardening step comprising at least one cycle of severe plastic deformation carried out using one of the following methods, or a combination of these methods: angular extrusion with equal channels, accumulative co-rolling, high pressure twisting, rotational die-forging, cyclic extrusion-compression or repeated corrugation and straightening so as to obtain a semi-finished product in which the crystallographic structure of the alloy is transformed into an ultra-fine grain structure, that is, i.e. less than 1um, preferably less than 500nm

The component obtained by the process of the invention has a hardness typically 25% to 50% higher than the hardness of the starting alloy. The process allows the component to be obtained in its final shape and hardness without the need for heat treatment on the final component to modify its hardness.

Example (s) of embodiment of the invention

• fournir un matériau brut comprenant au moins 80.0 % massique de platine et ayant une première dureté;
• réaliser une étape d'écrouissage de l'alliage de manière à obtenir un produit semi-fini ayant une seconde dureté, plus élevée que la première dureté; et
• réaliser une étape d'usinage du produit semi-fini afin d'obtenir le composant d'ornement; l'étape d'ecrouissage comportant au moins un cycle de déformation plastique sévère réalisé à l'aide de l'une des méthodes suivantes, ou d'une combinaison de ces méthodes : l'extrusion angulaire à canaux égaux, le colaminage accumulatif, la torsion à haute pression, le matriçage rotationnel, l'extrusion-compression cyclique ou la corrugation et redressement répétés de manière à obtenir un produit semi-fini dans lequel la structure cristallographique de l'alliage est transformée en une structure à grain ultrafins, c'est-à-dire inférieure à 1um, de préférence inférieure à 500 nm

According to one embodiment, a process for obtaining an ornamental component comprising at least 80% by mass of platinum, comprises the steps of:

• providing a raw material comprising at least 80.0 wt% platinum and having a first hardness;
• carrying out a step of hardening the alloy so as to obtain a semi-finished product having a second hardness higher than the first hardness; and
• performing a step of machining the semi-finished product in order to obtain the ornamental component; the hardening step comprising at least one cycle of severe plastic deformation carried out using one of the methods following, or a combination of these methods: angular equal-channel extrusion, accumulative co-rolling, high-pressure torsion, rotational die-forging, cyclic extrusion-compression, or repeated corrugation and straightening to obtain a semi-finished product in which the crystallographic structure of the alloy is transformed into an ultra-fine grain structure, i.e. less than 1um, preferably less than 500nm

The or severe plastic deformation cycles of step of hardening are intended to transform the crystal structure of platinum alloy into a grain structure ultrafine (English ultrafine grained UFG). By ultra-fine grain structure is meant a structure having a grain size of at least less than 1 µm in at least one grain orientation. Depending on the hardening step, for example the number of severe plastic deformation cycles, the grain size of the ultra-fine grain structure may be less than 500 nm, and even less than 200 nm, or even 100 nm.

In the context of severe plastic deformation, a very strong hydrostatic stress is introduced during implementation, delaying or even preventing the localization of the deformation and therefore the appearance of cracks. The deformation is more homogeneous than for conventional techniques such as rolling or drawing, where a texture related to the direction of deformation remains. In the usual cold deformation techniques, hardening is caused by the creation of dislocations (Frank-Read sources) which will pile up on the initial grain boundaries, to gradually form a structure of sub-joints (or cells) whose walls contain a very high density of dislocations. In materials that have undergone severe plastic deformation, the deformation is such that new grains are formed, with grain boundaries sharper than the cell walls and containing few dislocations. It is the very high density of grain boundaries that induces the mechanical properties of materials that have undergone severe plastic deformation. In severe plastic deformation, there is no variation of section nor of thickness, therefore the rate of deformation expressed with conventional calculations would be zero. However, the strain introduced is extremely large. For example, the deformation (or the relative elongation, denoted ε) can go up to 5, or even more.

According to one embodiment, the alloy comprises at least 80.0% of platinum and one or more of the following chemical elements: between 0% and 6% of ruthenium; between 0% and 5% copper; between 0% and 3% gallium.

The severe plastic deformation cycle (s) can be achieved using one of the methods, or a combination of these methods, comprising: equal channel angular pressing (ECAP ), angular extrusion according to equal channels (ECAP- accordance), high pressure torsion (high pressure torsion tube twisting or high pressure, or HPTT HPT), accumulative roll-bonding (accumulative roll bonding, ARB), repetitive corrugation and straightening (straightening and repetitive corrugation, SCR ), asymmetric rolling ( asymmetric rolling, ASR), cyclic extrusion-compression ( cyclic extrusion-compression, CEC), roller stamping (rotary swaging), or any other suitable method to obtain said structure with ultra-fine grains.

In one embodiment, the method comprises a step of shaping the alloy so as to give the semi-finished product a particular shape. For example, the semi-finished product may take the form of a plate, bar, ingot, billet, or any other shape advantageous for the subsequent machining step.

According to one embodiment, the method also comprises a step comprising one or more restorative heat treatments. Restorative heat treatment can be performed after the severe plastic deformation cycle, or between the severe plastic deformation cycles. The parameters of the restorative heat treatment are adjusted so as not to completely relax the platinum alloy, so as not to degrade the second hardness obtained during the work hardening step.

According to one embodiment, the restorative heat treatment (s) is carried out at a temperature below 600 ° C., above which the mechanical properties of the alloy drop rapidly and the ductility increases.

The restorative heat treatment (s) is preferably carried out at a temperature above 250 ° C and below 600 ° C. In fact, below 250 ° C., the restorative heat treatment may have little or no effect for treatment times which are less than 1 hour, the thermal activation not being sufficient.

The figure 1 reports the evolution of the elastic limit (Rm in MPa) and of the maximum stress (Rp0.2 in MPa) for an alloy comprising at least 80.0% of platinum as a function of the temperature. The figure 2 reports the evolution of the hardness (HV) of the same alloy as a function of temperature.

As these figures suggest, the restorative heat treatment (s) is preferably carried out at a temperature between 250 ° C and 500 ° C, or even between 250 ° C and 400 ° C.

Restoration heat treatment time can be one hour or less than one hour.

According to another embodiment, the method comprises one or more heat hardening treatments. The hardening heat treatment or treatments make it possible to further increase the second hardness of the semi-finished product by the precipitation of the alloying elements.

The process of the invention makes it possible to obtain the semi-finished product with a second hardness which is at least 25% and even 50% higher than the first hardness of the starting alloy.

The machining step may include a material removal process such as turning, milling, grinding, spark erosion, cutting, laser or water jet cutting, or any other suitable process. The machining step can also comprise a method of shaping by deformation, or by assembly.

The machining step can also include one or more finishing treatments, such as machining, satin-finishing, polishing, sandblasting, micro-blasting, etching or any other suitable mechanical process.

The machining step provides the component in its final shape and hardness. It is therefore not necessary to carry out heat treatment on the component obtained by the present process to modify its hardness. Indeed, the second hardness obtained for the semi-finished product by the hardening step corresponds to the desired hardness of the component.

The ornamentation component may comprise a horological component, for example a component of the movement, the bracelet or a covering component such as the case, the bezel, the back. The ornamentation component may also include a component for jewelry, jewelry, eyewear, writing instruments, luxury accessories (key ring, lighter, cufflink, tie bar, etc.) and others. decorative or functional applications where visual appearance is also important.

Example 1

In this first example, an alloy comprising about 95% platinum and about 5% ruthenium is provided, having a nominal first hardness of about 120Hv.

The alloy is subjected to the first forming step and to the hardening step comprising at least one cycle of severe plastic deformation, so as to increase the second hardness of the semi-finished product to a value of at least 250 Hv.

For example, when the hardening step is carried out by the accumulative co-rolling method, the hardened semi-finished product has a second hardness of about 250 Hv. When the hardening step is carried out by the high pressure twisting method, the semi-finished product has a second hardness greater than 400 Hv.

Example 2

In this first example, an alloy is provided comprising about 95% platinum, between 1% and 4% copper and between 1% and 4% gallium (for a total of about 5% copper and gallium), having a first nominal hardness of at least 200Hv.

For example, when the hardening step is carried out by the accumulative co-rolling method, the semi-finished product has a second hardness of about 300 Hv. The semi-finished product can then be subjected to a subsequent hardening heat treatment step so as to increase the second hardness to a value of about 400 Hv.

Example 3

This is the same alloy as in Example 2 but in which the work hardening step is carried out using the high pressure torsion method, allowing the semi-finished product to have a second hardness between 600 and 650 Hv.

The hardening step comprising at least one cycle of severe plastic deformation which can be followed by at least one restorative heat treatment, as described above.

The process for obtaining an ornamental component described here applies to any alloy comprising at least 80% by mass of platinum, and which may contain one or more of the following chemical elements: between 0% and 6% of ruthenium; between 0% and 5% copper; between 0% and 3% gallium.

Claims (18)

1. Method for obtaining an ornament component comprising at least 80% by mass of platinum, the method comprising:

   providing a raw material comprising at least 80.0% by mass of platinum and having a first hardness;

   carrying out a step of work hardening of the alloy in order to obtain a semi-finished product having a second hardness that is higher than the first hardness; and

   carrying out a step of machining of the semi-finished product to obtain the component;

   characterized in that the work hardening step comprises at least one cycle of severe plastic deformation brought about by equal-channel angular extrusion such that the crystallographic structure of the alloy is transformed into a structure having grains smaller than 1 µm, and preferably smaller than 500 nm.
2. Method for obtaining an ornament component comprising at least 80% by mass of platinum, the method comprising:

   providing a raw material comprising at least 80.0% by mass of platinum and having a first hardness;

   carrying out a step of work hardening of the alloy in order to obtain a semi-finished product having a second hardness that is higher than the first hardness; and

   carrying out a step of machining of the semi-finished product to obtain the component;

   characterized in that the work hardening step comprises at least one cycle of severe plastic deformation brought about by accumulative roll bonding such that the crystallographic structure of the alloy is transformed into a structure having grains smaller than 1 µm, and preferably smaller than 500 nm.
3. Method for obtaining an ornament component comprising at least 80% by mass of platinum, the method comprising:

   providing a raw material comprising at least 80.0% by mass of platinum and having a first hardness;

   carrying out a step of work hardening of the alloy in order to obtain a semi-finished product having a second hardness that is higher than the first hardness; and

   carrying out a step of machining of the semi-finished product to obtain the component;

   characterized in that the work hardening step comprises at least one cycle of severe plastic deformation brought about by high-pressure torsion such that the crystallographic structure of the alloy is transformed into a structure having grains smaller than 1 µm, and preferably smaller than 500 nm.
4. Method for obtaining an ornament component comprising at least 80% by mass of platinum, the method comprising:

   providing a raw material comprising at least 80.0% by mass of platinum and having a first hardness;

   carrying out a step of work hardening of the alloy in order to obtain a semi-finished product having a second hardness that is higher than the first hardness; and

   carrying out a step of machining of the semi-finished product to obtain the component;

   characterized in that the work hardening step comprises at least one cycle of severe plastic deformation brought about by rotational punching such that the crystallographic structure of the alloy is transformed into a structure having grains smaller than 1 µm, and preferably smaller than 500 nm.
5. Method for obtaining an ornament component comprising at least 80% by mass of platinum, the method comprising:

   providing a raw material comprising at least 80.0% by mass of platinum and having a first hardness;

   carrying out a step of work hardening of the alloy in order to obtain a semi-finished product having a second hardness that is higher than the first hardness; and

   carrying out a step of machining of the semi-finished product to obtain the component;

   characterized in that the work hardening step comprises at least one cycle of severe plastic deformation brought about by cyclic extrusion-compression such that the crystallographic structure of the alloy is transformed into a structure having grains smaller than 1 µm, and preferably smaller than 500 nm.
6. Method for obtaining an ornament component comprising at least 80% by mass of platinum, the method comprising:

   providing a raw material comprising at least 80.0% by mass of platinum and having a first hardness;

   carrying out a step of work hardening of the alloy in order to obtain a semi-finished product having a second hardness that is higher than the first hardness; and

   carrying out a step of machining of the semi-finished product to obtain the component;

   characterized in that the work hardening step comprises at least one cycle of severe plastic deformation brought about by repeated corrugation and

   straightening such that the crystallographic structure of the alloy is transformed into a structure having grains smaller than 1 µm, and preferably smaller than 500 nm.
7. Method according to one of Claims 1 to 6, further comprising at least one restoring heat treatment that is carried out following said at least one cycle of severe plastic deformation.
8. Method according to one of Claims 1 to 7, further comprising at least one hardening heat treatment that is configured to further increase the hardness of the semi-finished product that is hardened by precipitation of the alloy elements.
9. Method according to one of Claims 1 to 8, in which the alloy contains, by weight, at least 80.0% platinum and one or more of the following chemical elements: between 0% and 6% ruthenium; between 0% and 5% copper; between 0% and 3% gallium.
10. Method according to one of Claims 1 to 9, in which the alloy contains, by mass, approximately 95% platinum and approximately 5% ruthenium and has a first hardness of approximately 120 Hv; and in which, after the work hardening step, the semi-finished product has a hardness of at least 250 Hv.
11. Method according to Claim 10, in which the work hardening step is carried out using the accumulative roll bonding method such that the semi-finished product has a hardness of at least 250 Hv.
12. Method according to Claim 10, in which the work hardening step is carried out using the high-pressure torsion method such that the semi-finished product has a hardness of greater than 400 Hv.
13. Method according to one of Claims 1 to 8, in which the alloy contains, by mass, approximately 95% platinum, approximately 2% copper and approximately 3% gallium and has a first hardness of approximately 200 Hv; and in which, after the work hardening step, the semi-finished product has a hardness of at least 300 Hv.
14. Method according to Claim 13, in which the work hardening step is carried out using the accumulative roll bonding method such that the semi-finished product has a hardness of at least 300 Hv.
15. Method according to Claims 8 and 14, in which, after the hardening heat treatment, the semi-finished product has a hardness of at least 400 Hv.
16. Method according to Claim 13, in which the work hardening step is carried out using the high-pressure torsion method such that the semi-finished product has a hardness of at least 600 Hv.
17. Method according to one of Claims 1 to 14, further comprising a step of shaping the alloy so as to give the semi-finished product a specific shape, in particular a plate, a bar, an ingot or a billet.
18. Use of the ornament component obtained by the method according to one of Claims 1 to 17 in one of the following fields: watchmaking, jewellery, eyewear, writing instruments, luxury accessories or decorative applications.

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