Balance wheel for timepiece and method for manufacturing such balance wheel

Abstract

The invention relates to a balance (1) for a timepiece, comprising a rim (2), a hub (4) and at least one arm (8) connecting the hub (4) to the rim (2), at least a part of the balance (1) being made of an at least partially amorphous metal alloy, characterized in that said at least partially amorphous metal alloy is based on an element selected from platinum, zirconium and titanium and has a coefficient of thermal expansion of 7-12 ppm / DEG C. The invention also relates to a method for manufacturing such a balance by moulding, and to a resonator comprising such a balance and a single crystal quartz hairspring.

Description

[0001] This application is a divisional application of the Chinese invention patent application entitled “Balance wheel for clocks and watches and method for manufacturing such a balance wheel” with application date of December 3, 2018, application number 201880081961.6 and international application number PCT / EP2018 / 083295. Technical Field

[0002] The present invention relates to a balance wheel for a timepiece, comprising a rim, a hub and at least one arm connecting the hub to the rim, at least a portion of the balance wheel being made from a partially or completely amorphous metal alloy. The invention also relates to a method for manufacturing such a balance wheel and a resonator comprising such a balance wheel. Background Art

[0003] Such a balance wheel made of an amorphous metal alloy is disclosed, for example, in published European patent application No. EP2466396. In this patent application, the balance wheel is associated with a steel balance spring, and an iron-based amorphous metal alloy is used for the balance wheel due to its ferromagnetic properties. The problem that the invention, which is the subject of patent application EP2466396, therefore seeks to solve is to protect the balance spring from external interfering magnetic fields that could affect the frequency stability of the resonator.

[0004] The present invention addresses another parameter that can affect the frequency stability of a resonator, namely thermal variations, which were not addressed in patent application EP2466396. These thermal variations alter the stiffness of the balance spring, as well as the geometry of the balance spring and balance wheel, which in turn changes the spring constant and inertia, and thus the oscillation frequency. Watchmakers have long strived to develop temperature-stable oscillators and have explored and employed various approaches, including the Einvar alloy, which earned Charles-Édouard Guillaume the Nobel Prize. The elastic modulus of the Einvar alloy increases with temperature, compensating for the increase in balance inertia. Subsequently, the development of silicon oxide, and hence temperature-compensating silicon, has surpassed the performance of Einvar and offers the advantage of being less sensitive to magnetic fields. Similarly, single-crystal quartz balance springs provide thermal compensation for variations in balance inertia. However, unlike silicon oxide, where the oxide thickness can vary depending on the material used for the balance wheel, quartz is limited to materials with a thermal expansion coefficient of around 10 ppm / °C, corresponding to, for example, titanium and platinum. Major challenges with these materials are machinability and the control of fine structures and / or perfect finishes (e.g., mirror polish). For titanium, its relatively low density limits its use in large balance wheels, while for platinum, its high price limits its use to high-end luxury products. Summary of the Invention

[0005] One object of the present invention is to overcome these drawbacks by proposing a balance wheel made of a new material that allows it to be matched with a balance spring preferably made of single-crystal quartz, but also of silicon.

[0006] Another object of the invention is to propose a balance wheel made of a new material which allows simpler and more precise manufacturing, thereby for example reducing the dispersion of inertia and / or imbalance within the same production batch.

[0007] To this end, the invention firstly relates to a balance wheel for a timepiece, comprising a rim, a hub and at least one arm connecting the hub to the rim, at least a portion of the balance wheel being made of an at least partially amorphous metal alloy.

[0008] According to the invention, the at least partially amorphous metal alloy is based on an element chosen from platinum, zirconium and titanium and has a coefficient of thermal expansion comprised between 7 and 12 ppm / °C.

[0009] More specifically, the invention relates to a balance wheel for a timepiece, comprising a rim, a hub and at least one arm connecting the hub to the rim, at least a portion of the balance wheel being made from a partially or completely amorphous metal alloy, characterized in that the at least partially amorphous metal alloy is based on an element chosen from platinum, zirconium and titanium and has a coefficient of thermal expansion between 7 and 12 ppm / °C.

[0010] Preferably, the hub and the arms are made of the at least partially amorphous metal alloy and the rim is made of a first material having a higher density than the at least partially amorphous metal alloy from which the hub and the arms are made.

[0011] Preferably, said rim, said hub and said arms are made from said at least partially amorphous metal alloy.

[0012] Preferably, the rim comprises an over-molded first inertia-modulating element made of a second material having a density higher than that of the at least partially amorphous metal alloy.

[0013] Preferably, the rim comprises a housing for receiving a second inertia and / or imbalance adjustment element.

[0014] Preferably, the rim comprises a housing for receiving a decorative element and / or a lighting element.

[0015] Preferably, the hub comprises an integrated flexible centering element.

[0016] Preferably, the integrated flexible centering element is arranged on the inner edge of the hub.

[0017] Preferably, the arm carries an integrated third flexible inertial adjustment element.

[0018] Preferably, the arm, rim or hub has a structured surface finish.

[0019] Preferably, the at least partially amorphous metal alloy is platinum-based and has a coefficient of thermal expansion between 8 and 12 ppm / °C.

[0020] Preferably, the at least partially amorphous metal alloy based on platinum has the following composition in atomic percentages:

[0021] - platinum groups, the content of which constitutes the remainder,

[0022] -13-17% copper,

[0023] -3-7% nickel,

[0024] -20-25% phosphorus.

[0025] Preferably, the at least partially amorphous metal alloy is based on zirconium and has a coefficient of thermal expansion between 8 and 11 ppm / °C.

[0026] Preferably, the at least partially amorphous metal alloy based on zirconium has the following composition in atomic percentages:

[0027] - zirconium groups, the content of which constitutes the remainder,

[0028] -14-20% copper,

[0029] -12-13% nickel,

[0030] -9-11% aluminum,

[0031] -2-4% niobium.

[0032] Preferably, the at least partially amorphous metal alloy is titanium-based and has a coefficient of thermal expansion between 8 and 11 ppm / °C.

[0033] Preferably, the at least partially amorphous metal alloy based on titanium has the following composition in atomic percentages:

[0034] - titanium-based, the content of which constitutes the remainder,

[0035] -5-45% copper,

[0036] -2-25% nickel,

[0037] -2-30% zirconium,

[0038] -2-15% tin,

[0039] -0-5% silicon,

[0040] -0-5% hafnium.

[0041] The invention also relates to a method for manufacturing a balance wheel, wherein the rim, the hub and the arms are made of said at least partially amorphous metal alloy as defined above, based on an element chosen from platinum, zirconium and titanium, the method comprising the steps of:

[0042] a) manufacturing a mold having a negative form of the balance wheel;

[0043] -b) introducing said at least partially amorphous metal alloy based on an element chosen from platinum, zirconium and titanium into a mold, heating said metal alloy to a temperature between its glass transition temperature and its crystallization temperature for hot forming;

[0044] -c) cooling said metal alloy at a selected cooling rate so as to obtain a balance wheel made of said at least partially amorphous metal alloy based on an element chosen from platinum, zirconium and titanium,

[0045] - d) releasing the balance wheel obtained in step c) from its mould.

[0046] Preferably, the method comprises the step of overmoulding a first inertia adjustment element in the wheel rim.

[0047] The invention also relates to a method for manufacturing a balance wheel as defined above, comprising a hub and at least one arm made of an at least partially amorphous metal alloy based on an element chosen from platinum, zirconium and titanium, said method comprising the following steps:

[0048] a) making a mould with the negative form of the balance wheel,

[0049] a′) inserting a rim or rim elements into the mould, said rim or rim elements being made of a material having a higher density than the at least partly amorphous metal alloy based on an element selected from the group consisting of platinum, zirconium and titanium,

[0050] b) introducing said at least partially amorphous metal alloy based on an element chosen from platinum, zirconium and titanium into a mould, said metal alloy being heated to a temperature between its glass transition temperature and its crystallization temperature for hot forming,

[0051] c) cooling said metal alloy at a selected cooling rate so as to obtain a balance wheel made of said at least partially amorphous metal alloy based on an element chosen from platinum, zirconium and titanium,

[0052] d) releasing the balance wheel obtained in step c) from the mould.

[0053] Preferably, the method comprises the step of overmoulding a flexible centering element on the hub.

[0054] Preferably, the method includes the step of overmoulding a third flexible inertia adjustment element in the arm.

[0055] Preferably, the mold has a microstructure forming a decoration or a photonic network.

[0056] The invention also relates to a resonator comprising a balance wheel as defined above and a single-crystal quartz balance spring.

[0057] This at least partially amorphous metal alloy based on platinum, zirconium or titanium makes it possible to produce a balance wheel that can be paired with a single-crystal quartz balance spring.

[0058] The properties of amorphous metals make it possible to manufacture balance wheels made of at least partially amorphous metal alloys based on platinum, zirconium, or titanium using simplified manufacturing methods, such as casting or hot forming. Furthermore, due to the absence of dislocations, these at least partially amorphous metal alloys based on platinum, zirconium, or titanium exhibit a significantly higher elastic range than their crystalline counterparts. This property enables overmolding or integration into balance elements, not only to improve centering but also to adjust for inertia and / or imbalance. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Other features and advantages will become apparent from the following description given by way of non-limiting illustration with reference to the accompanying drawings, in which:

[0060] - Figure 1 is a perspective view of a balance wheel according to the invention.

[0061] - Figure 2 1 is a partial top view of a variation of the balance wheel according to the present invention.

[0062] - Figure 3 FIG. 4 is a partial top view of another variation of the balance wheel according to the present invention.

[0063] - Figure 4 It is along Figure 3 A sectional view of the axis AA; and

[0064] - Figures 5 to 10 1 is a partial top view of another variation of the balance wheel according to the present invention. DETAILED DESCRIPTION

[0065] See also Figure 1 , which shows a balance wheel 1 for a timepiece. This balance wheel 1 conventionally comprises a rim 2, continuous or discontinuous, defining the outer diameter of the balance wheel 1, and a hub 4 forming its central portion and provided with a hole 6 intended to receive an arbour (not shown) defining the pivot axis of the balance wheel 1. The hub 4 is securely connected to the rim 2 by arms 8. Here, the arms 8 are four in number and arranged at 90°. Balance wheels with two or three arms are also common, with the arms being arranged at 180° or 120°, respectively.

[0066] At least a portion of balance wheel 1 is made of a partially or completely amorphous metal alloy. By "at least partially amorphous" is meant a material capable of plastic deformation when heated to a temperature between its glass transition temperature and its crystallization temperature, and capable of solidifying into an at least partially amorphous phase.

[0067] According to the invention, the at least partially amorphous metal alloy is based on an element chosen from platinum, zirconium and titanium and has a coefficient of thermal expansion comprised between 7 and 12 ppm / °C.

[0068] In this specification, the expression "based on an element" means that the metal alloy contains at least 50% by weight of the element.

[0069] The at least partially amorphous metal alloy used in the present invention may be platinum-based and have a coefficient of thermal expansion in the range of 8-12 ppm / °C.

[0070] This at least partially amorphous metal alloy based on platinum may be composed of the following atomic percentages:

[0071] - platinum groups, the content of which constitutes the remainder,

[0072] -13-17% copper

[0073] -3-7% nickel

[0074] -20-25% phosphorus.

[0075] The at least partially amorphous metal alloy used in the present invention may also be zirconium-based and have a coefficient of thermal expansion of 8-11 ppm / °C.

[0076] This at least partially amorphous metal alloy based on zirconium may be composed of the following atomic percentages:

[0077] - zirconium groups, the content of which constitutes the remainder,

[0078] -14-20% copper

[0079] -12-13% nickel

[0080] -9-11% aluminum

[0081] -2-4% niobium.

[0082] The at least partially amorphous metal alloy used in the present invention may also be titanium-based and have a coefficient of thermal expansion in the range of 8-11 ppm / °C.

[0083] This at least partially amorphous metal alloy based on titanium may be composed of the following atomic percentages:

[0084] - titanium-based, the content of which constitutes the remainder,

[0085] -5-45% copper

[0086] -2-25% nickel

[0087] -2-30% zirconium

[0088] -2-15% tin

[0089] -0-5% silicon

[0090] -0-5% hafnium.

[0091] Ideally, the alloys used in the present invention do not contain any impurities. However, they may contain trace amounts of impurities that are usually unavoidable from the production of the alloys.

[0092] The platinum, titanium and zirconium based alloys used in the present invention have the advantage of a thermal expansion coefficient lower than 12 ppm / ° C. and higher than 7 ppm / ° C. They can therefore be used to manufacture at least a portion of a balance wheel to be matched with a single crystal quartz hairspring.

[0093] More preferably, the at least partially amorphous metal alloy based on platinum used in the present invention consists of the following atomic percentages:

[0094] 57.5% platinum, 14.7% copper, 5.3% nickel, 22.5% phosphorus.

[0095] The thermal expansion coefficient of this alloy is 11-12 ppm / °C.

[0096] More preferably, the at least partially amorphous metal alloy based on zirconium used in the present invention consists of the following atomic percentages:

[0097] 58.5% zirconium, 15.6% copper, 12.8% nickel, 10.3% aluminum, 2.8% niobium.

[0098] The thermal expansion coefficient of this alloy is 10.5-11 ppm / °C.

[0099] More preferably, the at least partially amorphous metal alloy based on titanium used in the present invention consists of the following atomic percentages:

[0100] 42.5% titanium, 7.5% zirconium, 40% copper, 5% nickel, 5% tin.

[0101] The thermal expansion coefficient of this alloy is 8-11 ppm / °C.

[0102] According to a first embodiment of the invention, rim 2, hub 4 and arms 8 are made of the same at least partially amorphous metal alloy based on platinum, zirconium or titanium as defined above. Advantageously, balance wheel 1 is monolithic, ie made in a single part.

[0103] For example, the balance wheel 1 can be made entirely of a platinum-based alloy as defined above. Since platinum has a high density (21,000 kg / m³), the at least partially amorphous platinum-based alloy used in the present invention also has a high density (15.5 g / cm³), and therefore it is not necessary to add elements made of dense materials to increase the inertia of the balance wheel.

[0104] The balance wheel 1 can also be made entirely of an at least partially amorphous zirconium- or titanium-based alloy as defined above. Since zirconium or titanium have relatively low densities, the at least partially amorphous zirconium- or titanium-based alloys used in the present invention also have relatively low densities (6.5 g / cm³ for zirconium and 5.5 g / cm³ for titanium), and therefore it is advisable to add elements made of denser materials to increase the inertia of the balance wheel, particularly if it is desired to manufacture a small balance wheel for a small movement. These elements make it possible to increase the inertia of the balance wheel while maintaining an attractive rim geometry and good aerodynamic properties.

[0105] Therefore, according to Figure 2 In the first variant shown, the rim 2 can include first overmolded inertial adjustment elements 10 made of a material having a higher density than the at least partially amorphous metal alloy. These first inertial adjustment elements 10 can be made, for example, of tungsten or tungsten carbide and obtained by overmolding.

[0106] according to Figure 3 In the second variant shown, the rim 2 may include housings 12 for receiving second inertia and / or unbalance adjustment elements 14, 15. As will be seen below, these housings 12 may advantageously be provided during the production of the balance wheel 1 by molding. The second inertia and / or unbalance adjustment elements 14, 15 may be, for example, inertia blocks, split inertia blocks, pins 14, split pins, or pins with unbalance 15 acting as inertia blocks. These elements are press-fitted or clamped in the corresponding housings 12. Figure 3 The pin 14 inserted into its housing 12 is shown, as is the pin with the unbalance 15 inserted into its housing 12 . Figure 4 Indicates along Figure 3 The cross-section along line AA of FIG. 1 shows a pin with an unbalance 15 inserted into a housing 12 arranged in the wheel rim 2 .

[0107] Obviously, these elements for increasing the inertia of the balance are preferably used with an at least partially amorphous zirconium- or titanium-based rim, but can also be used with a rim made of another material in a balance according to the invention.

[0108] In order to increase the inertia of the balance wheel, a thicker or wider rim may also be provided, especially in the case of a large balance wheel.

[0109] Figure 3 The housing 12 shown may also form a housing for accommodating decorative elements and / or lighting elements, such as tritium tubes (not shown).

[0110] According to another variant of the invention, hub 4 may comprise an integrated flexible centering element allowing the balance to self-center during its assembly on the arbour by elastic deformation of said flexible centering element.

[0111] according to Figure 5 , the integrated flexible centering element 16 is an elastic band arranged on the inner edge of the hub 4 so as to be positioned in the hole 6. Figure 6 , said integrated flexible centering elements 17 are arranged on the surface of hub 4 and distributed around hole 6. As will be seen below, flexible centering elements 16 and 17 can advantageously be put in place during the manufacture of balance wheel 1 by molding.

[0112] According to another variant of the invention, at least one of the arms 8 carries an integrated third flexible inertial adjustment element.

[0113] exist Figure 7 In the embodiment, the arm 8 ends at its end on the rim 2 side in two branches 8a, 8b, between which a housing 18 is formed, in which a third flexible bistable V-shaped inertial regulating element 19 for adjusting the frequency is integrated.

[0114] exist Figure 8 In the present invention, a third flexural buckling inertial regulating element 20 is used to regulate the frequency. To this end, it is made of a material having expansion characteristics different from those of the at least partially amorphous metal alloy based on platinum, zirconium or titanium of the balance wheel according to the invention, such as silicon or silicon oxide.

[0115] exist Figure 9 In the figure, the arm 8 ends at the end on the rim 2 side in three branches 8a, 8b, 8c, forming two shell cavities 18a, 18b between them, and a third flexible multi-stable inertial adjustment pawl element 22a, 22b is integrated in the shell cavities 18a, 18b for adjusting the frequency.

[0116] These three flexible inertial regulating elements 19 , 20 , 22 a , 22 b for regulating the frequency can advantageously be put in place during the manufacture of the balance 1 by moulding, as will be seen below.

[0117] These three flexible inertial adjustment elements 19, 20, 22a, 22b for adjusting the frequency can be used in both cases: when the entire balance wheel is made of an at least partially amorphous metal alloy based on zirconium, titanium or platinum according to the invention; and when the arms are made of an at least partially amorphous metal alloy based on zirconium, titanium or platinum, while the rest of the balance wheel, in particular the rim, is made of another material.

[0118] According to another variant of the invention, one of the arms 8, the rim 2 or the hub 4 has a structured surface. Only one of these elements may have a structured surface, or all the elements of the balance may have a structured surface; this structured surface may be the same or different. Figure 10 A balance wheel according to the invention is shown, in which the rim 2 has a structured surface state different from that exhibited by the arms 8. This structured surface state can be polished, satin-finished, sanded, circular-textured, sun-bleached, etc. Microstructures forming a photonic network can also be provided inside the mold used to manufacture the balance wheel, so that these microstructures can be replicated on the surface of the balance wheel. These microstructures can generate photonic crystals, giving the component diffraction arrays, specific colors, or holograms that can form security elements. These structures are introduced directly into the mold and replicated during the manufacture of the balance wheel by thermoforming, eliminating the need for finishing operations.

[0119] According to a second embodiment of the invention, the arms and hub of the balance wheel are made of the same at least partially amorphous metal alloy based on zirconium, titanium, or platinum, as defined above, and the rim is made of a material having a higher density than the at least partially amorphous metal alloy used for the arms and hub. This material can itself be an at least partially amorphous platinum-based metal alloy, as defined above, or another material. For example, the arms and hub of the balance wheel are made of an at least partially amorphous zirconium-based or titanium-based metal alloy, as defined above, to allow the balance wheel to be paired with a single-crystal quartz balance spring, and the rim is made of another material having a higher density than the at least partially amorphous zirconium-based or titanium-based metal alloy used for the arms and hub, to increase the inertia of the balance wheel.

[0120] Obviously, in this second embodiment of the invention, the rim can include the same first inertia adjustment element or the same housing for receiving the second inertia and / or imbalance adjustment element or the decorative and / or luminous element as described above with respect to the first embodiment of the invention. Similarly, the hub can include the same integrated flexible centering element as described above with respect to the first embodiment of the invention. Similarly, the arm can include the same third integrated flexible inertia adjustment element as described above with respect to the first embodiment of the invention. Similarly, the balance wheel element can have the structured surface conditions described above with respect to the first embodiment of the invention.

[0121] The invention also relates to a method for manufacturing a balance wheel 1, wherein the rim 2, the hub 4 and the arms 8 are made of a partially or completely amorphous platinum-, zirconium- or titanium-based metal alloy as defined above, the method comprising the following steps:

[0122] a) producing a mold having a negative form of the balance wheel, which can provide microstructures forming a decoration or a photonic network on the surface;

[0123] b) introducing said at least partially amorphous metal alloy based on an element chosen from platinum, zirconium and titanium into a mould, heating this metal alloy to a temperature between its glass transition temperature and its crystallization temperature, so as to be hot-formed in a balance wheel mould;

[0124] c) cooling said metal alloy at a selected cooling rate so as to obtain a balance wheel made of said partially or completely amorphous metal alloy based on an element chosen from platinum, zirconium and titanium;

[0125] d) releasing the balance wheel obtained in step c) from its mould.

[0126] In order to manufacture a balance wheel in a partially or completely amorphous metal alloy based on platinum, zirconium or titanium, it is advantageous to shape the metal by exploiting its properties in the at least partially amorphous state.

[0127] In fact, at least partially amorphous metals are very easy to form, allowing greater precision to be obtained when manufacturing parts with complex shapes. This is due to the special properties of amorphous metals, which are able to soften while remaining at least partially amorphous for a specific period of time within a given temperature range [Tg-Tx] specific to each alloy (for example zirconium-based alloys: Tg = 440°C, Tx = 520°C). Therefore, it can be formed at relatively low stresses and low temperatures, allowing the use of simplified processes such as hot forming. The use of such materials also allows fine geometries to be reproduced with high precision, since the viscosity of the alloy decreases sharply with temperature within the temperature range [Tg-Tx] and the alloy is thus molded to all the details of the negative mold. For example, for a platinum-based material as defined above, forming occurs at about 300°C with a viscosity of at most 10 3 Pa·second, the force is 1 MPa, not the viscosity at temperature Tg 10 12 The advantage of using a mold is that it can produce high-precision three-dimensional parts, which cannot be obtained by cutting or stamping.

[0128] One method used is the hot forming of an amorphous preform. This preform is obtained by melting metallic elements in a furnace, the metallic elements intended to form a partially or completely amorphous metal alloy based on platinum, zirconium, or titanium. The melting is carried out in a controlled atmosphere in order to obtain the lowest possible oxygen contamination of the alloy. Once the elements are melted, they are cast into a semi-finished product and then rapidly cooled to maintain the partially or completely amorphous state. Once the preform is formed, hot forming is performed to obtain the finished part. Hot forming is achieved by performing a pressing process within the temperature range between the glass transition temperature Tg and the crystallization temperature Tx of the metal alloy for a determined period of time to maintain the at least partially amorphous structure. This is done in order to preserve the elastic properties that are unique to amorphous metals.

[0129] Typically, for zirconium-based alloys and a temperature of 440°C, the pressing time should not exceed about 120 seconds. Thus, hot forming preserves the initial, at least partially amorphous state of the preform. The various final forming steps for the one-piece balance wheel according to the invention are then:

[0130] 1) Heat the mold with the negative form of the balance wheel to a selected temperature,

[0131] 2) inserting the at least partially amorphous metal preform between heated dies,

[0132] 3) applying a closing force to the mold to replicate the geometry of the mold on the at least partially amorphous metal preform,

[0133] 4) Wait for the selected maximum time,

[0134] 5) Open the mold,

[0135] 6) rapidly cooling the balance wheel to below the glass transition temperature so that the material remains in its at least partially amorphous state, and

[0136] 7) Remove the balance wheel from the mold.

[0137] Of course, the balance wheel can be made by casting or injection molding. This method involves casting or injecting a metal alloy heated to a temperature between its glass transition temperature and its crystallization temperature, so as to be at least partially amorphous, into a mold having the form of the final component. Once the mold is filled, it is rapidly cooled to a temperature below the glass transition temperature in order to prevent the alloy from crystallizing, thereby obtaining a balance wheel made of an at least partially amorphous metal alloy as defined above.

[0138] The mold can be reused or disassembled to release the parts. The advantage of this molding method is that it perfectly replicates the geometry of the balance wheel, including any decoration or surface structure. This reduces dispersion of inertia and allows for better centering within the same production batch of balance wheels. This molding method enables the production of balance wheels with attractive geometries, featuring sharp internal angles, raised rims and / or arm profiles, and a perfectly smooth finish. Discontinuous rims are also available. To achieve the highest quality, the mold is made of silicon using a deep reactive ion etching (DRIE) process. Obviously, the mold can also be produced using milling, laser, electrical discharge machining (EDM), or any other type of machining process.

[0139] The characteristic elastic properties of at least partially amorphous metals are used to overmold or integrate functional and / or decorative elements in the rim and / or arms and / or hub, for example by means of corresponding inserts placed in the mold before introducing the metal alloy, which is heated to a temperature between its glass transition temperature and its crystallization temperature so as to be at least partially amorphous.

[0140] More specifically, the method of the invention may comprise a step of overmolding the first inertia regulating element 10 in the rim 2 by means of an insert, wherein the insert is placed in a mold and the overmolding is carried out before introducing a metal alloy heated to a temperature between its glass transition temperature and its crystallization temperature so as to be at least partially amorphous.

[0141] The method of the invention may further comprise the step of overmolding flexible centering elements 16 , 17 on the inner edge or surface of the hub 4 .

[0142] The method of the invention may further comprise the step of overmolding a third flexible inertial adjustment element 19 , 20 , 22 a , 22 b in the arm 8 .

[0143] As mentioned above, the molding method also makes it possible to provide a mold with microstructures forming a decoration or a photonic network in order to obtain a structured surface state on the arms and / or the hub and / or the rim. It is also possible to add a logo to the mold.

[0144] The invention also relates to a method for manufacturing a balance wheel, wherein the hub and at least one arm are made of an at least partially amorphous metal alloy based on zirconium, titanium or platinum as defined above, and the rim is made of a material having a higher density than the density of said at least partially amorphous metal alloy used for the arms and the hub, said method comprising the following steps:

[0145] a) making a mould having a negative form of the balance wheel;

[0146] a′) inserting a rim or rim elements made of a material having a higher density than the at least partially amorphous metal alloy based on platinum, zirconium or titanium used for the arms and the hub into the mould,

[0147] b) introducing into a mould said at least partially amorphous metal alloy based on an element chosen from platinum, zirconium and titanium, this metal alloy being heated to a temperature between its glass transition temperature and its crystallization temperature, so as to be hot-formed in a balance wheel mould,

[0148] c) cooling said metal alloy at a cooling rate selected so as to obtain a balance wheel made of an at least partially amorphous metal alloy based on an element chosen from platinum, zirconium and titanium,

[0149] d) releasing the balance wheel obtained in step c) from its mould.

[0150] The invention also relates to a resonator comprising a balance wheel as defined above and a single-crystal quartz balance spring.

[0151] The balance wheel according to the invention is thus made of a material that allows the use of simple manufacturing methods while having a coefficient of thermal expansion that allows it to be matched to a single-crystal quartz hairspring. The balance wheel according to the invention also makes it possible to have at least an arm whose coefficient of thermal expansion allows it to be matched to a single-crystal quartz hairspring, while having a high inertia and maintaining a compact and attractive rim geometry, with a small volume, using a suitable rim that includes elements made of a higher-density material or is itself made of a higher-density material.

[0152] Heat treatment may also be performed to adjust the coefficient of expansion of the partially amorphous material in its final form by relaxing the amorphous structure (without crystallization).

[0153] The expansion coefficient can also be adjusted by locally controlled crystallization of the partially amorphous material in its final form.

Claims

1. A balance wheel (1) for a timepiece, comprising a rim (2), a hub (4) and at least one arm (8) connecting the hub (4) to the rim (2), at least a portion of the balance wheel (1) being made of a partially or completely amorphous metal alloy, characterized in that The at least partially amorphous metal alloy is based on an element selected from platinum, zirconium and titanium and has a coefficient of thermal expansion between 7 and 12 ppm / °C.

2. The balance wheel (1) according to claim 1, characterized in that The hub (4) and the arms (8) are made of the at least partially amorphous metal alloy, and the rim (2) is made of a first material having a density higher than the density of the at least partially amorphous metal alloy from which the hub (4) and the arms (8) are made.

3. The balance wheel (1) according to any one of claims 1 to 2, characterized in that The rim (2), the hub (4) and the arm (8) are made of the at least partially amorphous metal alloy.

4. Balance wheel (1) according to the preceding claim, characterized in that The rim (2) comprises an overmolded first inertia-modulating element (10) made of a second material having a density higher than that of the at least partially amorphous metal alloy.

5. Balance wheel (1) according to any one of the preceding claims, characterized in that The rim (2) comprises a housing (12) for receiving a second inertia and / or imbalance adjustment element (14, 15).

6. Balance wheel (1) according to any one of the preceding claims, characterized in that The rim (2) comprises a housing (12) for receiving a decorative element and / or a lighting element.

7. Balance wheel (1) according to any one of the preceding claims, characterized in that The hub (4) comprises integrated flexible centering elements (16, 17).

8. Balance wheel (1) according to the preceding claim, characterized in that The integrated flexible centering element (16) is arranged on the inner edge of the hub (4).

9. Balance wheel (1) according to any one of the preceding claims, characterized in that The arm (8) carries an integrated third flexible inertial adjustment element (19, 20, 22a, 22b).

10. Balance wheel (1) according to any one of the preceding claims, characterized in that The arm (8), the rim (2) or the hub (4) has a structured surface condition.

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