EP4092489A1 - Method for shaping a barrel spring made of amorphous metal - Google Patents

Abstract

Process for shaping a barrel spring formed from a monolithic ribbon of metallic glass. The process comprises a first step of forming a ribbon of metallic glass by a quenching technique on a wheel, as well as steps of shaping of the barrel spring, from this ribbon.

Description

The present invention relates to a method for shaping a barrel spring for a mechanism driven by a mainspring, in particular for a timepiece, formed from a metallic glass material.

We have already proposed in the EP0942337 a watch comprising an amorphous metal mainspring. In reality, only a blade formed from a laminate comprising ribbons of thicknesses ranging up to 50 μm in amorphous metal assembled with epoxy resin is described in this document. As a variant, an assembly of blades by spot welding of the two ends and of the point of inflection of the free form of the spring has been proposed.

The major problem with such a blade is the high risk of delamination of the laminate during its shaping and following the repeated winding and unwinding to which such a spring is subjected. This risk is all the more accentuated when the resin ages badly and loses its properties.

This solution does not make it possible to guarantee the functionality and the fatigue behavior of the spring. Moreover, the modeling of the theoretical shape of the proposed spring does not take into account the behavior of a stratified material.

The reason for choosing to use several thin blades assembled together is due to the difficulty of obtaining thicker metallic glass blades, whereas processes for the manufacture of ribbons of ten to thirty microns by quenching were known. fast, developed in the 1970s for amorphous ribbons used for their magnetic properties.

It is obvious that such a solution does not make it possible to meet the torque, reliability and autonomy requirements that a mainspring must satisfy.

• La bande est enroulée sur elle-même pour former une spirale serrée (déformation élastique) et ensuite traitée dans un four pour fixer cette forme. Ce traitement thermique est également essentiel pour les propriétés mécaniques car il permet d'augmenter la limite élastique du matériau, par une modification de sa structure cristalline (durcissement structurel par précipitation);
• le ressort en forme de spirale est estrapadé, donc déformé plastiquement à froid pour prendre sa forme définitive. Ceci permet aussi d'augmenter le niveau de contrainte à disposition.

As for traditional Nivaflex ^® alloy springs in particular, the initial strip of alloy is formed into a mainspring in two stages:

• The strip is rolled up on itself to form a tight spiral (elastic deformation) and then treated in an oven to fix this shape. This heat treatment is also essential for the mechanical properties because it makes it possible to increase the elastic limit of the material, by modifying its crystalline structure (structural hardening by precipitation);
• the spiral-shaped spring is strapped, therefore plastically deformed when cold to take on its final shape. This also increases the level of constraint available.

The mechanical properties of the alloy and the final shape are the result of the combination of these two steps. A single heat treatment would not achieve the mechanical properties desired for traditional alloys.

The fixing of crystalline metal alloys involves a relatively long treatment time (several hours) at a temperature high enough to induce the desired modification of the crystalline structure.

In the case of metallic glasses, the mechanical properties of the material are intrinsically linked to its amorphous structure and are obtained immediately after solidification unlike the mechanical properties of traditional alloy springs. Nivaflex ^® which are obtained by a series of heat treatments at different stages of their manufacturing process. Therefore, and contrary to the Nivaflex ^® alloy, subsequent hardening by heat treatment is not necessary.

Traditionally, only strapping makes it possible to give the spring an optimal shape which allows maximum stress of the band over its entire length once the spring is armed. On the contrary, for a metallic glass spring, the final optimal shape is only fixed by a single heat treatment, while the high mechanical properties are only related to the amorphous structure. The mechanical properties of metallic glasses are not changed by heat treatment or by plastic deformation, because the mechanisms are totally different from those encountered in a crystalline material.

The object of the present invention is to remedy, at least in part, the aforementioned drawbacks.

To this end, the subject of the present invention is a method for shaping the barrel spring according to claim 1.

The fact of making a mainspring in a monolithic ribbon of metallic glass makes it possible to derive all the advantages of this class of materials, in particular its ability to store a high density of elastic energy and to release it with a remarkably constant torque. . The values of the maximum stress and of the Young's modulus of these materials indeed make it possible to increase the ratio σ ² /E compared to traditional alloys, such as Nivaflex ^® .

Modes of execution of the method are defined by claims 2 to 11.

1. 1. Procédé pour la mise en forme d'un ressort de barillet formé d'un ruban monolithique en verre métallique, caractérisé en ce que :
   • on calcule la forme théorique libre à donner à ce ruban monolithique en verre métallique pour que chaque segment, une fois le ressort armé dans le barillet, soit soumis au moment de flexion maximum,
   • on met ce ruban en forme en lui donnant des courbures, caractéristiques de cette forme théorique libre, pour tenir compte d'une diminution des courbures une fois le ruban libéré,
   • on effectue la relaxation du ruban pour fixer sa forme en le chauffant,
   • on refroidit ce ruban.
2. 2. Procédé selon la proposition 1, selon lequel, on obtient la forme théorique libre du ressort de barillet au ruban monolithique en le disposant sur un posage approprié.
3. 3. Procédé selon l'une des propositions 1 et 2, selon lequel on effectue le fixage du ruban monolithique mis en forme en le soumettant à un chauffage dans une plage comprise entre -50K de la température de transition vitreuse et +50K de la température de cristallisation.
4. 4. Procédé selon l'une des propositions 1 à 3, selon lequel on effectue le fixage du ruban mis en forme en le chauffant puis en le refroidissant dans un intervalle de temps inférieur à 6 minutes.
5. 5. Procédé selon la proposition 1, dans lequel le rapport entre les courbures dudit ruban mis en forme avant le chauffage de relaxation et les courbures de la forme théorique libre se situe entre 100% et 140%.
6. 6. Procédé selon la proposition 5, dans lequel le rapport entre les courbures dudit ruban mis en forme avant le chauffage de relaxation et les courbures de la forme théorique libre se situe typiquement à 130%.

According to another aspect of the invention, a formatting method is defined by the following proposals:

1. 1. Method for shaping a barrel spring formed from a monolithic ribbon of metallic glass, characterized in that:
   • the theoretical free shape to be given to this monolithic ribbon of metallic glass is calculated so that each segment, once the spring is wound in the barrel, is subjected to the maximum bending moment,
   • this ribbon is shaped by giving it curvatures, characteristics of this theoretical free shape, to take into account a reduction in curvatures once the ribbon is released,
   • the ribbon is relaxed to fix its shape by heating it,
   • this ribbon is cooled.
2. 2. Method according to proposal 1, according to which, the theoretical free shape of the barrel spring with the monolithic ribbon is obtained by arranging it on an appropriate laying.
3. 3. Method according to one of proposals 1 and 2, according to which the fixing of the shaped monolithic ribbon is carried out by subjecting it to heating in a range between -50K of the glass transition temperature and +50K of the temperature of crystallization.
4. 4. Method according to one of proposals 1 to 3, according to which the fixing of the shaped tape is carried out by heating it then cooling it in a time interval of less than 6 minutes.
5. 5. Method according to proposal 1, in which the ratio between the curvatures of said ribbon shaped before the relaxation heating and the curvatures of the theoretical free shape is between 100% and 140%.
6. 6. Process according to proposition 5, in which the ratio between the curvatures of said ribbon shaped before the relaxation heating and the curvatures of the theoretical free shape is typically at 130%.

• La figure 1 est une vue en plan du ressort armé dans le barillet;
• la figure 2 est une vue en plan du ressort désarmé dans le barillet;
• la figure 3 est une vue en plan du ressort dans son état libre;
• la figure 4 est un diagramme armage-désarmage d'un ressort de barillet en verre métallique.

The accompanying drawings illustrate, schematically and by way of example, one embodiment of the process for shaping a barrel spring which is the subject of the invention.

• The figure 1 is a plan view of the spring cocked in the barrel;
• the figure 2 is a plan view of the spring disarmed in the barrel;
• the picture 3 is a plan view of the spring in its free state;
• the figure 4 is a winding-unwinding diagram of a metallic glass mainspring.

In the example set out below, the strips intended to form the barrel springs are produced by the wheel quenching technique (or Planar Flow Casting) which is a technique for producing metal strips by rapid cooling. A jet of molten metal is propelled onto a cold wheel which spins at high speed. The speed of the wheel, the width of the injection slot, the injection pressure are all parameters which will define the width and the thickness of the tape produced. Other ribbon-making techniques may also be used, such as the Twin Roll Casting.

The alloy used is Ni ₅₃ Nb ₂₀ Zr ₈ Ti ₁₀ Co ₆ Cu ₃ in this example. From 10 to 20g of alloy are placed in a distribution nozzle heated between 1050 and 1150°C. The slit width of the nozzle is between 0.2 and 0.8mm. The distance between the nozzle and the wheel is between 0.1 and 0.3mm. The wheel on which the molten alloy is deposited is a copper alloy wheel and driven at a speed of 5 to 20m/s. The pressure exerted to force the molten alloy out through the nozzle is between 10 and 50 kPa.

Only a good combination of these parameters made it possible to form ribbons with a thickness greater than 50 μm, typically >50 to 150 μm and with a length of more than one meter.

e :

Epaisseur du ruban [mm]

h :

Hauteur du ruban [mm]

σmax:

Contrainte maximale en flexion [N/mm2]

For a tape subjected to pure bending, the maximum elastic moment is given by the following relationship: $$\mathit{Mmax}=\frac{e^2\mathrm{<figure-callout\; id="6"\; label="h"\; filenames="imgf0001.png"\; state="\{\{state\}\}">h</figure-callout>}}{6}\mathit{\sigma max}$$

e :

Tape thickness [mm]

h:

Tape height [mm]

σmax:

Maximum bending stress [N/mm2]

The barrel spring releases its energy when it passes from the armed state to the disarmed state. The goal is to calculate the shape that the spring must have in its free state so that each section is subjected to the maximum bending moment in its armed state. The figures 1 to 3 below respectively describe the three mainspring configurations, namely armed, disarmed and free.

For the calculations, the spring in its charged state (see figure 1 ) is considered to be a spiral with the turns tight against each other.

rn :

Rayon à l'état armé du nième tour [mm]

rbonde :

Rayon de la bonde du barillet [mm]

n :

Nb de tours d'armage

e :

Epaisseur du ruban [mm]

In this case any point on the curvilinear abscissa can be written by: $$\mathrm{rn}=\mathrm{rbonde}+\mathrm{not}$$

rn:

Radius in the cocked state of the nth turn [mm]

round:

Barrel bung radius [mm]

not :

No. of winding turns

e :

Tape thickness [mm]

Ln :

Longueur de l'abscisse curviligne du nième tour [mm]

rn :

Rayon à l'état armé du nième tour [mm]

θ :

Angle parcouru [rad]. Dans le cas d'un tour θ=2π

Moreover the length of the curvilinear abscissa of each turn is given by: $$\ln =\mathrm{rn}\times \mathrm{\theta }$$

Ln:

Length of the curvilinear abscissa of the nth lap [mm]

rn:

Radius in the cocked state of the nth turn [mm]

θ:

Angle traveled [rad]. In the case of a turn θ=2π

$R_{\mathit{libre}}^n$

:

Rayon à l'état libre du nième tour [mm]

Mmax :

Moment max [N mm]

E :

Module de Young [N/mm²]

I:

Moment d'inertie [mm⁴]

The shape of the spring in its free state is calculated taking into account the differences in radii of curvature so that the spring is constrained to σ max over the entire length. $$\frac{1}{\mathit{rn}}-\frac{1}{R_{\mathit{free}}^{\mathrm{not}}}=\frac{\mathit{Mmax}}{\mathit{IS}}=\frac{2\sigma }{\mathit{eE}}$$

$R_{\mathit{free}}^{\mathrm{not}}$

:

Radius in the free state of the nth turn [mm]

Mmax:

Max moment [N mm]

E :

Young's modulus [N/ ^mm2 ]

I:

Moment of inertia [mm ⁴ ]

1. 1. Calculer le rayon à l'état armé du nième tour par la relation (2) avec n=l, 2, ....
2. 2. Calculer la longueur de l'abscisse curviligne du nième tour par la relation (3).
3. 3. Calculer le rayon à l'état libre du nième tour par la relation (4) .
4. 4. Pour finir calculer l'angle du segment du nième tour par la relation (3) mais en remplaçant rn par $R_{\mathit{libre}}^n$
   
   et en conservant la longueur de segment Ln calculée au point 2.

Therefore, to calculate the theoretical shape of the spring in the free state, we only need to calculate the following elements:

1. 1. Calculate the radius in the reinforced state of the nth turn by the relation (2) with n=l, 2, ....
2. 2. Calculate the length of the curvilinear abscissa of the nth turn by relation (3).
3. 3. Calculate the radius in the free state of the nth turn by the relation (4).
4. 4. Finally calculate the angle of the segment of the nth turn by the relation (3) but replacing rn by $R_{\mathit{free}}^{\mathrm{not}}$
   
   and keeping the segment length Ln calculated in point 2.

With these parameters, it is now possible to construct the spring in the free state so that each element of the spring is constrained to the σmax ( picture 3 ).

The metallic glass ribbon is obtained by rapid solidification of the liquid metal on a copper or alloy wheel with high thermal conductivity rotating at high speed. A minimum critical cooling rate is required to vitrify liquid metal. If the cooling is too slow, the metal solidifies by crystallization and loses its mechanical properties. It is important, for a given thickness, to guarantee the maximum cooling rate. The higher this will be, the less the atoms will have time to relax and the greater the concentration of free volume will be. The ductility of the ribbon is then improved.

The plastic deformation of metallic glasses, below about 0.7 x the glass transition temperature Tg [K], occurs heterogeneously through the initiation and then the propagation of slip bands. The free volumes act as germination sites for the slip bands and the higher their number, the less the deformation is localized and the greater the deformation before failure.

The Planar Flow Casting step is therefore decisive for the mechanical and thermodynamic properties of the tape.

Between the glass transition temperature Tg-100K and Tg, the viscosity decreases sharply with temperature, i.e. approximately one order of magnitude per 10K rise. The viscosity at Tg is generally equal to 10 ¹² Pa.s, independently of the alloy considered. It is then possible to model the viscous body, in this case the ribbon, to give it its desired shape, then cool it to permanently fix the shape.

Around Tg, thermal activation will allow the diffusion of free volumes and atoms within the material. The atoms will locally form denser domains, close to a crystalline structure at the expense of the free volumes, which will be annihilated. This phenomenon is called relaxation. The decrease in free volume is accompanied by an increase in Young's modulus and a decrease in subsequent ductility.

At higher temperatures (above Tg), the relaxation phenomenon may resemble annealing. By the thermal agitation, the relaxation is accelerated and causes a drastic embrittlement of the glass by annihilation of the free volume. If the treatment time is too long, the amorphous material will crystallize and thus lose its exceptional properties.

Hot forming is therefore a balance between sufficient relaxation to retain the desired shape and as little reduction in ductility as possible.

To achieve this, it is necessary to heat and cool as quickly as possible, and to maintain the ribbon at the desired temperature for a well-controlled time.

The Ni ₅₃ Nb ₂₀ Zr ₈ Ti ₁₀ Co ₆ Cu ₃ alloy used was selected for its excellent compromise between mechanical resistance (3 GPa) and its ability to vitrify (critical diameter of 3 mm and ΔT (=Tg-Tx) of 50°C, Tx designating the crystallization temperature). His elastic modulus is 130 GPa, measured in tension and bending.

• Résistance maximale σmax = 3000 MPa
• Déformation élastique εmax= 0.02
• Module élastique E = 130 GPa

Mechanical properties :

• Maximum resistance σmax = 3000 MPa
• Elastic deformation εmax= 0.02
• Elastic modulus E = 130 GPa

• Transition vitreuse Tg = 593°C
• Température de cristallisation Tx = 624°C
• Température de fusion Tm = 992°C

Thermodynamic properties :

• Glass transition Tg = 593°C
• Crystallization temperature Tx = 624°C
• Melting temperature Tm = 992°C

The ribbons produced by the Planar Flow Casting (PFC) technique have a width of several millimeters and a thickness of between 40 and 150 µm. Strips have been machined, using the wire EDM technique, to the typical width and length of a mainspring. The flanks were ground, after which the spring was shaped, based on the theoretical shape as calculated previously.

To carry out the shaping, a laying of the type generally used is used, on which the spring is wound to give it its free shape, determined by the theoretical shape as calculated above, taking into account a variation between the shape imposed by the laying and the free shape actually obtained. It has in fact been observed that the curvatures (being defined as the inverse of the radius of curvature) of the spring in the free state after shaping were reduced with respect to the curvatures of the shape of the laying. The laying curvatures must therefore be increased accordingly so that the free shape obtained corresponds to the theoretical shape. Furthermore, the ratio between the curvatures of the shaped ribbon before the relaxation heating and the curvatures of the theoretical free form depends on the heating parameters, the alloy and its state of initial relaxation, and is between 100% and 140%, typically 130% under the conditions used below.

The spring in its setting was then introduced into an oven heated to around Tg (590° C.) for a period of 3 to 5 minutes, depending on the setting used.

Other heating modes can be used, such as heating by Joule effect or a jet of hot inert gas for example.

Once the spring thus formed, a sliding flange for a self-winding watch spring in Nivaflex ^® alloy was riveted to its outer end, to enable winding and unwinding tests to be carried out. The sliding flange is necessary to ensure the function of such a spring, however its method of assembly to the blade as well as the material of the flange may vary.

The figure 4 shows the variation in torque as a function of the number of turns obtained with the spring calculated and shaped according to the method described in this document. This winding-unwinding curve is entirely characteristic of the behavior of a mainspring. In addition, the torque, the number of turns of development and the overall efficiency are fully satisfactory given the dimensions of the ribbon.

Claims (11)

Process for shaping a barrel spring formed from a monolithic ribbon of metallic glass, characterized in that the ribbon is produced by a quenching technique on a wheel. A method according to claim 1, wherein the strip is machined to length and width by a wire EDM technique. Method according to one of the preceding claims, in which flanks of the strip are ground. Method according to one of the preceding claims, in which the tape has a thickness greater than 50 µm. Method according to one of the preceding claims, according to which: - the theoretical free shape to be given to this metallic glass monolithic ribbon is calculated so that each segment, once the spring is wound in the barrel, is subjected to the maximum bending moment, - this ribbon is shaped by giving it curves, characteristics of this theoretical free shape, to take into account a reduction in curvatures once the ribbon is released, - the ribbon is relaxed to fix its shape by heating it, - this ribbon is cooled. A method according to claim 5, wherein the ratio of the curvatures of said shaped ribbon before the relaxation heating to the curvatures of the theoretical free shape is between 100% and 140%. A method according to claim 6, wherein the ratio between the curvatures of said shaped ribbon before the heating of relaxation and curvatures of the theoretical free form is typically at 130%. Method according to one of Claims 5 to 7, according to which the theoretical free shape of the barrel spring with the monolithic strip is obtained by arranging it on an appropriate setting. Process according to one of Claims 5 to 8, according to which the fixing of the shaped monolithic ribbon is carried out by subjecting it to heating in a range between -50K of the glass transition temperature and +50K of the crystallization temperature. . Process according to one of Claims 5 to 9, according to which the fixing of the shaped ribbon is carried out by heating it then by cooling it in a time interval of less than 6 minutes. Method according to one of the preceding claims, characterized in that a sliding flange is assembled to the spring, in particular by riveting.

Images