EP3377247B1 - Method for manufacturing a part from amorphous metal - Google Patents

Description

The present invention relates to a method of manufacturing a micromechanical part made of amorphous metal.

The technical field of the invention is the technical field of fine mechanics. More precisely, the invention will belong to the technical field of methods of manufacturing parts made of amorphous metal.

TECHNOLOGICAL BACKGROUND

Various methods are known for making micromechanical parts. Indeed, the latter can be produced by micromachining or stamping or by injection into a mold.

To produce parts from amorphous metal, it can also be envisaged to use micro-machining or stamping methods.

However, an advantageous solution consists in directly casting the part in amorphous metal so as to obtain by casting the final geometry or a geometry close to the final geometry requiring only a few alterations. The lack of crystal structure implies that the properties of the amorphous metal part (especially mechanical properties, hardness and polishability) do not depend on the manufacturing method. This is a major advantage over traditional polycrystalline metals for which the foundry blanks exhibit reduced properties compared to forgings.

However, for the production of micromechanical parts of very low thickness (0.5 to 2mm), drawbacks appear.

A first problem arises from the cooling of the mold. This drawback can be seen in two aspects. A first aspect is that cooling should not be too slow because there is then a risk of partial or total crystallization and therefore of losing the properties of amorphous metals. For certain micromechanical parts or certain trim parts, the presence of a single crystallite can be prohibitive for reasons of mechanical properties or visual appearance, given that such crystallites will inevitably appear during the finishing steps. It is therefore essential to have sufficiently rapid cooling during casting to guarantee the amorphicity of the part. For this reason, the molds are made of metal, for example steel or copper, allowing heat to be quickly extracted. Depending on the ability of the alloy chosen to become amorphous, it is possible with this method to obtain parts with a thickness of the order of 10mm.

The second aspect to consider comes from the fact that the cooling should not be too fast because there is a risk of solidification before the mold cavity is completely filled. However, with metal molds such as copper or steel, thermal energy is quickly dispersed, leading to a risk of premature solidification. These two contradictory aspects imply the following compromise: the thickness of the castings must be neither too small (risk of solidification before complete filling of the impression), nor too great (risk of crystallization). This is why this process is conventionally limited to parts with a thickness between about 2 and 10mm.

A second drawback is a shaping problem. This shaping problem comes from the small size of the mold and the footprint of the micromechanical part to be manufactured. For certain geometries, in particular hollowed out geometries that cannot be molded, it may be necessary to add inserts in the mold which must be eliminated after shaping, and lost. For complex shapes, the cost of these inserts and of the additional operations linked to them can become very high, making the process unusable industrially.

Another advantageous solution consists in using the shaping properties of amorphous metals. In fact, amorphous metals have the particular characteristic of softening while remaining amorphous in a given temperature range [Tg - Tx] specific to each alloy and not very high because these temperatures Tg and Tx are low. This then makes it possible to reproduce very precisely fine and precise geometries because the viscosity of the alloy decreases sharply and the latter can be easily deformed in order to match all the details of a mold.

However, for the production of micromechanical parts of very low thickness (0.5 to 2mm), the production of suitable molds is here again very complex and has the same limitations as in foundry.

In addition, at a temperature between Tg and Tx, the time available before the alloy eventually crystallizes is limited. If the geometry has many complexities of low thicknesses, the time required for the complete filling of the mold may be greater than the time available, resulting in partial or complete crystallization of the part and a loss of its mechanical properties in particular.

A similar known technique is LIGA technology. LIGA consists of three main stages of treatment; lithography, electroplating and casting. There are two main LIGA manufacturing technologies, the LIGA X-Ray technique, which uses X-rays produced by a synchrotron to create structures with a high aspect ratio, and the LIGA UV technique, a more accessible method that uses ultraviolet light to create structures with low aspect ratios.

• des rapports d'aspect élevés, de l'ordre de 100:1 ;
• des parois latérales parallèles avec un angle de flanc de l'ordre de 89,95 °;
• parois latérales lisses avec = 10 nm, adaptées pour les miroirs optiques ;
• hauteurs structurelles de dizaines de micromètres à quelques millimètres ;
• les détails structurels de l'ordre de micromètres sur des distances de centimètres.

Notable features of LIGA structures fabricated by the X-ray method include:

• high aspect ratios, of the order of 100: 1;
• parallel side walls with a flank angle of the order of 89.95 °;
• smooth side walls with = 10 nm, suitable for optical mirrors;
• structural heights from tens of micrometers to a few millimeters;
• structural details on the order of micrometers over distances of centimeters.

X-Ray LIGA is a microtechnology manufacturing process developed in the early 1980s. In this process, an X-ray sensitive photoresist polymer, typically PMMA (poly (methyl methacrylate)), bonded to an electrically conductive substrate, is exposed to parallel high-energy x-ray beams from a synchrotron radiation source through a mask partially covered with an x-ray absorbing material. Chemical removal of exposed (or unexposed) areas of the photoresistive polymer enables a three-dimensional structure to be obtained which can be filled by electrodeposition of metal. The resin is chemically removed to produce a metal mold insert. The mold insert can be used to produce polymer or ceramic parts by injection molding.

The main advantage of the LIGA technique is the precision obtained by the use of X-ray lithography (DXRL). This technique allows to have microstructures having high aspect ratios and high precision to be fabricated in a variety of materials (metals, plastics and ceramics).

The LIGA UV technique uses an inexpensive ultraviolet light source, such as a mercury lamp, to expose a photoresistive polymer, typically SU-8. Since heating and transmission are not a problem in optical masks, a simple chrome mask can be substituted for the sophisticated X-ray mask technique. These simplifications make the LIGA UV technique a much cheaper and more accessible technique than its X-ray counterpart. However, the LIGA UV technique is not as efficient in producing precision molds and is therefore used when the cost must be kept low and when very high aspect ratios are not required.

The drawback of such a method is that it does not allow the simple production of three-dimensional parts. Indeed, the manufacture of three-dimensional part via the LIGA process is possible but it is necessary to carry out several successive iterations of photolithography, and galvanic deposition.

Furthermore, the LIGA process poses a problem concerning the choice of materials. Indeed, two materials are necessary: a material for the substrate and a material that will be deposited. The material for the substrate must be photo-structurable so that plaster or zircon cannot be used. For the deposited material, it must be able to be deposited via electroplating so that metallic materials are the only possible ones. However, such materials generally have thermal characteristics such that they ensure good heat dissipation and therefore good cooling. This ability to properly dissipate thermal energy would cause, for an amorphous metal alloy shaped in the LIGA mold, too rapid hardening and would therefore prevent good formation of the parts.

Finally, the LIGA process for the manufacture of the mold is such as to limit the possible geometries since such a three-dimensional mold would require manufacturing layer by layer.

Patent documents are also known in the state of the art. CH707351A2 , EP2466394A1 and EP2835698A , which relate to other examples of the manufacturing process of these parts.

SUMMARY OF THE INVENTION

1. a) se munir d'un moule réalisé dans un second matériau, ledit moule comportant une empreinte formant le négatif du composant ;
2. b) se munir du premier matériau et le mettre en forme dans l'empreinte dudit moule, ledit premier matériau ayant subi au plus tard au moment de ladite mise en forme un traitement lui permettant de devenir au moins partiellement amorphe;
3. c) désolidariser le composant ainsi formé du moule ;

caractérisé en ce que le second matériau formant le moule présente une effusivité thermique allant de 250 à 2500 J/K/m²/S^0.5.The invention relates to a method of producing a first part which overcomes the drawbacks of the prior art by making it possible to obtain a method of manufacturing a component made of a first material. metallic, said first material being a material capable of becoming at least partially amorphous, said process comprising the following steps:

1. a) providing a mold made of a second material, said mold comprising an imprint forming the negative of the component;
2. b) providing the first material and shaping it in the imprint of said mold, said first material having undergone, at the latest at the time of said shaping, a treatment allowing it to become at least partially amorphous;
3. c) separating the component thus formed from the mold;

characterized in that the second material forming the mold has a thermal effusivity ranging from 250 to 2500 J / K / m ² / S ^0.5 .

In a first advantageous embodiment, step c) consists in dissolving said mold.

In a second advantageous embodiment, said first material is subjected to a rise in temperature above its melting point allowing it to locally lose any crystalline structure, said rise being followed by cooling to a temperature below its temperature. glass transition allowing said first material to become at least partially amorphous.

In a third advantageous embodiment, the shaping step b) is simultaneous with a treatment making said first material at least partially amorphous, by subjecting it to a temperature above its melting point followed by cooling to a temperature. temperature below its glass transition temperature allowing it to become at least partially amorphous, during a casting operation. This embodiment is characterized in that the critical cooling speed of the first material is less than 10K / s.

In a fourth advantageous embodiment, the shaping is done by injection.

In a fifth advantageous embodiment, the shaping is done by centrifugal casting.

In another advantageous embodiment, the second material is zircon having an effusivity of 2300 J / K / m ² / S ^0.5 .

In another advantageous embodiment, the second material is of the plaster type composed mainly of gypsum and / or silica, having an effusivity of between 250 and 1000 J / K / m ² / s ^0.5 .

In another advantageous embodiment, the first material has a critical cooling rate less than or equal to 10K / s.

The invention also relates to a component made from a first material being a metallic material capable of becoming at least partially amorphous, characterized in that it is manufactured using the method according to the invention.

The invention further relates to a timepiece or jewelry piece comprising the component according to the invention, said component is chosen from the list comprising a middle part, a bezel, a bracelet link, a gear train, a hand, a crown, an escapement system anchor or balance, tourbillon cage, ring, cufflink or earring or pendant.

BRIEF DESCRIPTION OF THE FIGURES

• les figures 1 à 6 représentent de manière schématique les étapes du procédé selon la présente invention.

The aims, advantages and characteristics of the method for producing a first part according to the present invention will appear more clearly in the following detailed description of at least one embodiment of the invention given solely by way of non-limiting example and illustrated. by the accompanying drawings in which:

• the figures 1 to 6 schematically represent the steps of the process according to the present invention.

DETAILED DESCRIPTION

The figures 1 to 6 represent the different steps of the process for producing a watch or jewelery component 1 also called the first part 1 according to the present invention. This first part 1 is made of a first material. This first part 1 can be a covering part such as a middle part, a bezel, a bracelet link, a ring, cufflinks or earrings or a pendant or a functional part such as a gear 3, a needle. , a crown, an anchor 5 or a balance 7 of the exhaust system 9, a tourbillon cage.

The first material is advantageously an at least partially amorphous material. More particularly, the material is metallic, by which it is understood that it comprises at least one metallic or metalloid element in a proportion of at least 50% by mass. The first material can be a homogeneous metal alloy or an at least partially or completely amorphous metal. The first material is thus chosen capable of locally losing any crystalline structure during a rise in temperature above its melting point followed by sufficiently rapid cooling to a temperature below its glass transition temperature, allowing it to become at least partially amorphous. The metallic element may or may not be valuable. The first stage, represented in the figure 2 , consists in providing a mold 10. This mold 10 has an imprint 12 which is the negative of the part 1 to be produced. This is a so-called lost wax mold. This type of mold consists of a mold 10 made of a material which can be destroyed or dissolved after use to release said part. The advantage of this type of mold is its ease of manufacture and release, which is independent of the geometry of the impression. It is thus possible to easily produce imprints of complex and / or hollowed out geometries, without inserts. This mold can be obtained by covering a wax or resin model, itself obtained by injection, by additive manufacturing, by machining, or by sculpture. This mold 10 comprises a conduit 14 so that the liquid metal can be poured into it.

This mold 10 is thus made of a second material. Advantageously, the material of the mold is chosen to have specific thermal properties. Indeed, the goal here is to have a mold for the lost wax casting which is made of a material allowing the amorphous material of the micromechanical part not to crystallize while completely filling the mold cavity.

Crystallization of amorphous metals occurs when these, when in a viscous or liquid state, are not cooled quickly enough to prevent atoms from structuring among themselves. For a given alloy, this characteristic is defined by the critical cooling rate, Rc, ie the minimum cooling rate to be observed between the melting temperature and the glass transition temperature in order to maintain an amorphous state of the material. Therefore, it becomes necessary to have a mold 10 made of a material which dissipates thermal energy sufficiently well to ensure a cooling rate R greater than Rc. Conventionally, foundry molds are made of steel or copper alloys in order to have a high R value.

However, for parts of small dimensions or having fine and complex details, this capacity to dissipate thermal energy should not be too great. In the event that this capacity is too great, the risk is that the first material forming the first part freezes before being able to completely fill the cavity 12 of the mold 10.

As such, the present invention proposes to use the thermal effusivity criterion E in combination with Rc.

où :

• λ : est la conductivité thermique du matériau (en W·m¹·K^-1)
• ρ : la masse volumique du matériau (en kg·m^-3)
• c : la capacité thermique massique du matériau (en J·kg^-1·K^-1)

L'effusivité se mesure alors en J/K/m²/s^0.5.The thermal effusivity of a material characterizes its ability to exchange thermal energy with its environment. It is given by: $$E=\surd \mathit{\lambda \rho c}$$

or :

• λ: is the thermal conductivity of the material (in W m ¹ K ^-1 )
• ρ: the density of the material (in kg m ^-3 )
• c: the specific heat capacity of the material (in J kg ^-1 K ^-1 )

The effusivity is then measured in J / K / m ² / s ^0.5 .

This effusivity makes it possible, depending on the thickness of the first part to be produced, to obtain cooling which guarantees an amorphous state of the material, that is to say R> Rc. Indeed, if the criterion of effusivity is important, the amorphicity is linked to the thickness of the part to be manufactured. It is easily understood that, for a given thickness, a high effusivity risks causing a solidification of the material before the latter has been able to fill the whole of the mold, whereas a too low effusivity risks causing crystallization. According to the invention, it will be considered that the effusivity will be chosen within a range going from 250 to 2500 J / K / m ² / s ^0.5 . As an example of a material, the effusivity of plaster-type materials is 250 - 1000 J / K / m ² / s ^0.5, while for zircon, it will be 2300 J / K / m ² / s ^0.5 .

With the effusivity characteristics chosen for the invention, it is possible to obtain a first part having a thickness of 0.5 mm or more without freezing the material before the imprint is completely filled. It is clear that parts or parts of parts less than 0.5mm thick can be correctly filled if they are point details and small dimensions.

The second step consists in providing the first material, that is to say the material constituting the first part 1. Once provided with the material, the rest of this second step consists in shaping it as visible to the eyes. figures 3 and 4 . For this, a casting process is used.

Such a process consists in taking the first material which was provided during the third step without having subjected it to a treatment making it at least partially amorphous and putting it under liquid form. This liquid form is effected by melting said first material in a pouring container 20.

Once the first material is in liquid form, it is poured into the cavity 2 of the mold. When the mold cavity 2 is filled or at least partially filled, the first material is then cooled so as to give it an amorphous shape. According to the invention, the cooling is carried out by dissipation of heat from the mold 10, that is to say by using only the thermal characteristics of the material constituting the mold, in other words the cooling is carried out only thanks to the effusivity of the mold and only the mold / air interface to give the amorphous or at least partially amorphous character to the metallic material of the component. The cooling is therefore carried out without the use of any quenching medium other than air or a gas, for example helium.

As a reminder, the material constituting the mold 10 will be chosen to have an effusivity within a range of 250 to 2500 J / K / m ² / s ^0.5 , this thermal effusivity of a material being the capacity of the latter to exchange thermal energy with its environment. Thus, the greater the effusivity, the greater the cooling will be, at equivalent thickness.

With such effusivity values, the cooling rate R is low compared to the molds traditionally used in metal. For comparison, the effusivity of steel is over 10,000 J / K / m ² / s ^0.5 and copper over 35,000 J / K / m ² / s ^0.5 . For this reason, it is necessary to use a first material having a low critical cooling rate Rc in order to guarantee an amorphous or partially amorphous state of the part to be produced. This critical cooling rate Rc will be less than 15 K / s. Alloys used are for example given by the compositions Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 (Rc = 10K / s), Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Rc = 1.4K / s) or even Pd43Cu27Ni10P20 ( Rc = 0.10K / s). Other alloys forming the first material can be example (composition in atomic%): Pd43Cu27Ni10P20, Pt57.5Cu14.7Ni5.3P22.5, Zr52.5Ti2.5Cu15.9Ni14.6AI12.5Ag2, Zr52.5Nb2.5Cu15.9Ni14.6AI12.5Ag2, Zr56Ti2CuFe22.5Ag4.5, Zr56Nb2Cu22.5Ag4.5Fe5AI10, Zr61Cu17.5Ni10Al7.5Ti2Nb2, and Zr44Ti11Cu9.8Ni10.2Be25. It will therefore be understood that a mold used in the present invention cannot be made of a metallic material.

With the effusivity characteristics chosen for the invention, it is thus possible to obtain a first piece of amorphous metal having a thickness of between 0.5mm and 1.4mm, it being understood, as explained above, that details of smaller thickness are achievable if they are punctual and limited in size. Similarly, parts or parts of parts thicker than 1.4mm can be made without crystallization if they are considered to be point details and small dimensions.

An advantage of casting a metal or alloy capable of being amorphous is having a low melting point. Indeed, the melting temperatures of metals or alloys capable of having an amorphous form are generally two to three times lower than those of conventional alloys when we consider compositions of the same types. For example, the melting temperature of the alloy Zr41.2Ti13.8Cu12.5Ni10Be22.5 is 750 ° C compared to 1500-1700 ° C of crystalline alloys based on zirconium Zr and titanium Ti. This is to prevent damage to the mold.

Another advantage is that the solidification shrinkage for an amorphous metal is very low, less than 1% compared to the 5-7% shrinkage for a crystalline metal. This advantage makes it possible to use the principle of casting without fear of surface defects or of significant changes in dimensions which would be the consequence of said shrinkage.

Another advantage is that the mechanical and polishability properties of amorphous metals do not depend on the process of workmanship as long as they are amorphous. Thus the parts obtained by casting will have the same properties as the forged, machined, or hot-deformed parts, which is a major advantage over crystalline metals whose properties depend strongly on the crystalline structure, itself linked to the history of the process for obtaining the part.

In a first alternative, the casting could be of the gravitational type. In such a casting, the metal fills the mold under the effect of gravity.

In a second alternative, the casting could be of the centrifugal type. Such centrifugal casting uses the principle that the mold is driven in rapid rotation. The liquid metal poured inside sticks to the wall by centrifugal force and solidifies. This technique allows centrifugation and pressure on the material which causes degassing and expels the impurities contained in the bath of liquid metal to the external part. Smaller cavities can be filled compared to simple gravitational casting.

In a third alternative, the casting could be of the injection type. Such injection casting uses the principle according to which the mold is filled by means of a piston which comes to apply a very large force to push the liquid metal. This thrust then makes it possible to introduce the liquid metal into the mold in order to better fill it. In other alternatives, the casting may be of the type by counter-gravity, by pressure molding, by vacuum casting.

The third stage, represented in the figure 5 , consists in separating the first part 1 from the mold 10. For this, the mold 10, in which the amorphous metal has been overmolded to form the first part 1, is destroyed by using a jet of water under high pressure, by dissolving in water or in a chemical solution, or by mechanical removal. In the case of using a chemical solution, this is chosen to specifically attack the mold 10. Indeed, the aim of this step is to dissolve the negative 1 without dissolving the first part 5 made of amorphous metal. For example, in the case of a mold made of plaster with a phosphate binder, a hydrofluoric acid solution is used to dissolve the mold. The end result is then obtaining the first piece of amorphous metal.

Then, it is planned to remove the excess material by mechanical or chemical means as shown in figure 6 .

It will be understood that various modifications and / or improvements and / or combinations obvious to a person skilled in the art can be made to the various embodiments of the invention described above without departing from the scope of the invention defined by the appended claims.

It can also be understood that the first step consisting in providing the negative 1 can also include the fact of preparing said negative Indeed, it is possible to decorate the negative 1 so that surface finishes can be directly produced on the first part . These surface finishes can be a “Geneva coast” decoration, beaded, diamond snailed or a satin finish.

Claims (10)

1. A method for manufacturing a component (1) having a thickness below 1.4 mm made of a first material, said first material being a metallic material that can become at least partially amorphous, and having a critical cooling rate below 15 K/s, said method comprising the following steps:

   a) providing a mold (10) made of a second material, said mold comprising a cavity (12) forming the negative of the component;

   b) providing the first material and forming it in the cavity of said mold, said first material having undergone, at the latest at the time of said forming, treatment allowing it to become at least partially amorphous;

   c) separating the component thus formed from the mold;

   characterized in that the second material forming the mold has a thermal effusivity from 250 to 2500 J/K/m²/s^0.5, and in that the treatment step allowing said first material to become at least partially amorphous comprises a step of cooling, which is only accomplished owing to the effusivity of the mold and only at the mold/gas interface.
2. The method of manufacture as claimed in claim 1, characterized in that step c) consists of dissolving said mold.
3. The method of manufacture as claimed in claims 1 or 2, characterized in that said first material is subjected to a temperature rise above its melting point, allowing it to lose any crystalline structure locally, said rise being followed by cooling to a temperature below its glass transition temperature allowing said first material to become at least partially amorphous, the first material having a critical cooling rate below 15 K/s.
4. The method of manufacture as claimed in claim 3, characterized in that the first material has a critical cooling rate less than or equal to 10 K/s.
5. The method of manufacture as claimed in one of claims 1 to 4, characterized in that the forming step b) is simultaneous with treatment making said first material at least partially amorphous, by subjecting it to a temperature above its melting point followed by cooling to a temperature below its glass transition temperature allowing it to become at least partially amorphous, during a casting operation.
6. The method of manufacture as claimed in claim 5, characterized in that forming takes place by injection.
7. The method of manufacture as claimed in claim 5, characterized in that forming takes place by centrifugal casting.
8. The method of manufacture as claimed in one of the preceding claims, characterized in that the second material is zircon having an effusivity of 2300 J/K/m²/s^0.5.
9. The method of manufacture as claimed in one of claims 1 to 8, characterized in that the second material is a plaster consisting predominantly of gypsum and/or silica, having an effusivity between 250 and 1000 J/K/m²/s^0.5.
10. The method of manufacture as claimed in one of claims 1 to 9, characterized in that the component (1) is a timepiece selected from the list comprising a caseband, a bezel, a bracelet link, a wheel (3), a hand, a crown wheel, pallets (5) or a balance wheel (7) of an escapement system (9), a tourbillon cage or a piece of jewelry selected from the list comprising a ring, a cuff link or an earring or a pendant.

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