CH715727B1 - Process for obtaining a micromechanical component in 18 carat gold alloy. - Google Patents

Abstract

The present invention relates to a process for obtaining a functional component intended for horological, jewelery or micromechanical applications, the process comprising the following steps: supplying a raw material (12) formed of an alloy comprising at least 750‰ by mass of gold and having a first hardness; performing a work hardening step of said material so as to obtain a semi-finished product having a second hardness higher than the first hardness; and a step of machining the semi-finished product to obtain the component; the strain hardening step comprising at least one cycle of severe plastic deformation such that the crystallographic structure of the alloy is transformed into a grain structure smaller than 10 µm.

Description

Technical area

The present invention relates to a process for obtaining a micromechanical component in gold alloy having high hardness and ductility. The component obtained by the method of the invention is intended for horological or micromechanical applications, in particular when the component is subjected to high mechanical stresses.

State of the art

[0002] It may be desirable to use gold alloy components, for example 18-carat 3N, 4N, 5N or gray gold for watchmaking or micromechanical applications, where these components are subjected to high mechanical stresses. . However, the mechanical properties of these types of alloys are often not satisfactory, in particular due to low hardness.

Brief summary of the invention

The present invention relates to a process for obtaining a functional component intended for horological or micromechanical applications, the process comprising the following steps: supplying a raw material formed from an alloy comprising at least 750‰ by mass of gold and having a first hardness; performing a work hardening step of said material so as to obtain a semi-finished product having a second hardness higher than the first hardness; and a step of machining the semi-finished product to obtain the component; wherein the strain hardening step includes at least one cycle of severe plastic deformation such that the crystallographic structure of the alloy is transformed into a grain structure smaller than 10 µm.

The present invention also relates to a component obtained by the method of the invention.

The component obtained by the method of the invention has better mechanical characteristics, for example higher hardness and ductility, than a component obtained conventionally.

Brief description of figures

Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which: FIG. 1 illustrates a rotary hammering device; Figure 2 shows a component; and Figures 3 to 8 compare a top view of the head of a screw obtained by a conventional method and by the method of the invention, before tightening (Figure 3), tightening to the nominal torque of 2.7 N.cm (figure 4), tightening up to the nominal torque of 5 N.cm (figure 5), tightening up to the nominal torque of 6.5 N.cm (figure 6), tightening up to the nominal torque of 7 N.cm (figure 7), tightening up to the nominal torque of 8.5 to 9 N.cm (figure 8).

Example(s) of embodiment of the invention

[0007] According to one embodiment, a method for obtaining a functional component intended for watchmaking or micromechanical applications, comprises the following steps: supplying a raw material formed of an alloy comprising at least 750‰ by mass of gold and having a first hardness; performing a work hardening step of said material so as to obtain a semi-finished product having a second hardness higher than the first hardness; and machining the semi-finished product to obtain the component;wherein the strain hardening step includes at least one cycle of severe plastic deformation such that the crystallographic structure of the alloy is transformed into a grain structure smaller than 10 µm .

[0008] The hardening step comprising at least one cycle of severe plastic deformation results in a grain structure where the size of the grains is not necessarily equal in the three dimensions of the grain. In particular, the hardening step comprising at least one severe plastic deformation cycle can produce grains having an elongated shape. Here, the grain structure is defined by the size of the grains according to their smallest dimension. For example, the smallest dimension in the case of an elongated grain is its cross section while the largest dimension is its longitudinal section.

[0009] Depending on the number of severe plastic deformation cycles, the transverse size of the grains of the ultrafine grain structure may be less than 10 μm, and even less than 5 μm, or even 1 μm.

[0010] 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 linked to the direction of deformation remains. In the usual cold deformation techniques, the hardening is generated by the creation of dislocations (Frank-Read sources) which will pile up on the initial grain boundaries, to gradually form a structure of sub-boundaries (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.

[0011] The grain size of a crystallographic structure of an alloy having undergone "at least one severe plastic deformation cycle" can depend on the composition of the alloy and the conditions of the severe plastic deformation method. However, grain size can be verified directly and successfully using tests and procedures known to those skilled in the art and not requiring an unreasonable amount of experimentation. For example, depending on the composition of the alloy comprising at least 750‰ by weight of gold, the number of severe plastic deformation cycles will be adjusted in order to obtain a structure with ultrafine grains, less than 10 µm.

[0012] According to a preferred form, the raw material can take the form of a bar, for example a bar having a diameter of approximately 10 mm. The raw material may be in the annealed state.

According to a particular form, the raw material can be formed from a 3N gold alloy comprising: 750‰ Au, 125‰ Cu and 125‰ Ag.

[0014] For example, such a conventional commercial grade 3N gold alloy having the characteristics below: Annealed* 145 550 375 40 Hardened 70% 240 890 810 2 Annealed + hardened** 230 750 630 16.5 Hardened 70% + hardened** 290 1010 970 1.5 *annealing corresponds to 650°C for 30min under an H2/N2 atmosphere then quenching in water. **thermal hardening is carried out at 280°C for 60min.

[0015] The work hardening at 70% can be achieved by a rolling and/or drawing method, typically after between 3 and 10 passes.

[0016] According to other possible shapes, the raw material can be formed from a gold alloy according to the classic commercial grades of 3N (750Au-125Cu-125Ag 3N), 4N (750Au-160Cu-90Ag 4N), 5N (750Au-205Cu-45Ag 5N) or white gold (750Au-125Pd-95Cu-30Ag). These compositions are given as an indication, other variants are also possible.

[0017] According to one embodiment, the severe plastic deformation cycle(s) are carried out using a rotary swaging method, also known as rotary shrinking.

[0018] Figure 1 schematically illustrates a rotary hammering device 10, capable of carrying out the severe plastic deformation cycle(s) by the rotary hammering method. Hammers 11 are positioned around the raw material, for example a bar 12. The hammers 11 perform radial movements (indicated by arrows 13) at high frequency. The radial movements 13 of the hammers 11 take place in the plane of the section of the material 12, perpendicular to the longitudinal axis 15 of the material 12. For example, the frequency of the radial movements varies from 1,500 to 10,000 per minute depending on the size of the device. The lateral stroke of the 11-stroke hammers is approximately 0.2 to 5 mm. Usually, the rotary hammering device 10 comprises four hammers 11. Depending on the application and the size of the device 10, it is possible to use sets of two, three, six or, in particular cases, up to with eight hammers 11. To avoid the formation of longitudinal burrs at the interstices between the hammers 11, the device 10 can impart a relative rotational movement between the hammers 11 and the work bar 12. In this case, the hammers 11 rotate around the longitudinal axis 15, in a clockwise or counterclockwise direction. During a deformation cycle, the material 12 is advanced in the direction of the longitudinal axis 15, for example in the direction indicated by the arrow 16.

[0019] Rotary hammering is an incremental forming process. According to one embodiment, the final deformation of bar 12 is reached after a succession of intermediate severe deformations (a succession of cycles of severe plastic deformation). For example, a bar 12 with an initial diameter of 10 mm can see its diameter reduced to 2 mm following successive deformations (successive cycles) carried out using the rotary hammering method, i.e. 96% rate of deformation (on the section). For example, successive cycles, each cycle making it possible to reduce the diameter of the bar 12 by approximately 1 mm, are carried out to reduce the diameter of the bar 12 from 10 mm to 5 mm. Successive cycles, each cycle making it possible to reduce the diameter of the bar 12 by approximately 0.5 mm, are carried out to reduce the diameter of the bar 12 from 5 mm to 2 mm.

[0020] The deformation is rather plane compression, which generates a rather complex state of stress with a great deal of compression. This is what delays cracking. For the 3N gold alloy comprising: 750‰ Au, 125‰ Cu and 125‰ Ag having undergone a deformation of 96% by the rotary hammering step according to the invention, a semi-finished product having a hardness of 350HV is obtained. Depending on the number of severe plastic deformation cycles, the transverse grain size of the ultrafine grain structure can be less than 10 microns, and even less than 5 microns, or even 1 micron.

[0021] According to one embodiment, the method further comprises a precipitation hardening heat treatment step (or alternatively, heat hardening treatment). For example, the thermal hardening can be carried out at a temperature of 280° C. to 300° C., for a time of 60 min to 8 hours. The thermal hardening can be carried out under an argon atmosphere. The heat hardening step can be carried out following the strain hardening step. The method may also include the heat hardening treatment.

[0022] The machining step can be carried out before and/or after the thermal hardening step.

[0023] The severe plastic deformation cycle or cycles make it possible to pass from a first hardness of 145 HV for the annealed raw material, to a second hardness of 350 HV.

[0024] Alternatively, the severe plastic deformation cycle(s) can be achieved using one of the methods, or a combination of these methods, including: equal channel angular pressing (ECAP) extrusion, equal channel conformal angular extrusion (ECAP-conform), high pressure torsion or high pressure tube twisting, HPT or HPTT, accumulative roll bonding (ARB), repetitive corrugation and straightening (repetitive corrugation and straightening, RCS), asymmetric rolling (ASR), cyclic extrusion-compression (CEC), rotary hammering, or any other method suitable for obtaining said ultrafine grain structure.

[0025] Depending on the method with which the severe plastic deformation cycle or cycles are carried out, the size and shape/geometry of the grains may change. For example, for processes derived from rolling and drawing, the grains will have an elongated shape. In all cases their size will be reduced compared to the raw material.

[0026] The machining step can include a material removal process such as turning, milling, grinding, spark erosion, cutting, laser or water jet cutting, or any other process. appropriate. The machining step can also include a shaping process by deformation, or by assembly. The machining step can also include one or more finishing treatments, such as machining, rolling, satin-finishing, polishing, sandblasting, microblasting, etching or any other suitable mechanical process.

[0027] Comparative tests were carried out in order to compare a component obtained with the process of the invention, comprising the hardening step comprising the severe plastic deformation cycle or cycles, with a component obtained by a process in which the The hardening step is carried out by a rolling and/or drawing method, typically after between 3 and 10 cycles up to 70% deformation rate. The tests were carried out for a component comprising a screw 20 having a threaded part 21 and a head 22 provided with a slot 23 (see FIG. 2). The diameter of the test screws is 1 mm.

The comparative tests include mechanical strength tests of the screw 20. More specifically, the comparative tests include tightening the screw 20 in a threaded steel plate up to the nominal torque of 2.7 N.cm, carried out with a torque screwdriver. The nominal torque of 2.7 N.cm corresponds to a reference value recommended for this type of screw. The comparative tests also include tightening the screw 20 in stages until it breaks and tightening the screw 20 at the maximum torque until it breaks.

The screw 20 obtained by the method of the invention and the screw obtained conventionally were machined in the same way.

Figures 3 to 8 compare a top view of the head 22 of a screw 20 obtained by a conventional method (denoted standard) and by the method of the invention (denoted invention). In each figure, the comparison is shown for three standard screws and three screws of the invention (corresponding to figures a, b and c).

Figure 3 corresponds to the situation before tightening.

Figure 4 corresponds to the tightening of the screw 20 to the nominal torque of 2.7 N.cm. No notable difference can be seen in the mechanical strength between the standard screw and the screw of the invention. The deformation of the slot 23 of the screw 20 negligible in both cases.

Figure 5 corresponds to the tightening of the screw 20 to the nominal torque of 5 N.cm. A difference is observed in the appearance of the deformation zone of the slot 23 between the standard screw and the screw of the invention.

Figure 6 corresponds to the tightening of the screw 20 to the nominal torque of 6.5 N.cm. In this case, the difference in the appearance of the deformation zone of the slot 23 between the standard screw and the screw of the invention is more marked. The deformation zone is more localized than for a nominal torque of 7 N.cm (see figure 7). The formation of sliding bands is observed on the head 22 of the screw of the invention.

Figure 7 corresponds to the tightening of the screw 20 to the nominal torque of 7 N.cm. From 7 N.cm, a rupture is observed at the level of the threaded part 21 for the standard screw. Otherwise, there is a very significant deformation of the slot 23 of the standard screw head. The appearance of the head 22 of the screw of the invention changes little compared to the application of a nominal torque of 6.5 N.cm.

[0036] Figure 8 corresponds to the tightening of the screw 20 up to the nominal torque of 8.5 to 9 N.cm and shows a breakage of the screw of the invention at the level of the threaded part 21, or even a deformation very important slot 23 of the head of the invention. The behavior of the screw of the invention at a torque of 8.5 to 9 N.cm is similar to that of the standard screw at a torque of 7 N.cm. For a torque of 8.5 to 9 N.cm, all the standard saws have suffered a break and are therefore not shown in the figure.

The average breaking value of the standard screw therefore corresponds on average to a nominal tightening torque of the order of 7 N.cm, while the average breaking value of the screw of the invention corresponds on average to a nominal tightening torque in excess of 8 N.cm.

The rupture at the level of the threaded part 21 could only be achieved once at a torque of 9 N.cm for the screw of the invention.

[0039] For the screw of the invention, the breaking point is significantly increased compared to the breaking point of the standard screw. According to the tests, the breakage limit of the screw of the invention is of the order of + 14%, if we base ourselves on the average 7 N.cm to 8 N.cm, or of the order of + 28%, based on the rupture of the threaded part 21 at 9 N.cm.

The component obtained by the method of the invention may comprise any functional component intended for horological or micromechanical applications. More particularly, the method is particularly suitable for obtaining such components which are subjected to high stresses. For example, the component can comprise a horological component such as an axle, a screw, a pin or a bar, etc.

The component obtained by the method of the invention has better mechanical characteristics, for example higher hardness and ductility, than a component obtained conventionally. The component obtained by the method of the invention therefore makes it possible to reduce after-sales service returns.

[0042] As the component obtained by the process of the invention can have a reduced diameter for equal strength, it can allow a reduction of the designs. The component also makes it possible to obtain an improved seal.

Reference numbers used in the figures

[0043] 10 rotary hammering device 11 hammer 12 raw material 13 radial movements 15 longitudinal axis 16 direction of advancement 20 component, screw 21 threaded part 22 head 23 slot

Claims (8)

1. Process for obtaining a functional component intended for horological or micromechanical applications, the process comprising the following steps: providing a raw material formed from an alloy comprising at least 750‰ by weight of gold and having a first hardness; performing a work hardening step of said material so as to obtain a semi-finished product having a second hardness higher than the first hardness; and machining the semi-finished product to obtain the component; characterized in that the hardening step includes at least one cycle of severe plastic deformation so that the crystallographic structure of the alloy is transformed into a structure with a grain of less than 10 μm. 2. Method according to claim 1, in which the crystallographic structure of the alloy is transformed into a grain structure smaller than 5 µm, or even 1 µm. 3. Method according to claim 1 or 2, wherein the one or more severe plastic deformation cycles are carried out using a rotary hammering method. 4. Method, according to one of claims 1 to 3, further comprising a precipitation hardening heat treatment step. 5. Method, according to claim 4, wherein said heat treatment is carried out at 280°C for 60 min. 6. Method according to claim 4 or 5, wherein the machining step is performed prior to the precipitation hardening heat treatment step. 7. Watch or jewelry component obtained by the process according to one of claims 1 to 5. 8. Component according to claim 6, comprising an axis, a screw, a pin or a bar.