CH715728B1 - Process for obtaining an 18 carat gold component for watchmaking and jewelery applications. - Google Patents

Abstract

The present invention relates to a method for obtaining an ornamental component comprising at least 750‰ by weight of gold, the method comprising the following steps: providing a raw material (16) comprising at least 750‰ by weight of gold and having a first hardness; carrying out a work hardening step of the alloy 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; 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. The component thus obtained has high hardness, good scratch resistance and good aesthetic durability.

Description

Technical area

The present invention relates to a process for obtaining a gold alloy component having good scratch resistance and aesthetic durability. The component obtained by the process of the invention is intended for horological and jewelery packaging applications.

State of the art

[0002] In watchmaking, jewelry, eyewear, and other applications, it is common to use alloys of precious metals, in particular gold. However, the properties of the precious metal alloy are often not satisfactory, in particular due to poor scratch resistance, due to low hardness, thus resulting in too low aesthetic durability.

Brief summary of the invention

The present invention relates to a process for obtaining an ornamental component comprising at least 750‰ by weight of gold, the process comprising the steps of: providing a raw material comprising at least 750‰ by weight of gold and having a first hardness; carrying out a work hardening step of the alloy 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; the strain hardening step comprising 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.

[0004] The invention also relates to a component obtained by the process, in particular a watch or jewelery casing component.

The component thus obtained has high hardness, good scratch resistance and good aesthetic durability.

Brief description of figures

[0006] Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which: FIG. 1 schematically illustrates a device configured to carry out one or more cycles of severe plastic deformation by an extrusion method angular with equal channels; FIG. 2 shows the evolution of the hardness as a function of time, during a thermal hardening treatment; Figure 3 shows a compression test for different samples; Figure 4 reports a tensile test for different samples; Figure 5 schematically illustrates a device configured to perform the severe plastic deformation cycle(s) by a high pressure torsion method; FIG. 6 shows the deflection as a function of the load applied during a bending test, for a sample of yellow gold; and FIG. 7 shows the deflection as a function of the load applied during a bending test, for a sample of red gold.

Example(s) of embodiment of the invention

[0007] A method of obtaining an ornamental component comprising at least 750‰ by mass of gold, the method comprising the following steps: providing a raw material comprising at least 750‰ by mass of gold and having a first hardness; carrying out a work hardening step of the alloy 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. In the following text, the grain size is specified relative to the cross section of the grains.

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

[0010] According to one embodiment, the method further comprises a heat treatment step of hardening by precipitation (or even 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 heat hardening step can be carried out following the hardening step or following the machining step.

[0011] The method may also include an annealing treatment. The annealing treatment can be carried out at a temperature of 650° C., for a time of 30 min. The annealing treatment is preferably carried out before the hardening step. Heat hardening and/or annealing can be performed under an argon atmosphere.

[0012] The step of machining the semi-finished product is carried out before and/or after the heat hardening step.

In the context of severe plastic deformation, a very high 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. In severe plastic deformation, there is no variation in section or thickness, so the rate of deformation expressed with conventional calculations would be zero. However, the deformation introduced is extremely large. For example, the deformation (or the relative elongation) can go up to 5, or even more.

[0014] 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 mass of gold, the number of cycles of severe plastic deformation will be adjusted in order to obtain a structure with ultrafine grains, less than 1 μm, preferably less than 500 nm .

According to a preferred form, the raw material can take the form of a bar, for example a bar having a diameter of approximately 10mm, 40mm or 60mm. The raw material may be in the annealed state (that is to say having undergone the annealing treatment, as described above).

[0016] According to a particular form, the raw material may be formed from an 18-carat gold alloy of the 4N type (pink gold, conventional commercial grade) comprising (% by weight): 75.0% Au, by weight, 16.0 % Cu and 9.0% Ag. For example, such a gold alloy has the characteristics below: Annealed* 155 550 335 40 Hardened 75% 245 920 770 2 Annealed + hardened** 280 850 750 5

Chart 1

[0017] where Rm is the maximum tensile strength, R0.2 corresponding to a relative elongation (deformation) ε=0.2% (where ε is the deformation), and A is the elongation at break. *annealing corresponds to 650°C for 30 min under an H2/N2 atmosphere then quenching in water. **thermal hardening is carried out at 280°C for 60min.

The raw material having undergone the annealing treatment (first line of table 1) is characterized by an average grain size of the order of 30 μm.

According to one embodiment, the cycle or cycles of severe plastic deformation are carried out using an equal channel angular extrusion method (hereinafter called ECAP, from equal channel angular pressing). Figure 1 schematically illustrates an ECAP device 10 configured to carry out the severe plastic deformation cycle(s) by the ECAP method. The raw material 16 is forced (pressing force) through a die, or mould, 11 comprising a channel 12 comprising a first portion 121 and a second portion 122 forming, at their intersection, a bend 13 having an intersection angle θ and an angle Ψ subtended by the arc of curvature at the point of intersection. The section of the channel 12 is equal to the inlet 14 and to the outlet 15. The complex deformation of the material 16 when it flows through the elbow 13 produces a very high deformation, by shearing under hydrostatic pressure. Because the cross section remains the same (which matches the cross section of channel 12), material 16 can be repeatedly forced through channel 12, introducing additional strain with each pass. The material 16 can also be rotated between consecutive passages in the channel 12 in order to activate different sliding systems and obtain an extremely large multidimensional deformation.

[0020] The cycle or cycles of severe plastic deformation by the ECAP method lead to a microstructural refinement down to the ultrafine grain (UFG) microstructure and consequently to improved mechanical properties, i.e. exceptionally high resistance. high associated with a still decent ductility as well as an increased fatigue limit of the semi-finished product thus obtained.

Other configurations of the matrix 11 of the ECAP device 10 are also possible. For example, the section of channel 12 can be circular or square with a diameter or diagonal, respectively of 12mm, 15mm, 20mm, 25mm, 30mm or 40mm. The intersection angle θ can be between 90° and 120°, for example 120° or 105°. Channel 12 temperature can be varied between TRT room temperature up to 500°C. The pressing force can be up to 700 kN with a pressing speed of 1 to 20 mm/s. Note that lower temperatures, higher forces and other speeds are also possible.

According to one embodiment, the method of the invention was applied to a raw material, that is to say a bar with a diameter of 12 mm and a length of 120 mm of the material formed from the 18 carat gold alloy described above, in the annealed state, that is to say having undergone the annealing treatment at 650°C for 30 min under an H2/N2 atmosphere then quenched in water (first line of table 1).

[0023] The cycle or cycles of severe plastic deformation by the ECAP method were carried out in order to obtain a semi-finished product. The parameters selected for the ECAP method are as follows: the intersection angle is 120° (example illustrated in figure 1), an equivalent deformation of ε=0.63 on each pass and a pressing speed of 1 mm/s. The passages were made with a lubricant, in particular based on graphite.

[0024] Figure 2 shows the change in hardness (Vickers hardness HV) as a function of time, during the heat hardening treatment carried out at a temperature of 280° C. on the semi-finished product. The curve "Sample A" corresponds to the raw material. The "sample B" curve corresponds to the semi-finished product, obtained with three cycles of severe plastic deformation by the ECAP method with a 120° bend 13 and at the TRT temperature of channel 12. The "sample C" curve corresponds to the semi-finished product. -finished, obtained with three cycles of severe plastic deformation by the ECAP method at the temperature of 350°C-380°C in channel 12.

Figure 2 shows that the heat hardening treatment makes it possible to increase the hardness of the semi-finished product from 265 HV to 345 HV for sample B and from 293 HV to 348 HV for sample C, in 1 hour. A longer heat hardening treatment time increases the hardness only slightly. In comparison, the hardness of the raw material having undergone the annealing treatment (first line of table 1) goes from 168 HV to 273 HV.

[0026] the grain size distribution for sample B is characterized by a maximum grain size of 30 μm and an average grain size of less than 10 μm, or even 5 μm or even 1 μm.

[0027] During the severe plastic deformation stage by the ECAP method, significant grain refinement occurs resulting in an average grain size below 1 µm with several sub-grains. However, the microstructure of the component obtained by the process may contain a small number of large grains (less than 30 μm) having a high disorientation, towards the center of the component.

The exact value of the average grain size can be difficult to estimate because of the presence of very fine grains in the structure and the detection limit (measured for example by a backscattered electron diffraction method). The very fine grains correspond to the fragmentation of the grains into a mosaic of sub-grains during the cycle or cycles of severe plastic deformation. It may then be practical to classify the grain boundaries according to the importance of the misorientation between two grains. In particular, low disorientation, or low angle (SAGB) grain boundaries, the misorientation of which is less than about 15°, reflect the presence of small subgrains. High-misorientation, or high-angle (HAGB) grain boundaries, with a misorientation equal to or greater than about 15° (the transition angle varies from 10 to 15 degrees depending on the material) correspond to grain boundaries before the or cycles of severe plastic deformation. High angle grain boundaries correspond to grain sizes above 1 µm.

[0029] For sample A, a markedly higher fraction of high-angle grain boundaries than of low-angle grain boundaries was measured. More specifically, a fraction of 0.022 of grain boundaries having an angle between 2° and 5°, a fraction of 0.022 of grain boundaries having an angle between 5° and 15° and a fraction of 0.956 of grain boundaries was measured. grains having an angle between 15° and 180°.

[0030] For sample B, a significantly higher fraction of low-angle grain boundaries than high-angle grain boundaries was measured. More specifically, a fraction of 0.667 of grain boundaries having an angle between 2° and 5°, a fraction of 0.239 of grain boundaries having an angle between 5° and 15° and a fraction of 0.095 of grain boundaries was measured. grains having an angle between 15° and 180°.

[0031] For sample C, a markedly higher fraction of low-angle grain boundaries than of high-angle grain boundaries was also measured. More specifically, a fraction of 0.697 of grain boundaries having an angle between 2° and 5°, a fraction of 0.223 of grain boundaries having an angle between 5° and 15° and a fraction of grain boundaries of 0.080 of grains having an angle between 15° and 180°.

The values measured for samples B and C confirm that the component obtained by the method of the invention results in a grain structure of less than 1 μm. The heat hardening treatment, after the stage of severe plastic deformation by the ECAP method, produces little change in the grain size. The microstructure induced by the stage of severe plastic deformation by the ECAP method is stable at 280°C at least for 24 hours.

[0033] Figure 3 reports the applied stress σ as a function of the deformation ε for a compression test carried out on sample A having undergone the annealing treatment (first line of table 1), sample B having undergone the step of severe plastic deformation but without the heat hardening treatment and sample C (sample B + hardening) after the heat hardening (carried out at 280°C for 60 min).

[0034] Figure 4 reports the applied stress σ as a function of the deformation ε for a tensile test carried out on sample A having undergone the annealing treatment (first line of table 1) and for sample C (sample B + hardening) after thermal hardening (carried out at 280°C for 60 min). In the latter case, an elastic limit value Rp0.2 of 1101 MPa and a maximum tensile strength value Rm of 1136 MPa are obtained. An elongation at break value of A%=2.2% is also obtained. For sample A, an elastic limit value Rp0.2 of 335 MPa and a maximum tensile strength value Rm of 550 MPa are obtained. An elongation at break value of A%=40% is also obtained.

[0035] For the raw material + annealed having also undergone thermal hardening (carried out at 280° C. for 60 min), one obtains (see table 1, line 3) a yield strength value Rp0.2 of 750 MPa, a maximum tensile strength value Rm of 850 MPa and an elongation at break value of A%=5%.

[0036] Parameters for severe plastic deformation performed using the ECAP method can be varied while achieving high hardness and good scratch resistance as well as good aesthetic durability.

[0037] Alternatively, the severe plastic deformation cycle(s) can be achieved using one of the methods, or a combination of these methods, including: the ECAP method, 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 (RCS), asymmetric rolling , ASR), cyclic extrusion-compression (CEC), rotary swaging, or any other suitable method for obtaining said ultrafine grain structure.

According to another embodiment, the severe plastic deformation cycle or cycles are carried out using a high pressure torsion (HPT) method. Figure 5 schematically illustrates an HPT device 20 configured to perform the severe plastic deformation cycle(s) by a high pressure torsion method. In the HPT method, a disk 21 of the material to be treated is placed between two dies 22. A large compressive stress 23 (usually several gigapascals) is applied, while one of the anvils 22 is rotated to create a twisting force 24.

The material having been subjected to the method of the invention consists of a 75Au-12.5Ag-12.5Cu alloy (yellow gold) and a 75Au-4.5Ag-20.5Cu alloy (red gold). A yellow gold formed raw material (Sample A - yellow gold) and a red gold formed raw material (Sample A - red gold) are prepared in the form of a disk of about 30 mm in diameter and about 10 mm thick. Five samples A - yellow gold and five samples A - red gold were tested. Samples A - yellow gold and samples A - red gold underwent corresponding annealing at 650° C. for 30 min under an H2/N2 atmosphere then quenched in water.

Samples A - yellow gold and red gold were subjected to 15 or 30 rotations of one of the anvils 22 (15 or 30 cycles of severe plastic deformation). The rotational speed of the anvil was 15 or 5 min/rotation (of the anvil). Compressive stress 23 of 4GPa was applied.

[0041] Table 2 shows the results for the tests carried out on the yellow gold alloy material. 1 4 15 15 2 4 15 30 3 4 15 15 4 4 5 15 5 4 15 15

Chart 2

[0042] Table 3 shows the results for the tests performed on the red gold alloy material. 1 4 15 15 2 4 15 30 3 4 15 15 3 4 5 15 5 4 15 15

Chart 3

Bending tests (3 points) were carried out on the samples having undergone 15 cycles of severe plastic deformation by the HPT method (samples B). The samples B were cut in a portion close to the periphery of the disk so as to have saturation conditions, that is to say a portion where the greatest gain in hardness is reached. Samples B, about 1 mm thick and about 1 mm wide, were oriented parallel to the deformation axis.

[0044] An average grain size of about 30 nm is measured for samples A and of about 20 nm for samples B.

[0045] Figure 6 shows the deflection (in μm) as a function of the stress (in MPa) for sample B - yellow gold, during a bending test. The rupture limit of the sample arrives for a stress of about 1750 MPa and a deflection of about 500 µm.

[0046] Figure 7 shows the deflection (in μm) as a function of the stress (in MPa) for sample B - red gold, during a bending test. The rupture limit of the sample arrives for a stress of about 2000 MPa and a deflection of about 325 µm.

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

[0048] Depending on the method with which the severe plastic deformation cycle or cycles are carried out, the size and the shape/geometry of the grains can 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 that of the raw material.

Reference numbers used in the figures

[0049] 10 ECAP device 11 mold 12 channel 121 first portion 122 second portion 13 bend 14 inlet 15 outlet 16 material 20 HPT device 21 disc 22 anvil 23 compressive stress 24 torsional force ε deformation θ intersection angle Ψ sub-angle stretched A% elongation at break R0.2deformation at ε=0.2% Rmmaximum tensile strength TRTroom temperature

Claims (5)

1. Process for obtaining an ornamental component comprising at least 750‰ by mass of gold, the process comprising the following steps: providing a raw material comprising at least 750‰ by weight of gold and having a first hardness; carrying out a work hardening step of the alloy 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 an average grain size 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, further comprising a step of carrying out a precipitation hardening heat treatment. 4. Method according to claim 2 or 3, wherein the semi-finished product machining step is performed before the precipitation hardening heat treatment step. 5. Watchmaking or jewelery casing component obtained by the method according to one of claims 1 to 4.