EP1594995B1 - Doped alloy of gold - Google Patents

Abstract

Alloy constituents expressed as preferred ppm by weight are: 100-1000 Zn; 100-1000 Ga; 100-100 Ta; 90-950 Pt and 5-100 Ru. Independent claims are included for the method of manufacture and object produced.

Description

The present invention relates to a gold alloy of at least 14 carats for the production of jewels by lost wax casting.

Investment casting is a method that allows for complex parts with a beautiful surface appearance and excellent dimensional accuracy. This technique consists first of all in waxing, by injection into tools, the replica of each of the desired pieces. The assembly of these models on casting channels also in wax constitutes a cluster; after having uniformly surrounded this cluster with a ceramic shell, the wax is melted, leaving its exact imprint in the ceramic, into which the molten metal is poured. After cooling, the shell is destroyed and the metal parts are separated and finished. The use of this technique for the casting of gold jewelry dates back to the early days of metallurgy, about 4000 BC It was not until its application in dental technology at the beginning of the twentieth century that were developed mold production and casting techniques as we know them today.

However, the various parameters of the casting by lost wax are difficult to control. Thus, it frequently happens that the parts obtained have the following disadvantages: irregular surface, porosity due to the reactions between the liquid metal and the coating (mold) leading to the release of gas, grouping grain refiner into "nests". These disadvantages are at the origin of many discharges of cast objects.

D. Ott, in "Optimizing gold alloys for the manufacturing process", Gold Technology, 34 spring 2002, pp. 37-44 , reviews the various addition or doping elements used to improve the properties of 14-gold and 18-carat gold-silver-copper alloys, including flowability, grain fineness, ductility, breaking strength and hardness. According to this author, the only elements used for this purpose in practice are zinc, silicon, iridium and cobalt.

Silicon is known to cause the formation of a stable, protective oxide layer around castings when it is added to small 14-carat gold alloys. The formation of this oxide layer makes it possible to avoid the porosities due to the reactions between the liquid metal and the coating, and to obtain a perfect surface for the 9, 14 and 18 carat gold alloys. However, the addition of silicon causes an increase in the size of the grains and a decrease in the breaking strength. These side effects are catastrophic in the case of 18-karat gold alloys, resulting in a hot brittleness of the alloy, a huge grain size due to inhibition of grain refiners, and an inhibition of grain refiners.

The object or problem of the invention is to find doping elements of a gold alloy of at least 14 carats with the advantages of silicon without the disadvantages mentioned above.

This problem is solved by the invention as defined in the attached set of claims.

According to the invention, the doping elements are Zn, Ga, Ta, Pt and Ru. Surprisingly, the presence of these elements makes it possible to avoid, during lost wax casting of gold alloys, the harmful interaction between the mold and the liquid metal, apparently through the formation of an oxide layer. protective impervious to gases. Ruthenium is a very efficient grain refiner, even at low levels.

The invention relates to a gold alloy of at least 14 carats, characterized in that it contains as doping elements, by weight, from 10 to 20,000, preferably from 100 to 1000, ppm Zn, from 10 to 20,000, preferably from 100 to 1000 ppm Ga, from 10 to 20,000, preferably from 100 to 1000, ppm Ta, from 10 to 10,000, preferably from 90 to 950, ppm Pt and from 10 to 5000, preferably from 5 to 100 ppm Ru.

The presence of these doping elements at these levels makes it possible to obtain castings without problems of porosity, with a surface state, a grain size and mechanical properties, in particular of excellent bending and breaking strength, excellent . The flowability of the alloy is further improved.

The gold alloy of at least 14 carats may be an alloy based on gold, silver and copper, especially a 14-carat alloy such as for example a yellow gold alloy 14 comprising, expressed in weight, 58-59% Au, 24-28% Ag and 13-17% Cu or a red gold alloy having, expressed by weight, 58-59% Au, 7-11% Ag and 30-34% Cu, an 18-carat alloy such as, for example, a yellow gold alloy comprising, expressed by weight, 75-76% Au, 10-14% Ag and 10-14% Cu, a pale yellow gold alloy comprising, expressed by weight, 75-76% Au, 14-18% Ag and 7-11% Cu, a pink gold alloy having, expressed by weight, 75-76% Au, 7-11% Ag and 14-18% Cu, a red gold alloy comprising, expressed by weight, 75-76% Au, 2-6% Ag and 18-22% Cu, a 22-carat alloy such as for example a yellow gold alloy comprising, expressed by weight, 91-92% Au, 3-7% Ag and 1-5% Cu, or a red gold alloy having, expressed by weight, 91-92% Au, 0-2% Ag and 6-10% Cu.

The gold alloy of at least 14 carats may also be fine gold alloy, particularly having, expressed by weight, 99-99.9% Au and 0-1% Cu. In this case it will suitably contain 10 to 10,000 ppm Zn, 10 to 10,000 ppm Ga, 10 to 10,000 ppm Ta, 10 to 10,000 ppm Pt and 10 to 5000 ppm Ru.

The gold alloy of at least 14 carats may also be a gray gold alloy, for example an 18 carat gray gold alloy having, expressed by weight, 75-76% Au, 8-12% Cu, 0 -4% In, and 11-15% Pd, or a 14-carat white gold alloy having, expressed by weight, 58-59% Au, 14-18% Ag, 12-16% Pd, and 6-10% Cu.

The same advantageous properties of the alloys are obtained by replacing the weight ratio of Ta specified above by an identical weight ratio of an element selected from the group consisting of Ti, Zr and Nb.

The gold alloy according to the invention is generally manufactured in ingots by casting under an inert atmosphere, for example nitrogen, constituent elements of the alloy, either in the pure state, or in the state of alloy, in ingot molds made of heat-resistant material such as example graphite. The alloy can then be shaped by continuous casting to obtain pads. Continuous casting is a process in which the molten alloy is fed into an open-ended graphite mold in which the metal solidifies to produce a bar of predefined dimensions. The solidified form is cooled and removed from the water-cooled mold at a controlled rate using rollers, and the material is sawn to the desired length. The pads directly usable in casting are then obtained by cutting and marking in the bar from the continuous casting.

The invention also relates to a process for manufacturing a gold alloy as defined above which comprises the casting under an inert atmosphere of the constituent elements of the alloy, either in the pure state, or in the state of 'alloy.

The preparation of cast articles by the lost wax casting technique is generally carried out in the following manner. The ingots are rolled and cut into small pieces, or if the alloy has been shaped by continuous casting, casting studs are used as such. The coating used consists of gypsum and silica. Dewaxing is carried out without steam at a temperature of 140 to 160 ° C, then the firing cycle is as follows: bearing at 200 ° C, rise of 5 ° C per minute, bearing at 650 ° C 45 minutes. The casting is then done by centrifugation, after melting in a graphite crucible, under nitrogen. The parts are then removed from the mold and stripped to remove the surface oxide. Correct and complete: generalize the poured object preparation protocol actually used.

The invention also relates to the use of the alloy defined above for the manufacture of jewels by lost wax casting.

The invention also relates to a cast object comprising this alloy.

The invention will be better understood with the aid of the following examples, given for illustrative purposes, without any limiting character.

In these examples, all percentages are by weight unless otherwise indicated. In addition, the temperature is the ambient temperature or is expressed in degrees Celsius, and the pressure is the atmospheric pressure.

On the other hand, all the examples form an integral part of the invention, as well as any characteristic of the description including the examples, which appears to be new with respect to any state of the art, and in the form of general characteristic and not a particular characteristic of the example.

The reading of these examples will be facilitated by reference to Figures 1 and 2 , and in Tables 1 and 2.

The Figures 1 and 2 respectively represent the diagram of a steering wheel for evaluating the surface condition, the flowability, the ductility, the porosity, the oxidation and the grain size of the alloy after casting, and a photograph of a part shaped harp to evaluate the hot resistance alloy.

Tables 1 and 2 respectively show the compositions of the standard and doped alloys, and the main characteristics of the moldings obtained from these alloys.

EXAMPLE 1 Preparation of Objects Cast in Alloys According to the Invention

Initially, alloy ingots of dimensions 80 × 50 × 5 mm ³ were cast under nitrogen in graphite ingot molds, from shot for gold and silver, copper plates, fine pieces of zinc and gallium, and 5% gold-tantalum and 5% platinum-ruthenium pre-alloys in thin strips.

The ingots were then rolled up to 1 mm thick. A 2 cm square plate for each alloy was used (after embedding and polishing) for spectrometric color analyzes.

The rolled plates were then cut into pieces about 1 cm apart. For lost wax casting, the coating used consists of gypsum and silica. Dewaxing is carried out without steam at 150 ° C, then the firing cycle is as follows: bearing at 200 ° C, rise of 5 ° C per minute, bearing at 650 ° C of 45 minutes. The casting is then done by centrifugation, after melting in a graphite crucible, under nitrogen. The parts are then removed from the mold and stripped to remove the surface oxide and analyzed according to the procedures below.

Table 1 specifies the composition of cast objects for four 18-carat gold alloys according to the invention, here called "yellow doped", "pale yellow doped", "pink doped" and "red doped", respectively corresponding to the alloys "standard yellow", "standard pale yellow", "standard pink" and "standard red" obtained in Example 2

Example 2 Preparation of Cast Objects in Standard Alloys and in Silicon-doped Alloy

These objects were manufactured as described above, with the difference in use when casting ingots, gold and silver shot and copper plates, and, where appropriate, thin pieces of zinc and silicon.

Table 1 specifies the composition of cast objects for four state-of-the-art 18-carat gold alloys known as "standard yellow", "standard pale yellow", "standard pink" and "standard red", and known silicon-doped alloy of composition close to standard yellow, called "yellow-Si".

Example 3 Study of the properties of cast objects

The color of the alloys was measured on a 2 cm square and 1 mm thick square plate according to the 3-dimensional measurement system named CIELab, CIE being the sign of the International Commission on Illumination, and Lab les trois coordinate axes. The L-axis measures the white-black component (black = 0, white = 100), the axis measures the red-green component (red = + a, green = -a) and the b-axis measures the yellow component -blue (yellow = + b, blue = -b). For more details on this measurement system, please refer to the article "The Color of Gold-Silver-Copper Alloys" by RM German, MM Guzowski and DC Wright, Gold Bulletin 1980, 13, (3), pages 113-116 . The human eye can distinguish a difference of 1 point on this scale.

The values obtained for this measurement (Table 2) show that the addition of doping elements in an alloy has no adverse influence on its color.

The properties of the alloys after casting by lost wax were evaluated for each alloy using two castings. The first piece ( Figure 1 ) consists of a steering wheel on which are placed a wafer of 1 cm ² surface and 1 mm thick and rods 2 cm in height and diameters 0.8, 0.6, 0.4 and 0.3 mm. On the steering wheel are placed 2 rods of each diameter, 8 rods. This first part makes it possible to evaluate the surface condition, the flowability, the bending, the ductility, the porosity, the oxidation as well as the grain size of the alloy after casting. The second piece is shaped like a harp ( Figure 2 ) and makes it possible to evaluate the hot resistance of the alloy.

The score given to the surface condition is calculated according to the following criteria: porosity and texture fineness of the wafer. The note 10 corresponds to a perfect surface state without flaws.

• aucun pore visible, ni creux : 0 points
• pores et creux visibles sur moins de 10% de la surface : 2 points
• pores visibles sur 10 à 50% de la surface : 4 points

The notation for the surface porosity is noted with the subtraction of the following points from 10:

• no visible pore or hollow: 0 points
• pores and hollows visible on less than 10% of the surface: 2 points
• pores visible on 10 to 50% of the surface: 4 points

• si l'extrémité de la plaquette est droite : 0 point
• si l'extrémité de la plaquette est faiblement dentelé : 1 point
• si l'extrémité de la plaquette est fortement dentelé : 2 points
• si la plaquette est dentelée au-delà de son extrémité : 4 points
• si la texture globale de la plaquette présente de fines vagues : 1 point
• si la texture globale de la plaquette présente de larges vagues : 4 points.

The second criterion concerning the fineness of the structure is noted with the subtraction of the following points from 10:

• if the end of the plate is straight: 0 points
• if the end of the wafer is slightly serrated: 1 point
• if the end of the plate is strongly serrated: 2 points
• if the plate is serrated beyond its extremity: 4 points
• if the overall texture of the wafer has fine waves: 1 point
• if the overall texture of the wafer has large waves: 4 points.

The small raised points present on the surface are due to surface defects of the coating and are independent of the alloy, however they are detrimental to the quality of the part. A perfect surface from the point of view porosity and delicacy of texture but presenting points in relief will obtain a note of 9.5 or 9 according to the size or the frequency of these points in order to distinguish it from a perfect surface and without points in relief.
In order for the alloy to be accepted from the point of view of its surface, the minimum score must be 9/10, and only the defects due to the quality of the coating will be tolerated (raised points).

Table 1 shows that the doped alloys according to the invention have a satisfactory surface state, improved with respect to the corresponding standard alloys (10/7, 9/7, 9.5 / 7, 9/6) and identical to the doped alloy. with silicon (10/10).

The various alloys have been subjected to a flowability test which makes it possible to determine the ease of an alloy to be cast in small diameter conduits. This property is important for the manufacture of jewelery pieces with fine parts that must be reproduced during casting. The rating given is derived from the average made on the heights of the 8 precious alloy rods after casting. The higher the score on 20, the better the flowability of the alloy.

According to Table 1, the doped alloys according to the invention have a better flowability than the corresponding standard alloys (14.12 / 9.40, 14.50 / 9.25, 16.90 / 9.40, 18.5 / 12.6) and the silicon-doped alloy (14, 12 / 9.0).

The bend test is used to simulate the crimping step at the jeweler. It is important that the crimping rods can be folded several times to allow the jeweler several tries without the entire piece having to be recumbed. The bent stems have a diameter of 0.8 mm in this test. The bend test consists of a first twist at 90 ° angle and the following are alternately opposed at 180 ° angle. A value of 1 corresponds to a break at 90 ° angle, a value of 2 corresponds to a break at 90 ° + 180 ° angle. The higher values correspond to an additional twist inverse to the previous one and 180 ° angle.

Table 1 shows that the objects cast in alloys doped according to the invention have a better folding than those in corresponding standard alloys (4/3, 4.5 / 3.5, 3/2, 2/1) or silicon-doped alloy (4). / 2).

Another test not mentioned in this table, the so-called enlargement test of the rings has shown that the doped alloys according to the invention are more ductile than the corresponding standard alloys and can withstand up to 24% elongation before rupture. Initially the cast rings had a diameter of 15.9 mm (number 10) and a section of 2 mm ² . The standard alloy without refiner supports a magnification of 2 numbers, and the doped alloy supports a magnification of 1

The hot breaking strength test is performed by casting a piece in the form of a harp ( Figure 2 The difference in the coefficient of expansion of the mold and the metal generates a tension able to cause the breaking of the metal according to its fragility. This test makes it possible to discriminate the fragile structures as well as the possible harmful pollutions of the metal. The score is awarded by subtracting 20, 1 point per broken stem. Only alloys rated 20/20 were selected.

Table 1 shows that the doped alloys according to the invention have excellent resistance to hot fracture, unlike the silicon-doped alloy.

• si des pores importants sont observables, la note 0 est automatiquement attribuée.
• si des pores de faible taille sont observés en surface (sur 200 µm d'épaisseur environ) : soustraction de 1 ou 2 points selon leur nombre
• si la surface est légèrement irrégulière : soustraction de 1 point
• si des pores sont mis à jour : soustraction de 1 point. Plus de points peuvent être enlevés à la note selon la gravité du problème.

The porosity state is noted by observing the wafer on the wafer under the light microscope. The score on 10 is given according to the number of pores and their size and the regularity of the surface:

• if significant pores are observable, the score 0 is automatically assigned.
• if small pores are observed on the surface (about 200 μm thick): subtraction of 1 or 2 points according to their number
• if the surface is slightly irregular: subtraction of 1 point
• if pores are updated: subtract 1 point. More points can be removed to the note depending on the severity of the problem.

The minimum acceptable score is 9/10. Parts with surface porosity are automatically rejected. The less porous the alloy, the better its mechanical properties and the easier it will be polishing.

Table 1 shows that the doped alloy according to the invention has a state of porosity identical to that of the alloy doped with silicon (10/10) and much better than each of the standard alloys (10/8, 10/6, 9/0 and 9/7).

The oxidation state is noted according to the appearance of the piece just after demolding. The more the part will have a uniform appearance close to the color of the alloy without black traces due to copper oxide, the more the score obtained will tend towards 10/10. Copper oxide is to be avoided in the realm of the possible because it does not protect the part against the gases and it is suspected to favor the reactions of degradation of the mold leading to the release of gaseous sulfur dioxide.

The parts resulting from doped alloys according to the invention have a uniform surface close to the color of the alloy without traces of copper oxide and therefore have an excellent oxidation state, much better than that of the parts resulting from standard alloys ( Table 1: 10/0, 10/0, 10/5, 10/10).

Finally, the ASTM grain size is given by the superposition of an ASTM grid on the photo of a metallographic grid of a casting after etching to reveal the grain boundaries. According to the ASTM conversion table, a size of 7 corresponds to an average grain diameter of 32 microns. ASTM 3 corresponds to an average diameter of 125 microns. The higher the ASTM value, the smaller the grains, the better the mechanical properties of the alloy and the easier the polishing.

Table 1 shows that the parts resulting from the doped alloys according to the invention therefore have a grain size identical to or finer than that of the parts resulting from the standard alloys (7/7, 7/7, 6 / 3-4, 6 / 6) or silicon-doped alloy (7 / 2-3).

The results obtained in the tests reported above thus show that the addition of the doping elements of the invention to an 18-carat gold alloy makes it possible for the lost-wax cast objects to improve the state of porosity. oxidation state, surface condition, folding and ductility, to maintain or reduce the size of grains, while maintaining the breaking strength when hot and the color of the alloy. In addition the flowability of the alloy is increased, which allows the manufacture of jewelry parts having fine parts. <u> Table 1: </ u> Compositions of standard and doped alloys, in percentages by mass. % weight At Ag Cu Ir Your ga Zn Pt Ru Yes standard yellow 75.03 12.5 12.45 0.02 - - - - - - standard pale yellow 75.03 15.97 8.95 0.05 - - - - - - standard rose 75.03 8.94 15.98 0.05 - - - - - - standard red 75.03 4.47 20.45 0.05 - - - - - - yellow doped 75.03 12.24 12.53 - 0.05 0.05 0.05 0.0475 0.0025 - pale yellow doped 75.03 15.71 9.06 - 0.05 0.05 0.05 0.0475 0.0025 - pink doped 75.03 8.86 15.91 - 0.05 0.05 0.05 0.0475 0.0025 - red doped 75.03 4.27 20.5 - 0.05 0.05 0.05 0.0475 0.0025 - If yellow 75.03 9.95 13.96 0.01 1.0 0.05 Color L, a, b Surface condition / 10 Coulability / 20 folding Breaking strength / 20 State of porosity / 10 Oxidation state / 10 ASTM grain size standard yellow 93.4, 3.6, 23.5 7 9.40 3 20 8 0 7 standard pale yellow 95.3, 0.16, 25.3 7 9.25 3.5 20 6 0 7 standard rose 92.7, 5.7, 21.2 7 9.4 1 20 0 5 3-4 standard red 91.5,8.4,18.1 6 12.6 1 20 7 5 6 yellow doped 94.9, 3.1, 23.0 10 14.12 4 20 10 10 7 pale yellow doped 95.2, 0.14, 25.3 9 14.50 4.5 20 10 10 7 pink doped 92.3, 5.7, 21.2 9.5 16.90 3 20 9 10 6 red doped 91.1, 8.5, 17.9 9 18.5 2 20 9 10 6 If yellow 93.3, 3.29, 21.96 10 9.0 2 6 10 10 2-3

Claims (12)

1. A gold alloy of at least 14 carats, characterized in that it contains as dopants, by weight, from 10 to 20,000, preferably from 100 to 1000 ppm Zn, from 10 to 20,000, preferably from 100 to 1000 ppm Ga, from 10 to 20,000, preferably from 100 to 1000 ppm Ta, from 10 to 10,000, preferably from 90 to 950, ppm Pt and from 10 to 5000, preferably from 5 to 100 ppm Ru.
2. The alloy as claimed in claim 1, which is a 14 carat alloy based on gold, silver and copper, selected from the group consisting of a yellow gold alloy having, expressed by weight, 58-59% Au, 24-28% Ag and 13-17% Cu, and a red gold alloy having, expressed by weight, 58-59% Au, 7-11% Ag and 30-34% Cu.
3. The alloy as claimed in claim 1, which is an 18 carat alloy based on gold, silver and copper, selected from the group consisting of a yellow gold alloy having, expressed by weight, 75-76% Au, 10-14% Ag and 10-14% Cu, a pale yellow gold alloy having, expressed by weight, 75-76% Au, 14-18% Ag and 7-11% Cu, a pink gold alloy having, expressed by weight, 75-76% Au, 7-11% Ag and 14-18% Cu, and a red gold alloy having, expressed by weight, 75-76% Au, 2-6% Ag and 18-22% Cu.
4. The alloy as claimed in claim 1, which is a 22 carat alloy based on gold, silver and copper, selected from the group consisting of a yellow gold alloy having, expressed by weight, 91-92% Au, 3-7% Ag and 1-5% Cu, and a red gold alloy having, expressed by weight, 91-92% Au, 0-2% Ag and 6-10% Cu.
5. The alloy as claimed in claim 1, which is a fine gold alloy having, expressed by weight, 99-99.9% Au, 0-1% Cu, from 10 to 10,000 ppm Zn, from 10 to 10,000 ppm Ga, from 10 to 10,000 ppm Ta, from 10 to 10,000 ppm Pt and from 10 to 5000 ppm Ru.
6. The alloy as claimed in claim 1, which is an 18 carat gray gold alloy having, expressed by weight, 75-76% Au, 8-12% Cu, 0-4% In and 11-15% Pd.
7. The alloy as claimed in claim 1, which is a 14 carat gray gold alloy having, expressed by weight, 58-59% Au, 14-18% Ag, 12-16% Pd and 6-10% Cu.
8. The alloy as claimed in one of the preceding claims which, instead of the specified Ta weight ratio, has an identical weight ratio of an element selected from the group consisting of Ti, Zr and Nb.
9. A cast object comprising an alloy as claimed in one of the preceding claims.
10. A method for manufacturing a gold alloy as claimed in one of claims 1 to 8, characterized in that it includes casting the constituent elements of the alloy, either in the pure state or in the alloy state, under an inert atmosphere.
11. The method as claimed in claim 10, characterized in that the alloy is formed by continuous casting.
12. Use of an alloy as claimed in one of claims 1 to 8 in the manufacture of jewelry by investment casting.

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