Abstract:
Gold alloy comprising, by weight, at least Gold Au≧33%, Iridium Ir≦0.4%, germanium Ge≦2%, 0.015% ≦silicon ≦0.3%, Phosphorus ≦0.02% and Copper Cu≦66%. The alloy can also comprise, in percentage by weight, Silver Ag≦34%, nickel Ni≦20% and Zinc Zn≦25%. In some variations the gold alloy can further comprise no more than 4% of at least one of the elements of the group constituted by cobalt, manganese, tin and indium, and no more than 0.15% of at least one of the deoxidizing elements of the group constituted by magnesium, silicon, boron and lithium. To the alloy can also be added at least one of the refining elements of the group constituted by ruthenium, rhenium and platinum in quantities not exceeding 0.4% by weight. The invention further relates to a master alloy for obtaining said gold alloy.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to gold alloys and to master alloys for obtaining them, mainly for the manufacturing of precious items such as jewellery and gold works, coins and medals.  
         BACKGROUND OF THE INVENTION  
         [0002]    One of the fundamental goals in jewelry is to obtain gold alloys exhibiting good fluidity (i.e. a good ability to fill and replicate wax patterns) at the moment of casting, and finished articles which have a bright outer surface and a good mechanical properties.  
           [0003]    Silicon deoxidized alloys are widely used for investment casting in jewelry manufacturing. The main advantages of silicon are primarily related to its strong affinity with oxygen, thus preventing zinc and copper oxidation.  
           [0004]    This gives rise to a protective ability of the silicon, which is manifested either during the production of grains, or the investment casting process.  
           [0005]    However, a considerable drawback of silicon when added to gold alloys is the effect on the enlargement of grain size, leading to brittleness and mechanical failures, with degradation of the mechanical properties (especially in the case of 18 carat gold alloys).  
           [0006]    In order to reduce the grain enlargement due to silicon additions, the addition of a suitable quantity of grain refiners elements such as iridium, ruthenium, cobalt, nickel and rhenium, is nowadays widely used.  
           [0007]    However, this solution too is not totally free of drawbacks.  
           [0008]    It is well known the unique ability of silicon to combine with the refining elements commonly used giving rise to the formation of silicides with consequent formation of high melting intermetallic silicide inclusions commonly known as “hard spots”.  
           [0009]    Such inclusions can appear on the surface of the finished piece after the final work process, entailing either the rework of the piece, or its discarding.  
           [0010]    Another solution is proposed in the patent U.S. Pat. No. 5,384,089, where gold alloys substantially silicon-free are disclosed. In this patent, the addition of germanium as a deoxidizing element for production of yellow gold alloys is disclosed.  
           [0011]    The main disadvantages related to the aforementioned invention are related to the inability of germanium to produce bright and oxidation free castings especially when melting and casting processes are carried out in presence of small amounts of oxygen. It must be pointed out how, even in high quality undervacuum casting equipments used for jewelry manufacturing, the oxygen presence in small amounts cannot be removed completely.  
           [0012]    This drawback emerges dramatically when “stone in place casting” has to be carried out. Articles produced in this way require a casting process able to produce oxidation free surfaces. In fact, brown oxidized casting can be result difficult in polishing treatments, with an impairment of reflectivity properties of gemstones. The aforementioned invention does not allow to obtain completely deoxidized surfaces.  
           [0013]    Furthermore, another drawback of the known alloys, is connected to the fact that during the casting process part of the alloy must be re-melt many times. In fact, in accordance with the known alloys, at each smelting it is necessary to add a new amount of deoxidizing elements, since the old one are combined with the oxygen.  
         SUMMARY OF THE INVENTION  
         [0014]    In this situation the technical task constituting the basis of the present invention is to provide gold alloys and master alloys for obtaining them which overcome the aforementioned drawbacks.  
           [0015]    In particular the technical task of the present invention is to provide gold alloys and master alloys for obtaining them, which exhibit excellent fluidity in the molten state, brightness and mechanical resistance.  
           [0016]    The specified technical task and the indicated aims are substantially achieved by gold alloys and master alloys for obtaining them, as described in the accompanying claims.  
           [0017]    Germanium, in the percentage of employment described in the present invention, possesses a remarkable ability to increase fluidity, this to be considered similar or superior to silicon. As described below, this property can be highlighted in comparative melting tests between silicon based alloys and germanium based alloys, the latter described in the present invention.  
           [0018]    In addition, the use of germanium and silicon allows at the same time to combine the positive effect of the silicon on brightness of the jewels and the high fluidity of the alloy provided by the germanium.  
           [0019]    Furthermore, the addition of phosphorus to an alloy comprising germanium and silicon, or only germanium, increases deoxidizing properties and reduces the amount of silicon and germanium removed by oxidation at every cycle of scrap re-use, this due to the higher affinity for oxygen of phosphorus, in comparison to silicon and germanium. As a consequence, the addition of new deoxidizing elements to the scraps to be recycled is largely reduced.  
           [0020]    Moreover, it has also been verified that germanium concentrations varying between 0.05% and 2% by weight have led to an increasing of fluidity deemed to be similar or even greater than silicon. Furthermore, on the basis of our studies, germanium does not show any effect on grain enlargement, even when used at significantly higher concentrations in comparison to silicon.  
           [0021]    Hence, in the case of germanium alloys, with low or no silicon content, mechanical properties of alloys are significantly improved as well, as can be observed from the data illustrated below.  
           [0022]    More precisely, Germanium based alloys show an improvement on ductility.  
           [0023]    Furthermore, the addition of Germanium and Phosphorus allows to reduce the minimum silicon amount to be added in order to obtain bright and oxidation free castings. As a consequence, the combined use of germanium or germanium and phosphorus with small quantities of silicon allows to obtain “clean and shiny” castings, with no degradation of mechanical properties as can be observed in traditional based alloys where the sole silicon in larger amounts is employed.  
           [0024]    Finally, low silicon contents reduce the possibility for hard spots formation.  
           [0025]    The ameliorative effect of germanium on mechanical properties can also be exhibited in nickel based white alloys, as is evident from the formulations described hereafter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    Further characteristics and the advantages of the present invention shall become more readily evident from the detailed description of some preferred, but not exclusive, embodiments of gold alloys and master alloys for obtaining them, and from the accompanying figures, in which:  
         [0027]    [0027]FIG. 1 shows in graph form the effect of different elements and compounds on the grain size of a gold alloy;  
         [0028]    [0028]FIG. 2 shows in graph form the effect of the quantity of silicon and germanium on the grain size of the gold alloy;  
         [0029]    [0029]FIG. 3 shows in graph form the effect of the elements and compounds of FIG. 1 on the tensile strength of the gold alloy obtained therewith; 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    A first gold alloy of the present invention comprises at least the following elements:  
         [0031]    gold: Au≧33%;  
         [0032]    iridium: Ir≦0,4%;  
         [0033]    germanium: Ge≦2%;  
         [0034]    phosphorus: P≦0.02%;  
         [0035]    copper: Cu sufficient to reach 100,  
         [0036]    in the indicated quantities, with reference to the total weight of the alloy.  
         [0037]    A second gold alloy further comprises silicon: 0.015% ≦Si≦0.3%, with reference to the total weight of the alloy.  
         [0038]    In order to better meet specific requirements, to complete the alloy several other chemical elements can also be used.  
         [0039]    More precisely, depending on requirements, the gold alloy can contain (with percentages expressed in weight):  
         [0040]    silver: Ag≦34%;  
         [0041]    nickel: Ni≦20%;  
         [0042]    zinc: Zn≦15%,  
         [0043]    which can be present simultaneously, or otherwise, without thereby departing from the scope of the present invention.  
         [0044]    In particular, the addition of nickel (or other elements having similar properties, and equivalent thereto) in suitable quantity, allows to obtain alloys of so-called white gold.  
         [0045]    Secondarily, the alloy can also contain, in a proportion not exceeding 4% by weight, at least one of the elements of the group constituted by cobalt, manganese, tin and indium.  
         [0046]    To improve the qualities of the alloy, one ore more deoxidizing elements such as magnesium, silicon, boron and lithium can also be added, each in a proportion not exceeding 0.15% by weight.  
         [0047]    Note that, even when silicon is added to the alloy, it is added only in small quantities (in particular not exceeding 0.05% by weight in 18 carat alloys, and not exceeding 0.15% by weight in 14 carat alloys) solely in order to guarantee the protection of the alloy against the formation of oxides, and not to improve its fluidity.  
         [0048]    Due to particular productive requirements in which a particularly reduced grain size is required, the alloy can also comprise refining elements such as ruthenium, rhenium and platinum in a suitable quantity and preferably not exceeding 0.4% by weight.  
         [0049]    For the production of precious objects, moreover, there are five preferential ranges for the quantity of gold present in the alloy.  
         [0050]    A first preferred range is the one associated with obtaining 18 carat gold, in which the quantity of gold present in the alloy is between 74% and 77% by weight.  
         [0051]    A second preferred range is the one associated with obtaining 14 carat gold, in which the quantity of gold present in the alloy is between 57% and 60% by weight.  
         [0052]    A third preferred range is the one associated with obtaining 8 carat gold, in which the quantity of gold present in the alloy is between 33% and 35% by weight.  
         [0053]    A fourth preferred range is the one associated with obtaining 9 carat gold, in which the quantity of gold present in the alloy is between 37% and 39% by weight.  
         [0054]    A fifth preferred range is the one associated with obtaining 10 carat gold, in which the quantity of gold present in the alloy is between 40% and 43% by weight.  
         [0055]    In regard to master alloys for obtaining the above gold alloys, they are composed at least by:  
         [0056]    Iridium: Ir≦0.4%;  
         [0057]    Germanium: Ge≦4%;  
         [0058]    Silicon: 0.03≦Si≦1.2%;  
         [0059]    Phosphorus: P≦0.1%;  
         [0060]    Copper: Cu sufficient to reach 100,  
         [0061]    in the indicated quantities, with reference to the total weight of the master alloy.  
         [0062]    Moreover, as stated, the master alloy can also comprise, as a weight percentage relative to the total weight of the master alloy:  
         [0063]    silver: Ag≦72%;  
         [0064]    nickel: Ni≦41%;  
         [0065]    zinc: Zn≦25%.  
         [0066]    Additionally, the master alloy can include, in quantities not exceeding 8% by weight, at least one of the elements of the group constituted by cobalt, manganese, tin and indium.  
         [0067]    The master alloy can further include, in quantities not exceeding 0.56% by weight, at least one of the deoxidizing elements of the group constituted by magnesium, silicon, boron and lithium.  
         [0068]    Advantageously, in some applications, at least one of the refining elements of the group constituted by ruthenium, rhenium and platinum can be inserted in the master alloy, in quantities not exceeding 0.96% by weight.  
       EXAMPLES  
       [0069]    Some examples of gold alloys which can be obtained with a composition in accordance with the present invention are set out below.  
       Example A  
       [0070]    A 14 carat yellow gold alloy whose composition in terms of weight percentage is as follows:  
                                                                                         Gold   58.5                with master alloy comprising (as a percentage           on the weight of the gold alloy):                Silver   8.0           Zinc   6.0           Iridium   0.01           Germanium   0.4           Phosphorus:   0.01           Silicon   0.06                Copper, sufficient to reach 100.                      
 
       Example B  
       [0071]    A 18 carat yellow gold alloy whose composition in terms of weight percentage is as follows:  
                                                                                         Gold   75.0                with master alloy comprising (as a percentage           on the weight of the gold alloy):                Silver   15.0           Iridium   0.01           Germanium   0.2           Phosphorus:   0.01                Copper sufficient to reach 100.                      
 
       Example C  
       [0072]    A 18 carat yellow gold alloy whose composition in terms of weight percentage is as follows:  
                                                                                         Gold   75.0                with master alloy comprising (as a percentage           on the weight of the gold alloy):                Silver   12.5           Zinc   0.5           Germanium   0.25           Silicon   0.04                Copper sufficient to reach 100 (in this           specific case 11.71%).                      
 
       Example D  
       [0073]    A 14 carat white gold alloy whose composition in terms of weight percentage is as follows:  
                                                                                         Gold   58.5                with master alloy comprising (as a percentage           on the weight of the gold alloy):                Nickel   8.5           Zinc   8.0           Iridium   0.01           Germanium   0.4           Phosphorus:   0.01                Copper sufficient to reach 100.                      
 
       Example E  
       [0074]    A 18 carat white gold alloy whose composition in terms of weight percentage is as follows:  
                                                                                         Gold   75.0                with master alloy comprising (as a percentage           on the weight of the gold alloy):                Nickel   7.5           Zinc   3.5           Iridium   0.01           Germanium   0.25           Phosphorus:   0.01                Copper sufficient to reach 100.                      
 
       Example F  
       [0075]    A 8 carat yellow gold alloy whose composition in terms of weight percentage  
                                                                                         Gold   33.3                with master alloy comprising (as a percentage           on the weight of the gold alloy):                Silver   13.0           Zinc   10.0           Germanium   0.4           Silicon   0.2           Iridium   0.02                Copper sufficient to reach 100 (in this           specific case 43.08%).                      
 
       Example G  
       [0076]    A 10 carat yellow gold alloy whose composition in terms of weight percentage is as follows:  
                                                                                         Gold   41.7                with master alloy comprising (as a percentage           on the weight of the gold alloy):                Silver   11.0           Zinc   8.7           Germanium   0.3           Silicon   0.15           Iridium   0.017                Copper sufficient to reach 100 (in this           specific case 38.13%                      
 
         [0077]    To obtain the five yellow gold alloys described in examples A, B, C, F and G, set out above, a preferential process comprises the following phases:  
         [0078]    melting in controlled atmosphere or in an inert gas such as argon, of the elements in the respective doses, inside graphite or ceramic crucibles at a temperature ranging between 880 and 940° C.;  
         [0079]    subsequent heating to a temperature ranging between 970 and 1030° C. before proceeding with casting;  
         [0080]    casting the material in appropriate dies;  
         [0081]    cooling the die in air;  
         [0082]    subsequent cooling of the die in water.  
         [0083]    To obtain instead the two white gold alloys described in examples D and E, set out above, a preferential process comprises the following phases:  
         [0084]    melting in controlled atmosphere or in an inert gas such as argon, of the elements in the respective doses, inside graphite or ceramic crucibles at a temperature ranging between 890 and 970° C.;  
         [0085]    subsequent heating to a temperature ranging between 980 and 1100° C. before proceeding with casting;  
         [0086]    casting the material in appropriate dies;  
         [0087]    cooling the die in air;  
         [0088]    subsequent cooling of the die in water.  
         [0089]    The present invention achieves important advantages.  
         [0090]    In the first place, laboratory tests conducted by the Applicant have shown that use of germanium in weight concentrations ranging between 0.05% and 2% leads to an increase in the fluidity of the alloy in the molten state that is even greater than the one brought about by the use of silicon alone in normal usage concentrations.  
         [0091]    Moreover, the increase in grain size consequent to the use of germanium was lesser than the one that takes place in traditional alloys containing silicon alone, as shown in FIGS. 1 and 2.  
         [0092]    [0092]FIG. 1 shows the variation in the dimensions of the crystal grain of the alloy as a result of the addition, thereto, of the elements and compounds indicated in the x-coordinate. It is evident that the influence of only silicon (Si) on the increase in grain size is considerably higher than the influence of germanium (Ge).  
         [0093]    [0093]FIG. 2 shows the effect of the concentration of silicon and germanium alone, on the grain dimension of the gold alloy. In this case, too, it is evident that a low concentration of silicon, in the graph from 0 to 300 ppm, entails a considerable increase in crystal grain size, even exceeding the size increase caused by additions of germanium in concentrations that are 10 times greater.  
         [0094]    This has positive repercussions on the mechanical behavior of the alloy, as can be seen in FIG. 3, which shows the (positive or negative) variation of the maximum load bearable by the alloy, following the addition to the alloy of equal quantities of the different elements or compounds indicated in the x-coordinate (good both germanium alone, and germanium plus Copper plus silicon).  
         [0095]    The use of germanium instead of, or together with, silicon also yielded positive effects on the percent of lengthening of the alloy following the tensile test.  
         [0096]    In regard to the combined use of germanium and silicon, respectively to improve the fluidity and decrease the oxidation of the alloy, very encouraging results were obtained.  
         [0097]    The combined use of these two elements gave rise to deoxidized alloys which at the same time show a very good mechanical behavior, generally better than the one exhibited by the alloys in which silicon is used both as a fluidizing element, and as a deoxidizing element.  
         [0098]    Then the use of Phosphorus together with germanium (alone or combined also with silicon) gives the possibility to maintain the level of germanium and silicon substantially constant for more successive fusions.  
         [0099]    In fact, if scrap amounts of alloys are molten together with new amount of alloys (generally 50% each one), phosphorus reaction with oxygen reduces the formation of silicon and germanium oxides. As a consequence, silicon and germanium content in alloys decreases less drastically during the re-cycling operations.  
         [0100]    In any case, where the use of refining elements becomes necessary to obtain even smaller grain dimensions, the formation of silicides does not take place, thanks to the small amounts of silicon (or in some cases its absence).  
         [0101]    It should further be noted that the present invention is relatively easy to implement and that also the cost connected to the implementation of the invention remains within the standards of the industry.  
         [0102]    The invention thus conceived can be subject to numerous modifications and variations, without thereby departing from the scope of the inventive concept that characterizes it.