The particularity of AgCu 92.5 alloy production technology at Kazakhstan Mint

The History of Kazakhstan Mint (hereafter KM) dates back to November 13, 1992; when the first sovereign Kazakhstan’s coin was minted. Kazakhstan Mint owns its birth to independence and sovereignty of Kazakhstan, as the national currency is of the attributes of statehood in any country.

Since 1996 the KM has developed the technology to produce “proof” quality coins of silver-copper alloy (AgCu 92.5), which is the top of monetary art. The result of this long-term work was the release of the “Millennium” coin in December 1999.

The problems caused by defects in the surface of precious metals products concern the producers of these high-priced products, as these defects not only increase the products’ cost and reduce the profitability of production, but also lower the company’s competitive capacity in the market economy.

These problems were in the spotlight at the 24th MINT DIRECTORS CONFERENCE held in Paris in May, 2006.

Several reports on the analysis of precious metals products surface defects were made at the technical section of the conference. An overview report [1] on the surface defects was presented; it contained the results of defects study carried out at 18 mints.

The report presented the classification of surface defects of the “proof”-quality coins such as floating fibers, excessive grain size, orange peel, cross contamination, blisters, tarnish, stains, peck marks, inclusions, struck in debris, stardust.

Many silver alloy compositions are known. Some of them are used for jewelry and flatware, while others are used in brazing compositions and as electrical conductors. Due to this silver alloys are of interest to manufacturers in the whole world [2-6].

A large number of products are made from the AgCu 92.5 alloy at KM. Minimizing the “proof” production defects is a large cost saving reserve. The defects of AgCu 92.5 alloy coin blanks and finished products caused by inclusions and porosity are one of the main types of defects.

The increasing demand for “proof”- quality finished products requires the optimization of production scheme. KM carries out research on technological improvement of “proof” quality coins production in cooperation with D. Serikbayev EKSTU experts.

Experimental procedure

We used two AgCu 92.5 alloy samples for our research: ingot 97/1 obtained by induction melting, continuous horizontal casting into a water-cooled kokille; the other sample, ingot Sh-139 was obtained by melting in an electric furnace and cast into a steel mold.

The obtained samples quality evaluation was performed with nondestructive evaluation (NDE): eddy-current testing, ultrasonic inspection, fluoroscopy and radiographic analysis. However, these methods were not effective for the detection of internal defects smaller than 3 mm.

We developed the procedure of AgCu 92.5 alloy quantitative metallography for its quality attestation (hereafter ÌÌÀ). According to this procedure, we made microslices from the samples. The evaluation of the sample’s surface contamination for inclusions was done with an optical microscope “Axiovert 200 MAT. Zeiss”, and a scanning electron microscope (SEM). The sample’s metal was given a quality rating based on the evaluation results.

The presence of gas porosity in the samples was measured by fracture test.

After studying the defects’ morphological peculiarities we studied the elemental composition of the inclusions with a scanning electron microscope microanalyzer ISM 5610.

The final quality evaluation of the AgCu 92.5 alloy was minting of the “proof” quality coins.

The Results and Discussion

Our research revealed that AgCu 92.5 alloy products manufactured at KM contain foreign inclusions of ferric oxide, silicon oxide, and complex oxides of aluminum, magnesium, and calcium. The inclusions are located on the alloy surface and often in a subsurface layer. These inclusions virtually always contain carbon. Polishing of blanks removes some of these inclusions as a part of the surface layer is taken off, but the defects in the subsurface layer are exposed. Similar processes take place as a result of motion of the metal caused by plastic deformation during minting. Foreign defects located on the surface disable embossing die and scratch the mirror surface of minted coins. We also found that there are three types of inclusions: "black", "gray" and "light" ones.

The examination of the received results showed that the "black" inclusions contain mostly carbon 82 % (mass). The source of theses inclusions is corrosion graphite mixer, crucible and coal.

In addition to the increased carbon content the "gray" inclusions contains oxygen up to 27 % (mass) and traces of Al, Si, Na which could get there as a result of a slight erosion of clay graphite crucible, Al₂O₃ top isolation.

Oxygen is the main component of the "light" inclusions; its content is up to 36 % (mass) at the high carbon content.

Examination of the gas porosity didn’t reveal any macroscopic porosity in ingot 97/1 samples. The presence of gas blister larger than 300 micron on the fracture of ingot Sh-139 is associated with the capture of the air in a steel casting mold.

The results of the “proof” quality coins minting showed that samples of AgCu 92.5 alloy coin blanks which have quality rating no higher than R-2 (by the metallographic analysis of AgCu 92.5 alloy) do not have defects of inclusions and porosity after minting.

Microstructure studies have shown that the structure of the alloy at room temperature is heterogeneous and consists of two phases: solid solutions based on silver and copper. These phases have a significant difference in chemical composition. Micro Roentgen-spectral analysis of these areas showed that the silver content in the light areas is 96-97 mass.%. In the dark areas it is 67 mass.% (Fig. 1). This submicroscopic liquation is only detected at the etching of polished sections.

Figure 1 - X-ray spectrum and elemental composition of solid solutions based on silver (A) and Cu (B)

AgCu 92.5 alloy structural and chemical micro heterogeneity (microliqua-tion) is due to the limited solubility of copper in silver. Below 700º C, the separation of β-phase, which is a solid solution of silver in copper, occurs.

Further studies of structural heterogeneity showed that enlarged β-phase areas often form around the inclusions. Such defects were assigned to endogenous inclusions, i.e. inherent to the technology, the equipment design, the materials of casting equipment (Fig. 2, Table 1).

Figure 2 - Endogenous "carbon oxide" inclusions in AgCu 92.5 alloy

Table 1 – The chemical composition of endogenous inclusions at. %

The basis of endogenous inclusions is carbon, oxygen, as well as silicon, aluminum, sodium, and magnesium. From the analysis of the elemental composition of the inclusions it can be assumed that they are microparticles (up to 30 mkm), formed by the erosion of the clay graphite crucible, around which a copper-rich zone forms.

The results indicate that despite the “deoxidation” of the alloy during smelting and the low content of oxygen in the alloy (less than 0.002%) there are micro volumes with the oxygen content up to 54 at%.

The values of their high oxygen content exceed the values required for the formation of Al, Si, Cu oxides. This suggests that much of the dissolved oxygen and carbon is in the form of CO ligands soluble in silver [9].

The carbon which is present in the clusters may participate in the processes of deoxidation and is a reducing agent for the copper oxide with the formation of carbon monoxide:

Cu₂O + C = Cu + CO

Areas enriched with CO and local accumulations of gas may appear in the areas of endogenous clusters. Besides, during the subsequent hydrogen annealing by the known mechanism of "hydrogen disease" [7, 8], the molecules of water formed in the alloy can react with carbon monoxide, which intensifies the formation of gas porosity:

CO + H₂O = CO₂ + H₂

Sometimes during polishing preparations such defects as "peck marks" appear in the microrelief irregularities where the remains of the high content of carbon and copper are detected (Fig. 3). These "peck marks", apparently appear as a result of treatment of clusters and gas micropores in the surface finishing.

Figure 3 - Electronic image of "peck marks" in the polished section in reflected electrons (A), X-ray spectrum (B), the elemental composition of the inclusion (C)

Thus, the detected clusters can be the cause of the "peck marks" in the polished surface of the product. But it remained unclear why this phenomenon occurs sporadically.

We assessed the crucible erosion (on the alloy "contamination"), according to its operating time.

The results of the assessment of the alloy "contamination" (according to MMA) are shown in Fig. 4.

The received results show that the alloy “contamination” by endogenous clusters increases with the crucible operating time. At that, the inclusions become larger from 15 to 30 micrometers (Fig. 5), which means the sizes of the clusters and areas enriched in oxygen and carbon increase, as well as the probability of occurrence of the “peck marks" defects during polishing.

Figure 4 - The relationship between the alloy “contamination” and the operating time of the crucible

Figure 5 – The structure of the alloy "contamination" of the 1st and the10th casting work

Conclusion

The research has revealed that foreign inclusions in AgCu 92.5 alloy produced at KM can be of exogenous and indigenous nature.

Some of the inclusions get into the metal from the outside: with the source melting stock, during plastic deformation, heat treatment, polishing, etc. from the industrial atmosphere. These are exogenous inclusions. The other part of inclusions is obviously of metallurgical nature. They are produced by physicochemical interaction of the melt and the melting crucible, casting tool set materials and atmosphere inside crucible. Such inclusions are indigenous. These inclusions are formed by the erosion of the clay graphite crucible, they increase the alloy “contamination” as the crucible operating time increases. Gas microporosity localizes in the cluster area as the result of carbon deoxidizing reaction.

When the sizes of the inclusions reach critical values (with the long term of the crucible use or in case of nonoptimal glazing), the formation of gas macroporosity and the “peck marks" defects in the blanks’ polished surface is possible.

The methods of nondestructive evaluation (NDE) of AgCu 92.5 alloy proved to be not effective for the diagnosis of defects smaller than 3 mm.

The developed method of AgCu 92.5 alloy metallographic analysis allows making a valid and prompt evaluation of the metal’s quality and forecasting the amount of defective goods at “proof” quality coins minting.

Continuous horizontal casting of AgCu 92.5 alloy partially excludes the possibility of macro shrinkage. It prevents the swelling of the metal caused by hydrogen disease at annealing in protective atmosphere.

The metal’s quality rating no higher than R-2 (by the metallographic analysis of AgCu 92.5 alloy) doesn’t lead to defects in “proof” quality goods like foreign inclusions or porosity.

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