The particularity of AgCu 92.5 alloy production technology at Kazakhstan Mint
Table of contents: The Kazakh-American Free University Academic Journal №4 - 2012
Author: Maslennikov Oleg, East Kazakhstan State Technical University in honor of D. Serikbayev, Republican State Enterprise "Kazakhstan Mint of the National Bank of the Republic of Kazakhstan", Kazakhstan
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  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.
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
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.
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 .
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
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
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
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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Table of contents: The Kazakh-American Free University Academic Journal №4 - 2012