Continuous Casting of Precious Metals
Gold
Gold is used in coinage and is a standard for monetary systems in many
countries. It is extensively used in jewelery, decoration, dental work,
plating and a wide range of electrical circuit applications.
The composition of alloys used in jewelery in most countries is denoted in
the 'carat system' (spelt Karat in the USA and on the Continent). This
system denotes the proportion of pure or 'fine gold' in the alloy. Pure gold
is designated 24 carat. The alloy gold is classified as 11, 18, 14, 9 carat
etc. which indicates the parts of pure gold in 24 parts of the alloy.
For 'hallmarking' purposes the composition or standard of the gold is also
expressed in parts per thousand (%o). Pure gold has a fineness of 1000. In
the UK the Hallmarking Act revised in January 1999 now includes 999 and 990
finenesses as well as916.6 fine (22 carat), 750 fine (18 carat), 585 fine
(14 carat), 375 fine (9 carat). Other hallmarks and fineness standards in
the EC are also allowed
Gold-Melting Characteristics
Gold is available commercially in several purities from 99.5% to 99.9+%,
but pure gold is too soft to be used for jewelery manufacture and its main
use has been for coinage and various industrial purpose. Most of the
commercially important colored alloys for jewelery and dental applications
are based on the gold-silver-copper system, frequently modified with the
addition of zinc and sometimes of nickel, for jewelery alloys. Palladium and
platinum are more commonly added to dental alloys. In general the alloys
have excellent continuous casting properties.
The Au-Ag-Cu alloys have a low vapour pressure and therefore there is
negligible volatilisation at the normal casting temperatures around 1200°C.
Zinc has a high vapour pressure, being above its boiling -point of 910°C,
and is a fairly common addition to the alloy golds. Losses are therefore to
be expected and allowed for.
Continuous Casting of Fine Gold
Fine gold 24 carat, with purity to '99.9% to 99.99%' is cast in strip or
rod form. It is extremely soft and ductile and as in all high-purity metals
can be slightly difficult to cast at higher speeds. The hardness increases
with trace impurities and rapidly rises with cold work. Typical values for
hardness and ductility (as elongation %) .
The metal generally cast in the form of strip in mints for medal production
and certain coinage. The metal is also cast as fine wire. A considerable
amount of the pure gold cast is used in the electronic and instrument
industry. Metal containment is mostly in high-purity graphite, this being
preferred to minimise trace element contamination. Silicon carbide is used
in certain induction-heated units on alloy gold. Die material is high-grade
graphite for all applications with the exception of high-palladium and
platinum-bearing alloys. The containment crucible is designed to give
drainage to the casting die to ensure complete metal recovery. Horizontal
casting is used in most cases with the exception of a few vertical
proprietary micro-casting units used for fine wire. Tilting horizontal units
are now being used in special cases to facilitate complete metal recovery.
Continuous Casting of Carat Gold
The full range of carat gold alloys is continuous cast as strip, rod and
tube. With the exception of special applications in tube casting and some
small-diameter rod casting the bulk of the material is cast in the
horizontal mode.
Gold Alloy 18 Carat
The 18-carat gold is used extensively in the manufacture of jewelery of all
kinds containing 75% gold and 25% alloying metals. In most compositions the
alloying metal is either silver or copper or a mixture of the two.
Gold Alloy 14 Carat
The 14-carat standard is fixed at 58.5% gold, known as 585. There is a wide
application for this alloy, having similar hardness and being less expensive
than the 18-carat gold. It is used extensively in industrial applications in
jewelery, pen nibs and slip rings and bushes on electrical instruments.
Gold Alloy 10 Carat
The 10-carat alloy containing 41.7% gold, known as 417, is used in similar
applications to the 14-carat alloy. Mainly used in the USA as a cheaper
version of 14 carat.
Casting of Gold Tube
Gold tube is extensively continuous cast, generally for the jewelery
industry in the manufacture of rings and also for industrial applications.
Metal can be cast in fine gold or any of the carat gold alloys.
Gold-Tin Alloys
The Au:Sn-80:20 (melting -point 280°C) eutectic alloy is fabricated in
the form of square or rectangular thin washers that are used for the final
closure of integrated circuit or hybrid packages and extensively in
semiconductor manufacture. Itr has good corrosion resistance, solderability
and wear resistance. It is used as thin strip around 0.5mm thick, and
because of the difficulty in breakdown rolling from the cast state (i.e. the
early rolling passes), it is desirable to cast the strip as thin as
possible.
Silver
In commercial applications, the special chemical properties, superior
thermal and electrical conductivity, high reflectivity,
malleability,ductility and good corrosion resistance give it a unique place
in industry.
Silver Alloys-Silver Copper
The silver-copper alloys are the most important of the industrial silver
alloys. The addition of copper increases the hardness of the as-cast alloy
and lowers the electrical conductivity.
Noble -Metal Brazing Alloys
The electronic and aerospace industries requirements have necessitated the
development of gold and palladium base brazing alloys. In the manufacture of
these alloys great care must be exercised in maintaining a low trace
impurity level.
Continuous Casting Au-Cu Brazing Alloys
Teh BS1845 gold: copper brazing alloys are generally continuously cast as
small-diameter rod or this strip. In precious-metal casting it is essential
to control composition to precise limits. For economic reasons it is good
casting practice to use pre-alloyed ingots or grain as feedstock. The alloy
system has a low vapour pressure, therefore there are negligible volatiles
at the casting temperatures.
Casting Sequence
The Au:Cu alloys form a continuous series of solid solutions. There are two
intermetallic compounds, CuAu, formed in the solid state around 400°C,
but these have no influence on the castability of the listed alloys. The 80%
Au alloy form a eutectic at a temperature around 905°C and thereafter
the liquids/solidus gap increases.For high-density precious metals it is
essential to use premium-quality high-density graphite in the construction
of the crucible.
Group Ag Silver Brazing Alloys
This group constitutes one of the most important series of silver-bearing
brazing alloys. Continuously cst product is generally in the form of
small-diameter rod or thin strip
Cu-P-Ag Brazing Alloys
The copper-phosphorus filler metals provide relatively low-cost alloys with
low melting points and on copper are self fluxing. These alloys fall into
three distinct groups straight copper-phosphorus, silver-copper-phosphorus
and modifications of the two groups. Although these alloys are not precious
metals, they have been listed in this section of the brazing alloy group as
most contain silver.
