For everyone interested in working with the new microalloyed
gold I am attaching the manufacturing guidelines.
John Bernardin
Manufacturing with PureGold
PureGold is a microalloy that has a gold content of 99.85 parts
per hundred. Therefore, to retain its purity and to ensure that
you do not alter its properties, care must be taken not to
contaminate the alloy as you are working with it. Even a very
small amount of foreign material will significantly alter its
properties as well as reduce the gold content below the level
required to mark your piece 24K. The piece should always be
pickeled before a heating operation such as age hardening or
before using any of the joining techniques that are available.
Alloys available: PureGold A
99.85 pure, this alloy exhibits the greatest work hardening
behavior of the two 24K alloys. It is usually used for
fabricated pieces where the maximum effect of work hardening and
age hardening is desired. PureGold A can also be cast.
However, it does require either a vacuum atmosphere or an inert
atmosphere such as argon gas over the molten metal.
PureGold C
99.85 pure, this alloy exhibits greater hardness after casting
and can be cast in air. It also exhibits excellent cold work
hardening and age hardening behavior.
Casting PureGold A
Solidus temperatuRe: 1067.99 oC (1954.38 oF)
Liquidus temperatuRe: 1076.25 oC (1969.25 oF)
PureGold A requires an inert atmosphere when in the molten
state. Therefore, a torch cannot be used to melt the alloy.
Resistance heating or induction melting devices work well but
the metal should be covered by a blanket of argon gas or the
melting should be carried out in a vacuum.
For the shop producing one-of-a-kind pieces, centrifugal casting
machines such as those used in the dental industry work very
well. Many of these have induction heaters, provide for an
argon flow over the molten alloy, and cast very rapidly once the
casting temperature is reached. Production casting in modern
machines that provide for vacuum operation or for an inert
nitrogen or argon atmosphere also work very well. With care,
PureGold can also be cast using a vacuum assist casting machine.
Casting PureGold C
Solidus temperatuRe: 1068.37 oC (1955.07 oF)
Liquidus temperatuRe: 1096.85 oC (2006.33 oF)
PureGold-C is easily cast in air. Due to it’s high melting
temperature, PureGold is best melted using induction heating.
However, resistance heating can be used as well. Torch melting
can be used if care is taken not to overheat the metal. Casting
temperature should be approximately 150 oC above the liquidus
temperature. Flask temperatures for a typical ring would be 600
oC. These conditions should be considered starting points for
casting and may have to be changed depending on the design being
cast.
Investment
Because of the high melting temperature of the PureGold
microalloys, investment should be phosphate based since gypsum
based investment material begins to decompose at the
temperatures of the molten metal. Excellent results are also
obtained by coating the wax model with a ceramic skin before
investing. Prepared ceramic slurries are used in casting finely
detailed medical implants and in the dental industry. These
ceramic slurries are readily available and produce an excellent
fine surface on the casting. The new rapid casting investments
also work very well.
Gating
For both PureGold-A and PureGold-C, there is very little
difference between liquidus and solidus. This very narrow
freezing range requires that close attention be paid to gating
the wax model. For most applications, the gating procedures
followed for platinum will work very well. Sprues should be
large (1.5 times the thickness of the thickest part of the
casting) and feed the heaviest sections of the wax model.
Sprues should feed directly from the button. After casting,
allow the flask to cool to room temperature before devesting.
Once the investment is removed from the cast piece it should be
age hardened by holding the piece at 250oC for three hours
followed by air cooling. The surface of the piece can then be
further hardened through the use of one of the magnetic pin
finishers that are available. Experience has shown that if one
attempts to cast PureGold using gating techniques used for 14K
alloys there will be porosity in the casting. However,
successful castings are easily obtained if one provides an
adequately sized gate and sets casting conditions for rapid
filling of the mold.
Joining PureGold
Small diameter wire and plate can most easily be joined by laser
welding or fusion welding. The process is rapid and very little
degradation in the hardness occurs. These processes also
overcome any degradation of karatage and any visible change of
color in the joint.
Fusion welding of PureGold using pure gold or the .9985 alloy as
filler material also works well but there can be some loss of
hardness at the joint if the piece has been hardened by cold
working followed by aging. The filler material should be thin
(0.1 to 0.5 mm) and melts down into the joint by applying
concentrated heat.
PureGold can be brazed using traditional brazes (what are
typically referred to as solders) with the best color match
obtained by 22K solder and a jewelers flux.
The best technique for joining PureGold with minimal loss in
hardness is through the use of the diffusion soldering
technique. Making a diffusion soldered joint involves placing a
thin layer of solder between the components to be joined and
heating the assembly to a temperature above the melting point of
the solder. A diffusion soldered joint can be made in air by
first dipping the solder foil into liquid flux (e.g. ARAX*)
prior to inserting it into the joint gap. The assembly is then
heated to about 250oC at which temperature the flux is
chemically active and aids wetting of the joint by the solder.
This is sufficient to “tack” the joining pieces together. The
joint is weak and full mechanical properties are not attained
until a heat treatment at 450oC has been completed. Final
joining is completed by heating the assemblage at 435=B115oC for 1
hour.
*ARAX flux is available from Solder Master, 6410 Independence,
Woodland Hills, CA 91367