Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF PRODUCING PRECIOUS METAL ALLOY OBJECTS
Technical field of the invention
The present invention relates to precious metal alloys and methods of
manufacturing
such. In particular the present invention relates to precious metal alloy
objects such
as jewellery and other precious metal containing objects, for example dental
implants
and decorative members, that are intended to be in contact with a human body.
Background of the invention
Precious metals are commonly used in jewellery or other objects which are
intended
to be in contact with the human body. One reason for this is that precious
metals are
less reactive than most elements. Another is their high economical value.
Moreover,
precious metals usually have an attractive lustre and high ductility. The most
well-
known precious metals are gold and silver, but other precious metals such as
platinum and palladium are commonly used for the same purposes.
Precious metal objects which are worn on the human body are subjected to wear
and
damage. The ductility of precious metals is an advantage since the risk for
fracture is
low, but precious metals have relatively low hardness making them susceptible
to
wear. To make them harder, and also due to the high cost of the precious
metals,
precious metals used in jewellery, implants, etc. are usually alloyed with
other
elements. The precious metals may also be alloyed to improve other properties
of the
precious metal, such as for example to obtain a certain lustre or colour or to
improve
the workability.
It is known that some people cannot wear jewellery or other decorative members
due
to hypersensitivity or allergy, which may cause dermatitis or allergic
reactions. The
allergenic potency of different elements differs and generally precious metals
have the
lowest potency. Among the alloying elements commonly used for gold, nickel has
been identified as having the highest allergenic potency. Therefore the nickel
release
in a synthetic sweat solution has been established as a measure on the
allergenicity
of a nickel-containing material, and a threshold level (0.2 g/cm2/week) below
which
an object may be considered non-allergic has been defined in the European
Union
"Nickel Directive" (94/27/EC). Similar threshold levels for other alloying
elements
have not been established, but it is likely that other alloying elements, even
silver,
copper and gold, may also cause sensitisation. Allergenic reactions or the
like may
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also occur due to impurities in the precious metals or metal alloys. The
impurities
may appear due to impurities of the raw materials used or due to the
manufacturing
of the alloy. For example impurities may be added if the precious metal or
metal alloy
is treated with an acid in a step following a casting step to remove oxides
formed on
the cast object. Irrespective of the reason for the sensitisation, a precious
metal
object can be regarded as biocompatible if the probability of causing
sensitisation is
below a certain degree.
One common belief is that allergenic reactions do not occur if only pure
alloying
elements of precious metals are used. Using conventional manufacturing methods
this does not necessarily yield a precious metal alloy that is non-allergenic
and more
important the semi-finished or finished product may not have e.g. the required
hardness, fracture toughness, workability, colour, etc. As mentioned above the
hardness of a precious metal or metal alloy is important to provide wear
resistance.
By way of example, a gold alloy comprising the alloying elements gold, silver
and
copper is usually manufactured by melting the alloying elements in a crucible
and
casting them in a mould to form a raw material that subsequently is subjected
to
further processing to form the final object. In manufacturing of a precious
metal alloy
object, the raw material is typically cold or hot worked and it may be
subjected to
heat treatments and/or cooling steps necessary to obtain certain material
properties
in the final object. This process is by no means simple, e.g. an increased
hardness
due to e.g. strain hardening during cold working of the raw material may cause
difficulties due to decreased workability and on the contrary hot working of
the raw
material may significantly decrease the workability of the alloy making it
difficult to
form the final object. Also the alloy may be brittle after the casting of the
raw
material, making additional annealing steps necessary.
Summary of the invention
The prior art has drawbacks with regard to being able to provide a precious
metal
alloy object that is biocompatible and has the desired material properties,
such as
high hardness and good workability.
The object of the present invention is to overcome the drawbacks of the prior
art.
This is achieved by a biocompatible precious metal alloy object and a method
for
manufacturing such as defined in the independent claims.
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The method for manufacturing a biocompatible precious metal alloy object
according to the present invention comprises the step of forming the
biocompatible
precious metal alloy object in a process chamber. The method further comprises
the step of providing a process gas of predetermined composition having a
water
content of less than 0.005kg H2O per kg process gas and an oxygen content of
less
than 5%. The process gas is provided in the process chamber at least during
said
forming of the biocompatible precious metal alloy object.
According to a first aspect of the present invention the step of forming the
biocompatible precious metal alloy object comprises the steps of melting
alloying
elements together in order to form the precious metal alloy, and casting the
molten
alloying elements of the precious metal alloy.
According to a second aspect of the present invention the step of forming the
biocompatible precious metal alloy object comprises the step of post-
processing a
precious metal alloy, i.e. a raw material, in the process chamber to form the
biocompatible precious metal alloy object. Preferably the raw material is
manufactured in accordance with the method of the present invention. The post-
processing may for example include soldering and/or welding.
According to one embodiment of the present invention a solder alloy, suitable
for
being used in the above mentioned soldering of the precious metal alloy raw
material or object, is manufactured in accordance with the method of
manufacturing the biocompatible precious metal alloy object according to the
first
aspect.
In one embodiment of the present invention the content of the process gas and
hence the environment in the process chamber is controlled by burning a flame
that is supplied with a hydrocarbon-containing gas. Thereby oxygen present in
the
process chamber is combusted.
The bulk of a biocompatible precious metal alloy object that has been
manufactured according to the method of the present invention has an oxygen
content of less than 5 g/g, preferably less than 3 g/g and more preferably
less
than lpg/g; and a hydrogen content of less than 0.05 g/g, preferably less than
0.0l g/g and more preferably less than 0.005 g/g.
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A biocompatible precious metal alloy object according to the present invention
preferably comprises 2% Ag. More preferably it is a gold alloy of more than 14
carat
or a silver alloy.
Thanks to the invention it is possible to provide a biocompatible precious
metal alloy
object which is not likely to cause sensitisation when in contact with a human
body.
It is a further advantage of the invention to provide a precious metal alloy
object
which has tailored material properties with regards to e.g. hardness and
workability.
Such an object can be used as a raw material that is subjected to post-
processing in
order to form a final precious metal alloy object having adequate material
properties
such as high hardness and high fracture toughness.
It is a yet further advantage of the invention to provide post-processing of a
biocompatible precious metal alloy raw materials in a dedicated workstation to
substantially maintain the tailored material properties of the biocompatible
precious
metal alloy raw material which preferably has been manufactured according to a
method in accordance with the present invention.
Embodiments of the invention are defined in the dependent claims. Other
objects,
advantages and novel features of the invention will become apparent from the
following detailed description of the invention when considered in conjunction
with
the accompanying drawings and claims.
Brief description of the drawings
Preferred embodiments of the invention will now be described with reference to
the
accompanying drawings, wherein:
Figs. la-d are schematic diagrams of embodiments of a method of
manufacturing a precious metal alloy object according to the present
invention;
Figs. 2a-b are schematic illustrations of process chambers according to the
present
invention;
Fig. 3 is a schematic illustration of a crucible arranged on a mould with an
intermediate pre-heater chamber according to the present invention;
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Fig. 4 is a schematic diagram of a method in accordance with the present
invention
for manufacturing a precious metal alloy comprising the step of evacuating the
mould; and
Fig. 5 is a schematic illustration of a process chamber suitable for post-
processing
5 according to the invention.
Detailed description of embodiments
During manufacturing of a precious metal alloy object the alloying elements
are
usually melted and subsequently cast to form a precious metal alloy object, a
so-
called raw material, which subsequently is subjected to post-processing,
including
e.g. forging, welding, soldering, casting, grinding, polishing or drawing, to
form a
precious metal alloy object such as a jewellery. One object of the present
invention is
to provide a method. for manufacturing of precious metal objects which are
biocompatible so that they do not cause sensitisation when carried in contact
with
the human body. Examples of such objects are jewellery (including piercing
jewellery), decorative members of other kind, dental implants, etc. as well as
the raw
material mentioned above. The precious metal alloy composition according to
the
present invention comprise of precious metal alloys compositions commonly used
for
e.g. jewellery, dental implants, and decorative members. Examples of such,
however
not limited to these, are gold (22K, 18K, 14K, etc.) and sterling silver.
Although a gold
alloy manufactured according to the present invention may be of a certain
carat it
may differ slightly in the content of the main alloying elements (Au, Ag, Cu)
and the
additional alloying elements may differ in content or composition to obtain
e.g. a
certain lustre. Furthermore, although the term alloy is used, the present
invention is
not limited to alloys comprising two or more materials. Also pure precious
metals
may be manufactured using the method of the present invention.
Referring to Figs. la-d, a method for manufacturing a biocompatible precious
metal
alloy object that is made of a precious metal alloy according to the present
invention
comprises the steps of:
- 100 forming the biocompatible precious metal alloy object in a process
chamber;
and
- at least during said forming 101 providing a process gas of predetermined
composition in the process chamber 11, wherein the process gas has a water
content
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of less than 0.005 kg H2O per kg of process gas and an oxygen content less
than 5%
oxygen.
In one embodiment of the present invention the step of forming further
comprises the
steps of-
- 102 melting alloying elements together in order to form the precious metal
alloy;
and
- 103 casting the molten alloying elements of the precious metal alloy,
wherein the
steps of melting and casting are carried out within the process chamber 11 in
a
controlled atmosphere comprising the process gas.
In another embodiment of the present invention step of forming comprises the
step
of 111 post-processing the precious metal alloy in the process chamber 11 to
form
the biocompatible precious metal alloy object. The post-processing is
preferably
performed on a precious metal alloy raw material that has been manufactured
according to the above mentioned steps of melting and casting. However, the
invention is not limited to this and suitable raw materials manufactured
according
to other methods can be used. The post-processing may be made in the same
process chamber 11 as used in the manufacturing of the raw material or in
another
process chamber such as a dedicated workstation chamber.
In one embodiment of the present invention the step of providing the process
gas
further comprises the step of 104 combusting oxygen of the process chamber 11
using a flame 19 that is supplied with a hydrocarbon-containing gas.
Referring to Figs. 2a-b, the process chamber 11 is preferably designed such
that a
controlled atmosphere that is separated from the ambient air can be provided
in the
process chamber 11. In one embodiment of the present invention the step of
providing the process gas comprises the step of generating an overpressure in
the
process chamber 11 in order to have a net flow of gas from within the process
chamber 11 to the outside, for example by using a check valve or a pump. A
suitable
overpressure can also be maintained by having a net flow through doors of an
airlock
system 28. This also automatically provides a controlled atmosphere in the
airlock
system.
Fig. 2a schematically illustrates a process chamber 11 according to one
embodiment
of the present invention. A process gas of predetermined composition is
provided in
the process chamber 11, preferably before and during melting and casting of
alloying
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elements, by combusting burning a flame 19 that is supplied with a hydrocarbon-
containing gas within the process chamber. The combustion process lowers the
oxygen content of the process chamber 11 to at least less than 5%, preferably
less
than 2% and more preferably to less than 1%. In addition dehydration means 21
may
be used. This limits the water content of the process gas to at least less
than 0.01 kg
H2O per kg air, preferably less than 0.005 kg H2O per kg air, and most
preferably less
than 0.001 kg H2O per kg air. The process chamber 11 may further comprises a
crucible 13 arranged on a mould 15, which, for example, may be a so-called
flask
comprising a plaster compound inside, which a skilled person is familiar with.
The
alloying elements are provided in the crucible 13 and melted. The mould 15 is
at
least partly filled by the molten alloying elements and after solidification
of the
molten alloying elements a precious metal alloy object is formed in the mould
15.
Fig. 2b schematically illustrates a process chamber 11 suitable for the
metling and
casting according to one embodiment of the present invention. A process gas of
predetermined composition in the process chamber 11 is accomplished by
supplying
a hydrocarbon-containing gas to a burning flame 19 within the process chamber
11.
By way of example the hydrocarbon-containing gas may be a mixture of oxygen
and
acetylene, i.e. a welding flame, wherein the oxygen/ acetylene ratio is
adjusted to give
a reducing flame (an over-rich mixture). The combustion process lowers the
oxygen
content of the process chamber 11 to at least less than 5%, preferably less
than 2%
and more preferably to less than 1%. In addition dehydration means 21 are used
to
limit the water content of the process gas to at least less than 0.01 kg H2O
per kg air,
preferably less than 0.005 kg H2O per kg air, and most preferably less than
0.001 kg
H2O per kg air. The process chamber 11 may further comprise a crucible 13
arranged
on a mould 15, which may be a so-called flask comprising a plaster compound.
The
alloying elements are provided in the crucible 13. Inductive heating by
inductive
heaters 25 may be used to melt the alloying elements, which subsequently are
supplied as a melt to the mould 15, for example through an openable and
closable
opening in the bottom of the crucible 13. After solidification of the melt a
precious
metal alloy object is formed in the mould 15.
In one embodiment of the present invention the step of providing said first
process
gas further comprises the step of supplying a protective gas such as nitrogen,
argon,
etc. to the process chamber 11. This protective gas can be used as means for
removing ambient air from the process chamber and also can function as an
inert
gas during melting and casting.
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In one embodiment of the present invention the step of providing said first
process
gas comprises the step of 106 drying the first process gas of predetermined
composition using dehydration means 21. This can be achieved, for example, by
water vapour in the first process gas being condensed onto a cold surface and
led to
a drain.
In one embodiment of the present invention the method further comprises the
step of
evacuating a gas from the mould 15 prior to the casting of the molten alloying
elements e.g. by connecting a vacuum pump to one end of the mould 15.
In one embodiment of the method according to the present invention the step of
evacuating further comprises drying of an inert gas, optionally pre-heating of
the
inert gas, and providing a flow of the optionally pre-heated inert gas through
the
mould before casting. The inert gas may be provided from the process gas of
pre-
determined composition. One alternative is to supply an inert gas of another
composition. Inert gas is for the purpose of this application interpreted to
mean a gas
having a water content of less than 0.005 kg H2O per kg air and an oxygen
content of
less than 5% oxygen.
In one embodiment of the invention the drying of the inert gas is obtained
using
dehydration means 21 in the form of e.g. a refrigeration drier. Gas from the
process
chamber 11 is pumped into the refrigeration drier, wherein water vapour in the
gas is
condensed and removed from the gas. The dried gas may then be fed back to the
process chamber 11.
Referring to Fig. 3, in one implementation of the method of the present
invention the
mould 15 is preheated, e.g. in a separate oven, to about 350-400 C.
Thereafter, a
pre-heater chamber 17, a mould 15 and a crucible 13 are assembled with the
mould
15 underneath the crucible 13. Alloying elements are provided in the crucible
13.
Heater means, for example, inductive heaters 25, are used to heat the crucible
13 to
a temperature which is sufficient to melt the alloying elements. The
temperature
depends on the composition of the alloying elements but may be about 900 C.
The
pre-heater chamber may be heated by heat transferred from the crucible 13. The
temperature of the pre-heater chamber 17 may be about 600 C. A pressure
gradient
is applied over the mould 15, e.g. by applying a vacuum pump to one end, i.e.
an
outlet, of the mould 15, in such way that the process gas of the process
chamber 11
is sucked into the pre-heater chamber 17 and gets preheated before entering
the
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mould 15. This gives a preheating of the mould 15 which is at least sufficient
for
maintaining the temperature obtained after the preheating. By supplying the
mould
through an inlet of the mould with a gas having a controlled composition to
provide a
flow of the gas through the mould the conditions for casting a biocompatible
object is
improved. Residual oxygen and water trapped in the mould may be forced out of
it.
By way of example the crucible may have an exit hole in the bottom, which
initially is
sealed using a rod. When the alloying elements have melted and reached the
desired
temperature the rod can be removed and the melt is poured down into the
preheated
mould 15. The method of the present invention results in precious metal
objects
having substantially no oxidation layer. One advantage with this is that no
subsequent treatment in an acid bath (as is commonly used in the prior art) is
required. Treatment in such acid baths is believed to be one source of
impurities
which may give sensitisation for a carrier of a precious metal alloy object
manufactured from the acid bath-treated raw materials.
Referring to Fig. 4, in one embodiment of the present invention wherein
alloying
elements are melted in a crucible 13 and a biocompatible precious metal alloy
object
is casted in a mould 15 within a process chamber 11 having an atmosphere of a
process gas of predetermined composition, the method comprises the steps of.
- optionally 107 pre-heating the mould 15 before casting in said mould 15,
- 108 pre-heating an inert gas in a pre-heater chamber 17 arranged in-between
the
mould 15 and the crucible 13, and
- 109 flowing the inert gas through the mould 15 by evacuating the inert gas
from the
one end of the mould 15.
A pre-heater chamber according to the invention may comprise a cylindrical
body
having holes around the perimeter to allow gas from the atmosphere of the
process
chamber to enter into a through bore which is open for the melted alloying
elements
to be supplied to the mould. Hence the gas enters the pre-heater chamber from
the
side and is sucked down into the mould.
As mentioned above, the step of casting comprises solidification of the melted
alloying elements in the mould 15. In one embodiment of the method of the
present
invention the cooling of the solidified precious metal alloy object resulting
from the
solidification of the molten alloying elements is made in a controlled
environment
such as an atmosphere of the process gas of predetermined composition in the
process chamber. The cooling may be performed e.g. within the process chamber
or
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in an adjacent chamber which can be entered from the process chamber without
exposing the mould to the ambient air.
In one embodiment of the method of the present invention the mould with the
solidified precious metal alloy object is quenched in an alcohol-containing
water bath
5 having a temperature of less than 5 C.
The bulk of the precious metal alloy object that has been manufactured
according
to a method in accordance with the present invention will have an oxygen
content
of less than 5 g/g, preferably less than 3 g/g and more preferably less than 1
g/g.
In addition, the bulk of the precious metal alloy object that has been
manufactured
10 according to the method of the present invention will have a hydrogen
content of
less than 0.05 g/g, preferably less than 0.01 g/g and more preferably less
than
0.005 g/g. The surface layer of the same precious metal alloy object will have
an
oxygen content of less than 30 g/g, preferably less than 20 g/g and more
preferably less than 10 g/g and a hydrogen content of less than 3 g/g,
preferably
less than 2pg/g and more preferably less than l g/g. The oxygen and hydrogen
content of the precious metal alloy object are important for their mechanical
properties, in particular if the cast precious metal alloy object is a raw
material that
is going to be worked by a goldsmith to form for example jewellery. High
hydrogen
content may, for example, give a hard and brittle alloy which is not easily
post-
processed by a goldsmith. This phenomenon is known in the field of metallurgy
as
hydrogen embrittlement. A method for testing the hydrogen and oxygen content
in
the surface layer comprises heating of the precious metal alloy object to a
temperature close to, but below, the melting temperature of the alloy and then
measuring the residual gases. At this temperature only gases originally
trapped in
the surface of the alloy object are released. The bulk values have been
obtained in
a similar way but by heating the alloy object to a temperature well above the
melting temperature so that gases originally trapped in the bulk of the alloy
object
are released.
In one embodiment of the present invention the precious metal alloy object
comprises at least 2% Ag. Examples of such precious metal alloys are 18 carat
gold,
14 carat gold, Sterling silver etc.
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Referring to Fig. 5, the advantageous properties of the precious metal alloy
object of
the present invention may be ruined by improper treatment of e.g. a goldsmith
in
his post-processing to form e.g. jewellery of the precious metal alloy object,
i.e. a
raw material, which has been manufactured in accordance with the method of the
present invention. Hence, in one embodiment of the present invention a process
chamber that is a dedicated workstation chamber for post-processing of a
precious
metal alloy in accordance with the method of the present invention is
provided. The
precious metal alloy is preferably manufactured according to the method of the
present invention, but this embodiment is not limited to this. In one
embodiment of
the present invention the workstation chamber is a glove box, i.e. a closed
chamber
having two gloves extending into the chamber.
Any kind of machining that normally is performed on precious metal alloys
objects
can benefit from being performed within the workstation chamber. In
particular, if
biocompatible precious metal alloy has been formed e.g. using the method of
the
present invention, the properties of that alloy can be maintained using this
workstation. Using conventional techniques there is an overwhelming risk that
the
advantageous properties are ruined. Examples of machining that can be
performed
are cold working, hot working, soldering, drawing, forging, polishing, etc.
In one embodiment of the invention the method further comprises the step of
soldering and/or welding of a precious metal alloy object, which preferably
has
been melted and cast according to the method of the present invention, in the
process gas of the process chamber or the dedicated workstation chamber. A
typical solder for soldering precious metal alloy objects of the present
invention is a
precious metal alloy itself. Preferably the solder is fabricated in the same
way as
the precious metal alloy object of the present invention in a process chamber
having a process gas of predetermined composition, i.e. having a water content
of
less than 0.005kg H2O per kg process gas and an oxygen content of less than
5%.
The method for manufacturing a biocompatible precious metal alloy object can
be
used to manufacture a solder alloy as well. A method for manufacturing a
solder
according to the present invention comprises the steps of providing a process
gas of
predetermined composition in a process chamber, the process gas having a water
content of less than 0.005 kg H2O per kg air and an oxygen content less than
5%
oxygen; melting solder elements; and casting the molten solder elements to
form the
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solder, by way of example in the form of a rod or a block, wherein the steps
of melting
and casting are carried out within the process chamber. Preferably the step of
providing further comprises the step of combusting oxygen of the process
chamber
using a flame that is supplied with a hydrocarbon-containing gas. By way of
example
the hydrocarbon-containing gas may be a mixture of oxygen and acetylene, i.e.
a
welding flame, wherein the oxygen/ acetylene ratio is adjusted to give a
reducing
flame. The combustion process lowers the oxygen content of the process
chamber.
Dehydration means may be used to limit the water content of the process gas.
In one
implementation of the method for manufacturing of a solder alloy the process
chamber comprises a crucible arranged on a mould. The solder elements are
provided in the crucible. Heating, for example by inductive heaters may be
used to
melt the alloying elements, which subsequently are supplied to the mould, by
way of
example through an opening in the bottom of the crucible. After solidification
of the
melt a solder alloy is formed in the mould. Optionally the step of providing
further
comprises the step of supplying a protective gas such as nitrogen, argon, etc.
to the
process chamber. This protective gas can be used as means for removing ambient
air
from the process chamber and also work as an inert gas during melting and
casting.
While the invention has been described in connection with what is presently
considered to be the most practical and preferred embodiments, it is to be
understood that the invention is not to be limited to the disclosed
embodiments, on
the contrary; it is intended to cover various modifications and equivalent
arrangements within the appended claims.
