Note: Descriptions are shown in the official language in which they were submitted.
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COPPER-BORON MASTER ALLOY AND ITS USE IN MAKING SILVER-COPPER ALLOYS
FIELD OF THE INVENTION
This invention relates to master metal compositions adapted for alloying with
silver, to processes for making silver alloys using the master metal
compositions, and to
the optional further treatment of the alloys to make shaped articles and/or to
effect
precipitation hardening thereof.
SUMMARY OF THE INVENTION
The invention provides copper-based master alloys for alloying with silver,
said
master alloys containing germanium, boron and optionally other alloying
ingredients
including silver and/or zinc and/or silicon and/or indium.
The invention further provides substantially pure copper or a copper alloy
(e.g. a
Cu-Ge or Cu-Zn-Ge or Cu-Ge-Si or Cu-Ge-Zn-Si alloy) containing up to 2 wt%
boron
introduced into the copper by means of a compound that is decomposable in situ
in
molten copper to form boron. Said compounds may be selected from the group
consisting of alkyl boron compounds, boron hydrides, boron halides, boron-
containing
metal hydrides, boron-containing metal halides and mixtures thereof.
Decomposition in
situ is believed superior to current methods of making copper-boron master
alloys by
rapid melting together of copper and finely divided boron, which tends to give
rise to
boron hard spots. In some embodiments, usable master alloys are therefore
obtainable
which can impart greater boron content to the alloys in which they are
incorporated
while keeping development of hard spots to low acceptable levels. Boron
contents of
such alloys may be up to the 2 wt% level of currently available Cu-B alloys,
or may be
less where boron in the resulting precious metal alloy is being used as a
grain refiner.
Some embodiments provide Ag-Cu-Ge-B, Ag-Cu-B, Ag-Cu-B-Si or Ag-Cu-Ge-B-Si
containing silver in an amount sufficient to facilitate melting or casting of
the copper
e.g. 1-30 wt% Ag, typically 1-25 wt% Ag and more typically 10-25 wt% Ag.
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The invention relates in one embodiment to a master metal composition adapted
for alloying with silver to give a silver alloy containing at least 77 wt % Ag
and at least
0.5 wt% Ge, said master metal comprising Cu, Ge and 0.001-0.5, typically 0.005-
0.3
wt% boron together with any further ingredients for said alloy and any
impurities.
The invention further provides a process for making a silver alloy containing
at
least 77 wt% Ag, 1-7.2 wt% Cu, at least 0.5 wt% Ge and 0.005-0.3 wt% B
together with
any further ingredients for said silver alloy and any impurities, comprising
the step of
melting together fine silver and the master metal composition as aforesaid.
The invention provides in a yet further embodiment a process for making a
master alloy used in the manufacture of silver articles, which process
comprises melting
copper and optionally germanium or other alloying ingredients, and adding
boron to the
melt in the form of a compound selected from the group consisting of alkyl
boron
compounds, boron hydrides, boron halides, boron-containing metal hydrides,
boron-
containing metal halides and mixtures thereof. The present invention is
applicable e.g.
to the manufacture of master alloys e.g. Cu-Ge-B master alloys and Cu-B master
alloys.
Use of a master alloy provides a number of technical benefits. Boron is a very
light element that is easily lost in the melting process. If the boron level
in the alloy is
too high or the boron has not been dissolved properly the result is boron hard
spots,
which appear as drag marks in the surface of the silver when the piece is
polished.
However, more boron than is needed is routinely added to compensate for its
loss
during melting. What happens to the additional boron is at present unknown.
One
possibility is that it may react with oxygen present in the silver. Another
possibility is
that it may react with the material of a graphite crucible in which the alloy
is typically
melted. A third possibility is that it may diffuse towards the surface of the
melt and
become oxidized by any atmospheric oxygen present. However, the combination of
germanium and boron in the master alloy is believed to exhibit a protective
effect and
germanium may protect boron in the same manner that it protects copper.
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In some embodiments, order of addition of the alloying ingredients may be
significant. It is difficult to add the germanium first to a copper alloy and
then to add the
boron. The problem is that when using copper boride as the source of boron a
much
higher temperature is required to dissolve the boron into the alloy and the
germanium
content of the alloy may therefore be put at risk by overheating. The present
master
alloy is therefore normally made by melting together the highest melting
elements first
and progressively working through the lower melting temperature elements.
Alternatively, boron is added e.g. as a boron hydride or metal boron hydride,
which
decomposes in contact with the molten metal of the master alloy and disperses
boron
into the alloy with reduced opportunity for development of hard spots and the
like.
The invention further provides a method for casting a master alloy containing
at
least Cu and B including the steps of
(a) forming a precursor master melt containing at least Cu;
(b) dispersing throughout said master melt a compound selecting from the group
consisting of alkyl boron compounds, boron hydrides, boron halides, boron-
containing
metal hydrides, boron-containing metal halides and mixtures thereof; and
(c) allowing the melt to solidify.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Master alloys
The master alloy may comprise 80-95 wt % Cu (or of Cu together with further
ingredients for said alloy as set out below) and 20-5 wt % Ge. A preferrred
class of such
alloys comprises 80-86.7 wt % Cu (or of Cu together with further ingredients
for said
alloy) and 20-13.3 wt % Ge. A still more preferred class of alloys comprises
82-84.55
wt % Cu and any further ingredients for said alloy and 15.5-18 wt% Ge. Alloys
with
about 0.03 wt% B can give desirable boron contents in the silver alloys into
which they
are incorporated. A preferred class of the master alloys comprises only
copper,
germanium, boron and impurities.
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The master alloy may provide the whole of the copper required for the silver
alloy. Alternatively, the silver alloy may be made by melting together copper
and a
master alloy of the above defined genus.
Incorporation of boron into copper metal or copper master alloys
The master alloy precursor to which boron is added may be pure copper, Cu-Ge,
or Cu or Cu-Ge further comprising small amounts of casting adjuvants e.g. Si
or Ag to
facilitate casting and prevent development of surface cracking and porosity.
The copper
or alloy will normally be at a nominal temperature for casting or pouring e.g.
about
1150-1200 C. The melting temperature influences the kinetics of boron
evaporation
which determines the final boron concentration in the cast master alloy. The
selected
temperature should be sufficiently above the liquidus temperature of the alloy
to prevent
freezing in a die during continuous casting or freezing in a grain box during
grain
making. While the alloys are readily cast at atmospheric pressures, higher or
lower
pressures should not affect the benefits of the invention, but will affect the
kinetics of
boron evaporation. Furthermore higher boron content is desirable for master
alloys
which may be melted with precious metal to make casting grain and then further
melted
to form rod, wire, or investment casting.
In an embodiment, sufficient boron is added to the master alloy so that an
effective amount remains in the cast precious metal alloy or master alloy for
effective
grain refinement and deoxidation. Typically, the boron content is between 100
ppm and
1600 ppm for a master alloy, with a nominal boron content in the cast master
alloy of
about 250 ppm being more typical. Typically, from 0.01% to 0.16% of boron
added to
the precursor alloy melt is effective.
Boron is incorporated into the present master alloys for use, in the eventual
silver alloys, as an oxygen scavenger and/or as a grain refiner. It may be
added as a
metal boride e.g. copper boride. Alternatively it may be added e.g. to the
molten master
alloy e.g. Cu, Cu-Ge, Ag-Cu-Ge, Ag-Cu-Si or Ag-Cu-Ge-Si containing at least 50
wt%
Cu and optionally containing incidental ingredients by bubbling a gaseous
borane e.g.
diborane into the master alloy in admixture with a non-reactive gas such as
argon, by
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introducing into the master alloy a borane which is solid at ambient
temperatures e.g.
decaborane B1oH14 (m.p 100 C, b.p. 213 C), or by adding an alkylated borane
e.g.
triethylborane or tri-n-butyl borane, although the latter reagents are
spontaneously
combustible and require care in handling. Preferably, however, the boron is
added as a
5 metal borohydride, e.g. a borohydride of an alkali metal, a pseudo-alkali
metal or an
alkaline earth metal, e.g. lithium borohydride. Sodium borohydride is
especially
preferred because it is widely available commercially and can be obtained in
the form of
relatively large pellets which are convenient to handle during precious metal
melting
operations.
As previously explained, the boron compound may be introduced into molten
copper or copper alloy in the gas phase, advantageously in admixture with a
carrier gas
which assists in creating a stirring action in the molten copper or copper
alloy and
dispersing the boron content of the gas mixture into said alloy. Suitable
carrier gases
include, for example, hydrogen, nitrogen and argon. The gaseous boron compound
and
the carrier gas may be introduced from above into a vessel containing molten
copper or
copper alloy e.g. a crucible in a copper-melting furnace, a casting ladle or a
tundish
using a metallurgical lance which may be a elongated tubular body of
refractory
material e.g. graphite or may be a metal tube clad in refractory material and
is immersed
at its lower end in the molten copper or alloy. The lance is preferably of
sufficient
length to permit injection of the gaseous boron compound and carrier gas deep
into the
molten copper or copper alloy. Alternatively the boron-containing gas may be
introduced into the molten copper or copper alloy from the side or from below
e.g.
using a gas-permeable bubbling plug or a submerged injection nozzle. For
example,
Rautomead International of Dundee, Scotland manufacture horizontal continuous
casting machines in the RMK series for the continuous casting of semi-finished
products. The copper or alloy to be heated which may be is placed in a solid
graphite
crucible, protected by an inert gas atmosphere which may for example be oxygen-
free
nitrogen containing <5 ppm oxygen and <2 ppm moisture and may be heated by
electrical resistance heating using graphite blocks. Such furnaces have a
built-in facility
for bubbling inert gas through the melt.
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Addition of small quantities of thermally decomposable boron-containing gas to
the inert gas being bubbled through the melt readily provides from a desired
few ppm to
some hundreds or even thousands of ppm of boron into the molten metal or
alloy. The
introduction of the boron compound into the copper or copper alloy as a dilute
gas
stream over an period of time, the carrier gas of the gas stream serving to
stir the molten
copper or alloy, rather than in one or more relatively large quantities is
believed to be
favourable from the standpoint of avoiding development in the metal or alloy
of boron
hard spots, with the result that the resulting boron-containing alloy can
serve as a master
alloy for precious metal alloy manufacture with reduced development of hard
spots.
Compounds which may be introduced into molten copper or alloy thereof in this
way
include boron trifluoride, diborane or trimethylboron which are available in
pressurised
cylinders diluted with hydrogen, argon, nitrogen or helium, diborane being
preferred
because apart from the boron, the only other element is introduced into the
alloy is
hydrogen. A yet further possibility is to bubble carrier gas through the
molten copper or
alloy thereof to effect stirring thereof and to add a solid boron compound
e.g. NaBH4 or
NaBF4 into the fluidized gas stream as a finely divided powder which forms an
aerosol.
The boron compound may also be introduced into the molten copper or copper
alloy in the liquid phase, either as such or in an inert organic solvent.
Compounds which
may be introduced in this way include alkylboranes or alkoxy-alkyl boranes
such as
triethylborane, tripropylborane, tri-n-butylborane and methoxydiethylborane
which for
safe handling may be dissolved in hexane or THF. The liquid boron compound may
be
filled and sealed into containers of copper foil resembling a capsule or
sachet using
known liquid/capsule or liquid/sachet filling machinery and using a protective
atmosphere to give filled capsules sachets or other small containers typically
of capacity
0.5-5 ml, more typically about 1-1.5 ml. As an alternative the capsules or
sachets may
be of a polymer e.g. polyethylene or polypropylene. The filled capsules or
sachets in
appropriate number may then be plunged individually or as one or more groups
into the
molten copper or alloy thereof. A yet further possibility is to atomize the
liquid boron-
containing compound into a stream of carrier gas which is used to stir the
molten copper
or copper alloy as described above. The droplets may take the form of an
aerosol in the
carrier gas stream, or they may become vaporised therein.
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Also as previously explained, preferably the boron compound is introduced into
the molten copper or copper alloy in the solid phase, e.g. using a solid
borane e.g.
decaborane B10H14 (m.p. 100 C, b.p. 213 C). However, the boron is preferably
added
in the form of either a boron containing metal hydride or a boron containing
metal
fluoride or other halide. When a boron-containing metal hydride is used,
suitable
metals include sodium, lithium, potassium, calcium, zinc and mixtures thereof.
When a
boron-containing metal fluoride is used, sodium is the preferred metal. Most
preferred
is sodium borohydride, NaBH4 which has a molecular weight of 37.85, contains
28.75%
boron and can be obtained in the form of relatively large pellets which are
convenient to
handle during precious metal melting operations.
Boron may be lost as vapour from molten copper or copper alloy at elevated
temperatures and it may be necessary to make sequential additions of boron to
maintain
an adequate concentration for grain refining. To enable better mixing into the
copper or
copper alloy, the boron compound may be wrapped in a thin copper foil or thin
foil of
an inert material (i.e. a material which decomposes in the molten silver
substantially
without residue), such as paper or plastics sheet. Preferred metal for the
foil is copper,
but silver may also be used since it assists casting properties. The foil
preferably has a
thickness of from about 0.01 mm to about 0.3 mm to enable the foil-wrapped
boron
compound to be well submerged in the molten copper or alloy before the foil
melts
through releasing the boron compound. Once released, the constituents of the
boron
compound combine with oxygen in the melt to effectively deoxidize the melt and
the
boron is believed to react (although the effectiveness of the invention does
not depend
on the accuracy of this theory) with some of the elements in the melt to form
discrete
insoluble particles dispersed throughout the base material which act as
nucleation sites
promoting the formation of fine grains that are uniform in size and resist
growth.
When boron is added to molten metal e.g. as diborane, the compound
decomposes to boron and hydrogen e.g.
B2H6-> 2B(s) + 3H2 (g).
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The hydrogen is effective to deoxidize the melt
When sodium borohydride is first added to the molten metal, the initial
reaction
is believed to be decomposition of the boron containing grain refiner.
(1) NaBH4(s)->Na(g)+B(s)+2H2(g)
After decomposition, the sodium, hydrogen and boron are all effective to
deoxidize the
melt as follows:
(2) Na(g)+0.502(g) ->Na20(s)
(3) H2(g)+0.502(g) ->H20(g)
(4) B(s)+0.502(g)+0.5H2(g) ->HBO(g)
To achieve a uniform casting, the boron may be dispersed throughout the molten
metal
by stirring for in excess of 1 minute and typically for from 1-5 minutes.
Stirring may be
by any means which does not contaminate the molten metal such as with a
graphite
stirring rod.
The resulting master alloy is then cast by a method suitable for forming a
desired product. One such useful product is casting grains. Casting grains are
particles
that are sold to jewellery manufacturers who then investment cast the grains
of master
alloy with grains of precious metal to form a desired article of jewellery.
Subsequent to
stirring, molten master alloy is poured into a grain box which is a container
with
openings in the bottom, through which the liquid metal flows to make the
desired shape
and size of grains. The grain box may be made from materials similar to a
melting
crucible, such as, but not limited to, graphite, clay/graphite, ceramic and
silicon carbide.
The molten master alloy is formed into discreet droplets in the grain box as
it flows
through the openings and is then solidified into roughly spherical particles
in grain tank
containing water into which the master alloy droplets fall and solidify. The
master alloy
casting grain is then removed from the grain tank and dried e.g. by
centrifugal force and
hot air. The resulting roughly spherical grains have a typical diameter of
from about 0.1
mm to about 5 mm.
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Alloys that may be made from the present master alloys
The present master alloys may be used to make alloys of silver.
The present master alloys may be used to make silver/germanium alloys having
an Ag content of at least 75% by weight, a Ge content of between 0.5 and 3% by
weight, the remainder being copper apart from any incidental ingredients and
impurities, which alloy contains boron as a grain refiner. If desired, the
copper content
may also be substituted, in part, by one or more incidental ingredient
elements selected
from Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Mn, Ni, Pb, Pd, Pt, Si, Sn, Ti,
V, Y, Yb,
Zn and Zr, provided the effect of germanium in terms of providing firestain
and tarnish
resistance is not unduly affected. The weight ratio of germanium to incidental
ingredient
elements may be from 100: 0 to 80: 20, preferably from 100: 0 to 60: 40. The
term
"incidental ingredients" permits the ingredient to have ancillary
functionality within the
alloy e.g. to improve colour or as-moulded appearance, and includes amounts of
the
metals or metalloids Si, Zn, Sn or In appropriate for "deox".
The alloys that may be made according to the invention include coinage grade,
800-grade (including 830 and 850 grades and the like) and standard Sterling
silver and
an alloy of silver containing an amount of germanium that is effective to
reduce firestain
and/or tarnishing. The ternary Ag-Cu-Ge alloys and quaternary Ag-Cu-Zn-Ge or
Ag-
Cu-Ge-Si alloys that can suitably be made by the method of the present
invention are
those having a silver content of at least 80%, and most preferably at least
92.5%, by
weight of the alloy, up to a maximum of no more than 98%, preferably no more
than
97%. The germanium content of the Ag-Cu-(Zn)-Ge or Ag-Cu-(Si)-Ge alloys should
be
at least 0.1 wt%, preferably at least 0.5 wt%, more preferably at least 1.1
wt% The
germanium content is most preferably not more than 1.5%, by weight of the
alloy, more
preferably no more than 4 wt% up to a maximum of preferably no more than 6.5
wt%.
Silicon, in particular, may be added to silver alloys e.g. in an amount of up
to 0.5
wt %, typically 0.5-3 wt %, more usually 0.1-0.2 wt%, and is conveniently
provided in
the form of a copper-silicon master alloy containing e.g. about 10 wt% Si.
When
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incorporated e.g. into casting grain of a silver-copper-germanium ternary
alloy it can
provide investment castings that appear bright immediately on removal from the
mould.
It may be added to casting grain e.g. before investment casting or it may be
incorporated
into the silver at the time of first melting to form an alloy.
5
The remainder of ternary Ag-Cu-Ge alloys, apart from impurities, incidental
ingredients and any grain refiner, will be constituted by copper, which should
be present
in an amount of at least 0.5%, preferably at least 1%, more preferably at
least 2%, and
most preferably at least 4%, by weight of the final alloy. For an '800 grade'
ternary
10 silver alloy, for example, a copper content of 18.5% is suitable.
Appropriate levels of
copper are incorporated into the master alloy, copper usually comprising at
least 50 wt%
of said master alloy.
The remainder of quaternary Ag-Cu-Zn-Ge alloys, apart from impurities and any
grain refiner, will be constituted by copper which should be present in an
amount of at
least 0.5%, preferably at least 1%, more preferably at least 2%, and most
preferably at
least 4%, by weight of the alloy, and zinc which should be present in a ratio,
by weight,
to the copper of no more than 1: 1. Therefore, zinc is optionally present in
the silver-
copper alloys in an amount of from 0 to 100 % by weight of the copper content.
For an
'800 grade' quaternary silver alloy, for example, a copper content of 10.5%
and zinc
content of 8% is suitable. Where present, zinc may be incorporated into the
master
alloy.
In addition to silver, copper and germanium, and optionally zinc, the silver
alloys preferably contain a grain refiner to inhibit grain growth during
processing of the
alloy, and this grain refiner is added as part of the master alloy. Suitable
grain refiners
include boron, iridium, iron and nickel, with boron being particularly
preferred. The
grain refiner, preferably boron, may be present in the Ag-Cu-(Zn)-Ge or Ag-Cu-
(Si)-Ge
alloys in the range from 1 ppm to 100 ppm, preferably from 2 ppm to 50 ppm,
more
preferably from 4 ppm to 20 ppm, by weight of the alloy.
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In a preferred embodiment, the silver alloy is a ternary alloy consisting,
apart
from impurities and any grain refiner, of 80% to 96% silver, 0.1 % to 5%
germanium
and 1% to 19.9% copper, by weight of the silver alloy. In a more preferred
embodiment, the silver alloy is a ternary alloy consisting, apart from
impurities and
grain refiner, of 92.5% to 98% silver, 0.3% to 3% germanium and 1% to 7.2%
copper,
by weight of the alloy, together with 1 ppm to 40 ppm boron as grain refiner.
In a
further preferred embodiment, the silver alloy is a ternary alloy consisting,
apart from
impurities and grain refiner, of 92.5% to 96% silver, 0. 5% to 2% germanium,
and 1%
to 7% copper, by weight of the alloy, together with 1 ppm to 40 ppm boron as
grain
refiner. A particularly preferred ternary silver alloy being marketed under
the name
ArgentiumTM comprises 92.7-93.2 wt% Ag, 6.1-6.3 wt% Cu and about 1.2 wt% Ge.
Particular known silver alloys that may benefit from incorporation of boron as
Cu-B or Cu-Ge-B using the master alloys of the invention include the
following:
(i) US-A-3811876 (Harigawa et al., K. K. Suwa Seikosha, the disclosure of
which
is incorporated herein by reference) which discloses silver alloys in which
Sn, In and Zn
are disclosed as synergistically reducing tarnish. It describes and claims
alloys
consisting essentially of 4-10 wt% Sn, 0.5 - 12 wt% In, and 0.1 - 5 wt% Zn,
the
remainder being silver. It also alleges that mechanical strength and tarnish
resistance
may be further increased by adding Ti, Zr, Be, Cr, Si, Al, Ge and/or Sb which
protect
the surface of silver alloys by oxidizing preferentially and forming stable
oxides.
Amounts of such additional elements less than 0.001 wt% are ineffective. If
more than 1
wt% Ti, Zr, Be, Cr or Si is added, the alloy is said to become brittle and
insoluble
components are said to form that interfere with polishing. Additions of 0.001 -
5 wt%
Al, Ge and Sb are said to promote tarnish resistance without reducing
workability. The
alloy is stated not to suffer from firestain because of the absence of copper,
but is soft.
(ii) US-A-4973446 (Bernhard et al., United Precious Metal Refining, the
disclosure
of which is incorporated herein by reference) which discloses a silver alloy
composition
of the Sn, In, Zn type that also contains copper and boron. It comprises 89-
93.5 wt%
Ag, 0.01-2 wt% Si, about 0.001-2 wt% B, about 0.5-5 wt% Zn, about 0.5-6 wt%
Cu,
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about 0.25-2 wt% Sn, and about 0.01-1.25 wt% In. Silicon is added as a de-
oxidant.
Boron is added to reduce the surface tension of the molten alloy, and to allow
it to blend
homogeneously. Zinc is added to reduce the melting point of the alloy, to add
whiteness, to act as a copper substitute, to act as a deoxidant, and to
improve fluidity of
the alloy. Copper is added as a conventional hardening agent for silver, as
well as acting
as the main carrying agent for the other materials. Tin is added to improve
tarnish
resistance, and for its hardening effect. Indium is added as a grain-refining
agent, and to
improve the wetability of the alloy. Silver must be present in the necessary
minimal
percentage to qualify as either coin silver or sterling silver. Master alloys
for use in
creating the above silver alloy compositions are also disclosed and may
comprise 0.91-
30.77 wt% Si, 0.001-30.77 wt% B, 4.54-76.93 wt% Zn, 4.54-92.31 wt% Cu, 2.27-
30.77
wt% Sn, and 0.09-19.24 wt % In. A typical master alloy comprises about 25 wt%
Zn,
about 54 wt% Sn, about 0.75 wt% In, about 19.44 wt% Cu, about 0.135 wt% B, and
about 0.675 wt% Si. In the experience of the present inventors, although
tarnish
resistance is exhibited to some extent, together with some firestain reduction
on
investment casting, firestain resistance on soldering or annealing is not
obtained
because of the copper content. The disclosure if US-A-5039479 (Bernhard et al,
also
incorporated herein by reference) is similar.
(iii) GB-B-2255348 (Rateau, Albert and Johns; Metaleurop Recherche, the
disclosure of which is incorporated herein by reference) which discloses a
silver alloy
that maintains the properties of hardness and lustre inherent in Ag-Cu alloys
while
reducing problems resulting from the tendency of the copper content to
oxidise. The
alloys are ternary Ag-Cu-Ge alloys containing at least 92.5 wt% Ag, 0.5-3 wt%
Ge and
the balance, apart from impurities, copper. The alloys are stainless in
ambient air during
conventional production, transformation and fmishing operations, are easily
deformable
when cold, easily brazed and do not give rise to significant shrinkage on
casting. They
also exhibit superior ductility and tensile strength. Germanium exerts a
protective
function that is responsible for the advantageous combination of properties
exhibited by
the new alloys, and was in solid solution in both the silver and the copper
phases. The
microstructure of the alloy is constituted by two phases, a solid solution of
germanium
and copper in silver surrounded by a fllamentous solid solution of germanium
and silver
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and copper. The germanium in the copper-rich phase inhibits surface oxidation
of that
phase by forming a thin GeO and/or Ge02 protective coating that prevents the
appearance of firestain during brazing and flame annealing. Furthermore the
development of tarnish is appreciably delayed by the addition of germanium,
the surface
turning slightly yellow rather than black and tarnish products being easily
removed by
ordinary tap water. The alloy is useful inter alia in jewellery.
(iv) US-A-6168071 (Johns, the disclosure of which is incorporated herein by
reference) which describes and claims inter alia a silver/germanium alloy
having an Ag
content of at least 77% by weight, a Ge content of between 0.5 and 3% by
weight, the
remainder being copper apart from any impurities, which alloy contains boron
as a grain
refiner at a concentration of up to about 20 parts per million. If desired, an
alkyl boron
compound, boron hydride, boron halide, boron-containing metal hydride, boron-
containing metal halide and mixture thereof may be used to provide the boron
content
of the alloy instead of the disclosed CuB master alloy.
(v) US-A-6406664 (Diamond, the disclosure of which is incorporated herein by
reference) which discloses a silver alloy said to be firestain and tarnish
resistant and
comprising 92.5-96 wt% Ag, 0.1-0.38 wt% Ge, 0.5-3.8 wt% Sn, 0.001-0.008 wt% B,
0.001-0.1 wt% Ni, balance copper, the boron being used as a grain refiner, the
tin and
nickel content being said to permit the amount of germanium to be reduced. The
alloy is
said to be capable of being age hardened, soldered, welded, formed, cast and
mechanically worked. The product is stated not to shrink, to be non-porous,
and to
exhibit no fire scale as a result of processing involving elevated
temperatures.
(vi) US 6726877 (Eccles, the disclosure of which is incorporated herein by
reference) which discloses an allegedly fire scale resistant, work hardenable
jewellery
silver alloy composition comprising at least 86 wt% Ag, 0.5 - 7.5 wt% Cu, 0.07
- 6 wt%
of a mixture of Zn and Si wherein 0.02 - 2 wt % Si and 0.01 - 2.0 wt% Ge are
present.
The alloy may also include rheology modifying and other additives to aid in
improving
the castability and/or wetting performance of the molten alloy. For example,
about up to
3.5% by weight of a modifying additive selected from In, B or a mixture
thereof may be
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14
added to the alloy to provide grain refinement and/or provide greater
wettability of the
molten alloy. The compositions may be formed by the addition of a master alloy
to fine
silver, the master alloy comprising e.g. 52.5 - 99.85 wt % Cu, 0.1 - 35 wt %
Zn and 0.05
- 12.5 wt% Ge.
(vii) US 6841012 (Croce; Steridyne Laboratories, the disclosure of which is
incorporated herein by reference) which discloses an allegedly tarnish
resistant silver
alloy, comprising: at least about 85% by weight of silver, and a balance of
said alloy
including zinc, copper, indium tin and iron and optionally further comprising
at least
one of gold, silicon, manganese, boron, bismuth, cobalt, chromium and lead.
The
presence of zinc is said to add to the whiteness of the alloy. Copper is said
to act as a
conventional hardening agent and to add malleability. Indium is said to add
brilliance,
ductility, and facilitates casting of the alloy. Tin is said to add to the
hardness,
malleability, ductility, and solderability of the alloy. Iron is said to add
to the hardness
of the alloy. Boron is said to contribute to the elimination of fire scale.
(viii) US 6913657 (Ogasa, the disclosure of which is incorporated herein by
reference) which discloses alloys of a variety of precious metals. In one
embodiment it
discloses a hard precious metal alloy member consisting essentially of a
silver alloy, the
silver alloy having a silver content of not less than 80.0 wt %, and
containing
gadolinium in an amount of not less than 50 ppm but less than 15,000 ppm.
Boron in
amounts of 0.01-0.1 wt% is added to some of the alloys.
(ix) US-A-2004/0219055 (Croce, the disclosure of which is incorporated herein
by
reference) which discloses anti-tarnish silver alloys of the Zn, Cu, In, Sn
family the
alloy having at least 85 wt% Ag and the balance also including Fe. Boron is an
optional
ingredient.
Post-treatment of articles made using the master alloys
Ag-Cu-Ge silver alloy workpieces and shaped articles made from the above
master alloys and heated to an annealing temperature may be self-hardening on
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controlled air cooling, so that products of useful hardness can be obtained
without the
need for reheating to effect annealing and/or precipitation hardening. The use
of
reheating to e.g. 180-350 C, and preferably 250-300 C, to develop further
hardness is,
however, also possible according to the invention. Over-aging of Ag-Cu-Ge
silver
5 alloys during precipitation hardening does not cause a significant drop-off
of the
hardness achieved. Processing workpieces is possible, for example as part of
soldering
or annealing in a mesh belt conveyor furnace or in investment casting, reduces
the
number of process steps required to produce articles of a required hardness
and in
particular eliminates quenching e.g. with water which is required for Ag-Cu
Sterling
10 silver.
A surprising difference in properties exists between conventional Sterling
silver
alloys and other Ag-Cu binary alloys on the one hand and Ag-Cu-Ge silver
alloys on the
other hand, in which gradual cooling of the binary Sterling-type alloys
results in coarse
15 precipitates and little precipitation hardening, whereas gradual cooling of
Ag-Cu-Ge
alloys results in fine precipitates and useful precipitation hardening,
particularly where
the silver alloy contains an effective amount of grain refiner. Furthermore,
the addition
of germanium to sterling silver changes the thermal conductivity of the silver
alloy,
compared to standard sterling silver. The International Annealed Copper Scale
(IACS)
is a measure of conductivity in metals. On this scale the value of copper is
100%, pure
silver is 106%, and standard sterling silver 96%, while a sterling alloy
containing 1.1%
germanium has a conductivity of 56%. The significance of this is that the
Argentium
sterling and other germanium-containing silver alloys do not dissipate heat as
quickly as
standard sterling silver or their non-germanium-containing equivalents, a
piece will take
longer to cool, and precipitation hardening to a commercially useful level
(preferably to
Vickers hardness 110 or above, more preferably to 115 or above) can take place
during
natural air cooling or during slow controlled air cooling.
Thus after the master metal defined above has been incorporated into a silver
alloy starting with e.g. 999 or 9999 fine silver from a bullion manufacturer,
the resulting
alloy may be subjected to the further steps of annealing and/or brazing a
shaped article
of the alloy in a furnace, and hardening by subsequent air cooling. Thus the
alloy may
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be annealed and/or brazed by heating in a furnace at 600-680 C, preferably 600-
660 C
and more preferably 600-650 C. The annealing may be during investment casting,
and
hardening may be by air-cooling the investment or allowing it to air cool. The
final
product may be an article of jewellery or giftware.
The ability of the present silver alloys to precipitation harden enables the
copper
content of the alloy to be reduced. Even though an alloy of lower copper
content may be
relatively soft as cast, reheating at a low temperature e.g. 200-300 C may
bring the
hardness up to the level of normal sterling silver or better. This is a
significant
advantage because the copper content is actually the most detrimental part of
the alloy
from the standpoint of corrosion resistance, but in a standard sterling alloy
less copper
means unacceptably low hardness. If the copper content is reduced, the silver
content
may simply be increased which is a preferred option. Other possibilities
include
increasing the germanium content or adding zinc or another alloying element.
Silver
alloy of Ag 973 parts per thousand and containing about 1.0 wt% Ge, balance
copper,
has been successfully precipitation hardened by gradual air cooling from an
annealing
temperature, and it is believed that Ag-Cu-Ge alloys with silver content above
this level
are also precipitation hardenable. The copper in a master alloy may be
adjusted
according to the silver content.
The benefit of not having to quench to achieve the hardening affect is a major
advantage of silver alloys that can be made from the present master alloys.
There are
very few times in practical production that a silversmith can safely quench a
piece of
nearly finished work. The risk of distortion and damage to soldered joints
when
quenching from a high temperature would make the process not commercially
viable. In
fact standard sterling can also be precipitation hardened but only on
subsequent
quenching and this is one reason why precipitation hardening is not used for
sterling
silver.
How the invention may be put into effect will now be further described, by way
of illustration only, in the following Examples.
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Example 1
A master alloy is made by melting together 79 wt% Cu, 18 wt% Ge and 3 wt%
of a Cu/B alloy containing 2 wt% boron. The Cu is melted together with the
Cu/B
master alloy. High temperatures can be used because there are no other
elements to
damage. The temperature is then lowered and the germanium is added just above
the Ge
melting point. Melting is therefore in descending order of melting
temperatures i.e.
copper/copper-boron master alloy/germanium. The resulting master alloy
comprises,
apart from impurities, and with a 50% boron loss on melting, about 82 wt% Cu,
about
18 wt% Ge and about 0.03 wt% boron, together with any impurities.
There is then added 72g of the above master alloy and 928 g of 9999 purity
fine
silver which when melted together just above the melting point of the fine
silver (e.g. at
about 960-1200 C) with a 50% boron loss gives the desired
silver/copper/germanium
ternary alloy of composition about 92.8 wt% Ag, 5.90 wt% Cu, 1.30 wt% Ge and
about
11 ppm boron. The master alloy is weighed and placed in a crucible for melting
and the
fine silver is weighed and placed in the crucible, which is then heated to
melt the silver
and the master alloy under a protective cover of natural gas to prevent
unnecessary
oxidation. Silver has a known affinity for oxygen, which affmity increases
with
temperature. When exposed to air, molten silver will absorb about twenty-two
times its
volume of oxygen. Like silver, copper also has a great affinity for oxygen,
typically
forming copper oxide. Thus, in forming or re-melting sterling silver and other
silver-
copper alloys, care must be taken to prevent oxidation. When the mixture
becomes
molten, it may be stirred e.g. with a carbon rod and poured through a tundish
into water,
so that the silver becomes solidified into shot-like granules or pellets of
diameter about
3-6 mm which is the form in which sterling silver is typically sold.
The resulting alloy granules are used in investment casting using traditional
methods and is cast at a temperature of 950-980 C and at a flask temperature
of not
more than 676 C under a protective atmosphere. The investment material which
is of
relatively low thermal conductivity provides for slow cooling of the cast
pieces.
Investment casting with air-cooling for 15-25 minutes followed by quenching of
the
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investment flask in water after 15-25 minutes gives a cast piece having a
Vickers
hardness of about 70 which is approximately the same hardness as sterling
silver. The
products exhibit excellent tarnish and firestain resistance and have a fine
grain structure
due to their boron content. It has been found that a harder cast piece can be
produced
by allowing the flask to cool in air to room temperature, the piece when
removed from
the flask having a Vickers hardness of about 110. Contrary to experience with
Sterling
silver, where necessary, the hardness can be increased even further by
precipitation
hardening e.g. by placing castings or a whole tree in an oven set to about 300
C for 20-
45 minutes to give heat-treated castings of approaching 125 Vickers. The
germanium
content is towards the upper limit of that presently considered desirable in a
0.925 type
alloy.
As an alternative, the master alloy and fine silver in the form of granules
can be
mixed together in a crucible, and poured straight into the investment mould,
giving
similar results to those described above.
Example 2
The fine silver granules and the master alloy of Example 1 in the proportions
set
out in that example are formed into sheet by continuously casting at 1150-1200
C.
Pieces of the sheet are brazed together to form shaped articles by passage
through a
brazing furnace and are simultaneously annealed. Precipitation hardening
develops
without a quenching step by controlled gradual air-cooling in the downstream
cooling
region of the furnace. For this purpose, it is desirable that the material
should spend at
least about 8-30 minutes in the temperature range 200-300 C which is most
favourable
for precipitation hardening. Articles which have been brazed in a furnace in
this way
and gradually cooled can achieve hardness of 110-115 Vickers.
Example 3
A second master alloy is made by melting together 81.5 wt% Cu, 15.5 wt% Ge
and 3 wt% of a Cu/B alloy containing 2 wt% boron. The resulting master alloy
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comprises, apart from impurities, and with a 50% boron loss on melting about
84.5 wt%
Cu, about 15.5 wt% Ge and about 0.03 wt% boron, together with any impurities.
There is then added 72g of the second master alloy and 928 g of 9999 silver
which when melted together at about 960-1200 C with a 50% boron loss gives the
desired silver/copper/germanium ternary alloy of composition about 92.8 wt %
Ag, 6.08
wt % Cu, 1.12 wt % Ge and about 11 ppm boron. The subsequent performance of
the
alloy is similar to that of Example 1. The germanium content is towards the
lower limit
of that presently considered desirable in a 0.925 type alloy.
Example 4
A master alloy is made by melting together copper and germanium in the
proportions given in Example 1. The copper is melted by heating in a gas-fired
furnace
or an induction furnace to about 1150 under a charcoal melt cover which gives
a
reducing atmosphere. The germanium is added to the copper by wrapping pieces
of the
germanium in copper foil and plunging the wrapped germanium to the bottom of
the
melt using a graphite or plumbago stirring rod. When addition of the copper is
complete
the temperature is lowered to 1100 C, pellets of sodium borohydride to give
0.5 wt%
boron are wrapped in copper foil and are plunged to the bottom of the melt
using a
graphite or plumbago stirring rod as described above. The sodium borohydride
decomposes with evolution of hydrogen over a period of 1-2 minutes leaving
boron and
some sodium in the melt.
After boron addition, the crucible is pivoted to permit the molten alloy to be
poured into a tundish whose bottom is formed with very fine holes. The molten
alloy
pours into the tundish and runs through the holes in fine streams which break
into fine
pellets which fall into a stirred bath of water and become solidified and
cooled. The
cast pellets are removed from the bath and dried to give a master alloy as
casting grain.
The above master alloy can be used in the manufacture of Ag-Cu-Ge alloys
containing
boron as melt refiner e.g. using the procedures of the preceding Examples.
Dispersion
of boron into the master alloy using the borohydride is very effective, and
the resulting
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silver alloys can contain up to 20 ppm boron, or if desired above 20 ppm boron
without
development of hard spots.
In particular, the procedure of the Example may be used to manufacture Ag-Cu-
5 Ge casting grain for Sterling-type alloys containing about 40 ppm boron.
Boron loss on
re-melting reduces the boron content of the final cast alloy to 20 ppm or
below, which is
still an effective amount for grain refinement, and offers the possibility of
producing
cast, investment cast or other products having more consistent microstructure
and
properties.
Example 5
The procedure of Example 4 is repeated except that prior to addition of the
boron, silicon is added in an amount that will impart to the intended final
alloy 0.05 -
0.2 wt% Si as incidental ingredient.
Example 6
A master alloy is made by melting together 56 wt% Cu, 28 wt% Ag, 13 wt% Ge
and 3 wt% of a Cu/B alloy containing 2 wt% boron. The Cu (m.p. 1085 C) is
melted
together with the Cu/B master alloy. High temperatures can be used because
there are
no other elements to damage. The temperature is then lowered and the silver
(m.p.
962 C) is added followed by the germanium which is added just above the Ge
melting
point (m.p. 938 C). Melting is therefore in descending order of melting
temperatures i.e.
copper/copper-boron master alloy/silver/germanium. The resulting master alloy
comprises about 0.03 wt% boron.
There is then added 100g of the above master alloy and 900 g of 9999 purity
fine silver which when melted together just above the melting point of the
fine silver
(e.g. at about 960-1200 C) with a 50% boron loss gives the desired
silver/copper/germanium ternary alloy of composition similar to that in
Example 1.
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Addition of the master alloy to the fine silver is as described in Example 1,
and it is
formed as described in that example into alloy granules are used in investment
casting
as described in Example 1.
Example 7
A master alloy is made by melting together 59 wt% Cu, 28 wt% Ag and 13 wt%
Ge. Sodium borohydride is then introduced into the alloy as described in
Example 4 to
give a boron content of about 1000-1100 ppm. The master alloy is used to make
a
Sterling grade jewellery or silversmithing alloy as described in Example 7.
Example 8
In a modification of the procedure in Example 7, the sodium borohydride is
wrapped in silver foil and introduced into said master alloy.
