Note: Descriptions are shown in the official language in which they were submitted.
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Machinable copper-based alloy and method for producing the same
Field of the invention
The present invention concerns an alloy based on copper, nickel,
tin, lead and its production method. In particular, though not exclusively,
the present invention concerns an alloy based on copper, nickel, tin, lead
easily machined by turning, slicing or milling.
Description of related art
Alloys based on copper, nickel and tin are known and widely
used. They offer excellent mechanical properties and exhibit a strong
hardening during strain-hardening. Their mechanical properties are further
improved by known heat-aging treatments such as spinodal decomposition.
For an alloy containing, by weight, 15% of nickel and 8% of tin (standard
alloy ASTM C72900), the mechanical resistance can reach 1500 MPa. These
alloys also offer good stress relaxation resistance, and high corrosion
resistance in air.
Another advantage of these materials is their excellent
formability, combined with favorable elastic properties, brought by their
high yield stress. Moreover, these alloys offer a good resistance against
corrosion and an excellent resistance to heat relaxation. For this reason, Cu-
Ni-Sn springs do not lose their compression force with age, even under
vibrations and high heat or stress.
These favorable properties, combined with good thermal and
electrical conductivity, mean that these materials are widely used for
making highly reliable connectors for telecommunications and the car
industry. These alloys are also used in switches and electrical or
electromechanical devices or as supports of electronic components or for
making bearing friction surfaces subjected to high charges.
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Good machinability in these alloys is usually obtained by adding
lead, which is distributed as a fine dispersion of inclusions in the alloy
matrix. Unfortunately such lead additions also increase markedly the alloy's
warm shortness, which can lead to problems both in processing and in
service.
The loss in ductility of Cu-based alloys at intermediate
temperature (300 C-700 C) is a long-known problem and has been
reviewed by R. V. Foulger and E. Nicholls, in "Metals Technology" 3, pages
366-369 (1976), and by V. Laporte and A. Mortensen, in "International
Materials Reviews", in press (2009). The onset of grain boundary sliding in
this temperature range results in the formation of voids and cavities at
grain boundaries and changes the normally ductile fracture of copper and
its alloys to intergranular brittle failure. This phenomenon was observed for
pure copper but is much more pronounced when embrittling alloying or
impurity elements are present in the alloy. At higher temperatures,
exceeding this critical range, dynamic recrystallization can restore
ductility.
The presence of molten Pb inclusions in such Cu-alloys can cause
liquid metal embrittlement (LME), particularly at high strain rates. At the
same time lead contents as low as 18 ppm were reported to embrittle grain
boundaries of Cu-Ni alloys, and alloys that had been exposed to lead gas at
800 C have failed in a brittle manner, showing that lead can also cause
solid-state grain boundary embrittlement; this is, contrary to LME, more
severe at low strain rates. Other elements that are known to cause grain
boundary embrittlement in Cu-alloys are sulfur and oxygen.
Brief summary of the invention
An object of the invention is therefore to propose a metallic
product composed of a Cu-Ni-Sn-Pb-based alloy which overcomes at least
some limitations of the prior art.
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Another object of the invention is to provide a metallic product
composed of a Cu-Ni-Sn-Pb-based alloy with enhanced tensile properties
and having good machinability.
According to the invention, these objectives are achieved by
means of a system and method comprising the features of the independent
claims, preferred embodiments being indicated in the dependent claims
and in the description.
These aims are also achieved by means of an alloy containing
between 1 % and 20% by weight of Ni, between 1 % and 20% by weight of
Sn, between 0.5% and 3% by weight of Pb in Cu which represents at least
50% by weight of the alloy; characterized in that the alloy further contains
between 0.01 % and 5% by weight P or B, alone or in combination.
In an embodiment of the invention, the alloy further contains
between 0.01 % and 0.5% by weight of P or B alone or in combination.
In a preferred embodiment of the invention, the alloy comprises
9% by weight of Ni, 6% by weight of Sn, 1 % by weight of Pb.
The alloy of the invention is characterized by a yield strength
Rp0.2 and a maximum stress Rm essentially above 180 MPa and 333 MPa,
respectively, measured at 400 C after heat treatment at 800 C for about
one hour, followed by a quench in water or in air. The alloy is also
characterized by a Hv hardness essentially above 190, after a heat
treatment at 800 C for about one hour and subsequent aging at 320 C for
about twelve hours.
These aims are also achieved by a production method of a
metallic product composed of the alloy of the invention and comprising the
steps of: obtaining a first slug of said alloy having a homogeneous
structure; annealing said alloy at a temperature comprised between 690 C
and 880 C for homogenizing and improving the alloy cold forming
properties; cooling at a cooling speed comprised between 50 C/min and
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50000 C/min, depending on the transversal dimension of said product and
composition of said alloy; and cold forming.
The present invention also encompasses a metallic product
composed of the alloy of the invention and produced with the method of
the invention, the product being characterized by mechanical resistance
comprised between 700-1500 N/mm2, a Hv hardness comprised between
250 and 400, and a machinability index greater than 70 %, in relation to
standard ASTM C36000 brass.
The machinable metallic product can be fabricated without
fissuring and has excellent mechanical and tensile properties at
intermediate temperature (300 C-700 C).
In the present description of the invention, all % are expressed in
% by weight even if not explicitly mentioned in the text.
Brief Description of the Drawings
The present invention will be better understood by reading the
attached claims and the description given by way of example and
illustrated by the attached figures, in which:
Fig. 1 represents a metallographic section of a B-containing Cu-
Ni-Sn-Pb alloy according to the invention; and
Fig. 2 represents a metallographic section of a P-containing Cu-
Ni-Sn-Pb alloy according to the invention.
Detailed Description of possible embodiments of the Invention
In an embodiment of the invention, Cu-based alloys comprise
between 1 % and 20% by weight of Ni, between 1 % and 20% by weight of
Sn, and Pb in a ratio that can vary between 0.1 % and 4% by weight, the
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remainder being constituted essentially of Cu, with the unavoidable
impurities being typically comprised in an amount of 500 ppm or less.
Lead being essentially insoluble in the other metals of the alloy,
the product obtained will comprise lead particles dispersed in a Cu-Ni-Sn
5 matrix. During machining operations, the lead has a lubricating effect and
facilitates the fragmentation of the slivers.
The quantity of lead introduced in the alloy depends on the
degree of machinability that one strives to achieve. Generally, a quantity of
lead up to several percents by weight can be introduced without the alloy's
mechanical properties at normal temperature being modified. However,
above the lead melting point (327 C), the liquid lead strongly weakens the
alloy. Alloys containing lead are thus difficult to make, on the one hand
because they have a very strongly pronounced tendency towards fissuring
and, on the other hand, because they can exhibit a two-phased
crystallographic structure containing an undesirable weakening phase.
Consequently, in the alloy of the invention, lead content is preferably
between 0.5% and 3% or 0.5% and 2% by weight, even more preferably
between 0.5% and 1.5% by weight.
The alloy composition can optionally further comprise between
0.1 % and 1 % of an element such as Mn, introduced in the composition as
deoxidizer. The Cu alloy can also comprise other elements, such as Al, Mg,
Zr, Fe, or a combination of at least two of these elements, in place of Mn or
in addition to Mn. The presence of these elements can also improve the
spinodal hardening of the Cu alloy. Alternatively, devices preventing the Cu
alloy from oxidizing can be used.
In another embodiment, part of the Cu content of the alloy of
the present invention can be replaced by other elements, such as Fe or Zn,
at a ratio for example up to 10%.
In yet another embodiment of the invention, the Cu-based alloy
contains at least 0.01 % by weight of an additional alloying element chosen
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among Al, Mn, Zr, P (phosphorus) or B (boron). Alternatively, the Cu-based
alloy of the invention contains at least 0.01 % by weight of a mixture of at
least two additional elements chosen among Al, Mn, Zr, P or B.
In a preferred embodiment of the invention, the Cu-based alloy
contains between 0.01 % and 5% by weight of P or B.
In a more preferred embodiment of the invention, the Cu-based
alloy contains 9% by weight of Ni, 6% by weight of Sn, 1 % by weight of
Pb, and between 0.02% and 0.5% of P or B.
The influence of addition of P and/or B on the mechanical
properties at intermediate temperatures of Cu-Ni-Sn-Pb alloys was
investigated. To this end, metallic products composed of a Cu-based alloy
containing about: 9% by weight of Ni, 6% by weight of Sn, 1 % by weight
of Pb, and about between 0.02 and 0.5% of P or B, were prepared from
pure constituents (pre-alloys Cu3P and CuZr: 99.5% by weight, Al: 99.9% by
weight, all others: 99.99% by weight) in a semi-continuous casting unit
(capacity: 30kg) under a cover of argon.
The composition of the different alloys investigated, measured by
inductively coupled plasma (ICP) analysis, is given in Table 1, where the
compositions are reported in % by weight, and the balance is Cu. The value
of Zr was not detectable with the ICP method.
Ni Sn Pb Al Mn Zr B P Fe Co
Al CuNi9Sn6 8.907 6.230 1.025 0.002 0.004
A2 CuNi9Sn6Pbl 9.231 6.083 0.009 0.004
B1 CuNi9Sn6Pbl + 0.5 Al 8.810 6.104 0.997 0.515 0.002 0.005
B2 CuNi9Sn6Pbl + 0.5 Mn 8.960 5.979 0.968 0.474 0.005
B3 CuNi9Sn6Pbl + 0.25 Zr 8.917 6.300 0.995 0.002 0.25 0.008 0.005
B4 CuNi9Sn6Pbl + 0.3 B 8.950 6.096 0.963 0.020 0.002 0.325 0.016 0.18
B5 CuNi9Sn6Pbl + 0.5 P 8.915 6.259 0.997 0.002 0.478 0.004
C1 CuNi9Sn6Pbl + 0.03 B 9.480 6.250 0.890 0.003 0.02
C2 CuNi9Sn6Pbl + 0.1 P 9.170 6.300 0.920 0.027 0.075
Table 1 - Composition of alloys
The metallic products were cast into cylindrical bars, 12 mm in
diameter, and subsequently swaged in three steps down to a diameter of
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7.5 mm. From these bars cylindrical tensile test samples having a gauge
length of 30 mm and a diameter of 4 mm were machined. Samples were
homogenized at 800 C for one hour in air and quenched in water.
Alloys C1 and C2 were added to this list in order to examine
whether with a lower content of alloying additions the characteristics for
machinability and high strength can be reached as well. In contrast to alloys
denoted B, samples of alloys C1 and C2 were cooled in air after annealing
at 800 C for 1 h.
Figs. 1 and 2 represent SEM micrographs of a metallographic
section of the respectively B-containing (B4) and P-containing (B5) alloys,
according to the invention. Both alloys B4 and B5 show hard second phase
particles 1, rich in Ni, Sn, and either B or P respectively formed when B or P
is added to the Cu-based alloy. Hard second phase particles 1 rich in Ni, Sn,
and Zr are also formed (not shown) when Zr is added to the Cu-based alloy.
The second phase 1 is harder than the rest of the Cu-based alloy matrix.
Alloys B4 and B5 are also characterized by a grain size, here essentially 35
pm in average diameter, smaller by a factor near two than that in other
alloys not containing B or P. The alloys C1 and C2 with the lower B or P
content, respectively, also exhibit second phase particles 1 although in a
decreased amount (micrograph not shown). The second phase particles 1
are distributed evenly in the microstructure and are few micrometres in
size. Pb inclusions 2 appear in white in Figs. 1 and 2.
Table 2 reports Vickers hardness (HV10) test values measured for
the alloys 131 to B5, after heat-treating at 800 C for about one hour and
subsequent aging at 320 C for about 10 and for 12 h. The test values are
compared with values obtained for the alloy A2. The highest increase in
hardness was found for the alloys B4 and B5 according to the invention.
Time [h] A2 B1 B2 B3 B4 B5
0 98 105 99 102 114 114
10 177 137 161 179 167 190
12 160 138 160 177 188 208
Table 2 - Vickers hardness (HV10) in Hv
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In Table 3, yield strength (R,,,.,) and maximum stress (Rm) values
are reported for Al to B5 alloy samples. The values were obtained by
performing hot tensile tests after heat treatment at 800 C for about one
hour, followed by a quench in water or in air. Tensile tests were conducted
with a servo-hydraulic testing machine (MFL 100 kN) at 400 C at a strain
rate of 10-2 s'. The samples were heated rapidly using a lamp furnace
(Research Inc., Model 4068-12-10), reaching the stabilised testing
temperature within less than 2 min, so as to minimize the occurrence of
phase transformations during the heat-up period. Due to both rapid
heating and high strain rate, fracture of the samples was obtained after
not more than three minutes' hold at 400 C.
Al A2 B1 B2 B3 B4 B5
Rp 0.2 [MPa] 229 161 - 166 184 190
Rm [MPa] 422 184 158 134 198 333 334
Table 3 - Yield strength (Rp 0.2) and maximum stress (Rm) in MPa
Lead added to CuNi9Sn6 alloy significantly embrittles the alloy.
Improved yield strength (Rp 0.2) and maximum stress (Rm) values are obtained
for alloys B4 and B5 of the invention compared to the values obtained for
the other Pb-containing alloys A2 to B3 without P and/or B addition. Yield
strength and maximum stress values obtained for alloys C1 and C2 with
reduced amounts of B (0.03wt.%) and P (0.lwt.%), respectively 160 MPa
and around 300 MPa at 400 C, were also improved compared to the values
of alloys A2 to B3 at that temperature.
SEM investigations of longitudinal cuts of broken samples (not
shown) of alloys C1 and C2, after fracture in the hot tensile tests above,
showed that the second phase particles 1 are often situated adjacent to the
Pb inclusions 2 (see Figs. 1 and 2) and that failure is intergranular,
suggesting that fracture does not nucleate at the larger second phase
particles 1.
Table 3 reports qualitatively the susceptibility to quench-crack
formation of alloys A2 to B5. In Table 3, the sign "+" denotes the presence
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of cracks, with increasing number and depth going from "+" to "+++",
while "0" stands for the absence of any cracks. Quenching experiments
were performed on the as-cast alloy A2 to B5 samples by first heat treating
the samples at 800 C for one hour and dropping the samples into a bath of
water at room temperature, or of oil held at 80 C or alternatively at 180 C.
Alloy sample surfaces were afterwards examined optically for cracks. Table
3 shows that the alloys B4 and B5 according to the invention are the least
susceptible to quench-crack formation.
water oil 80 C oil 180 C
A2 +++ ++ +
B1 +++ + +
B2 ++ + +
B3 +++ + +
B4 + 0 0
B5 + 0 0
Table 3
The machinability characteristics of the alloys B4 to C2 according
to the invention, tested by drilling, accounting for cutting speed, feed and
chip length, were found to be similar to that of the other alloys not
containing P or B. Alloy B5 was found to have best machinability
characteristics compared to the other alloys of the group Al to C2.
The above results suggest that the hard second phase particles 1
do not represent preferred nucleation sites for intergranular voiding in the
alloy but rather impede grain boundary sliding, which is one of the
principal reasons for intermediate temperature (300 C - 700 C)
embrittlement in copper alloys, without nucleating voids. Moreover, in the
Zr, B- and P-containing alloys (B3, B4, B5, C1, C2) of the invention, Pb
inclusions 2 show a marked tendency to be situated adjacent to the solid 13-
or P-containing second phase precipitates 1, and have rather irregular,
complex shapes. This can result in low energy interfaces between molten
lead inclusions 2 and the hard second phase 1 at intermediate
temperatures, such that Pb "wets" the second phase particles 1. This
increases the applied stress necessary for the attainment of instability of
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molten Pb inclusions 3, delaying the fracture of the B- and P-containing
alloy making it both stronger and more ductile, and possibly yielding
improved tensile properties at intermediate temperatures. In other words,
the added elements, such as P, B or Zr, in the Cu-based alloy cause the
5 formation of the hard second phase 1 that presents, in contact with molten
Pb, a low interfacial energy, such as to stabilize the particles against shape
change under the application of stress. Higher tensile properties of B4 and
B5 in comparison to A2 and the remaining B-series alloys (Table 2) can also
be explained by the difference in grain size where both B and P acting as
10 grain refiners, and load-bearing by the less ductile second phase 1.
Clearly, the alloys B4, B5, C1 and C2 of the invention solve, to a
significant degree, the intermediate temperature embrittlement that is
caused by the addition of lead to improve the machinability of the
CuNi9Sn6 alloy. The leaded B3 to C2 alloys retain their attractive free-
machining attributes.
In an embodiment of the invention, a machinable metallic
product, composed of the Cu-based alloy of the invention, is obtained by a
method comprising a continuous or semi-continuous casting process. In the
method, a first slug is extruded, for example, to a diameter that can be
comprised typically between 25 mm to 1 mm. The alloy is then cooled, for
example, by a stream of compressed air or by water spray or any other
suitable means able to reach a suitable cooling speed that is preferably
sufficiently high to limit the formation of the fragilizing second phase and
fast enough in order to prevent fissuring, as will be discussed below.
The material of the first slug then undergoes one or several cold
forming operations, e.g. by rolling, wire-drawing, stretch-forming,
hammering, or any other cold deformation process. After the cold forming
step, a second slug is annealed, typically in a through-type furnace or
removable cover furnace, at an annealing temperature that must lie within
the range where the alloy is one-phased. In the case of the Cu alloy of the
invention having one of the compositions described above, the annealing
temperature is comprised between 690 C and 880 C. The annealing step, or
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heat homogenizing treatment step, is used, among other, to induce
ductility, refine the structure by making it homogeneous, and improve cold
forming properties of the alloy.
In a variant of the embodiment, the second slug can undergo an
annealing or heat homogenizing treatment step prior to the cold forming
process.
During the annealing step, at least partial recrystallization will
occur with the second slug, where new strain-free grains nucleate and
grow to replace those deformed by internal stresses. After the annealing
step the second slug is cooled, again, at a cooling speed that is preferably
sufficiently high to limit the formation of the fragilizing second phase and
fast enough in order to prevent fissuring.
One or several successive steps of cold forming process can be
performed, each cold forming step being followed by an annealing and
cooling step, in order to obtain successive slugs having desired diameters
and shapes.
After the successive cold forming, annealing and cooling steps, a
final slug can be wire-drawn or stretch-formed to a final diameter and/or
shape to obtain a machinable product. A spinodal decomposition heat
treatment, or hardening, can then be finally performed on the machinable
product or on the machined pieces in order to obtain optimal mechanical
properties. The latter heat treatment can take place before or after the
final machining.
The cooling step after the extrusion and/or annealing treatment
must occur at a speed sufficiently slow to prevent fissuring of the alloy due
to internal constraints generated by the temperature differences during
cooling, but sufficiently fast to limit the formation of a two-phased
structure. If the speed is too slow, a considerable quantity of second phase
can appear. This second phase is very fragile and greatly reduces the alloy's
deformability. The critical cooling speed required to avoid the formation of
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too large a quantity of second phase will depend on the alloy's chemistry
and is greater for a higher quantity of nickel and tin.
Moreover, during cooling, transitory internal constraints are
generated within the alloy. They are linked to temperature differences
between the surface and the center of the slug, or product. If these
constraints exceed the alloy's resistance, the latter will fissure and is no
longer usable. Internal constraints due to cooling are all the higher the
more the product's diameter is large. The critical cooling speeds to avoid
fissuring thus depend on the product's diameter. In the method of the
present invention, cooling, after the extrusion and/or annealing steps, is
performed at a cooling speed comprised between 50 C/min and
50000 C/min.
Copper-nickel-tin alloys have a long solidification interval leading
to a considerable segregation during the casting operation. During the
continuous or semi-continuous casting process, the molten alloy can be
stirred in order to obtain a greater regularity for the cast metal, in respect
to its surface state and its internal properties, such as segregation and
shrinkage. Moreover, when the molten alloy is melted and cast, a dendrite
structure is generated and a fine-grained alloy cannot be obtained.
The copper alloy can be stirred electromagnetically in order to
agitate the melt. Such magnetic forces are able to produce sufficient
stirring of the slug allowing for a reduction in the number of segregation
centers and obtaining the Cu-based alloy having fine equiaxed crystals with
average grain size being essentially below 5 mm.
Alternatively, the molten Cu alloy in the slug can be agitated
mechanically using ultrasonic energy in order to produce cavitation and
acoustic streaming within molten material. Other type of mechanical
stirring can also be used such as forced gas mixing, and physical mixing such
as oscillating or shaking the molten alloy, or mechanical devices such as a
rotor, a propeller, or a stirring pulsing jet. Alternatively, the
electromagnetic stirring can be used in combination with mechanical
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stirring or, the ultrasonic stirring can be used in combination with
mechanical stirring.
In another embodiment of the invention, first slugs of the Cu-
based alloy having a diameter up to 320 mm are produced using a
sprayforming process, such as the process known as the "Osprey" method
and described in patent EP0225732. Here, using atomized particle sizes in
the size range of 1-500 microns, alloy with an average grain size below 200
microns could be obtained. The sprayforming method makes it possible to
obtain an almost homogenous microstructure presenting a minimal degree
of segregation. Other types of slugs, such as ingot, disc or bar having a
rectangular section can also produced with the sprayforming process. The
spraying of the molten metal or metal alloy particles is performed under a
desired atmosphere, preferably under an inert atmosphere, such as
Nitrogen or Argon.
Alternatively the metallic product can be obtained by a static
billet casting method or any other suitable method.
The Cu-based alloy product is characterized by a tensile strength
comprised between 700-1500 N/mm2 (700-1500 MPa), measured at room
temperature, after the annealing treatment and cooling steps; a Vickers
hardness (HV1 0) comprised between 250 and 400, measured after the
annealing treatment and cooling steps; and a machinability index greater
than 70 %, in relation to standard ASTM C36000 brass. Moreover, the Cu-
based alloy product can be machined easily due to the facilitated
elimination of chips generated during turning and can be advantageously
used for machining operations requiring, in particular, a turning step, or a
free-cutting step, a stamping step, a bending step, a drilling step, etc.
The Cu-based alloy product of the invention can be
advantageously used in order to obtain a product having the shape of rods,
wires having circular or any other profile shape, strips, for example rolled
strips, slabs, ingots, sheets, etc. The Cu-based alloy product can also be
used
advantageously for the fabrication of the whole or part of a machined
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piece, such as electrically conductive pieces having, for example, a high
elastic limit above 700 N/mm2, such as connectors, electromechanical pieces,
parts in telephony, springs, etc., or micromechanical pieces in applications
such as micromechanics, horology, tribology, aeronautic, etc., or any other
pieces in diverse applications.
The method of the present invention makes it possible to
produce a machinable Cu-Ni-Sn-based products containing up to several
percent by weight of Pb and between 0.01 % and 0.5% of P and/or B,
without it fissuring during fabrication, and having excellent mechanical
and tensile properties.
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Reference Numbers and Symbols
1 second phase particle
2 Pb inclusions
Rp 0.2 yield strength
R,m maximum stress
