U.S. patent application number 13/267973 was filed with the patent office on 2012-03-29 for machinable copper-based alloy and method for producing the same.
This patent application is currently assigned to SWISSMETAL- UMS USINES METALLURGIQUES SUISSES SA. Invention is credited to Natanael Dewobroto, Doris Empl, Laurent Felberbaum, Vincent Laporte, Andreas Mortensen, Andreas Rossoll, Emmanuel Vincent.
Application Number | 20120073712 13/267973 |
Document ID | / |
Family ID | 42235291 |
Filed Date | 2012-03-29 |
United States Patent
Application |
20120073712 |
Kind Code |
A1 |
Dewobroto; Natanael ; et
al. |
March 29, 2012 |
MACHINABLE COPPER-BASED ALLOY AND METHOD FOR PRODUCING THE SAME
Abstract
Alloy containing between 1% and 20% by weight of Ni, between 1%
and 20% by weight of Sn, between 0.5%, 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
of P or B alone or in combination. The invention also pertains to a
metallic product having enhanced mechanical resistance at
intermediate temperatures (300.degree. C. to 700.degree. C.) and
excellent machinability. The metallic product of the invention can
be advantageously used for the fabrication of connectors,
electromechanical, or micromechanical pieces.
Inventors: |
Dewobroto; Natanael;
(Yverdon-les-Bains, CH) ; Empl; Doris; (Weiz,
AT) ; Felberbaum; Laurent; (Petit Lancy, CH) ;
Laporte; Vincent; (Maulaucene, FR) ; Mortensen;
Andreas; (Saint-Saphorin-sur-Morges, CH) ; Rossoll;
Andreas; (Lausanne, CH) ; Vincent; Emmanuel;
(St-Imier, CH) |
Assignee: |
SWISSMETAL- UMS USINES
METALLURGIQUES SUISSES SA
Dornach
CH
|
Family ID: |
42235291 |
Appl. No.: |
13/267973 |
Filed: |
October 7, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2009/054250 |
Apr 8, 2009 |
|
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13267973 |
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Current U.S.
Class: |
148/554 ;
148/433; 148/684; 420/471; 420/472; 420/473 |
Current CPC
Class: |
C22C 9/02 20130101; C22C
9/06 20130101; C22C 1/02 20130101; C22F 1/08 20130101 |
Class at
Publication: |
148/554 ;
148/684; 148/433; 420/471; 420/472; 420/473 |
International
Class: |
C22F 1/08 20060101
C22F001/08; C22C 9/06 20060101 C22C009/06 |
Claims
1. Alloy containing between 1% and 20% by weight of Ni, between 1%
and 20% by weight of Sn, between 0.5%, 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
of P or B, alone or in combination.
2. The alloy according to claim 1, wherein the alloy further
contains between 0.01% and 0.5% by weight of P or B alone or in
combination.
3. The alloy according to claim 1, wherein said alloy comprises 9%
by weight of Ni, 6% by weight of Sn, 1% by weight of Pb.
4. The alloy according to claim 3, wherein said alloy has a yield
strength R.sub.p 0.2 essentially above 180 MPa, measured at
400.degree. C. after heat treatment at 800.degree. C. for about one
hour, followed by a quench in water or in air.
5. The alloy according to claim 3, wherein said alloy has a maximum
stress R.sub.m essentially above 333 MPa, measured at 400.degree.
C. after heat treatment at 800.degree. C. for about one hour,
followed by a quench in water or in air.
6. The alloy according to claim 3, wherein said alloy has a Hv
hardness essentially above 190, measured after a heat treatment at
800.degree. C. for about one hour and subsequent aging at
320.degree. C. for about twelve hours.
7. The alloy according to claim 1, wherein said alloy comprises a
second phase containing Ni, Sn, and either B or P respectively,
after a heat treatment at 800.degree. C. for about one hour,
followed by a quench in water or in air.
8. Production method of a metallic product composed of an alloy
containing between 1% and 20% by weight of Ni, between 1% and 20%
by weight of Sn, between 0.5%, 3% by weight of Pb in Cu which
represents at least 50% by weight of the alloy; the alloy further
containing between 0.01% and 5% by weight of P or B, alone or in
combination; the method comprising the steps of: a) obtaining a
first slug of said alloy having a homogeneous structure; b)
annealing said alloy at a temperature comprised between 690.degree.
C. and 880.degree. C. for homogenizing and improving the alloy cold
forming properties; c) cooling at a cooling speed comprised between
50.degree. C./min and 50000.degree. C./min, depending on the
transversal dimension of said product and composition of said
alloy; and d) cold forming.
9. The method according to claim 8, wherein step a) of claim 8 is a
continuous casting process for extruding a first slug of said alloy
with a diameter comprised between 25 mm and 1 mm.
10. The method according to claim 8, wherein said alloy in first
slug is stirred electromagnetically or mechanically in order to
obtain said alloy with fine equiaxed crystals with average grain
size being essentially below 5 mm.
11. The method according to claim 8, wherein step a) of claim 8 is
a sprayforming process and wherein said first slug is formed with a
diameter up to 320 mm and an average grain size below 200
microns.
12. The method according to claim 8, wherein said cold forming step
comprises a rolling, wire-drawing, stretch-forming, hammering
process.
13. Metallic product obtained from a production method of a
metallic product composed of an alloy containing between 1% and 20%
by weight of Ni, between 1% and 20% by weight of Sn, between 0.5%,
3% by weight of Pb in Cu which represents at least 50% by weight of
the alloy; the alloy further containing between 0.01% and 5% by
weight of P or B, alone or in combination; the method comprising
the steps of: a) obtaining a first slug of said alloy having a
homogeneous structure; b) annealing said alloy at a temperature
comprised between 690.degree. C. and 880.degree. C. for
homogenizing and improving the alloy cold forming properties; c)
cooling at a cooling speed comprised between 50.degree. C./min and
50000.degree. C./min, depending on the transversal dimension of
said product and composition of said alloy; and d) cold forming;
wherein said metallic product has a tensile strength comprised
between 700-1500 MPa, measured at room temperature after the
annealing and cooling steps b) and c).
14. The product according to claim 13, wherein said product has a
Hv hardness comprised between 250 and 400, after the annealing and
cooling steps b) and c) of claim 13.
15. The product according to claim 13, wherein said product has a
machinability index greater than 70%, in relation to standard ASTM
C36000 brass.
16. The product according to claim 13, wherein the product has the
shape of a rod, wire, strips, slab, ingot, and sheet.
17. The product according to claim 13, wherein the product is used
for the fabrication of the whole or part of machined electrically
conductive pieces or mechanical or micromechanical pieces.
Description
FIELD OF THE INVENTION
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] The loss in ductility of Cu-based alloys at intermediate
temperature (300.degree. C.-700.degree. 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.
[0007] 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.degree. 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
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] The alloy of the invention is characterized by a yield
strength R.sub.p0.2 and a maximum stress R.sub.m essentially above
180 MPa and 333 MPa, respectively, measured at 400.degree. C. after
heat treatment at 800.degree. 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.degree. C. for about one hour and subsequent aging at
320.degree. C. for about twelve hours.
[0015] 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.degree. C. and 880.degree. C. for
homogenizing and improving the alloy cold forming properties;
cooling at a cooling speed comprised between 50.degree. C./min and
50000.degree. C./min, depending on the transversal dimension of
said product and composition of said alloy; and cold forming.
[0016] 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/mm.sup.2, a Hv hardness
comprised between 250 and 400, and a machinability index greater
than 70%, in relation to standard ASTM C36000 brass.
[0017] The machinable metallic product can be fabricated without
fissuring and has excellent mechanical and tensile properties at
intermediate temperature (300.degree. C.-700.degree. C.).
[0018] 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
[0019] 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:
[0020] FIG. 1 represents a metallographic section of a B-containing
Cu--Ni--Sn--Pb alloy according to the invention; and
[0021] 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
[0022] 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 remainder being constituted essentially of Cu, with the
unavoidable impurities being typically comprised in an amount of
500 ppm or less.
[0023] Lead being essentially insoluble in the other metals of the
alloy, the product obtained will comprise lead particles dispersed
in a Cu-Ni-Sn matrix. During machining operations, the lead has a
lubricating effect and facilitates the fragmentation of the
slivers.
[0024] 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 .degree.
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.
[0025] 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.
[0026] 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%.
[0027] In yet another embodiment of the invention, the Cu-based
alloy contains at least 0.01% by weight of an additional alloying
element chosen 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.
[0028] In a preferred embodiment of the invention, the Cu-based
alloy contains between 0.01% and 5% by weight of P or B.
[0029] 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.
[0030] 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: 30 kg) under a cover
of argon.
[0031] 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.
TABLE-US-00001 TABLE 1 Composition of alloys Ni Sn Pb Al Mn Zr B P
Fe Co A1 CuNi9Sn6 8.907 6.230 1.025 0.002 0.004 A2 CuNi9Sn6Pb1
9.231 6.083 0.009 0.004 B1 CuNi9Sn6Pb1 + 0.5 Al 8.810 6.104 0.997
0.515 0.002 0.005 B2 CuNi9Sn6Pb1 + 0.5 Mn 8.960 5.979 0.968 0.474
0.005 B3 CuNi9Sn6Pb1 + 0.25 Zr 8.917 6.300 0.995 0.002 0.25 0.008
0.005 B4 CuNi9Sn6Pb1 + 0.3 B 8.950 6.096 0.963 0.020 0.002 0.325
0.016 0.18 B5 CuNi9Sn6Pb1 + 0.5 P 8.915 6.259 0.997 0.002 0.478
0.004 C1 CuNi9Sn6Pb1 + 0.03 B 9.480 6.250 0.890 0.003 0.02 C2
CuNi9Sn6Pb1 + 0.1 P 9.170 6.300 0.920 0.027 0.075
[0032] The metallic products were cast into cylindrical bars, 12 mm
in diameter, and subsequently swaged in three steps down to a
diameter of 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.degree. C. for one hour
in air and quenched in water.
[0033] 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.degree. C. for 1
h.
[0034] 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 .mu.m 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.
[0035] Table 2 reports Vickers hardness (HV10) test values measured
for the alloys B1 to B5, after heat-treating at 800.degree. C. for
about one hour and subsequent aging at 320.degree. 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.
TABLE-US-00002 TABLE 2 Vickers hardness (HV10) in Hv 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
[0036] In Table 3, yield strength (R.sub.p 0.2) and maximum stress
(R.sub.m) values are reported for A1 to B5 alloy samples. The
values were obtained by performing hot tensile tests after heat
treatment at 800.degree. 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.degree. C. at a
strain rate of 10.sup.-2 s.sup.-1. 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.degree. C.
TABLE-US-00003 TABLE 3 Yield strength (R.sub.p 0.2) and maximum
stress (R.sub.m) in MPa A1 A2 B1 B2 B3 B4 B5 R.sub.p 0.2 [MPa] 229
161 -- 166 184 190 R.sub.m [MPa] 422 184 158 134 198 333 334
[0037] Lead added to CuNi9Sn6 alloy significantly embrittles the
alloy. Improved yield strength (R.sub.p 0.2,) and maximum stress
(R.sub.m) 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 (0.03 wt. %) and P (0.1 wt. %), respectively 160 MPa and around
300 MPa at 400.degree. C., were also improved compared to the
values of alloys A2 to B3 at that temperature.
[0038] 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.
[0039] Table 3 reports qualitatively the susceptibility to
quench-crack formation of alloys A2 to B5. In Table 3, the sign "+"
denotes the presence 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.degree.
C. for one hour and dropping the samples into a bath of water at
room temperature, or of oil held at 80.degree. C. or alternatively
at 180.degree. 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.
TABLE-US-00004 TABLE 3 water oil 80.degree. C. oil 180.degree. C.
A2 +++ ++ + B1 +++ + + B2 ++ + + B3 +++ + + B4 + 0 0 B5 + 0 0
[0040] 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 A1 to C2.
[0041] 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.degree. C.-700.degree. 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 B- 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 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 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 grain
refiners, and load-bearing by the less ductile second phase 1.
[0042] 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.
[0043] 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.
[0044] 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.degree. C. and 880.degree. C.
The annealing step, or 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.
[0045] In a variant of the embodiment, the second slug can undergo
an annealing or heat homogenizing treatment step prior to the cold
forming process.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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 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.
[0050] 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.degree. C./min
and 50000.degree. C./min.
[0051] 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.
[0052] 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.
[0053] 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 stirring or,
the ultrasonic stirring can be used in combination with mechanical
stirring.
[0054] 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.
[0055] Alternatively the metallic product can be obtained by a
static billet casting method or any other suitable method.
[0056] The Cu-based alloy product is characterized by a tensile
strength comprised between 700-1500 N/mm.sup.2 (700-1500 MPa),
measured at room temperature, after the annealing treatment and
cooling steps; a Vickers hardness (HV10) 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.
[0057] 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 piece, such as electrically
conductive pieces having, for example, a high elastic limit above
700 N/mm.sup.2, 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.
[0058] 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.
REFERENCE NUMBERS AND SYMBOLS
[0059] 1 second phase particle [0060] 2 Pb inclusions [0061]
R.sub.p 0.2 yield strength [0062] R.sub.m maximum stress
* * * * *