Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Cu-Ni-Zn-Mn alloy
Field
[0001] The present invention generally relates to wrought Cu-Ni-Zn
(nickelsilver) alloys, more particularly to Cu-Ni-Zn-Mn alloys mainly for the
use in areas where machining operations are substantial.
Description of related art
[0002] With regard to the current market situation the trend goes from
ball point pens typically filled with oil-based inks of relatively high
viscosity
towards roller-ball pens with inks of lower viscosity. These new lower
viscosity inks are mainly water-based gel-inks. Compared to oil-based inks,
gel-inks have the advantage of allowing a greater variety of bright colors
and can have glitter effects, as they usually contain pigments that sink into
the paper. Driven by stylistic arguments and reducing the ink consumption
the trend in writing instruments goes towards finer pens, which can be
more easily be realized with low viscosity inks, in particular with roller-
ball
pens. Reducing pen tips to dimensions smaller than 1.6 mm diameters cause
stringent consequences with respect to the strength of the tip material. In
order that a tip can bear the same load with finer tip dimensions, higher
strength values of the alloy must be assured. Therefore, so far only stainless
steel have been used as tip material for the finest tips, while Cu-based
alloys are regarded as not being suitable due to their inferior strength.
Another common mistrust of Cu-Ni-Zn alloys compared to stainless steels is
the resistance against corrosion in water-based gel inks. The invented alloys
presented here aim to represent an alternative to stainless steel alloys used
in pen tips, which show mechanical properties (strength and ductility) as
good those of stainless steels and corrosion properties, which are suitable
for pen tip applications, where gel-based inks are used.
[0003] Introduction
The alloy family Cu-Zn-Ni originally imported from China in 17th century has
later in the 18th century been recognized almost parallel in France (1819),
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Germany (1823) and England (1832) as a copper-nickel-zinc alloy and given
the names "Maillechort" ¨ the later after their Lionese inventors Maille and
Chorier, "Neusilber" and "Nickelsilver". In recent times nickelsilver is
known for its good combination of properties and the silvery color has
promoted the alloy to be successfully used in various applications. Today
most commercially available Cu-Zn-Ni alloys contain between 10-25% Ni,
which due to its complete solubility in Cu increases not only the strength of
the alloy (by solid solution strengthening, see below) but also elastic
modulus and the corrosion resistance. On the other hand, Cu-Ni-Zn alloys of
grey color bear significant disadvantages, which are related to the effect of
'fire cracking' [H.W. Schlapfer, W. Form Metal Science 13 (1979); H.W.
Schlapfer, W. Form Metall, 32, 135 (1978)] that is related to the high
internal stresses in the pure mono-phased alpha alloys containing lead. The
term fire cracking describes a kind of liquid metal embrittlement, which
occurs in certain leaded alpha phase alloys, when cold deformed and
annealed, whereby an explosive intergranular fracture occurs during or
after the annealing process.
[0004] To circumvent this difficulty successive alloy development
progress led to the partial replacement of Ni with Mn, which allowed to
maintain the grey color, meanwhile changing the alloy from a pure alpha
alloy to a duplex like alpha/beta structure, which is not prone to fire
cracking as internal stresses are released at the phase boundaries. Mn has a
more limited solubility in Cu than Ni, but can be alloyed up to
approximately 15 wt.% in Cu-Zn alloys resulting similarly to Ni in a grey
color appearance of the alloy (e.g. see US5997663).
[0005] Nowadays generations of Cu-Ni-Zn-Mn alloys often contain about
10-25 wt.% Ni, and 3-7 wt.% Mn. The field of applications ranges from
writing instruments, to eye glass frames, keys, applications in watch
industry, fittings, fine tooling applications and several other areas, where
free-machining operations are frequent or inevitable resulting in large
quantities of waste material in form of chips (up to 50 cY0). Commonly lead
in quantities of 1.0 to 3.0 wt.% are alloyed to alloys, where free-machining
operations are required, which significantly improving their machinability.
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[0006] Lead-free alloys
Pressured by new legislations demanding for environmentally friendly and
nontoxic element additions the demand for lead-free products in particular
in applications of free-machining is constantly increasing. As a consequence
new solutions have to be found in order to secure the recycling path of Cu-
based alloys containing lead substituting elements
[0007] The most prominent current alternatives to Pb as a chip breaker
in free-machining Cu-alloys are: Bismuth, Silicon and Tellurium. Bismuth has
similar properties and behaviour with Cu-alloys as lead, i.e. low melting
point (Pb: 327 C, Bi: 271 C), miscible in the liquid and immiscible in the
solid, high density (Pb: 11.3 g/cm3, Bi: 9.78 g/cm3), a lubrication effect
during machining and so represents an excellent chip breaker as is Pb.
However, due to the incompatibility of Bismuth with certain Cu-based
alloys (high internal stress causing stress corrosion cracking) a replacement
of Pb with Bi in die-castings and wrought products is not recommended.
Alloys containing bismuth are also more difficult to recycle, because
recycling is
done unmixed and so far fully developed recycling does only exist for lead
containing copper alloys [Adaptation to Scientific and Technical Progress of
Annex
II Directive 2000/53/EC; J. Lohse, S. Zangl, R. Gra, C.O. Gensch, O. Deubzer.
oko-
Institut e.V. (2008)1. Bismuth is industrially rated as less toxic than lead
and
other neighboring heavy metals, however injection of large doses can cause
kidney damage. Furthermore, it is considered that its environmental impact
is small, due in part to the low solubility of its compounds
[http://en.wikipedia.org/wiki/Bismuth; Fowler, B.A.. "Bismuth" in Friberg, L..
Handbook on the Toxicology of Metals (2nd ed.). Elsevier Science
Publishers. (1986),117]. Nonetheless bismuth has found its way into brass
products as chip breaker mainly in Asia. Several patents describe the effect
of Bi as a chip breaker in free-machining wrought copper alloys
[U55167726; [P1790742].
[0008] Alternatively, silicon has been suggested, as an element addition
to favor chip breaking in brasses, but is due to the less-favorable chip form,
the absence of a self-lubricating effect causing higher wear damage on
tools and the associated difficulty to recycle such chips neither an easy
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choice for a free-machining Cu-based alloy. Furthermore the risk of Fe-Si
precipitates during casting of brasses containing low Fe concentrations
further reduces the machinability. Silicon containing free-machining
brasses, which show high strength levels than Si-free leaded versions of
free-machining brasses are nowadays available and are covered in large
parts by the patent family [EP1038981; EP1452613]. Apart from its effect on
machinability Silicon has the strongest influence in the Cu-Zn diagram to
shift the phase boundary between alpha and alpa+beta towards the beta
rich side (Guillet Zn equivalent of 10; see: [L. Guillet and A. Portevin,
Revue
de Metallurgie Memoirs XVII, Paris, (1920), 561]) and has a positive
influence on the strength, wear resistance and corrosion resistance.
[0009] Other known alternative Pb replacements in copper alloys are
based on additions of Tellurium, Calcium and Graphite acting in form of
intermetallics or particles as chip breakers [W02008/093974; W09113183].
Copper Tellurium alloys (C14500) contain 0.4-0.7 wt.% Te with minor
additions of P and Ag and the rest being Cu. They form CuTe-intermetallics
with a satisfactory chip breaking effect. Unfortunately, the alloy is not an
easy to manufacture alloy due to the high sensitivity of forming oxides
causing embrittlement. In addition, in brasses, Te forms brittle ZnTe
intermetallics as well results in unfavorable properties. Graphite containing
Cu-alloys are expensive due to high production cost via spray casting
technology. Little or no information is available on Ca-containing Cu-alloys
[W02008/093974], in particular with respect to Cu-Ni-Zn or Cu-Ni-Zn-Mn
alloys.
Summary
[0010] It is part of the aim of this invention to introduce new
microstructural design solutions for the alloy, which allows having, even in
the absence of lead as a chip breaker, good machinability performances in
free-machining operations. This can be solved on the one hand by adjusting
the microstructure related to its partitioning of the alpha/beta phases
and/or by additions of minor alloying elements forming precipitates with
one of the major alloying elements. The minor alloying elements foreseen
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for this task are Fe, Al, Ca, Sn, P, and Si. Although it is known, that duplex
structures, or precipitates favor chip breaking compared to a monophase
structure, our multi-path approach is new with regard to the field of
application as well as the alloy family of Cu-Ni-Zn-Mn alloys. First
5 mentioned approach in the invention relating to the fine needle like
precipitates of beta or beta" phase in alpha mother grains is conceptually
new and can be applied not only to this alloy family, but basically all Cu-Zn
alloys, where part of the microstructure is in a metastable condition with
respect to the phase transformation. The second approach of using
precipitation of supersaturated solutions is a well-known process to
increase the strength, but here it fulfills for this specific family of alloys
and
specific application two tasks: hardening and chip breaking and thus can be
considered as novel. Lastly adding Ca as chip breaker has so far not found
notation in combination with the fields of applications and the alloy family
considered mentioned here.
[0011] Conventionally, there are four different hardening mechanisms
known in single phased metals: Precipitation hardening, cold deformation
hardening, solid solution strengthening, and grain size strengthening (Hall-
Petch strengthing). Industrially mainly the first two mechanisms are of
importance. Precipitation hardening is typically used in low-alloyed Cu-
alloys where high electrical conductivity paired with moderate strength is
requested. Spinoidal decomposition can be regarded as a special variation
of precipitation hardening out of a supersaturated solid solution and finds
application in Cu-alloys mainly in alloys containing substantial amounts of
Sn or Ti. Cold deformation hardening is typically used for increasing the
strength in rods, profile and wire products independent of the type of
alloys. Solution hardening can be regarded as a side-effect when adding
additional elements for improving different properties of the alloys, but is
as such not of great relevance. Finally grain size hardening is industrially
and technically difficult to control and its hardening contribution becomes
evident only at grain sizes smaller than about 10 micrometers, sizes difficult
to achieve in industrial production.
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[0012] Similar to duplex steels, brasses or nickel-silver alloys having
a
certain range of Zn content exhibit a duplex alpha (face-centered cubic, fcc)
and beta (body-centered-cubic, bcc) structure, which apart from
representing a fifth mode of increasing strength, is also beneficially
influencing the machinability, grain size stability and hot workability.
Current commercially available leaded Cu-Ni-Zn-Mn alloys range in the Ni
content from 5 to 25 wt.%, a Mn 0-7 wt.%, Zn 25- 40 wt.% and rest Cu and
impurities typically < 1 wt.%. According to Guillet's rule [L. Guillet and A.
Portevin, Revue de Metallurgie Memoirs XVII, Paris, (1920), 561] Mn shows
with a factor of 0.5 only a slight influence towards the beta rich side in the
phase diagram, while Ni exhibits a factor of -1.2 keeping the phase diagram
on the alpha-rich side, and thus almost in balance for a Mn content of 6
wt.% and Ni content of 12 wt.%. Thus, as a first approximation the
complicated 4 component system Cu-Zn-Ni-Mn can in this case be treated as
the Cu-Zn binary phase diagram. However, as shown below for more
precise estimates on a multicornponent phase diagram, more advanced
thermodynamic software tools are required. With increasing Ni and Mn
content the strength increases. Typical tensile strength values for cold
drawn materials are 700 - 800 MPa, while in fewer cases values up to 900
MPa can be found for strongly cold drawn wires, however that typically
goes at the expense of ductility, so that tensile elongations are limited to
-1%.
[0013] In this application we aim to combine these mechanisms in a
novel family of Cu-Ni-Zn-Mn alloys in such a way, that high strength and
sufficient ductility can be achieved. Hereto the Zn, Al, Ca, Mn, Si, Ni, Sn,
Fe
content is adjusted to have a sufficiently high beta content at elevated
temperatures, which can be later reduced by thermo-mechanical heat
treatments for increasing the cold deformability, followed by a
precipitation hardening process, where on the one hand fine precipitates
of beta or beta" (tetragonal distorted bcc structure) are nucleating in alpha-
mother grains, while on the other hand intermetallic precipitates are
forming. This yields a strong increase in tensile strength higher than usually
reached in Cu-Ni-Zn-Mn alloys. In these classical compositions a trade of is
required between cold deformability, which increases the strength and the
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ductility which remains. Here however, the strengthing is only in part
resulting from cold deformation (increase in dislocation density and point
defects), but from the precipitation strengthening. Thus in final processing
steps only moderate deformation has to be applied, reaching much higher
strength values with still good plasticity. The following detailed description
of the invention addresses the above mentioned points in more detail.
[0014] Corrosion properties
Dezincification is understood as the dissolution of Zn in Cu-Zn alloys and
can be regarded as the most severe corrosion effect in Cu-alloys. More
precisely Zn dissolves by a di-vacancy diffusion process leaving a "hole" in
the crystal lattice of the surface layers [J.Y. Zou, D.H. Wang, W.C. Qiu,
Electrochmica Acta, 43, (1997), 1733-1737]. Thus, Cu-alloys free of Zn show
superior corrosion resistance than brasses. In analogy, alpha brasses are
more corrosion and dezincification resistant than the Zn-rich beta-brasses.
Cu-Ni-Zn alloys show in comparison to brasses similar corrosion resistance as
alpha brasses, but have due to the higher nickel content a better tarnish
resistance and resistance to stress corrosion cracking. Little information is
available on the corrosion properties and the influence of minor alloying
elements in Cu-Ni-Zn alloys, but can be extrapolated from the effects
known to brasses. There different alloying elements have been reported to
improve corrosion resistance and retard dezincification in brasses as
summarized in Ref. [D.D. Davies, "A note on the dezincification of brass
and the inhibiting effect of elemental additions", Copper Development
Association Inc., 260 Madison Avenue, New York, NY 10016, (1993), 7013-
0009]. Minor additions of arsenic, phosphorous or antimony are known to
show improved corrosion resistance in all-alpha brasses. Duplex brasses
where the beta phases are completely enclosed by alpha grains do exhibit
also a beneficial effect on the resistance to dezincification. Al-containing
alpha brasses are well known to show improved corrosion resistance
(Admiralty or Naval brasses) and even dezincification in duplex brasses was
reported to be retarded when adding up to 2 wt.% Al. The influence of tin
on the dezincification and corrosion of brasses is more ambiguous as it has
a positive effect in beta but a negative effect in alpha grains. However in
combination with Al additions an amount of up to 1 wt.% Sn has been
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reported to improve corrosion and dezincification resistance. Silicon
exhibits a positive effect when added below the level of precipitation of Si-
rich precipitates in alpha grains of brasses, which lies at around 0.5 wt.%.
Above this level of Silicon corrosion and dezincification increases as it does
for iron additions. Finally the influence of lead shows positive effects in
alpha brasses, but only if Pb-compounds are forming a passivation layer [S.
Kumar, T.S.N. Sankara Narayanan, A. Manimaran, M. Suresh Kumar, Mater.
Chem. & Phys.106, (2007), 134-141], while it shows reducing performance in
duplex brasses.
[0015] The present invention aims also for applications where corrosion
properties can be of crucial importance, in particular in solutions where
crevice conditions are present. This is for instance the case in ball pen tips
where the gap between the ball and the surrounding pen socket is of the
order of few micrometers distance and the ink is not constantly stirred
(during storage of the pen tip). In water-based gel-inks this may locally
lower the pH of the ink and cause local corrosion attack. The right choice of
elements and the appropriate microstructure to reduce corrosion is thus
detrimental to the lifetime of a pen tip.
[0016] More generally the invention also aims to increase the
dezincification and the corrosion resistance in mild and medium active
solutions to levels which are common to stainless steels, with the goal to
replace them in applications, where a combination of high strength, good
corrosion resistance and improved machinability are the main parameters
for the materials choice.
[0017] The invention relates to age hardenable high-strength Cu-Zn-Ni-
Mn-based alloys with superior mechanical properties and excellent
machinability suitable for applications, where intensive free-machining
operations are required as for example for the production of pen tips and
reservoirs for writing implants of reduced tip dimensions. However, the
range application goes beyond the production of writing instruments and
does in general extent to all applications where heavy free machining
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operations are required. The composition of the invented alloy is given as
follows:
Cu: 42 - 48 wt.%
Zn: 34 - 40 wt.%
Ni: 9 - 14 wt.%
Mn: 4 - 7 wt.%
Pb: 0 - 2.0 wt.%
Al: 0 - 1 wt.%
Sn: 0 - 2 wt.%
Fe: 0 - 0.5 wt.%
Si: 0 - 1.0 wt.%
Ca: 0 - 1.5 wt.%
As: 0 - 0.15 wt.%
P: 0 - 0.3 wt.%
[0018] The invention of the alloy aims to satisfy the current needs for
lead-free machinable Cu-Ni-Zn-Mn alloys suitable free-machining
operations as required for example in writing applications. In addition, the
invented alloys exhibit an attractive combination of high strength with
sufficient ductility required for subsequent operations or safety margins.
While the flow stress reaches values comparable to those of typical stainless
steels used for pen tip and other free-machining applications, sufficient
cold-formability is often still required in order to perform further bending
operations or other cold-deformation steps, such as the insertion of the pen
ball onto the tip socket. However, in contrary to stainless steels the
machinability of this alloy family is superior due to the precipitation
hardened phases. Additions of arsenic as well as minor additions of P, Si, Al
and Sn demonstrate beneficial effects on the corrosion resistance.
[0019] The copper alloy disclosed herein exhibits machinability
performance (easier chip handling, less tool consumption) superior to that
of stainless steel used in pen tip and also other applications allowing for a
higher production rate of parts per hour. When subjected to a special low-
temperature heat treatment, the alloy has a unique microstructure, which
even in absence of lead, is leading to a good machinability performance
superior to that of typical stainless steels used in pen tips. The alloy that
is
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an ecologically friendly, lead-free free-machining Cu-Ni-Zn-Mn alloy free of
harmful elements.
Brief Description of the Drawings
[0020] The invention will be better understood with the aid of the
5 description of an embodiment given by way of example and illustrated by
the figures, in which:
Figl shows an optical microscopy images of samples heat treated
at 350 C (Fig. la) and 450 C (Fig. lb) of alloy N 1;
Fig. 2 shows an optical image of the longer screw-like chips of
10 alloy N 1 produced with the Citizen long turning machine;
Fig. 3 shows an optical microscopy images of the as-cast structure
(Fig. 3a) and the cold deformed annealed (450 C) (Fig. 3b) of
alloy N 3;
Fig. 4 shows pseudo-binary phase diagram (Fig. 4a) and phase
fraction diagram for a specific composition (Fig. 4b) of alloy N 3.
Fig. 5 represents a screw-type and curly type chips shown for two
types of alloys of the alloy N 3;
Fig. 6 shows machining tests with Mikron Multistar made at 100
Hz on alloy N 3 with composition A annealed at 450 C (Fig. 6a
and 6b); and alloy N :1 (Fig. 6c and 6d), chip length of the leaded
alloy N :1 being smaller than that in alloy N 3;
Fig. 7 shows as-extruded microstructure (Fig. 7a) and after 2
cycles of cold deformation and annealing at 650 C) (Fig. 7b) of
alloy N 5; heat treated alloy at 540 C followed by 350 C (Fig. 7c)
and 400 C (Fig. 7d) low temperature heat treatment of alloy N 5;
and
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Fig. 8 shows optical microscopy image of a sample annealed at
540 C followed by a second annealing process at 400 C (Fig. 8a);
secondary electron microscopy image of alloy with NiSn
preciptates in beta phase matrix and at boundary to alpha grains
(Fig. 8b) both of alloy N : 6.
Detailed Description of possible embodiments of the Invention
[0021] The present invention generally relates to wrought Cu-Ni-Zn
(nickel-silver) alloys, more particularly to Cu-Ni-Zn-Mn alloys mainly for the
use in areas where machining operations are substantial. The present
invention relates also to leaded, leadless or lead-free free-machining Cu-
Ni-Zn-Mn alloys particularly suited for applications in areas where free
machining operations are heavily involved, such as writing instruments, eye
glass frames, medical tools, electrical connectors, locking systems, fine
tooling, fasteners and bearing for automotive industry, without restriction
to other fields of application. In addition, the present invention aims to
replace wrought steel products in various applications where high strength
and sufficient ductility combined with excellent free-machinability are
required with or without the presence of lead.
[0022] The present invention has among the above mentioned various
fields of applications particular focus on writing instruments, where the tip
material is in direct contact with the ink and the ball material. Nowadays a
number of ball materials, such as various types of tungsten carbide hard-
metal balls with different binders (Co, Co+Ni+Cr), different types of steels
and different types of ceramic balls are on the market, while the type of
inks can be separated into mainly gel-based and oil-based inks and to a
lesser extent inks based on other liquids. The Cu-Ni-Zn-Mn alloy family
presented here can be combined with all possible combinations of ball or
ink materials.
[0023] The objective of the present invention is to provide a new high-
strength Cu-Ni-Zn-Mn alloy family that thanks to a special thermo-
mechanical treatment and an optimized alloy composition reaches
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mechanical properties comparable with those of wrought stainless steel
alloys. The leaded variations exhibit excellent machinability and are thus
promising candidates for all applications, where high strength, good
ductility and excellent machinability are of utmost importance, i.e. writing
instruments, eye glass frames, keys, applications in watch industry, fittings
and other fine tooling and free-machining applications, without restricting
other fields of application. The lead-free variations persuade on the one
hand by their duplex alpha beta structure, and on the other hand by the
precipitates both result in a significant improvement of the machinability
with respect to untreated Pb-free Cu-Ni-Zn-Mn alloys. In addition, the lead-
free variations do not contain any user unfriendly amounts of elements,
which either may be harmful for human and/or environment.
[0024] The
present invention is realized by providing seven different Cu-
Ni-Zn-Mn alloys on a basis of copper, zinc, nickel, manganese and other
elements. The compositions of the alloys presented here and in the granted
patent family EP1608789B1 are optimized for special applications, where
apart from production costs the appearance of the alloy is as important as
the mechanical properties, machinability and corrosion properties.
Different dimensions and geometrical forms can be produced from these
alloys, such as wires, strips, rods, tubes and various profiles and square
shapes. In particular wire drawn products such as pen tips for writing
instruments are addressed, which after a hot deformation process are
typically drawn down to the final diameter in successive cold drawing and
heat treatment steps. In this respect the Mn content of the alloy is limited
to the range of 4 ¨ 7 wt.%. Higher levels of Mn show a negative effect
during cold-forming, while a lower Mn content increases the risk of fire
cracking and too low beta content during warm extrusion processes. Apart
from the high cost of Ni, a higher Ni content (>14 wt.%) is pushing the
phase diagram towards a purely mono-phase alloy even at elevated
temperatures. A lower Ni content (< 9.0 wt.%) bears the risk that the silvery
color turns gradually into a yellowish one and has to be increased in Cu
content in order to maintain the balance between alpha and beta phases.
In cases where steels are aimed to be substituted a silvery appearance of
the Cu-Ni-Zn-Mn alloys is of great importance. The Zn content is chosen in a
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range that allows to vary the microstructure (fraction of beta content) from
0% to approximately 50% 10%. Zn content > 40 wt.% show a to high
amount of beta suitable for cold drawing, while a lower content than 34
wt.% makes hot extrusion processing difficult. The content of Pb is kept at
a minimum level to assure good to excellent machinability. The copper
alloy is of grey or silver color / appearance typical for Cu-Ni-Zn-Mn alloys
sometimes having a nuance of a pale yellowish tone.
[0025] For the alloys presented in this invention a thermodynamic model
approach has been applied in order to have a better estimate on the phase
fields and the influence of alloying elements on the phase fields than it is
possible with the Guillet role of thumb used in brasses [J. Agren, F. H.
Hayes, L. Hoglund, U.R. Kattner, B. Legendre, R. Schmid-Fetzer: Applications
of Computational Thermodynamics. Z.Metallkunde 93, (2002), 128-142].
This is clearly a more refined approach than common alloy design
approaches and has demonstrated to be a fine tool to evaluate the stability
of each phase as a function of temperature.
[0026] The machinability of most of the alloys described below has been
measured on a Citizen long turning lathe and a Mikron Multistar turning
machine. The following machine parameters have been used: (see Table 1).
[0027] First alloy:
The first alloy is based on the granted patent EP1608789 applications and
consists of 42 - 48 wt.% Cu, 34 - 40 wt.% Zn, 9 - 14 wt.% Ni, 4 - 7 wt.%
Mn, <0.5 wt.% Fe, <0.03 wt.% P and <2.0 wt.% Pb.
[0028] Said granted patents mentioned above are based on the idea
that thanks to a special thermal heat treatment an alloy with an alpha-beta
structure stable at elevated temperature and thus suitable for hot
deformation processes can be modified into a pure alpha alloy when
annealed at temperatures between 630-720 C resulting in improved cold
formability and better corrosion resistance due to the mono-phased
structure. The associated chemical variations of the major elements are
balanced out in order to guarantee the said microstructural transformation
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from a duplex to a mono-phase alpha alloy. According to Guillet's rule of
thumb for the Zn equivalent in brasses, Mn is almost insensitive to the
variation, while Ni is showing an alpha stabilizing effect. Our
thermodynamic calculations in the multi-component system shows that for
minor elements such as Fe a content of 0.5 wt. % increases the beta phase
fraction of the alloy by about -5-10%, without changing the slope of the
curves, while at intermediate temperatures of about 400 C Fe provokes a
co-existence of the gamma phase (< 5 % volume fraction) in an alpha/beta
matrix. Phosphorous is added in order to increase the corrosion resistance.
Citizen
Parameter i 11 III
Turning speed ¨4000 ¨8000 ¨10000
[1/min]
Lengthwise depth 0.01 0.02 0.02
[mm]
Facing depth [mm] 0.01 0.01 0.01
Interpolation [mm] 0.001 0.001 0.001
Cutting speed 25 40 50
[m/min]
Mikron Multistar
Frequency [Hz] 85 95 100
Turning speed ¨16000 ¨18'000 ¨19'000
[1/min]
Parts per minute 120 140 120 140 120 140
Feed moderate high Low- moderate Low-
low
moderate moderate
Table 1: Machining test parameters used for the alloys included in the
present invention.
[0029] The first invention presented here builds-up on the processing
parameters used for the above mentioned granted patents, i.e. EP16087891
which allow the formation of a mono-phase alpha Cu-Ni-Zn-Mn alloy. Its
primary aim was to develop an alloy suitable for pen tip applications,
where the corrosion resistance is superior with respect to duplex phased
Cu-Ni-Zn-Mn alloys. This can only be guaranteed in purely mono-phase
state not allowing for microstructural conditions allowing galvanic
corrosion leading to localized microstructural determined crevice
conditions.
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[0030] Compared to aforementioned alloys developed in the patent
family EP1608789, the alloy presented here is in addition subjected to heat
treatments at lower temperatures of 300-450 C (also called "low
temperature heat treatment" below) allowing for a fine precipitation of
5 beta and/or beta' precipitates. This precipitates are showing a needle
like
morphology and are oriented along the primary crystallographic axis of the
fcc mother grains. Figs. la and lb shows micrographs with the low
temperature heat treated alloys having fine precipitates of beta' and beta,
respectively. Note that the phase boundary between beta and beta' (its
10 tetragonal distorted variation) lies between 400 and 450 C. More
particularly, Figs. la and lb shows samples heat treated at 350 C (a) and
450 C (b) of alloy N 1.
[0031] It must be mentioned that the concept of low temperature heat
treatments is commonly applied to Cu-alloys that are age hardenable, i.e.
15 where a supersaturated solid solution is present with minor additions of
elements. Here on the other hand not a chemical driving force for
precipitation is used, but the energy difference between the beta and beta'
phase. This is often applied to steels, where a martensitic transformation
causes an increase in the strength of the alloy. In this invention, this
concept has been adopted, whereby the transformation cannot be induced
by plastic deformation.
[0032] In order to determine the precise temperature range of heat
treatments a special thermodynamic software tool has been applied, which
allows calculating the phase stability fields in a multi-component system as
a function of temperature and chemical composition [J. Agren, F. H. Hayes,
L. Hoglund, U.R. Kattner, B. Legendre, R. Schmid-Fetzer: Applications of
Computational Thermodynamics. Z.Metallkunde 93, (2002), 128-142].
[0033] Said alloy results in improved hardness and tensile strength of
850 ¨ 950 MPa with remaining elongation levels of 2-10 % compared to the
same alloy not subjected to the low temperature heat treatment (see Table
2). Even higher strength and ductility might be reachable by further
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optimization of thermo-mechanical treatment as it was done for unleaded
alloys (see further down).
[0034] The machinability of said alloy exhibits thanks to the higher
strength, the uniform partition of lead particles and the fine beta
precipitates an excellent machinability (> -90% with respect to CuZn39Pb3
= 100%), which makes it an interesting candidate for replacing stainless
steels in pen tip applications. Most often chips were very short (< lmm
length) in particular when machining with the Mikron Multistar (at all
conditions set in Table 1). But also favorable screw-shape chips.
[0035] Fig. 2 shows an optical image of the longer screw-like chips of
alloy N 1 produced with the Citizen long turning machine.
[0036] Second alloy
The second alloy of the present invention has a very similar chemical
composition as the first mentioned alloy, however including arsenic, i.e. of
42 - 48 wt.% Cu, 34 - 40 wt.% Zn, 9- 14 wt.% Ni, 4 - 7 wt.% Mn, <0.5
wt.% Fe, <0.03 wt.% P, <2.0 wt.% Pb and 0.01 - 0.15 wt.% As.
[0037] The second invention presented here builds-up on the processing
parameters used for the above mentioned granted patents, i.e. EP1608789,
which allow the formation of a mono-phase alpha Cu-Ni-Zn-Mn alloy.
[0038] Apart from Arsenic, the same influences on the variations of
chemistry are present in this invented alloy as in the first alloy presented
above.
[0039] As mentioned in the background to the invention, As is used in
brasses as a corrosion inhibitor, which due its fast diffusion in alpha
brasses
migrates to the di-vacancies and inhibits further corrosion of the surface
layer [J.Y. Zou, D.H. Wang, W.C. Qiu, Electrochmica Acta, 43, (1997), 1733-
1737]. In the Cu-Ni-Zn-Mn alloy presented here, the presence of As also
improves the corrosion resistance, which shows in aqueous solutions with <
1 wt. % NaCI and in water-based inks an increased corrosion potential and
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a lower corrosion rate as compared to the alloy without the addition of As.
This in turn has also a positive effect on the ink, as fewer ions are taken up
by the ink, which might lower their performance.
Annealing temperature [ C] Vickers
hardness [Hv5]
6h 1h 5h 10h 24h
Initial (650) 240
239
350 249 265 272 265
350 253 260 268 265
400 244 253 239
232
400 241 254 239 234
Tensile tests
Yield Ultimate Total
strength tensile elongation
strength
[MPa] [A]
650 C, 2h ; 0 2.3 mm 177 453 47
450 C, 6h ; 0 2.3 mm 537 730 14
350 C, 6h ; 0 2.3 mm 738 815 7.3
450 C, 6h ; 0 1.6 mm 663-681 858-877 7-9
450 C, 6h + 350 C, 6h ; 0 1.6 mm 734 941 4
Table 2: Vickers hardness tests on samples annealed at 350 and 400 C for 1,
5, 10 and 24 hours compared to normal annealing temperatures for
recrystallisation.
[0040] The low level As additions does not exhibit any difference in the
microstructural appearance of the alloy and it exhibits the same mechanical
properties and machinability performance as the version without As (First
alloy).
[0041] Third alloy
The third alloy of the present invention is unleaded and contains the
following chemical composition: 45 - 48 wt.% Cu, 37 - 40 wt.% Zn, 9 - 14
wt.% Ni, 4- 7 wt.% Mn, <0.5 wt.% Fe, <0.03 wt.% P, <0.15 wt.% As and
<0.1 wt.% Pb.
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[0042] One aim of the present alloy invention was to increase the beta
content of the microstructure to a level, which shows good machinability
suitable for turning operations. This is realized by an increased Zn content
as compared to the alloy composition of the first and second alloy of the
present invention. Fig. 3a shows the as-extruded microstructure of the
duplex phased alloy.
[0043] A second goal of the invention of this alloy was to increase the
mechanical properties of the alloy by low temperature heat treatment
steps during wire cold deformation. Fig. 3b shows the microstructure of
such a cold deformed and annealed microstructure, where a heat
treatment of 450 C has been applied.
[0044] Zn content below 37.5 % reduces the amount of beta during hot
extrusion (-800 C) to a volume fraction close to zero percent, while with a
content of Zn > 39% the beta phase fraction reaches about 30% at this
temperature. However at lower temperature annealing its content
increases to almost 50% and thus reduces the ability to strongly cold
deform the material. Increasing the Mn content and reducing the Ni
content at the same Cu : Zn ratio increases stability of the beta phase at
high temperatures suitable for hot extrusion, which can be reversed at
intermediate annealing temperatures (-600 C). More particularly, an
optical microscopy images of the as-cast structure is shown in Fig. 3a and
the cold deformed annealed (450 C) is shown if Fig. 3b for alloy N 3.
[0045] As described in the aforementioned invention of the first alloy
the same low temperature heat treatment has been applied. According to
the thermodynamic calculations shown in Figs. 4a and 4b the face-centered
cubic (fcc) structure (alpha) is solidifying first followed by a body-centered-
cubic phase (beta). At about 420 C the beta phase is partially transforming
into a beta prime phase (b), which is in accord with the microstructural
observations of low temperature heat treatments (see Fig. 1 and Fig. 3b).
The phase MnNi phase displayed in Figure 4 could not be manifested in the
microstructure, due to too low reaction kinetics. The same is the case for
low volume fraction phases thermodynamically stable at low temperatures,
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but due to low reaction kinetics not appearing. More particularly, Figs. 4a
and 4b show pseudo-binary phase diagram (a) and phase fraction diagram
for a specific composition (b) of alloy N 3.
[0046] Said microstructure has been achieved with a Zn content of 38
and 39 wt.%. Lower Zn content lowers the amount of beta phase
significantly, while Zn larger than 40 wt.% are showing a too low density
of alpha grains.
[0047] Mechanical strength of this alloy reaches values between 850 ¨
1050 MPa and tensile elongations of 2 ¨ 20 %. Such high strength values
combined with good tensile elongations have not been reported so far to
the knowledge of the inventors. One main key ingredient in achieving an
optimum combination between strength and ductility is to perform two
cycles of a low temperature heat treatment after significant cold
deformation. This cyclic heat treatment allows for a maximum driving force
to precipitate fine beta needles, at the expense of decreasing the
dislocation density, which allows for further cold deformation. Meanwhile
recrystallization and grain growth of alpha grains is kept to a minimum so
that softening effects are avoided.
Condition Tensile tests
Yield Ultimate Total
strength tensile elongation
strength
[MPa]
Composition A; 450 C, 6h ; 0 2.3 mm 640 815 19
Composition A; 450 C, 6h ; + 350 C, 2h 809 904 18.9
; 0 2.3 mm
Composition A; 450 C, 6h ; 0 1.6 mm 687-702 891-898 12
Composition A; 450 C, 6h + 350 C, 2h ; 724-809 848-904 8-19
0 1.6 mm
Composition B ; 350 C, 6h ; 0 1.6 mm 815-835 1020-1040 1
Composition B ; 450 C, 6h + 350 C, 2h ; 895-929 1000-1016 2-4
0 1.6 mm
Table 3. Tensile test data for alloy N :3
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[0048] Fig. 5 shows screw-type and curly type chips shown for two types
of alloys of the alloy N 3.
[0049] Due to the uniform dispersion of softer and harder phases in the
microstructure a good machinability (> 70% with respect to CuZn39Pb3 =
5 100%) is reached. Chip length is significantly longer than in the leaded
alloys, however not affecting significantly the machining performance.
Note that the surface quality is significantly better compared to the surface
of the leaded alloy N :1 (see Figure 6).
[0050] Figs. 6a to 6d represent machining tests with Mikron Multistar
10 made at 100 Hz on alloy N 3 with composition A annealed at 450 C (Figs.
6a and 6b); and alloy N :1 (Fig. 6c and 6d). Chip length of the leaded alloy
N :1 is smaller than that in alloy N 3.
[0051] Forth alloy
The forth alloy of the present invention is also unleaded and contains the
15 following chemical composition: 45 - 48 wt.% Cu, 36 - 40 wt.% Zn, 9 - 14
wt.% Ni, 4 - 7 wt.% Mn, < 0.5 wt.% Fe, < 1.5 wt.% Ca, <1.0 wt.% Si, <1.0
wt.% Al, <0.03 wt.% P, <0.15 wt.% As and <0.1 wt.% Pb.
[0052] The main focus of this alloy was to introduce Ca into the
material
for it to act as a chip breaker when forming precipitates with Cu, Si, Al and
20 Fe. In absence of Fe, Al and Si additions of Ca forms precipitates with
Cu as
has been demonstrated in the patent application W02008/093974.
Additions of at least one of the other alloying elements Si, Al or Fe further
improve the machinability of this alloy.
[0053] The main difficulty with this type of alloy is the avoidance of
oxidation of Ca as it strongly reacts with oxygen. This can be avoided by
pre-alloying of Ca with Zn in inert atmosphere. Subsequent alloying with a
pre-alloy of Cu-Mn incl. the above mentioned amounts of Fe, Si, Al.
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[0054] Fifth alloy
The fifth alloy of the present invention can be unleaded and has the
following chemical composition: 43.5 - 48 wt.% Cu, 36 - 40 wt.% Zn, 9 - 12
wt.% Ni, 5 - 7 wt.% Mn, <1.0 wt.% Al, <0.5 wt.% Sn, <0.5 wt.% Fe, <0.03
wt.% P, < 0.15 wt.% As and <2.0 wt.% Pb.
[0055] The main focus of this alloy was to generate a variation of the
aforementioned unleaded Cu-Ni-Zn-Mn alloy (N : 3) that is on the one hand
age hardenable, i.e. forms secondary precipitates from a supersaturated
solid solution matrix and on the other hand is suitable for hot and cold
deformation, i.e. allows to be transformed from a duplex rich in beta
structure into a duplex structure poor in the beta phase fraction. This was
realized by including additions of Fe, Al and Sn.
[0056] Technically and economically speaking high beta phase fraction
in the alloy during extrusion is beneficial as it allows for lowering the
extrusion force and temperature. Subsequent cold drawing steps require
however a high volume fraction of alpha grains, which if the chemistry is
optimized can be achieved with dedicated heat treatment steps. This
metallurgic ally difficult task has been fulfilled satisfactorily by the
addition
of Al and Sn.
[0057] The as-extruded microstructure shows a very fine recrystallized
two-phased structure, with grain sizes well below 20 mm (Fig. 7a). Al acts in
this respect as effective grain growth inhibitor. Subsequent heat treatments
above 600 C exhibit some grain growth. Low temperature heat treatments
exhibit a peak hardening at 350 C with Vickers hardness values of > 250 HV
(see Table 4 and Figure 7).
[0058] Figs. 7a to 7d show as-extruded microstructure (Fig. 7a) and
after
2 cycles of cold deformation and annealing at 650 C) (Fig. 7b) of alloy N 5.
Heat treated alloy at 540 C followed by 350 C (Fig. 7c) and 400 C (Fig. 7d)
low temperature heat treatment of alloy N 5.
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[0059] Cycles of annealing (-600-700 C) and cold deformation
treatments cause an alteration in the microstructure with an increasing
content of the beta volume fraction to -50%, whereby the alpha grains
form the matrix surrounded by beta grains. When successively annealed at
lower temperatures <450 C fine precipitates in form of needles nucleate
(Fig. 7c and 7d).
[0060] According to thermodynamic simulations Ni-Aluminides are
formed right after having reached the solidus curve and maintain a
constant level of about 0.02 % and thus act as strong grain growth
inhibitors as mentioned before. In addition Al has a strong effect on the
variation of the beta fraction reaching a minimum value at around 600 C
which towards higher and lower temperatures is increasing.
[0061] The tensile properties of the alloy show values ranging from 850
- 900 MPa with elongations of 2-12 % (see Table 4).
[0062] Sixth alloy
The sixth alloy of the present invention is also age hardenable and has the
following chemical composition: 43.5 - 48 wt.% Cu, 36 - 40 wt.% Zn, 9 - 12
wt.% Ni, 5 - 7 wt.% Mn, <1.0 wt.% Al, <2.0 wt.% Sn, <0.5 wt.% Fe, Si <0.2
wt.%, <0.03 wt.% P, < 0.15 wt.% As and <2 wt.% Pb.
[0063] The main focus of this alloy was to evaluate the influence Sn in
the system, which has been added to provoke precipitation of NiSn phases.
[0064] A strong increase in the beta fraction with increasing Sn content
has been observed, which allows for very low extrusion temperatures
resulting in a high volume fraction of beta phase. Laboratory heat
treatment and drawing tests have shown that this volume fraction can be
decreased significantly allowing for subsequent good cold formability.
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Vickers hardness measurements for various combinations of heat treatments
500 / 4h 540 / 4h 560 / 4h 580 / 4h 600 / 4h 640 / 4h
175 171 171 157 157 146
500 / 4h 540 / 4h 560 / 4h 580 / 4h 600 / 4h 640 / 4h
300 / 8h 300 / 8h 300 / 8h 300 / 8h 300 / 8h 300 / 8h
232 232 227 227 241 234
500 / 4h 540 / 4h 560 / 4h 580 / 4h 600 / 4h 640 / 4h
350 / 8h 350 / 8h 350 / 8h 350 / 8h 350 / 8h 350 / 8h
252 244 237 206 221 234
500 / 4h 540 / 4h 560 / 4h 580 / 4h 600 / 4h 640 / 4h
400 / 8h 400 / 8h 400 / 8h 400 / 8h 400 / 8h 400 / 8h
223 206 221 214 225 212
Tensile tests:
Ultimate
Cold work Annealing Annealing Yield tensile Total
reduction temperature time strength strength elongation
[cY0] [ C] [h] [MPa] [MPa]
46 650 6 284 581 32.9
42.3 650 2 435 686 27.3
37.5 500 5 693 871 11.7
6 350 6 692 899 2.4
Table 4: Mechanical testing results of alloy N :5.
[0065] Low temperature age hardening tests have shown a maximum
hardening at -350 C. The scanning electron microscopy (SEM) image shown
in Fig.8 shows the material in the overaged condition heat treated at
400 C, where NiSn precipitates are visible as white dots in the beta phase
and localized to the phase boundary.
[0066] Figs. 8a and b show optical microscopy image of a sample
annealed at 540 C followed by a second annealing process at 400 C (Fig.
8a); Secondary electron microscopy image of alloy with NiSn preciptates in
beta phase matrix and at boundary to alpha grains (Fig. 8b) both of alloy
N : 6.
[0067] Vickers hardness measurements revealed a hardness of 230 - 240
HV for the age hardening at 350 C, while values between 220 - 230 HV
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were measured for heat treatments at 300 and 400 C comparable with
values given in Table 4 for alloy N :5, but slightly lower.
[0068] Seventh alloy
The seventh alloy of the present invention is also an age-hardenable alloy
and has the following chemical composition: 43.5 - 48 wt.% Cu, 36 - 40
wt.% Zn, 9 - 12 wt.% Ni, 5 - 7 wt.% Mn, <0.1 wt.% Al, <0.1 wt.% Sn, <0.5
wt.% Fe, <1.0 wt .% Si, <0.3 wt.% P, <0.15 wt.% As and <2.0 wt.% Pb.
[0069] Again as the alloy inventions N : 4 and 5, this invention aims
for
an age hardenable Cu-Ni-Zn-Mn alloy that apart from precipitations of
alpha in beta or vice versa also contains typical alloying elements suitable
for age hardenability. Here Silicon and Phosphorus are chosen as
candidates.
[0070] Silicon has the strongest effect of all alloying elements on the
alpha beta phase boundary in brasses and thus has to be added to the alloy
with great care. Thermodynamic simulations have shown that additions of
up to -0.5 wt.% are still tolerable with respect to the balance of alpha/beta
ratio (3:1, at 800 C), while a Si content of 1.0 wt.% reverses the fraction of
alpha/beta completely for a Zn content of 37 wt.%.
[0071] Similar as the Ni-Aluminides precipitates in the previous
mentioned alloy (N : 5) here, Ni5Si2 precipitates are formed right after
temperature has been lowered to below the solidus curve. However their
detection is a non-trival task and was not successful with the instruments at
hands. In low-alloyed copper the precipitates are nucleating and growing
to rounded platelets [D. Zhao, Q.M. Dong, B.X. Kang, J.L. Huang, Z.H. Jin,
Mater. Sci. Eng. A361, (2003). 93-99].
[0072] Additions of Phosphorous beyond the level used for de-oxidation
is common in copper alloys containing either Fe or Ni. Such alloys are
known for their excellent performance with respect to the combination of
high conductivity paired with high strength. Typically they form small 20 -
50 nm sized circular particles of Fe2P [M. Motohisa, J. Jpn. Copper Brass Res.
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Assoc. 29. (1990), 224-233; D.P. Lu, J. Wang, W.J. Zeng, Y. Liu, L.Lu, B.D.
Sun,
Mater. Sci. Eng. A421, (2006), 254-259] or hexagonal platelets, of NiP2
having sizes of 50-150 nm [J.S. Byun, J.H. Choi, D.N. Lee, Scripta Mater. 42,
(2000), 637-643].
5 [0073] The age hardened stage of these alloys show a high mechanical
resistance reaching hardness values beyond 250 HV and tensile strength
above 1000 MPa with tensile elongation of 1 ¨ 5 %.
