Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ALUMINIUM ALLOY FOR MAKING FIN STOCK MATERIAL
FIELD OF THE INVENTION
The invention relates to an aluminium alloy for making fin stock material. Fin
stock material is used in heat exchanger devices. The fin stock material is
used for
making for instance corrugated fins, by which the heat from the heat exchanger
must
be removed. Furthermore, the invention relates to fin stock material made from
the
aluminium alloy according to the invention, and to a brazed heat exchanger
having
to fins made of this alloy. Moreover, the invention relates to a method of
producing the
fin stock alloy and the brazed heat exchanger.
DESCRIPTION OF THE RELATED ART
In the prior art, aluminium alloys are used for fins in heat exchanger
applications
because of their desirable combination of strength, weight, thermal
conductivity,
brazeability, corrosion resistance and formability.
Heat exchangers from aluminium can be fabricated by stacking aluminium alloy
sheets, which have been formed to a desired configuration, to form fluid
passageways or tubes, and by securing aluminium alloy fins between the fluid
2o passageways by brazing. The aluminium alloy sheets used to make the fluid
passageways and/or the aluminium alloy used for the fins are provided with a
low
melting clad layer. The bonding between the alloy clad sheets and the fins is
achieved by melting the cladding or filler material of the sheets and/or fin
material.
As a brazing method, typically vacuum brazing or controlled atmosphere brazing
is
used. To improve the corrosion resistance of the fluid passageways, fin
materials are
used which are electrochemically anodic (less noble) relative to the fluid
passageways material, so that this fin material has a sacrificial anode
effect.
An example of an aluminium alloy for making fin material is given in
International patent application no. WO OI/36697. This alloy has the following
3o composition, in weight percent:
Si 0.7 - I.2
Mn 0.7 - 1.2
Mg up to 0.35
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Fe up to 0.8
Zn up to 3.0
Ni up to 1.5
Cu up to 0.5
Ti up to 0.20
In up to 0.20
Zr up to 0.25
V up to 0.25
Cr up to 0.25
others up to 0.05 each, and up to 0.15 in total.
Al balance.
This disclosed alloy is said to have an improved post-braze 0.2%-yield
strength (also
referred to as 0.2%-offset proof stress or 0.2% PS) over conventional alloys
for the
same application.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an aluminium alloy for
making
fin stock material which can be used for heat exchangers, and which has an
improved
thermal conductivity.
2o It is another object of the invention to provide such an aluminium alloy,
which
has a strength which is at least as good as the strength of conventional
aluminium
alloys for making fin stock material.
It is still another object of the invention to provide such an aluminium
alloy,
which has a corrosion potential which is at least as negative as the corrosion
potential
of conventional aluminium alloys for making fin stock material.
In one aspect of the invention one or more of these objects are reached with
an
aluminium alloy for malting fin stock material, having the composition in
weight
percent:
Si <_ 1.2
Mn <_ 0.05
Mg <_ 0.05
Fe _< 2.0
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0.5 _< Ni <_ 1.5
0.05 <_ Cu 51.0
0.5 <_ Zn <_ 4.0 and/or 0.1 <_ Sn _< 2.0 and/or 0.01 <_ In <_ 0.5
V <_ 0.40 and/or Ti <0.01 and/or Cr <0.01 andlor Zr <0.01
other elements up to 0.05 each, up to 0.15 in total
Al balance.
This aluminium alloy has a very good thermal conductivity, thereby improving
the heat exchange properties of the fins made from this aluminium alloy .
Moreover,
this aluminium alloy has satisfactory mechanical properties in the post-brazed
l0 condition, such as tensile strength and corrosion potential.
Although this aluminium alloy is primarily intended as fin stock material for
heat exchangers, it may be used for other parts of heat exchange units, such
as tube
plate, or other uses.
The heat exchanger market, particularly in the automotive industry, requires a
balance of properties for fin stock alloys, which includes strength,
conductivity,
formability, brazeability and corrosion potential. If one of these properties
should be
improved where the other properties must remain as good as they are, often
many of
the alloying elements in the composition must be changed in relation to each
other.
In the present case, it is the merit of the invention that has been seen that
the
conductivity of the alloy could be improved by decreasing the solid solution
in the
alloy through carefully selecting the content of certain alloying elements.
This has
resulted in the following limitations of the alloying elements in the
aluminium alloy
according to the present invention. All percentages are by weight.
Si is an important alloying element in the alloy according to the invention;
it is
expected that Si improves the strength of the alloy by solid solution
hardening and
precipitation hardening. Because the solid solution in the alloy should be as
low as
possible for the required conductivity, the amount of Si should not be higher
than 1.2
%. When the amount of Si is higher, too much Si will remain in solid solution,
resulting in a lower conductivity. A more preferred range for Si is 0.4 to 0.8
%.
3o Within this range the required combination of strength and conductivity is
reached
best.
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Mn is an important alloying element in conventional alloys for making fin
stock
material. Mn is normally added for strength. In the alloy according to this
invention,
the Mn content is kept very low so as to reduce the amount of solid solution
in the
alloy. Preferably Mn <_ 0.03 %, and more preferably Mn 5 0.01 %, thereby
improving
the conductivity as much as possible. Mn may also be absent.
Mg increases the strength of the alloy significantly, but has a detrimental
effect
on controlled atmosphere brazeability because it tends to interact with the
flux
material. For this reason the Mg content has a maximum of 0.05 %, and
preferably
Mg <_ 0.03 %, and more preferably Mg S 0.01 %, to keep the Mg content as low
as
possible. Mg may also be absent.
Fe is an alloying element that is present in all known aluminium alloys. Fe is
added for post-braze strength. It is supposed to form precipitates with Al, Ni
and Si.
The solid solubility of Fe in A1 is extremely low; therefore, Fe can be used
to
improve the strength without compromising the conductivity. Preferably Fe is
in the
range of 0.3 % to 1.6 %, and more preferably in the range of 0.7 % to 1.3 %,
so as to
reach a preferred strength without compromising the formability.
Ni is also present to improve the post-braze strength of the alloy. Like Fe,
the
solid solubility of Ni in A1 is extremely low; therefore, Ni can be used to
improve the
strength without compromising the conductivity. However, when the Ni content
is >
2 %, the formability becomes too low. Ni is preferably present in the range of
of 0.~
to 1.2 % because in this range the best combination of strength and
formability is
found.
Cu is present in the alloy according to the invention to improve the post-
braze
strength of the alloy. The amount of Cu is preferably restricted to the range
of 0.1 %
to 0. ~ %, and more preferably to the range of 0.1 % to 0.6 %, so as to reach
the
required strength. However, Cu is believed to increase the corrosion potential
of the
alloy, whereas the corrosion potential should be low to allow the fin material
to act
as a sacrificial anode. For this reason, at least one of the elements Zn, Sn
or In should
be present.
Zn, Sn or In, or a combination of these three elements, are present to
counteract
the effect of Cu on the corrosion potential of the alloy. The amount of these
elements
must therefore be higher than zero, taking into account the stronger effect of
Sn and
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especially In as compared to Zn. The amounts of Zn, Sn and In should not be
higher
than necessary and therefore preferably Zn is in the range of 1.0 % to 3.0 %
and/or
Sn is in the range of 0.1 % to 1.0 % and/or In is in the range of 0.01 % to
0.05 %.
Preferably, only Zn is present, but Zn can be (partly) replaced by Sn and/or
In.
Ti, V, Cr and Zr are to be avoided as much as possible, because they have a
negative effect on the conductivity of the alloy. Preferably, these elements
are below
0.01 % each.
It is expected that all elements present in the aluminium are detrimental to
the
conductivity of the alloy. Both impurities and intentionally added elements
should
therefore be as low as possible, the intentionally added elements being added
in so
far as they are needed to reach the desired properties.
In a second aspect of the invention there is provided fin stock material made
from the aluminium alloy as specified above, wherein the fin stock material
has a
post-braze conductivity of at least 26 MSIm (45 % IACS), and preferably at
least 29
MSIm (50 % IACS). A conductivity of more than 45 % IACS is good and a
conductivity of more than 50 % IACS is very good in comparison to conventional
fin
stock material for heat exchangers.
Preferably, the fin stock material has a corrosion potential between -750 mV
and
-950 mV versus SCE (ASTM G69), more preferably between -750 mV and -850 mV
2o according to SCE (ASTM G69). The indication SCE means that the voltage in
mV
has been measured in relation to a saturated calomel electrode. These values
for the
corrosion potential give a good sacrificial anode effect when this fin stock
material is
used in heat exchangers.
According to a preferred embodiment the fin stock material has a post braze
UTS (Ultimate Tensile Strength) between 135 and 155 MPa, andlor a 0.2% PS > 50
MPa. This strength is sufficiently high for normal use of fin stock material.
According to a third aspect of the invention, there is provided a brazed heat
exchanger having fins made of an aluminium alloy according the first aspect of
the
invention, or having fins made of fin stock material according to the second
aspect of
the invention.
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EXAMPLES
The aluminium alloy and fin stock material in accordance with the invention
will
now be illustrated by non-limitative and comparative examples.
On a laboratory scale nine alloys have been cast with solidification rates in
a
range comparable to solidification rates in industrial scale DC-casting. These
alloys
are manufactured on a laboratory scale, but the aluminium alloy in accordance
with
this invention can be manufactured on an industrial scale using various
standard
industrial scale DC-casting and continuous aluminium casting methods, followed
by
i0 hot andlor cold rolling.
The chemical compositions of the nine alloy examples are given in table l,
some
relevant properties after brazing simulation are given in table 2.
Table 1. Chemical composition, in wt%, of the aluminium alloys tested, the
balance
is aluminium and impurities.
alloySi Mn Mg Fe Ni Cu Zn Ti Zr V
1 0.70 <0.010.01 1.05 0.94 0.15 1.52 <0.01<0.01 <0.01
2 0.70 <0.01<0.01 0.87 0.99 0.30 1.99 <0.01<0.01 <0.01
3 0.48 <0.01<0.01 0.92 1.05 0.50 2.50 <0.01<0.01 <0.01
4 0.49 0.16 0.01 0.90 1.02 0.31 1.99 <0.01<0.01 <0.01
5 0.78 0.26 <0.01 0.76 0.66 0.51 0.51 0.03 <0.01 <0.01
6 0.78 0.96 <0.01 0.76 0.73 0.25 1.01 0.03 0.106 <0.01
7 0.76 0.97 0.11 0.29 0.71 0.25 0.20 0.13 <0.01 <0.01
8 0.79 0.99 <0.01 0.31 0.71 0.25 1.49 0.03 <0.01 0.15
9 1.07 ~ 0.21 ~ ~ ~ ~ ~ ~ <0.01~
0.92 0.31 0.49 0.25 0.20 0.02 <0.01
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Table2. Properties after simulated brazing cycle.
alloy conductivitycorrosion potentialUTS 0.2% PS
[%IACS] [mV SCE] [MPa] [MPa]
1 49.3 -783 137 53
2 51.4 -779 143 59
3 46.0 -786 149 57
4 43.2 -778 134 54
42.8 -732 142 56
6 42.3 -770 156 70
7 40.5 -748 166 76
8 36.6 -805 154 67
9 43.3 -747 161 69
The nine different chemistries as specified in table 1 were cast and sawn to
pieces with a thickness of 80 xnm, and thereafter preheated to a temperature
below
5 540°C, the alloys were not homogenised. Subsequently hot rolled at a
temperature
below 540°C and cold rolled to a thickness of 0.15 mm. After inter
annealing, the
pieces were cold rolled to a thickness of 0.1 mm.
In the alloys 1 to 4 of table l, no Ti, V, Cr or Zr is present. The Zn present
in the
alloys can be (partly) replaced by Sn and/or In, as is known in the art.
1o As can be seen, the alloys 4 to 9 are comparative examples that do not fit
in the
alloy ranges according to the invention. The amount of Mn is too high. In
alloys 6.7
and 8 additions of Zr, Ti and V, respectively, are also present. Despite the
fact that
the Mn level of alloys 4 and 5 is increased as compared to alloys 1, 2 and 3,
the
strength is not significantly increased. This is attributed to the decrease in
Ni. Alloys
6, 7 and 8 show that with the Ni level of alloys 4 and 5 high strength can be
reached
when Zr, Ti or V additions are present. However, due to the additions of Mn,
Zr, Ti
or V in the alloys 4 to 9, the conductivity of these alloys is comparatively
low.
The three alloys 1, 2 and 3 having a composition according to the invention
clearly have a high conductivity, a sufficiently high strength and a corrosion
potential
2o within the required range to get the sacrificial anode effect.
