Note: Descriptions are shown in the official language in which they were submitted.
CA 02629816 2008-04-24
ALUMINIUM ALLOY FOR EXTRUSION AND DRAWING
PROCESSES
Field of the Invention
The invention relates to an aluminum alloy having good elevated temperature
properties
and, more particularly, to an aluminum alloy extrudable or drawable into
relatively thin
wall tubing.
Description of the Prior Art
Aluminum alloy extrusions are widely used in heat transfer applications
including air
conditioning, refrigeration and automotive applications. Charge air coolers
for heavy
duty diesel engines have typically used extruded tubing in AA 3003 alloy (in
weight %,
max. 0.60 Si, max. 0.70 Fe, 0.05 - 0.20 Cu, 1.0 - 1.5 Mn, max. 0.10 Zn, other
elements
max. 0.05 each and max 0.15 in total, the remainder being AI) which has
performed
satisfactorily in this application in terms of ease of making the tube and in-
service
properties.
However, with changes to environmental legislation, the operating temperatures
of
charge air coolers are increasing and recirculation of a proportion of the
engine exhaust
gases is becoming necessary. AA 3003 alloy cannot meet these new service
requirements and a new aluminum alloy is required with the combination of
improved
strength over AA 3003, the ability to be furnace brazed and resistance to
exhaust gas
corrosion.
At the same time, it has to be possible to extrude the alloy into thin wall
tubing at
commercially acceptable production rates. Often alloy additions made to
aluminum to
improve strength can be detrimental to the ability to extrude fast, often
termed
extrudability. The main factors controlling this are the alloy flow stress at
extrusion
temperature and the alloy melting point or solidus.
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Many aluminum alloys are processed into brazing sheet which can be fabricated
into
tubing by folding and resistance welding. This production route via hot and
cold rolling,
does not have the same limitations as the extrusion route and a wider range of
alloy
compositions can be successfully produced in sheet form.
The attraction of an extruded product is that the tube wall thickness can be
varied
around the periphery which is not possible with a sheet product. The challenge
is
therefore to develop an alloy composition which gives the maximum strength
increase
for the minimum loss of extrudability while at the same time having adequate
acidic
corrosion resistance.
BRIEF SUMMARY OF THE INVENTION
It is therefore an aim of the present invention to address the above mentioned
issues.
According to a general aspect, there is provided an extrudable aluminum alloy
composition comprising, in weight percent, between 0.60 and 0.90 manganese,
between 0.45 and 0.75 copper, between 0.05 and 0.24 magnesium, less than 0.30
iron,
less than 0.30 silicon, less than 0.05 titanium, less than 0.05 vanadium, and
a Cu/Mg
ratio higher or equal to 3.
According to another general aspect, there is provided an extrudable aluminum
alloy
composition consisting essentially of, in weight percent, between 0.60 and
0.90
manganese, between 0.45 and 0.75 copper, between 0.05 and 0.24 magnesium, less
than 0.30 iron, less than 0.30 silicon, less than 0.05 titanium, less than
0.05 vanadium,
and a Cu/Mg ratio higher or equal to 3.
According to another general aspect, there is provided aluminum alloy heat
exchanger
extruded or drawn tubes comprising an aluminum alloy composition having, in
weight
percent, between 0.60 and 0.90 manganese, between 0.45 and 0.75 copper,
between
0.05 and 0.24 magnesium, less than 0.30 iron, less than 0.30 silicon, less
than 0.05
titanium, less than 0.05 vanadium, and a Cu/Mg ratio higher or equal to 3.
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According to still another general aspect, there is provided an extruded or
drawn
aluminum alloy tubing comprising an aluminum alloy composition having, in
weight
percent, between 0.60 and 0.90 manganese, between 0.45 and 0.75 copper,
between
0.05 and 0.24 magnesium, less than 0.30 iron, less than 0.30 silicon, less
than 0.05
'titanium, less than 0.05 vanadium, and a Cu/Mg ratio higher or equal to 3.
According to a further general aspect, there is provided a process to
manufacture a heat
exchanger, comprising: extruding one tubing having an aluminum alloy
composition
having, in weight percent, between 0.60 and 0.90 manganese, between 0.45 and
0.75
copper, between 0.05 and 0.24 magnesium, less than 0.30 iron, less than 0.30
silicon,
less than 0.05 titanium, less than 0.05 vanadium, and a Cu/Mg ratio higher or
equal to
3; and brazing at least one extruded tubing section to at least one heat
exchanger
component.
According to a further general aspect, there is provided a heat exchanger
comprising a
plurality of extruded or drawn tube sections having an aluminum alloy
composition
having, in weight percent, between 0.60 and 0.90 manganese, between 0.45 and
0.75
copper, between 0.05 and 0.24 magnesium, less than 0.30 iron, less than 0.30
silicon,
less than 0.05 titanium, less than 0.05 vanadium, and a Cu/Mg ratio higher or
equal to
3.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graph showing the tensile properties and extrusion breakthrough
pressure for
the seven (7) aluminum alloys presented in Table 1;
Fig. 2 is a graph showing the strength/extrudability index versus the Cu/Mg
ratio for the
aluminum alloys 1 to 4 presented in Table 1;
Fig. 3 includes Fig. 3a and Fig. 3b and are micrographs of cross sections of
alloys 1 and
4 respectively after corrosion attack; and
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Fig. 4 is a graph showing the effect of copper addition on UTS at 290 C,
corrosion
depth, extrudability (AP) and alloy melting point.
It will be noted that throughout the appended drawings, like features are
identified by
like reference numerals.
DETAILED DESCRIPTION
The aluminum alloy contains, aside from aluminum and inevitable impurities,
the
following amounts of alloying elements. In an embodiment, it contains between
0.60 and
0.90 wt% of manganese (Mn), between 0.45 and 0.75 wt% of copper (Cu), between
0.05 and 0.24 wt% of magnesium (Mg), less than 0.30 wt% of iron (Fe), less
than 0.30
wt% of silicon (Si), less than 0.05 wt% of titanium (Ti), less than 0.05 wt%
of vanadium
(V), and a Cu/Mg ratio higher or equal to 3, i.e. the ratio between the copper
content
and the magnesium content in weight percent. In an embodiment, the Cu/Mg ratio
is
lower than 15.
The aluminum alloy has adequate corrosion resistance, elevated temperature
strength
15. and extrudability (or drawing properties). It can thus be used for any
extruded or
drawing applications requiring at least one of these properties. In an
embodiment, the
aluminum alloy provides a strength improvement of at least 10% over aluminum
alloy
AA 3003. For example and without being limitative, the aluminum allow has a
tensile
strength (UTS) and a proof strength (PS), tested according to ASTM E21 at 290
C for
instantaneous testing, higher or equal to 48 and 35 MPa respectively.
For example, it can be used as a billet for the production of tubing either by
extrusion or
drawing. For example and without being limitative, the end product can be thin
wall
<1.5mm extruded or drawn tubes assembled into a brazed heat exchanger. The
brazing
step can be, for instance, a vacuum brazing or controlled atmosphere brazing
(CAB)
wherein at least one tubing section is brazed to heat exchanger components
such as
manifold, internal and external corrugated fins, etc. These heat exchangers
can be used
in charge air coolers, for instance.
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It can also be used in non brazed applications and any other applications
where
adequate properties at relatively high temperature are desirable.
Manganese additions are beneficial for strengthening; however, at relatively
high
content, manganese is also detrimental to extrudability. Moreover, its
strengthening
efficiency is relatively poor and does not provide improvement for corrosion
resistance.
In an embodiment, the manganese content of the aluminum alloy ranges between
0.60
and 0.90 wt%, in another embodiment, it ranges between 0.65 and 0.85 wt%, and,
in
still another embodiment, it ranges between 0.70 and 0.80 wt%.
A low magnesium content, such as below 0.25 wt%, is beneficial to
extrudability and
strength efficiency. Moreover, a deliberate addition is necessary for
acceptable
corrosion resistance. Thus, in an embodiment, the magnesium content of the
aluminum
alloy ranges between 0.05 and 0.24 wt%, in another embodiment, it ranges
between
0.05 and 0.20 wt% and, in still another embodiment, it ranges between 0.05 and
0.12
wt%.
Strength and extrusion pressure increase with copper content but the
strength/extrudability efficiency remains relatively stable. A minimal copper
content is
necessary for good corrosion resistance; however, if the copper content is too
high, the
alloy solidus becomes too low for successful furnace brazing. Moreover, the
combination of copper and magnesium additions can give a good combination of
strength and strength/extrudability efficiency. Thus, in an embodiment, the
copper
content of the aluminum alloy ranges between 0.45 and 0.75 wt%, in another
embodiment, it ranges between 0.50 and 0.60 wt%, and, in still another
embodiment, it
ranges between 0.57 and 0.63 wt%.
A low magnesium content combined with a higher copper content is desirable
since it is
advantageous to both strength and extrudability. Moreover, it promotes
resistance to
corrosion. This combination of copper and magnesium can be represented by the
copper and magnesium ratio. A Cu/Mg ratio above or equal to 3.0 is also
beneficial. In
an embodiment, the Cu/Mg ratio is below 15.
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High iron levels are detrimental to corrosion resistance while low iron levels
are known
to be beneficial for the quality of the extruded surface finish. In an
embodiment, the iron
content of the aluminum alloy is below 0.30 wt%, in another embodiment, it is
below
0.25 wt% and, in still another embodiment, it ranges between 0.15 and 0.25
wt%.
In an embodiment, the silicon content is maintained below 0.30 wt% to make the
material compatible with sacrificial brown band formation due to silicon
diffusion from
the filler metal during brazing. In an embodiment, the silicon content of the
aluminum
alloy is below 0.30 wt%, in another embodiment, it is below 0.20 wt% and, in
still
another embodiment, it ranges between 0.05 and 0.15 wt%.
The addition of nickel has no benefits on high temperature strength and can
lead to
relatively significant corrosion attack. In an embodiment, it should be kept
below 0.05
wt%.
Additions of titanium and vanadium have no beneficial effects on either
strength or
corrosion resistance and both elements are known to be detrimental to high
temperature flow stress and therefore extrudability. However, titanium is used
as a grain
refiner during casting, either on its own or when added in conjunction with
boron to form
TiB2, when added in a concentration up to 0.05 wt%. In an embodiment, the
titanium
content of the aluminum alloy is below 0.05 wt%, in another embodiment, it is
below
0.03 wt%a, and, in still another embodiment, it is ranges between 0.005 and
0.025 wt%.
In an embodiment, the vanadium content of the aluminum alloy is below 0.05
wt%, in
another embodiment, it is below 0.03 wt% and, in still another embodiment, it
is below
0.02 wt%.
It is appreciated that the alloying element content for a particular alloying
element can
be selected from any of the above-described embodiments and it can differ from
the
embodiment of another alloying element. For example and without being
limitative, in an
embodiment of the aluminum alloy, the aluminum alloy can have a manganese
content
ranging between 0.60 and 0.90 wt% and a magnesium content ranging between 0.05
and 0.12 wt%.
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Experiment 1
A series of six (6) experimental alloys were direct-chill (DC) cast as 101
millimeter (mm)
diameter billet. The compositions are shown in Table 1. Alloy AA 3003 was
included as
a reference material as it is currently used as extruded tubing in heat
exchangers such
as charge air coolers. All the alloys were cut into billets and homogenized
for four (4)
hours at 600 C. The homogenization step was followed by a controlled cooling
at a
cooling rate of 150 C per hour to decrease the alloy flow stress and make it
more
extrudable or drawable. Alloys A to D and reference alloy 3003 were extruded
consecutively, starting with four (4) billets of 3003 followed by two (2)
billets of each
experimental composition. The billets were preheated to 500 C and extruded at
a ram
speed of 15 mm per second (sec.) into a strip profile 3 mm x 41.7 mm. The
strip was
quenched using fans to give a quench rate of 8 C per sec. Extrusion pressure
was
monitored during the test and peak extrusion pressures were recorded for the
second
billet of each alloy to avoid any carry over effects from the previous
variant.
Extrusion pressure is a commonly used measure of ease of extrusion. An alloy
with
lower extrusion pressure is said to be more extrudable and this reflects the
ability to
extrude the alloy at higher speed and lower cost or into more complex shapes.
Alloys E and F were extruded separately and extrusion pressures were not
recorded.
Sections of the strip were given a simulated braze cycle heating at 15 C per
minute
(min) to 600 C, holding for two (2) minutes then still air cooling to room
temperature.
Tensile samples were machined and then tested at 290 C (according to ASTM
E21)
after holding for five (5) minutes at the test temperature. A second set of
samples were
exposed for 1000 hours at 290 C and then tensile tested at 290 C to
investigate the
effects of long term thermal exposure.
Referring now to Table 1, reproduced below, aluminum alloy compositions are
shown
that were manufactured and tested in the first experiment. Their measured
extrudability
and mechanical properties are given in Table 2, also reproduced below.
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The AA 3003 base alloy is a typical aluminum composition used for extruded
tubing in a
range of heat transfer applications and was used as a reference aluminum alloy
in all
experiments.
Aluminum alloy 1 was of similar composition to materials used commercially for
brazing
sheet which can be fabricated into tubing by folding and resistance welding.
This alloy,
designed for sheet products, has a high manganese content and is difficult to
extrude
into hollow relatively thin walled tubing and was also included for
comparative purposes.
Aluminum alloys 5 and 6 included additions of Ti and V respectively
comparatively to
reference alloy AA 3003. Such additions have been used in extruded heat
exchanger
tubing alloys for various reasons and might have been expected to give some
improvement in high temperature strength through solid solution strengthening.
The remaining aluminum alloys compositions, i.e. aluminum alloys 2 to 4,
contained
varying levels of magnesium, copper and manganese. In comparison to aluminum
alloy
1, aluminum alloy 2 had a lower manganese content while aluminum alloy 3 had a
manganese content similar to aluminum alloy 2 but a lower copper content.
Finally,
aluminum alloy 4 had a copper content similar to aluminum alloys 1 and 2, a
manganese content similar to aluminum alloys 2 and 3 but a lower magnesium
content
than aluminum alloys 1 to 4.
Table 1: Aluminum alloy Compositions.
in weiht% wt%
Alloy Cu Mg Mn Fe Si Ti V Cu/Mg
1 0.62 0.24 1.50 0.20 0.10 0.02 < 0.01 2.6
2 0.61 0.24 0.76 0.20 0.10 0.02 < 0.01 2.5
3 0.29 0.24 0.75 0.18 0.09 0.02 < 0.01 1.2
4 0.61 0.14 0.74 0.19 0.10 0.02 < 0.01 4.4
AA 3003 0.08 < 0.01 1.06 0.57 0.22 0.02 < 0.01 .
5 0.07 <0.01 1.01 0.17 0.21 0.17 <0.01
6 0.08 < 0.01 1.01 0.19 0.09 0.02 0.20
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Table 2: Extrudability and Mechanical Properties.
All Strengths measured at 290 C in MPa.
PS
BTP UTS (1000
Alloy si AP (%) UTS PS (1000 hr hr)
1 2200 37 66.8 50.6 58.9 41.1
2 1980 23 61.6 41.8 55.1 36.1
3 1900 18 56.4 37.1 52.2 33.4
4 1750 9 56.0 38.6 51.5 34.0
AA 3003 1610 0 43.7 32.0 37.0 23.6
42.8 29.9
6 37.5 27.1
Table 2 summarizes extrusion results and tensile properties for all aluminum
alloys
presented in Table 1. It includes extrusion breakthrough pressure (BTP) in
psi, relative
5 BTP increase versus reference aluminum alloy AA 3003 (AP) as a measure of
BTPalloy - B TP3003
extrudability OP = x 100, tensile strength (UTS) and proof strength
B TP3003
(PS) tested at 290 C for instantaneous testing and after 1000 hour exposure.
Low APs
are associated with relatively easily extrudable alloys. Finally, Figure 1
shows UTS and
PS for instantaneous testing and after 1000 hour exposure as well as BTP
versus the
aluminum alloy composition.
From Figure 1, it was observed that the 1000 hour UTS and PS properties show
the
same trends as the instantaneous UTS and PS properties except there is a loss
of
strength in every case.
Aluminum alloys 5 and 6, which include additions of Ti and V respectively, did
not give
any significant benefit over reference alloy AA 3003 while alloys 1 to 4
exhibited some
improvement in strength at 290 C comparatively to reference alloy AA 3003.
Moreover,
alloys 1 to 4 had higher BTP than reference alloy AA 3003 with alloy 4 having
the
second lowest BTP.
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Table 3, reproduced below, shows values of strength improvement / extrusion
pressure
increase or "strength / extrudability efficiency" versus reference alloy AA
3003, i.e.
(Strength(alloy) - Strength (AA 3003)) / AP(alloy). The units are MPa per
percentage of
extrusion pressure increase (MPa /%). Surprisingly, aluminum alloy 4
consistently gave
the highest ranking for all strength measurements. In some cases, the strength
/
extrudability efficiency ratio doubled that of the next best performing
aluminum alloy, i.e.
alloy 3.
Table 3: Strength Improvement over 3003/ AP rankings.
Strength / Extrudability ncy
Alloy UTS/ext PS/ext UTS/ext 1000 hr PS/ext 1000 hr
1 0.63 0.51 0.60 0.48
2 0.78 0.43 0.79 0.54
3 0.71 0.28 0.84 0.54
4 1.41 0.76 1.67 1.20
Table 4, reproduced below, shows the percentage of strength loss with long
term
exposure at 290 C. i.e. Strengthinst - Strenqth,ooonr .000. Aluminum alloys 3
and 4
Strength;nsr
exhibited a reduced loss of strength compared to the stronger aluminum alloys
1 and 2
and also as compared with reference alloy AA 3003.
Table 4: Percenta e of Strength Loss Durin 1000 Hour Ex osure at 290 C.
Alloy UTS % dro PS % dro
1 12 19
2 11 14
3 7 10
4 8 12
AA 3003 15 26
Overall, aluminum alloy 4 gave the best balance of extrudability and strength
improvement over reference alloy AA 3003 with minimal influence of exposure
time at
elevated temperature.
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The effects of the various elements in these tests can be interpreted as
follows.
Aluminum alloy 1, which has the highest manganese content, is similar to a
brazing
sheet alloy, which has high strength but has poor extrudability and cannot be
extruded
into thin wall (< 1.5 mm) tubing, such as and without being limitative heat
exchanger
tubes. Reducing the manganese level from 1.5 wt% to about 0.75 wt%, as in the
case of
aluminum alloy 2, gave some loss of strength but extrudability, measured by
AP,
improved along with the strength/extrudability factor (see Table 3). Also the
loss of
strength with increasing exposure time on strength at 290 C was reduced (see
Table 4).
Reducing the magnesium content from 0.25 wt% to 0.15 wt% (alloy 2 versus alloy
4)
resulted in some loss of strength but this is compensated by the improvement
in
strength/extrudability factor (see Table 3) and a further reduction in the
effect of
increased exposure time at 290 C (see Table 4).
Reducing the copper content from 0.61 wt% in aluminum alloy 2 to 0.29 wt% in
aluminum alloy 3, while maintaining the magnesium content at 0.25, gave a loss
of
strength with only moderate improvement in extrudability. The combination of
lower Mg
and higher Cu contents in alloy 4 gave about the same strength levels as alloy
3 but
with markedly reduced extrusion pressure giving a significant improvement in
strength/extrudability factor.
If the Cu/Mg ratio of the different alloys is considered, as shown in Figure
2, the
strength/extrudability index, either for proof strength or tensile strength
improves in a
near linear fashion with increasing Cu/Mg ratio. A Cu/Mg ratio higher or equal
to 3
gives improved performance.
Neither of the elements titanium and vanadium added at the 0.17 and 0.20 wt%
level in
aluminum alloys 5 and 6 respectively gave any improvement in mechanical
properties.
Both elements should be maintained lower or equal to 0.05 wt% as they are
known to
be detrimental to high temperature flow stress.
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Titanium is added as a grain refiner during casting, either on its own or in
conjunction
with boron, to produce a fine cast grain structure and prevent formation of
feather grains
which can produce non uniform deformation during extrusion.
Experiment 2
A further series of aluminum alloys, including some of those from experiment
1, were
tested using a corrosion test designed to assess resistance to attack by
diesel engine
condensate. Recirculation of diesel engine exhaust back through the
turbocharger, the
charge air cooler, and the engine is becoming increasingly common to meet
emission
legislation. As the air mixed with exhaust gas is cooled, condensation occurs
within the
heat exchanger. Resistance to corrosion by this condensate containing nitric
and
sulphuric acids is an important property for tubing materials. Examples of
typical
condensate compositions are available in the literature for example in SAE
#05P-660 -
2004- Assessment of Corrosivity Associated with Exhaust Gas Recirculation in a
Heavy
Duty Diesel Engine - Kass et al.
The new alloys were cast as 101 mm diameter ingots and homogenized at
temperatures ranging between 580 and 620 C. The homogenization step was
followed
by a controlled cooling at a cooling rate of 100 C per hour. Aluminum alloys
1 to 4 were
the same alloys included in experiment 1. They were extruded under the same
conditions and given the same braze cycle simulation as described in
experiment 1.
Aluminum alloy 7 had similar copper, manganese, iron, silicon, titanium, and
nickel
contents to alloys 2 to 4 but also had a lower magnesium content. Aluminum
alloy 8 had
similar copper, magnesium, iron, silicon, titanium, and nickel contents to
alloy 7 but had
a higher manganese content. Aluminum alloy 9 had similar magnesium, manganese,
iron, silicon, titanium, and nickel contents to alloy 4 but had a lower copper
content.
Aluminum alloy 10 had similar copper, magnesium, manganese, silicon, and
titanium
contents to alloy 7 but had higher iron and nickel contents. Finally, aluminum
alloy 11
had similar copper, magnesium, manganese, silicon, and titanium contents to
alloy 7
but had a lower iron content and a higher titanium content.
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Samples with approximate dimensions 50 mm x 7 mm x 3 mm were cut from the
extruded lengths, given a braze simulation and degreased in ethyl alcohol.
These were
immersed in a solution of distilled water acidified with 2 milliliter (ml) per
liter nitric and
102 ml per liter sulphuric acids held with a pH of 0.3 at 80 C. The
composition of the
test solution was derived from published data on diesel engine condensates.
The time
of exposure was seven (7) days. At the end of the test the samples were rinsed
and
any corrosion deposits brushed off. The maximum depth of corrosion attack was
measured with an optical microscope using the procedure described in ASTM G46.
Tensile properties were also measured at 290 C after 1 hour exposure according
to
ASTM E21.
Alloys 4, 8 and AA 3003 were extruded separately in order to compare extrusion
pressures. The breakthrough pressure increase over AA 3003 was calculated
along
with the strength/extrudability efficiency as described in experiment 1. The
results are
presented in Table 5, reproduced below.
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CA 02629816 2008-04-24
Results with significant improvements over reference alloy AA 3003 in terms of
corrosion depth or tensile strength (UTS) are in italic in the appropriate
columns
Table 5.
In terms of the corrosion results, aluminum alloys 3 and 4 experienced the
shallowest attack of the alloys tested and were the only alloys to undergo
less attack
than reference alloy AA 3003. In contrast, aluminum alloys 1 and 2 with a
higher
combined copper and magnesium content exhibited more attack.
Figure 3 shows metallographic cross sections through the corrosion sites on
aluminum alloys 1 and 4. Alloy 1 tended to give deeper more localized attack
whereas alloy 4 gave broader shallower pits which is usually preferable.
There was no significant effect of decreasing the manganese content from alloy
1 to
alloy 2, which had approximately 50 % less Mn than alloy 1. This result is
somewhat
surprising as manganese additions are generally thought to improve corrosion
resistance.
Alloy 10 with an addition of nickel and iron gave the most severe attack of
all the
samples. Alloy 11 with a titanium addition still exhibited inferior corrosion
resistance
to alloy 4. Alloy 9 also gave inferior corrosion performance compared to alloy
4
indicating that the copper addition to alloy 4 is beneficial in this respect.
Alloy 7
suffered more attack than alloy 4 indicating the magnesium present in alloy 4
is also
beneficial. Alloy 8 is a higher manganese containing alloy without a magnesium
addition but the results were still inferior to alloy 4.
In terms of high temperature tensile strength, the alloys that gave more than
a 10%
improvement over AA 3003 are in italic in the UTS column in Table 5. These
include
alloys 1-4, but experiment 1 has already indicated that while alloys 1-3 can
achieve
attractive strength levels this is at the expense of extrudability. Alloy 8,
containing
1.19 wt% manganese and without a magnesium addition, also gave a useful
strength
increase, which is similar to the Alloy 4 strength increase. However, although
alloy 8
did not contain magnesium, which is well known to be detrimental to high
temperature flow stress, in a direct comparison it gave an increase in
extrusion
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pressure of 28% versus 9% for alloy 4 with a resulting low value of
strengthening
efficiency, which can be attributed to the increased manganese content.
Overall, the results indicate that a combined but limited copper and magnesium
addition, such as is the case with alloys 3 and 4, promotes resistance to the
simulated condensate attack.
Additions of iron, titanium, and nickel are detrimental to condensate
corrosion
resistance and there is no benefit from increased manganese levels. Moreover,
increased manganese contents are detrimental to extrudability.
Although alloys 3 and 4 gave similar corrosion resistance, the significant
benefit in
strengthening efficiency of alloy 4 make its composition more advantageous for
extruded tube applications.
Experiment 3
A further series of alloys were cast and homogenized as described in
experiment 1.
As for the previous experiment, the homogenization step was followed by a
controlled cooling at a cooling rate of 100 C per hour. The compositions were
designed such that they were similar to alloy 4 but with varying levels of
copper
addition. As for the previous experiments, reference alloy AA 3003 was
included as a
control and aluminum alloy 9 was the same alloy included in experiment 2.
The billets were extruded into a 1.3 mm "I-beam" profile with an extrusion
ratio of
120:1. The billets were preheated to 500 C and extruded at a ram speed of 10
mm
per sec. Three billets of each alloy were extruded in sequence to obtain
accurate
ram pressure data in order to assess extrudability. Coupons were again given a
simulated brazing cycle and corrosion resistance and tensile strength at 290
C were
measured as described in experiment 2. The pressure increase versus the
reference alloy AA 3003 and resulting strengthening efficiency were
calculated.
Table 6 summarizes the alloy compositions and test results, ranked in terms of
increasing copper content. Corrosion and UTS results superior to reference
alloy AA
3003 are in italic in the appropriate columns. Some of the results in Table 6
are
plotted in Figure 4, and will be described in more details below.
OR File No. 11014-317CA - 16 -
CA 02629816 2008-04-24
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CA 02629816 2008-04-24
In terms of corrosion attack, only alloy 4 with 0.60 wt% copper experienced
less
attack than reference alloy AA 3003. Alloys 9 and 12 with a copper content
less than
0.40 wt% exhibited significantly more attack again indicating that the
presence of
copper at the levels present in alloy 4 is beneficial. As shown in Figure 4,
the
corrosion attack was relatively level above this point.
Tensile strength and extrusion pressure both increased fairly linearly with
increasing
copper. Strengthening efficiencies (UTSeff) were similar for alloys 4 and 12-
14.
However, one detrimental feature of increasing copper levels is the depression
of the
alloy melting point. This has some impact on extrudability as it limits the
maximum
temperature and resulting extrusion speed at which the product can leave the
extrusion die. Also for commercial brazing applications, where braze
temperatures
can range from 600-620 C, it is undesirable to melt the tube alloy so a
minimum
solidus temperature of 630 C is usually desired which translates to a maximum
copper content of approximately 0.80 wt%.
Based on the experiments conducted, aluminum alloy 4 gave the best combination
of extrudability, elevated temperature strength, condensate corrosion
resistance and
brazeability.
The aluminum alloy can be homogenized as a billet, prior to the extrusion and
drawing steps. The homogenization step can be carried out at a temperature
above
570 C and below the aluminum alloy melting point up to 8 hours. Then, the
alloy can
be cooled at a cooling rate below 200 C per hour. In an alternative
embodiment, the
homogenization step is carried out at a temperature ranging between 590 C and
the
aluminum alloy melting point for 2 to 8 hours and the alloy is cooled down at
a
cooling rate below 150 C per hour. The homogenization step followed by a
controlled cooling decreases the alloy flow stress and makes it more
extrudable or
drawable. The lower limit for the homogenization temperature is determined by
the
temperature at which the extrusion pressure starts to rise due to formation of
fine
dispersoids.
The effects of each element can be summarized as follows in the following
paragraphs.
OR File No. 11014-317CA - 18 -
CA 02629816 2008-04-24
As mentioned above, at high levels, manganese is detrimental to extrudability
and
although some elevated strengthening can be achieved, it has a poor
strengthening
efficiency. Manganese content should be lower than 1.20 wt% in an embodiment.
Less than 0.25 wt% magnesium is beneficial to extrudability and strength
efficiency
but a deliberate addition is necessary for acceptable corrosion resistance.
A copper addition of approximately 0.60 wt% combined with a magnesium addition
of less than 0.25 wt% gives a good combination of strength and strength
efficiency.
A Cu/Mg ratio higher or equal to 3.0 is beneficial in this respect. In an
embodiment,
the Cu/Mg ratio is below 15. Strength and extrusion pressure increase linearly
with
copper content but the strength efficiency remains relatively stable. A
minimum
addition of 0.40 wt% copper is necessary for good corrosion resistance but
above
0.80 wt% copper, the alloy solidus becomes too low for successful furnace
brazing.
High iron levels of approximately 0.60 wt !o were shown to be detrimental to
corrosion resistance. There were no measurable effects on extrudability or
strength
but lower levels are known to be beneficial for the quality of the extruded
surface
finish. In an embodiment, an upper limit of 0.30 wt% is applicable.
In an embodiment, the silicon content is maintained below 0.30 wt% to make the
material compatible with sacrificial "brown band" formation due to silicon
diffusion
from the filler metal during brazing.
An addition of 0.48 wt% nickel gave the highest level of corrosion attack in
the tests
conducted. The element has no benefits on high temperature strength and, in an
embodiment, it should be kept below 0.05 wt%.
Deliberate addition of 0.15 wt% titanium and 0.20 wt% vanadium had no
beneficial
effects and both elements are known to be detrimental to high temperature flow
stress and therefore extrudability. In an embodiment, titanium and vanadium
contents should be below 0.05 wt% and titanium can be added as a grain refiner
during casting within this range.
Thus, in an embodiment, the aluminum alloy includes between 0.60 and 0.90 wt%
manganese, between 0.05 and 0.24 wt% magnesium, between 0.45 and 0.75 wt%
OR File No. 11014-317CA - 19 -
CA 02629816 2008-04-24
copper, below 0.30 wt% iron, below 0.30 wt% silicon, below 0.05 wt% titanium,
and
below 0.05 wt% vanadium. The Cu/Mg ratio is above or equal to 3. Moreover, the
aluminum alloy includes less than 0.05 wt% nickel. The remainder is aluminum
and
inevitable impurities.
It is appreciated that, in alternative embodiments, the range for one or
several
alloying elements can vary from the one described above. For example and
without
being limitative, the aluminum alloy can include between 0.65 and 0.85 wt% or
between 0.70 and 0.80 wt% Mn, between 0.05 and 0.20 wt% or between 0.05 and
0.12 wt% Mg, between 0.50 and 0.60 wt% or between 0.57 and 0.63 wt% Cu, below
0.25 wt% or between 0.15 and 0.25 wt% Fe, below 0.20 wt% or between 0.05 and
0.15 wt% Si, below 0.03 wt% or between 0.005 and 0.025 wt% Ti, and below 0.03
wt% or below 0.02 wt% vanadium.
The aluminum alloy described above can be used as a billet for the production
of
heat exchanger extrusions. For example and without being limitative, the end
product can be thin wall <1.5mm extruded or drawn tube assembled into a brazed
heat exchanger. The brazing step can be, for instance a vacuum brazing or
controlled atmosphere brazing (CAB) wherein at least one tubing section is
brazed to
heat exchanger components such as manifold, internal and external corrugated
fins,
etc. These heat exchangers can be used in charge air coolers, for instance.
It is appreciated that the aluminum alloy can be used for any extruded
application
where corrosion resistance, elevated temperature strength and extrudability
are
required. It also includes non brazed applications and any other applications
where
good elevated properties are desirable.
The embodiments of the invention described above are intended to be exemplary
only. The scope of the invention is therefore intended to be limited solely by
the
scope of the appended claims.
OR File No. 11014-317CA - 20 -
