Note: Descriptions are shown in the official language in which they were submitted.
CA 02297116 2000-O1-14
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High corrosion resistant aluminium alloy ~'onl~'ining zirconium
The invention relates to an improved aluminium alloy and more paficularly to
an aluminium
alloy which contains controlled amounts of defined compounds and is
characterized by the
combination of high extrudability and high corrosion resistance.
In the automotive industry, aluminium alloys are used in a number of
applications,
especially for tubing because of the extrudability of the alloys combined with
relatively high
strength and low weight
Espeaalfy useful are aluminium alloys for use in heat exchangers or air
conditioning
condensers. In this application the alloy must have a good strength, a
sufficient corrosion
resistance and good extrudability.
A typical alloy used in this application is AA 3102. Typically this alloy
contains
approximately 0,4396 by weight Fe, 0,12~o by weight Si and 0,25% by weight Mn.
In W097/46726 there is described an aluminium alloy containing up to 0,039'o
by weight
copper, between 0,05 - 0,12% by weight silicon, between 0,1 and 0,5°!o
by weight
manganese, between 0,03 and 0,30 °!o by weight titanium between 0,06
and 1,096 weight
zinc, less than 0,01% by weight of magnesium, up to 0,509~o by weight iron,
less than 0,01
by weight nickel and up to 0,509'o by weight chromium.
In W097I46726 it is claimed that there is no positive effect of Cr on the
corrosion
resistance. It should also be noted that in the same patent, the lower level
of manganese is
0,1 % by weight
According to WO 91.14794, a number of elements may be present as normal
impurities,
inGuding Zn, Zr, Ni V and Cr, the maximum amount per element being 0,05 by
weight
percent In the table on page 5 an example is given according to which the
total amount of
impurities is 0,041 9'o by weight and Zr is only present as 0,001 9~ by weight
increasing the
total amount of impurities to 0,15 9'o by weight would result in a maximum
presence of Zr of
0,0036585 % by weight Apparently some of the impurities may be present up to a
local of
0,05 9~o by weight, such as Zn and Cr, but not all of them.
AMENOEO SHEET
CA 02297116 2000-O1-14
P 98055 WO . :..: ': :.. ~~~la~..: ,.,~ .
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Acccording to US-A-4749627 Mn might be rpesent a~ art tmpurtty, up to an
amount of 0,03
by weight
The alloy according to D2 is intended to be used as finstock, and no
indication is given
about its extrudability. From table 6 it becomes clear (see no. 10,11 and 19)
that the
corrosion resistance is due to the high amount of Zn present in the alloy, and
not as a
result of the balanced presence of different elements, as eludicated by the
alloys A - E
according to the invention.
There is a constant need for having aluminium alloys, having the combination
of excellent
extrudability and superior corrosion resistance. F~ccellent extrudability is
required to
minimize production costs at the extrusion plant, including tower extrusion
pressure and
higher extrusion speeds.
tt is therefore an object of the invention to provide an aluminium alloy
composition which
exhibits superior corrosion resistance and improved extrudability while
maintaining the
strength of the at this moment commercial aluminium alloys. For that reason
the aluminium
AH~ENOED SHEEN
CA 02297116 2000-O1-14
WO 99/04050 PCT/EP98/04956
alloy according to the present invention includes controlled amounts of iron,
silicon,
manganese, zirconium, chromium and zinc.
It is a further object of the present invention to provide an aluminium-based
alloy suitable for
use in heat exchanger tubing or extrusions.
(t is another object of the present invention to provide an aluminium-based
alloy suitable for
use as finstock for heat exchangers or in foil packaging applications,
subjected to corrosion,
for instance salt water.
These objects and advantages are obtained by an aluminium-based alloys,
consisting about
0,06-0,25 % by weight of iron, 0,05-0,15 % by weight of silicon up to 0,70 %
by weight of
copper, up to 0,10% by weight of manganese, 0,02 to 0,20% by weight zirconium,
up to
0,18% by weight chromium, up to 0,70 % by weight of zinc, 0,005 to 0,02% by
weight
titanium, for grain refining puroses, up to 0,02 % by weight of incidental
impurities and the
balance aluminium, said aluminium-based alloy exhibiting high corrosion
resistance, good
extrudability and acceptable tensile strength.
Preferably the iron content of the alloy according to the invention is between
about 0,06-0,15
by weight. In this way the corrosion resistance and the extrudability is
optimal, as both
characteristics are substantially reduced with high iron content.
In order to optimize the resistance against corrosion, the zirconium content
is preferably
between 0,10-0,18 % by weight. In this range the extrudability of the alloy is
practically not
influenced by any change in the amount of zirconium.
Preferably also the chromium. content is between 0,10-0,18 % by weight. An
increase in
chromium content results in an increased resistance against corrosion, within
this range the
extrudability is slightly reduced but still within an acceptable range.
Zinc will in even small consentration, negatively affect the anodizing
properties of AA 6000
alloys. In view of this polluting effect of zinc, the level of Zn should be
kept fow to make the ,
alloy more recycleable and save costs in the cast house. Otherwise, zinc has
.a positive
effect on the corrosion resistance up to at least 0,7 % by weight, but for the
reason given
above the amount of zinc is preferable between 0,10 - 0,18 % by weight.
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Although copper may be present to up to 0,50 % by weight, it is preferred to
have the copper
content below 0,01 % by weight in order to have the best possible
extrudability. In some
circumstances it might be necessary to add copper to the alloy to control the
corrosion
potential, making the product less electo negative, to avoid galvanic
corrosion attack of the
product. It has been found that copper increases the corrosion potential with
some 100mV
' for each % of copper added, but at the same time decreases the extrudability
substantially.
The invention also relates to an aluminium product obtained by means of
extrusion and
based upon an aluminium alloy according to the invention.
Normally after casting, the alloy will be homogenized by means of an heat
treatment at
elevated temperatures, e.g. 550-610°C during 3-10 hours. It has been
found that by such a
heat treatment the extrudability was slightly improved, but the corrosion
resistance was
negatively influenced.
According to the invention the aluminium product is characterized in that the
only heat
treatment of the aluminium alloy after casting is the preheating immediately
before extrusion.
Such preheating takes place at lower temperatures than the homogenization step
and only
takes a few minutes, so that the characteristics of the alloy with respect to
extrudability and
corrosion resistance are hardly effected.
In an effort to demonstrate the improvements associated with the inventive
aluminium-based
alloy over known prior art alloys, properties related to mechanical
properties, corrosion
resistance and extrudability were investigated.
The following description details the techniques used to investigate the
properties and
discussion of the results of the investigation.
A number of alloys according to the invention have been prepared, which alloys
are listed
below in table 1 the alloys A - E. In table 1 the composition of these alloys
has been
indicated in % by weight, taking into account that each of these alloys may
contain up to
0,02 % by weight of incidental impurities. In table 1 is also shown the
composition of the
traditional 3102-alloy.
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All these alloys have been prepared in the traditional way . The extrusion of
the billet after
preparation of the alloy was preceded by a preheating to temperatures between
460-490°C.
Table 1: Chemical composition of the different alloys
Alloy Fe Si Mn Zr Cr Zn
A 0,10 0,08 0,07 0,18 0,11 0,00
B 0,12 0,07 0,07 0,12 0,11 0,10
C 0,12 0,07 0,07 0,14 0,14 0,17
D 0,13 0,07 0,07 0,10 0,13 0,19
E 0,11 0,07 0,09 0,07 0,00 0,24
3102 0,43 0,12 0,25 - - -
In order to evaluate the improvements obtained by the alloys according to the
invention, a
number of tests were executed and the results thereof are shown in Table 2.
Table 2: Properties of the alloys listed in Table 1
Alloy UTS YS Elongation Die force Max force SWAAT
MPa MPa % tons tons days
A 87,60 67,60 38,50 5094 6319 35
B 84,20 64,70 35,00 5115 6245 83
C 87,60 68,00 35,50 5130 6305 90
D 85,00 65,20 35,50 5078 6168 67
E 80,50 56,00 36,00 4734 5078 35
3102 86,20 65,50 37,20 5008 6025 10
For investigation of the properties of these alloys, a set of billets was cast
and their
composition determined by means of electron spectroscopy. For this analysis
use was
made of an instrument of make BAIRD VACUUM, and the standards used were
supplied by
Pechiney.
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The extrudability is related to the die force and the maximum extrusion force
indicated as
max force. Those parameters are registered by pressure transducers mounted on
the
press, giving a direct read out of these values.
For determining the corrosion resistance of these alloys, use is made of the
so-called
' 5 SWAAT-test. The test sample was an extruded tube with a wall thickness of
0,4mm.
This test was performed according to ASTM-standard G85-85 Annex A3, with
alternating 30
minutes spray periods and 90 minutes soak periods at 98% humidity. The
electrolyte is
artificial sea water acidified with acetic acid to a pH of 2,8 to 3,0 and a
composition
according to ASTM standard D1141. The temperature is kept at 49°C. The
test was run in
a Liebisch KTS-2000 salt spray chamber.
In order to study the evolution of corrosion behaviour samples from the
different materials
were taken out of the chamber every third day. The materials were then rinsed
in water and
subsequently tested for leaks at a applied pressure of 10 bars. If e.g. a
sample was found to
be perforated after 35 days comparative samples were introduced in the chamber
and left
for 35 days before first inspection, in order to confirm the result. In the
column SWAAT the
number of days before perforation are indicated.
The test as described are in general use with the automotive industry, where
an acceptable
performance is qualified as being above 20 days.
The testing of mechanical properties was carried out on a Zweck Universal
Testing
Instrument (Module 167500) and in accordance with the Euronorm standard. In
the testing
the E-module was fixed to 70000NImm2 during the entire testing. The speed of
the test was
constant and 10 Nlmm2 per second until Rp0,2 was reached, whilst the testing
speed from
Rp0,2 until fracture was 40% Lolmin, Lo being the initial gauge length.
The results of table 2 show that both the mechanical properties, extrudability
in terms of die
force and maximum force as well as corrosion resistance are alloy dependent.
First of all,
the corrosion resistance of the alloys A-E is superior compared to the 3102
alloy. The
extrudability is in general comparable to the 3102 alloy, and the same applies
to the
mechanical properties. When the SWAAT data of the alloys C, D and E are
analysed it is
seen that the best combinations appear when both Cr, Zr and Zn are present
(alloy C).
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Alloy E which contains no Cr, and alloy A which does not contain Zn give
results which are
much better than the acceptable limit of 20 days, however, the corrosion
resistance is
significantly lower than the alloys B, C and D. This clearly shows that both
Cr and Zn should
be present in a long life alloy in order to optimize the corrosion resistance.
In addition,
comparing the results from alloy C and D indicate the importance of Zr.
Increasing the
Zr-content improves the corrosion behaviour in a significant way.
It should therefore be emphasized that the optimum and especially the
corrosion resistance
is the result of the right combination of the elements Cr, Zr, Mn and Zn.
The extrudability is affected by small additions of the different alloying
elements. By
introducing Cr and Zr it is seen that the die force and the maximum force
increases (i.e. the
extrudability is reduced). Zinc, on the other hand, does not affect the
extrudability in any
significant way which as such is well known.
The mechanical properties in terms of ultimate tensile strength and yield
strength are seen
to be significantly improved when Cr is added. In that case the new alloys
match the 3102
alloy properties.
The corrosion test have been performed on samples taken at different location
of the coil.
About 10 samples were taken from the very start of the coil (from the front of
the billet), 10
samples from the middle part of the coil (middle part of the billet) and 10
samples from the
end of the coil (end of the billet). Each sample was about 50 cm long. The
results were very
consistent which means that there is no effects on the corrosion resistance
related to
extrusion speed and material flow during the exterusion of one billet, for the
extrusion
parameters used.
Additional work has been done to evaluate the effect of the different alloying
elements, which
is also shown in the annexed Figures 1 - 6, in which
Fig. 1 shows the influence of the Fe-content on the characteristics of the
alloy
according to the invention.
Fig. 2 shows the influence of the Mn-content on the characteristics of the
alloy
according to the invention.
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Fig. 3 shows the influence of the Zr-content on the characteristics of the
alloy
according to the invention.
Fig. 4 shows the influence of the Cr-content on the characteristics of the
alloy
' S according to the invention.
Fig. 5 shows the influence of the Zn-content on the characteristics of the
alloy
according to the invention.
Fig. 6 shows the influence of the Cn-content on the characteristics of the
alloy
according to the invention.
In the Figures 1 - 5 the x-axis represents the content of the alloying agent
expressed in % by
weight, whereas the y-axis is a relative representation of the different
properties, the square
dots being used to represent the ultimate tensile strength in MPa, the black
triangular dots
being used to represent the entrudability expressed in ktons and using the die
force as
representative measurement, and the white triangular dots being used to
represent the
SWAAT-test results expressed in days.
As shown in Fig. 1 the corrosion resistance is reduced in a significant way
with higher
Fe-contents (keeping Si-content at the same level of 0,08 % by weight). This
effect
especially occurs at Fe-contents in the range of 0,2 - 0,3 % by weight. At the
same time the
extrudability is significantly reduced with higher Fe-contents. It should be
noted that a
reduction of 2-3% of the extrudability (expressed as 2-3% increase of the
break through
pressure) is an unacceptable increase for an extrusion plant. Otherwise an
increase of the
Fe-content results in an increase of the tensile strength.
As becomes clear for Fig. 2, below 0,30 % by weight of Mn any change in the
content of Mn
has practically no effect on the resistance against corrosion (keeping Fe and
Si constant at
0,15 and 0,08 % wy weight respectively). An increase in the Mn-content results
in a reduction
of the extrudability and easily results in an unacceptable extrudabiity.
Otherwise the
mechanical properties improve with an increase of the Mn-content.
If Fe, Si and Mn are kept at a constant level of 0,15, 0,08 and 0,08 % by
weight, an increase
of the Zr-content from 0,07 to 0,15 % by weight will result in an improved
resistance against
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WO 99/04050 PCT/EP98/04956
corrosion as shown in Fig. 3. At the same time the extrudability is only
decreased slightly,
whereas the tensile strength is increased.
The effect of changes in the Cr-content from 0,08 to 0,12 % by weight, while
maintaining
Fe, Si and Mn at the same level as in Fig. 4, is that the corrosion resistance
is increased, the
extrudability is slightly reduced , and the mechanical properties somewhat
increased. This is
shown in Fig. 4.
The influence of Zn, white keeping Fe, Si and Mn at the same level of 0,15,
0,08 and 0,08
by weight respectively, is illustrated in Figure 5. In this figure titanium
(Ti) was also present
at a constant level of 0.15 wt%. (Titanium and zirconium is believed to affect
the corrosion
resistance in the same way, as supported in the results presented in table 2
above)).
The effect of Zn is practically zero with respect to the extrudability and the
mechanical
properties, but the corrosion resistance is increased with increased Zn-
content.
The use of Cu is optional and dependent upon the actual use of the alloy. In
Fig.6 there is
shown a diagram showing the influence of the Cu-content on the extrudability
and on the
corrosion potential. On the X-axis is shown the amount of Cu in % by weight,
whereas the
left Y-axis is the extrusion force expressed in kN and the right Y-axis is the
corrosion
potential expressed in mV according to ASTM G69. The upper line in the graph
is the
evolution of the corrosion potential, whereas the lower line is the evolution
of the extrusion
force.
From this graph it will be clear that a decreasing Cu-content results in an
increase in
extrudability, whereas an increase of Cu with 1 % by weight makes the
corrosion potential
100 mV less negative.
Normally it might be preferred to use an alloy with the smallest possible
amount of copper,
as copper has a negative influence of the inherent resistance against
corrosion of the bare
tube, and strongly influences the extrudability in a negative sense.
However in situations where the extruded product, such as a heat exchanger
tube, must be
connected to another product, such as a header with a clad containing no Zinc,
it is possible
by way of Cu additions to modify the corrosion potential of the extruded
product in such a
s
