Note: Descriptions are shown in the official language in which they were submitted.
CA 02409870 2002-11-20
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Corrosion resistant aluminium alloy
The present invention is directed to a group of corrosion resistant and
extrudable aluminium
alloys with improved elevated temperature strenght, especially to a AA3000
series type
'aluminium alloy including controlled amounts of titanium, vanadium and
zirconium for improved
extrudability and/or drawability.
In the prior art, aluminium is well recognized for its corrosion resistance.
AA1000 series
aluminium alloys are often selected where corrosion resistance is needed.
In applications where higher strengths may be needed, AA1000 series alloys
have been
replaced with more highly alloyed materials such as the AA3000 series types
aluminium alloys.
AA3102 and AA3003 are examples of higher strength aluminium alloys having good
corrosion
resistance.
Aluminium alloys of the AA3000 series type have found extensive use in the
automotive
industry due to their good combination of strength, light weight, corrosion
resistance and
extrudability. These alloys are often made into tubing for use in heat
exchanger or air
conditioning condenser applications.
One of the problems that AA3000 series alloys have when subjected to some
corrosive
environments is pitting corrosion. This type of corrosion often occurs in the
types of
environments found in heat exchanger or air conditioning condenser
applications and can result
in failure of an automotive component where the corrosion compromises the
integrity of the
aluminium alloy tubing.
In a search for aluminium alloys having improved corrosion resistance, more
highly alloyed
materials have been developed such as those disclosed in U.S. Patent nos.
4,649,087 and
4,828,794. These more highly alloyed materials while providing improved
corrosion
performance are not conducive to extrusion due to the need for extremely high
extrusion forces.
U.S. Patent no. 5,286,316 discloses an aluminium alloy with both high
extrudability and high
corrosion resistance. This alloy consists essentially of about 0.1 - 0.5 % by
weight of
manganese, about 0.05 - 0.12 % by weight of silicon, about 0.10 - 0.20 % by
weight of titanium,
about 0.15 - 0.25 % by weight of iron, with the balance aluminium and
incidental impurities.
The alloy preferably is essentially copper free, with copper being limited to
not more than 0.01
%.
CONFIRMATION COPY
CA 02409870 2002-11-20
WO 01/90430 PCT/EPO1/05920
Although the alloy disclosed in U.S. Patent no. 5,286,316 offers improved
corrosion resistance
over AA3102, even more corrosion resistance is desirable. In corrosion testing
using salt water
- acetic acid sprays as set forth in ASTM Standard G85 (hereinafter SWAAT
testing),
condenser tubes made of AA3102 material lasted only eight days in a SWAAT test
environment
before failing. In similar experiments using the alloy taught in U.S. Patent
no. 5,286,316, longer
duration than AA3102 were achieved. However, the improved alloy of U.S. Patent
no.
5,286,316 still failed in SWAAT testing in less than 20 days.
Accordingly, it is a first object of the present invention to provide an
aluminium alloy having
improved combinations of corrosion resistance and hot formability.
A still further object of the present invention is to provide an aluminium
alloy which has good
both hot- and cold- formability and corrosion resistance.
Other objects and advantages of the present invention will become apparent as
a description
thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention
provides a
corrosion resistant aluminium alloy consisting essentially of, in weight
percent, 0,05 -1,00 % of
iron, 0,05 - 0,60 % of silicon, less than 0,50 % of copper, up to 1,20 % of
manganese, 0,02 -
0,20 % of zirconium, up to 0,50 % of chromium, 0,02 to 1,00 % of zinc, 0,02 -
0,20 % of
titanium, 0,02 - 0,20 % of vanadium, up to 2,00 % of magnesium, up to 0,10 %
of antimony, up
to 0,02 % of incidental impurities and the balance aluminium.
Considering in more detail the amounts of the individual components, iron
preferably is between
0,05 - 0,55 %, more preferably, between 0,05 - 0,25 %. Reducing the Fe content
improves the
corrosion resistance. Silicon is preferably between 0,05 and 0,20 %, more
preferably, not more
than 0,15 %. Copper is below 0,50 %, as this elements normally negatively
influences the
extrusion speed and the corrosion resistance. BUt in some circumstances some
copper might
be needed to adjust the electro-potential of the allay. Preferablly the Cu-
content is below 0,05
% by weight. Zirconium is preferably between 0,02 and 0,18 %. Zn should always
be present in
at least 0,02 % by weight in order to improve the general level of corrosion
resistance and
preferably zinc content is between 0,10 and 0,50 %, more preferably between
0,10 and 0,25 %.
Ttitanium is preferably between 0,02 and 0,15 %, and vanadium is preferably
between 0,02 and
0,12 %. The preferred amount of manganese is highly dependent on the intended
use of the
article because manganese impacts extrudability, especially with thin
sections.
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For applications with these type of alloys in which the corrosion resistance
and excellent
extrudability is the primary concern, manganese is preferably present in
amounts between 0,05
- 0,30 %~ by weight. Fe is preferably present in amounts between 0,05 - 0,25 %
by weight. For
these applications the preferred amount of chromium is between 0,02 and 0,25%.
The
magnesium amount is preferably below 0,03 %. Zn is preferably present in
amounts between
0,10 - 0,5 % by weight. By making an appropriate selection of the amount of
these elements it
is possible to have an alloy with good extrusion characteristics, with good
mechanical properties
and superior corrosion resistance.
When the alloy is intended to be used in applications, in which after
extrusion further
deformation processes will be used in order to obtain a final product, such as
cold deforming as
e.g. drawing and/or bending, and where higher strength is required, it is
preferred to have the
amount of manganese between 0,50 and 0,80 % by weight. In this application
chromium is
preferably between 0,02 and 0,18 % by weight and magnesium below 0,30 % by
weight, for
brazeability reasons. The Fe content should be kept low for improved corrosion
resistance. To
further improve corrosion resistance 0,10 - 0,5 % Zn is added. Likewise,
controlled additions of
V, Zr and Ti each not more than 0,2 % by weight are made to further
improve,corrosion
resistance.
If the alloy is to be used in high temperature applications the role of V, Ti
and especially Zr
becomes important. The amounts added of each of these elements will depend on
the
functional requirements, however, the amount of zirconium is preferably
between 0,10 and
0,18 % by weight. Further it is preferred in these applications to use post
heat treatment of the
cast alloy in that it is heated to a temperature of between 450 and
550°C with a heating rate of
less than 150 °C/hour, and maintain the alloy at that temperature for
between 2 and 10 hours.
The final product may also for certain applications and especially after cold
working, require a
"back annealing" treatment consisting of heating the work piece to
temperatures between 150
and 350 degrees Centigrade and keep at temperature for between 10 and 10000
min.
Imaroved corrosion resistance.
Zr and Ti in solid solution, are used separately to improve corrosion
resistance in low alloy
highly extrudable alloys e.g. for use in extruded tubes for automotive A/C
systems. The useful
maximum additions of Zr and Ti when added separately is less than 0,2% by
weight. Above this
level primary compounds are formed that reduces the level of these elements in
solid solution.
In addition, the primary compounds from Zr and Ti (AI3Zr, AI3Ti) may initiate
pitting corrosion
as they are more noble than the AI matrix.
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4
Both Zr and Ti will upon solidification go through a peritectic reaction. The
product of this
reaction is revealed as a highly concentrated region of the elements in the
centre of the grain
(large positive partition ratio). These regions or zones will upon rolling or
extrusion form a
lamellae structure parallel to the surt'ace of the work piece and slow down
the corrosion in the
through thickness direction.
Additions of both Zr and Ti in combination, will give larger and more
concentrated zones and
hence improve corrosion resistance.
V is an element with much the same behaviour and effect as Zr and Ti, buff has
up to now not
been used much in these type of alloys. V will improve the mechanical
properties in the same
way as Zr and Ti, but do not have the same effect on corrosion unless the Zr-
content is higher
than the V-content.
Combination of all three elements will give the most optimal balance of the
corrosion, strength
and workability properties, if the total content of Zr, Ti and V is kept below
0,3 % by weight.
.
Improved elevated temperature mechanical proaerties and formability.
The transition elements such as Zr, Ti, and V are known to improve formability
by increasing
the work hardening coefficient ("n"). The "n" increases with increased amount
of the transition
elements almost linearly up to some 0,5%. By combining Zr, Ti and V up to
0,45% of the
transition elements may be added without the formation of deleterious primary
particles of the
type AI3Zr, as opposed to below 0,2% if only one of the elements is added. But
it has found
otherwise that above a total level of 0,3 % by weight some characteristics are
negatively
influenced.
Zr, Ti and V, and in particular Zr are known to impede the tendency of
recrystalization, provided
optimum heat treatment before high temperature processing. The ability to
retard
recrystalization is related to the number and size of small coherent/semi-
coherent precipitates
that are stable at temperature up to 300- 400 degrees Centigrade for prolonged
times. The fine
polygenized structure that will result from back annealing at temperatures in
the 150 to 350
degrees Centigrade range will have higher mechanical strength than the
corresponding
recrystalized structure resulting in the absence of such transition elements.
The density of these precipitates increases with increased amount of fihe
transition elements,
therefore combining the three elements would improve the mechanical property
in the
temperature range from ambient temperature to approx. 400 degrees Centigrade.
CA 02409870 2002-11-20
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Experimental results.
In order to prove the above mentioned statements about improvement of a number
of
characteristics of the alloys according to the invention, a number of
experiments has been
5 carried out, which are described below. From these results it will become
clear that there is an
additive effect of the simultaneous use of the three elements Zr, Ti and V up
to a content of
maximum 0,3 % by weight. It is supposed that this is due to the comparable
behavior of the
different elements Zr, Ti and V in an aluminium alloy, such as solvability,
crystal structure, etc...,
which at the same time allowing higher effective amounts to be used than in
case of only one or
two of these elements.
Billets with different content of Zr, V and Ti were cast using the laboratory
casting equipment at
Sunndalsora. For each alloy, four billets with a diameter of 95 mm and a
length of 1.1 m were
produced. At the beginning of the casting the casting speed was 115 mmlmin,
increasing to
240 mmlmin after 15 cm cast billet. The temperature in the launder was set to
be 705°C and
the temperature was recorded during casting. Grain refiner (TisB-wire) were
added in the
furnace before the casting.
After casting each billet were cut, producing three samples for extrusion and
two samples for
spectrographic analysis (first one sample for spectrographic analysis, then
two samples for
extrusion, then the second sample for spectrographic analysis (i.e. ~in the
middle of the billet)
and finally the third sample for extrusion). Samples from the as-cast material
(~middie of the
billet) was etched to reveal feathery crystals, in addition samples were
prepared to show grain
structure and particle structure. Hardness and conductivity measurements were
carried out for
each alloy on specimens (2 cm x 2 cm x 1 cm) that were grinded to a grit size
of 2000.
Extrusion experiments were carried out with a 8 MN vertical extrusion press,
producing a tube
with outer diameter 6 mm. Four extrusion trials we're carried out for each
alloy variant and the
first three were cooled in air while the fourth were cooled in water. Samples
for further
investigations were taken from the first, the third and the fourth extrusion
trial. The samples
were taken from close to the end of the extruded profile, avoiding the very
end (~2 m).
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6
In the following a presentation of the results from the investigations of the
as-cast material and
the extruded material will be given. The extruded material was tested in a
SWAAT-test and in
addition mechanical testing was carried out. A presentation of the results
from these tests will
also be given.
The results of the experiments are partly given as diagrams in the annexed
drawings, partly as
tables. In the figures there is:
Fig. 1 a diagram showing for the alloys 1-11 in the Y-axis the electrical
conductivity (in
MS/m) in function of the total amount of Ti, V and Zr (wt % in X-axis),
Fig. 2 a diagram showing for the alloys 1-11 in the Y-axis the main extrusion
force (in
kN) in function of the total amount of Ti, V and Zr (wt% in X-axis),
Fig. 3 a diagram showing for the alloys 1-11 in the Y-axis the yield strength
(round dots)
and the ultimate tensile strength (square dots) in function of the total
amount of Ti,
V and Zr (wt% in X-axis).
Fig. 4 a diagram showing for the alloys 41-56 in the Y-axis the electrical
conductivity (in
MS/m) in function of the total amount of Ti, V and Zr (wt % in X-axis),
Fig. 5 a diagram showing for the alloys 41-56 in the Y-axis the break through
pressure
(in kN) of the alloy as cast in function of the total amount of Ti, V and ZR
(wt % in
X-axis),
Fig. 6 a diagram showing for the alloys 41-56 in the Y-axis the breakthrough
pressure (in
kN) of the alloy after homogenizing at 470°C for 1 hour in function of
the total
amount of Ti, V and Zr (wt % in X-axis),
Fig. 7 a diagram showing for the alloys 41-56 in the Y-axis the yield strenght
(in MPa) of
the alloy after extrusion in function of the total amount of Ti, V and ZR (wt
% in
X-axis),
Fig. 8 a diagram showing for the alloys 41-56 in the Y-axis the ultimate
tensile strenght
(in MPa) of the alloy after extrusion' in 'function of the total amount of Ti,
V and ZR
(wt % in X-axis),
Fig. 9 a diagram showing for the alloys 41-56 in the Y-axis yield strenght (in
MPa) of the
alloy after extrusion and subsequently homogenizing at 470°C for 1 hour
in
function of the total amount of Ti, V and ZR (wt % in X-axis),
Fig. 10 a diagram showing for the alloys 41-56 in the Y-axis the ultimate
tensile strenght
(in MPa) of the alloy after extrusion and subsequently homogenizing at
470°C for
1 hour in function of the total amount of Ti, V and ZR (wt % in X-axis),
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7
Fig. 11 a diagram showing for the alloys 41-56 in the Y-axis the ultimate
tensile strenght
,, (in MPa) of the alloy after homogenizing at 470°C for 1 hour and
subsequently
extrusion in function of the total amount of Ti, V and ZR (wt % in X-axis),
1. The as-cast material
The as-cast material represents the starting point for the extrusion process
and the following
mechanical and corrosion testing. An investigation of the starting material
has been carried out,
and the results are shown in the following. Samples from the as-cast material
were investigated
to find the actual chemical composition and to reveal the microstructure
(grain structure and
particle structure) in the various alloys. The chemical composition of the
material was obtained
by spectrographic analysis, and the results are listed in Table 1 (alloys 1-11
), Table 2 (alloys
20-35) and Table 3 (alloys 41-56).
20
30
CA 02409870 2002-11-20
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CA 02409870 2002-11-20
WO 01/90430 PCT/EPO1/05920
9
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CA 02409870 2002-11-20
WO 01/90430 PCT/EPO1/05920
Table 3
Allo Si Fe Mn M Ti V Zr TiVZr
41 0.142 0.210 0.5197 0.191 0,003 0,002 0,006 0,011
42 0.129 0.196 0.4962 0.171 0,012 0,002 0,168 0,182
43 0.142 0.210 0.5050 0.186 0,140 0,002 0,002 0,144
5 44 0.132 0.2 0.4719 0.174 0,131 0,002 0,193 0,325
45 0.136 0.204 0.5027 0.178 0,013 0,118 0,002 0,133
46 0.132 0.199 0.4741 0.163 0,012 0,115 0,218 0,345
47 0.135 0.204 0.4949 0.172 0,153 0,121 0,002 0,276
48 0.130 .199 0.465 0. 0 0,084 0,197 0,409
0 9 162 ,128
49 0.129 _ _ _ _ 0,038 0,196 0,291
_ _ 0.162 0,057
0.2 0.474_2
50 0.134 0.204 _ 0.166 0,078 0,042 0,096 0,216
0.4907
51 0.133 0.204 0.4921 0.165 0,079 0,041 0,097 0,216
10 52 0.132 0.204 0.4891 0.161 0,079 0,041 0,097 0,217
53 0.450 0.210 0.5181 0.162 0,003 0,001 0,002 0,006
54 0.417 0.201 0.4796 0.147 0,042 0,035 0,157 0,233
55 0.134 0.204 1.0234 0.154 0,003 0,000 0,012 0,014
56 0.125 0.193 0.9462 0.138 0,043 0,036 0,183 0,262
Conductivity measurements were also carried out, and the results are shown,in
Figure 1 and 4.
The electrical conductivity is seen to decrease approximately linearly with
increasing content of
the alloying elements Zr, Ti and V. And, as seen from the figures, the effect
of the various
alloying elements is additive for this property.
2. The extrusion testing
To investigate the effect of the addition of Ti, V and Zr on the force during
extrusion, all the
alloy variants were extruded with the same extrusion conditions and the max
force on the ram
were measured. The temperature in the container and the ram speed was recorded
during the
trials and were found to be 430°C and 1.3-1.9 mm/s respectively. The
temperature in the
container and the ram speed were not seen to be stable from one experiment to
the next. The
values of the maximum force as found from the' experiments are shown in
Figures 2, 5 and 6.
The values shown in the figure are averages of four extrusion trials.
3. Mechanical testing of the extruded tubes
The results from the tension testing of the extruded tubes are shown in
Figures 3,.4. As can be
seen from the table and the figure, the variations in stress with changing
alloy are small. The
stress at max load is seen to increase slightly with increasing content of
alloying elements,
while the effect on the yield stress is not clear. This qualitative
evalutation of the results were
confirmed by a statistical analysis.
CA 02409870 2002-11-20
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11
4. SWAAT-Testing
The specimens for the SWAAT testing were taken from the first of the four
extrusion trials for
each alloy. Specimens of 30 cm length were cut from the extruded tubes and
then placed in a
SWAAT chamber. The results from the SWAAT test are shown Tables 4 and 5.
Table 4
Alloy Hours
to
failure
1 1232
2 1280
3 1416
4 1176
5 1064
6 1288
7 608
8 1184
9 1352
10 1352
11 I 16561
Table 5
alloy Hours
to
failure
20 1760
21 1512
22 1320
23 1440
24 >2100
25 >2100
26 936
27 2088
28 1592
29 1452
30 1712
31 1944
32 >2100
33 1872
34 1716
35 >2100
CA 02409870 2002-11-20
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12
in Table 6 is shown the effect of the addition of the elements Ti, Zr and V
and heat treatment.
Table 6
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