Note: Descriptions are shown in the official language in which they were submitted.
CA 02356486 2001-06-26
WO 00/50656 PCT/EP00/01518
1
Extrudable and drawable, high corrosion resistant aluminium alloy.
The invention relates to a high corrosion resistant aluminium alloy,
especially an alloy
intended to be used for manufacture of automotive air conditioning tubes for
applications as
heat exchanger tubing or refrigerant carrying tube lines, or generally fluid
carrying tube lines.
The alloy has extensively improved resistance to pitting corrosion and
enhanced mechanical
properties especially in bending and endforming.
The introduction of aluminium alloy materials for automotive heat exchange
components is
now widespread, applications including both engine cooling and air
conditioning systems. In
the air conditioning systems, the aluminium components include the condenser,
the evapora-
tor and the refrigerant routing lines or fluid carrying lines. In service
these components may
be subjected to conditions that include mechanical loading, vibration, stone
impingement and
road chemicals (e.g.. salt water environments during winter driving
conditions). Aluminium
alloys of the AA3000 series type have found extensive use for these
applications due to their
combination of relatively high strength, light weight, corrosion resistance
and extrudability. To
meet rising consumer expectations for durability, car producers have targeted
a ten-year
service life for engine coolant and air conditioning heat exchanger systems.
The AA3000
series alloys (like AA3102, AA3003 and AA3103), however, suffers from
extensive pitting
corrosion when subjected to corrosive environments, leading to failure of the
automotive
component. To be able to meet the rising targets/requirements for longer life
on the automo-
tive systems new alloys have been developed with significantly better
corrosion resistance.
Especially for condenser tubing, 'long life' alloy alternatives have recently
been developed,
such as those disclosed in US-A-5,286,316 and WO-A-97/46726. The alloys
disclosed in
these publications are generally alternatives to the standard AA3102 or AA1100
alloys used
in condenser tubes, i.e. extruded tube material of relatively low mechanical
strength. Due to
the improved corrosion performance of the condenser tubing the corrosion focus
have
shifted towards the next area to fail, the manifold and the refrigerant
carrying tube lines.
Additionally, the tendency towards using more under vehicle tube runs, e.g.
rear climate
control systems, requires improved alloys due to the more heavy exposure
towards the road
environment. The fluid carrying tube lines are usually fabricated by means of
extrusion and
final precision drawing in several steps to the final dimension, and the
dominating alloys for
this application are AA3003 and AA3103 with a higher strength and stiffness
compared to
the AA3102 alloy. The new requirements have therefore created a demand for an
aluminium
alloy with processing flexibility and mechanical strength similar or better
than the
AA3003/AA3103 alloys, but with improved corrosion resistance.
CA 02356486 2008-07-30
26625-401
2
An aspect of this invention is to provide an extrudable, drawable and
brazeable aluminium
alloy that has improved corrosion resistance and is suitable for use in thin
wall, fluid carrying
tube lines. It is a further object of the present invention to provide an
aluminium alloy suitable
for use in heat exchanger tubing or extrusions. It is another object of the
present invention to
provide an aluminium alloy suitable for use as finstock for heat exchangers or
in foil packag-
applications, subjected to corrosion, for instance salt water. A still further
object of the
ing
present invention is to provide an aluminium alloy with improved formability
during bending
and end-forming operations.
An aspect of the invention relates to an aluminium-based alloy, consisting of
0,05 -
0,15 % by weight of silicon, 0,06 - 0,35 % by weight of iron, 0,01 - 1,00 % by
weight of
manganese, 0,02 - 0,60 % by weight of magnesium, 0,05 - 0,70 % by weight of
zinc, 0 - 0,25
% by weight of chromium, 0 - 0,20 % by weight of zirconium, 0 - 0,25 % by
weight of
titanium, 0 - 0,10 % by weight of copper, up to 0,15 % by weight of other
impurities, each not
greater then 0,Q3 % by weight and the balance aluminium.
Preferably the manganese content is between 0,50-0,70 % by weight, more
preferably 0,62 -
0,70 % by weight. The addition of manganese contributes to the strength,
however, it is a
major point to reduce the negative effect manganese have with respect to
precipitation of
manganese bearing phases during final annealing, which contributes to a
coarser final grain
size.
Addition of magnesium, preferably 0,15-0,30 % by weight, and more preferably
0,25 - 0,30 %
by weight, results in a refinement of the final grain size (due to storage of
more energy for
recrystallization during deformation) as well as improvements the strain
hardening capacity
of the material. In total this means improved formability during for instance
bending and
endforming of tubes. Magnesium also has a positive influence on the corrosion
properties by
altering the oxide layer. The content of magnesium is preferably below 0,3 %
by weight due
to its strong effect in increasing extrudability_ Additions above 0,3 % by
weight are generally
incompatible with good brazeability.
In view of the polluting effect of zinc (ex. even small zinc concentrations
negatively affect the
anodising properties of AA6000 series alloy), the level of this element should
be kept low to
make the alloy more recycleable and save cost in the cast house. Otherwise,
zinc has a
strong positive effect on the corrosion resistance up to at least 0,70 % by
weight, but. for the
CA 02356486 2008-07-30
26625-401
3
reasons given above the amount of zinc is preferably between 0,10 - 0,30 % by
weight, more
preferably 0,20 - 0,25 % by weight.
Preferably the iron content of the alloy according to the invention is between
0.06-0.22 % by
weight. In general, a low iron content, preferably 0,06 - 0,18 % in weight, is
desirable for
improved corrosion resistance, as it reduces the amount of iron rich particles
which generally
creates sites for pitting corrosion attack. Going too low in iron, however,
could be difficult
from a casthouse standpoint of view, and also, has a negative influence on the
final grain
size (due to less iron rich particles acting as nucleation sites for
recrystallization). To counter-
balance the negative effect of a relatively low iron content in the alloy
other elements has to
be added for grainstructure refinement. However, another preferred iron
content for many
practical applications is 0,18 - 0,22 % by weight, giving a combination of
excellent corrosion
properties, final grain size and casthouse capability.
The silicon content is between 0,05-0,12 % by weight, more preferably between
0,06 - 0,10
% by weight. It is important to keep the silicon content within these limits
in order to control
and optimise the size distribution of AlFeSi-type particles (both primary and
secondary parti-
cles), and thereby controlling the grain size in the final product.
For recycleability some chromium in the alloy is desirable. Addition of
chromium, however,
increases the extrudability and influences negatively on the tube drawability
and therefore
the level is preferably 0,05-0,15 % in weight.
In order to optimise the resistance against corrosion, the zirconium content
is preferably
between 0,02-0,20 % in weight, more preferably between 0,10-0,18% in weight.
In this range
the extrudability of the alloy is practically not influenced by any change in
the amount of
zirconium
Further optimising of the corrosion resistance can be obtained by adding
titanium, preferably
between 0,10-0,25 % by weight. No significant influence on the extrudability
is found for
these titanium ievels.
The copper content of the alloy should be kept as low as possible, preferably
below 0.01 %
by weight, due to the strong negative effect on corrosion resistance and also
due to the
negative effect on extrudability even for small additions.
CA 02356486 2008-07-30
26625-401
3a
Another aspect of the invention relates to an aluminium
based, corrosion resistant alloy consisting of 0,05 - 0,15 %
by weight of silicon, 0,06 - 0,35 % by weight of iron,
0,01 - 1,00 % by weight of manganese, 0,15 - 0,30 % by
weight of magnesium, 0,05 - 0,70 % by weight of zinc,
0 - 0,25 % by weight of chromium, 0 - 0,20 % by weight of
zirconium, 0 - 0,25 % by weight of titanium, 0 - 0,10 % by
weight of copper up to 0,15 % by weight of other impurities,
each not greater than 0,03 % by weight and the balance
aluminium.
CA 02356486 2001-06-26
WO 00/50656 PCT/EP00/01518
4
In an effort to demonstrate the improvements associated with the inventive
aluminium-based
alloy over known prior art alloys, the extrudability, drawability, mechanical
properties,
formability parameters and corrosion resistance were investigated for a series
of alloy
compositions, see Table 1. The alloys have been prepared in a traditional way
by DC casting
of extrusion ingots. Note that the composition of the alloys have been
indicated in % by
weight, taking into account that each of these alloys may contain up to 0.03 %
by weight of
incidental impurities. Compositions were selected with varying amounts of the
different major
elements. Note that alloy 1 in Table 1 is the composition of the standard
AA3103 alloy, which
is used as reference alloy in the investigation.
Table 1: Chemical composition of alloys (% by weight).
Alloy Fe Si Mn Mg Cr Zn Cu Zr Ti
1 0,54 0,11 1,02 - - - 0,03 - 0,01
2 0,24 0,08 0,67 0,29 - - - - -
3 0,23 0,09 0,70 0,29 0,10 - - - -
4 0,24 0,08 0,70 0,27 0,22 - - - -
5 0,21 0,08 0,68 0,28 - 0,25 - - -
6 0,20 0,08 0,67 0,27 0,07 0,24 - - -
7 0,25 0,13 0,67 0,05 0,04 0,16 - - 0,17
8 0,22 0,10 0,74 0,29 - 0,13 - -
9 0,21 0,10 0,72 0,25 0,10 0,12 - 0,19
10 0,22 0,10 0,71 0,27 0,12 0,22 - 0,20
11 0,23 0,09 0,70 0,26 0,01 0,11 0,08 -
12 0,22 0,10 0,50 0,26 - 0,22 - -
13 0,55 0,10 0,69 0,27 - 0,21 - -
14 0,21 0,05 0,68 0,27 0,06 0,25 - - -
The following description details the techniques used to investigate the
properties, followed
by a discussion of the obtained results.
The composition of the billets were determined by means of electron
spectroscopy. For this
analysis a Baird Vacuum Instrument was used, and the test standards as
supplied by Pechi-
ney, were used.
CA 02356486 2001-06-26
WO 00/50656 PCT/EP00/01518
Extrusion billets were homogenised according to standard routines, using a
heating rate of
100 C/hr to a holding temperature of approximately 600 C, followed by air
cooling to room
temperature.
Extrusion of the homogenised billets were carried out on a full scale
industrial extrusion
5 press using the following conditions:
billet temperature: 455 - 490 C
extrusion ration: 63 : 1
ram speed: 16.5 mm/sec
die: three hole
extrudate: 28 mm OD tube (extrudate water cooled)
The extrudability is related to the die pressure and the maximum extrusion
pressure (peak
pressure}. Those parameters are registered by pressure transducers mounted on
the press,
giving a direct rpad out of these values.
The extruded base tube were finally plug drawn in totally six draws to a final
9.5 mm OD tube
with a 0.4 mm wall. The reduction in each draw was approximately 36 %. After
the final draw
the tubes were soft annealed in a batch furnace at temperature 420 C.
Testing of mechanical properties of annealed tubes were carried out on a
Schenk Trebel
universal tensile testing machine in accordance with the Euronorm standard. In
the testing
the E-module was fixed to 70000 N/mm2 during the entire testing. The speed of
the test was
constant at 10 N/mm2 per second until YS (yield strength) was reached, whilst
the testing
from YS until fracture appeared was 40 % Lo/min, Lo being the initial gauge
length.
Corrosion potential measurements were performed according to a modified
version of the
ASTM G69 standard test, using a Gamry PC41300 equipment with a saturated
calomel
electrode (SCE) as a reference. The tube specimens were degreased in acetone
prior to
measurements. No filing or abrasion of the tube specimen surface was
performed, and the
measurements were done without any form of agitation. Corrosion potentials
were recorded
continuously over a 60 minute period and the values presented represents the
average of
those recorded during the final 30 minutes of the test.
To demonstrate the improved corrosion resistance of the inventive aluminium
alloy composi-
tion over known prior art alloys, the corrosion resistance was tested using
the so-called
CA 02356486 2001-06-26
WO 00/50656 PCT/EP00/01518
6
SWAAT test (Acidified Synthetic Sea Water Testing). The test was performed
according to
ASTM G85-85 Annex A3, with alternating 30 minutes spray periods and 90 minutes
soak
periods at 98 % humidity. The electrolyte used was 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 in the chamber was kept at 49 C. The test was run in a Erichsen
Salt Spray
Chamber (Model 606/1000).
In order to study the evolution of corrosion behaviour, samples from the
different alloys were
taken out of the chamber every third day. The materials were then rinsed in
water and
subsequently tested for leaks by immersing tube specimens in water and
applying a
pressure of 1 bars. The test as described is in general use within the
automotive industry,
where an acceptable performance for condenser tubing is qualified as being
above 20 days
exposure. Data presented from the SWAAT corrosion testing is the 'SWAAT life';
first tube
sample out of totally 10 tube samples (each 0,5 m long) to perforate in the
test:
It was found that during extrusion of the different alloys, the extrusion
pressures obtained for
the tested alloys were equal or maximum 5-6 % higher compared with the 3103
reference
alloy (equals alloy 1). This is regarded as a small difference and it should
be noted that all
alloys were run at the same billet temperature and ram speed (no press-
parameter optimisa-
tion done in this test).
Surface finish after extrusion, especially on the interior of the tube, is
particularly important in
this application because the tube is to be cold drawn to a smaller diameter
and wall thick-
ness. Surface defects may interfere with the drawing process and result in
fracture of the
tube during drawing. All the alloys investigated during the tests showed good
intemal surface
appearance.
Concerning drawing, most of the alloys drew well, i.e. same speed and
productivity as for
standard alloy 1. Note that a number of other alloys than given in Table 1
also were tested
but they were not able to withstand the required number of draws without
serious fracturing,
and they were therefore excluded from further consideration. Basically the
reason for these
alloys having difficulties in drawing was related to microstructural features
being incompatible
with heavy drawing reductions (i.e. large grains or particle phases). Alloys
surviving more
than five draws have been included in this consideration.
Table 2 summarises the results of the draw ability test.
CA 02356486 2001-06-26
WO 00/50656 PCT/EP00/01518
7
Table 2.
Alloy Intended No. of draws without Comment
no. of serious fracturing of
draws tube
1 6 6 OK
2 6 6 OK
3 6 6 OK
4 6 6 OK
6 6 OK
6 6 6 OK
7 6 6 OK
8 6 6 OK, periodically breaks during last draw
9 6 5 considerable effort to finish last draw
6 6 OK
11 6 5 considerable effort to finish last draw
12 6 6 OK, periodically breaks during last draw
13 6 5 breaks at last draw
14 6 5 considerable effort to finish last draw
The characteristics of the alloys after annealing is given in Table 3.
CA 02356486 2001-06-26
WO 00/50656 PCT/EP00/01518
8
Table 3.
Alloy YS UTS Elong. n-value* Grain-size** SWAAT Corr. pot.
life
MPa MPa A10 (%) um 1 st out mV SCE
1 48 108 41.2 0.23 141 3 -730
2 51 113 36.1 0.24 82 7 -769
3 52 115 36.1 0.24 56 15 -755
4 53 117 37.1 0.23 66 15 -760
46 112 36.0 0.25 88 57 -769
6 51 113 36.6 0.24 79 41 -782
7 42 99 43.0 0.24 92 30 -830
8 49 112 37.8 0.24 83 32 -797
9 57 119 33.9 0.22 48 32 -814
51 121 36.9 0.23 59 49 -819
11 51 112 37.1 0.23 48 28 -812
12 63 106 37.2 0.22 59 25 -745
13*** 156 169 2.0 - - 21 -770
14 49 116 34.6 0.24 46 50 -775
* n-value means strain hardening exponent, obtained by fitting a Ludwik law
expression to
the true stress-strain curve in the region between yield and uniform strain.
5 ** grain size measured along the drawing direction on longitudinal tube
cross sections.
*** alloy is tested in H14 temper condition.
From the results in Table 3 it can be seen that the mechanical properties,
grain size and
corrosion resistance are strongly alloy dependent. First of all, concerning
mechanical proper-
ties the test alloys in general shows slightly higher UTS and YS values
compared with the
10 reference alloy 1. The measured n-values also are slightly higher which
indicates better
formability due to improved strain distribution during forming. Note also the
refinement in
grain structure obtained for the Long Life test alloys which influences in a
positive way on the
formability with less risk 'orange peel' effects after extensive forming.
In terms of corrosion resistance (i.e. SWAAT life) of all the test alioys are
superior compared
to the standard alloy 1. Tubes of alloy 1 are seen to fail after only 3 days,
while significantly
longer lifetimes are found for the test alloys. A major feature in obtaining
increased corrosion
CA 02356486 2001-06-26
WO 00/50656 9 PCT/EP00/01518
lifetime, is a low iron content in the alloy. Additional elements like
zirconium, titanium and
especially zinc introduces a second level of corrosion protection by altering
the oxide layer
and changing the corrosion attack morphology. For alloys 5, 6, 10 and 14 a
more than 10
times improvement in corrosion resistance is obtained compared with reference
alloy 1,
which is really a significant improvement. The superior corrosion resistance
obtained in case
of the test alloys is attributable in art to the mode of corrosion attack
being limited to gener-
ally a laminar type. This extends the time required for corrosion to penetrate
through a given
thickness and thereby providing a long life alloy.
Concerning electrochemical corrosion potentials it can be seen from Table 3
that the test
alloys generally have a more negative potential (more anodic) as compared to
the reference
alloy 1. Adding zinc, zirconium and/or titanium strongly drags the potentials
to more negative
values. The fact that these Long Life alloys have a more negative potential is
important infor-
mation with respect to corrosion design criteria, i.e. the importance of
selecting appropriate
material combinations in application were the tube is connected to a
fin/header material (for
instance in a qondenser), is emphasised. In order for the tube not to behave
sacrificial
towards the fin/header, materials being more anodic than the Long Life tube
needs to be
selected.
