Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02533428 2006-O1-23
GS 030029~~10
HIGH-STRENGTH ALLOY FOR HEAT EXCI-IANGERS
The invention relates to a cold age-hardenable aluminium
alloy for heat exchangers, a method for producing a cold
age-hardenable aluminium strip and an aluminium strip or
sheet.
Heat exchangers consisting of aluminium or aluminium
alloys are increasingly being used in the automobile
field. In this case, the use of aluminium instead of the
previously commonly used nonferrous metal heat exchangers
has almost halved the weight of heat exchangers with
comparable size and performance. Heat exchangers of
aluminium or an aluminium alloy are nowadays used in the
motor vehicle mainly for cooling the cooling water, oil
and in air-conditioning systems. Heat exchangers f or
motor vehicles are usually made of aluminium strips or
sheets, the individual pre-fabricated components of the
heat exchanger such as fins, tubes and distributors for
example, being joined together by brazing. The loads
acting in practical use on components thus manufactured
and built into motor vehicles as a result of intermittent
vibrations, more sustained vibrations, corrosion attack
and similar are considerable. This particularly applies
to the fins via which the heat is removed. Despite the
considerable loads and increasing operating pressures of
the heat exchangers in motor vehicles, there continues to
be a trend towards saving weight in motor vehicles and
therefore towards a further reduction in the wall
thickness of the heat exchanger. However, this results in
further increasing strength requirements for the
aluminium alloy of the heat exchangers, especially after
brazing. On the one hand, vacuum brazing without flux and
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inert-gas brazing using non-corrosive fluxes are
available for brazing heat exchangers. The cold age-
hardenable aluminium alloys used so far for the vacuum
brazing of heat exchangers, for example, the aluminium
alloy AA6063 (AlMgO, ?Si), AA6061 (AlMglSiCu) or AA6951
(AlMgO, 6SiCu), have relatively high magnesium contents
in order, on the one hand, to prevent any oxidation of
the molten aluminium solder on the components to be
brazed as a result of "gettering" during the brazing
process in vacuum and thereby ensure a perfect brazed
joint without flux and on the other hand, to achieve high
strength values of the brazed heat exchangers during
natural ageing after brazing. A disadvantage with this
method however is that it is cost-intensive to maintain
the gas protection and the purity requirement for the
components to be soldered. The alternative inert-gas
brazing (also called CAB - controlled atmosphere brazing)
certainly requires less expenditure under these aspects
and additionally makes it possible to achieve up to 200
shorter brazing cycles but it is not possible to use the
aluminium alloy having high magnesium contents known from
vacuum brazing since the magnesium reacts with the non-
corrosive fluxes during the brazing. This can only be
prevented by using more expensive caesium-containing
fluxes. It is furthermore possible to use high-copper-
content aluminium alloys (Cu content > 0.50) which
however tend to form heat cracks during casting and thus
impose increased requirements on the casting of the
rolling bars which should be considered to be critical
from economic aspects. In addition, at elevated Cu
i
contents there is a risk of sensitivation for pitting or
grain-boundary corrosion if copper is present in suitably
precipitated form in the structure. Finally, in i:~ert-gas
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brazing an aluminium alloy with an intermediate cladding
can be used as a diffusion barrier layer so that a cold
age-hardenable aluminium alloy with relatively high
magnesium contents is used as core material. Howev a r,
intermediate cladding with a diffusion barrier lave r is
associated with additional costs so that economica 1
production of heat exchangers likewise cannot be
achieved.
The fabrication of heat exchangers by brazing of
components consisting of the aforementioned aluminium
alloys is known, for example, from US Patent
Specification US 4,214,925.
Starting from the previously indicated prior art, it is
thus the object of the present invention to provide a
cold age-hardenable aluminium alloy for heat exchangers,
a method for producing an aluminium strip for heat
exchangers and a corresponding aluminium strip or sheet
which has high strength values after natural agein g
following brazing.
According to a first teaching of the present invention,
the object derived and indicated above is solved f or an
aluminium alloy by said aluminium alloy having the
following alloying components in wt. o:
Si -<< 0.70
0.1% <- Mg < 1%
Fe <-0.30
0.080 < Cu < 0.20
Ti -<0.20
Mn -<<0.1%
Cr <-0.1o
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~n <- 0.1%,
unavoidable accompanying elements individually to a
maximum of O.lo and in total to a maximum of 0.150
and aluminium as the remainder.
It has surprisingly been shown that heat exchangers
consisting of an aluminium alloy containing the alloying
fractions specified above, after natural ageing at room
temperature following brazing, have the necessary
strength for use in motor vehicles, especially the yield
point RP~;,2, without further heat treatments being
necessary. The reason for this is the combination of the
Si and Mg contents according to the invention which form
finely distributed precipitates of the type Mg2Si in the
aluminium alloy according to the invention and result in
an increase in strength as a result of natural ageing at
room temperature. This increase in strength by natural
ageing is further improved by adding copper in the
claimed range of 0.08 wt.o to 0.2 wt. o.
Limiting the Fe content to a maximum of 0.3 wt.% ensures
that Si is present in the aluminium alloy in a dissolved
state. Furthermore, the low Cu contents of 0.2 wt.o
maximum on the one hand ensure that the increase in
strength during natural ageing can be increased and on
the other hand, this limitation of the Cu content reduces
the sensitivity of the strength of the aluminium alloy to
the cooling rate after brazing. Likewise, the Mn content
must be limited to a maximum of 0.1 wt.o to limit the
dependence of the strength of the aluminium alloy on the
cooling rate after brazing. In contrast, Cr contents of
0.1 wt.o maximum increase the strength and the corrosion
resistance of the aluminium alloy according to the
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invention. In addition, a Ti content of 0.2 wt.% maximum
has a positive effect on the resistance to corrosion of
the aluminium alloy according to the invention since the
Ti alloying element contributes to the grain refining of
the structure of the aluminium alloy and thus makes the
corrosion attack uniform. In order to avoid the negative
effect of zinc on the corrosion of the aluminium alloy
according to the invention, the 2n content must be
restricted to a maximum of 0.1 wt. o.
According to a first advantageous embodiment, the
strength of the aluminium alloy according to the
invention can be further increased by natural ageing
after brazing by the aluminium alloy containing Si, Mg
and Cu as the principal alloying elements.
In order to avoid softening of the components of a heat
exchanger to be brazed during brazing, it is advantageous
for carrying out a perfect brazing process if the solidus
temperature of the aluminium alloy does not go below 610°C
since brazing is usually carried out at temperatures up
to 600°C. According to the invention this is achieved by
the total of the alloying fractions of Si, Mg and Cu not
exceeding 1.2 wt. o. In this-case, alloying elements
generally bring about a reduction in the solidus
temperature where Si causes a reduction in the solidus
temperature of the aluminium alloy a factor of 1.2
greater than Mg and Mg in turn causes a reduction in the
solidus temperature a factor of 3.5 more effective than
Cu.
This does not apply to the alloying element Ti so that an
increase in the solidus temperature of the aluminium
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alloy according to the invention can be achieved b y the
aluminium alloy having Ti as an alloying component.
If the upper limit of the claimed alloy is exhauste d for
magnesium, the brazing of heat exchangers fabricate d from
this alloy is preferably effected by vacuum brazing.
Inert-gas brazing using caesium-containing fluxes i s also
possible here to a limited extent. Inert-gas brazin g
using caesium-containing fluxes is especially simplified
by the alloying fraction of magnesium not exceeding 0.8
wt.°.
In addition, with a low Mg content up to a maximum of 0.3
wt.o, the aluminium alloy according to the invention i_s
readily suitable for inert-gas brazing using non-
corrosive fluxes since a reaction with the fluxes only
takes place to a limited extent and the use of more
expensive caesium-containing fluxes can be dispense d
with.
A particularly advantageous embodiment of the aluminium
alloy according to the invention is obtained whereb y
after processing and brazing and after natural ageing for
approximately 30 days at room temperature the aluminium
alloy has particularly high strength values. This
material property ensures a particularly inexpensive
fabrication process since the natural ageing as part of
the transport process already ensures a very good
strength without further measures.
According to a second teaching of the present invention,
the object derived and indicated above is solved
according to the method by
CA 02533428 2006-O1-23
- a rollinuJ bar being cast from an aluminium
alloy according to the invention in a
conventional bar casting method,
- the rolling bar being homogenised at 500 to
600°C for more than 6 h, especially for more
than 12 h, and being hot-rolled at at least
400°C, preferably 450°C, to form a strip,
wherein the final temperature during the hot
rolling is at least 300°C,
- the hot-rolled strip being cold-rolled .to final
thickness and being then subjected to soft
annealing at at least 300°C, preferably 350°C.
As a result of the homogenisation of the rolling bar cast
by the conventional bar casting method at temperatures of
500 to 600°C for more than 6 hours, especially for more
than 12 hours, it is achieved that even sluggishly
diffusing elements such as manganese and chromium are
precipitated in a finely dispersed fashion during cooling
of the melt. As a result of the hot rolling at at least
400°C an optimised structure of the hot strip is produced
with regard to the deformability and corrosion resistance
where the final rolling temperature during hot rolling
must be at least 300°C in order to achieve sufficient
deformability of the rolling bar on the one hand and
optimised structure formation during the hot rolling on
the other hand. In this case, the hot strip final
thickness can be less than 9 mm, for example. In order to
facilitate the forming of the strip produced by the
method according to the invention into pre-fabricated
CA 02533428 2006-O1-23
components for the heat exchangers, for example, fins,
tubes or distributors, the strip which has been cold-
rolled to a maximum final thickness of 2 mm by cold
rolling is subjected to subsequent soft annealing at at
least 300°C, preferably 350°C.
As a result of the combination of the alloy composition
of the aluminium alloy in conjunction with the process
features described previously, heat exchangers can be
fabricated on the basis of conventional alloying elements
(Mg, Si, Cu) which, after inert-gas brazing and natural
ageing for about 30 days at room temperature, have yield
points of RP~,2 >- 65 MPa and are thus particularly well-
suited for the enormous loads in motor vehicles. In
addition, inert-gas brazing without using caesium-
containing fluxes can be used to fabricate the heat
exchangers so that economical fabrication is possible.
If the hot rolling and/or cold rolling takes place in a
reversing or unidirectional mode on single- or multiple-
stand rollers, the method according to the invention can
be carried out using conventional means and devices with
regard to the reducing rolling.
A particularly high process safety during brazing of the
heat exchanger can be achieved by cladding the rolling
bar with an aluminium solder after homogenising. The
aluminium strip fabricated from this rolling bar has a
uniform layer of aluminium solder which during brazing,
results in particularly homogeneous and uniform brazed
joints, for example, between the fins, tubes and
distributors of the heat exchanger. If only one side of
the aluminium strip according to the invention is clad
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with an aluminium solder, the other side can be cla d or
coated with an alloy serving as corrosion protectio n, for
example.
Advantageously used as aluminium solder is an aluminium
alloy having a silicon content of 6-13 wt. o, especi ally
an AlSi7 or A1Si10 alloy, which during inert-gas brazing
have a particularly good wetting capacity with aluminium
solder with regard to the oxide layers remaining in non-
oxidising atmospheres on the components of the heat
exchanger to be brazed.
Finally, the object derived and indicated above is solved
according to a third teaching of the present invention by
an aluminium strip or sheet for fabricating heat
exchangers which is produced by the method according to
the invention. As has already been stated, an aluminium
strip or sheet produced by the method according to the
invention has improved strength values, especially yield
point, after natural ageing following brazing so that the
wall thicknesses of the heat exchanger can be further
reduced. In addition, inert-gas brazing using non-
corrosive fluxes can be used to fabricate the heat
exchangers without using caesium-containing fluxes.
The aluminium strip or sheet advantageously has a maximum
thickness of 2 mm, especially 1 mm. As a result of the
higher strength compared with conventional materials,
when using the aluminium strip according to the
invention, the strip thickness can be further reduced and
thus material can be saved during the fabrication of heat
exchangers and a further reduction in the weight of the
heat exchangers can be achieved. In this case, the
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operating safety of the heat exchanger is not impaired,
even at r~igher operating pressures, because of the higher
strength of the aluminium alloy.
There are now a plurality of possibilities for
configuring and further developing the cold age-
hardenable aluminium alloy for heat exchangers according
to the first teaching of the invention, the method for
producing a cold age-hardenable aluminium strip for heat
exchangers according to the second teaching of the
invention and the aluminium strip or sheet according to
the inUention for fabricating heat exchangers according
to the third teaching of the invention. For this purpose,
for example, reference is made on the one hand to the
claims subordinated to claims l, 5 and 9, on the other
hand to the description of an exemplary embodiment of a
method for fabricating a cold age-hardenable aluminium
strip for heat exchangers according to the second
teaching of the invention in conjunction with the
drawing.
In the drawing the single figure is a schematic diagram
showing the production path for implementing an exemplary
embodiment of a method for fabricating a cold age-
hardenable aluminium strip for heat exchangers according
to the second teaching of the invention.
The production path shown in the single figure comprises
the bar casting 1 from an aluminium alloy in a first
step. In this case, the aluminium alloy of the exemplary
embodiment has the following alloying components in wt.%
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0 < Si <-0.
. 70
60 0,
~
0.12 <_ Fe <-0.30,
0.08 <- Cu -<0.20%,
0.04%<- Mn < 0.080,
0.120< Mg <-0.300,
Cr < 0.050,
Zn < 0.050,
0.080<- Ti < 0.20a,
B < 50
ppm,
unavoidable accompanying elements to a maximum of
0.03° and to a maximum of 0.1~ in total and
aluminium as the remainder.
The low boron content of 50 ppm maximum improves the
recyclability of the aluminium alloy. The rolling bars
cast using the DC method from the aluminium alloy just
described are then homogenised in a homogenisation stage
2. Particularly good results with regard to the
homogenisation of the rolling bar were achieved at a
temperature of 575°C for more than 6 h, especially 12 h.
Following homogenisation the rolling bars are then hot-
rolled on a tandem stand 3a to a thickness of 7 mm, for
example, wherein in particular the final temperature
during hot rolling must be higher than 300°C, preferably
330°C, in order to ensure optimised structure formation
during the hot rolling. Alternatively, however, the hot
rolling can be carried out on a reversing stand 3 and
wound onto a reel which is not shown and hot rolling in
the tandem stand 3a can be dispensed with. The subsequent
cold rolling to a final thickness of about 1 mm takes
place on single- or multiple-stand rollers 4. Dike the
hot rolling the cold rolling can alternatively also be
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carried out in reversing mode on a reversing stand. As a
result of final soft annealing at about 350°C in a batch
furnace 5, the aluminium strip is converted to a st ate of
lowest possible strength and high elongation to
facilitate subsequent forming work during fabrication of
the heat exchanger components.
Alternatively to the exemplary embodiment of the method
according to the invention for producing a strip for heat
exchangers which has just been described, after
homogenisation in the homogenisation stage 2 the rolling
bar can be clad with an aluminium solder, for example of
an AlSi7 or AlSilO alloy, to avoid subsequent application
of an aluminium solder before the brazing of the heat
exchangers fabricated from the strip according to the
invention. For this purpose the rolling bar must be
heated to an initial rolling temperature of at least
400°C, preferably 450°C, before the hot rolling. When
brazing heat exchangers fabricated from aluminium strip
or sheet according to the invention, especially when
using inert-gas brazing particularly high strength values
of the heat exchanger, in particular values for the yield
point of RPo,2 >- 65 MPa can be achieved without using
caesium-containing fluxes at temperatures up to 600°C and
typical cooling rates of 30°C/min from 600°C to 200°C as
well as natural ageing of about 30 days at room
temperature after brazing. The cooling from 200°C to room
temperature need not to take place in an exactly defined
manner.
