U.S. patent application number 10/683455 was filed with the patent office on 2004-06-24 for high strength aluminium fin material for brazing.
Invention is credited to Stenqvist, Torkel.
Application Number | 20040118492 10/683455 |
Document ID | / |
Family ID | 20289240 |
Filed Date | 2004-06-24 |
United States Patent
Application |
20040118492 |
Kind Code |
A1 |
Stenqvist, Torkel |
June 24, 2004 |
High strength aluminium fin material for brazing
Abstract
The invention refers to an aluminium alloy, a clad or unclad
material for brazed products containing said alloy as a core, as
well as a method of producing materials to be used in brazed
products from said alloy. The material is suitable for controlled
atmosphere brazing (CAB) using fluxes that manage higher Mg levels
in the materials. The alloy is intended as a fin-stock material for
brazed products, such as heat exchangers. The alloy comprises
0.5-1.0 wt-% silicon, 0.25-0.6 wt-% magnesium, 0.3-0.7 wt-%
manganese, and 0.05-0.25 wt-% zirconium, and optionally up to 4%
Zn, the balance consisting of aluminium and unavoidable impurities.
The method for producing the material comprises the steps of
subjecting said alloy to a casting process and subjecting the cast
alloy to hot rolling and a cold rolling process, possibly followed
by an annealing process.
Inventors: |
Stenqvist, Torkel;
(Finspang, SE) |
Correspondence
Address: |
SWIDLER BERLIN SHEREFF FRIEDMAN, LLP
3000 K STREET, NW
BOX IP
WASHINGTON
DC
20007
US
|
Family ID: |
20289240 |
Appl. No.: |
10/683455 |
Filed: |
October 14, 2003 |
Current U.S.
Class: |
148/692 ;
148/415; 420/541; 420/544 |
Current CPC
Class: |
C22F 1/04 20130101; C22C
21/08 20130101; C22C 21/00 20130101; C22C 21/02 20130101; C22F
1/047 20130101; C22F 1/05 20130101 |
Class at
Publication: |
148/692 ;
420/544; 420/541; 148/415 |
International
Class: |
C22C 021/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 14, 2002 |
SE |
0203009-6 |
Claims
1. An aluminium alloy for brazed products with high strength,
characterized in that the alloy comprises 0.5-1.0 wt-% silicon,
0.25-0.6 wt-% magnesium, 0.3-0.7 wt-% manganese, and 0.05-0.25 wt-%
zirconium, and optionally up to 4% Zn, the balance consisting of
aluminium and unavoidable impurities, the Fe in said impurities
being controlled up to 0.3%.
2. An aluminium alloy according to claim 1, characterized in that
the manganese content is 0.4-0.7 wt-%.
3. An aluminium alloy according to claim 1, characterized in that
the manganese content is 0.5-0.7 wt-%.
4. An aluminium alloy according to any of claims 1-3, characterized
in that the silicon content is 0.6-0.9 wt-%.
5. A clad material for brazed products with high strength,
characterized in that the core alloy comprises 0.5-1.0 wt-%
silicon, 0.25-0.6 wt-% magnesium, 0.3-0.7 wt-% manganese, and
0.05-0.25 wt-% zirconium and optionally up to 4% Zn, the balance
consisting of aluminium and unavoidable impurities, the Fe in said
impurities being controlled up to 0.3%.
6. A clad material according to claim 5, characterized in that the
manganese content is 0.4-0.7 wt-%.
7. A clad material according to claim 5 or 6, characterized in that
the manganese content is 0.5-0.7 wt-%.
8. A clad material according to any of claims 5-7, characterized in
that the silicon content is 0.6-0.9 wt-%.
9. A method of producing a material from the alloy of any of claims
1-8, characterized by the steps of subjecting said alloy to a
casting process, subjecting the obtained material to a hot rolling
process subjecting the obtained material to a cold rolling
process.
10. The method as claimed in claim 9, characterized in that after
said casting process the material is scalped and clad with at least
one additional layer.
11. The method as claimed in claims 9 or 10, characterized in that
after said cold rolling the material is subjected to an annealing
process.
12. Use of the material in any of claims 1-8 to produce a fin-stock
for heat exchangers.
13. Use of the material in any of claims 1-8 in a brazing process
where an inert atmosphere is used.
14. Use of the material in any of claims 1-8 in a brazing process
using a controlled atmosphere and a suitable flux.
Description
[0001] The present invention is related to an aluminium alloy
intended as a fin-stock material for brazed products, such as heat
exchangers, either as a braze clad material containing said alloy
as a core or unclad. The alloy is heat treatable (precipitation
hardenable) The material obtained has got a high strength after
brazing, especially after an artificial ageing treatment, and gives
a high corrosion resistance to the brazed product as it is
sacrificial to the tube. The material may be used to make products
by any brazing method, in particular the controlled atmosphere
brazing method (CAB) when a flux that allows alloys with Mg is
used.
[0002] To attain an aluminium alloy with a very high strength
precipitation hardenable alloys must be used as the precipitation
hardening mechanism gives the highest strength of the aluminium
alloys. In present commercial brazing applications only the AlMgSi
system can be used as the alloys using the other precipitation
hardenable systems have a too low melting point compared to the
melting interval of the standard braze filler alloys.
[0003] When aluminium components are heated in air, the surface
layer oxidises and forms aluminium oxide. Even in the protective
atmosphere in the CAB process some oxygen and water vapour are
present, which will oxidise the surface. A flux is therefore
provided to disrupt the aluminium oxide, and protect the surface
during the brazing. When alloys containing Mg is brazed magnesium
oxides are formed, which will not be broken down by the common
fluxes. Magnesium fluoride compounds are found as residues,
indicating a consumption of the flux by Mg.
[0004] Present manufacturers using CAB has therefore been limited
to use alloys without Mg as fins, and common fin alloys are
modified AA3003 variants with 1 to 3 wt-% Zn. The Zn is added to
provide a sacrificial action of the fin, thereby protecting the
tube from corrosion attacks. Another well-used alloy is the FA6815
from SAPA Heat Transfer [protected by patent SE-510272]. However
with the advent of fluxes (WO-8604007, EP-0091231) that tolerate
and allow higher Mg levels, alloys containing Mg may be brazed in
CAB furnaces.
[0005] In combination with Si, Mg may form small precipitates that
increase the strength of the alloy considerably. This mechanism is
called age hardening. The AA6xxx series of alloys are based on the
Mg and Si precipitates, but alloys in that series are generally not
suitable for brazing, as most of them has a too high Mg content.
Others are without Mn, reducing the sag resistance of the alloy.
Age hardening alloys have not yet been utilised in a fin stock
material to be brazed in the CAB process.
[0006] A challenge today is to manufacture light-weight components
for the automotive market. A lot of research is therefore directed
to reduce the weight of heat exchangers by using thinner strip. The
new invention shows a higher strength compared to the presently
used alloys, while the corrosion protection of the tubes is
retained. This will allow thinner fins with retained strength of
the brazed product, thereby reducing the weight compared to
products brazed today. Alternatively, the higher strength can be
utilised to give a more rigid brazed product that will withstand
higher stresses, such as vibrations or pulsation of the internal
pressure.
[0007] In AlMgSi alloys small Mg.sub.2Si precipitates form during
ageing, causing the strength to increase. Thus the trivial solution
to increase the strength would be to increase the Mg and the Si
contents, allowing more Mg.sub.2Si to form. However, Mg reacts with
the flux during brazing and this limits the amount of Mg. This is
also true for the new fluxes that is said to tolerate Mg, but at a
much higher level (levels of 0.66 wt.-% Mg has been reported in the
literature [Garcia J, Massoulier C, Faille Ph, VTMS 5, Nashville,
Tenn., May 14-17, 2001]).
[0008] U.S. Pat. No. 6,120,848 discloses a method for providing the
flux onto the surface of a brazing sheet, by mechanically embedding
the flux in the cladding. It is mentioned that CsF fluxes allows a
higher Mg in the core materials.
[0009] In U.S. Pat. No. 5,771,962 a modified flux, containing
caesium and/or lithium fluoride, and a modified braze cladding is
used to braze the standard 1000, 3000, 5000 or 6000 alloys
containing up to 3 wt-% Mg by CAB (Controlled Atmosphere Brazing).
The braze cladding on the tube surfaces contains, apart from the
main alloying element silicon, lithium, magnesium, sodium, and
optionally caesium.
[0010] In U.S. Pat. No. 6,234,243 an attempt is made to increase
the strength of aluminium heat exchangers tube material by adding
Mg, and protecting the braze cladding with an intermediate Al--Li
alloy layer. To improve the brazeability Cs is added to the braze
cladding alloy, and finally a modified flux with Cs and Li is
recommended. This is an expensive way to achieve high strength.
[0011] The effort put into trying to find a way to increase the
strength of fin materials above that achievable with the standard
aluminium alloys shows, exemplified by the above patent documents,
shows that there has been a long felt need for a stronger alloy
that may be brazed using CAB. One example can be found in the
patent WO 01/36697 (Corus), where a fin material with the following
composition (in wt.-%) is disclosed: 0.7-1.2 Si (0.75-1.0
preferred), up to 0.8 Fe (0.2-0.45 preferred), up to 0.5 Cu
(0.2-0.4 preferred), 0.7-1.2 Mn (0.8-1.0 preferred), up to 0.35 Mg
(0.2-0.35 preferred), up to 3 Zn, up to 0.25 Zr (0.05-0.15
preferred), up to 0.2 In (0.01-0.1 preferred), up to 1.5 Ni
(0.3-1.2 preferred), up to 0.2 Ti, and up to 0.25 of Cr and V. It
is stated in the patent that a key feature is the, compared to the
standard AA3xxx alloys, relatively high Si content in combination
with a medium Mn content, increasing the strength. The alloy is not
reported to be age hardenable, and the amount of Mn will certainly
make the material quench sensitive, i.e. the age hardening
response, if any, will be low.
[0012] None of the above documents discloses a high strength, heat
treatable aluminium alloy suitable as fin-stock for CAB according
to this invention.
SUMMARY OF THE INVENTION
[0013] The object of the present invention is to provide a high
strength, heat treatable, aluminium alloy while keeping the Mg
content sufficiently low for brazing in a CAB furnace using a flux
that tolerates Mg. Another object is to provide a material with a
sufficient corrosion sacrificial action to protect another material
brazed to the invention. Preferred applications are fins for heat
exchangers, like automotive radiators, heaters, or charge-air
coolers. Other applications are not excluded.
[0014] An important aspect considering the strength of heat
treatable brazing alloys is the quench sensitivity. A quench
sensitive alloy must be cooled rapidly after the solutionising
treatment (i.e. the brazing operation) in order to keep the Mg and
Si atoms in solid solution. A high Mn content will increase the
quench sensitivity.
[0015] The invention will now be described in more detail,
references below being made to the accompanying drawings
wherein
[0016] FIG. 1 shows the yield (Rp0.2) and the tensile strength (Rm)
of the invented alloy compared to two reference materials after a
braze simulation with cooling rates of 0.7.degree. C./s and
2.5.degree. C./s, natural ageing at room temperature.
[0017] FIG. 2 shows the tensile strength (Rm) of the invented alloy
after a braze simulation with a cooling rate of 0.7.degree. C./s
and 2.5.degree. C./s, artificial ageing at different
temperatures.
[0018] FIG. 3 shows the yield strength (Rp0.2) of the invented
alloy after a braze simulation with a cooling rate of 0.7.degree.
C./s and 2.5.degree. C./s, artificial ageing at different
temperatures.
[0019] FIG. 4 shows the sagging test jig.
[0020] Note that standard materials available presently are not age
hardenable after a controlled atmosphere brazing cycle. The tensile
strength of the standard material AA3003 is about 110 MPa and that
of the state of the art FA6815 is about 135 MPa. The yield strength
of the standard material AA3003 is about 40 MPa and that of the
state of the art FA6815 is about 50 MPa. The alloy of the invention
has a notably higher strength than the presently available
alloys.
[0021] The reason for the limitation of the composition of the
alloy according to the present invention and its range will now be
described.
[0022] The concentration of silicon should be 0.5-1.0 wt-%,
preferably 0.6-0.9 wt-%. Below 0.5 wt-% the ageing response is low,
above 1.0 wt-% the solidus temperature of the alloy is
significantly lowered.
[0023] Magnesium increases the strength by forming Mg.sub.2Si
precipitates during ageing, but lowers the brazeability by reacting
with the flux, even when the flux contains Cs or Li. The Mg content
could therefore be as low as 0.25 wt-% and as high as 0.6 wt-%.
Below the lower limit the alloy would not give a sufficient number
of the Mg.sub.2Si precipitates, and a high strength would not be
obtained. The more Mg the higher the strength, but with a too high
level the brazeability will be reduced, and furthermore there is a
risk for incipient melting of the material at the brazing
temperature. However, the preferred Mg level will depend on both
the used flux and the used tube material. Different flux mixtures
are available which have different tolerances to Mg [Garcia et al.,
cited previously]. Thus, if a flux with a high Mg tolerance and a
tube material without Mg is used, then an alloy with 0.5-0.6 wt-%
could be used, while another flux with less tolerance and/or a tube
material with some Mg would limit the Mg in the fin.
[0024] Optionally Zn is added, up to 4%. Zn will increase the
sacrificial action of the fin material and the level must be
optimised together with the tube material.
[0025] To improve the sagging resistance 0.1-0.3 wt-% zirconium,
preferably 0.05-0.25 wt-% is added to the alloy. Zr is distributed
as small Al.sub.3Zr in the material. They will inhibit the
recrystallisation, giving large grains of the material after
brazing. Below 0.05 wt-% this effect is negligible, above 0.3 wt-%
coarse precipitates are formed which will reduce the effect and the
workability of the material.
[0026] Manganese in solid solution increases the strength, however
the quench sensitivity is also increased. Thus, a low Mn content is
beneficial to the strength if cooling rates are low. Furthermore,
Mn is beneficial to the sagging resistance and corrosion
resistance. The Mn content should be 0.3-0.7 wt-%, preferably
0.4-0.7 wt-%, most preferably 0.5-0.7 wt-%.
[0027] Fe has an adverse effect on the corrosion resistance and in
higher amounts on the sagging resistance. It is therefore limited
to 0.3 wt-%.
[0028] Copper is avoided in the alloy. Even though copper will
further increase the strength, it has a negative influence on the
corrosion behaviour. The electrochemical potential will increase,
thereby reducing the anodic action and the protection of the tubes
by the fin material.
[0029] Nickel is also avoided in the alloy. Nickel increases the
risk for obtaining small grains in the product, and thereby reduce
the sagging resistance.
[0030] The claimed material is produced by casting an aluminium
alloy according to the invention, and thereafter subjecting the
obtained material to a hot rolling and a cold rolling process.
After said casting process the material may be scalped and clad
with at least one additional layer. The material may be
interannealed between two cold rolling passes, and partially or
fully annealed after the final cold rolling step. The annealing
step may also be omitted.
EXAMPLE 1
[0031] An alloy was designed according to the composition described
above. The actual composition is shown in Table 1, together with
the limits for two reference materials. The material was first
scalped, hot rolled, and then cold rolled down to 0.1 mm, with an
interannealing. Samples for tests were extracted at 0.5 mm (not
interannealed) and in the final gauge.
[0032] The 0.5 mm material was braze simulated with two different
heating cycles, basically giving cooling rates of 2.5.degree. C./s
and 0.7.degree. C./s between 400 and 200.degree. C. This represents
both an optimal cooling rate and one usually surpassed by brazing
furnaces in practice.
[0033] The increase in strength with time at room temperature is
shown in FIG. 1. It is compared with the strength of the standard
material AA3003 and the high strength fin material FA6815, the
present state of the art. The increase in strength after natural
ageing is substantial for the new material, even though the cooling
rate after brazing is not optimal.
1 Designation Si Fe Cu Mn Mg Zn Zr New invention 0.88 0.27 0.02
0.69 0.26 0.7 0.12 Reference AA3003 + Zn <0.6 <0.7 0.05-0.20
1.0-1.5 <0.05 * <0.05 Reference FA6815 0.5-1.0 <0.7
<0.10 1.4-1.8 <0.05 1.2-1.8 0.05-0.20 Reference AA6063
0.20-0.6 <0.35 <0.10 <0.10 0.45-0.9 <0.10 <0.05 *
The Zn in fin materials may vary, depending on the desired anodic
protection of the tube. Common levels of Zn in fins are between 1
and 2.5 wt. -%. Zn has only marginal effects on other properties
like strength and sagging.
EXAMPLE 2
[0034] Material from the same braze simulations as in Example 1 was
artificially aged at different temperatures after a delay of one
day. Three temperatures were used: 160, 180, and 195.degree. C. The
tensile strength of the samples is shown in FIG. 2 and the yield
strength in FIG. 3. As can be seen, tensile strengths of 250 MPa
and yield strengths surpassing 200 MPa may be obtained.
[0035] The material show a substantial age hardening response, and
yield strengths more than three times as high as that of the
standard materials in brazing of today were achieved.
EXAMPLE 3
[0036] The sagging resistance of the material was measured by
mounting thin strip samples (gauge 0.1 mm) in a special jig,
allowing 60 mm lever length (FIG. 4). The material in the jig was
then subjected to a braze cycle with a 10 minute dwell at
600.degree. C. The deflection is measured when the material has
cooled off.
[0037] The mean deflection for the new invention was 27.6 mm, which
may be compared to 17 to 23 mm for FA6815 and 35 to 40 mm for the
standard heat treatable material AA6063. The new invention shows a
reasonable sagging resistance.
* * * * *