Note: Descriptions are shown in the official language in which they were submitted.
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DC CAST AI ALLOY
This invention relates to an aluminium alloy, particularly one which has
been DC cast and is used for finstock in heat exchangers.
The automotive heat exchanger market requires finstock alloys that offer,
at low cost, a balance of physical and chemical properties, i.e. strength,
formability, sag resistance, corrosion resistance, thermal conductivity and
brazeability.
Aluminium alloy heat exchangers, provided with header plates, tank units,
tubes for coolant (water based) and fins for improved heat exchange, are very
widely used in the automobile industry and elsewhere. Generally the fins are
joined to the tubes by brazing. In order to reduce corrosion of the tubes with
consequent leakage of coolant, it has been common practice to make the fins
electronegative relative to the tubes so that the fins act as sacrificial
anodes.
This has been achieved by adding Zn, Sn or In to the metal used to form the
fins.
Nevertheless, there is a need to balance the sacrificial effect with the
need to maintain thermal performance throughout the service life of the heat
exchanger. If fins corrode too rapidly, the heat transfer characteristics are
severely compromised. Additionally, use of a higher strength fin material in a
particular construction can offer the opportunity to downgauge the fin and/or
the tube material to achieve lower weight targets. The use of continuously
cast materials can, because of the very high solidification rates
(>10°C/sec),
be utilised to achieve the higher conductivity levels required when
downgauging.
WO-A-00/05426 discloses a high conductivity aluminium fin alloy. The
invention disclosed therein relates to continuous strip casting of an AI
alloy,
where the finstock has a conductivity after brazing of greater than 49.0%
IACS. During continuous casting a thin strip is produced which cools quickly.
AI-Fe-Si alloys are disclosed in, for example, the following documents:
GB 1524355, GB 1524354, WO-A-00/05426, JP-A-2000169926, JP-A-
080218143, J P-A-060145861, J P-A-040154931, J P-A-010195257. The
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compositions of the alloys exemplified in these documents varies, but there is
no disclosure of DC cast alloys.
According to a first aspect of the present invention, there is provided a DC
cast alloy of composition (in wt%):
Fe 0.8 - 1.5
Si 0.7 - 0.95
Mn 0.2 - 0.5
Zn 0.2 - 0.8
Mg up to 0.2
Cu upto0.2
Ti <0.1
B <0.01
C <0.01
Unavoidable
impurities
up
to
0.05
each,
0.15
total
AI balance.
The alloy could, for example, be used in header plate, or side support
applications, and may have other uses, but it is primarily intended as a
finstock alloy for heat exchangers.
Preferably, Cu is present in the range 0.05 - 0.2, even more preferably
0.1 - 0.15. When present, Cu is included as a solid solution strengthening
component.
Mg is preferably present as a strengthening component. At high
concentrations, an undesired Mg0 deposit is formed on the metal surface
during brazing. The Mg concentration is controlled at levels where this is not
a problem. Higher levels of Mg will, in the presence of K3AIF6 and KAIF4 flux
mixtures, lead to the formation of high melting point aluminium fluorides
which
have a deleterious effect on clad fluidity. Preferably, Mg is present in the
range 0.05 - 0.2, even more preferably 0.1 - 0.15.
The strengthening effect of the Cu and Mg has to be balanced against the
reduction of thermal conductivity caused by these elements.
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Fe is preferably present in the range 0.8 - 1.4, although an even more
preferred upper limit of the range is 1.35. Higher levels cause the formation
of
excessively large intermetallic particles.
Si above the upper limit of 0.95% reduces the solidus of the alloy to below
610°C which is unacceptable as it is too close to the brazing
temperature.
The lower limit is determined by the amount of Si necessary to achieve the
required post braze strength.
Mn if present in amounts more than 0.5% has a deleterious effect on the
thermal conductivity. Below 0.2%Mn there is insufficient strengthening effect.
Zn is added to make the finstock more electro-negative than, and
sacrificial to, the tube material of the heat exchanger. Above 0.8%Zn, the fin
becomes too electro-negative. Such high levels of Zn are detrimental to the
thermal conductivity of the fin. Below 0.2%Zn, the fin is not sufficiently
electro-negative to be sacrificial. The preferred range is 0.5 to 0.7%Zn.
According to a second aspect of the present invention, there is provided a
DC cast aluminium alloy finstock having a composition (in wt%):
Fe 0.8 - 1.5
Si 0.7 - 0.95
Mn 0.2 - 0.5
Zn 0.2 - 0.8
Mg up to 0.2
Cu up to 0.2
Ti <0.1
B <0.01
C <0.01
Unavoidable
impurities
up
to
0.05
each,
0.15
total
AI balance.
The finstock sheet may be clad with a layer of aluminium alloy that is rich
in silicon. Suitable alloys are AA4343 and AA4045 although other silicon rich
alloys may be used. The silicon rich layer may optionally contain additions
such as Zn or Sn or In that render the fin more electronegative.
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This finstock may have the above-mentioned preferable or optional
features. Preferably the finstock has a conductivity after brazing of at least
45% IACS.
The aluminium alloys of this invention are DC cast, and the reasons for
this are described in further detail below.
There is also provided a brazed heat exchanger having fins of the above-
mentioned alloy.
According to a further aspect of the present invention, there is provided a
method of producing an ingot from an alloy of composition (in wt%):
Fe 0.8 - 1.5
Si 0.7 - 0.95
M 0.2 - 0.5
n
Zn 0.2 - 0.8
Mg up to 0.2
Cu upto0.2
Ti <0.1
B <0.01
C <0.01
Unavoidable
impurities
up
to
0.05
each,
0.15
total
AI balance,
which method comprises DC casting the alloy to form an ingot.
In this aspect, the present invention is therefore aimed at producing the
ingot by conventional DC casting, thereby avoiding the need for continuous
casting (for example belt casting or twin roll casting). DC casting produces a
thick ingot that cools slowly during solidification. Continuous casting
produces
a thin, 3 - 20mm strip that cools quickly. The difference in cooling rate has
a
profound effect on the metallurgical structure of the cast product. These
differences remain even after processing the ingot down to very thin gauge
foil. By including DC casting, the process of the present invention produces a
material that has properties comparable to those of continuous cast material.
The control of the particle size of the DC cast material is important in
obtaining these properties. In DC cast products, the cooling rate is
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approximately 1 - 5°C per second depending on the ingot thickness.
Finstock
derived from the alloy should have high thermal conductivity after brazing
(which typically occurs at 595°C to 605°C for times of about 2 -
10 minutes)
combined with good strength (UTS) and corrosion potential. For ease of
measurement, the thermal conductivity is usually implied by measuring the
electrical conductivity. Both are reduced by elements in solution so there is
a
need to reduce the amount of soluble elements going into solution during
brazing. This is achieved by controlling the particle size of the
intermetallics
and the dispersoids in the rolled sheet and the chemical composition of the
sheet.
The high strength achieved by the DC cast material allows downgauging,
to below 100wm, for example below 75pm, thus providing the ability to
achieve new and improved lightweight fins. Additionally, the composition of
the alloy has been chosen to maximise the absorption of brazing sheet scrap
to ensure a low-cost production route, and maximise recyclability.
Preferably, the method further comprises the steps of heating or
homogenising the DC 'cast ingot, hot rolling, cold rolling, and
(inter)annealing,
and may further comprise the step of cold rolling after the interannealing
step.
The final thickness reduction during cold rolling is preferably between 25 and
45%.
The homogenising step may be a two-stage homogenisation comprising
heating the ingot to 580 - 620°C, holding for up to 8 hours, cooling to
460 -
500°C and holding for up to 8 hours.
Alternatively, the heating step may be a simple heat to roll step
comprising heating to 460 - 540°C, and holding for up to 8 hours. This
simple
heat to roll step is not sufficient to bring about complete homogenisation of
the
ingot. It is intended to bring the ingot to a uniform temperature to
facilitate hot
rolling.
The ~ hot rolling is preferably performed to 2.5 to 5.Omm, with an exit
temperature from the hot mill of typically 280 - 360°C. Where an
interannealing step is present, subsequent cold rolling to 50 - 700p,m is
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preferred. Cold rolling to final gauge without an intermediate anneal is
followed by a final anneal.
The (inter)annealing step may be a single stage process at 250 -
450°C,
preferably holding at this temperature for 2 to 4 hours. Alternatively, it may
be
a two-stage process comprising heating at 300 - 500°C (preferably
holding for
up to 4 hours) and subsequently cooling to 200 - 350°C (preferably
holding for
up to 4 hours).
Alternatively, annealing may be carried out in a continuous annealing
furnace with the strip being fed through as a single strand to greatly
increase
the heating rates and reduce the holding times required. Higher annealing
temperatures and shorter annealing times may thus be achieved.
The method may further comprise the step of brazing, wherein the particle
size of the intermetallics and/or dispersoids present in the rolled product is
sufficiently large such that the reduction in IACS is less than 5% units
during
brazing.
During the brazing cycle, small (sub-micron) dispersoids dissolve readily
and increase the solute level. Rapid cooling after brazing retains
substantially
all of the solute in solution and this reduces the IACS value and the thermal
conductivity. Increasing the mean dispersoid size reduces the amount
dissolved during the brazing cycle and hence reduces the reduction in IACS
and thermal conductivity. Dispersoid size is increased by using special
homogenising and/or annealing treatments that coarsen the alpha AI(Fe,
Mn)Si dispersoids and probably also coarsen some of the intermetallic
particles. The homogenising treatment preferably comprises consecutive
treatments at two temperatures, a first high temperature treatment followed by
a lower temperature treatment. Annealing treatments may follow the same
pattern.
In the absence of these special homogenising or annealing treatments,
the dispersoids present in the sheet generally have a mean size substantially
below one micron (as determined by the mean linear intercept method). The
special homogenising and annealing treatments increase the mean
dispersoids size, for example to greater than 0.5 microns preferably greater
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than 1 ~.m and ideally greater than 2p.m but less than 1 Op.m. Thermodynamic
calculations reveal that these particles are unlikely to transform during the
brazing cycle.
The intermetallic particles (containing Fe Mn and Si together with other
elements) are substantially larger than the dispersoids; their size is
determined during solidification and possible break up during rolling.
According to a further aspect of the present invention, there is provided a
method of producing an aluminium alloy finstock from an alloy comprising (in
wt%):
Fe 0.8 - 1.5
Si 0.7 - 0.95
Mn 0.2 - 0.5
Zn 0.2 - 0.8
Mg up to 0.2
Cu up to 0.2
Ti <0.1
B <0.01
C <0.01
Unavoidable
impurities
up
to
0.05
each,
0.15
total
AI balance,
which method comprises DC casting the alloy, heating or homogenising,
hot rolling, cold rolling, and annealing or interannealing.
The invention will now be described, by way of example, with reference to
the following figures and examples, and in which:
Figure 1 is a flow diagram showing various processes of the present
invention;
Figure 2 is a graph showing post-brazed strength trends;
Figure 3 is a graph showing corrosion potential trends (the left hand
column of each pair relates to interannealed, and the right hand column
relates to back annealed, i.e. annealed after cold rolling to final gauge);
Figure 4 is a graph showing post-brazed mechanical properties;
Figure 5 is a graph showing corrosion potentials; and
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Figure 6 is a graph showing the relationship between post-brazed UTS
and grain size.
Refierring to Figure 1, there is shown various treatments of the DC ingot
covered by the present invention, including the preferred conditions of each
step.
Example 1
Ingots approximately 1350mm x 600mm and weighing in excess of
7500kg were DC cast with the following composition:
Si Fe Mn Cu Mg ~Zn Ti B
0.85 1.25 0.45 0.15 0.15 0.60 <0.05 <0.01
One ingot (route 1, Figure 1 ) was heated to 520°C and held for
about 4
hrs before hot rolling to 3.Omm, with an exit temperature of approximately
325°C. The coil was then cold rolled to a transfer gauge of 400pm and
then
cold rolled to a gauge of 95pm, interannealed at 270°C for 2 hours at
temperature, and further cold rolled to a fiinal gauge of 63Nm (final
reduction of
about 34%).
Post-Brazed Properties:
UTS: 138MPa
IACS: 46% IACS
Potential: -755mV (According To: ASTM G69 Standard Test
Method For Measurement Of Corrosion Potentials Of
Aluminium Alloys)
Example 2
Material produced according to Example 1 as far as the transfer gauge
(400pm) was taken and subjected to a range of interanneal temperatures
between 200 and 400°C for about 2 hrs before cold rolling to 63pm
gauge.
Post-brazed strength trends are shown in Figure 2 - interannealed route.
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Corrosion potential trends are shown in Figure 3 - interannealed route.
Conductivity values are approx. 45% IACS.
Additionally, material at transfer gauge was cold rolled directly to 63pm
and given a partial anneal (back anneal) at temperatures between 200 and
400°C.
Post-brazed strength trends are shown in Figure 2 - back annealed route.
Corrosion potential trends are shown in Figure 3 - back annealed route.
Conductivity values are approx. 45.5% IACS.
Example 3
Material produced according to Example 1 as far as the transfer gauge
(400pm) was taken and cold rolled to the following gauges:
253 pm, 125 pm, 95 pm and 89 pm
and interannealed (2 hrs at 270°C) before cold rolling to a final gauge
of
63 pm. Final rolling reductions were:
>70%, 50%, 34% and 30% respectively.
Post-brazed mechanical properties are shown in Figure 4 (between 135-
140MPa).
Corrosion potenfiials are shown in Figure 5 and are largely unaffected by
interanneal gauge.
Conductivity is about 45% IACS.
Interanneal gauge is varied to maximise post-brazed grain size which
benefits sag resistance during the braze cycle. For example, the thicker the
interanneal gauge the smaller the grain size after brazing. Furthermore, the
aspect ratio of the grains (length in the cold working direction to the
thickness
direction) increases as the interanneal gauge is reduced.
Example 4
Material produced according to the composition noted in Example 1 was
taken and heated to 460°C and held for approximately 4 hours before hot
rolling to 3.Omm gauge. The coil was then cold rolled to a transfer gauge of
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400p,m and subsequently cold rolled and interannealed at 360°C, to a
final
gauge of 63~,m. The final pass reductions were in the range 45 to 25%.
Figure 6 shows the relationship between post-brazed UTS and grain size.
To achieve maximum UTS the average grain size needs to be less than
50~,m. However, reducing the post-brazed grain size does reduce the sag
resistance of the material.
A balance between post-brazed grain size, UTS and sag resistance can
be achieved by selecting specific combinations of:
- preheat temperature and soak time
- interannealing gauge
- final cold reduction.
