Development of Aluminum Alloy Fin Stock for Heat Exchangers
Using Twin-Roll Continuous Casting Method
by Akira Kawahara *, Akio Niikura * and Takeyoshi Doko *The authors have developed and put into practical use an aluminum alloy fin
stock for automobile heat exchangers that satisfies the requirements for highstrength and high thermal conductivity. This fin stock is an Al-Fe-Ni-Si alloy manufactured by thetwin-roll continuous casting method, whereby the mechanical strength is increased by finely anddensely enhancing dispersion of intermetallic compounds.
The fin stock developed here with a thickness of 0.06 mm has, after brazing process at 600°C for 3 min, a tensile strength of 130 MPa and an electrical conductivity of 50 % IACS. For
comparison, 3000-series alloys widely used for automobile heat exchangers have, under thesame conditions, approximately a tensile strength of 110 MPa and an electrical conductivity of 40% IACS. Thus the new fin stock has achieved a strength increase while maintaining an electricalconductivity equivalent to that of 1000-series alloys, enabling thickness reduction of fin stocks forheat exchangers.

ABSTRACT1.

INTRODUCTION

There has been a requirement to improve fuel economy ofautomobiles aimed at preventing global warming and so
on, raising a strong demand for weight reduction of heatexchangers --one of the major automobile components.
Aluminum alloys are commonly used as a material
for heat exchangers, since they have high thermal
conductivity and their specific strength is superior to othermetal materials such as copper. Recently, much researchand development work is going on to lessen the thicknessof aluminum alloy members in order to further reduce theweight of heat exchangers.
Figure 1 shows the schematic of a radiator, one of theautomobile heat exchangers made of aluminum alloy. Finstock is one of the main members of heat exchangers,
and it is necessary to improve its mechanical strength andthermal conductivity so as to reduce its thickness. Usuallyheat exchangers of aluminum alloy are manufactured
using the brazing method, whereby aluminum alloy
members are heated up to approximately 600°C, which
is close to their melting point. This process makes it
difficult to improve the mechanical strength and thermal
conductivity of the fin stock, because of the fact that theheating for brazing is equivalent to annealing for thealuminum alloy leading to mechanical strength reduction
and that the heating also acts as a solution treatment thus
reducing thermal conductivity.

*

2nd Research Dept., Metals Research Center

Addressing such tasks, we have gained, throughprevious research work, a technological knowledge ofadding nickel to Al-Fe-Si alloys thereby improving themechanical strength without decreasing the thermal
conductivity. It is thought that, in this alloy system, themechanical strength is increased due to the dispersion
enhancement of intermetallic compounds in the Al-FeNi-Si system, while the thermal conductivity does notdecrease because of the small solid solubility of the added
element into the aluminum matrix 1). The Al-Fe-Ni-Si alloy
thus developed has superior mechanical strength andthermal conductivity after brazing, and is in production asa fin stock material for radiators 2).
Since then, heat exchanger manufacturers continued
Resin tank

Header plate
Tube

Fin

Figure 1

Schematic view of heat exchanger.

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Development of Aluminum Alloy Fin Stock for Heat Exchangers Using Twin-Roll Continuous Casting Method

to study the issue of thinning fin stocks, creating arequirement for a new fin stock having a thermal
conductivity equivalent to, and a mechanical strength
higher than, the fin stock in production. To meet thisrequirement, we pursued the study of mechanical strength
improvement based on dispersion enhancement. This
paper reports on the development of an Al-Fe-Ni-Si alloy
fin stock whereby the twin-roll continuous casting method
was applied.

2.

REQUIREMENTS FOR FIN STOCK
CHARACTERISTICS

Figure 2 illustrates the manufacturing process of a radiator
made of aluminum alloy 2). The fin stock is corrugated,assembled with other members such as the tube stock,

Tube

Fin

Header plate

InsertResin tankBrazing

Assembly

Figure 2

2.1 Tensile Strength
During the use of a heat exchanger, the pressureof cooling water swells the tube. If the mechanicalstrength of the fin withstanding the swell is insufficient,the fin would buckle, resulting in breakage of the tube.
Consequently, the tensile strength of the fin stock
should desirably be high. But commonplace methods ofstrengthening aluminum alloys are difficult to be applied
effectively because of the heating for brazing. The methodof enhancing the tensile strength will be described in detailin Section 3.
2.2 Thermal Conductivity
To improve the thermal efficiency of heat exchangers, it
is desirable that the thermal conductivity of constituting
materials be high. But elements added to aluminum alloys
degrade the thermal conductivity, when they dissolve into
the aluminum matrix at brazing.
2.3 Brazeability
Fin stocks are required not to be eroded by filler metal
during brazing process. If the molten filler metal erodes
the fin stock as shown in the right-hand photo of Figure
3, the mechanical strength of fin-tube joints would be
degraded, resulting in deformation or buckling of the
fin stock during the brazing process, or insufficiencyof supporting the swelling of the heat exchanger tubes
during the use.
As fin stocks become thinner, it is more likely that they
are severed by the filler metal due to erosion 3). Moreover,at the brazing process of volume production lines, heatexchangers of various sizes are heated all together, sothat the temperatures of small heat exchangers tend torise higher than 600°C, readily resulting in defects due toerosion.

Core

Gasketand subsequently brazed at temperatures near 600°C.
The heating temperature of 600°C is close to the melting
point of the aluminum alloy, so that various influences
are exerted on its characteristics. Those that are moststrongly influenced will be described below.

Manufacturing process of heat exchanger made ofaluminum alloy 2).
a)

b)

Fin

Filler metalTube

200 µ mFigure 3 Microphotographs of fin-tube joint. Left: Good joint. Right: Erosion
by filler metal causing buckling of the fin stock can be seen.

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3.

INVESTIGATION OF IMPROVINGMECHANICAL STRENGTH

We investigated performance requirements for fin stocks
focusing primarily on the improvement of tensile strength,
considering this is the most important factor for thickness
reduction of fin stocks. Below will be described thetechniques for improving the tensile strength together with
its effects on other characteristics.
Solid-solution hardening is known as one of themechanical strength improving methods for aluminumalloys. For example, 5000-series alloys take advantageof the solid-solution hardening by magnesium. However,
it is impossible to use magnesium as an adding element
to fin stock alloys, because the element reacts withfluorine, a component of brazing flux, during heating
for brazing thus ruling out jointing. Although silicon
and copper also improve the mechanical strength bydissolving into aluminum, they lower the melting point of
the alloy simultaneously. When the melting temperatureof a fin stock alloy is lowered approaching the brazing
temperature, the erosion by filler metal mentioned in
Section 2.3 can occur more readily. Therefore, silicon andcopper cannot be added into heat exchanger members inlarge amounts.
On the other hand, as crystallized grain structures
are made finer, the proof strength and tensile strength
increase in general. However, finer recrystallization grain
structure of fin stocks for heat exchangers corresponds
to an increase in the grain boundaries that constitute thepath for fast diffusion of the molten filler metal, leading
to easy erosion. In addition, slippage deformation
along grain boundaries is considered to dominate thedeformation of aluminum alloys at elevated temperatures.
Thus finer recrystallization grain structure of fin stocks, in
effect, results in a reduction of sag resistance mentioned
in Section 2.4.
What is more, work hardening cannot be used to
improve the mechanical strength of heat exchanger
members, because the fin stock is annealed by theheating for brazing.
For the reasons mentioned above, it is thought that
dispersion enhancement is the most effective techniqueof improving the mechanical strength of fin stocks without
degrading their thermal conductivity, brazeability, andother characteristics.
The 3000-series alloys comprise manganese-added

65

60

55

90

Tensile strength(MPa)

2.4 Sag ResistanceAt the brazing process, heat exchangers are fastenedusing steel wires and the like to keep their geometricalshape, so that they are firmly squeezed due to thedifference of thermal expansion between steel andaluminum at 600°C. Consequently, fin stocks musthave superior sag resistance (buckling resistance) inorder to protect the heat exchangers from deformation
due to buckling in spite of the fastening by steel wire attemperatures near 600°C.

Electrical conductivity(%IACS)

Development of Aluminum Alloy Fin Stock for Heat Exchangers Using Twin-Roll Continuous Casting Method

85
80
75
0

0.2

0.4

0.6

Ni addition (mass%)

Figure 4

Changes in tensile strength and electricalconductivity after brazing of Al-0.1 wt%Si-0.5 wt%Fe
alloy when nickel is added 2).

aluminum alloys whereby Al-Mn intermetallic compounds
are precipitated to improve the mechanical strength. But
manganese has a large solid solubility with aluminumalloys at 600°C, and because of this, the thermal
conductivities of 3000-series alloys after brazing are
considerably lower than 1000-series alloys and the Al-Fe
alloys, making it unfavorable to apply these alloys to fin
stocks for heat exchangers where much emphasis isplaced on thermal conduction efficiency.
We paid attention to the fact that the maximum solid
solubility of iron in the Al-Fe alloys is as small as 0.05
wt% 5), and that that of nickel in the Al-Ni alloys is also
small as comparable to iron 6). Figure 4 shows changes
in the tensile strength and electrical conductivity of an
Al-0.1 wt%Si-0.5 wt%Fe alloy when nickel is added 2).
Meanwhile, this paper uses electrical conductivity as anindex to represent thermal conductivity, because, as is
known, the two are in a proportionality relation and theformer is easier to measure experimentally. It was found,as shown in Figure 4, the mechanical strength of an
Al-Fe-Ni-Si alloy after brazing can be improved without
degrading the thermal conductivity by increasing theaddition of nickel. Thus an Al-Fe-Ni-Si alloy with superior
thermal conductivity and mechanical strength wasdeveloped, and was put to practical use in 1995.
Table 1 shows the changes in fin stock characteristics
when various improvement methods for mechanicalstrength are applied to.

4.

INVESTIGATION OF TWIN-ROLL
CONTINUOUS CASTING METHOD

We found that, as a result of our investigations later than1995 to improve the mechanical strength of fin stock, theaddition of iron and nickel above a certain extent did not
improve the mechanical strength but reduced the size ofrecrystallization grain structure after heating for brazing.
Thus it was concluded that development of a fin stock

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Development of Aluminum Alloy Fin Stock for Heat Exchangers Using Twin-Roll Continuous Casting Method

Table 1

Strengthening methods of aluminum alloys and associated problems when applied to fin stock.

Strengthening method

Addition of magnesium
Solid-solution hardening

Dispersion hardening

ProblemTechnique

Addition of copper or silicon

Incapable of being brazed
Thermal conductivity decreases
Erosion resistance against filler-metal decreases

Addition of manganese

Thermal conductivity decreases significantly after brazing

Addition of iron and nickel

None
Imparted strain is annealed by heating for brazing

Work hardening

Impartment of strain

After brazing, strain cannot be imparted because fin is jointed with
tube

Grain size reduction

Increase of cold rolling

Erosion resistance against filler-metal decreases

reduction ratio

Buckling resistance decreases

Nozzle chip

Casting rollCast sheet

Figure 5

Molten metal

Schematics of solidification area of molten metalin CC method.

suitable for thickness reduction could not be achieved
solely by changing alloy composition.
We paid attention to the fact that higher speeds at
molten alloy casting reduce the grain size of precipitation
phase 7), and proceeded to investigate the manufacturingof fin stock for heat exchangers using twin-roll continuous
casting method (hereafter called “CC method”) whereby
a higher chilling speed at casting was expected to beobtained.
Figure 5 is a schematic diagram of the solidification
area of molten metal in the CC method. In this method,molten metal of aluminum alloy is continuously supplied
between the casting rolls to be cast into a sheet a
few millimeters thick, and is directly wound into a coil.
Because the molten metal is directly chilled with thewater-cooled casting roll, the chilling speed at casting ishigher than the direct chill method (hereafter called “DC
method”) commonly used in industrial casting. Figure 6
shows the result of SEM observation of a sheet cast by
a CC casting machine, whereby it was confirmed thatcoarse intermetallic compounds larger than 10 μm were
not present in the whole area.
The SEM image of Figure 6 was used to estimate thechilling speed at casting. It has been experimentallyconfirmed, and a theoretical explanation is provided, thatin many alloy systems a relationship given below exists
between dendrite arm spacing (DAS) d and chilling speedc at casting 7):

d =A ×c -n
where, A and n are constants specific to the alloy
system10 µ mFigure 6

SEM image of cast sheet.

The values of A and n are identified as 33.4 and 0.33,
respectively 8), in an Al-Fe-Si alloy which resembles thealloy system developed here. Since d is seen to be about
4 μm from the SEM image in Figure 6, the chilling speed
at casting is calculated, using the equation above, as
600~700°C/sec. This is almost 100 to 1000 times thechilling speed in the DC method of 0.5~5.0°C/sec.
Figure 7 gives the tensile strength of fin stocks
manufactured by adding varied amounts of iron plus nickel
to an Al-Si alloy. The fin stocks were prepared by castingAl-0.5 wt%Si alloys added with varied amounts of ironplus nickel using the DC and CC methods, respectively,
hot-rolling only the ingot cast by the DC method, and coldrolling down to 0.06 mm through intermediate annealing.
The fin stocks were heated at 600°C for 3 min, tensile
tested using JIS Standard specimens, and their tensile
strengths were plotted with respect to the total additionof iron plus nickel. It can be seen that the fin stock by DC
method does not significantly increase in strength when
the total addition of iron plus nickel exceeds 2.0 wt%.
In contrast, the fin stock by CC method increases in thetensile strength as the total addition of iron plus nickel
increases. Furthermore, the fin stock cast by the CC
method has higher tensile strengths than the fin stock by
DC method, when their total addition of iron plus nickel isalmost equivalent.

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Table 2140130
Fin stock by CC method

120

110

100

90

1

1.5

2

2.5

3

Total addition of iron plus nickel (wt%)

Figure 7

Si

Fe

Ni

Zn

Al

0.5

1.7

1.1

0.6

Balance

Relationship between tensile strength after brazing
and total addition of iron plus nickel in Al-0.5 wt%Si
alloy.

CHARACTERISTICS OF THE
DEVELOPED FIN STOCK

Thus the application of the CC method was confirmed
to improve the tensile strength. To improve othercharacteristics such as electrical conductivity, we further
investigated the alloy composition, casting conditions, andcold-rolling conditions after casting, thereby succeeded indeveloping an aluminum fin stock for heat exchangers thatis effective in thickness reduction. Table 2 shows a typicalchemical composition of the new alloy developed here.
5.1 Thermal Conductivity and Tensile Strength
Figure 8 shows the tensile strength and electricalconductivity after heating for brazing of the developed
fin stock, in comparison with those of 3000-series alloy(A3003 alloy added with 1.5 wt% zinc), 1000-series alloy(A1050), and Al-Fe-Ni-Si alloy developed by the authors
previously, all manufactured by the DC method. The finstocks are unified into a thickness of 0.06 mm.
The developed fin stock has a tensile strength of about
130 MPa, which is higher than that of the previously
developed Al-Fe-Ni-Si alloy by about 20 MPa, andan electrical conductivity of 50 %IACS equivalent to1000-series alloys.
Figure 9 and Figure 10 show the results of SEM andTEM observations of the fin stocks after heating at 600°C,
respectively. In Figure 9, while intermetallic compounds
are hardly seen to exist in the 3000-series alloy (Figure9-c), they do exist in the Al-Fe-Ni-Si alloy (Figure 9-b)
and abundantly exist, with a fine and dense distribution,in the developed fin stock (Figure 9-a) using the CC
method. In Figure 10, some intermetallic compounds in
the Al-Fe-Ni-Si fin stock manufactured by the DC method
are seen to be more than 5 μm in size, suggesting thatthe mechanical strength by the DC method did not
improve despite the increased addition of iron and nickel,
because the intermetallic compounds coarsened insteadof growing in number. In contrast, most of the intermetallic
compounds in the developed fin stock are not larger than
1 μm. It is thus supposed that, because the CC method
has a high chilling speed at casting, the intermetallic

New alloy
developed here

1000-series alloy

50
Al-Fe-Ni-Si alloy
by DC method

45

40
3000-series alloy

35

5.

Typical chemical composition of fin stock alloy
developed here.

55
Fin stock by DC method

Electrical conductivity (%IACS)

Tensile strength after brazing (MPa)

Development of Aluminum Alloy Fin Stock for Heat Exchangers Using Twin-Roll Continuous Casting Method

80

90

100

110

120

130

140

Tensile strength (MPa)Figure 8

Relationship between tensile strength and electricalconductivity after heating for brazing.

compounds did not coarsen despite the addition of ironplus nickel to a high concentration, thereby achieving adispersion enhancement in number to result in strength
improvement.
5.2 Brazeability
Figure 11 shows the macrostructures of the developed
fin stock, Al-Fe-Ni-Si fin stock by DC method, and
3000-series alloy fin stock. These observations werecarried out by dipping the specimens, after they were
heated corresponding to brazing, into aqua regia. The
recrystallized grain size of the fin stock by DC method isseen to be around 0.2~1 mm, while that of the developed
fin stock is coarsened to more than 5 mm.
As was stated in Section 3, the recrystallized grain sizeof fin stocks for heat exchangers should desirably belarge, as is the case in the developed fin stock. Dispersed
precipitates larger than about 5 μm in size readily
accumulate strain around themselves, often turning into
a nucleus generating site for recrystallization. In view ofthe fact that the developed fin stock has few intermetallic
compounds larger than 5 μm in size, this fin stock is
supposed to have fewer recrystallization generation sites
in comparison with the fin stocks manufactured by the
DC method. On the other hand, the developed fin stock
has many intermetallic compounds smaller than 1 μm,
which is considered to impede moving of subgrains or
grain boundaries at recrystallization of aluminum alloys.
Therefore, because the developed fin stock has fewerrecrystallization generation sites and also the move ofgrain boundaries is impeded, recrystallization is retarded
up to high temperatures, and as a result, the recrystallized
grain size becomes larger after heating for brazing.
Figure 12 shows the cross-sectional microphotographsof fin-tube joints brazed at 610°C for 3 min. The
thicknesses for the developed fin stock and other fin
stocks are 0.06 mm and 0.07 mm, respectively. Whereas

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Development of Aluminum Alloy Fin Stock for Heat Exchangers Using Twin-Roll Continuous Casting Method

a)

b)

c)

10 µ mFigure 9

SEM images of fin stocks after heating for brazing.
a) Fin stock developed here, b) Al-Fe-Ni-Si fin stock by DC method, c) 3000-series alloy

a)

b)

c)

1 µm

Figure 10 TEM images of fin stocks after heating for brazing.
a) Fin stock developed here, b) Al-Fe-Ni-Si fin stock by DC method, c) 3000-series alloy
a)

b)

c)

10 µ mFigure 11 Macrostructures of fin stocks after heating for brazing.
a) Fin stock developed here, b) Al-Fe-Ni-Si fin stock by DC method, c) 3000-series alloyin the joint using the fin stock by DC method the filler
metal erodes the fin stock thus severing it, the developed
fin stock has, in spite of its smaller thickness, a soundjoint without being eroded by the filler metal. Thus thedeveloped fin stock has superior resistance against filler
metal erosion at brazing.
5.3 Sag Resistance
Because slippage deformation along grain boundaries
is considered to dominate the deformation of aluminumalloys at elevated temperatures over 500°C, it would
be effective to make recrystallization grain structurecoarsen in order to prevent the deformation of fin stock
at elevated temperatures. As mentioned earlier, the finstock developed here has a coarse recrystallization grain
structure, and it is thought to have high sag resistance,
accordingly.
Sag resistance of a fin stock can be evaluated bymeasuring fin pendency 9), which becomes smaller, thethicker the fin stock is, and the larger the recrystallized
grain size becomes. The fin pendency was measured
using the method shown in Figure 13, whereby acantilever specimen with an arm length of 50 mm washeated up to 600°C at a speed of 50°C/min, kept for 5min, and cooled down to room temperature to make ameasurement. The specimens used were 0.06 mm and0.07 mm in thickness for the developed fin stock and theothers, respectively.
The sag value of each specimen is shown in Figure 14,
in which the developed fin stock is seen to have superior
sag resistance than those by DC method despite its
reduced thickness.
5.4 Corrosion Resistance
The potential of fin stock is designed to be less noble thanthat of tube so that the former corrodes faster than thelatter. Corrosion resistance, (i.e., penetration lifetime of
the tube) of heat exchangers are designed based on thisFurukawa Review, No. 24

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Development of Aluminum Alloy Fin Stock for Heat Exchangers Using Twin-Roll Continuous Casting Method

a)

b)

c)

200 µ mFigure 12 Cross-sectional microphotographs of brazed fin-tube joints.
a) Fin stock developed here, b) Al-Fe-Ni-Si fin stock by DC method, c) 3000-series alloy
35

50 mm

Sag
(mm)

Jig
Specimen

Figure 13 Sag measuring apparatus.

Sag distance (mm)

30
25
20
15
10

6.

IN CONCLUSION

An aluminum alloy fin stock for automobile heatexchangers has been developed. The fin stock developed
here has, since twin-roll continuous casting method
is applied for its casting, a metallurgical structure inwhich intermetallic compounds are distributed finely
and densely. Due to the dispersion enhancement byintermetallic compounds, the developed fin stock has atensile strength of 130 MPa after heating for brazing and,simultaneously, an electrical conductivity of 50 %IACS
after heating for brazing that is equivalent to 1000-series
alloys. These characteristics have been achieved bythorough investigations of the casting method and alloy
composition, whereby added elements scarcely dissolve
into the aluminum matrix and precipitation of intermetallic

0.07mm0.07mmAl-Fe-Ni-Si alloyfor DC method

3000-series
alloy

5
0

sacrificial corrosion prevention by fin stock over tube, andusually zinc is added to the fin stock for heat exchangers
to make the potential of the material less noble. In thedeveloped fin stock also, the needed addition of zinc is
calculated for a tube material to be incorporated, and 0.6
wt% of zinc is added accordingly.
The fin stock cold-rolled down to 0.06 mm wascorrugated, brazed with the tube material, and thus asimulation sample of mini cores with three tubes was
prepared. The sample was subjected to the salt spray(SS) test and the copper accelerated acetic acid salt spraytest (CASS) test to evaluate the penetration lifetime of the
tube material. As a result, it was shown that the sacrificial
lifetime of the developed fin stock was equivalent to thatof fin stocks by DC method, resulting in an equivalent
penetration lifetime of the tubes.

t=
0.06mmDeveloped alloy

Figure 14 Sag value of fin stocks kept at 600°C for 5 min.

compounds are prevented to grow in size. Also thedeveloped fin stock has superior characteristics suchas filler metal erosion resistance and sag resistance,
because of the coarse recrystallization grain structure
after brazing in comparison to the fin stock by DC method.
The developed fin stock shows superiority in termsof important characteristics for thickness reduction, as
was stated above, and has been in practical application
for automobile radiators. Application of this material toheat exchangers other than radiators is also expected toenable weight reduction and miniaturization of these heatexchangers.

REFERENCES
1) Niikura et al.: Furukawa Electric Review, 100(1997), 117 (inJapanese)2) K. Takeuchi et al.: Journal of Japan Institute of Metals,
39(2000), 81 (in Japanese)
3) Niikura et al.: Proceedings of 92nd Spring Conference ofJapan Institute of Light Metals, (1997), 279 (in Japanese)4) M. Asano et al.: Aluminum Brazing Handbook, Japan Light
Metal Welding & Construction Association, (1992), (inJapanese)
5) J. K. Edger: Trans. Amer. Inst. Min. Met. Eng., 180(1949),
225
6) R. Krsnik et al.: Solid State Commun. 12(1973), 891
7) T. Okamoto: Journal of Japan Institute of Metals, 17(1978),731 (in Japanese)
8) I. Miki et al.: Journal of Japan Institute of Light Metals,
25(1975), 1 (in Japanese)
9) T. Doko et al.: Furukawa Review, 9(1991), 47

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