Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~Z30~1~
HEAT EXCHANGER CORE OF ALUMIN~M MATERIAL
AND METHOD FOR MANUFACTURE THEREOF
FIELD ~F INVENTION AND PRIOR ART
This invention relates to a heat exchanger core
made of aluminum material and to a method for the manufac-
ture thereof. More particularly, the present invention
relates to an improved heat exchanger core made of aluminum
material having a tube for passage of liquid joined by
brazing with corxugated fins coated in a furnace with a
brazing filler and to a method for the manufacture thereof.
Recently, an increasing fraction of heat
exchangers such as automobile radiators, evaporators for
automobile air-conditioners, condensers and heaters have
come to be formed of aluminum material which features light
weight, low cost, and high thermal conductivity in the place
of copper alloy material which used to find popular accept-
ance owing to high thermal conductivity and high resistance
to corrosion.
The heat exchangers formed of aluminum material
prove beneficial for use in automobiles which are urged to
come in lowered weight and price. Unfortunately, the
aluminum material is susceptible to corrosion and particu-
larly vulnerable to galvanic corrosion. In the case of heat
exchangers such as automobile radiators and condensers for
automobile air-conditioners which are installed in an
atmosphere disposed to rise to high temperatures and suffer
the occurrence of dirt, a medium capable of encouraging
corrosion, the phenomenon of pitting corrosion occurs
frequently before long. The pitting corrosion naturally
lZ301~;~
degrades these heat exchangers in performance and, in an
extreme case, completely disables them.
In the heat exchangers of aluminum material which
comprise a tube and corrugated fins, sites of pitting
corrosion are distributed exclusively in their tubes. Since
the heat exchangers each have corrugated fins joined to a
tube and these components are generally made of aluminum
materials of dissimilar grades having dissimilar electrode
potentials (for example, the tube made of 1050 and the
corrugated fins of BA12PC), the potential difference between
the two materials causes pitting corrosion to occur prepon-
derantly near the fillets formed of webs of the corrugated
fins. If the number of pits so bored in the tube by the
corrosion is small, the pits cause the tube to suffer from
leakage of liquid without fail and, consequently, shorten
notably the service life of the heat exchanger as a whole.
Heretofore, the occurrence of the pitting corro-
sion has been precluded in the case of a radiator, for
example, by integrally joining the tube, corrugated fins,
and a seat plate by brazing to complete a heat exchanger
core and subsequently applying a protective coating on the
surface of the core by chromate treatment or electrodeposi-
tion.
In this case, the desired prevention of the
pitting corrosion is achieved when the protective coating is
formed in a perfect state and it is enabled to retain the
initial state permanently. In actuality, however, the
formation of the protective coating in a flawless state is
extremely difficult. Not infrequently, part of the protec-
tive coating is peeled by some physical impact exerted as
B -2-
lZ30~L1X
when the heat exchanger is being handled in transit or during
installation in an automobile. Thus, the protective coating
offers no perfect solution to the problem of pitting corrosion.
Recently, it has been proposed to solve the afore-
mentioned problem by causing the core material for corrugated
fins to contain zinc in a prescribed concentration thereby
lowering the potential of the corrugated fins and allowing
the corrugated fins to function as a sacrifice electrode
and actively undergo corrosion and consequently keeping the
tube from pitting corrosion. From the practical point of
view, however, this method is not advantageous because the
seat of this electrolytic corrosion is deprived of uniformity
because of particular structure of union between the tube
and the corrugated fins and, as the result, the possibility
of the tube similarly yielding to pitting corrosion is not
necessarily remote.
_BJECTS OF ASPECTS OF THE INVENTION
An object of an aspect of this invention, therefore,
is to provide a novel heat exchanger core of aluminum material
and a method for the manufacture thereof.
An object of an aspect of this invention is to pro-
vide a heat exchanger made of aluminum material and improved
in durability, which is obtained by causing corrugated fins
coated in a furnace with brazing filler to be joined by brazing
to a tube and a method for the manufacture thereof.
BRIEF DESCRIPTION OF THE INVENTION
The objects described above are accomplished by a
heat exchanger core formed by causing corrugated fins of
aluminum material coated with brazing filler to be joined by
-- 3 --
.~
123011Z
brazing to a tube of aluminum material, which heat exchanger
core has in the surface of the tube thereof a zinc concen-
tration (in % by weight) and a zinc diffusion depth ~in ~m)
falling in the range defined by the following formula I:
l < Y _ 0.15X - 8 (I)
The objects are also attained by a method for the
manufacture of a heat exchanger core of aluminum material,
comprising the steps of causing corrugated fins of aluminum
material coated with brazing filler to be joined by brazing
at a temperature in the range of about 580 to about 620 C
under pressure of 10 2 to 780 Torrs by the use of a non-
corrosive flux to a tube of aluminum material coated with a
zinc-containing layer, and causing the zinc in the aforemen-
tioned zinc~containing layer to be diffused into the wall of
the tube to an extent such that the zinc concentration (in ~
by weight) and the zinc diffusion depth (in um) in the
surface of the tube will fall in the range defined by the
following formula I:
l < Y < 0.15X - 8 (I)
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 i5 a perspective view illustrating a
typical heat exchanger core according to the present
invention.
Fig. 2 is a magnified view of the essential part
of the heat exchanger core of Fig. l.
Fig. 3 is a graph showing the range in which the
zinc concentration and the zinc diffusion depth in the
surface of the tube of the heat exchanger core according
to the present invention fall, and
Fig. 4 is a graph showing the relation between the
--4--
#~
1230~
time of standing and the maximum pit depth in CASS test
conducted on various heat exchanger cores.
DETAILED DESCRIPTION OF THE INVENTION
The expression "heat exchanger core" as used in
this invention embraces the cores of radiators for automo-
biles, those of evaporators for car coolers, those of
condensers for coolers, those of car heaters, etc., which
each comprise a tube and heat transfer fins and, in the
cores of radiators and car heaters, further comprise a seat
plate, a reinforcement, etc. All these components are
formed of aluminum material.
For example, the serpentine type heat exchanger
has an appearance as illustrated in Fig. 1. This evaporator
1 is constructed by zigzagging a flattened tube 4 incorpo-
rating therein a multiplicity of holes 3 for passing coolant
and nipping corrugated fins 5 between the adjacent webs of
the zigzagged tube 4. The coolant which enters the evapora-
tor 1 through an inlet side conduit 6, flows through the
interior of the tube 5, and departs from the evaporator 1
through an outlet side conduit 7, therefore, exchanges heat
with the air flowing along the fins 5. All the components
of the evaporator are formed of aluminum material.
For the pasage of heat medium in the heat exchang-
er of this invention, there can be used a multiplicity of
straight tubes circular, elliptic or rectangular in cross
section, a zigzagged (serpentine) flat tube containing a
multiplicity of parallelly spaced continuous holes for
passage of heat medium, or a tube-forming member composed of
a multiplicity of tube units each produced by joining two
3~ tray-shaped plates (pieces) around their flange parts in the
--5--
~, "
';
~Z3011X
manner of a cream puff so as to give rise therein to a
passage for heat medium.
On the outer surface of these liquid passing
tubes, a zinc containing layer is formed, and the formation
of the zinc containing layer is usually conducted, for
example, by plating to precipitate a zinc layer on the ou~er
surface of the liquid passing tube made of aluminum
material. The precipitation of zinc to the surface of
aluminum material can be effected by a chemical plating
method, an electroplating method, flame spray coating
method, depositing method, etc. A uniform zinc coating is
formed on the surface of aluminum material by immersing this
aluminum material in a treating liquid such as, for example,
a zinc salt bath containing 50 to 500 g/liter, preferably
200 to 400 g/liter, of sodium hydroxide, 5 to 100 g/liter,
preferably 20 to ~0 g/liter, of zinc oxide, and the balance
of water at a temperature of not more than 60 C, preferably
in the range of 20 to 50 C, for 0.5 to 10 minutes,
preferably 0.5 to 5 minutes.
The aluminum material to be used for liquid
passing tube as well as fins are, one containing at most
0.3% by weight, preferably at most 0.1% by weight of zinc,
for example, are aluminum materials rated as 1050, 1070,
1100, 1200, 3003, 3004, 3005, 3200, 5005, 6951, etc.
The brazing filler to be us~ed during the course of
brazing i9 an aluminum-silicon alloy ~having a silicon
content of about 4.5 to about 13.5%) having a lower melting
point than the aluminum material to be used. Concrete
examples are aluminum materials rated as 4034 (silicon
content 4.5 to 6.0%), 4045 (silicon content 9.0 to 11.0%),
3011~
4343 (silicon content 6.8 to 8.2~), 4047 ~silicon content
11.0 to 13.0%), etc. By reason of workability, such brazing
filler is used as deposited in the form of clad on at least
one of the members of aluminum material to be joined.
The flux to be used in the method of this inven-
tion is a mixture of potassium tetrafluoroaluminate (KAlF4)
with potassium hexafluoroaluminate (K3AlF6), both complexes
of potassium fluoride (KF) and aluminum fluoride (AlF3).
Normally, this flux is used in the form of a~ueous slurry.
This mixture is obtained by dissolving AlF3 and KF in
respective amounts constituting an accurate ratio, cooling
the resultant dissolved mixture, pulverizing the cooled
substance to a suitable particle diameter, and suspending
the produced powder in water to afford a dilute slurry.
Generally, this particle diameter is below 100 mesh,
desirably below lS0 mesh, and more desirably below 200 mesh.
The preparation of the aforementioned mixture can otherwise
be effected by preparing RAlF4 and K3AlF6 separately and
mixing the complexes in a prescribed ratio. The method for
the preparation of KAlF4 is disclosed in Brosset Z. Anorg.
Algem. Chemie, 239, 301-304 ~1938).
A typical method for the preparation of the flux
comprises adding 2 parts of water to 1 part of the pulveriz-
ed mixture of complexes thereby producing a dilute slurry
and optionally adding a small amount of surfactant. The
relative ratio of KF and AlF3 to be used in the preparation
of the flux is desired to be such that the resultant mixture
will acquire a melting point as close to the eutectic as
possible. The flux to be used in this invention, therefore,
substantially consists of a mixture of K3AlF6 and KAlF~ in
B
._
1~30~
respective amounts such that the KF/AlF3 (weight) ratio will
range from 40 : 60-to 50 : 50. The flux contains virtually
no unaltered KF.
The members fabricated of aluminum material which
has undergone the treatment for surface precipitation of
zinc and which optionally has been clad with brazing filler
such as the tube produced for use in the condensers for car
cooler by fabricating inserted a tube of aluminum material
and treating the tube for surface precipitation of zinc and
the corrugated fins fabricated, similarly for the condenser
of aluminum material having brazing filler deposited in the
form of clad on either or both of the surfaces, are tied in
a prescribed structure, optionally with the aid of a jig,
then coated with the aforementioned flux in an application
ratio of 0.5 to 50 g/m2, preferably 2 to 10 g/m2, placed in
an oven, and brazed therein at a temperature below the
melting point, desirably at a temperature in the range of
about 580 to about 620 C, and preferably in the range of
590 to 610C. In this case, the site of brazing is desired
to be enveloped with a non-oxidative atmosphere of nitrogen
or argon, for example. The pressure falls in the range of
10 2 to 780 Torrs, preferably 740 to 780 Torrs. The time is
1 to 7 minutes, preferably 2 to 4 minutes.
When the heat exchanger core is assembled by
brazing as described above, the tube 4 and the corrugated
fins 5 are bonded into union by brazing and, at the same
time, a zinc-diffused layer 8 is formed on the outer surface
side of the tube 4 as illustrated in Fig. 2. In this case,
the surface zinc concentration (in % by weight) and the zinc
diffusion depth ~in ~m) of the zinc diffused layer 8 are
~Z3011~:
required to fall in the range indicated by the hatch lines
of Fig. 3. This range is expressed by the following formula
I.
1 < Y < 0.15X - 8 (I)
In the range mentioned above, the preferred
portion is indicated by the crossline in Fig. 3. This
preferred range is expressed by the following formula II.
1.5 _ Y _ 0.15X - 10 tII)
The maximum depth of the zinc diffused layer is
50~, preferably 30%, of the wall thicknessof the tube.
In the tube 4 of the heat exchanger core
constructed as described above, therefore, the zinc diffused
layer 8 functions as a sacrifice amode. ~he sacrificial
corrosion proceeds very gradually throughout the entire
volume of the zinc diffused layer 8 and, only thereafter,
the core material begins to yield to corrosion. Unlike the
heat exchanger core of the conventional structure in which
the tube yields to pitting corrosion before long, therefore,
the heat exchanger core in accordance with invention enjoys
a long service life.
For the sake of thls invention, the relation
between the zinc concentration and the zinc diffusion depth
in the zinc diffused layer 8 formed in the tube 4 has been
limited to the range of the formula I for the following
reason: The zinc diffusion is effected by the action of the
heat used during the brazing. If this relation deviates
from the range, particularly 80 as to be expressed by the
furmula Y > 0.15X - 8, defective brazing ensues and the
layer of sacrificial corrosion to be formed fails to grow to
a sufficient thickness and, as the result, the zinc diffused
_g_
,~,
123~ 2
layer 6 wholly yields to sacrificial corrosion and sustain
pits of corrosion after it has been in service only briefly.
The maximum depth of the zinc diffused layer has
been fixed at 50% of the wall thickness of the tub~ because
the tube, after the zinc diffused layer has wholly underg~ne
the sacrificial corrosion, loses mechanical strength and
therefore tends to sustain cracks under the influence of
vibration, pressure, etc. if the depth exceeds 50%. Then
the lower limit of the surface zinc concentration in the
zinc diffused layer has been fixed at 1% because the
galvanic corrosion caused on the fillets during the brazing
cannot be prevented so much as to jeopardize the resistance
of the layer of corrosion when the zinc concentration is
less than 1~.
Now, the present invention will be described more
specifically below with reference to working examples.
Examples 1 4 and Controls 1-2
Tubes were fabricated of aluminum material shown
in Table 1. These tubes were immersed in a zinc salt bath
formed of an aqueous solution having the composition shown
in Table 1 under the conditions shown in Table 1 to effect
precipitation of zinc at a rate shown in Table 1 on the
surface of the tube. These tubes were washed with water,
and dried. Separately, corrugated fins were fabricated of
aluminum foil of 3003 having aluminum material of 4045
deposited in the form of clad on both surfaces thereof.
Each thickness of clad layer were 0.016 mm and total
thickness of the clad layers and the fin was 0.16 mm. The
dried tube~ and the corrugated fins were assembled with the
aid of a jig. The a~semblies were coated with an aqueous
--10--
,,
7 ,~
~Z3C)~2
slurry of a finely pulverized (less than 200 mesh) mixture
of potassium tetrafluoroaluminate and potassium hexafluoro-
aluminate [KF/AlF3 tweight) ratio 45 : 551 at an application
rate of 5 g/m2 (as solids). The assemblies were then placed
in an oven and heated therein under a blanket of nitrogen
gas under the conditions shown in Table 1 to effect brazing.
Consequently,there were obtained condensers for car cooler.
The tubes of the condensers thus producd were
tested for internal diffusion of zinc with an X-ray
microanalyzer. The results were as shown in Table 2. The
tests were conducted on a total of five points randomly
selected.
The tubes of these heat exchanger cores were
subjected to the CASS test (JIS H-8681) for 1200 hours. The
results were as indicated by the curve A (Example 1), curve
B (Example 2), curve C (Example 3), curve D (Example 4),
curve E (Control 1) and curve F (Control 2) in Fig. 4.
,_-
lZ30~2
. , . C
a~ c c ~ c c
.
C ~ ~ ~ ~ ~ ~ ,~
~,, X X X X X XX
. ~:
~ U u o ~ U aJ
N ~ E~O o o o o o ,~
C Q)O O O o o o ~)
O ~ ~ ~
m o_ u7~D ~D ~ ~ In
~^ ~C
a) o~ a
~ ~O U~ oU~ ~ o
.,, ~ _ . . . . . . ~,
0 C~ In ~D~1 0 U~ U~
O :1 O r-l h
Q~ O C .,,
a ~ N
_
C
~ ~ ~ OU~
C-- . . . . . o
._1 ~ ~ ~1_ ~ o ~ O
~ a~ ~
a ,' 0
U~
a
I U ~
Q. C
a~ ~ ~ o o o o o o o
U
~ c ~ U~
E~
~ ~ _~
m ~
~ o
I -I m c
o~o~ O O O O O O
~--z
o
u c au ~
o ~ C
s--~ oo o o o o o o ~ o
c~ o o ~1--r~ o
0 -~ ~ ~I ~
m 0 3
Q~ C
O ~ o O o o
~-1In OU''\ 10 ~ 1~
~UO o o o o o
al~ 1 C
O
~ ~ a ~ ~ a ~
o a~ o o o o ~ ~o
. . . . . D.
~/ ~ O ~1
0~O.)aJ ~ ~1 ~ H O
_I ~ ~ --I ~ O O
Q QC~ ~Q~
a r~ a c e
U~ X X X X o o o
E~ ~~3 ~t) U
--12--
12301~2
Table 2
Zinc concentration in Depth of diffusion
Sample surface layer (~ by weight) of zinc (~m)
Example 1 2.4 115
Example 2 2.4 87
Example 3 6.5 102
Example 4 1.0 92
Control 1 0.5 80
Control 2 3.1 50
As described above, in the production of the heat
exchanger core of this invention, the zinc-containing
deposited in advance on the outer surface of the tube is
caused to form the zinc diffused layer satisfying the range
of the aforementioned formula I by the action of the heat
used during the brazing in the furnace. Since the zinc
diffused layer functions as a sacrifice electrode and
intentionally undergoes pitting corrosion, the tube is
prevented from pitting corrosion and the heat exchanger core
is protected substantially against degradation of function.
So far as the range defined by the aforementioned formula I
is strictly observed, the heat exchanger core enjoys service
life exceeding the automobile's service life.
Moreover, in accordance with the present
invention, since the zinc diffused layer is formed by the
heating action during the union of the tube and the
corrugated fins by brazing, there is an advantage that the
tube is only required to be coated with zinc in advance and
the formation of the zinc diffused layer does not call for
any exclusive heating work.
