Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR MANUFACTURE OF HEAT EXCHAN OER
WITH ALUMINUM M~TERIAL
FIELD OF INVENTION AND PRIOR ART
This invention relates to a method for the
manufacture of a heat exchanger with aluminum material.
More particularly, this invention relates to a method for
the manufacture of a heat exchanger with aluminum material
excellent in resestance to corrosion.
Recently, heat exchangers such as radiators for
automobiles, evaporators for car coolers and condensers for
car coolers have been advancing toward preponderant use of
aluminum material for reduction in weight. The condenser
for a car cooler, for example, is manufactured by fabricat-
ing both the tube and the heat transfer fins with aluminum
material clad with brazing filler or fabricating one of them
with aluminum material clad with brazing filler and the
other simply with aluminum material, tying the fabricated
components as with jigs, and brazing them with a corrosive
flux in an oven.
Heretofore, in the manufacture of a heat exchanger
with aluminum material by the brazing process with a flux,
it has been customary for the component parts fabricated of
aluminum material to be coated with flux consisting of an
alkaline earth metal salt containing zinc chloride and an
alkali metal salt and then brazed by the use of a brazing
filler of Al-Si alloy (Si content 5 to 12%) at a temperature
below the melting point of aluminum material (about 580 to
6~0 C). By the brazing, the component parts are joined and,
at the same time, the zinc is precipitated to the surface of
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the aluminum material (3ZnC12 + 2Al > 3Zn + 2AlC13) and
diffused toward the center of the aluminum material because
of the heat of brazing. The zinc-diffused layer exhibits
the effect of a sacrificial anode, which goes to improve the
resistance of brazed heat exchanger to corrosion.
When this process is adopted, however, since the
flux to be used possesses high hygroscopicity, the brazed
heat exchanger must be immediately washed with hot water,
pickled with nitric acid, washed with cold water, etc. to a
sufficient extent. These treatments add greatly to the
complexity of process. The zinc chloride contained in the
flux is utilized for incorporating the zinc-diffused layer
in the aluminum material. In any structure of the heat
exchanger's complexity, it is difficult to apply this flux
uniformly on its entire surface. Thus, the amount of zinc
diffused in the aluminum material is heavily dispersed. The
heat exchanger, therefore, locally fails to manifest the
effect of a sacrificial anode in resisting corrosion and
suffers development of pinholes in the wall.
OBJECTS OF THE INVENTION
It is, therefore, an object of an aspect of this in-
vention to provide a novel method for the manufacture of a heat
exchanger using aluminum material.
An object of an aspect of this invention is to provide a
method for the manufacture of a heat exchanger with aluminum
material excellent in resistance to corrosion.
BRIEF DESCRIPTION OF INVENTION
The objects described above are attained by a
method for the manufacture of a heat exchanger with aluminum
material, characterized by the steps of causing precipita-
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tion of zinc in an amount of 0.5 to 20 gjm2 on the surface
of at least a liquid passing tube or members making up the
liquid passing tube, of the heat exchanger formed of
aluminum material, subsequently applying on the surface a
flux formed of a mixture of potassium tetrafluoroaluminate
(KAlF4) with potassium hexafluoroaluminate tK3AlF6), and
brazing the heat exchanger by the use of brazing filler
formed of an aluminum-silicon alloy having a lower melting
point than the aluminum material at a tempera-ture of about
580 to about 620 C.
BRIEF DESCRIPTION OF T~E DRAWINGS
Fig. 1 is a sectional view of a superposition type
heat exchager;
Fig. 2 is a perspective view of a serpentine type
heat exchanger;
Fig. 3 is a graph showing the data of a test for
resistance to corrosion conduc-ted on a heat exchanger
manufactured by the method of this invention and a heat
exchanger manufactured by the conventional method: and
Fig. 4 is a graph showing relation between amount
of precipitated zinc and zinc concentration in surface
layer.
DETAILED DESCRIPTION OF THE INVENTION
The expression "heat exchanger manufactued by the
method of this invention" embraces radiators for automo-
biles, evaporators for car coolers, condensers for car
coolers, car heaters, etc., which each comprise a tube and
heat tranfer fins and, in radiators, further comprise a seat
plate, a reinforcement, etc. All these components are
formed of aluminum material.
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The laminate type heat exchanger illustrated in
Fig. 1, for example, is a typical evaporator in an automo-
bile air conditioner. This evaporator 1 is consturcted by
superposing a multiplicity of tube units 5 each formed by
joining two tray-shaped plates (pieces) 2 around their
peripheral flanges 3 after the manner oE a cream puff to
embrace therein coolant passages 4 and setting corrugated
fins 7 in place in the spaces 6 separating the depressed
walls of the tube units 5. The coolant which enters the
evaporator 1 through an inlet side conduit 8, flows through
the tube units 5, and departs from the evaporator 1 through
an outlet side conduit 9, therefore, exchanges heat with the
air flowing on the fins 7.
The serpentine type heat exchanger has an
appearance as illustrated in Fig. 2. This evaporator 11 is
constructed by zigzagging a flattened tube 14 incorporating
therein a multiplicity of holes 13 for passing coolant and
nipping corrugated fins 15 between the adjacent webs of the
zigzagged tube 14. The coolant which enters the evaporator
11 through an inlet side conduit 16, flows through the
interior of the tube 15, and departs from the evaporator 11
through an outlet side conduit 17, therefore, exchanges heat
with the air flowing along the fins 15.
All the components of the evaporator are formed of
aluminum material.
In the method of this invention, the components
made of aluminum material, particularly the liquid passing
tube or the members forming the liquid passing tube, are
caused to precipitate zinc on the surface in an amount of
0.5 to 2Q g/m2, preferably 5 to 15 g/m2. If the amount of
zinc thus precipitated is less than 0.5 g/m2, the treated
surface fails to manifest the effect of a sacrificial anode
sufficiently and offers satisfactory resistance to corrosion
after the brazing. If the amount of zinc precipitated
exceeds 20 g/m2, the cost is increased without any further
improvement in resistance to corrosion., Besides, the zinc
concentration is so high as to accelerate the elution of the
zinc-diffusd layer and impair the effect of a sacrificial
anode.
The precipitation of zinc to the surface of
aluminum material can be effected by a chemical plating
method, an electroplating method and the like. 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, oE sodium hydroxide,
to 100 g/liter, preferably 20 to 80 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 50C, 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, for example, are aluminum
materials rated as 1050, 1070, I100, 1200, 3003, 3004, 3005,
3200, 5005, 6951, etc.
The brazing filler to be used during the course of
brazing is 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 maerials rated as 4034 (silicon
content 4.5 to 6.0%), 4045 (silicon content 9.0 to 11.0%),
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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 aqueous 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 150 mesh, and more desirably below 200 mesh.
The preparation of the aforementioned mixture can otherwise
be effected by preparing KAlF4 and K3AlF6 separatel~ 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, therfore,
substantially constists of a mixture of K3AlF6 and KAlF4 in
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respective amounts such that the KE/~lF3 (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 condenser of a car
cooler by fabricating a tube of aluminum material and
treating the tube for surface precipitation of zinc and the
heat transfer 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
.J 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 610 C. 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
Torr to 20 Torrs above the atmospheric pressure,
preferably in the rclnge of atmospheric pressure + 20 Torrs.
~ w, the method of this invention will be
described more specifically below with reference to working
examples.
Example 1
A tube 1.0 mm in wall thickness was fabricated of
aluminum material 1050. This tube was immersed in a zinc
salt bath formed of an aqueous solution of 356 g of sodium
z~
hydroxide (NaOH) per liter and 60 g of zinc oxide (ZnO) per
liter at a temperature of 20 C for 5 minutes to effect
precipitation of zinc at a rate of 12 g/m2 on the surface of
the tube. This tube was washed with water, and dried.
Separately, heat transfer fins were fabricated of aluminum
foil of 3003 having aluminum material of 4343 deposited in
the form of clad on both surfaces thereof. The dried tube
and the heat transfer fins were assembled with the aid of a
jig~ The assembly was coated with an aquieous slurry of a
finely pulverized (less than 200 mesh) mixture of potassium
tetrafluoroaluminate and potassium hexafluoroaluminate
[KF/AlF3 (weight) ratio 45 : 55] at an application rate of 5
g/m2 (as solids). The assembly was then placed in an oven
and heated therein under a blanket of nitorgen gas at 600C
under atmospheric pressure to effect brazing. Consequetly,
there was obtained a condenser.
The tube of the condenser thus producd was tested
for internal diffusion of zinc with an X-ray microanalyzer.
The results were as shown in Table 1. The test was conduct-
ed on a total of five points randomly selected.
Table 1
Zinc concentration in Depth of diffusion
Point of test surface layer (% by wei~ht) of zinc (~m)
1 4.9 80
2 4.5 85
3 4.5 80
4 4.4 80
4.5 85
The tube of this condenser was subjected to the
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C~SS test (JIS El-~681) Eor 12nO hours. The results were as
indicated by the curve A in Fig. 3.
Example 2
The procedure of Example 1 was repeated, with that
the amount of æinc precipitated was changed to 5 g/m2, to
produce a condenser. When the -tube of this condenser was
subjected to the same test as involved in Example 1, the
results were as indicated by the curve A in Fig~ 3.
Example 3
The procedure of Example 1 was repeated, except
that the amount of zinc precipitated was changed to 15 g/m2,
to produce a condenser. When the tube of this condenser was
subjected to the same test as involved in Example 1, the
results were as indicated by the curbe A in Fig. 3.
Example 4
The procedure of Example 1 was repeated, with that
the amount of æinc precipitated was changed to 0.5 g/m2, to
produce a condenser. When the tube of this condenser was
subjected to the same test as involved in Example 1, the
results were as indicated by the curve B in Fig. 3.
Example 5
The procedures of Example 1 were repeated, with
that the amounts of zinc precipitated were varied from 5
g/m2 to 20 g/m2 using 3 g/m2 of flux at a temperature of 600
C under atmospheric pressure, to produce a condenser. The
relation between amount of precipitated zinc and zinc
concentration in surface layer is as shown in Fig. 4.
Control
A condenser was produced by following the
procedure of Example 1, except that the treatment for zinc
_g_
precipitation was omitted. The tube of this condenser was
subjected to the same test as in Example 1. The results
were as indicated by Curve B Fig. 3. It is noted from the
data that the tube .sustained through holes.
As described above, the method of this invention
for the manuEacture of a heat exchanger with aluminum
material comprises causing precipitation of 0.5 to 20 g/m2
of zinc on the surface of at least the liquid passing tube
or members making up the liquid passing tube, of the heat
exchanger fabricated of aluminum material, then applying on
the surface the flux, a mixture of potassium tetrafluoro-
aluminate with potassium hexafluoroaluminate, and brazing
the assembled heat exchanger by the use of brazing filler of
an aluminum-silicon alloy having a lower melting point than
the aluminum material at a temperature of about 580 to
about 620 C. The zinc initially precipitated on the surface
of the aluminum material, therefore, is diffused from the
surface toward the center of the aluminum material by virtue
of the heat of brazing and distributed therein at a certain
concentration gradient. At this time, the zinc content in
the surface zone of the aluminum material falls in the range
of 0.2 to 10% by weight as shown in Fig. 4 and the depth of
zinc diffusion falls in the ragne of 30 to 200 ~m. Owing to
the effect of a sacrificial anode manifested by the zinc
uniformly diffusd in the surface zone, therfore7 the hea-t
exchanger of aluminum material is rendered resistant to
corrosion at least in the liquid passing tube. Thus, the
heat exchanger enjoys notably enhanced resistance to
corrosion. Further, since the aforementioned flux is
minimally soluble in water and not hygroscopic, it is
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incapable of corroding the aluminum material. The flux
remaining on the surface, therefore, is not required to be
removed at the cost of time and labor.
