CA1230112A - Heat exchanger core of aluminum material and method for manufacture thereof. - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A heat exchanger core comprising corrugated fins of aluminum material coated with brazing filler and tubes of aluminum material having said corrugated fins joined thereto by brazing, which heat exchanger core has formed on said tube a surface whose zinc concentration Y (in % by weight) and zinc diffusion depth X (in µm) fall in the range expressed by the following formula I.
1 ? Y ? 0.15X - 8 (I)

Description

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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 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 .~

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 #~

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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 ~, "
';

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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
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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 ,, 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.

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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.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A heat exchanger core comprising corrugated fins of aluminum material coated with brazing filler and tubes of aluminum material having said corrugated fins joined thereto by brazing, which heat exchanger core has formed on said tube a surface whose zinc concentration Y (in % by weight) and zinc diffusion depth X (in µm) fall in the range expressed by the following formula I.
1 ? Y ? 0.15X - 8 (I)

2. A heat exchanger core according to Claim 1, wherein the maximum zinc diffusion depth is 50% of the wall thickness of said tube.

3. A heat exchanger core according to Claim 2, wherein said zinc concentration Y (in % by weight) and zinc diffusion depth X (in µm) fall in the range expressed by the following formula II.
1.5 ? Y ? 0.15X - 10 (II)

4. A heat exchanger core according to Claim 3, wherein the maximum zinc diffusion depth is 50% of the wall thickness of said tube.

5. A heat exchanger core according to Claim 1, wherein said aluminum material has a zinc content of not more than 0.3% by weight.

6. A method for the manufacture of a heat exchanger core of aluminum material, which comprises 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 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 said zinc-containing layer to be diffused into the wall of said tube to an extent such that the zinc concentration (in % by weight) and the zinc diffusion depth (in µm) in the surface of said tube will fall in the range defined by the following formula I.
1 ? Y ? 0.15X - 8 (I)