HEAT EXCHANGER TUBE, HEAT EXCHANGER, AND BRAZING PASTE

Abstract

A heat-exchanger tube has a tube main body composed of an aluminum alloy. A coating is applied onto a surface of the tube main body. The coating contains a powder mixturewhich includes: 1 g/m.sup.2 or more and 7 g/m.sup.2 or less of an Si powder, 0.2 g/m.sup.2 or more and 4.0 g/m.sup.2 or less of a Zn powder, 0.5 g/m.sup.2 or more and 5.0 g/m.sup.2 or less of a first flux powder composed of a compound that contains Zn, and 5 g/m.sup.2 or more and 20 g/m.sup.2 or less of a second flux powder composed of a compound that does not contain Znand a binder. The total amount of the powder mixture in the coating is 30 g/m.sup.2 or less. The proportion of the binder in the coating is 5-40 mass %.

Claims

1. A heat-exchanger tube, comprising: a tube main body composed of an aluminum alloy; and a coating applied onto a surface of the tube main body; wherein, the coating contains a powder mixturewhich includes: 1 g/m.sup.2 or more and 7 g/m.sup.2 or less of an Si powder, 0.2 g/m.sup.2 or more and 4.0 g/m.sup.2 or less of a Zn powder, 0.5 g/m.sup.2 or more and 5.0 g/m.sup.2 or less of a first flux powder composed of a compound that contains Zn, and 5 g/m.sup.2 or more and 20 g/m.sup.2 or less of a second flux powder composed of a compound that does not contain Znand a binder; the total amount of the powder mixture in the coating is 30 g/m.sup.2 or less; and the proportion of the binder in the coating is 5-40 mass %.

2. The heat-exchanger tube according to claim 1, wherein the content of the first flux powder is 0.5 g/m.sup.2 or more and less than 3.0 g/m.sup.2.

3. The heat-exchanger tube according to claim 1, wherein the first flux powder is composed of KZnF.sub.3.

4. The heat-exchanger tube according to claim 1, wherein the second flux powder is composed of a KAlF compound.

5. The heat-exchanger tube according to claim 1, wherein the Si powder has a maximum particle size of 100 m or less.

6. The heat-exchanger tube according to claim 1, wherein the Zn powder has a maximum particle size of 100 m or less.

7. The heat-exchanger tube according to claim 6, wherein the maximum particle size of the Zn powder is 50 m or less.

8. The heat-exchanger tube according to claim 6, wherein the maximum particle size of the Zn powder is 30 m or less.

9. The heat-exchanger tube according to claim 1, wherein the aluminum alloy has a chemical composition in which Cu is 0.05 mass % or less.

10. The heat-exchanger tube according to claim 9, wherein the aluminum alloy further contains Mn: 0.1-1.2 mass %.

11. The heat-exchanger tube according to claim 9, wherein the aluminum alloy further contains at least one element selected from the group consisting of Zr: 0.01-0.30 mass %, Cr: 0.01-0.30 mass %, Ti: 0.01-0.30 mass %, and Sr: 0.01-0.10 mass %.

12. The heat-exchanger tube according to claim 11, wherein the aluminum alloy further contains 0.05-0.30 mass % of Si.

13. A heat exchanger manufactured using the heat-exchanger tube according to claim 1, wherein a fin, and a header, which are composed of aluminum alloy, are joined to the heat-exchanger tube by brazing the coating applied onto the surface of the tube main body.

14. (canceled)

15. A brazing paste, containing: a powder mixturewhich includes 1 part by mass or more and 7 parts by mass or less of an Si powder, 0.2 parts by mass or more and 4.0 parts by mass or less of a Zn powder, 0.5 parts by mass or more and 5.0 parts by mass or less of a first flux powder composed of a compound that contains Zn, and 5 parts by mass or more and 20 parts by mass or less of a second flux powder composed of a compound that does not contain Znand a binder; wherein, the binder content is 5-40 mass % with respect to the total of the powder mixture and the binder.

16. The brazing paste according to claim 15, wherein the content of the first flux powder is 0.5 parts by mass or more and less than 3.0 parts by mass.

17. The heat-exchanger tube according to claim 9, wherein the aluminum alloy further contains at least one element selected from the group consisting of Zr: 0.01-0.30 mass %, Cr: 0.01-0.30 mass %, Ti: 0.01-0.30 mass %, and Sr: 0.01-0.10 mass %.

18. The heat-exchanger tube according to claim 17, wherein the aluminum alloy further contains 0.05-0.30 mass % of Si.

19. The heat-exchanger tube according to claim 9, wherein the aluminum alloy further contains 0.05-0.30 mass % of Si.

20. The heat-exchanger tube according to claim 10, wherein the aluminum alloy further contains 0.05-0.30 mass % of Si.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is an oblique view of a heat-exchanger tube according to working example 1.

[0035] FIG. 2 is an oblique view of a heat exchanger that was prepared using the heat-exchanger tube according to working example 1.

MODES FOR CARRYING OUT THE INVENTION

[0036] The configuration of the above-mentioned tube is explained in detail below.

[0037] (Tube Main Body)

[0038] The configuration of the tube main body is not particularly limited and can be selected as appropriate in accordance with the application, the required characteristics, etc. For example, the tube main body can be formed by extrusion and can be configured as an extruded multi-hole pipe having a plurality of refrigerant passageways internally. In addition, the tube main body may have a shape such as a simple tube shape. In this case, the tube main body may be manufactured by extrusion or may be manufactured by bending a plate material.

[0039] The tube main body is composed of an aluminum alloy. Here, the above-mentioned aluminum alloy is a general term that includes both pure aluminum and aluminum alloys. Aluminum alloys of various chemical compositions can be utilized as the aluminum alloy used in the tube main body.

[0040] For example, a chemical composition that is restricted to Cu: 0.05% (mass %, likewise below) or less, the balance being Al and unavoidable impurities, can serve as the chemical composition of the above-mentioned aluminum alloy. The preferred content ranges and the like of each element based on this chemical composition are explained below. [0041] Cu (Copper): 0.05% or less

[0042] Cu is mixed in, to a certain extent, as an unavoidable impurity. If the Cu content of the tube main body is large, then there is a risk that the intrinsic corrosion resistance of the tube main body will decrease. Consequently, from the viewpoint of increasing corrosion resistance, the Cu content is preferably restricted to 0.05% or less.

[0043] It is known in the conventional art that, when Zn is added to Al, the electric potential is made lower, and when Cu or Si is added, the electric potential is made higher. The present inventors discovered that, in the case in which Zn, Cu, and Si coexist, the effect of making the electric potential higherowing to the synergistic effect of Cu and Sibecomes large as compared to the case in which Cu or Si are present alone; therefore it becomes difficult to obtain the effect of increasing corrosion resistance using Zn.

[0044] That is, if brazing is performed using the above-mentioned tube, a Zn-diffusion layer is formed by the diffusion of Zn in the depth direction from the surface of the tube main body. Furthermore, simultaneous with the diffusion of Zn, Si supplied from Si particles also diffuses in the depth direction, and thereby an Si diffusion layer is formed. Consequently, if the Cu content included in the tube main body is excessively large with respect to the Zn concentration in the above-mentioned Zn-diffusion layer, then the effect of Zn making the electric potential lower is offset by the effect of the Si diffusion layer and Cu making the electric potential higher; therefore it becomes difficult to make the electric potential on the surface side of the tube main body much lower than the electric potential of the deep part.

[0045] On the other hand, if the amount of Zn powder in a coating is increased in order to make the effect of Zn causing the electric potential to be lower to be greater than the effect of the Si diffusion layer and Cu making the electric potential higher, then the total amount of the oxide film present on the surface of the Zn powder also increases. In this case, the content of the above-mentioned second flux powder must be increased in order to eliminate that portion of the oxide film that was added, and consequently the above-mentioned coating becomes thick. As a result, the clearance between the above-mentioned tube and the above-mentioned fin created by the melting of the above-mentioned coating due to the heating becomes large, and consequently there is a risk that the decrease in the dimensions of the entire core of the heat exchanger will become excessive.

[0046] Accordingly, to avoid such a problem and to increase corrosion resistance, it is necessary to both increase the intrinsic corrosion resistance of the tube main body by decreasing the Cu content and to form an electric-potential gradient in which the surface side of the tube main body is a sufficiently lower potential than that of the deep part. By restricting the Cu content to 0.05% or less, such an electric-potential gradient can be implemented and thereby corrosion resistance increased. From the same viewpoint, it is more preferable to restrict the Cu content to 0.03% or less, and yet more preferable to restrict the Cu content to 0.01% or less.

[0047] Mn (Manganese): 0.1-1.2%

[0048] The above-mentioned aluminum alloy may further contain Mn: 0.1-1.2%. Mn functions to increase strength by solid soluting Mn in the Al matrix. By making the Mn content 0.1% or more, a sufficient strength-increasing effect can be obtained. On the other hand, if the Mn content is more than 1.2%, then workability during extrusion decreases, and consequently there is a risk that the efficiency of production of the above-mentioned tube main body will decrease. Accordingly, from the viewpoint of combining both strength and productivity, the Mn content is preferably 0.1-1.2% and more preferably 0.2-1.0%. [0049] Zr (Zirconium): 0.01-0.30%

[0050] The above-mentioned aluminum alloy may further contain Zr: 0.01-0.30%. When the aluminum alloy of the tube main body recrystallizes due to the heating during brazing, Zr functions to coarsen the recrystallized grains and thereby reduce the grain-boundary density. If the Zr content is 0.01% or more, then it is possible to prevent the liquid-phase filler material of the AlSi alloy produced during the brazing from infiltrating into the crystal-grain boundaries of the matrix and thereby to prevent preferential corrosion at the grain boundaries from occurring. On the other hand, if the Zr content is more than 0.30%, then huge crystallized products are produced during casting and there is a risk that it will become difficult to manufacture a suitable tube main body. Accordingly, the Zr content is preferably 0.01-0.30%.

[0051] Cr (Chromium): 0.01-0.30%

[0052] The above-mentioned aluminum alloy may further contain Cr: 0.01-0.30%. Cr functions to coarsen the recrystallized grains and thereby to decrease grain-boundary density, the same as in Zr. By setting the Cr content to 0.01% or more, it is possible to prevent the occurrence of preferential corrosion at the grain boundaries. On the other hand, if the Cr content is more than 0.30%, then huge crystallized products are produced during casting, and consequently there is a risk that it will be difficult to manufacture a suitable tube main body. Accordingly, the Cr content is preferably 0.01-0.30%.

[0053] Ti (Titanium): 0.01-0.30%

[0054] The above-mentioned aluminum alloy may further contain Ti: 0.01-0.30%. If the aluminum alloy contains Ti, then high-concentration regions, in which the Ti concentration is comparatively high, and low-concentration regions, in which the Ti concentration is comparatively low, are alternately layered in the wall-thickness direction. The low-concentration regions tend to corrode more than the high-concentration regions, and consequently the progress of corrosion in the wall-thickness direction is reduced by the formation of the low Ti concentration regions in a laminar manner. As a result, pitting-corrosion resistance and grain-boundary-corrosion resistance are increased. In addition, Ti functions to increase strength at room temperature and at high temperature. To sufficiently obtain these effects, the Ti content is preferably 0.01% or more. On the other hand, if the Ti content is more than 0.30%, then huge crystallized products are produced during casting, and consequently there is a risk that it will become difficult to manufacture a suitable tube main body. Accordingly, the Ti content is preferably 0.01-0.30%.

[0055] Sr (Strontium): 0.01-0.10%

[0056] Sr functions to refine the structure of the eutectic that crystallizes when the liquid-phase filler material solidifies during cooling and to distribute that eutectic structure evenly. This eutectic structure becomes the anode site, and consequently the corrosion configuration can be made into a planar shape by the distribution of the eutectic structure. As a result, pitting-corrosion resistance can be increased. To sufficiently obtain the effect of increasing pitting-corrosion resistance, the Sr content is preferably 0.01% or more. On the other hand, if the Sr content is more than 0.10%, then AlSiSr compounds crystallize, and consequently there is a risk that the refinement of the eutectic structure will become insufficient.

[0057] Accordingly, the Sr content is preferably 0.01-0.10%.

[0058] Si (Silicon): 0.05-0.30%

[0059] By coexisting with Mn, Si precipitates an AlMnSi intermetallic compound as heating is applied during brazing. The AlMnSi intermetallic compound functions to coarsen the recrystallized grains, which makes it possible to reduce the grain-boundary density of the aluminum alloy. If the Si content is 0.05% or more, then AlMnSi intermetallic compounds can be sufficiently precipitated. As a result, the liquid-phase filler material produced during brazing can be prevented from infiltrating the crystal-grain boundaries of the aluminum alloy, and thereby the occurrence of preferential corrosion at the grain boundaries can be further inhibited. On the other hand, if the Si content is more than 0.30%, then the electric potential of the tube main body becomes higher owing to solid soluting Si, and consequently there is a risk that corrosion resistance will decrease. Accordingly, the Si content is preferably 0.05-0.30%.

[0060] Zr, Cr, Ti, and Sr may be added independently or as a composite. If these elements are added as a composite, then the additive effects of the elements can be obtained.

[0061] Homogenization Treatment

[0062] If the tube main body is manufactured by extrusion using an aluminum alloy that contains Mn, then it is preferable to use an aluminum alloy that has been subjected to a homogenization treatment under the conditions below.

[0063] In a first aspect of the homogenization treatment, a homogenization treatment is performed in which an aluminum alloy ingot having the above-mentioned chemical composition is held at a temperature of 400 C.650 C. for 2 hours or more. In this case, the coarse crystallized products formed during casting are decomposed or granulated, and thereby it is possible to homogenize the heterogeneous structures, such as segregation layers, produced during casting. As a result, resistance during extrusion can be reduced and thereby extrudability can be increased. In addition, the surface roughness of the product after extrusion can be reduced.

[0064] If the hold temperature in the homogenization treatment is below 400 C., then there is a risk that coarse crystallized products, the above-mentioned heterogeneous structures, or the like will remain, and there is a risk that such will lead to a decrease in extrudability, an increase in surface roughness, or the like. The higher the hold temperature in the homogenization treatment, the more that the hold time can be shortened and thereby the more that productivity can be increased. However, if the hold temperature is above 650 C., there is a risk that such will lead to the melting of the ingot. Accordingly, the hold temperature in the homogenization treatment is preferably 400 C.-650 C. From the same viewpoint, the hold temperature in the homogenization treatment is preferably 430 C.-620 C.

[0065] In addition, from the viewpoint of sufficiently performing the homogenization, the hold time in the homogenization treatment is preferably 3 hours or more. On the other hand, if the hold time is more than 24 hours, then the homogenization effect becomes saturated, and consequently it is difficult to obtain an effect commensurate with the hold time. Accordingly, the hold time in the homogenization treatment is preferably 3-24 hours.

[0066] In a second aspect of the homogenization treatment, a first homogenization treatment, in which the ingot is held at a comparatively high temperature, and a second homogenization treatment, in which the ingot is held at a temperature lower than that of the first homogenization treatment, may be performed in combination. In this case, hot extrudability can be further increased. In addition, in the extrusion, aluminum pieces that have accumulated in the die are discharged from the die when they reach a certain size, and thereby defects are produced by the adhesion of these aluminum pieces to the surface of the tube; however, the number of the above-mentioned defects can be reduced by utilizing the above-mentioned second aspect.

[0067] The first homogenization treatment is preferably performed by holding the ingot at a temperature of 550 C.-650 C. for 2 hours or more. In this case, the coarse crystallized products formed during casting not only decompose or granulate but can also actively form solid solutes. Solid solute formation tends not to progress if the hold temperature in the first homogenization treatment is below 550 C. The higher the hold temperature in the first homogenization treatment, the shorter that the hold time becomes and thereby the more that the productivity can be increased. However, if the hold temperature in the first homogenization treatment is above 650 C., then there is a risk that the ingot will melt. Accordingly, the hold temperature in the first homogenization treatment is preferably 550 C.-650 C. From the same viewpoint, the hold temperature in the first homogenization treatment is more preferably 580 C.-620 C.

[0068] In addition, by making the hold time in the first homogenization treatment 2 hours or more, solid soluting of the crystallized products can be implemented sufficiently. If the hold time is less than 2 hours, then the above-mentioned effect becomes insufficient. On the other hand, if the hold time is more than 24 hours, then it is difficult to obtain an effect commensurate with the hold time. Accordingly, the hold time in the first homogenization treatment is preferably 2 hours or more and is more preferably 3-24 hours.

[0069] The second homogenization treatment is preferably performed by holding the ingot at a temperature of 400 C.-550 C. for 3 hours or more. In this case, the Mn that is solid soluted in the matrix is precipitated, and thereby the amount of solid solutes of Mn can be reduced. As a result, the deformation resistance in the extrusion decreases and thereby extrudability can be increased. If the hold temperature in the second homogenization treatment is below 400 C., then the amount of precipitated Mn becomes small, and consequently there is a risk that the effect of decreasing deformation resistance will become insufficient. On the other hand, if the hold temperature in the second homogenization treatment is above 550 C., then the Mn tends not to precipitate, and consequently there is a risk that the effect of decreasing deformation resistance will become insufficient.

[0070] In addition, by setting the hold time in the second homogenization treatment to 3 hours or more, Mn can be sufficiently precipitated. If the hold time is less than 3 hours, then Mn precipitation becomes insufficient and consequently there is a risk that the effect of reducing deformation resistance will become insufficient. From the viewpoint of increasing extrudability, the hold time preferably is made long; however, if the hold time is more than 24 hours, then it is difficult to obtain an effect commensurate with the hold time. Accordingly, the hold time in the second homogenization treatment is preferably 3 hours or more, is more preferably 3-24 hours, and is yet more preferably 5-15 hours.

[0071] In the above-mentioned second aspect, the first homogenization treatment and the second homogenization treatment may be performed successively, or the ingot may be temporarily cooled between the first homogenization treatment and the second homogenization treatment. In either case, the above-mentioned effect can be obtained. Furthermore, performing the first homogenization treatment and the second homogenization treatment successively means that, after the first homogenization treatment is complete, the second homogenization treatment is started without cooling the ingot to a temperature lower than the hold temperature in the second homogenization treatment. If the ingot is cooled after the first homogenization treatment, then the second homogenization treatment can be performed by, for example, reheating the ingot after cooling it to 200 C. or lower.

[0072] (Coating)

[0073] The coating present on the surface of the tube main body contains a powder mixturewhich contains an Si powder, a Zn powder, a first flux powder composed of a compound that includes Zn, and a second flux powder composed of a compound that does not include Znand a binder. The coating can be formed by applying a paste, in which the powder mixture and the binder are mixed in a solvent, onto the tube main body and then drying the solvent. The application of the paste can be performed by, for example, a roll-coating method or the like.

[0074] Powder Mixture: 30 g/m.sup.2 or less

[0075] The total amount of the powder mixture contained in the coating is 30 g/m.sup.2 or less. If the total amount of the powder mixture is more than 30 g/m.sup.2, then the reduction of the volume caused by the melting of the powder mixture during brazing becomes excessively large. As a result, the clearance between the tube main body and the fin becomes large, and consequently there is a risk that the dimensions of the heat exchanger after brazing will shrink excessively. To avoid such a problem and from the viewpoint of increasing the dimensional accuracy of the resulting heat exchanger, the total amount of the powder mixture contained in the coating is 30 g/m.sup.2 or less. [0076] Si Powder: 1 g/m.sup.2 or more and 7 g/m.sup.2 or less

[0077] The Si powder reacts with the Al of the tube main body owing to the heating during brazing, thereby producing liquid-phase filler material composed of an AlSi alloy. Thereby, the tube can be joined to the fin, the header, etc.

[0078] The Si powder content is 1 g/m.sup.2 or more and 7 g/m.sup.2 or less. If the Si powder content is less than 1 g/m.sup.2, then the amount of liquid-phase filler material becomes insufficient, and consequently joint failures tend to occur. On the other hand, if the Si powder content is more than 7 g/m.sup.2, then the proportions of the Zn powder, the first flux powder, and the second flux powder with respect to the overall powder mixture become excessively small, and consequently the amount of Zn diffused into the tube main body, the total amount of the flux component, etc. become insufficient. As a result, there is a risk that problems, such as a decrease in corrosion resistance or a decrease in brazeability, will occur.

[0079] In addition, in this case, there is a risk that problems, such as the Si powder being unable to react with the Al of the tube main body and therefore remaining in the liquid-phase filler material, or the corrosion of the tube main body becoming remarkable as the liquid-phase filler material is produced, will occur. From the viewpoint of avoiding such problems, the Si powder content is 1 g/m.sup.2 or more and 7 g/m.sup.2 or less. From the same viewpoint, the Si powder content is preferably 2 g/m.sup.2 or more and 6 g/m.sup.2 or less.

[0080] The maximum particle size of the Si powder is preferably 100 m or less, more preferably 75 m or less, yet more preferably 50 m or less, and in particular preferably 35 m or less. If coarse Si powder is present, then there is a risk that melt holes will be produced owing to local melting of the AlSi eutectic during brazing. By restricting the maximum particle size of the Si powder to the above-specified ranges, the fluidity of the liquid-phase filler material produced during the addition of heat in brazing can be increased, and the formation of melt holes can be prevented by preventing erosion of the tube main body.

[0081] Furthermore, the maximum particle size of the Si powder is set to a value that is measured by a laser diffraction method. This applies likewise to the Zn powder, which is described below.

[0082] Zn Powder: 0.2 g/m.sup.2 or more and 4.0 g/m.sup.2 or less

[0083] The Zn powder functions as a Zn source for forming the Zn-diffusion layer on the surface of the tube main body. By using it in combination with the first flux component as the Zn sources, the Zn powder can form a Zn-diffusion layer that has a high concentration and is deep. In addition, as described above, by using the Zn powder in combination with the first flux powder, the total amount of the powder mixture can be reduced, and thereby a decrease in the dimensions of the overall core of the heat exchanger can be prevented.

[0084] The Zn powder content is 0.2 g/m.sup.2 or more and 4.0 g/m.sup.2 or less. If the Zn powder content is less than 0.2 g/m.sup.2, then the amount of Zn that diffuses into the tube main body becomes insufficient, even considering the amount of the Zn supplied from the first flux powder, and this leads to a decrease in corrosion resistance. If the Zn powder content is more than 4.0 g/m.sup.2, then the Zn concentration of the fillet formed at the junction with the fin becomes excessively high, and consequently preferential corrosion tends to occur more in the fillet than in the Zn-diffusion layer of the tube main body. As a result, there is a risk that the fin will detach at an early stage owing to corrosion. Accordingly, from the viewpoint of both increasing corrosion resistance and preventing the detachment of the fin, the Zn powder content is 0.2 g/m.sup.2 or more and 4.0 g/m.sup.2 or less. From the same viewpoint, the Zn powder content is preferably 0.2 g/m.sup.2 or more and 3.8 g/m.sup.2 or less.

[0085] The maximum particle size of the Zn powder is preferably 100 m or less, more preferably 50 m or less, yet more preferably 30 m or less, and in particular preferably 15 m or less. If coarse Zn powder is present, then there is a risk that melt holes will occur owing to local melting of the AlZn eutectic during brazing. By restricting the maximum particle size of the Zn powder to the above-specified ranges, the formation of melt holes can be prevented. In addition, if the maximum particle size of the Zn powder is in the above-specified ranges, then the liquid phase Zn tends to spread evenly when the Zn particles melt, and consequently the Zn-diffusion layer formed on the tube main body tends to become even.

[0086] First Flux Powder: 0.5 g/m.sup.2 or more and 5.0 g/m.sup.2 or less

[0087] The first flux powder is composed of a compound that contains Zn. As described above, the first flux powder produces the flux component(s) and Zn by reacting with the Al of the tube main body. By using it in combination with the Zn powder and the second flux powder, the first flux powder can reduce the total amount of the powder mixture while maintaining the amount of Zn and the amount of flux supplied, as described above; in turn, the thickness of the coating can be reduced. In addition, the oxide film present on the surface of the Zn powder is effectively eliminated, and consequently a Zn-diffusion layer that has a high concentration and is deep can be formed.

[0088] To obtain the effect of reducing the thickness of the coating and to completely react with the Al of the tube main body, the content of the first flux powder is 0.5 g/m.sup.2 or more and 5.0 g/m.sup.2 or less. If the content of the first flux powder is less than 0.5 g/m.sup.2, then the thickness of the coating cannot be sufficiently reduced. On the other hand, if the content of the first flux powder is more than 5.0 g/m.sup.2, then the coating becomes thick and unreacted first flux powder tends to remain in the vicinity of the surface-layer part of the coating. From the same viewpoint, the content of the first flux powder is preferably 0.5 g/m.sup.2 or more and less than 3.0 g/m.sup.2.

[0089] For example, a KZnF compound, such as KZnF.sub.3, can be used as the first flux powder. In addition, the average particle size of the first flux powder is not particularly limited; for example, a first flux powder having an average particle size of approximately 5 can be used. It is noted that the average particle size is set to a value that is measured by a laser diffraction method. This applies likewise to the second flux powder, which is described below.

[0090] Second Flux Powder: 5 g/m.sup.2 or more and 20 g/m.sup.2 or less

[0091] The second flux powder is composed of a compound that does not contain Zn. By using it in combination with the first flux powder and the Zn powder, the second flux powder can easily achieve both a reduction in the total amount of the powder mixture and the formation of a Zn-diffusion layer that has a high concentration and is deep, as described above.

[0092] The content of the second flux powder is 5 g/m.sup.2 or more and 20 g/m.sup.2 or less. If the content of the second flux powder is less than 5 g/m.sup.2, then, even considering the amount of the flux component produced by the first flux powder, the total amount of the flux component becomes insufficient, and consequently brazeability decreases. In addition, in this case, there is a risk that the effect of eliminating the oxide film on the Zn powder surface will become insufficient, and consequently there is a risk that corrosion resistance will decrease. If the content of the second flux powder is more than 20 g/m.sup.2, then the effect of eliminating the oxide film on the Zn powder, the tube main body, and the like will become saturated and, moreover, the total amount of the powder mixture will become excessive, and consequently the thickness of the coating cannot be sufficiently reduced. Accordingly, to reduce the thickness of the coating while ensuring brazeability, the content of the second flux powder is 5 g/m.sup.2 or more and 20 g/m.sup.2 or less. From the same viewpoint, it is preferably 6 g/m.sup.2 or more and 18 g/m.sup.2 or less.

[0093] For example, a KAlF compound, such as KAlF.sub.4, K.sub.2AlF.sub.5, or K.sub.3AlF.sub.6, can be used as the second flux powder. These compounds may be used alone or in combination. The average particle size of the second flux powder is not particularly limited; for example, a second flux powder having an average particle size of approximately 5 m can be used.

[0094] Binder: 5-40 mass %

[0095] For example, an acrylic resin, a urethane resin, or the like can be used as the binder. The proportion of the binder content with respect to the overall coating (with respect to 100 mass % of the total amount of the above-mentioned powder mixture and binder) is 5-40 mass %. If the binder content is less than 5 mass %, then detachment of the coating tends to occur. On the other hand, if the binder content is more than 40 mass %, then the thermal decomposition of the binder becomes insufficient, and there is a risk that undecomposed binder and the like will remain when brazing is performed. As a result, there is a risk that brazeability will be decreased.

[0096] (Paste)

[0097] The paste for forming the above-mentioned coating preferably contains the above-mentioned Si powder, the above-mentioned Zn powder, the above-mentioned first flux powder, the above-mentioned second flux powder, and the above-mentioned binder. In addition, the paste may contain a solvent or the like in order to adjust the coatability onto the tube-main-body part. The preferred aspects of these powders are as described above.

[0098] The content of each component in the paste can be appropriately set such that it is in the above-specified ranges in the above-mentioned coating state. That is, the content of the Si powder, the content of the Zn powder, the content of the first flux powder, and the content of the second flux powder can be set to 1 part by mass or more and 7 parts by mass or less, 0.2 parts by mass or more and 4.0 parts by mass or less, 0.5 parts by mass or more and 5.0 parts by mass or less, and 5 parts by mass or more and 20 parts by mass or less, respectively. In addition, the binder content may be 5 mass % or more and 40 mass % with respect to the total mass of the above-mentioned four types of powders and the binder.

[0099] In addition, as described above, from the viewpoint of obtaining the effect of reducing the thickness of the coating and completely reacting with the Al of the tube main body, the content of the first flux powder in the paste is preferably 0.5 parts by mass or more and less than 3.0 parts by mass.

[0100] (Heat Exchanger)

[0101] The heat exchanger, in which tubes having the configuration described above are used, can be prepared by bringing fins, which are composed of aluminum alloy, into contact with the coating, then assembling other members, such as headers, and then heating and brazing these. The atmosphere, heating temperature, and times during brazing are not particularly limited, and the brazing method is also not particularly limited.

[0102] Any well-known alloy can be used as the aluminum alloy used in the fins, as long as it has a strength and corrosion resistance sufficient for use in the heat exchanger.

WORKING EXAMPLES

Working Example 1

[0103] Working examples of the above-mentioned heat-exchanger tubes, and heat exchangers manufactured by using the above-mentioned heat-exchanger tubes, are explained below. In the present example, tubes 1 shown in FIG. 1 were prepared using 11 types of alloys A1-A11 having the chemical compositions shown in Table 1. Subsequently, heat exchangers 2 shown in FIG. 2 were assembled using the resulting tubes 1, and the brazeability and corrosion resistance of the resulting 11 types of heat exchangers 2 were evaluated. The details are explained below.

[0104] <Preparation of Tubes 1>

[0105] Billets having the chemical compositions shown in Table 1 were each heated at 600 C. for 10 hours to perform a homogenization treatment. After the homogenization treatment was completed, the billets were cooled to room temperature and then reheated to 450 C. and hot extrusion was performed. By the above, tube main bodies 10, each comprising a plurality of refrigerant passageways 11 and having a cross section perpendicular to the extrusion direction that exhibits a flat shape, were manufactured, as shown in FIG. 1. Separate from the manufacture of the tube main bodies 10, a paste for forming coatings 12 was prepared by mixing the Si powder, the Zn powder, the first flux powder, the second flux powder, and the binder with a solvent.

[0106] The above-mentioned paste was applied, using a roll coater, to the flat surfaces of each tube main body 10 obtained as mentioned above. Subsequently, the solvent was removed by drying the paste, thereby forming the coating 12 on each tube main body 10. It is noted that the content of each component of each coating 12 is as below. [0107] Si powder (maximum particle size of 15 m): 4 g/m.sup.2 [0108] Zn powder (maximum particle size of 15 m, average particle size of 3.4 m): 1.5 g/m.sup.2 [0109] First flux powder (KZnF.sub.3): 2.5 g/m.sup.2 [0110] Second flux powder (a mixture of KAlF.sub.4 powder and K.sub.3AlF.sub.6 powder): 9 g/m.sup.2 [0111] Binder: 25 mass % of overall coating

[0112] The tubes 1 as shown in FIG. 1 were obtained by the above.

[0113] <Preparation of Fins 3>

[0114] Fins 3, having a corrugated shape, were prepared by corrugating plate materials, each composed of an Al(1.2%)-Mn(1.5%)-Zn alloy and having a thickness of 0.1 mm. It is noted that the fin pitch was set to 3 mm, and the fin height was set to 7 mm.

[0115] <Preparation of Headers 4>

[0116] Brazing sheets, each made of an aluminum alloy and clad with a filler material, were each formed into a pipe shape such that the filler material was on the outer side.

[0117] Subsequently, holes, into which the tubes were inserted, were formed on a side surface. By the above, the headers 4 were obtained.

[0118] <Assembly of Heat Exchangers 2>

[0119] Both ends of each tube 1 were inserted into the headers 4 while alternately stacking the tubes 1 and the fins 3, thereby assembling the prescribed shape shown in FIG. 2. By heating and brazing in this state, the tubes 1, the fins 3, and the headers 4 were joined, and thereby the heat exchangers 2 were obtained. Furthermore, the brazing was performed in a nitrogen-gas atmosphere, the temperature of the tubes 1, the fins 3, and the headers 4 was increased to 600 C. at a temperature-increase rate of an average of 50 C./min, and the temperature of 600 C. was held for 3 min, after which the temperature was lowered to room temperature.

[0120] Brazeability and corrosion resistance were evaluated using the 11 types of heat exchangers 2 (test bodies 1-11) obtained by the above. The evaluating methods are explained in detail below.

[0121] <Brazeability Evaluation>

[0122] The joined state of the fins 3, the presence/absence of external-appearance defects such as discoloring, and the presence/absence of melting of the fins 3 were checked by visual observation. The results thereof are shown in Table 2. Furthermore, those for which there was no problem in the visual observation are denoted in Table 2 as Good.

[0123] <Corrosion-Resistance Evaluation>

[0124] The SWAAT test stipulated in ASTM G85 Annex A3 was performed for 1000 hours on each test body. After the tests were complete, the maximum corrosion depth was measured by observing a cross section of each sample using a microscope, and the presence/absence of the detachment of the fins 3 was judged visually. The results thereof are shown in Table 2. It is noted that those having a maximum corrosion depth of 0.05 mm or less were judged to be A+, those having a maximum corrosion depth of more than 0.05 mm and less than 0.10 mm were judged to be A, those having a maximum corrosion depth of more than 0.10 mm and less than 0.20 mm were judged to be B, and those having a maximum corrosion depth of more than 0.20 mm were judged to be C.

[0125] As can be understood from Table 1 and Table 2, test bodies 1-11 showed satisfactory results for both brazeability and corrosion resistance. In particular, test body 1 and test bodies 6-9 showed superior corrosion resistance because they contain Zr, Cr, Ti, or Sr in addition to Mn, Cu and Si and because the content of these elements are within the above-specified ranges.

Working Example 2

[0126] The present example is an example of heat exchangers 2 in which the content of each component in the coating 12 was modified by modifying the composition of the paste for forming the coating 12. In the present example, the tube main bodies 10 were prepared using alloy Al in working example 1, and the tubes 1 were obtained by forming coatings B1-B25 having the compositions shown in Table 3; otherwise, the heat exchangers 2 were prepared the same as in working example 1. Using the resulting 25 types of heat exchangers 2 (test bodies 21-45), brazeability and corrosion resistance were evaluated the same as in working example 1. The results thereof are shown in Table 4.

[0127] As can be understood from Table 3 and Table 4, test bodies 21-35 show satisfactory results for both brazeability and corrosion resistance because coatings B1-B15 having the above-specified compositions were used.

[0128] In test body 36, a portion was created at which a fin 3 and a tube 1 could not be joined because coating B16, which has a low Si powder content, was used, and consequently the joint failed.

[0129] In test body 37, liquid-phase filler material was excessively produced because coating B17, which has a high Si powder content, was used. As a result, melting of a fin 3 after brazing was confirmed.

[0130] Test body 38 shows corrosion resistance equivalent to test body 28, which has approximately the same total amount of Zn. Nevertheless, the content of the first flux powder in test body 38 is low, and consequently the total amount of the powder mixture in test body 38 is greater than that in test body 28. As a result, shrinkage of the dimensions of the heat exchanger after brazing became excessively large.

[0131] In test body 39, coating B19, in which the content of the first flux powder is high, was used, and consequently unreacted first flux powder remained after brazing and produced discoloring.

[0132] In test body 40, coating B20, in which the content of the second flux powder is low, was used; consequently a portion was created in which a fin 3 and a tube 1 could not be joined; therefore, the joint failed. In addition, discoloring occurred in test body 40.

[0133] In test body 41, coating B21, in which the content of the second flux powder is high and the total amount of the powder mixture is more than 30 g/m.sup.2, was used; consequently, shrinkage of the dimensions of the heat exchanger after brazing became excessively large.

[0134] In test body 42, coating B22, in which the Zn powder content is low, was used, and consequently corrosion resistance was insufficient.

[0135] In test body 43, coating B23, in which the Zn powder content is high, was used, and consequently preferential corrosion of the fillet occurred and detachment of a fin 3 occurred.

[0136] In test body 44, coating B24, in which the binder content is low, was used, and consequently peeling of the coating occurred when assembling the tube main bodies 10 into the heat exchanger. As a result, a fin 3 had joint failure.

[0137] In test body 45, coating B25, in which the binder content is high, was used, and consequently the fluidity of the liquid-phase filler material during brazing decreased. As a result, a fin 3 had joint failure. In addition, discoloring of the surfaces of the tubes 1 caused by un-decomposed binder residue was observed.

TABLE-US-00001 TABLE 1 Chemical Composition (mass %) Alloy Mn Cu Zr Cr Ti Sr Si Al A1 0.82 0.05 0.03 <0.01 <0.01 <0.01 0.12 Bal. A2 0.05 0.03 <0.01 <0.01 <0.01 <0.01 0.03 Bal. A3 0.39 0.05 <0.01 <0.01 <0.01 <0.01 0.03 Bal. A4 0.82 0.04 <0.01 <0.01 <0.01 <0.01 0.20 Bal. A5 0.97 0.03 <0.01 <0.01 <0.01 <0.01 0.12 Bal. A6 0.74 0.04 0.13 <0.01 <0.01 <0.01 0.12 Bal. A7 0.83 0.03 <0.01 0.12 <0.01 <0.01 0.12 Bal. A8 0.69 0.02 <0.01 <0.01 0.15 <0.01 0.12 Bal. A9 0.68 0.03 <0.01 <0.01 <0.01 0.06 0.12 Bal. A10 0.97 0.03 <0.01 <0.01 0.02 <0.01 0.03 Bal. A11 0.86 0.01 <0.01 <0.01 <0.01 0.05 0.35 Bal.

TABLE-US-00002 TABLE 2 Corrosion-Resistance Evaluation Max. Corrosion Presence/Absence Alloy Brazeability Depth of Detachment of Test Body Used Evaluation (mm) Judgment Fin 3 Test Body 1 A1 Good 0.04 A+ None Test Body 2 A2 Good 0.11 B None Test Body 3 A3 Good 0.12 B None Test Body 4 A4 Good 0.06 A None Test Body 5 A5 Good 0.07 A None Test Body 6 A6 Good 0.03 A+ None Test Body 7 A7 Good 0.04 A+ None Test Body 8 A8 Good 0.04 A+ None Test Body 9 A9 Good 0.03 A+ None Test Body 10 A10 Good 0.08 A None Test Body 11 A11 Good 0.07 A None

TABLE-US-00003 TABLE 3 Powder Mixture Si Powder First Second Zn Powder Total Amount Max. Flux Flux Max. Avg. Amount Binder of Particle Powder Powder Particle Particle of Powder Proportion Coating Content Size Content Content Content Size Size Mixture in Coating Content Applied Coating (g/m.sup.2) (m) (g/m.sup.2) (g/m.sup.2) (g/m.sup.2) (m) (m) (g/m.sup.2) (mass %) (g/m.sup.2) (g/m.sup.2) B1 1 13 2.5 9 1.6 14 3.5 14.1 15 2.5 16.6 B2 7 10 2.5 10 1.5 15 3.2 21.0 30 9.0 30.0 B3 3 97 2.3 9 1.0 14 3.3 15.3 25 5.1 20.4 B4 6 75 2.4 8 1.4 13 3.5 17.8 10 2.0 19.8 B5 4 31 2.5 9 1.5 13 3.2 17.0 35 9.2 26.2 B6 3 12 0.5 11 1.4 14 3.3 15.9 5 0.8 16.7 B7 4 12 1.0 10 1.3 13 3.1 16.3 30 7.0 23.3 B8 3 13 2.8 9 3.0 13 2.9 17.8 25 5.9 23.7 B9 3 12 3.0 9 1.3 13 3.1 16.3 20 4.1 20.4 B10 3 13 5.0 8 1.2 12 3.0 17.2 30 7.4 24.6 B11 4 13 2.4 5 0.2 13 3.2 11.6 40 7.7 19.3 B12 3 12 2.2 19 3.8 15 3.5 28.0 25 9.3 37.3 B13 5 10 2.5 8 1.4 98 15.3 16.9 25 5.6 22.5 B14 5 13 2.3 9 1.2 49 7.7 17.5 25 5.8 23.3 B15 3 12 2.1 10 1.3 30 5.2 16.4 25 5.5 21.9 B16 0.7 12 2.0 12 1.7 15 3.2 16.4 25 5.5 21.9 B17 8 11 2.1 10 1.4 13 3.3 21.5 25 7.2 28.7 B18 3 14 0.3 23 4.0 12 3.1 30.3 25 10.1 40.4 B19 4 11 6.0 10 1.6 11 3.0 21.6 35 11.6 33.2 B20 5 12 2.3 3 1.5 11 3.0 11.8 25 3.9 15.7 B21 4 12 2.1 23 1.5 14 3.3 30.6 30 13.1 43.7 B22 3 14 2.0 6 0.1 14 3.2 11.1 30 4.8 15.9 B23 4 15 2.5 18 4.4 15 3.6 28.9 30 12.4 41.3 B24 6 12 2.5 10 1.3 13 3.2 19.8 3 0.6 20.4 B25 5 11 2.0 10 1.2 14 3.2 18.2 45 14.9 33.1

TABLE-US-00004 TABLE 4 Corrosion-Resistance Evaluation Max. Corrosion Presence/Absence Coating Brazeability Depth of Detachment of Test Body Used Evaluation (mm) Judgment Fin 3 Test Body 21 B1 Good 0.04 A+ None Test Body 22 B2 Good 0.04 A+ None Test Body 23 B3 Good 0.07 A None Test Body 24 B4 Good 0.05 A+ None Test Body 25 B5 Good 0.04 A+ None Test Body 26 B6 Good 0.08 A None Test Body 27 B7 Good 0.07 A None Test Body 28 B8 Good 0.04 A+ None Test Body 29 B9 Good 0.05 A+ None Test Body 30 B10 Good 0.03 A+ None Test Body 31 B11 Good 0.10 A None Test Body 32 B12 Good 0.02 A+ None Test Body 33 B13 Good 0.04 A+ None Test Body 34 B14 Good 0.05 A+ None Test Body 35 B15 Good 0.05 A+ None Test Body 36 B16 Fin joint failure 0.04 A+ None Test Body 37 B17 Melting of fin 0.05 A+ None Test Body 38 B18 Large dimensional 0.04 A+ None shrinkage Test Body 39 B19 Discoloring 0.03 A+ None Test Body 40 B20 Fin joint failure, 0.04 A+ None discoloring Test Body 41 B21 Large dimensional 0.04 A+ None shrinkage Test Body 42 B22 Good 0.21 C None Test Body 43 B23 Good 0.01 A+ Present Test Body 44 B24 Fin joint failure 0.04 A+ None Test Body 45 B25 Fin joint failure, 0.05 A+ None discoloring