Brazing sheet for flux-free brazing, method for flux-free brazing and method for producing heat exchanger

Abstract

Brazing sheet for flux-free brazing, wherein an outermost surface brazing filler metal layer, consisting of an AlSi-based alloy containing 4 to 12% Si in mass %, and an intermediate brazing filler metal layer, consisting of an AlSiMg-based alloy containing 1% or more and less than 4% Si and 0.1 to 5.0% Mg in mass %, are cladded on one side or both sides of a core material, and wherein aluminum members are joined to each other without using flux in a non-oxidizing gas atmosphere under normal pressure with an oxygen concentration of 300 ppm or less, using the brazing sheets.

Claims

1. A brazing sheet for flux-free brazing, wherein an outermost surface brazing filler metal layer, consisting of an AlSi-based alloy comprising 4 to 12% Si in mass %, and an intermediate brazing filler metal layer, consisting of an AlSiMg-based alloy comprising 2.5% or more and less than 4% Si and 0.1 to 5.0% Mg in mass %, are cladded on one side or both sides of a core material.

2. The brazing sheet for flux-free brazing according to claim 1, wherein cladding ratios of the outermost surface brazing filler metal layer and the intermediate brazing filler metal layer are each 1 to 30% per each side relative to an overall thickness of the brazing sheet.

3. The brazing sheet for flux-free brazing according to claim 1, wherein 0.01 to 0.5% Bi in mass % is contained in a brazing filler metal of the intermediate brazing filler metal layer.

4. The brazing sheet for flux-free brazing according to claim 1, wherein 0.01 to 0.5% Bi in mass % is contained in a brazing filler metal of the outermost surface brazing filler metal layer.

5. The brazing sheet for flux-free brazing according to claim 1, wherein in the outermost surface brazing filler metal layer, a first number of Si particles having a circle equivalent diameter of at least 1.75 m accounts for at least 25% of a second number of Si particles having a circle equivalent diameter of at least 0.8 m, as observed in the direction of the surface layer.

6. The brazing sheet for flux-free brazing according to claim 1, wherein the intermediate brazing filler metal layer has fewer than 100,000 per mm.sup.2 of Si particles having a circle equivalent diameter of 0.25 m or more, as observed in a cross section of the brazing filler metal layer.

7. A method for flux-free brazing of aluminum members, wherein the aluminum members are joined to each other without using flux in a non-oxidizing gas atmosphere with an oxygen concentration of 300 ppm or less, using brazing sheets for flux-free brazing according to claim 1.

8. A method for producing a heat exchanger, wherein aluminum members are joined to each other without using flux in a non-oxidizing gas atmosphere with an oxygen concentration of 300 ppm or less, using brazing sheets for flux-free brazing according to claim 1.

9. The brazing sheet for flux-free brazing according to claim 5, wherein the intermediate brazing filler metal layer has fewer than 100,000 per mm.sup.2 of Si particles having a circle equivalent diameter of 0.25 m or more, as observed in a cross section of the brazing filler metal layer.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a brazing sheet for flux-free brazing in one embodiment of the present invention.

(2) FIG. 2 shows a perspective view of an aluminum-made automobile heat exchanger in one embodiment of the present invention.

(3) FIG. 3 shows the brazing evaluation model in examples of the present invention.

DESCRIPTION OF EMBODIMENT

(4) The following describes one embodiment of the present invention on the basis of the attached drawings.

(5) Of the aluminum alloys for the brazing filler metal, that for the outermost surface layer is prepared to a composition containing, in mass %, Si: 4 to 12%, containing, as desired, Bi: 0.01 to 0.5%, and with Al and inevitable impurities as the balance, and that for the intermediate layer is prepared to a composition containing, in mass %, Si: 1% or more and less than 4%, containing Mg: 0.1 to 5.0%, containing, as desired, Bi: 0.01 to 0.5%, and with Al and inevitable impurities as the balance. Moreover, as aluminum alloys for the brazing filler metal, others such as Fe, Cu, Mn, Ca, Li, and Be can be contained in known amounts as the brazing filler metal.

(6) Moreover, the aluminum alloy for the core material is prepared to a composition containing, in mass %, Mn: 0.1 to 3.0%, Si: 0.1 to 1.2%, Cu: 0.1 to 3.0%, and with Al and inevitable impurities as the balance. Moreover, as the aluminum alloy for the core material, others such as Si, Mn, Fe, Mg and Bi can be contained in known amounts.

(7) For the present invention, the composition of the aluminum alloy for the core material is not particularly limited, but alloys with positively added Mg and Si can be used preferably since they can achieve a significantly high strength of the material by precipitating Mg.sub.2Si etc. With the conventional brazing method using a fluoride-based flux, since the flux becomes inactive by reacting with Mg and generating Mg fluoride of high melting point, the brazability was decreased, and it was difficult to apply it to high strength Mg added alloys since Mg is consumed by this reaction, but with flux-free brazing, it is possible to utilize high strength Mg added alloys.

(8) An aluminum alloy with Zn can be cladded as a sacrificial anticorrosion layer between any clad layers, or on the core material surface wherein the brazing filler metal is not cladded.

(9) A clad material joined by performing hot rolling and cold rolling on these alloys, and superposing the intermediate brazing filler metal layer and the outermost surface brazing filler metal layer on one side or both sides of the core material, is obtained.

(10) Going through the above process allows to obtain a brazing sheet 1 for a heat exchanger wherein an aluminum alloy brazing filler metal 3 is cladded on both sides of an aluminum alloy core material 2, as shown in FIG. 1. The aluminum alloy core material 2 corresponds to the aluminum alloy member of the present invention. The aluminum alloy brazing filler metal 3 consists of an intermediate brazing filler metal layer 3a and an outermost surface brazing filler metal layer 3b.

(11) Each intermediate brazing filler metal layer 3a and outermost surface brazing filler metal layer 3b have a thickness of 1 to 30% to the total thickness of the brazing sheet 1.

(12) The brazing sheet 1 can be used as the tube, header, tank, outer fin or inner fin of the heat exchanger.

(13) On the other hand, as the object member to be brazed, an aluminum alloy containing, for example, in mass %, Mg: 0.1 to 0.8%, containing Si: 0.1 to 1.2%, and with Al and inevitable impurities as the balance can be prepared and processed into an appropriate shape. The object member to be brazed corresponds to the aluminum member of the present invention. The composition of the object member to be brazed is not particularly limited in the present invention and one with the appropriate composition can be used.

(14) It is desirable that the brazing sheet 1 be adjusted so that the outermost surface brazing filler metal layer 3b is located on the outermost surface, the average film thickness of the surface oxide film is 15 nm or less, and the average film thickness of the MgO film in the surface oxide film is 2 nm or less.

(15) Moreover, it is desirable that the object member to be brazed be adjusted so that, at least on the joining face, the average film thickness of the surface oxide film is 15 nm or less and the thickness of the MgO film inside the film is 2 nm or less.

(16) The surface oxide film can be adjusted by temperature and time for various heat treatments such as homogenization after the casting, soaking before hot rolling and annealing after cold rolling.

(17) The brazing sheet 1 and the object member to be brazed are arranged so that the intermediate brazing filler metal layer 3a and the outermost surface brazing filler metal layer 3b are interposed flux-free between the aluminum alloy core material 2 and the object member to be brazed. These are assembled to form an assembly of aluminum alloys for brazing. Therefore, the brazing sheet 1 corresponds to the brazing sheet for flux-free brazing of the present invention.

(18) The assembly is arranged in a heating furnace with a non-oxidizing atmosphere under normal pressure. The non-oxidizing gas can be composed using nitrogen gas, or inert gas such as argon, or reducing gas such as hydrogen and ammonia, or a mixed gas thereof. The pressure of the atmosphere inside the brazing furnace is basically the normal pressure, but it can be, for example, a medium to low vacuum of about 100 kPa to 0.1 Pa in the temperature range before the melting of the brazing filler metal in order to improve the gas replacement efficiency inside the product, or a positive pressure of about 5 to 100 Pa more than the atmospheric pressure in order to suppress the inclusion of outside air (atmospheric air) in the furnace. The heating furnace does not need to have a closed space, and can be a tunnel type having a carrying-in port and a carrying-out port for brazing filler metals. Even in such a heating furnace, the non-oxidizing property can be maintained by continuously blowing off an inert gas in the furnace. As the non-oxidizing atmosphere, an oxygen concentration of 300 ppm or less in volume ratio is desirable.

(19) Under the above atmosphere, brazing and joining are performed, for example, by heating at a temperature rising speed of 10 to 200 C./min, and under the heat treatment conditions wherein the arrival temperature of the assembly is 580 to 620 C.

(20) In the brazing conditions, the brazability is improved by suppressing the growth of the oxide film on the material surface, since the faster the temperature rising speed is, the shorter the brazing time is. Brazing is possible if the arrival temperature is at least higher than the solidus temperature of the brazing filler metal, but the fluid brazing filler metal increases by getting close to the liquidus temperature, and a good joining state is more easily obtained with a joint having an open part. However, when the temperature is too high, braze erosion progresses more easily and the structural dimensional accuracy of the assembly after brazing decreases, and so it is not preferable.

(21) FIG. 2 shows an aluminum-made automobile heat exchanger 4 forming a fin 5 using the brazing sheet 1, and using an aluminum alloy-made tube 6 as the object member to be brazed. The aluminum-made automobile heat exchanger 4 is obtained by incorporating the fin 5 and the tube 6 to a reinforcement 7 and a header plate 8, and by flux-free brazing.

Example 1

(22) The brazing filler metals of the composition shown in Table 1 and Table 2 (Al and the inevitable impurities are the balance), and an aluminum material cladded with a core material of JIS A3003 were prepared.

(23) For the aluminum clad materials, the brazing filler metals of the various compositions were set at a cladding ratio of 5% and finished so as to have a 0.25 mm thickness of a temper equivalent to H14. Moreover, as the object member to be brazed, a corrugate fin 11 of JISA3005 alloy, H14 aluminum bare material (0.1 mm thick) was prepared.

(24) TABLE-US-00001 TABLE 1 Composition of the outermost surface layer brazing filler metal (mass %) No. Si Mg Bi 1 5 2 7 3 9 4 11 5 5 0.02 6 7 0.02 7 11 0.02 8 5 0.1 9 7 0.1 10 11 0.1 11 5 0.3 12 7 0.3 13 11 0.3 14 9 1.0 15 9 1.5 0.1

(25) TABLE-US-00002 TABLE 2 Composition of the intermediate layer brazing filler metal (mass %) No. Si Mg Bi 1 1.5 0.5 2 2.5 0.5 3 3.5 0.5 4 1.5 1.0 5 2.5 1.0 6 3.5 1.0 7 1.5 1.5 8 2.5 1.5 9 3.5 1.5 10 1.5 2.5 11 2.5 2.5 12 3.5 2.5 13 1.5 4.0 14 2.5 4.0 15 3.5 4.0 16 1.5 1.5 0.02 17 2.5 1.5 0.02 18 3.5 1.5 0.02 19 1.5 1.5 0.1 20 2.5 1.5 0.1 21 3.5 1.5 0.1 22 1.5 1.5 0.3 23 2.5 1.5 0.3 24 3.5 1.5 0.3 25 2.5 26 2.5 0.1

(26) A 20 mm wide tube 12 was produced using the aluminum clad material, the tube 12 and the corrugate fin 11 were combined, and a 15-steps tube and a 300 mm long core 10 as shown in FIG. 3(a) were made as the brazing evaluation model. The core was heated to 600 C. in a brazing furnace in a nitrogen atmosphere (oxygen content of 80 ppm), and the brazing state was evaluated.

(27) Brazability

(28) Joining Rate

(29) The joining rate was found with the following formula, and the superiority/inferiority of each sample was evaluated.
Fin joining rate=(total brazing length of the fin and the tube/total contact length of the fin and the tube)100

(30) The classification was made according to the following criteria and the results are shown in Table 3 and 4. Fin joining rate after brazing Excellent: 98% or more, Good: 90% or more and less than 98%, Fair: 80% or more and less than 90%, Poor: less than 80%

(31) Evaluation of the Joining Part Width

(32) Since the brazing joining state is to confirm not only the above joining rate, but also the improvement of the fillet forming capacity that is the purpose of the present invention, the width W of the joining part 13 as shown in FIG. 3(b) was measured on 20 points in each sample, and the superiority/inferiority was evaluated based on its average value. The classification was made according to the following criteria and shown in Table 3 and 4. Excellent: 0.8 mm or more, Good: 0.7 mm or more and less than 0.8 mm, Fair: 0.6 mm or more and less than 0.7 mm, Poor: less than 0.6 mm

(33) Whereas all the examples showed a good brazability, a sufficient joining was not obtained in the comparative examples.

(34) TABLE-US-00003 TABLE 3 Outermost surfaces Intermediate layer brazing filler layer brazing filler Brazing filler metal metal the proportion metal the number Outermost of Si particles of of si particles of surface Intermediate 1.75 m or more 0.25 m or more Joining layer No. layer No. [%] [piece/mm.sup.2] Brazability part width Example 1 1 8 10 45000 2 2 8 20 45000 3 3 8 30 45000 4 4 8 40 45000 5 5 8 15 45000 6 6 8 25 45000 7 7 8 35 45000 8 8 8 25 45000 9 9 8 30 45000 10 10 8 35 45000 11 11 8 30 45000 12 12 8 35 45000 13 13 8 35 45000 14 3 1 30 30000 15 3 2 30 35000 16 3 3 30 40000 17 3 4 30 35000 18 3 5 30 40000 19 3 6 30 45000 20 3 7 30 40000 21 3 9 30 50000 22 3 10 30 50000 23 3 11 30 60000 24 3 12 30 70000 25 3 13 30 60000 26 3 14 30 70000 27 3 15 30 80000 28 3 16 30 55000 29 3 17 30 58000 30 3 18 30 60000 31 3 19 30 53000 32 3 20 30 56000 33 3 21 30 58000 34 3 22 30 50000 35 3 23 30 54000 36 3 24 30 58000 37 7 2 35 35000 38 7 5 35 40000 39 7 8 35 45000 40 7 11 35 60000 41 7 17 35 58000 42 7 20 35 56000 43 7 23 35 54000

(35) TABLE-US-00004 TABLE 4 Outermost surfaces Intermediate layer brazing filler layer brazing filler Brazing filler metal metal the proportion metal the number Outermost of Si particles of of si particles of Surface Intermediate 1.75 m or more 0.25 m or more Joining layer No. layer No. [%] [piece/mm.sup.2] Brazability Partwidth Comparative 1 14 9 35 50000 X X example 2 15 9 30 50000 X X 3 3 25 30 40000 X X 4 3 26 30 30000 X X 5 14 25 35 40000 X X 6 14 26 35 30000 X X 7 15 25 30 40000 X X 8 15 26 30 30000 X X

(36) The present invention was described on the basis of the above embodiment and examples, but the present invention is not limited to the content of the above embodiment and examples, and as long as it does not deviate from the claimed invention, the content of the above embodiment and examples can be changed appropriately.

INDUSTRIAL APPLICABILITY

(37) Using the present invention makes brazing joining possible for automobile heat exchangers, but also for aluminum-made heat exchangers, coolers, heat sinks, radiators etc. used in railway vehicles, airplanes, electronic components such as inverters and CPUs, various plants, industries and air conditioning appliances, without using flux. Since flux is not used in the present invention, there is no decrease in the surface quality or the surface chemical convertibility due to flux residues after brazing, and no trouble with the electric element due to contamination caused by the residues.

REFERENCE SIGNS LIST

(38) 1 brazing sheet 2 aluminum alloy core material 3 aluminum alloy brazing filler metal 3a intermediate brazing filler metal layer 3b outermost surface brazing filler metal layer 4 aluminum-made automobile heat exchanger