BRAZING SHEET FOR FLUX-FREE BRAZING AND METHOD FOR PRODUCING SAME

A brazing sheet for flux-free brazing, comprising a core material, a brazing material disposed on at least one surface of the core material, and a thin skin material disposed on the brazing material, wherein the core material is made of an aluminum alloy having a higher melting point than that of the brazing material; the brazing material is made of an Al—Si—Mg based alloy and has a thickness of 25 to 250 μm; the thin skin material is made of an aluminum alloy having a higher melting initiation temperature than the brazing material and containing substantially no Mg, and has a thickness of 5 to 30 μm; and a content of an oxide existing at an interface between the brazing material and the thin skin material is 0.1 ppm or less in weight ratio with respect to the entire clad material. The present invention provides a brazing sheet for flux-free brazing, which has a thin skin material, with uniform brazing characteristics, and enables stable joining.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiment of the present invention will be described based on Examples described below. In this embodiment, while plurality of core materials, brazing materials and thin skin materials were combined and manufacturing conditions were adjusted, brazing sheets for flux-free brazing were manufactured and their joining characteristics were evaluated.

First, for each component of the core materials, brazing materials and thin skin materials, alloy ingots having the composition in Table 1 were prepared with a semi-continuous casting method.

In manufacturing the brazing sheet, a core material ingot was subjected to a homogenization treatment, and the upper and lower surfaces were subjected to facing. As for the brazing material, an ingot was hot-rolled to adjust its thickness. As for the thin skin material, an ingot was hot-rolled and cool-rolled to adjust its thickness. Then, the respective members having an adjusted thickness was used to prepare brazing sheets of the combinations in Table 2 in the two manufacturing processes “a” and “b” described below. The thicknesses of the thin skin material and the brazing material were measured with an optical microscope. Incidentally, a three-layer material is a material in which the filler alloy and the thin skin were joined on only one surface, and a five-layer material is a material in which the filler alloy and the thin skin were joined on both surfaces of the core material. In the five-layer material, each thickness of the filler alloy and the thin skin is set to the same on both surfaces.Manufacturing method a: After a core material ingot, a brazing material subjected to the oxide film removal step described later and a thin skin material were combined and preheated, hot rolling was performed to prepare a clad material. The conditions of the preheating of hot rolling were to maintain at 430 to 475° C. for 1 to 2 hours. In some of the conditions, dry air with a dew point of −5° C. or lower was flowed into the furnace.Manufacturing method b: a thin skin material and a brazing material subjected to the oxide film removal step described later were preheated and hot-rolled to make a pre-clad material. Thereafter, the pre-clad material was further combined with a core material and hot rolling was performed to prepare a predetermined clad material. The condition of the preheating of each hot rolling was to maintain at 460° C. for 2 hours.

The plate thickness after the hot rolling was set to 5.0 mm. For some samples, characteristics as a brazing sheet were tested with use of this hot-rolled material. The other samples were cold-rolled to a final thickness to be 0 materials with annealing at 380° C.×1 hour.

In Examples of the present invention, there is performed a treatment for removing a surface film of the Al—Si—Mg alloy brazing material after the thickness is adjusted. The following was carried out as its method: (1) etching with sodium hydroxide solution and nitric acid desmut; (2) sulfuric acid etching; and (3) mechanical polishing. The detailed condition of each method is as follows.

(1) Etching with sodium hydroxide solution and nitric acid desmut

Immersion in 10 g/L of sodium hydroxide solution (55° C.) for 30 seconds, and desmutting with 250 g/L of nitric acid, and thereafter washing with water and drying;

Immersion in 50 g/L of sulfuric acid (60° C.) for 60 seconds and thereafter washing with water and drying; and

Surface polishing with a steel wire brush and thereafter the sulfuric acid etching similar to (2).

Samples of the brazing sheet were collected from eight positions in the length direction of the final rolled sheet and the center and positions of 40 mm from both ends of the rolled sheet in the width direction (total 24 positions). Thicknesses of the thin skin material and the brazing material were measured with an optical microscope. The original materials for evaluation in Examples 1 to 8 and Comparative Examples 1 to 6 had a width of 500 mm and a length of 60 to 80 m. In Examples 9 and 10 and Comparative 7, the original materials had a width of 1000 mm and a length of 200 m or more. In Comparative 8, the original material had a width of 250 mm and a length of 10 m.

As for a weight ratio of the oxide on the brazing sheet, both surfaces were polished while the interface between the thin skin material and the brazing material on one surface was included to prepare a foil sample of 0.09 to 0.1 mm, and oxides of aluminum and magnesium were extracted from the foil sample with an iodine methanol method, and then quantitative analysis was carried out. The amount of the oxide was converted into a weight ratio with respect to the initial sample weight before the polishing.

After the joint joining as shown inFIG. 1is performed, brazing characteristics of the respective brazing sheets are evaluated based on a joining rate (a rate of a portion other than voids) of the joining surface having a contact length L and based on a degree of formation of a fillet. The brazing is conducted in a nitrogen gas atmosphere having an oxygen concentration of 20 to 50 ppm or less with heating of 600° C.×5 min. The brazing sheet and the counterpart material were degreased with solvent washing. After drying, they were made up to a predetermined sample shape and then fixed with a stainless-steel jig to be subjected to a brazing test. After brazing of 24 joint specimens of each material, based on texture observation of a cross section of the center in the width direction of the adhered potion, a joining rate was calculated as a rate (%) of a portion except voids. As for evaluation criteria, a joining rate of 96% or more was defined as OK. Further, it was examined whether, among the 24 samples, the fillet at the periphery of the adhered portion was present uninterruptedly over the entire circumference, and then evaluation was performed based on the number of uninterrupted samples. In this evaluation, the case that the fillet is sound was defined as OK in all samples. The results of this brazing characteristics evaluation are shown in Table 2.

Based on these evaluation results, in all of the brazing sheets according to Examples 1 to 14, the content of the oxide existing at the interface between the brazing material and the thin skin material was 0.1 ppm or less. These brazing sheets also had good uniformity in thickness of the thin skin material because of the decreased oxide content. The difference between the maximum and minimum thicknesses was about 0.2% or less with respect to all of the brazing sheets. These Examples were also excellent in the brazing characteristics, and the joining rate and soundness of joining portion (fillet forming) were good. Meanwhile in Comparative Examples 1, 2, 7 and 8 in which the oxide content exceeds 0.1 ppm, there were variations in thickness of the thin skin material, and the brazing joining characteristics were also not preferable.

In view of composition of the brazing material, Mg concentration and Si concentration of the brazing material were set within the suitable range in Examples 1 to 14, and preferable brazing joining characteristics were shown. On the other hand, it was found from the results of Comparative Examples 3 and 4 that the joining characteristics were inferior when the Mg concentration in the composition of the brazing material was out of the suitable range, and further it was found from the results of Comparative Examples 5 and 6 that defective joining occurred in the Si concentration lower than the suitable range while erosion due to the brazing material occurred in the concentration higher than the suitable range and, also in this case, the brazing characteristics were inferior. In Examples 1, 6 and 11 to 14, there were applied the brazing materials to which Bi, Zn, In, Sn and Cu are added as an additive element, but these additive elements did not adversely affect the brazing joining characteristics and therefore it was confirmed that the respective brazing sheets had good joining characteristics.

Further, in view of effectiveness of the oxide film removal treatment in the manufacturing process, because the joining characteristics were good in Examples 1 to 14 in which the oxide film removal treatment was appropriately performed and because the oxide content at the interface was so high as to cause inferior joining characteristics in Comparative Examples 1, 2, 7 and 8, effectiveness of the oxide film removal treatment can be confirmed. Furthermore, there is a tendency that the oxide content is lowered with application of a low oxidizing dry air as a condition of the preheating before hot rolling. However, since the oxide content is sufficiently low even in the atmosphere, it is considered that atmosphere of the preheating has high flexibility. Incidentally, the two following methods were performed as the manufacturing process: a method in which the core material, the brazing material and the thin skin material were stacked and rolled (Method a); and a method via a pre-clad material (Method b). However, either method is not superior to the other and therefore it can be confirmed that both of them are useful.

INDUSTRIAL APPLICABILITY

The present invention is a brazing sheet for flux-free brazing having uniform brazing characteristics without flux. The present invention is useful for manufacture of brazed structures for a heat exchanger or the like, and the present invention enables stable joining in the manufacture thereof and contributes to ensuring product quality.