FIG. 1 shows a front view of a parallel-flow-type condenser as an example of an aluminum alloy heat exchanger. Extruded multiple-holed tubes are used as tubes (extruded multiple-holed flat-shape tubes) 1, serving as refrigerant passages; each of the tubes 1 has a corrugated fin 2 brazed thereto, that is a radiator, and that is made of a brazing sheet fin clad with a filler alloy (parts of the repeated sections of the corrugated fins are omitted in FIG. 1); and the tubes 1 and the corrugated fins 2 are integrated by the brazing process, to produce a core 5, that forms the major part. 3 indicates header pipes, through which the tubes 1 communicate with one another. In recent years, in order to make heat exchangers small-sized and light in weight, some production methods are proposed. Among the methods is a production method in which, in place of the corrugate fins 2, which are limited in how thin they can be made, bear fins are used, which are thinner; the outside of the tubes 1 is covered with a filler alloy by metal-spraying, and they are combined and integrated by brazing (JP-A-63-34495 ("JP-A" means unexamined published Japanese patent application) and JP-A-2-84261, alternatively a method in which tubes and fins whose surfaces are metal-sprayed with a filler alloy are assembled into a core and brazing is carried out (JP-A-6-200344).
When it is attempted to produce a heat exchanger by using tubes made by metal-spraying the above extruded tubes with a filler alloy, however, no heat exchanger is industrially satisfactory for practical use in brazing. The reason for that seems to be attributed to the occurrence of the following two problems with brazing.
First is depletion of the filler alloy mentioned. This is a failure apt to occur when conventional brazing is carried out using brazing sheets having a smaller amount of a filler alloy, and this failure is liable to occur particularly in the case of tubes metal-sprayed with a filler alloy. Since the cost of metal-spraying with a filler alloy is high, when a coating of a filler alloy layer is formed by metal-spraying, the thickness of the filler alloy layer is generally 50 g/m.sup.2 or less, and this quantity is 1/2 or less of the quantity of the brazing sheet (the cladding ratio of the filler alloy being 10%), whose thickness is 0.4 mm, that is used generally as an electric-resistance weld pipe in heat exchangers. Accordingly, when the metal-sprayed filler alloy is not microscopically and uniformly deposited at the part where the amount of the filler alloy is small, depletion of the filler alloy sometimes occurs as a failure in which the part having a smaller amount of the filler alloy cannot be brazed (no fillet is formed).
Second is the occurrence of locally unbonded parts of heat exchangers. The occurrence of locally unbonded parts is a phenomenon peculiar to tubes metal-sprayed with a filler alloy, which is illustrated schematically in FIG. 2. When tubes 1 metal-sprayed with a filler alloy, and corrugated fins 2, are combined to form a core having several stages, the brazing can be carried out by heating, to effect the brazing with no problems. However, if a core 5 of multi-stage having about 40 stages is intended on a practical scale a part D, where the corrugated fin is not brazed to the tube at all, occurs at places. FIG. 4 indicates a side plate view.
If there is such a locally unbonded part D where a tube 1 and a fin 2 are not connected with each other, not only is the strength of the core 5 lowered as a whole, but also the sacrificial corrosion-preventive (cathodic-protection) effect of the tube 1 by the fin 2 cannot be secured, and the corrosion resistance is lowered greatly. The cause of the occurrence of the locally unbonded part D has not yet been fully clarified.
The most simple conceivable way for preventing the depletion of a filler alloy from occurring is a method wherein the amount of a filler alloy is increased satisfactorily. If there is a satisfactory amount of a filler alloy, the filler alloy goes to everywhere, even if the deposited amount of the filler alloy is uneven. When the amount of a filler alloy is increased, however, locally unbonded parts are liable to occur accordingly, as described below in the case of multi-stage one as mentioned above. Therefore, it is difficult to increase the amount of deposition of a filler alloy.
To solve this, it is conceivable to supply a filler alloy as uniformly as possible. The uniform deposition of a filler alloy should lead to the formation of fillets with a minimum amount of the filler alloy. However, the uniform deposition of a filler alloy was technically difficult in the conventional metal-spraying.
To prevent locally unbonded parts from occurring, it is recommended, in theory, to make the filler alloy applied thinly and to reduce microscopic projections and recesses (unevenness), so as to increase the filling density of the filler alloy. Namely, the thinner the filler alloy is, the smaller the change in the thickness of the tube before and after the brazing is made, and therefore locally unbonded parts are not likely to occur, even in a case of multi-stage. However, since thin application of a filler alloy means a decrease in the amount of the filler alloy, depletion of the filler takes place, even though locally unbonded parts can be prevented from occurring. Further, if the applied filler alloy has microscopic unevenness, gaps are liable to occur between the fin and the tube after the brazing, because the fin is in contact with the projections (projected areas) of the filler alloy when the core is assembled into a heat exchanger, as described above. The filling density of the filler alloy mentioned above is defined such that, when the filler alloy is present fully between the surface of a tube and the maximum height of the deposited filler alloy shown in FIG. 3 the filling density of the filler alloy is 100%. There is a tendency that the more deeply the unevenness of the filler alloy are formed, the smaller the filling density is. In the case of brazing sheets conventionally used in brazing, the filling density is about 100%. On the other hand, in the case of tubes metal-sprayed with a filler alloy, the metal-sprayed amount of the filler alloy is about 25 g/m.sup.2 (the possible minimum amount in brazing with a conventional Al/12% Si alloy), with the height of the filler alloy being 50 .mu.m or more (the height of the filler alloy (average filler alloy height) is about 8 .mu.m if the filler alloy is deposited with the filling density being 100%), and the filling density is 20% or less, which is a great decrease.