JPH0355237B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to the composition of a brazing flux for assembling aluminum heat exchangers with good corrosion resistance. Prior Art Flux brazing, which involves interposing an aluminum brazing alloy layer with a melting point lower than the melting point of the aluminum materials to be joined between the aluminum materials to be joined, generally uses chloride as the main component. Fluxes are used in the atmosphere. These fluxes are water-soluble in nature, generally hygroscopic, and corrode aluminum, including aluminum braze alloys, in the presence of water. Therefore, post-brazing residues of such flux must be removed by cleaning after the brazing process. As a fluxless brazing method that performs brazing without using flux, vacuum brazing method,
Alternatively, inert atmosphere brazing methods are already known, but these require high vacuum or high purity inert gas as well as a special aluminum material for the materials to be joined. Furthermore, compared to the flux brazing method, it is inferior in that stricter precision is required due to the clearance of the brazed portion, and the aluminum material that can be used is not a common one. It is also virtually insoluble in water after brazing,
Fluoride fluxes are also known that leave a residue on the braze surface that is non-hygroscopic and non-corrosive to aluminum. That is, according to British Patent No. 1055914,
A flux containing a mixture of AlF ₃ and KF has been proposed. Furthermore, Japanese Patent Publication No. 58-27037 proposed a flux consisting of KAlF ₄ and K ₃ AlF ₆ as a flux that is non-hygroscopic before brazing and substantially water-insoluble after brazing. ing. This flux is obtained by melting the mixture using KF and AlF as raw materials and pulverizing the solidified product, and the solidified product contains KAlF ₄ and
K ₃ AlF ₆ is generated. Raw material components KF and AlF ₃
It is recommended that the ratio be as close to the eutectic composition of both components as possible. KF−AlF ₃ or K ₃ AlF ₆ −KAlF ₄ or
These fluoride fluxes, which have the substantial chemical formula K ₃ AlF ₆ -AlF ₃ or KAlF ₄ , K ₃ AlF ₆ -KAlF ₄ or KAlF ₄ in the as-brazed state, or combinations thereof, are similar to conventional chloride fluxes. In addition to the advantages of system fluxes, the residue is non-hygroscopic and non-corrosive to aluminum.
It has the great advantage of not requiring cleaning after brazing. These fluxes can also be applied in any form for brazing, and are particularly suitable for torch brazing or furnace brazing in the form of an aqueous slurry. However, these fluxes have the disadvantage that the operating temperature is slightly higher than that of chloride fluxes, making it difficult to obtain good joints. The action mechanism of flux in brazing has not been fully elucidated, but as a flux,
Generally, before the brazing alloy starts melting, flux covers the brazing surface and destroys and removes the surface oxide film, increases the wettability of the base metal, and helps the brazing alloy flow to the brazing surface. Also, it is necessary to have the ability to prevent re-oxidation by blocking contact with air. Therefore, it is significant in brazing to moderately lower the operating temperature of the flux. Purpose An object of the present invention is to have a flux that has a lower operating temperature than conventional fluoride-based fluxes used as fluxes for brazing aluminum materials, thereby making it possible to form good joints.
An object of the present invention is to provide a brazing flux for assembling an aluminum heat exchanger, which can improve the corrosion resistance of joints and is substantially water-insoluble and non-hygroscopic. Composition In order to achieve the above object, the present invention adds BiF ₃ to the composition of conventional fluid-based flux to lower the working temperature of additive-free flux and to improve the reaction during brazing. By mixing the thus produced trace amount of Bi into the brazing alloy, brazing properties are improved, thereby forming a good joint. Furthermore, on the aluminum alloy surface
Corrosion resistance is also improved by forming an Al-Bi alloy. That is, the gist of the present invention is essentially that KF43~
48% by weight, AlF ₃ 51-56% by weight, and BiF ₃ 0.05-
A flux for brazing aluminum heat exchangers with good corrosion resistance, which is obtained by pulverizing a molten solidified mixture of 1% by weight. Here, the term "substantially" means that the fluoride used as a raw material does not necessarily have to be of high purity; generally, impurities that are more mixed in than commercially available fluorides are allowed, but only as pure substances. This is because only the weight ratio of the three substances when converted into KF, AlF ₃ and BiF ₃ is important for the flux function. Substantial KF in _BiF3- free flux
Preferred values for the weight ratio of AlF ₃ are known from known literature (e.g. B. Phillips et al., J. Amer. Ceram. Soc,
Vol. 49 (1966), page 623), it can be easily understood from the KF-AlF ₃ system binary phase diagram, and as shown, the eutectic composition, that is, KF45.8% by weight -
AlF ₃ 54.2% by weight (KF55 mol% - AlF ₃ 45 mol%)
Or, it is a ratio corresponding to before or after that. The present invention further provides for a mixture or molten mixture having substantially this weight ratio of 99.95 to 99% by weight.
The improvement is remarkable when 0.05 to 1% by weight of BiF ₃ is added. Addition of BiF ₃ not only lowers the working temperature of the flux, but also reduces BiF ₃ to Bi through a chemical reaction during brazing, and a trace amount of Bi is added to the brazing alloy.
As is known, the presence of a trace amount of Bi in the brazing alloy improves brazing properties. Furthermore, Bi generated from _BiF3 in the flux
forms an Al-Bi alloy by diffusion on the aluminum alloy surface, and since this potential is less base than that of ordinary aluminum alloys, it improves the corrosion resistance of brazed parts through the sacrificial anode effect, and improves the corrosion resistance of brazed parts. It also significantly improves eating habits. Therefore, the present invention is particularly useful in the assembly of brazed aluminum heat exchangers where they are used in severely corrosive environments. If the amount of BiF ₃ added is less than 0.05% by weight, the effect of improving brazing properties will be poor, and if it exceeds 1% by weight, intergranular corrosion cracking will occur in the Al-Bi alloy, and
Since it is not economical, the upper limit is set at 1% by weight. Examples and effects thereof will be described below. Example 1 Using industrial KF and industrial AlF ₃ and taking into account their respective purity, substantially KF45.8% by weight -
BiF ₃ was added to both components in the same way as the flux prepared by mixing the AlF ₃ at a ratio of 54.2% by weight and melting it (the melting temperature was set at approximately 100°C above the eutectic temperature of 562°C). It was prepared by mixing the mixture so that the ratio was essentially 45.6% by weight of KF - 53.9% by weight of AlF ₃ - 0.5% by weight of BiF ₃ and melting the mixture.
The action temperature of each BiF _3- added flux was measured by the following method. That is, a certain amount of both of the prepared fluxes was placed in a platinum crucible, heated and melted in an argon atmosphere, and then
Thermal analysis was carried out by a cooling method at a cooling rate of 1° C./min to measure the liquidus temperature and solidus temperature, thereby determining the operating temperature. The measurement results are shown in Table 1. Thus, it was confirmed that the addition of BiF ₃ lowered the solidus temperature and lowered the flux action temperature.

[Table] Example 2 Using the additive-free flux shown in Example 1 and using the method shown in Example 1, substantially KF45.71 was added.
Each component was mixed in a ratio of weight% - AlF ₃ 54.10% by weight - BiF ₃ 0.19% by weight, and using a BiF ₃ -added flux prepared by melting the mixture,
Inert atmosphere brazing (fluxless brazing is not possible in this atmosphere) was performed, as well as standard fluxless inert atmosphere brazing in an inert atmosphere. The bonding states of these three examples were compared. The results are shown in Table 2.

[Table] Regarding the appearance and cross-sectional shape of the fillet, in the case of BiF ₃ -added flux, it is obtained with no discoloration compared to fluxless brazing and additive-free flux brazing, and when comparing the fillet cross-sectional area, the average value and Looking at the variation, it can be seen that the most stable brazing can be obtained with the BiF ₃ -added flux. Example 3 Brazing samples using the BiF _3- added flux and non-additive flux shown in Example 1 were subjected to a dry-wet alternating test (immersed in 40°C 3% NaCl water at pH 3 for 30 minutes, then immersed in 50°C air for 30 minutes). After drying for one month, the corrosion resistance was compared. As shown in Table 3, when the BiF ₃ -added flux is used, the pitting corrosion resistance is significantly improved.

[Table] Example 4 The water solubility of the BiF ₃ -added flux and non-additive flux shown in Example 2 was measured, and these were compared with the water solubility of the main components of the chloride flux (literature values). I did it. The results are shown in Table 4, and in the case of the BiF ₃ -added flux, the flux is on the same level as the non-additive flux, and is extremely low compared to the chloride component.

【table】

[Table] Effects The effects of the flux of the present invention are summarized as follows:
It is as follows. (1) Since BiF ₃ is added, the operating temperature of the flux is low and a good joint is formed. (2) By adding BiF ₃ , a small amount of Bi enters the aluminum alloy solder and helps form a good joint. Furthermore, on the aluminum alloy surface,
An Al-Bi alloy is formed by diffusion, and its sacrificial anode effect improves the corrosion resistance of brazed parts. (3) Characteristics of fluoride flux. Non-hygroscopic and virtually insoluble in water. Therefore, the flux of the present invention has great utility in brazing for assembling corrosion-resistant aluminum heat exchangers.

Claims (1)

[Claims] 1. Aluminum heat exchanger with good corrosion resistance, which is obtained by crushing a molten solidified mixture of 42 to 48% by weight of KF, 51 to 56% by weight of AlF ₃ and 0.05 to 1% by weight of BiF ₃ Flux for brazing vessels.