GB2119407A - Fluoaluminate brazing flux - Google Patents

Abstract

A flux for brazing aluminium articles, essentially consisting a mixture of water insoluble fluoaluminate complexes containing Al, F, K and Li atoms, or a mixture thereof with aluminium fluoride, having a following composition as expressed in terms of simple compound; about 53-62 wt% of aluminium fluoride, about 35-44 wt% of potassium fluoride, about 2-7 wt% of lithium fluoride and up to about 2 wt% of entrained impurities.

Description

SPECIFICATION Brazing flux This invention relates to a reactive flux useful in joining by brazing surfaces of aluminum and aluminum alloy (hereinafter referred to collectively as "aluminum articles"), to one another and to other metal articles such as copper and stainless steel, for example. More particularly, this invention is to provide a fluoride flux based on AlF3-KF-LiF ternaly system.

Generally the technique of brazing has been applied assembling metal articles.

In the construction of automotive parts, for example, the technique of brazing has been employed for the manufacture of structural members such as intake manifolds and cylinder heads as well as heat exchangers for condensers, evaporators, and engine oil coolers. The technique has also been adopted for the manufacture of rectifier heat sinks for use in electronic devices. In various other industrial fields, it has also found widespread acceptance.

When, for example, two aluminum articles are mutually joined by brazing, a flux is used for the purpose of removing oxide film on the surfaces to be joined and thereby enabling the underlying metallic aluminum surface to come into good metallic contact. At the same time, the flux improves the wet spreading or flowing property of the molten brazing metal over the joining surfaces.

Heretofore, a chloride flux has been used for this purpose.

Since a chloride flux is soluble in water and hygroscopic, it has a disadvantage that when the flux itself of the reaction residue resulting from brazing is permitted to adhere to and remain on the articles being joined, it causes corrosion of the aluminum article at the joint. For this reason, where brazing has been effected with a chloride flux,-the brazing operation has had to be followed by a step to clean the joined articles to remove the residual flux adhering to the joint. If the brazed product being cleaned happens to possess a narrow joint part, there is a possibility that some flux will remain there and eventually lead to corrosion.

It has been proposed that the problem of corrosion suffered by a chloride flux can be avoided by a reactive flux of the aluminum fluoride based on (AlF3)-potassium fluoride (KF) binary system, as is disclosed by B.P. No. 1,055,914.

However, since this flux has a high melting point in the range of 570 to 5800C, this flux has been accepted for actual use in limited industrial fields.

Generally, it is considered desirably if not necessary in a brazing technique, in order to achieve a stable product quality and a reliable brazing operation, that, on the one hand, the melting point of the brazing alloy should be 10 to 400C lower than the melting point of the articles to be joined, while on the other hand, the melting point of the flux should be equal to or 10 to 2000 lower than the melting point of the brazing metal.

From this point of view, any reduction in the melting point of the flux itselt can be expected to increase the variety of aluminum alloy which can be joined by brazing and, as a consequence, widen the range of products that can be assembled by brazing.

Further in the case of conventionally proposed AlF3-KF flux, if this flux is used for brazing aluminum articles containing magnesium, the wet spreading or flowing property exhibited by the brazing metal on aluminum articles will suffer and the brazing operation itself will be impaired so much as to render achievement of the desired joint strength impractible. From a practical point of view, therefore, this flux has been applicable to the soldering of aluminum alloys having a magnesium (Mg) content of only up to about 0.4 wt.% It has been discovered that problems suffered by the conventional AlF3-KF type flux are reduced in accordance with the invention by providing and AlF3-KF Li type flux.To be specific, the present invention provides a flux which compromises, as expressed in terms of simple compounds 53.0 to 62.0 wt% of aluminum fluoride (AIF3), 35.0 to 44.0 wt% of potassium fluoride (KF), and 2.0 to 7.0 wt% of lithium fluoride (Li F), said compounds existing in the flux form of a mixture of fluoaluminate complexes or mixture thereof with aluminum fluoride.

The flux having a composition in the range defined above has a melting point roughly in the range of 490 to 56000. It has been found capable of brazing aluminum articles having a Mg content of up to 2 wt%. In the course of brazing, this flux produces very little insoluble reaction residue and forms a brazed joint of highly desirable qualities.

The contents of AIF3 and KF should not deviate appreciably from their respective lower and upper limits. For, when they depart from the started ranges, the resultant flux produces in use a white insoluble residue and consequently impairs union by soldering and the melting point of the flux increases above the desired range.

The content of LiF should not fall short of about 2.0 wt%. When it is less than this level, the flux in use, depending on the properties of AIF, and KF present, produces a white insoluble residue and fails to manifest the desired brazing property with aluminum alloys containing magnesium. The LiF content should not, on the other hand, exceed about 7.0% wt%. Above this level, the flux produces in use a black insoluble residue and consequently impairs the union by soldering and the melting point of the flux itself is elevated.

In the flux of this invention, the components thereof are desired to be present in the form of a mixture of fluoaluminate complexes as such or in admixture with aluminum fluoride. As distinct forms of such fluoaluminate complexes, such structures as KAIF4, K3AlF6, K2LiAlF4, and LIAIF, have been identified. Depending on the method to be used for the preparation of the present fluxes, compounds such as K2AlF6 H20 which are less stable than the aforementioned complex salts may possibly be produced in trace quantities. The presence of such trace compounds is essentially immaterial, because they are converted.into the aforementioned complex salts when the temperature is elevated during the brazing operation work.

in the flux of the present invention, the fluorides are fundamentally desired to be present in the form of fluoaluminate complex salts, because the fluoaluminate complexes have better effects upon the homogeneity and stability of the flux and the reliability of its melting point, for example.

A flux prepared by simply blending simple fluoride compound is subject to various problems such as separation of their components during storage due to differences in specific gravity, the effects of the different diffusion speeds of the fused compound during brazing upon the reliability of the flux in terms of homogeneity and melting point, and the creation of a defective upon due to the occurrence of insoluble residues resulting from delicate variation in composition.

Thus, a flux prepared in this manner is not suitable in practice for the purpose of brazing operation.

Fluoaluminate complexes can be produced by the so-cailed fusion process of the wet process.

The fusion process produces fluoaluminate complexes by fusing the mixture of simple fluoride compounds having aforementioned compositions of their complexes and converting the resultant mixture into a fused coagulation. The wet process produces fluoaluminate complexes by dissolving hydroxides of the metallic components of the flux (Al(OH)3, KOH, and uOH) in an aqueous hydrofluoric acid solution, allowing the hydroxides to react with one another and with the acid to form fluoaluminates and desiccating the resultant complexes.

The fluxes of the present invention can be manufactured according to a desired composition by a one step method which is based on the fusion process or the wet process. Alternatively, they may be manufactured by a more complicated method which comprises preparing individual fluoaluminate complex in separate steps, homogeneously mixing these fluoaluminates in amounts calculated to give a desired fluoride composition of this invention, converting the resultant mixture into an aqueous slurry, and again desiccating the slurry. Other similar methods can also be employed as the method of manufacture is not critical.

While fluxes of this invention can be manufactured by any of the various methods described above, it is desirable to control the purity of the raw materials to be used and adjust the manufacturing conditions so that the inevitably entrained impurities in the ultimate flux do not exceed the approximate level of 2 wt%. The particle diameter of the flux powder is desired to be as small as possible. Normally, it is appropriate to keep this particle diameter below 74.

The fluxes of this invention have melting points lower than the melting point of conventional AlF3-KF fluxes. They promote an advantageous wet spreading or flowing action of the brazing metal on magnesium containing aluminum alloys because of their specific percentage composition and their specific form described above. The present fluxes are, therefore, useful in an expanded range of applications for solderable aluminum materials.

By means of the flux of the present invention, virtually all aluminum alloy products, whether prefabricated product or cast,exclusive or Al-Cu type alloys (A2XXX alloys, Al-Mg alloys containing more than about 2 wt% of Mg, and Al alloys having a melting temperature of not more than 53000 can be brazed independently of their manufacturing steps. The flux can be applied not only to the brazing of two aluminum articles but also to the brazing of an aluminum article to a copper, a stainless steel or other metal article.

Brazing with the flux of the present invention can be effected in ways similar to those used in brazing operations with the conventional AlF3-KF type flux. Before brazing, the surfaces to be joined are subjected to known preliminary treatments, such as de-greasing and coarsening or abrading.

The, the flux is applied to the surface by any suitable method which includes depositing the flux in its original powdered form on the surfaces being joined; mixing the powdered flux with a liquid medium such as water which isincapable of forming a residue during the brazing operation to make a slurry (containing 5 to 60 dry wt.% of flux), applying this slurry to the surface by any suitable technique (brushing; spraying, showering, or immersing) in an amount to give a flux layer of 3 to 15 g/m2, and thereafter drying the applied -slurry; or mixing the brazing metal in powdered form with the flux and subjecting the resultant -mixture to the same steps as in any of the preceding methods. Once the surfaces are covered with flux, the brazing metal is placed on the flux.

The brazing metal with which the flux of the present invention is advantageously used in aluminum alloy having a melting point in the neighborhood of 510 to 5700 C. Examples of preferred alloys include the Al~5% Si~26% Cu type alloy (mop. 517 to 53000) as well as A4043, A4045, A4047, A4145, A4245, A4343 and A4543. Such an alloy can be used in a suitable form such as, for example, powder or granules, a wire or rod, a plate, a punched plate, or clad on a core, with the selection thereof made in due consideration of the contour of the surfaces to be joined and the ease of brazing work to be involved.

Then, the articles whose joining surfaces have already received the brazing metal thereon are held with a suitable jig in a prescribed relative position. While surrounded by an atmosphere of inert gas such as nitrogen gas, preferably kept dry by control of the moisture content below the level of about 250 ppm, the materials are held at a brazing temperatures of 530 to 58000 for 3 to 20 minutes and until the brazing treatment is completed.

For the brazing operation, proper brazing equipment and devices suitable for the particular shape of articles to be joined and the desired productivity can be adopted. Generally, a torch brazing unit, a high frequency brazing unit, and a furnace brazing unit (either for batchwise and continuous operation) are available.

The fluxes of the present invention when used in the brazing operation described above has been demonstrated to be equivalent or superior to conventional AlF3-KF type fluxes in all areas of evaluation such as their ability to promote wet spreading or flowing of the brazing metal on aluminum surfaces, ability to inhibit formation of residue during the brazing, and ability to effectively mechanically join the surfaces to be brazed. Since the flux of this invention has a lower melting point than the conventional flux, it expands the range of aluminum products that can be effectively brazed. Particularly, manganese containing aluminum alloy articles which have never been brazed by use of any conventional flux are rendered brazability to some extent by the flux of this invention.Since the flux is composed of water insoluble fluoaluminate complexes, there is no necessity for removing any residual flux from the joint region, such as by a cleaning treatment.

The brazing procedure using the flux of this invention is simple in nature and is free from the tendency toward corrosion of the brazed product as compared with the brazing by use of the conventional chloride flux, which are further outstanding characteristicsfrom a practical point of view.

Now, to illustrate the manner of use of the flux of this invention, typical cases in which the flux of this invention was used in the brazing of fabricated articles ånd cast articles will be described below as working examples.

EXAMPLE 1 K3AlF6, AIF3, and LIAIF, were used as raw materials. They were mixed in the proportions of 57.7 parts by weight of K3AlF5, 32.4 parts by weight of AIR3, and 9.9 parts by weight of WAIF,, fused and solidified. The resultant solid was crushed to produce a powdered flux.

This powdered flux had a melting point of 55800. When it was analyzed by X-ray diffraction, the presence of K3AlF4, KAIF4, and Li3AlF6 was confirmed. In terms of chemical composition, the powdered flux was found by chemical analysis to consist of 56.3 wt% of Al F3, 39.0 wt% of KF, and 4.7 wt% of LiF.

This powdered flux was mixed with water to prepare an aqueous slurry having a slurry content of 10 dry wt%. The aqueous slurry was spray applied to a flat tube of A1050 alloy degreased in advance with trichloroethylene, and then dried at 1 5000 for five minutes. Subsequently, a corrugated fin member of an Al-i 0%Si-4%Cu (equivalent to AA4145 alloy) alloy clad on the opposite surfaces of a core of A3003 was integrally combined with the flat tube mentioned above. With a conveyor type continuous brazing furnace, the combined both articles were held at 57000 for five minutes in an atmosphere of nitrogen gas. Consequently the both articles were connected to form a plurålity of automotive condensers.

The condensers thus produced were tested for quality. The ratio of adhesion between the corrugated fin and the flat tube was 98% or over on the average and the pressure resistance was 100 to 150 kg/cm2. They were further subjected to an accelerated corrosion test in a brine spray.

Even after 1,000 hours of such exposure, occurrence of harmful holes of corrosion was not observed. They showed ample brazing strength to prove the outstanding brazing property of the flux of this invention. Further the flux produced no apparent adverse effect upon corrosion resistance of the product. Thus, the flux was shown to satisfy all the properties expected of a flux.

EXAMPLE 2 By the wet method which comprised mixing the starting materials in the form of hydroxides in an aqueous hydrofluoric acid solution and causing the hydroxides to react with one another, there was prepared a flux consisting of 59.3 wt% of AIF3, 37.6 wt% of KF, and 3.1 wt% of LiF. The produced flux was found to have a melting point of 55000. When this flux was analyzed by X-ray diffraction, the presence of KAIF4, K3AlF6, and Li3AIFa was confirmed.

This flux was mixed with water to produce an aqueous slurry having a flux content of 50 dry wt%. This aqueous slurry was applied with a brush to joint surfaces of the two halves of several diecast intake manifolds of Al 2%Si-O.8%Mg alloy (equivalent to ADCI alloy) roughened in advance with a wire brush. The. applied slurry was dried at 20000 for five minutes.

Subsequently, a brazing metal prepared by punching a plate of Al-1 0%Si#4%0u-1 0Zn alloy (equivalent to AA4245 alloy) in the shape conforming to the joining surfaces was mounted on one of the joining surfaces and the manifold halves were combined. The manifold thus combined with the solder material interposed between the joint surfaces thereof were placed in a batch type brazing furnace filled with an atmosphere of nitrogen gas and heated at 56000 for 10 minutes to effect brazing.

The intake manifold thus obtained was tested for quality. In the airtightness test, no leakage was detected under an inner pressure of 2.0 kg/cm2.

The pressure resistance was found to fall in the range of 60 to 100 kg/cm2. In the accelerated corrosion test with brine spray, no occurrence of holes due to corrosion was observed after 100 holes of the text. The flux was thus shown to satisfy all the properties expected of a flux.

Claims (3)

1. A brazing flux consisting essentially of mixture of fluoaluminate complexes containing Al, F, K, and Li atoms, or a mixture thereof with aluminum fluoride said atoms being present in the following proportions expressed in terms of simple compounds: aluminum fluoride about 53~62% potassium fluoride about 35-44% lithium fluoride about 2- 7% said flux containing up to about 2% entrained impurities, all percentages being by weight.

2. The flux of Claim 1 in the form of dry granules having a particle size of up to about 74.

3. The flux of Claim 1 in the form of a slurry in a fluid medium containing about 5~50% dry weight of solid flux material.