Specialized metallurgical processes – compositions for use therei – Processes – Consolidating metalliferous material by agglomerating,...
Reexamination Certificate
2000-04-25
2001-04-24
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Consolidating metalliferous material by agglomerating,...
C075S751000, C075S753000, C148S024000, C148S026000, C423S116000, C423S465000
Reexamination Certificate
active
06221129
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for brazing metallic materials, an aqueous flux suspension and a novel flux for use in a brazing process and a preparation process for the novel flux.
During brazing, metal materials are joined at a temperature of above 500° C. with the aid of a molten additional metal (solder). The melting point of the solder is lower than the melting point of the materials, so that the solder joints of many metals can be separated in a nondestructive manner by melting the solder again.
During soldering, oxides and other interfering top layers on the metal surface present a problem: the surface must be metallically pure so that a satisfactory solder joint forms. This is achieved using fluxes, which are generally brushed on, sprayed on or applied as a solder covering on the material.
Fluxes based on alkali metal salts, preferably potassium salts, of complex aluminum fluorides are particularly suitable, for example for brazing light metal materials, in particular alloyed (for example with magnesium) or unalloyed aluminum materials, since they are non-corrosive and non-hygroscopic. Fluxes of this type which are, for example, mixtures of KAlF
4
and K
3
AlF
6
or mixtures of KAlF
4
and K
2
AlF
5
are already known, and the last mentioned compound can optionally be present as the hydrate.
The surface-cleaning action of a flux is evident from, inter alia, the fact that, after melting, the solder runs on the surface of the component or of the components. The better the surface of the component has been cleaned by the flux, the more readily the solder runs on that surface.
SUMMARY OF THE INVENTION
It was an object of the present invention to provide a flux which improves the flow behavior of the solder on the surface of metal materials, in particular based on light metal materials, such as aluminum.
A further object was to provide a corresponding aqueous suspension of the flux, a novel flux and a process for preparing the flux.
These and other objects have been achieved in accordance with the present invention by providing a process for brazing metal workpieces comprising applying a flux composed of alkali metal salts of complex aluminum fluorides, wherein said flux comprises irreversibly dehydrated K
2
AlF
5
.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The process according to the invention for brazing metallic materials, in particular light metal materials, using a flux based on alkali metal salts, preferably potassium salts, of complex fluorides of aluminum is characterized in that a flux which contains irreversibly dehydrated K
2
AlF
5
is used. The term “irreversibly dehydrated K
2
AlF
5
” is explained below.
The thermal behavior of hydrated K
2
AlF
5
(K
2
AlF
5
.H
2
O) has already several times been the target of scientific investigations. Bukovec and Bukovec,
Thermochimica Acta
92 (1985), pages 689 to 692, have investigated the dehydration of the hydrate at temperatures up to about 200° C. Tananaev and Nekhamkina,
Izvest. Sektora Fiz
.-
Khim. Anal. Akad. Nauk S.S.S.R.
20 (1950), pages 227 to 237 (
Chemical Abstracts
. Vol. 48, 1954, Abstract No. 8012a) have observed that, in the thermogram, the hydrate exhibits, at 145 to 165° C., an endothermic effect which is due to the dehydration and, at 230 to 260° C., an exothermic effect which is due to the recrystallization of the anhydrous salt. Wallis and Bentrup have published an X-ray and thermoanalytical investigation of the thermal dehydration of K
2
AlF
5
.H
2
O in
Z. Anorg. und Allg. Chem
. 589 (1990), pages 221 to 227. It was found that the hydrate is converted reversibly at a temperature in the range from 90 to 265° C. into K
2
AlF
5
which crystallizes in the tetragonal system. At 265 (±10)° C., irreversible formation of K
2
AlF
5
crystallizing in the orthorhombic system occurs (designated as “Phase II” in the publication). Under quasi-isobaric conditions, the irreversibly dehydrated product Phase II was present at temperatures as low as 228° C. Such a K
2
AlF
5
crystallized in the orthorhombic system is designated as “irreversibly dehydrated K
2
AlF
5
” within the scope of the present invention.
The results of the scientific investigations have however not been used at all in the technology of flux preparation. Willenberg, U.S. Pat. No. 4,428,920, discloses the preparation of a flux by combining fluoroaluminic acid and a potassium compound (used in a substoichiometric amount), for example KOH (or potassium hydroxide solution). The melting points were determined for products which had been dried at 120° C. Kawase, U.S. Pat. No. 4,579,605, discloses a flux for soldering aluminum, which flux consists of KAlF
4
and K
2
AlF
5
.H
2
O or K
2
AlF
5
. This flux is prepared, for example, by dissolving aluminum hydroxide in hydrofluoric acid and adding potassium compounds. A drying temperature of 100° C. is stated as being sufficient. Meshri, U.S. Pat. No. 5,318,764, discloses various methods for preparing a flux based on potassium salts of complex fluorides of aluminum; for example, by combination of aluminum oxide trihydrate with KF or KHF
2
and HF, or of aluminum fluoride trihydrate and KF or KHF
2
, or by combination of aluminum oxide trihydrate and potassium hydroxide solution and subsequent addition of HF. According to the Examples, drying is carried out at 150° C.
The advantage of using fluxes which contain K
2
AlF
5
irreversibly dehydrated according to the invention is that the solder exhibits very much better flow behavior than fluxes without irreversibly dehydrated K
2
AlF
5
.
The soldering process is carried out in a known manner. The flux is advantageously applied in the form of an aqueous suspension, which is likewise a subject of the present invention, to one or more of the metal materials to be joined. Advantageously, the suspension contains 3 to 60% by weight of the flux; the remainder of the flux up to 100% by weight comprises water and any impurities. After application to the material or materials or component or components, the suspension of the invention forms an especially loose layer which resembles snow in appearance. Such a loose flocculant layer completely covering the desired area of the material is very advantageous with respect to application techniques. The metal materials are then heated, for example in an oven or by means of a burner, first the flux and then the solder melting and a solder joint forming. If desired, soldering can be carried out in an inert gas atmosphere, for example in a nitrogen atmosphere. However, the soldering process can also be carried out in an atmosphere comprising air.
The soldering process according to the invention can be modified. For example, according to the teachings of U.S. Pat. Nos. 5,100,048 and 5,190,596, the disclosures of which are incorporated herein by reference, or WO 92/12821, respectively, a metal which is advantageously present in finely powdered form (for example in the form of particles having a size of less than 1000 &mgr;m, preferably in the range from 4 to 80 &mgr;m) and which forms a solderable eutectic mixture with one or both metal surfaces to be soldered can be mixed with the flux. Silicon is particularly suitable, and copper and germanium are also suitable. Thus, it is possible to use a flux which contains 10 to 500 parts by weight of finely powdered metal per 100 parts by weight of the complex aluminum fluorides. In addition, it is also possible to admix further metals in finely powdered form, which modify the surface properties of the soldered parts or of the eutectic mixture which forms. Thus, finely powdered iron, manganese, nickel, zinc, bismuth, strontium, chromium, antimony or vanadium can be admixed, as described in WO 92/12821.
According to the teaching of WO 93/15868, it is also possible to apply to the metal parts to be joined a metallic coating in which the flux is then embedded. For example, zinc layers or layers of zinc/aluminum alloy are very suitable.
The invention also relates to a flux which contains irreversibly dehydrated K
2
AlF
5
. Since the pure, irreversibly
Belt Heinz-Joachim
Borinski Alfred
Rudolph Werner
Sander Ruediger
Andrews Melvyn
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Solvay Fluor und Derivate GmbH
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