Fluxing agents

Details Fluxing agents Fluxing agents

2000-11-15

2002-08-13

Jenkins, Daniel J. (Department: 1742)

Metal treatment

Compositions

Fluxing

C148S026000

Reexamination Certificate

active

06432221

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to new fluxes for brazing aluminum and aluminum alloys, to a brazing method and brazed components.
Structural components (such as the radiator of automobiles or heat exchangers) of parts made from aluminum or aluminum alloys can be produced by brazing (hard soldering) these parts. Advantageously, a flux composed of a fluoroaluminate, which frees the surface of the parts that are to be brazed from oxide adhesions, is used. Fluxes composed of potassium fluoroaluminate are particularly suitable for aluminum or aluminum alloys containing little magnesium. Such a method is disclosed in the British patent 1,438,955. The production of appropriate fluxes is described, for example, by Willenberg, U.S. Pat. No. 4,428,920 and Meshri, U.S. Pat. No. 5,318,746 as well as by Kawase, U.S. Pat. No. 4,579,605.
Fluxes, which contain fluoroaluminates of cesium are disclosed, for example, by Suzuki in U.S. Pat. No. 4,670,067 and by Shimizu in U.S. Pat. No. 5,171,377. Such fluxes, which may additionally also contain potassium fluoroaluminate fluxing agents, are particularly suitable for brazing aluminum alloys having a higher magnesium content.
U.S. Pat. No. 4,906,307 discloses a process for brazing components formed of aluminum alloys. In accordance with one embodiment it is envisioned that a fluxing agent will be used which contains K
2
SiF
6
, ZnF
2
, NaF and AlF
3
. Solder plated components are brazed.
For brazing, the fluxing agent (for example, in the form of a suspension) as well as a brazing metal are applied on the components which are to be joined. The components are brought together into the desired position and heated. Initially the fluxing agent melts and cleans the surface; after that, the solder melts. Subsequently, the parts are allowed to cool.
U.S. Pat. No. 5,190,596 teaches that, instead of a brazing metal, a metal can be added to the fluxing agent. During the brazing, this metal forms a eutectic with the aluminum. Suitable metals are copper, zinc and germanium and especially silicon.
The addition of certain metal fluorosilicates in particular amounts can make the brazing metal superfluous (see EP-A-810 057 and German patent application 196 36 897.9). The latter patent application discloses that a mixture of potassium fluoroaluminate fluxing agent and potassium fluorosilicate, in which the latter is contained in an amount of 6 to 50%, makes a brazing metal superfluous.
In the British patent 1,438,955, mentioned above, it is explained that smaller amounts of alkali metal zinc fluorides, up to 5 mole percent, can be tolerated in the fluxing agent. However, their presence does not bring about any advantages in relation to lowering the melting point. Instead, all have the effect of raising the melting point. Haramaki (U.S. Pat. No. 4,645,119) discloses fluxing agents composed of potassium fluoroaluminate, which contain 3 to 30 wt.-% of ZnF
2
, optionally in the form of KZnF
3
. The zinc fluoride decomposes at the brazing temperature and the metallic zinc covers the brazed parts or the whole surface of the components which are to be brazed to one another and provides the aluminum with improved protection against corrosion.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a new application method as well as new fluxes, which can be employed for this method. This objective is accomplished by the inventive method, the new flux and the new fluxing agent.
The inventive method for brazing of aluminum and aluminum alloys using a fluxing agent composed of complex fluorides envisions that the fluxing agent contains alkali fluorozincate or mixtures of alkali fluoride and zinc fluoride as the flux, and that brazing is effected at a temperature ranging from 420° C. to 590° C., and wherein alkali refers to potassium, cesium or rubidium.
The alkali fluorozincate or mixtures of alkali fluoride and zinc fluoride have the effect of a fluxing agent at these temperatures. This is a totally unexpected finding. Surprisingly, however, the alkali fluorozincates act as fluxing agents even when the brazing is carried out at temperatures, which are far below the melting point of the alkali fluorozincate employed. The melting points of KZnF
3
and K
2
ZnF
4
, for example, are at temperatures of 870° C. and 737° C. Therefore, at temperatures below 600° C., brazing ought not to occur. The following explanation may be advanced. In the absence of solder-forming components, such as silicon, an Al-Si eutectic is formed. Because of electrochemical processes, this generates an alkali fluoroaluminate fluxing agent in situ. For example, it is assumed that KZnF
3
+Al (from the Al—Si alloy) react with formation of KAlF
4
or KF and AlF
3
and Zn metal. However, this is only proposed explanation, which might explain the phenomena, such as the brazing process and the formation of Zn.
Mixtures of alkali fluoride (or alkali fluorides) and zinc fluoride provide usable brazings. The molar ratio of alkali fluoride to zinc fluoride may be in the range of about 1:1, for example, from 1:1.05 to 1.05:1. However, one of the two components, particularly the zinc fluoride, can also be present in a larger excess. Preferably, however, alkali fluorozincates are used, since they provide better brazings.
In the context of the present invention, “alkali fluorozincate” refers to compounds of the general formula (MF)
x
.(ZnF
2
)
y
, in which M=K, Rb, Cs and 0<x≦4 and 0<y≦4. “Alkali fluoride” refers to the fluorides of potassium, rubidium and cesium.
Preferably, x and y are whole numbers, namely, independently of one another, 1, 2, 3, or 4; however, x and y may also be in a sub-stoichiometric relationship to one another. In that case, either x, y or both are then larger than 0, but do not represent a whole number. In this case, it is preferred if y is larger than x.
In the context of the present invention, the term “fluxing agent” refers to those compounds which have the desired surface-cleaning effect (especially the removal of oxide layers) during the brazing. The fluxing agent may consist of alkali fluorozincate; other fluxing agents are then not included. The fluxing agent can also contain other fluxing agents besides alkali fluorozincate. For example, the fluxing agent may represent a mixture of alkali fluorozincate and alkali fluoroaluminate, such as potassium fluoroaluminate and/or cesium fluoroaluminate. The alkali fluorozincate may be present as a pure compound or as a mixture of alkali fluorozincates, such as pure potassium fluorozincate or pure cesium fluorozincate. Moreover, the compounds may exist in one or more phases. For example, pure KZnF
3
or also mixtures of KZnF
3
and KZnF
4
can be used. However, appropriate mixtures with different alkali metal cations can also be used.
Preferred flurozincates include potassium fluorozincate and cesium fluorozincate. These can, of course, also be contained as a mixture.
If cesium fluorozincate is contained as the only fluorozincate in the fluxing agent, it is present in an amount of 5 wt.-% or more. Preferably, the alkali fluorozincate is contained in an amount of more than 30 wt.-%, and especially in an amount of 50 wt.-% or more, in the fluxing agent. The percentages are relative to the fluxing agent, which is set at 100 wt.-%. If the fluxing agents are not pure alkali fluorozincates, other fluxing agents represent the remainder of the mixture up to 100 wt.-%, especially fluxing agents composed of potassium and/or cesium fluoroaluminate.
The fluxing agent frequently can be used as such, without the addition of auxiliary materials. For example, solder-plated aluminum sheet can be brazed with pure fluxing agents. Aside from fluxing agents, the ready-to-use compositions may comprise auxiliary materials. The fluxing agents may also contain auxiliary materials, such as binders, dispersants, brazing metal, brazing metal precursors, solder-forming materials, such as metal fluorosilicates, especially alkali fluorosilicates, or stabilizers. In the inventive method, fluxing agents of pure alkali m

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