Usually aluminum brazing performed in air or other atmosphere, containing oxygen, and therefore requires the use of chemical flux. Fluxes become active until the temperature of soldering and are in a molten state at all temperatures brazing. They penetrate through the oxide film, air is displaced and ensure wetting of the base metal solder.
Flux: good and perfect
A good soldering flux should:
- begin to melt at a sufficiently low temperature, to minimize product oxidation;
- It is completely melted by the time, when the solder starts to melt;
- spread along the joint and solder, to protect them from oxidizing gases;
- penetrate through and under the oxide film;
- remain liquid until the solder hardens;
- easily removed after soldering is complete.
The ideal flux for brazing in a furnace or burner to melt at a temperature only slightly below solder melting temperature. Thus he provides a uniform wetting and spreading of solder in the minimum time. Flux, which is used for soldering by dipping in molten solder, It has the structure, that it remains molten and stable at all temperatures solder melting. Solder dipping requires less active fluxes, than when soldering burner, since the parts to be completely immersed in the flux during the soldering process. Therefore, the surface detail is fully protected from oxidation by oxygen.
Composition fluxes for aluminum brazing solid
a flux, are used in the brazing of aluminum and its alloys, brazing, usually consist of a mixture of chlorides and fluorides of alkali and alkaline earth metals. Sometimes they contain aluminum fluoride or cryolite (3NaF AlF3). Composition fluxes selected so, that they have a favorable balance of melting range, density, reactivity, pickling capacity and cost. Reducing the content of fluoride fluxes reduces their effectiveness for removing oxides, while too great a concentration undesirably broad melting interval.
Flux composition may affect the color or appearance of the finished solder joint. Some fluxes intensive herbs and coarsening surface seam. a flux, chlorides which contain zinc and other heavy metals, These metals may be deposited on the base metal and painted in its darker color.
Fluxes are typically supplied as a dry powder in moisture proof containers. In such a package may be stored for a long time. When the container is opened with a flux, it should be taken strict measures to prevent contamination of the flux of atmospheric moisture. Container flux must be perfectly clean and be of aluminum, glass or ceramic - and never steel.
Methods for applying flux for brazing aluminum
Fluxes for brazing aluminum brazing is applied dry or mixed with tap water or alcohol and applied to the soldering by brush, spraying or dipping. Often the product is sprinkled with dry flux or heated bar is dipped into solder flux dry. Though, basically, flux may be mixed with tap water to a paste, alcohol use, often considered preferable. Under the influence of the flux of water vapor can be formed chemical compound, which can then be difficult to remove from the surface of the soldered seam and products.
Usually pause to 45 minutes superposition between subsequent soldering flux and considered normal. At the same time, Still recommend apply flux immediately before soldering. The aqueous paste of flux must be fresh or, at least, be prepared at least once per shift.
Improve the wetting effect of flux addition of wetting "agents", such as water. A mixture of two-thirds flux and one-third water (by weight) usually works well for brush application. For spraying or dipping need more water - the proportion is chosen from experience.
Источник: Aluminum and Aluminum Alloys, ASM International, 1996