Aluminium brazing fluxes

Kawakatsu I., Osawa T., Erosion of aluminium based alloys by molten brazing flux. Journal of Japan Institute of Metals, vol. 37, 1977, p. 435. 

Potassium fluorometalates and in particular fluorozincate and fluorosilicate have applications in the field of aluminium brazing as so-called reactive fluxes. The nomination reactive stands for the redox reaction that takes place between the flux (KZnp3 or K2SiF6) and the aluminium surface to be brazed. These reactions give rise to formation of elemental Zn or Si and potassium fluoroaluminate, which acts as the actual flux. The elements formed in this way combine and alloy with aluminium, diffusing through the... 

U. Seseke, New developments in non-corrosive fluxes for innovative brazing. First International Congress Aluminium Brazing, Diisseldorf, May 2000, conference proceedings. 

The fluxes used when brazing are alkali halide and borate mixtures and compounds, and they have two main functions first, to dissolve the oxide film on the component surface or at least to degrade its adhesion by penetration of naturally occurring flaws and electrolytic action at the oxide-substrate interface, and secondly to prevent formation on the liquid surface of oxide skins which would restrict braze flow. Fluxes can be contained in a bath held at the brazing temperature in which the, usually aluminium, component is placed or else applied as a paste to surfaces of the component or braze. 

The most common flux application method in CAB is by spraying an aqueous suspension. Constantly agitated flux slurries with concentrations of approximately 5-35 % solids are pumped from tanks to fluxing booths. All aluminium surfaces involved in the brazing process are coated with the slurry, resulting in a uniform flux layer. Excess flux slurry is removed with a high-volume air blow the excess is then collected, recycled and reused in the fluxing booth. 

Aluminium oxide thickness increases with temperature, time at temperature and particularly in the presence of moisture. Oxide formation will affect clad fluidity [78]. There is only a small drop in braze ability when the oxide thickness increases from 40 to 220 A at 5 g/m flux load. However, there is an appreciable drop in clad fluidity as the aluminium... 

One method involves a mixture of flux with silicon powder, the NOCOLOK Sil Flux process [95]. At brazing temperature, the silicon powder diffuses into the aluminium substrate and generates Al-Si filler metal for joint formation. Sil flux can be applied with a binder to specific component surfaces, e.g. extruded tubes. In this case, the filler metal would be supplied from the tube and a clad fin sheet is not necessary. 

Controlled atmosphere brazing using non-corrosive potassium fluoroaluminate flux is the dominant process for making all aluminium heat exchangers. 

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