Their use is essential in the soldering/brazing processes. Heating a metal surface accelerates the formation of oxides, the result of chemical combination between the hot metal and oxygen in the air. This oxide layer has to be removed before brazing, and the formation of a new oxide layer has to be prevented or they will inhibit the filler metal from wetting and bonding to the surfaces. Generally, fluxes play the following roles:
- Fluxes dissolve surface oxides
- Fluxes permit faying surfaces to wet
- Fluxes encourage filler metal flow
- Fluxed prevent re-oxidation during the brazing process
Fluxes should be selected to fit the brazing application and the specific metals and alloys to be joined, and the following main types are available on the market:
- Non-corrosive potassium flux paste for Al-Si brazing
- Non corrosive caesium flux paste for Al-Zn soldering .
Fluxes are typically applied just before brazing in order to reduce the risk of dry-out and flake off. Further, it is essential that the filler material has a higher melting point than the chosen flux.
Non-corrosive potassium flux paste for Al-Si brazing
Non-corrosive flux paste (based on potassium fluoroaluminate salts KAlF4 K3AlF6) can be used with filler AA4047 in a controlled atmosphere furnace, torch, or induction brazing processes for joining a wide variety of aluminium alloys. Adding potassium to the conventional fluoride fluxes allows for better brazing of aluminium magnesium alloys such as 6061 and 6063. No post-braze cleaning operations are required. Potassium flux paste and its residues are non-hygroscopic and non-corrosive.
Non-corrosive caesium flux paste for Al-Zn soldering
Special flux paste with caesium aluminium salts (CsAlF4) and a proprietary binder are available to be used in atmosphere, furnace, torch or induction brazing processes for joining a wide variety of aluminium alloys using low temperature filler alloys of zinc/aluminium that performs in the temperature range from 382-to-500°C. The flux and its residues are non-hygroscopic and non-corrosive.
