What is a brazing flux?
The functions of brazing flux
Normal brazing of aluminium is usually performed in air or other oxigen-containing atmospheres. Therefore, it requires the use of a chemical flux.
The functions of brazing flux are to:
- become active before brazing temperature is reached
- melt over the entire brazing range
- penetrate the film of oxide
- exclude air
- promote wetting of the base metal by filler metal .
What is a good brazing flux?
- Begin to melt at a temperature low enough to prevent oxidation of the base metal
- Be enough molten at the time the filler metal melts
- Flow over the joint and the filler metal to shield them from oxidation
- Penetrate the oxide film
- Lower the surface tension between the solid and liquid metal to help wetting
- Remain liquid until the filler metal has solidified
- Be easy to remove after brazing
Brazing fluxes composition
Aluminium brazing fluxes usually:
- consist of mixture of alkali and alkaline earth chlorides and florides
- sometimes, contain aluminium floride or cryolite
- have small amounts of one or more of the clorides of many metals and substances to improve the performance of fluxes.
The use of brazing fluxes
Brazing fluxes storage
Brazing flux is received in dry powder form in sealed, moisterproof containers. It can be stored for long periods if the sea is maintained.
Once a flux container is opened, strict care must be followed to prevent contamination of the flux by atmospheric moisture.
Way of working with brazing fluxes
Dry, water-mix or alcohol-mix
Aluminium brazing fluxes can be apply dry, or they cam be mixed with tap water or alcohol and applied by painting, sprying, or dipping. Dry flux can be sprinkled on the work, or a heated filler rod can be dipped into dry flux.
The wetting action of a flux can be much improved be the use of a wetting agent. A mixture of two-thirds flux and one-third water usually is good for painting. Sprying or dippingrequire more water.
45 minutes is considered the maximum time lapse between the application of flux and following brazing. But, the applicationof flux is recomended immidiately prior to brazing. Wet flux mixtures should be freshly prepared at last once in each shift .
Sometimes to prevent filler metal from flowing beyond a certain area is needed. Stopoffs suitable for it consist of a mixture of equal parts by weight of:
- a medium-heavy engine oil
- finely powdered graphite
- benzene or naphta or slurries of refractory oxides.