FLUXES SELECTION
FLUXES SELECTION
Most metals and alloys react with the gaseous atmosphere surrounding them. A layer of difficult to melt non-metallic compounds and in particular oxides form on their surface, The intensity of this process increases with temperature. This layer, present both on the surface of bonded elements as well as the melted filler metal, to a large extent hinders the spreading of the filler metal which results in the inability to create a durable bond between the brazed elements. Fluxes should be used in order to ensure the required purity of the bonded surfaces and the filler metal.
The primary requirements for the correct flux operation include:
- the flux should not enter into chemical reactions with the filler metal (except for reaction brazing),
- the flux should sufficiently clean oxides from the surfaces of the brazed object and filler metal and protect them against atmospheric influence during brazing.
- the flux melting temperature should be below that of the filler metal and its evaporation temperature – higher,
- fluxes in liquid form and gas form should improve the wetability of the surface of the shape, or improve the spreading of the filler metal,
- fluxes should maintain their properties and chemical composition during the temperature increase under brazing,
- the flux should not cause a strong corrosion and should not emit toxic gasses whilst brazing,
- the flux should easily flow to the surface of the liquid filler metal and allow for the formation of a regular and smooth piece of filler metal surface during filler metal settling.
- after brazing flux slag covering the bond should be as easy as possible to remove [2],
- mutual attraction between the liquid flux and brazed metal should be less than that between the brazed metal and liquid filler metal, which is a pre-condition for the flux easily flowing to the surface
Due to their use, fluxes fall into two primary groups:
- Flux for soldering
- Flux for brazing
Flux for soldering
Flux for soldering classification by their main ingredients acc. to PN-EN 29454-1:2000
Flux type | Cover | Activator | Form |
---|---|---|---|
Resin1 | 1 Colophony | 1 Without activator 2 Halide activator * 3 Non-Halide activator | Liquid A Solid B Paste C |
2 Non-colophony (resin) | |||
Organic1 | 1 Water soluble | ||
2 Not water soluble | |||
Non-organic 3 | 1 Salts | 1 With ammonium chloride 2 Without ammonium chloride |
|
2 Acids | 1 Phosphoric acid 2 Other acids |
||
3 Alkalis | Amines and/or ammonium | ||
* Other activators may be used. |
Flux for brazing
Flux for brazing in accordance with PN-EN 1045:2001
Class | Type | Melting temp. range | Characteristics ingredients | Use designation |
---|---|---|---|---|
FH1) | FH10 | 550-880°C | boron compounds, simple fluorides | General use flux |
FH11 | 550-800°C | boron compounds, simple and complex fluorides, chlorides | brazing copper alloys and aluminium | |
FH12 | 550-850°C | boron compounds, free boron, simple and compound fluorides | brazing stainless steel and other steels and hard metals | |
FH20 | 700-1000°C | boron compounds, fluorides | General use flux | |
FH21 | 750-1100°C | boron compounds | General use flux | |
FH30 | 550-880°C | boron compounds, phosphates, silicates | brazing using copper and nickel filler metals | |
FH40 | >1000°C | chlorides, fluorides | brazing when the presence of boron is unwanted | |
FL2) | FL10 | >550°C | hygroscopic fluorides, mainly lithium compounds | brazing aluminium and its alloys |
FL20 | >550°C | non-hygroscopic fluorides | ||
1)Flux for brazing heavy metals 2)Flux for brazing light metals |