Powder
Dry flux for electrostatic or wet spraying in continuous furnaces, dipping or dusting. Easy to meter in automated lines; common for aluminium heat exchanger brazing.
Brazing consumables guide
Brazing Flux: Types, Classes and How to Choose
Brazing flux is a chemical compound applied to a joint before heating. As it melts, it dissolves the oxide film on the base metal and filler, shields the hot surfaces from air and lets the molten filler wet and flow through the joint. The right flux depends on three things: the base metal, the filler alloy and its brazing temperature, and the heating method. This guide explains the main flux families for aluminium, silver brazing, copper alloys and steels, and how to select and use them. Metline supplies aluminium brazing fillers, including flux-cored Al-Si and Zn-Al rod; the other families are explained for comparison and for mixed-metal assemblies.
- Job of flux
- Remove oxide, protect, aid wetting
- Aluminium flux
- Fluoroaluminate or chloride
- Silver brazing flux
- Fluoride-borate (AWS FB3)
- Forms
- Powder, paste, slurry, cored or coated rod
What brazing flux does
Every engineering metal carries an oxide film. On copper it is thin and easily reduced; on stainless steel it is rich in chromium oxide; on aluminium it is a tough, stable layer that re-forms instantly in air. Molten filler cannot wet an oxide, so without flux, or a vacuum or reducing atmosphere that does the same job, the filler balls up instead of flowing into the gap.
- Cleaning: the molten flux dissolves or breaks up the oxide on the base metal and filler surfaces.
- Protection: it forms a liquid blanket that keeps oxygen away while the joint is hot.
- Wetting: it lowers the interfacial tension so the filler spreads and is drawn into the joint by capillary action.
- Indication: with many torch fluxes, the change from a white crust to a clear liquid shows the operator that the joint is approaching brazing temperature.
A flux must be fully molten and active slightly below the solidus of the filler and remain active until the filler has flowed, but it must not be exhausted by long heating. Matching this activity window to the filler's brazing range is the single most important selection rule.
Main flux families by base metal
| Base metal | Typical flux chemistry | Approximate activity range | Residue |
|---|---|---|---|
| Aluminium (furnace / CAB) | Potassium fluoroaluminate (KAlF4-K3AlF6) | Melts about 565 – 572 °C; used to about 610 °C | Thin, non-corrosive, usually left in place |
| Aluminium with magnesium | Caesium-modified fluoroaluminate | Slightly lower melting; tolerates more Mg in the base metal | Non-corrosive |
| Aluminium (torch, dip) | Chloride-fluoride mixtures (Li, Na, K chlorides with fluorides) | About 550 – 620 °C | Hygroscopic and corrosive; must be washed off |
| Copper, brass, steel with silver filler | Potassium fluoroborates, fluorides and borates | About 550 – 870 °C | Corrosive; remove with hot water |
| Stainless steel, carbide | Fluoride-borate with boron additions | About 550 – 925 °C | Remove after brazing |
| Steel, nickel alloys with copper or brass filler | Borax and boric acid based | About 750 – 1200 °C | Glassy; removed mechanically or by pickling |
| Copper-to-copper with phosphorus filler | Usually none needed | — | Phosphorus in BCuP filler acts as its own flux on copper |
Ranges are indicative of common products. The exact activity range for any flux is set by its classification and data sheet.
For aluminium, the non-corrosive potassium fluoroaluminate flux used in controlled atmosphere brazing (often called Nocolok-type flux) transformed heat exchanger manufacturing because its residue does not attack the part and need not be washed. Chloride fluxes still dominate manual torch and repair work because they are very active at lower temperatures, but their residues absorb water and must be removed completely.
How brazing fluxes are classified
Two systems are in common use. AWS A5.31 classifies fluxes for brazing and braze welding by filler type, form and activity range; ISO 18496 (which replaced EN 1045 in Europe) groups fluxes by base metal and residue behaviour.
| AWS A5.31 class | Typical use | Activity range |
|---|---|---|
| FB1 (A, B, C) | Aluminium and aluminium alloys with Al-Si filler | Roughly 560 – 615 °C, depending on sub-class |
| FB3-A | General silver brazing of steels, copper and copper alloys; paste | 565 – 870 °C |
| FB3-C | As FB3-A with boron additions for longer heating and stainless steel | 565 – 925 °C |
| FB3-D | High-temperature brazing with copper and nickel fillers | 760 – 1205 °C |
| FB4-A | Aluminium bronze and other alloys with tenacious oxides | 595 – 870 °C |
| ISO 18496 / EN 1045 type | Description |
|---|---|
| FH10 | Multi-purpose flux for heavy metals, around 550 – 800 °C; residues corrosive |
| FH11 | For copper-aluminium alloys (aluminium bronze), similar range |
| FH12 | For stainless and high-alloy steels and carbides, contains boron |
| FH20 / FH21 | Higher-temperature fluxes, about 700 – 1100 °C, for copper and brass fillers |
| FL10 | Aluminium flux with hygroscopic chlorides; corrosive residue must be removed |
| FL20 | Aluminium flux with non-hygroscopic fluorides; residue non-corrosive |
Check the current edition of the standard and the supplier data sheet for exact ranges; the tables summarise the general structure.
Flux forms and how they are applied
Paste
Flux mixed with water or a binder for brushing onto torch and induction joints. The standard form for silver brazing.
Slurry
Powder suspended in water or a carrier for spray, roll or dip coating, often with a binder so it stays on before brazing.
Flux-cored rod or wire
Filler with flux inside a seam or tube. One consumable for manual and repair brazing, with no separate fluxing step.
Flux-coated rod
Filler coated on the outside with flux; quick to use for torch brazing of copper, brass and steel.
Pre-fluxed rings and preforms
Filler rings or shapes carrying a measured amount of flux, placed on the joint before automatic heating.
Filler paste
Metal powder and flux in one paste, dispensed onto complex or hard-to-reach joints.
Pre-fluxed brazing sheet
Aluminium brazing sheet supplied with flux already applied, removing the fluxing step at the assembler.
Flux selection for aluminium brazing
Aluminium brazing is the most demanding case because the gap between the filler's melting range (about 577 – 615 °C for Al-Si fillers) and the base metal's solidus is narrow. Flux must be fully active in that window and cope with magnesium, which forms oxides and reacts with fluoroaluminate flux.
| Situation | Recommended approach |
|---|---|
| Heat exchangers in CAB furnaces (3003 core, 4343/4045 clad) | Standard potassium fluoroaluminate powder or slurry, about 3 – 5 g/m² of joint surface as a common starting point |
| Base metal with more magnesium (some 6xxx alloys) | Caesium-modified fluoroaluminate, or vacuum brazing without flux |
| Fine fins, screen-printed or localised fluxing | Paste or slurry with binder, applied only where joints form |
| Torch or induction repair | Chloride-fluoride paste or flux-cored Al-Si rod; wash residue thoroughly |
| Aluminium to copper | Special low-temperature flux and Zn-Al or Al-Si filler matched to the joint; see our aluminium-to-copper guide |
| Vacuum brazing (4004/4104 clad) | No flux; magnesium in the cladding disrupts the oxide in vacuum |
How to use brazing flux correctly
- Clean firstFlux removes oxide, not oil or heavy scale. Degrease parts and remove heavy oxide mechanically or chemically before fluxing.
- Apply the right amountCoat both joint surfaces and the filler. Too little flux is exhausted before the filler flows; too much leaves heavy residue and can trap voids.
- Heat the parts, not the fluxBring the base metal up to temperature evenly so the filler melts on contact with the work. Overheating burns out the flux and blackens it.
- Watch the fluxFor torch work, flux turning clear and fluid signals that the joint is near brazing temperature; then add filler.
- Remove residueWash corrosive fluxes off with hot water, brushing or ultrasonic cleaning soon after brazing. Non-corrosive aluminium flux residue is normally left on furnace-brazed parts.
- Store properlyKeep containers closed and dry. Chloride fluxes absorb moisture; water-based pastes can dry out or freeze.
Common flux-related brazing defects
| Problem | Likely flux cause | Fix |
|---|---|---|
| Filler balls up and will not wet | Flux inactive at that temperature or exhausted | Match flux range to filler; reduce heating time |
| Black, burnt flux | Overheating or long heating | Use a higher-range or boron-modified flux; heat faster |
| Voids and porosity in the joint | Flux trapped in the gap | Use less flux; allow a vent path; check joint clearance |
| Corrosion after service | Corrosive residue not removed | Improve washing or change to a non-corrosive flux |
| Poor joints on Mg-bearing aluminium | Standard flux reacting with magnesium | Use caesium-modified flux or vacuum brazing |
| White powder on furnace-brazed parts | Excess fluoroaluminate application | Reduce flux load and improve uniformity |
Health and safety
Most brazing fluxes contain fluorides, and some contain boron compounds. Fumes from heated flux can irritate the eyes, nose and lungs, so brazing should be done with local exhaust ventilation, and operators should avoid skin contact with pastes and powders. Always read the safety data sheet, keep flux away from food areas, and handle residue wash water according to local rules.
References and standards
- AWS A5.31M/A5.31 – Specification for Fluxes for Brazing and Braze Welding
- ISO 18496 – Brazing: fluxes for brazing, classification and technical delivery conditions
- AWS Brazing Handbook
- ASM Handbook, Volume 6: Welding, Brazing, and Soldering
Brazing flux: frequently asked questions
Can I use silver brazing flux on aluminum?
No. Silver brazing fluxes are active at temperatures above the melting range of aluminium filler and cannot remove aluminium oxide effectively. Aluminium needs a fluoroaluminate or chloride-fluoride flux designed for Al-Si fillers around 577 – 615 °C.
Do I need flux to braze copper to copper?
Not with copper-phosphorus (BCuP) filler: the phosphorus fluxes copper by itself. Flux is needed when brazing copper to brass, steel or other metals, or when using silver fillers without phosphorus.
Must flux residue be removed?
Residues of chloride aluminium fluxes and fluoride-borate silver brazing fluxes are corrosive and must be washed off. The non-corrosive potassium fluoroaluminate residue from furnace brazing of aluminium is normally left in place.
What is the difference between FB3-A and FB3-C flux?
Both are silver brazing fluxes. FB3-C contains boron additions that extend its activity to a higher temperature and give it more endurance for longer heating cycles and stainless steels.
Why does my flux turn black?
Black flux usually means it has been overheated or heated too long and is exhausted. Heat the part faster and more evenly, or use a flux with a higher or longer activity range.
Is flux needed for vacuum brazing?
No. In vacuum brazing of aluminium, magnesium in the filler cladding disrupts the oxide; in vacuum or hydrogen brazing of steels, the atmosphere prevents oxidation.
Need aluminium brazing filler matched to your flux?
Send the base metals, filler alloy, heating method and volume to sales@metline.in, and our team will reply with suitable Al-Si or Zn-Al brazing materials.
