Dry powder
For electrostatic application on CAB lines, or mixed in-house into slurry. Particle size affects how the powder sprays and adheres.
Brazing consumables
Aluminium Brazing Flux: Types, Use & Selection
Aluminium brazing flux removes and holds back the tough oxide film on aluminium so that molten aluminium-silicon filler can wet and flow into the joint. The main choice is between non-corrosive potassium fluoroaluminate flux, used in controlled-atmosphere furnace brazing of heat exchangers, and corrosive chloride flux, used mainly for torch and repair work. This guide explains how each works, which suits which alloy and process, and how to apply, store and buy it.
- Main type
- K-Al-F (non-corrosive)
- Melts
- approx. 560 – 575 °C
- Forms
- Powder, slurry, paste, cored rod
- Standards
- ISO 18496 · AWS A5.31
Why aluminium needs a brazing flux
Aluminium forms an oxide skin within moments of exposure to air. The film is only nanometres thick, but it melts above 2,000 °C and is chemically stable, so molten filler metal will simply bead up on it instead of wetting the surface.
Aluminium brazing is done with aluminium-silicon fillers (4343, 4045, 4047) that melt between about 577 °C and 615 °C, only a few tens of degrees below the solidus of the parent metal. There is no time or temperature margin to "burn through" the oxide. The flux must melt just before the filler, dissolve or loosen the oxide, protect the cleaned surface from re-oxidising and then get out of the way as the filler flows.
Two routes avoid flux entirely: vacuum brazing, which uses magnesium in the filler to disrupt the oxide under high vacuum, and some newer flux-free controlled-atmosphere processes. Everything else, from automotive radiators made by the million to a technician repairing an AC line with a torch, relies on flux.
The main types of aluminium brazing flux
| Type | Chemistry | Residue | Typical process | Notes |
|---|---|---|---|---|
| Non-corrosive fluoride | Potassium fluoroaluminates (KAlF4 with K2AlF5 / K3AlF6) | Thin, insoluble, non-hygroscopic; normally left in place | Controlled-atmosphere (CAB) nitrogen furnace brazing; also flame and induction | The standard for heat exchangers. Tolerates only low magnesium in the parent alloy. |
| Caesium-modified fluoride | Fluoroaluminates with caesium (Cs-Al-F or K-Cs-Al-F) | Non-corrosive | CAB and flame brazing of Mg-bearing alloys | Lower melting and more tolerant of magnesium; costs more. |
| Lithium- or zinc-modified fluoride | Fluoroaluminates with Li or reactive zinc additions | Non-corrosive | CAB | Used to improve post-braze corrosion behaviour or to deposit zinc for sacrificial protection. |
| Chloride (corrosive) | Alkali chlorides with a fluoride activator | Hygroscopic and corrosive; must be washed off | Torch, dip and repair brazing | Very active, but any residue left behind will attack aluminium. |
Under ISO 18496 aluminium fluxes form class FL, which separates hygroscopic, corrosive chloride-based fluxes from non-hygroscopic fluoride-based fluxes whose residues are not corrosive. In North America, AWS A5.31 classifies aluminium brazing fluxes in its FB1 group by form and activity range. Check the classification on the supplier’s data sheet against your procedure.
How fluoride flux works in CAB furnace brazing
Most aluminium brazing by volume is controlled-atmosphere brazing (CAB) of heat exchangers: radiators, condensers, evaporators, charge-air coolers and battery cooling plates. Parts are made from brazing sheet clad with 4343 or 4045 filler, assembled, fluxed, dried and passed through a continuous nitrogen furnace.
- DegreaseRemove forming oils, by thermal degreasing or washing. Residual oil prevents flux from adhering and causes poor joints.
- FluxApply flux as a water-based slurry by spraying or dipping, or electrostatically as dry powder. The target is a uniform coat, commonly in the region of 3 – 5 g/m² on the surfaces to be joined, adjusted by part design and trials.
- DryDrive off water in a drying oven before the brazing furnace, so moisture does not enter the nitrogen atmosphere.
- HeatIn nitrogen with low oxygen and dew point, the flux melts at about 560 – 575 °C, dissolves the oxide and spreads over the joint.
- BrazeAt around 590 – 610 °C the Al-Si cladding melts and flows into joints by capillary action. Time at peak temperature is kept short to limit silicon diffusion into the core.
- CoolThe filler solidifies; a thin, adherent flux residue remains on the surface and is normally not removed.
Magnesium: the main limit on fluoride flux
Magnesium in the parent alloy diffuses to the surface during heating and reacts with potassium fluoroaluminate flux, forming high-melting magnesium fluoride compounds that poison the flux. Above roughly 0.3 – 0.5 % Mg, standard non-corrosive flux struggles to produce sound joints in CAB. This is why typical CAB core alloys (3003, 3005 and modified 3xxx grades) are low in magnesium.
| Parent alloy group | Typical Mg | Standard K-Al-F flux | Better option |
|---|---|---|---|
| 1xxx (1050, 1100) | Very low | Good | Standard flux |
| 3xxx (3003, 3102) | Very low | Good | Standard flux |
| 3005 / modified 3xxx | Low (up to about 0.6 % in 3005) | Usually workable at low Mg contents | Standard or Cs-modified flux, by trial |
| 6063 / 6060 | 0.35 – 0.9 % | Difficult | Cs-modified flux; higher flux load |
| 6061 / 6951 | 0.4 – 1.2 % | Poor in CAB | Cs-modified flux, or vacuum brazing |
| 5xxx (5052, 5083) | 2.2 – 4.9 % | Not brazeable with flux | Joining by welding or adhesive instead |
| 7xxx | High Zn and Mg | Generally not brazeable | Not recommended |
Mg ranges are Aluminum Association composition limits. Whether a specific part brazes well also depends on joint design, heating rate and flux load; always confirm by trial.
Powder, slurry, paste and flux-cored filler
Water-based slurry
Flux suspended in water, sometimes with a binder, for spray or dip fluxing of whole cores.
Flux paste
Flux in a binder for brushing, dispensing or screen printing onto specific joints. Useful for cold plates and local brazing.
Filler + flux paste
Al-Si filler powder premixed with flux, applied directly to joints that have no clad filler.
Flux-cored or flux-coated rod
Filler rod or wire with flux inside or on the surface, for flame and induction brazing of tubes and repairs.
Pre-fluxed brazing sheet
Clad sheet with flux applied at the mill, removing the fluxing step from the customer’s line.
For torch and repair work, combine flux with a suitable filler: see our aluminium brazing rods and flux-core aluminium brazing rods.
Common flux problems and how to avoid them
| Problem | Likely cause | Remedy |
|---|---|---|
| Filler beads up and will not flow | Flux load too low, oil residue, or Mg-poisoned flux | Clean better; raise flux load; use Cs-modified flux for Mg alloys |
| Joints good at edges, poor in centre of core | Uneven slurry application or insufficient drying | Check spray coverage and drying oven |
| Excessive white residue | Flux load far above need | Reduce and control application weight |
| Corrosion after torch brazing | Chloride flux residue left on the part | Wash thoroughly in hot water; switch to non-corrosive flux where possible |
| Erosion of thin fins | Too long at peak temperature | Shorten the brazing cycle; review filler thickness |
Storage, handling and safety
- Keep containers sealed and dry. Chloride fluxes absorb moisture readily; fluoride powders cake if damp.
- Re-mix slurries before use, because flux settles out of suspension.
- Fluoride fluxes release fumes during brazing; use local exhaust ventilation and follow the supplier’s safety data sheet.
- Use dust masks or respirators when handling dry powder, and avoid skin and eye contact.
- Observe the shelf life on the label, and treat brazing wastes and washing water according to local regulations.
References and standards
- ISO 18496 – Brazing: fluxes for brazing, classification and technical delivery conditions
- AWS A5.31M/A5.31 – Specification for Fluxes for Brazing and Braze Welding
- AWS Brazing Handbook – aluminium brazing chapter
- The Aluminum Association – International Alloy Designations and Chemical Composition Limits
- ASM Handbook, Vol. 6 – Welding, Brazing and Soldering: brazing of aluminium alloys
Aluminium (aluminum) brazing flux: frequently asked questions
What flux is used for brazing aluminium?
For production furnace brazing, a non-corrosive potassium fluoroaluminate flux. For torch repairs, either a non-corrosive fluoride flux or a more active chloride flux whose residue must be washed off. Caesium-modified fluoride flux is used when the alloy contains magnesium.
Does non-corrosive flux residue need to be removed?
Normally no. The residue is a thin, insoluble, non-hygroscopic layer that does not attack aluminium and is left on most heat exchangers. Chloride flux residue, by contrast, must always be removed.
Can I braze 6061 with standard flux?
Usually not reliably in CAB, because the magnesium in 6061 poisons standard fluoroaluminate flux. Use a caesium-modified flux and confirm by trial, or consider vacuum brazing.
At what temperature does aluminium brazing flux melt?
Potassium fluoroaluminate fluxes melt at about 560 – 575 °C depending on their exact composition, just below the 577 °C eutectic at which Al-Si fillers begin to melt. Caesium-modified fluxes melt somewhat lower.
How much flux should be applied in CAB brazing?
Loads commonly fall in the region of 3 – 5 g/m² on joint surfaces, but the right value depends on part design, alloy and furnace; establish it by brazing trials and control it with regular weight checks.
Can flux be used for aluminium-to-copper joints?
Yes, with suitable fillers and fluxes, but the joint behaves differently from aluminium-to-aluminium brazing. See our aluminium-to-copper brazing guidance before choosing materials.
Need aluminium brazing flux?
Send the alloys, brazing process, application method and quantity. We reply with matching flux, filler options and delivery terms.
