How to braze aluminium.
Surface preparation, heat control and why the filler balls up instead of flowing. Almost every failed aluminium braze comes down to one of three causes, and all three are avoidable.
Brazing, welding, soldering.
All three join metal. The difference is temperature, and whether the parent metal melts.
- Welding melts the parent metal so the two parts fuse into one piece. Strongest, and most destructive of thin material.
- Brazing melts only the filler, conventionally above about 450 °C. The filler wets and bonds to solid parent metal.
- Soldering does the same below that temperature with weaker fillers.
Brazing aluminium is popular precisely because the parts never melt. A radiator core or a thin-wall tube survives a braze and does not survive a weld — and at 380 to 420 °C an ordinary propane torch is enough, so no welding equipment is needed.
Oxide is the whole problem.
Aluminium grows a tough oxide skin within seconds of exposure to air. Filler will not wet through it. This single fact explains most failed joints.
The consequence is a timing one that people routinely get wrong: clean the joint immediately before heating it, not at the start of the job. A joint cleaned and then left while you set up the torch has already re-oxidised.
- Degrease first if oil is present. Abrading oily metal grinds the oil into the surface.
- Then abrade mechanically — stainless brush, emery or a scraper — back to bright metal.
- Then heat, straight away. No pause, no coffee.
- Use a brush kept for aluminium only. One that has been on steel transfers contamination into the joint.
Watch the flux, not the metal.
Steel tells you its temperature by glowing. Aluminium does not — it gives almost no colour warning and then collapses. That is what catches people who learned on steel, and it is why aluminium is considered difficult when it is really just different.
Use the flux as your gauge. When the flux in the rod turns clear and flows, the joint is at temperature. A second reliable indicator is touching the rod to the work periodically: when it begins to melt on contact with the metal rather than in the flame, you are there.
Heat the work, not the rod. This is the single most important technique point. Brazing filler flows toward heat and wets hot, clean metal. If you melt the rod in the flame and drip it onto cold metal, it balls up and sits there — the classic symptom, and always the same cause. Take the flame back to the work and start again.
Three failures and their causes.
| Symptom | Cause | Fix |
|---|---|---|
| Filler balls up | Work too cold; rod heated in the flame | Heat the parent metal until the rod melts on contact with it |
| Will not stick at all | Oxide or oil on the surface | Degrease, abrade to bright metal, heat immediately |
| Part collapses | Too much heat, or rod too heavy for the section | Smaller rod, less flame, keep moving. See rod sizes |
A joint that has failed twice is usually telling you something about the parent metal rather than the technique. Repeated reheating of thin material work-hardens and thins it further — at that point, replacement is the right call.
Ventilate, and mind the heat.
Brazing flux gives off irritant fumes. Work in open air or with forced ventilation and keep your face out of the plume — anyone doing this all day needs proper local extraction rather than an open door. Eye protection and gloves throughout, and remember that brazed parts stay dangerously hot far longer than they look hot.
On pressure systems: never assume a repair holds because it looks good. Pressure-test before recharging. See HVAC and radiator repair.
Common questions.
What is the difference between brazing, welding and soldering?
All three join metal; the difference is temperature and whether the parent metal melts. Welding melts the parent metal so the two parts fuse into one. Brazing melts only the filler, above about 450 °C conventionally, and the filler wets and bonds to solid parent metal. Soldering does the same below that temperature with weaker fillers. Brazing aluminium is popular precisely because the parts never melt — which is what makes thin tube and radiator cores survivable.
How do I prepare an aluminium surface for brazing?
Mechanically, and immediately before you heat it. Remove paint, oil and corrosion, then abrade back to bright metal with a stainless brush, emery or a scraper. Aluminium grows a new oxide skin within seconds of exposure, and filler will not wet through oxide — so cleaning a joint and then leaving it until after lunch means cleaning it again. Degrease first if there is oil present, because abrading oily metal just grinds the oil in.
How do I know when the joint is at the right temperature?
Watch the flux, not the metal. Aluminium gives almost no colour warning before it collapses — unlike steel, it does not glow red first, which is what catches people out. The flux in the rod turning clear and flowing is the signal that the joint is ready. Touching the rod to the work periodically and feeling it start to melt on contact is the other reliable indicator.
Why does the filler ball up instead of flowing into the joint?
Because the work is too cold and the rod is being heated directly by the flame. Brazing filler flows towards heat and wets clean, hot metal — so the parent metal must be brought to temperature, and the rod then melts on contact with the work rather than in the flame. If you find yourself melting rod into a cold joint, stop, take the flame back to the work, and start again.
Can a brazed aluminium joint be re-brazed if it fails?
Yes, but clean it out properly first. Old flux residue and the previous filler both interfere with a new joint wetting, so the area needs mechanically cleaning back — ideally with the old filler removed rather than added to. Repeated reheating of thin material also work-hardens and thins it, so a joint that has failed twice is usually telling you the parent metal is the problem.
Is brazing aluminium safe to do without extraction?
Work in ventilated space and keep your face out of the plume. Brazing flux gives off fumes that are irritant, and the practical minimum is open air or forced ventilation, eye protection, and gloves — the parts stay hot far longer than they look hot. Anyone doing this all day should have proper local extraction rather than an open door.
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