Aluminium is awkward to braze because its working range sits close to its melting point, and the metal gives almost no visual warning as it approaches. A filler intended for aluminium flows well below the parent's melting point, but the margin is small enough that careless heat will collapse the part.
This is the defining constraint. Everything else, from cleaning to flux to torch technique, exists to keep the assembly inside that window. On steel there is room to be approximate. On aluminium there is not.
The oxide layer on aluminium forms instantly and melts at a far higher temperature than the metal beneath it. Filler will not wet through it, so the oxide must be removed and kept away during heating. That is what flux does, and it is why fluxes for aluminium are more aggressive than those used on steel.
The same property explains why the oxide is hard to see. It is thin, transparent and self-healing, so a surface that looks bright can still be unusable. Mechanical cleaning immediately before heating, followed by a suitable flux, is the only reliable approach.
Aluminium does not glow red before it melts, so colour is a poor guide and the usual steel instinct fails. Watch the flux instead: it changes appearance as it becomes active, and that change is the practical signal that the assembly is ready.
Watch the part's surface as well. A sudden dulling or a slight sag warns that the metal is close to collapse. On thin sheet the transition from ready to ruined is quick, and the only defence is to bring the heat up slowly and keep the torch moving.
Feed filler into the joint once the flux is active, and let it run rather than pushing it. If the filler sits on the surface and refuses to wet, stop and reassess. More heat will not fix an oxide problem, and it will destroy a thin part.
Where the joint is a lap or a tee, a small fillet at the edge is enough. On aluminium the filler is generally softer than the parent, so a large cosmetic fillet adds mass without adding strength and increases the heat the part must absorb.
Brazing aluminium produces less distortion than fusion welding because the parent never becomes liquid, but it is not distortion-free. Local heating still causes expansion, and thin sheet still buckles if the heat is concentrated. Clamping and a moving heat pattern remain necessary.
Long seams are best run in stages with cooling between them. Trying to complete a seam in one continuous traverse usually ends with a buckled panel and a joint that has to be straightened before it is usable. Patience here is cheaper than rework.
Pressurised vessels, structural brackets and anything machined after joining should be welded. For light sheet, small fittings and parts that can be cleaned and inspected, brazing is defensible, and the TIG aluminium welding route covers everything else.
That includes the aluminium fuel tanks we produce, where fusion in an inert atmosphere is the only defensible method. A brazed joint on a tank that flexes under fill and drain cycles is a leak waiting to happen, and the repair is rarely neat.
