Welding aluminium: oxide, AC TIG and pulsed MIG

Aluminium punishes habits learned on steel. It melts at half the temperature but needs more heat, it gives no colour warning before it collapses, and it is covered in an oxide that melts at three times the temperature of the metal.

The oxide film

Every aluminium surface carries a layer of Al₂O₃ that reforms within minutes of being removed. It melts at about 2050 °C against 660 °C for the metal, so the arc meets a refractory skin over a pool that is already liquid underneath. That is what produces a wandering arc, black smut in the weld and oxide inclusions.

Two removals, and both are needed. Mechanically: a stainless wire brush kept exclusively for aluminium, used immediately before welding — not an hour before. Electrically: the electrode-positive half of the AC cycle in TIG blasts the film off the surface ahead of the arc. That cathodic cleaning is the whole reason aluminium is TIG welded on alternating current.

Heat, and the lack of warning

Aluminium conducts heat about five times faster than steel, so it draws heat away from the joint and a cold start will not fuse. Preheating to 100–150 °C helps on thicker sections — but no higher, because above about 200 °C the heat-treatable alloys begin to lose their strength permanently.

The part that catches everyone: aluminium does not glow before it melts. Steel warns you by going red; aluminium goes from looking cold to collapsing into a hole with nothing in between. The only warning is the surface going shiny and liquid, which is roughly one second before it is too late.

TIG settings

AC, with the balance set for more electrode-negative time when you want penetration and more electrode-positive when the surface needs cleaning — around 70/30 is a normal starting point. AC frequency narrows the arc cone as it rises: 100–120 Hz for a tight arc on thin work.

Tungsten: pure (green) or zirconiated (white) if the machine gives a sine wave, since both hold the balled end that suits it; lanthanated (blue) ground to a point on a modern square-wave inverter. Diameter with a margin over the DC figure, because AC runs the electrode hotter — the tungsten calculator allows for this.

Gas: pure argon, and more of it than on steel — 8–14 l/min — because argon has to displace air from a pool that oxidises instantly.

MIG settings

Pure argon, spray or pulsed transfer, and never short arc: short arc on aluminium gives cold lapping that looks like a weld and is not.

The feeding problem dominates everything. Aluminium wire is soft enough to buckle under the pressure needed to push it down a normal torch, so: U-groove drive rolls, PTFE liner, minimum roll pressure, the shortest torch you can work with, laid out straight. For anything beyond occasional work a spool gun or a push-pull torch is not a luxury.

Porosity

Hydrogen dissolves readily in molten aluminium and almost not at all in solid aluminium, so everything dissolved during welding comes out as the weld freezes. The sources are moisture on the surface, oil, and hydrated oxide on material stored outdoors.

The defences are the boring ones: degrease with solvent, brush immediately before welding, keep the filler rod clean and dry, and do not weld cold material that has condensation on it. See also porosity in welds.

Updated: 14 August 2026