TIG welding aluminum: AC, settings, and what goes wrong
Why aluminum needs AC, how to set balance and frequency, starting amperages by thickness, which filler to pick — and the three faults that cause most rejects.
Aluminum punishes almost every habit carried over from steel. It melts at less than half the temperature, conducts heat several times faster, carries an oxide skin that will not melt at all, and takes up hydrogen the moment it goes liquid. Each of those has an answer, and the answers are what this page is about.
The oxide is the whole problem
Aluminum melts at about 660 °C. The aluminum oxide that forms on its surface within seconds of exposure to air melts at roughly 2050 °C.
That gap is the entire difficulty. Heat the joint and the metal underneath goes liquid while the skin above it stays solid — a tough, transparent film sitting on top of a molten pool, keeping the two sides of the joint apart and trapping whatever is underneath.
Two things deal with it: mechanical cleaning before the arc, and alternating current during it.
Why AC, and what balance and frequency do
On alternating current, each cycle does a different job:
- Electrode negative (EN) puts heat into the workpiece. This is the penetrating half.
- Electrode positive (EP) reverses the flow and the arc strips oxide from the surface — the cathodic cleaning effect, visible as the bright etched band alongside the bead. It also heats the tungsten, which is why an all-positive setting would destroy the electrode.
Two controls on any modern inverter shape that:
Balance sets the ratio. More EN means deeper penetration and a cooler electrode; more EP means more cleaning. Most work sits at 65 to 75% EN. Go toward more cleaning only when the material is genuinely dirty or heavily oxidized — and note that cleaning past what the joint needs just wastes heat and rounds the tungsten.
Frequency sets how tight the arc is. Low frequency, around 60 Hz, gives a broad soft arc and a wide cleaning band. High frequency, 120 to 200 Hz and beyond, narrows and stiffens the arc — better control on thin material and in corners, at the cost of a harsher sound and a narrower cleaning zone.
If a machine only offers one of the two, balance is the one that matters more.
Tungsten and cup
On the old transformer machines, pure tungsten (green) was standard: it forms the balled tip that AC used to need. On a modern inverter, 2% lanthanated (blue) or ceriated tungsten works better — it holds a truncated point, starts more reliably, and carries more current for its diameter.
Grind the point along the axis of the electrode, never across it. Cross-ground grinding marks steer the arc sideways.
Use a gas lens if there is one available. Aluminum welding benefits from the wider, calmer shielding envelope more than most work, and it lets you stick out further into a corner without losing the shield.
Starting settings by thickness
Everything below is a starting point for a butt joint in the flat position, on pure argon. A qualified procedure overrides all of it.
| Thickness | Current (AC) | Tungsten | Filler |
|---|---|---|---|
| 1.6 mm | 60–80 A | 1.6 mm | 1.6 mm |
| 3.2 mm | 125–160 A | 2.4 mm | 2.4 mm |
| 4.8 mm | 180–225 A | 2.4 mm | 3.2 mm |
| 6.4 mm | 225–275 A | 3.2 mm | 3.2 mm |
The underlying rule of thumb is about 40 A per millimeter of thickness, or one amp per thousandth of an inch. It holds well enough up to around 6 mm; above that, joint preparation and preheat start to matter more than the number.
Two corrections the table cannot make for you:
- Aluminum needs more current at the start than at the end. Its conductivity pulls heat out of a cold plate, and once the part is warm the same setting will burn through. A foot pedal or a downslope solves this; a fixed amperage on a long weld does not.
- Out of position, drop the current and expect the pool to want to fall out of the joint.
Filler: 4043 or 5356
Nine times out of ten it is one of these two.
4043 carries silicon. It runs more fluid, wets in more easily, and is less prone to cracking on the 6xxx alloys used for extrusions. It is the forgiving rod.
5356 carries magnesium. It is stronger, stiffer — which matters if it is being machine-fed — and it anodizes to a color close to the base metal, where 4043 turns dark. Use it with 5xxx base alloys, and where the joint is structural.
Where the drawing does not say, ask what matters: appearance after anodizing and strength point to 5356, ease of running and crack resistance on extrusions point to 4043.
The three faults that cause most rejects
Porosity. Aluminum dissolves hydrogen when molten and rejects it on solidifying. The hydrogen comes from moisture, oil, cutting fluid, or a filler rod that has been handled with bare hands. Degrease first with acetone, then remove the oxide mechanically — and clean in that order, because brushing an oily surface just pushes the oil into the metal.
Oxide inclusions. A stainless brush used on steel will transfer iron into aluminum and seed both inclusions and corrosion. Keep a brush that touches nothing else. Clean shortly before welding; the oxide starts reforming immediately.
Distortion and burn-through. Aluminum’s expansion is roughly twice that of steel, and its low melting point leaves little margin. Tack more often than feels necessary, use a backing bar on thin sheet, and keep the heat input down — which usually means traveling faster rather than turning the current down.
Where hot wire changes the picture
Aluminum is one of the applications where mechanically fed filler earns its keep most clearly, for two reasons that have nothing to do with speed.
The first is pool control. Manual TIG on aluminum asks the welder to dip a soft rod into a pool that is barely holding its shape, on a material that gives almost no visual warning before it collapses. A feeder delivers the same amount of filler every second, and the operator manages torch angle and travel instead.
The second is heat. Distortion and burn-through on aluminum are heat problems, and heat is a function of how long the arc stays over any one spot. Feeding the filler hot and continuously lets the weld move faster, so less energy goes into the plate for the same joint.
That is the combination TIP TIG is built around — a hot, mechanically oscillated wire on a conventional TIG arc, running on the same 100% argon. On aluminum it addresses the porosity, the pool control and the distortion at once, which is why it is one of the process’s strongest applications. The metals and alloys guide covers where else that applies.
In short
Aluminum needs AC because its oxide will not melt; balance controls how much cleaning you get, frequency controls how tight the arc is. Start at roughly 40 A per millimeter and expect to back off as the part heats. Pick 4043 for ease, 5356 for strength and color. And accept that most aluminum defects are decided before the arc is struck — by how clean the joint was, and how dry.