What gas for TIG welding?

The short answer

Why TIG is simpler than MIG here

In MIG, the shielding gas is partly a chemical participant. CO2 in a 75/25 mix breaks down in the arc and that reaction is what broadens penetration and drives fusion out to the toes of the bead. Change the gas and you change the weld, which is why 75/25 and 100% argon are not interchangeable in either direction.

In TIG, the gas has a different job. The tungsten electrode is not consumed, and it is extremely sensitive to anything reactive: oxygen and CO2 both oxidize it, which erodes the tip, destabilizes the arc and drops tungsten into the weld.

So TIG demands a fully inert gas, and argon is the cheap inert gas. That single requirement is why TIG gas selection is short.

The exceptions, in the order you will meet them

Argon/helium for thick aluminum. Helium produces a hotter arc at higher voltage and gives a wide, deep bead, which suits aluminum and copper alloys where heat conducts away from the joint quickly. Adding helium effectively buys penetration and travel speed you could otherwise only get from more amperage.

Two practical costs come with it. Helium is lighter than air, so it needs noticeably higher flow rates to maintain the same shielding — which is also why it empties a cylinder faster. And it is more expensive than argon to begin with.

For a home shop welding aluminum up to around a quarter inch, straight argon with AC balance set correctly is enough, and the helium question only becomes real on thick plate or on a machine that is out of amperage. The aluminum TIG roundup covers the machine side.

Argon/hydrogen for austenitic stainless. A 95% argon, 5% hydrogen mix cleans surface oxides, stabilizes the arc and produces a deeper, broader penetration profile on austenitic grades — the 300 series, so 304 and 316. It produces a visibly brighter, cleaner weld face.

It is also the gas with the sharpest restriction on this page, which is the next section.

Gases that will ruin a TIG weld

CO2, pure or in a mix. This is the most common mistake, and it comes from owning a 75/25 MIG bottle and wanting to avoid a second cylinder. The CO2 oxidizes the tungsten, the tip erodes within seconds, the arc wanders, and tungsten inclusions end up in the weld. The bottle sitting there is not the right bottle.

Oxygen, in any proportion. Oxygen is deliberately added to some MIG mixes. In TIG it does the same thing to the electrode that CO2 does, faster.

Nitrogen, as a general shield. Nitrogen has legitimate specialist uses as a backing gas on duplex stainless and as a plasma cutting gas, but it is not a TIG shielding gas for a home shop and it embrittles several common metals.

Welding-grade argon that has sat with the valve open. Less obvious and worth stating: a cylinder left open with a leaking regulator can be contaminated by back diffusion. Close the valve at the bottle, not just the regulator.

Flow rate, cup size and the two failure modes

Flow rate is where more TIG problems originate than gas selection, because there are two ways to be wrong and most people only know about one.

Too little flow leaves the puddle partly unshielded. On steel you get a dull gray or black weld face; on stainless a gray, rough, sugared surface; on aluminum a dirty, gray, hard-to-wet puddle.

Too much flow is the one that surprises people. Above a certain velocity the gas stream goes turbulent coming out of the cup and begins drawing air into itself, so the weld looks exactly as though it were under-shielded. Winding the flow up because the weld looks contaminated is the classic way to make it worse.

Flow is set in cubic feet per hour and the right figure scales with cup size, not with amperage. A larger cup needs more flow to fill it; a small cup on thin stainless wants very little. Start from the figure in your torch or machine manual, and judge by the color of the weld rather than by the number on the gauge.

A gas lens is the genuine upgrade here. It replaces the collet body with a layered mesh screen that straightens the flow, which gives better coverage at lower flow and allows more tungsten stickout for getting into corners. The TIG settings chart has the starting ranges.

Back purging, and when it stops being optional

The shielding gas protects the top of the weld. On full-penetration joints, the back of the weld is also molten and also exposed to air.

On mild steel nobody worries about this. On stainless it matters: an unpurged stainless root oxidizes into a gray, crystalline, sugary surface that is both weak and a corrosion site, and no amount of top-side shielding prevents it. On titanium it is absolute.

Purging means filling the back side with argon — inside a pipe, or in a simple purge chamber taped around the joint — before and during the weld. For exhaust tube and stainless pipework this is the difference between a joint that lasts and one that cracks at the root. The exhaust roundup covers where that bites in practice.

One cylinder or two?

If you only TIG: one cylinder of 100% argon covers everything.

If you MIG steel and TIG: two cylinders — a 75/25 argon/CO2 for steel MIG and a 100% argon for TIG — but only one regulator, because both use a CGA-580 valve outlet in the US. Swapping gases means changing the bottle, not buying a second regulator.

If you MIG aluminum as well, the 100% argon bottle serves both the TIG torch and the spool gun, so there is still no third cylinder. Cylinder sizes covers how big each should be — and for a second, less-used gas the answer is usually smaller than you expect.

The question that comes before this one

This page assumes you have accepted that a cylinder is part of the purchase. If you have not — or if you have read that "TIG welding flux" is a way around it — does TIG welding need gas answers that directly. The short version is that TIG is the one process on this site with no flux anywhere in it, so there is no self-shielded version and no gasless shortcut.