TIG welding: arc, tungsten and filler
Process 141, GTAW in the American numbering. A non-consumable tungsten electrode, an inert shield and a filler rod added by hand — the slowest process in the shop and the one that produces the best-looking welds.
Why it is different
In every other arc process the electrode is also the filler, so heat and deposition are locked together: more filler means more heat, whether you want it or not. TIG separates them. The arc supplies heat, the rod supplies metal, and you decide how much of each. That is the whole reason the process exists, and it is why it owns thin sheet, root runs and anything where the weld will be seen.
Polarity and current type
DC− for steel, stainless, titanium and copper: the electrode stays relatively cool and the heat goes into the work, so penetration is deep and the tungsten lasts.
AC for aluminium and magnesium, for the cleaning action described above. Modern inverters let you set the balance — more electrode-negative time for penetration, more electrode-positive for cleaning — and the frequency, which narrows or widens the arc cone.
DC+ is essentially never used: it puts the heat into the electrode and melts it.
Tungsten and grinding
Lanthanated (blue) covers almost everything on both AC and DC. Pure tungsten (green) is the traditional AC choice and forms the balled end that suits it. Thoriated red electrodes are best avoided: the grinding dust is mildly radioactive and lanthanated types match their performance. Diameter and grind angle come from the current — the tungsten calculator gives both, along with the cup size and gas flow.
Grind along the axis. The grinding marks guide the arc, so marks running across the tip make the arc wander, and no amount of technique fixes it.
Gas and purging
Argon, 6 to 12 l/min, with a post-flow long enough to protect the tungsten and the cooling pool — a rough rule is one second per 10 A. Argon with 2 to 5 % hydrogen is used on austenitic stainless to raise the heat and brighten the surface, and on nothing else.
On stainless and titanium pipe the underside of the root needs its own shield, because a root run that oxidises turns grey and crumbly and is scrap. Titanium is unforgiving here: any straw or purple colour means the shielding was insufficient.
The technique
Arc length equal to about the electrode diameter — long arcs waste heat and widen the bead. The torch leans about 15° in the direction of travel; the rod comes in at a shallow angle from the front, dipped into the leading edge of the pool and withdrawn, without ever touching the tungsten. Touching the tungsten means stopping and regrinding: the contamination will not burn off, and it puts tungsten inclusions in the weld.
On thin stainless the failure mode is heat, not technique. Tack often, use copper backing where you can, and drop the current as the plate warms up. The practical settings are in how to weld thin sheet.
What it costs
Time. TIG deposits a fraction of what MIG/MAG does, and on a priced job that dominates everything else — feed both into the cost per metre and the gap is obvious. Which is the argument for using TIG where it earns its keep: the root run, the visible weld, the thin section and the material that tolerates nothing else.
Frequently asked questions
Why AC for aluminium?
Because of the oxide film. Aluminium oxide melts at about 2050 °C while the metal under it melts at 660 °C, so the film has to be broken up before anything can be welded. The electrode-positive half of the AC cycle does that — the cathodic cleaning action — while the electrode-negative half puts the heat into the work. The balance control decides how the cycle is split.
What is the weld turning grey and dull?
Oxidation from too little gas or too much stick-out. On stainless a straw or light-blue tint is normal; grey and powdery means the shield failed. Check the flow rate, the post-flow time and how far the tungsten protrudes from the cup — and on pipe, whether the root needs purging.
Do I need a foot pedal?
Not to learn, but it changes what you can do. Being able to drop the current as the plate heats up is what keeps the last 50 mm of a thin-sheet seam from blowing out, and it is how a crater is filled without leaving a crack in it.
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