1.4 GTAW — Gas Tungsten Arc Welding (TIG)

Gas Tungsten Arc Welding (GTAW)

Gas Tungsten Arc Welding (GTAW), or TIG, produces the highest-quality, most precise welds of any manual arc process. It uses a non-consumable tungsten electrode under inert-gas shielding; filler metal, if used at all, is fed separately by hand or wire feeder. Because nothing in the arc but the gas and the puddle is reactive, GTAW leaves no slag and almost no spatter, making it the choice for root passes, thin sheet, and exotic alloys in aerospace, nuclear, and pipe work.

Tungsten is chosen because it has the highest melting point of any metal, ~6,170°F (3,410°C), so it sustains the arc without melting into the weld. Welds made without filler are called autogenous (common on thin stainless and tube). Filler rods carry the same A5.18 (ER70S-x) family for steel or matching specs for stainless/aluminum (for example, ER308L for 304 stainless or ER4043/ER5356 for aluminum). On root passes in pipe and stainless, the inside of the joint is often back-purged with argon to keep the unfused root side from oxidizing — a step the CWI verifies on corrosion-critical work.

Current Type and Polarity

The most-tested GTAW concept is matching current type to metal:

Exam trap: AC for aluminum. The DCEP half-cycle provides cathodic (oxide) cleaning that breaks up the tenacious aluminum-oxide skin (Al₂O₃ melts ~3,700°F, far above aluminum's ~1,220°F). DCEN for steel for maximum penetration.

A closely related point is arc starting. Touch-starting (scratch start) risks tungsten pickup, so quality GTAW uses high-frequency (HF) start to ionize the gap without contact, or a lift-arc start. On AC aluminum, continuous high frequency stabilizes the arc as it reverses. The CWI should also recognize balance control on modern AC machines, which adjusts the ratio of cleaning (EP) to penetrating (EN) time in each cycle — more EP cleans a dirtier oxide but erodes the tungsten faster.

Tungsten Electrodes and Shielding Gas

Tungsten electrodes are identified by an AWS A5.12 class and a color band; pure tungsten balls up for AC aluminum, while oxide-added types start easier and carry more current on DC.

Only inert gases shield GTAW — argon (most common, stable arc, good on thin material), helium (hotter, deeper, less stable), or Ar/He mixes for thick aluminum and copper. Active gases like CO₂ are forbidden: they would oxidize and rapidly destroy the tungsten and contaminate the weld. , Ar + 2–5% H₂) is sometimes used on austenitic stainless to increase heat and clean the bead, but never on carbon or hardenable steel because of hydrogen-cracking risk.

Tungsten preparation also affects the weld: a pointed/tapered tip is ground for DC steel to concentrate the arc, while a balled end forms naturally on pure tungsten for AC aluminum. Electrode diameter is matched to amperage — an oversized tungsten at low current wanders, and an undersized one at high current overheats and spits tungsten into the pool.

Applications and Discontinuities

GTAW is unmatched where quality outranks speed:

• Root passes on pipe and pressure vessels (then filled/capped with SMAW or FCAW)
• Thin sheet and tube where burn-through and distortion must be controlled
• Exotic and reactive alloys — titanium, zirconium, Inconel, Monel
• Stainless where a clean, corrosion-resistant root is critical (often back-purged with argon)

The signature GTAW defect the CWI looks for is tungsten inclusion — bits of tungsten in the weld when the electrode dips into the puddle or the gas/arc-start is mishandled. Tungsten inclusions show as bright white spots on radiographs (tungsten is denser than steel, so it blocks more radiation). Tint/oxidation ("sugaring") on an unpurged stainless root and porosity from lost gas coverage are other inspection flags. Worked heat input: 120 A, 12 V, 5 ipm → (12 × 120 × 60)/5 = 17,280 J/in (17.3 kJ/in) — the low currents reflect GTAW's modest 1–3 lbs/hr deposition.