3.4 TIG/GTAW Process

3.4 TIG/GTAW Process

Quick Answer: TIG/GTAW (ISO 4063 141) uses a non-consumable tungsten electrode under inert gas (argon and/or helium). Filler wire is added separately when needed. Use DCEN for steels; use AC for aluminium to gain oxide cleaning. Inspectors focus on gas coverage, polarity, tungsten condition, and avoiding tungsten inclusions.

Process Overview

Gas Tungsten Arc Welding (GTAW), widely called TIG (Tungsten Inert Gas), produces an arc between a tungsten electrode and the workpiece while inert gas shields the electrode, arc, and pool. The tungsten is not meant to melt into the weld. Filler metal, if required, is fed by hand or cold/hot wire feeder as a separate consumable.

Why fabricators choose TIG:

• Outstanding arc and heat control for thin materials and precise roots
• No slag and very low spatter when properly executed
• Excellent for stainless, nickel alloys, titanium, aluminium, and other quality-critical joints
• Clean cosmetic beads for process piping and food/pharma service (with correct purge practices)

Trade-offs: lower deposition rate than GMAW/FCAW/MMA for heavy fill passes; higher skill demand; sensitivity to draughts and contaminated gas or base metal.

Power source: typically constant current (CC), often with high-frequency (HF) or lift-arc starting and, for aluminium, AC output with balance control on modern inverters.

Inert Gas Shielding

Primary gases:

• Argon — most common; good arc initiation and stability for manual TIG on a wide range of metals.
• Helium — higher arc voltage and heat for a given current; used pure or mixed with argon for thicker sections or higher travel speeds.
• Argon–hydrogen mixes appear in some stainless/nickel procedures (specialised; only when WPS allows).

Gas also protects the hot tungsten. Inadequate flow, leaks, or wind cause electrode oxidation (discoloured, balling/eroded tip) and weld porosity or oxidation. For pipe and tube, backing gas / purge on the root side prevents sugaring (oxidised stainless roots) and is a frequent hold-point item for inspectors.

Current Type and Polarity

DCEN (electrode negative) — steels and many alloys

• About two-thirds of arc heat enters the work, giving deeper, narrower penetration and cooler tungsten.
• Standard choice for carbon steel, low-alloy steel, stainless, nickel, titanium, copper (with appropriate gas and filler).
• Tungsten is ground to a point (with grind marks along the axis) for a focused arc.

DCEP (electrode positive) — limited use

• Places more heat in the electrode, risking tungsten overheating at useful currents.
• Provides strong cathodic cleaning of oxides but is rarely used alone for production TIG on thick work.

AC — aluminium and magnesium

• Alternating half-cycles provide a compromise: EN half-cycle penetrates; EP half-cycle provides oxide cleaning essential for aluminium’s refractory Al₂O₃ film.
• Modern AC machines allow balance (more EN vs more EP time) and often amplitude control. More EP cleans better but consumes tungsten faster; more EN penetrates more with less cleaning.
• Continuous HF or advanced square-wave designs stabilise the arc through zero crossings.

Classic exam point: aluminium TIG → AC (cleaning + weld); steel TIG → DCEN.

Tungsten Electrode Types (Overview Level)

Tungsten electrodes are classified by oxide additions that improve electron emission and arc stability. At IWI-S overview depth, know the practical themes:

Diameter must suit current: undersized electrodes overheat and drop tungsten into the pool; oversized electrodes can give an unstable, wandering arc at low current. Colour codes help identification but always confirm against packaging and WPS—codes are not universal across every market legacy stock.

Filler Wire Addition and Autogenous Welds

• Autogenous TIG — fusion without added filler; common on thin sheet and some edge joints when chemistry and fit-up allow.
• With filler — matching or over-alloyed wires (procedure-specified) prevent underfill and control weld metal composition. Steel examples often follow ER-type designations; stainless and aluminium have their own families (e.g. ER308L, ER4043, ER5356—use only as WPS examples).
• Filler must be kept clean; touching the hot tip on contaminated plate or dipping tungsten into the pool destroys quality.

Root Passes and High-Quality Joints

TIG roots are common in pressure piping, hygienic tubing, and critical butt welds because the welder can see the pool and control penetration. Inspector focus:

• Correct purge quality (oxygen content limits when specified)
• Full root fusion without suck-back or excessive penetration
• Smooth root bead free of oxidation (especially stainless)
• Transition to fill processes (MMA, GMAW, SAW) only when the WPS allows process combinations and interpass conditions are met

Heat input may still be limited by metallurgy—even a “beautiful” TIG bead can violate toughness or corrosion requirements if travel speed and amperage drift.

Tungsten Inclusions — Imperfection Risk

If the tungsten electrode touches the pool, or if the tip breaks or overheats, particles of tungsten can freeze in the weld metal. Under ISO 6520 these appear as tungsten inclusions (a metallic inclusion type). They are dense and often visible on radiographs as bright (high-absorption) spots.

Prevention and inspection angles:

• Use HF or lift-arc starts instead of crude scratch starts where quality demands it
• Maintain correct stick-out of tungsten from the ceramic nozzle (too long loses gas protection)
• Regrind contaminated tips; do not continue with a dirty, alloy-fouled electrode
• Reject and repair per acceptance standard when inclusions exceed limits—do not “weld over and hope”

Inspector Focus Points for TIG/GTAW

1. Process 141 and CC power mode confirmed.
2. Current type/polarity — DCEN for steels; AC for aluminium as specified.
3. Gas — type, purity, flow; backing gas when required; draught protection.
4. Tungsten — correct type/diameter, ground geometry, clean tip, safe handling for thoriated stock if still used.
5. Filler — correct designation and cleanliness; no unauthorised substitutions.
6. Parameters — current (and pulse settings if any), arc voltage/length practice, travel speed within heat-input limits.
7. Technique risks — electrode dipping, inadequate purge, oxidation colours on stainless outside procedure limits.
8. NDT implications — tungsten inclusions on RT; surface oxidation or undercut on VT.

Comparison Snapshot vs Other Arc Processes

Closing Note for IWI-S Candidates

TIG questions on WTE papers often test polarity by material, why argon, and tungsten contamination. Inspection papers connect those same ideas to purge hold points, inclusion acceptance, and WPS fidelity. Master the control logic—non-consumable electrode, inert gas, separate filler—and the defect pattern that follows when control is lost.