TIG (GTAW) Welding Metallurgy: Arc Physics, Shielding Gases, Filler Metals, and Weld Pool Control

Gas Tungsten Arc Welding (GTAW) — universally known as TIG (Tungsten Inert Gas) welding — uses a non-consumable tungsten electrode to generate a sustained plasma arc while a separate filler rod is fed manually or automatically into the molten pool. Inert shielding gas flowing through the torch nozzle protects the weld zone from atmospheric contamination. Of all fusion arc processes, TIG produces the highest metallurgical quality welds: no flux, no spatter, full operator control over heat input, and applicability across virtually every weldable engineering alloy from carbon steel to titanium and nickel superalloys.

Arc Physics and Heat Generation Mechanisms

The TIG arc is a sustained plasma discharge maintained between the non-consumable tungsten electrode and the workpiece. In DCEN operation, electrons are emitted from the hot tungsten cathode by thermionic emission and accelerated through the plasma column toward the anode (workpiece). Plasma column temperatures reach 10,000–20,000 K — far above the boiling point of any engineering metal.

Heat Transfer Modes

Three mechanisms transfer energy from the arc to the workpiece:

• Electron condensation (dominant): Electrons impinging on the anode surface release their kinetic energy plus the work function of the anode material. This accounts for approximately 60–75% of workpiece heating in DCEN.
• Plasma radiation: The hot plasma column radiates across the UV, visible, and IR spectrum. Contribution increases with arc current and gap length.
• Conduction through shielding gas: Hot gas in contact with the workpiece surface conducts heat. This component is strongly influenced by shielding gas choice — helium’s superior thermal conductivity makes it far more effective than argon in this mode.

Heat Input Calculation

Heat Input (HI) = (Current × Voltage × 60) / (Travel Speed × 1000)   kJ/mm

Where:
  Current      = Welding current [A]
  Voltage      = Arc voltage [V]
  Travel Speed = Travel speed [mm/min]

Example: 120 A, 12 V, 100 mm/min
  HI = (120 × 12 × 60) / (100 × 1000)
     = 86,400 / 100,000
     = 0.864 kJ/mm

Thermal efficiency factor (η) for GTAW ≈ 0.60–0.80
Corrected HI = η × HI = 0.70 × 0.864 ≈ 0.605 kJ/mm net

TIG welding spans the lowest heat input range of any arc process: typically 0.1–1.5 kJ/mm. This narrow, precisely controllable heat input window is fundamental to TIG’s suitability for thin-gauge material, root pass welding in thin-walled pipe, and heat-sensitive alloys where overheating causes microstructural degradation. For a deeper treatment of HAZ thermal cycles, see the HAZ microstructure guide.

Polarity Selection

Tungsten Electrode Types and Current Capacity

The electrode composition determines arc starting characteristics, maximum current capacity before tip degradation, and suitability for DCEN versus AC service. Electrode diameter governs maximum usable current: exceeding the limit causes tip melting and tungsten contamination of the weld pool.

Electrode Tip Geometry

For DCEN welding, the tungsten tip is ground to a truncated cone: the included grinding angle controls arc cone angle and spot size on the workpiece. A sharper point (15°–30° included angle) produces a narrower, more focused arc — preferred for precise, low-current applications on thin material. A blunter angle (60°–90°) increases the stable current range. For AC welding with pure tungsten, the tip is not ground: the arc self-conditions the electrode tip into a hemispherical ball during welding, which is the stable AC geometry.

Shielding Gas Selection and Behaviour

Shielding gas performs three functions simultaneously: it excludes atmospheric oxygen, nitrogen, and hydrogen from the weld pool; it influences arc voltage and heat distribution; and it determines weld pool fluidity and bead profile geometry. Gas selection cannot be made independently of base metal, joint geometry, and productivity requirements.

Back Purge Requirements

For all stainless steel and titanium pipe and tube welds, the inside surface of the joint must be protected by a flowing purge gas during welding and until the metal cools below the critical oxidation threshold. Failure to back purge stainless steel root beads causes chromium oxide formation and potential sensitisation; failure to purge titanium causes embrittlement from oxygen and nitrogen interstitial pickup.

Purge flow rate for standard pipe: 10–20 L/min at the start, reducing once the internal volume is purged. The pipe ends are sealed with purge dams or tape, leaving a small vent hole downstream. Purge gas is maintained until weld temperature drops below 200 °C for SS and 150 °C for titanium.

Filler Metal Selection: Composition Matching and Overalloying

Filler metal selection in TIG welding follows two governing principles. Composition matching uses a filler that closely replicates the base metal chemistry to produce weld metal with equivalent mechanical properties, corrosion resistance, and post-weld heat treatment response. Overalloying intentionally adds excess quantities of specific elements to compensate for dilution by the base metal, evaporative losses of volatile elements during arc transfer, or microstructure control requirements in the solidified weld.

Overalloying in Practice: 2205 Duplex Stainless Steel

The 2205 duplex base metal contains approximately 22% Cr, 5.5% Ni, 3% Mo with a target ferrite/austenite ratio of 40-60/60-40. During welding, rapid cooling from the austenite-plus-ferrite field through the single-phase ferrite field and back produces weld metal with excess ferrite (sometimes >70%). Elevated ferrite content reduces toughness and corrosion resistance. ER2209 filler is overalloyed in nickel (9% vs. 5.5% in base metal) and nitrogen to promote austenite nucleation during cooling, restoring the target dual-phase microstructure in the as-deposited weld.

Weld Pool Dynamics and Solidification

The TIG weld pool is a free surface liquid metal system in which heat input, surface tension gradients, electromagnetic (Lorentz) forces, arc pressure, and buoyancy simultaneously drive fluid flow. The resulting flow pattern determines penetration depth, bead geometry, and the distribution of dissolved gases and alloying elements in the solidified weld metal.

Marangoni Flow and Sulphur Effects

Surface tension-driven flow (Marangoni convection) is the dominant flow mechanism in TIG pools. Its direction depends on the sign of the surface tension-temperature gradient (dγ/dT):

• Low-sulphur steel (<30 ppm S): dγ/dT is negative. Surface tension is highest at the cooler pool periphery, driving outward radial flow — producing a wide, shallow pool.
• High-sulphur steel (>100 ppm S): Sulphur adsorption reverses the dγ/dT sign to positive at high temperatures. Surface tension-driven flow reverses inward — concentrating heat at the pool centre and producing deep, narrow penetration.

This explains the well-documented variability in TIG penetration for nominally identical procedures when welding steel heats of different sulphur content — a critical concern in welding procedure qualification where penetration must be guaranteed. See the HAZ microstructure article for the downstream effect on CGHAZ grain growth.

Solidification Mode and Hot Cracking

TIG weld metal in austenitic stainless steels solidifies in one of four modes depending on Cr/Ni equivalent ratio (WRC-1992 diagram): A (fully austenitic), AF (austenite primary), FA (ferrite primary), or F (fully ferritic). The FA mode — ferrite primary solidification — is preferred because:

• Ferrite tolerates greater concentrations of hot crack-promoting impurities (S, P) than austenite.
• Residual ferrite (typically 4–12 FN) transforms partially to austenite on cooling, but remains as fine stringers that arrest hot crack propagation.

Fully austenitic solidification (mode A) is required only when ferrite is prohibited by service conditions (cryogenic, very high corrosion environments) — and hot cracking risk must then be controlled by very low impurity filler wire and tight heat input limits. The WRC-1992 diagram and FN calculation are covered in the delta ferrite and WRC-1992 article.

Common TIG Weld Defects, Causes, and Prevention

Tungsten Inclusions

The highest-severity defect specific to GTAW. Dense tungsten fragments (19.3 g/cm³) embedded in the fusion zone are detected as highly radio-opaque spots on radiographs. Any TIG weld failing radiographic examination for tungsten inclusions must be excavated and repaired; post-weld heat treatment does not dissolve or neutralise tungsten inclusions.

Prevention: Use correct electrode diameter for the current level; never exceed the rated amperage; use HF or lift-arc starting — never scratch-start; immediately replace contaminated electrodes. For critical pressure boundary welds, a contamination event during welding is a mandatory record entry.

Porosity

Gas bubbles entrapped in solidifying weld metal, visible as rounded or elongated voids on radiograph. Primary TIG porosity causes are shielding gas contamination (moisture, oil), inadequate flow rate, turbulent gas delivery (excessive flow rate through undersized nozzle), draughts, or contaminated base metal or filler.

Prevention: Use gas purity ≥99.998% Ar; degrease base metal and filler rod with acetone immediately before welding; verify gas flow at torch nozzle (10–15 L/min for 10 mm ceramic nozzle); use a gas lens (collet body with wire mesh) to laminarise shielding flow and extend effective coverage.

Crater Cracks

Hot cracks nucleating at the arc termination point where the pool freezes under tension with a shrinkage cavity. The concave crater is a stress concentration that opens as the weld cools. Prevention requires a current decay (crater fill) function on the machine — tapered reduction of current over 1–3 seconds while maintaining filler addition until the pool is filled flat. Alternatively, the welder can circle back over the crater before breaking the arc.

Lack of Fusion

Incomplete bonding between weld metal and base metal sidewalls or between passes. In TIG welding, the primary cause is insufficient heat input — the arc melts filler but fails to bring the base metal groove face to its fusion temperature before the pool advances. Correct by increasing current, reducing travel speed, or improving joint preparation to ensure adequate access for the arc to the groove root.

GTAW in Context: Arc Process Comparison

Industrial Applications and Procedure Qualification

Root Pass Welding in Pressure Piping

GTAW is universally specified for root pass welding of pressure piping in power generation, petrochemical, and pharmaceutical industries. The root pass establishes internal weld bead geometry, full penetration, and the critical corrosion-exposed surface. GTAW’s precise heat control and absence of flux or spatter make it the only process reliably delivering consistent root pass quality in restricted-access pipe bore positions (5G, 6G). Root passes are subsequently filled by SMAW, GMAW, or FCAW fill and cap passes depending on project specification. Procedure qualification follows ASME Section IX; thickness and diameter qualification ranges comply with Table QW-451. See the comprehensive HAZ microstructure guide for how root pass heat input selection affects downstream HAZ characteristics.

Aerospace and Titanium Fabrication

TIG welding of titanium alloys (Ti–6Al–4V, Ti–3Al–2.5V, commercially pure grades) demands stringent atmospheric exclusion — not only front shielding through the nozzle but trailing shields covering the solidifying weld metal and HAZ as the torch advances, and back purge as described above. Titanium’s high reactivity requires gas purity of ≥99.999% and clean handling protocols. Qualification testing for aerospace titanium welds typically includes tensile testing, bend testing, macrographic examination, and hardness surveys per AMS or customer-specific requirements.

Orbital TIG Welding: Pharmaceutical and Semiconductor Standards

Orbital GTAW under ASME BPE (Bioprocessing Equipment) standard is mandatory for pharmaceutical process tubing. Closed weld head systems rotate around fixed 316L SS tube, delivering precisely controlled current waveforms (often pulsed with peak/background current programming) to manage the variable heat extraction as the arc traverses the tube circumference. Qualification per ASME BPE requires visual examination of OD and ID bead, dimensional checks of bead width and ID concavity (0.0–0.25 mm), and surface finish verification. The high repeatability eliminates the human factors that dominate manual root pass quality and produces documented, traceable weld records for validation (IQ/OQ/PQ) documentation required under FDA cGMP regulations.