Gas Tungsten Arc Welding (GTAW / TIG)

Gas Tungsten Arc Welding (GTAW), also known as Tungsten Inert Gas (TIG) welding, is a precision arc welding process that uses a non‑consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas, usually argon or helium. GTAW is valued for its ability to produce high‑quality, clean welds on a wide range of metals, including stainless steel, aluminum, magnesium, and exotic alloys.

Overview

GTAW differs from processes like SMAW or FCAW because the electrode does not melt. Instead, the arc between the tungsten electrode and the workpiece generates heat to melt the base metal. Filler metal may be added separately, or the joint may be fused autogenously. The inert shielding gas prevents oxidation and contamination, resulting in welds with excellent mechanical properties and appearance. GTAW is slower and requires more skill, but it is the process of choice for critical applications where weld quality is paramount.

Apparatus and Working

Apparatus

  • Power source: Constant current AC/DC welding machine with high‑frequency start capability.
  • TIG torch: Holds the tungsten electrode and directs shielding gas to the weld zone.
  • Tungsten electrodes: Pure tungsten, thoriated, ceriated, or lanthanated depending on application.
  • Shielding gas: Argon (most common), helium, or mixtures for arc stability and penetration.
  • Filler rods: Separate consumable rods matched to base metal composition.
  • PPE: Auto‑darkening helmet, gloves, jacket, and proper ventilation for ozone/fume control.
  • Accessories: Gas regulator, flowmeter, foot pedal or fingertip amperage control.

Working Steps

  1. Joint preparation: Clean base metal thoroughly; remove oxides, oil, and contaminants.
  2. Electrode setup: Grind tungsten to a point (for DC) or ball end (for AC aluminum welding).
  3. Parameter selection: Choose polarity (DCEN for steels, AC for aluminum), current, and gas flow rate.
  4. Arc initiation: Use high‑frequency start or lift‑arc to strike arc without contaminating tungsten.
  5. Welding: Maintain short arc length; add filler rod manually into leading edge of weld pool if required.
  6. Shielding gas coverage: Hold torch in place briefly after arc termination to protect cooling weld.
  7. Inspection: Clean weld bead; check for porosity, cracks, or lack of fusion.

Principle

GTAW is based on the principle of generating heat through an electric arc between a non‑consumable tungsten electrode and the workpiece. The tungsten electrode has a very high melting point (~3,422 °C), so it does not melt under normal welding conditions. Shielding gas flows around the arc and weld pool, displacing air and preventing contamination. Filler metal, when used, is added separately into the molten pool. The result is a clean, precise weld with minimal spatter and excellent metallurgical quality.

Advantages and Disadvantages

Advantages

  • Produces high‑quality, precise, and clean welds with excellent appearance.
  • Suitable for thin materials and critical joints.
  • Can weld a wide range of metals, including aluminum, stainless steel, and exotic alloys.
  • No slag formation; minimal post‑weld cleaning required.
  • Excellent control over heat input and weld pool.

Disadvantages

  • Slower process compared to GMAW or FCAW; lower deposition rates.
  • Requires high operator skill and coordination.
  • Equipment cost is higher due to specialized power sources and torches.
  • Not ideal for outdoor welding (shielding gas can be disrupted by wind).
  • Limited productivity for thick sections without multiple passes.

Applications

  • Aerospace: Welding thin‑wall tubing, aluminum alloys, and critical components.
  • Nuclear and power plants: High‑integrity welds on stainless steels and alloys.
  • Automotive and motorsport: Fabrication of exhausts, roll cages, and lightweight structures.
  • Food and pharmaceutical: Sanitary stainless steel piping and vessels.
  • General fabrication: Precision welding of thin sheets and non‑ferrous metals.

Process Variants

  • DCEN (Direct Current Electrode Negative): Most common; deep penetration for steels and stainless steels.
  • DCEP (Direct Current Electrode Positive): Rare; shallow penetration, used for special cleaning action.
  • AC GTAW: Alternating current for aluminum and magnesium; provides cleaning action on oxide layer.
  • Pulsed GTAW: Alternates between high and low current for better control on thin materials.
  • Orbital GTAW: Automated TIG welding for pipes and tubes with consistent quality.

Welding Defects in GTAW

  • Tungsten inclusion: Contamination from electrode touching weld pool.
  • Porosity: Caused by inadequate shielding gas or contamination.
  • Lack of fusion: Insufficient heat input or poor technique.
  • Cracking: Hot or cold cracks due to improper filler selection or restraint.
  • Oxidation/discoloration: Inadequate shielding gas coverage or post‑flow.

Procedure and Quality Control

  • WPS/PQR compliance: Follow qualified procedures for base metal, filler, and parameters.
  • Tungsten preparation: Grind electrodes properly; avoid contamination.
  • Shielding gas control: Maintain correct flow rate and post‑flow duration.
  • Filler selection: Match filler rod chemistry to base metal for strength and corrosion resistance.
  • Inspection: VT for bead quality; PT/MT for surface cracks; RT/UT for critical joints.