Introduction to TIG/GTAW Welding
Gas Tungsten Arc Welding (GTAW), universally known as TIG (Tungsten Inert Gas) welding, uses a non-consumable tungsten electrode to generate the welding arc while a separate filler rod is fed manually or mechanically into the molten weld pool. The weld zone is shielded from atmospheric contamination by an inert gas — argon, helium, or their mixtures — flowing through the torch nozzle. TIG welding produces the highest quality, most metallurgically clean welds of any arc process, making it the standard for root passes in critical pipework, aerospace fabrication, and exotic alloy joining.
Arc Physics and Heat Generation
The TIG arc is a sustained plasma discharge between the tungsten electrode and the workpiece, sustained by thermionic emission of electrons from the hot tungsten tip (for DCEN polarity). Arc temperatures reach 10,000–20,000 K in the plasma column. Heat transfer to the workpiece occurs by:
- Electron condensation heating: Electrons emitted from the cathode (electrode in DCEN) accelerate through the arc and condense at the anode (workpiece), releasing kinetic energy + work function energy. This is the dominant heating mechanism in DCEN.
- Radiation from the plasma column
- Conduction through the gas
The heat distribution between electrode and workpiece depends on polarity:
| Polarity | Electron Flow | Heat Distribution | Use Cases | |
|---|---|---|---|---|
| DCEN (direct current electrode negative) | From electrode (−) to workpiece (+) | ~70% to workpiece, 30% to electrode | Steel, stainless, Ti, Ni alloys — standard polarity | |
| DCEP (electrode positive) | From workpiece (−) to electrode (+) | ~30% to workpiece, 70% to electrode | Aluminium only (cathodic cleaning); large electrode required | #f9f6f0 |
| AC (alternating current) | Alternates each cycle | ~50/50 average | Aluminium and magnesium — combines cathodic cleaning (EP half) with workpiece penetration (EN half) |
Tungsten Electrode Types
Electrode type significantly affects arc stability, maximum current capacity, and contamination risk:
| Electrode Type | Colour Code | Composition | Best For | Max Current (3.2mm) | |
|---|---|---|---|---|---|
| Pure tungsten | Green | 99.5% W | AC welding of Al/Mg — forms stable ball end | 150A AC | |
| 2% Thoriated | Red | 98% W + 2% ThO₂ | DCEN; best arc starts; radioactive | 250A DCEN | #f9f6f0 |
| 2% Ceriated | Grey | 98% W + 2% CeO₂ | DCEN & AC; best alternative to thoriated | 250A DCEN | |
| 1.5% Lanthanated | Gold | 98.5% W + 1.5% La₂O₃ | General purpose DCEN & AC; no radioactivity | 250A | #f9f6f0 |
| Zirconiated | White | 99.2% W + 0.8% ZrO₂ | AC only; higher current capacity | 180A AC |
Shielding Gas Selection
Shielding gas profoundly affects arc behaviour, weld pool fluidity, bead profile, and oxide cleaning action:
| Gas | Thermal Conductivity | Arc Voltage | Penetration Profile | Best Applications | |
|---|---|---|---|---|---|
| Argon (Ar) | Low | Lower (~10–15V) | Wide, shallow | Most metals; standard for all manual TIG | |
| Helium (He) | High (~6× Ar) | Higher (~15–25V) | Deep, narrow | Copper, aluminium; automated high-speed | #f9f6f0 |
| Ar + 25% He | Intermediate | Intermediate | Improved penetration over Ar | Thick section Ni alloys, copper | |
| Ar + 2% H₂ | — | Slightly higher | Better wetting | Austenitic SS only; not for hardenable steels | #f9f6f0 |
| Ar + 5% H₂ | — | Higher | Deepest; narrow profile | Automated orbital SS pipe welding — productivity gain |
Back purge gas is essential for all stainless steel and titanium pipe welds — flowing argon through the pipe interior (typically 10–20 L/min) prevents oxidation of the weld root. Adequate purging is confirmed by oxygen measurement (target <50 ppm for austenitic SS, <20 ppm for titanium) or by weld root colour: silver = excellent; gold = acceptable; blue = marginal; black = reject (heavy oxidation, loss of corrosion resistance).
Filler Metal Selection
Filler metal selection follows two principles: composition matching (matching filler to base metal for colour and properties) and overalloying (adding extra alloying elements to compensate for dilution, evaporation losses, or microstructure control).
| Base Metal | AWS Filler | Key Composition | Reason for Choice | |
|---|---|---|---|---|
| 304L SS | ER308L | 18-8 + low C | Matches 304; low C prevents sensitisation | |
| 316L SS | ER316L | 18-12-2 Mo + low C | Mo matches 316L; low C critical | #f9f6f0 |
| 321/347 SS | ER347 | 18-9 + Nb | Nb stabilised — no sensitisation risk | |
| 2205 Duplex | ER2209 | 22-9-3 + N | Extra Ni and N restore austenite/ferrite balance | #f9f6f0 |
| Inconel 625 | ERNiCrMo-3 | Ni-22Cr-9Mo-3.5Nb | High strength corrosion-resistant Ni alloy | |
| 4130/4140 | ER80S-D2 | C-Mn-Mo | Provides matching yield strength; low H₂ | #f9f6f0 |
| Ti-6Al-4V | ERTi-5 | Ti-6Al-4V | Matching composition; must be spooled Ar-purged | |
| Al 6061 | ER4043 | Al-5Si | Si reduces cracking; acceptable colour match | #f9f6f0 |
Common Defects in TIG Welding and Prevention
Tungsten inclusions: Tungsten spatter from overheated or dipped electrode lodges in the weld pool. Prevent by: correct electrode type and amperage, avoid electrode contact with pool (use HF start), grind contaminated tip before continuing. RT or UT detection required on critical welds.
Porosity: Caused by contamination (moisture, oil, oxide), inadequate shielding, or turbulent gas flow. Prevent by: clean base metal and filler (degrease with acetone), check gas purity (≥99.998% Ar), avoid draughts, correct nozzle diameter and gas flow (10–15 L/min for 10mm nozzle).
Crater cracks: Hot cracks at arc stop if pool solidifies with insufficient feed metal. Prevent by: crater fill function on machine (tapered current reduction while adding filler), or circle back over the crater.
Frequently Asked Questions
Q: What causes the ‘walking the cup’ technique and when is it used?
A: Walking the cup involves resting the TIG torch ceramic nozzle (cup) on the joint and tilting/rotating it to advance the weld without a freehand torch movement. It provides extremely stable arc length control and produces consistent bead width and penetration. Used for pipe root passes in process piping and for orbital weld applications where repeatable geometry is critical.
Q: Why is orbital TIG welding used for pharmaceutical and semiconductor tubing?
A: Orbital TIG welding produces fully automated, repeatable, fully penetrated root passes in small-bore stainless steel tubing without the variability of manual welding. For pharmaceutical (sanitary) and semiconductor (ultra-high purity) tubing, internal weld bead smoothness, freedom from crevices, and certified corrosion resistance are mandatory — requirements only consistently met by orbital welding with qualified procedures.
Conclusion
TIG welding’s combination of precise heat control, high metallurgical quality, and versatility across virtually all weldable alloys makes it irreplaceable for critical fabrication. Understanding arc physics, shielding gas effects, filler selection, and quality control requirements allows the welding engineer to specify and qualify TIG procedures that meet the most demanding structural, corrosion, and dimensional requirements. See also: Welding Austenitic Stainless Steel and Post-Weld Heat Treatment.
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