1.4 GTAW — Gas Tungsten Arc Welding (TIG)
Key Takeaways
- GTAW uses a non-consumable tungsten electrode with inert gas shielding (argon or helium only)
- AC current is required for aluminum and magnesium — the DCEP half-cycle removes oxide layers
- DCEN (straight polarity) is used for steel, stainless, and most other metals — provides deepest penetration
- GTAW produces the highest quality welds but has the lowest deposition rate (1–3 lbs/hr)
- Filler metal is added separately — autogenous welds (no filler) are possible
- Common applications: root passes on pipe, thin materials, exotic alloys, aerospace
Gas Tungsten Arc Welding (GTAW), commonly called TIG welding, produces the highest quality welds of any manual arc welding process. It uses a non-consumable tungsten electrode and external inert gas shielding, with filler metal added separately by hand or by automatic wire feed.
Process Fundamentals
The arc is struck between a non-consumable tungsten electrode and the workpiece. The tungsten electrode does not melt into the weld — it only serves to sustain the arc. Filler metal, if needed, is added separately as a rod or wire fed into the weld pool.
Key characteristics:
- The tungsten electrode has a melting point of approximately 6,170°F (3,410°C) — the highest of any metal
- Shielding is provided by an inert gas (argon, helium, or mixtures) flowing from the torch nozzle
- Can be performed with or without filler metal (autogenous welds use no filler)
- Produces the cleanest, most precise welds with minimal spatter and no slag
GTAW Current Types
| Current Type | Electrode Polarity | Characteristics | Best For |
|---|---|---|---|
| DCEN (DC Straight) | Electrode negative | Deep penetration, narrow bead, 70% heat at workpiece | Steel, stainless, nickel alloys, copper alloys |
| DCEP (DC Reverse) | Electrode positive | Shallow penetration, oxide cleaning action, 70% heat at electrode | Rarely used alone (overheats tungsten) |
| AC (Alternating) | Alternates EN/EP | Balanced penetration and cleaning | Aluminum, magnesium (oxide removal critical) |
| Pulsed DC | DCEN with pulsing | Better control of heat input and puddle | Thin materials, root passes, out-of-position |
For the Exam: AC is used for aluminum and magnesium because the DCEP half-cycle provides a cathodic cleaning action that breaks up the tenacious aluminum oxide layer. DCEN is used for steels because it provides maximum penetration.
Tungsten Electrode Types
| AWS Color Code | Composition | Primary Use |
|---|---|---|
| Green | Pure tungsten (EWP) | AC welding of aluminum |
| Red | 2% thoriated (EWTh-2) | DC welding of steel, stainless (mildly radioactive) |
| Gold | 1.5% lanthanated (EWLa-1.5) | AC or DC — versatile, non-radioactive alternative to thoriated |
| Gray | 2% ceriated (EWCe-2) | Low-amperage DC, good arc starting |
| Orange | Tri-mix (rare earth) | Universal — good performance on AC and DC |
Shielding Gases for GTAW
| Gas | Properties | Common Use |
|---|---|---|
| 100% Argon | Most common, stable arc, good for thin material | Most GTAW applications |
| 100% Helium | Higher heat, deeper penetration, less stable arc | Thick sections, copper alloys |
| Ar/He mixtures | Balanced heat and stability | Thick aluminum, automation |
Note: Only inert gases (argon, helium) are used in GTAW. Active gases like CO₂ would contaminate the tungsten electrode.
Applications
GTAW excels in applications requiring:
- Root passes on pipe and pressure vessels (followed by fill/cap with SMAW or FCAW)
- Thin materials where distortion and burn-through control is critical
- Exotic alloys (titanium, Inconel, Monel, reactive metals)
- Stainless steel where corrosion resistance of the root side is critical
- Aluminum and magnesium welding
- Precision work in aerospace, nuclear, and semiconductor industries
Advantages and Limitations
| Advantages | Limitations |
|---|---|
| Highest quality welds | Slowest manual arc process (low deposition rate, 1–3 lbs/hr) |
| No slag, minimal spatter | Requires high operator skill |
| Precise heat control | Requires external shielding gas (wind-sensitive) |
| Welds almost any metal | More expensive equipment |
| Can weld with or without filler | Not economical for thick sections |
| Excellent for thin materials | Tungsten inclusions possible if electrode contacts puddle |
Why is AC current used for GTAW on aluminum?
What type of electrode does GTAW use?
Which shielding gas type is acceptable for GTAW?