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
1.4 GTAW — Gas Tungsten Arc Welding (TIG)
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?