AWS Gas Tungsten Arc Welding (GTAW/TIG) Fundamentals 1 — Questions and Answers

Question 1: What type of tungsten electrode is recommended for welding aluminum with GTAW using AC power?

2% Thoriated (red band) tungsten
Pure tungsten (green band) or zirconiated tungsten (white band) (Correct answer)
2% Ceriated (gray band) tungsten
Lanthanated (gold band) tungsten

Correct answer: Pure tungsten (green band) or zirconiated tungsten (white band)

For AC GTAW on aluminum, pure tungsten (green) or zirconiated tungsten (white) is preferred because AC's electrode-positive half cycle forms a balled tip that is stable and compatible with these electrode types.

During GTAW with AC on aluminum, the electrode-positive half cycle provides oxide cleaning action (breaks up the aluminum oxide layer) but also heats the tungsten. Thoriated or ceriated tungsten maintain a pointed tip ideal for DCEN (used on steel/stainless) but tend to deteriorate on AC. Pure tungsten and zirconiated tungsten form a stable hemispherical ball under AC conditions, which provides consistent arc initiation. Zirconiated tungsten offers better current-carrying capacity and is preferred for production AC welding on aluminum.

Question 2: The primary advantage of GTAW over SMAW for welding thin stainless steel is:

Higher deposition rate
Precise heat control, no slag, and superior weld quality with no contamination (Correct answer)
Lower equipment cost
Faster travel speed

Correct answer: Precise heat control, no slag, and superior weld quality with no contamination

GTAW provides precise heat control via foot pedal or finger control, produces no slag, uses a non-consumable electrode that doesn't contaminate the weld, and delivers the cleanest possible welds for thin stainless.

GTAW (TIG) advantages for thin stainless steel and critical applications: non-consumable tungsten electrode eliminates electrode stub contamination; no flux or slag means no slag inclusions and no post-weld cleaning; separate filler addition gives the welder independent control of heat and filler; variable amperage via foot pedal allows precise pool control; produces the lowest spatter of any arc process; and creates welds with excellent appearance, dimensional accuracy, and mechanical properties. These advantages justify the lower productivity compared to SMAW or GMAW for precision work.

Question 3: In GTAW, 'high-frequency (HF) arc initiation' is used to:

Increase travel speed by pre-ionizing the arc path
Initiate the arc without contact between the tungsten and base metal, preventing tungsten contamination (Correct answer)
Preheat the base metal before welding
Increase penetration on thick sections

Correct answer: Initiate the arc without contact between the tungsten and base metal, preventing tungsten contamination

High-frequency arc initiation ionizes the arc gap between tungsten and base metal, allowing the arc to jump across without the tungsten touching the work — preventing tungsten contamination of the weld.

High-frequency arc starting superimposes a high-voltage, high-frequency electrical spark on the welding circuit. This ionizes the gas in the gap between tungsten and base metal, creating a conductive path that allows the main welding arc to establish without contact. This is critical in GTAW because touching the tungsten to the base metal causes tungsten inclusions (hard, brittle specks in the weld) that must be removed and the tungsten re-prepared. Most AC GTAW machines use continuous HF to maintain arc stability during the polarity reversal; DCEN machines use HF only for starting.

Question 4: What is the purpose of post-flow (post-purge) in GTAW?

Cool the tungsten slowly to prevent cracking
Continue shielding gas flow after arc extinguishment to protect the hot tungsten and weld pool from oxidation (Correct answer)
Purge the hoses before the next weld
Reduce gas consumption by gradually decreasing flow

Correct answer: Continue shielding gas flow after arc extinguishment to protect the hot tungsten and weld pool from oxidation

Post-flow continues argon shielding for several seconds after the arc stops, protecting the still-hot tungsten electrode and solidifying weld metal from atmospheric oxygen and nitrogen contamination.

Tungsten oxidizes rapidly above 1200°F (650°C). After arc extinguishment, the tungsten and weld remain hot enough to react with atmospheric oxygen and nitrogen, causing tungsten oxidation (turning blue/black), embrittlement, and arc instability in subsequent welds. Post-flow time depends on tungsten diameter and welding parameters (typically 5–15 seconds at 1 second per 10 amps). Similarly, back purge gas maintains shielding on the back side of stainless steel and titanium welds during and after welding to prevent carbide precipitation and oxidation.

Question 5: The correct tungsten electrode preparation for GTAW DCEN (direct current electrode negative) on carbon steel is:

Balled tip
Blunt flat end
Ground to a sharp point or taper (Correct answer)
Random preparation — shape doesn't matter

Correct answer: Ground to a sharp point or taper

For DCEN GTAW, the tungsten should be ground to a sharp taper or point, which concentrates the arc for deep, narrow penetration — the opposite of the balled tip needed for AC.

Tungsten preparation significantly affects arc characteristics. DCEN (electrode negative, used for steel, stainless, titanium, copper): grind tungsten to a point or taper at 2-3x the electrode diameter length; this concentrates the arc column, providing a narrow, deep penetrating arc. AC (used for aluminum): pure or zirconiated tungsten forms a hemispherical ball under AC conditions — don't grind to a point. DCEP (electrode positive): rarely used for GTAW; produces shallow, wide penetration with excessive tungsten heating. Proper preparation is critical for arc stability and weld profile.

Question 6: Back purging in GTAW of stainless steel tubing is performed to:

Increase arc energy by filling the tube with argon
Prevent oxidation ('sugaring') on the back side of the root pass (Correct answer)
Reduce the amount of filler metal needed
Allow faster travel speeds

Correct answer: Prevent oxidation ('sugaring') on the back side of the root pass

Back purging fills the inside of stainless steel tubing with inert gas (argon or nitrogen) to displace oxygen from the back side of the root pass, preventing oxidation ('sugaring') that reduces corrosion resistance.

When GTAW root passes are deposited on stainless steel pipe without back purging, oxygen in the pipe interior causes severe oxidation on the back surface of the weld — called 'sugaring' (looks like brown/black crystalline sugar). This oxidized surface depletes chromium from the root surface, destroying corrosion resistance and creating stress concentration points. Back purging with argon or nitrogen-hydrogen mix maintains oxygen below 10-20 ppm at the root. Purge time before welding and purge flow rate during welding are specified in the WPS. Critical for sanitary, pharmaceutical, and food-grade piping applications.

AWS Gas Tungsten Arc Welding (GTAW/TIG) Fundamentals 1

What type of tungsten electrode is recommended for welding aluminum with GTAW using AC power?