AWS Gas Tungsten Arc Welding (GTAW/TIG) 2 — Questions and Answers

Question 1: What type of tungsten electrode is most commonly recommended for welding aluminum using AC current in GTAW?

Pure tungsten (EWP) (Correct answer)
2% thoriated tungsten (EWTh-2)
Ceriated tungsten (EWCe-2)
Lanthanated tungsten (EWLa-1)

Correct answer: Pure tungsten (EWP)

Pure tungsten (EWP) is traditionally recommended for AC GTAW on aluminum because it forms a balled end that aids in oxide cleaning during the electrode positive half-cycle.

In AC GTAW, the electrode positive (EP) half-cycle provides cathodic cleaning of the aluminum oxide layer, while the electrode negative (EN) half-cycle provides heat into the base metal. Pure tungsten (EWP) is the traditional choice for AC aluminum welding because it readily forms a hemispherical ball at the tip, which is optimal for stable AC arc characteristics. Thoriated and ceriated tungstens are preferred for DC applications due to better electron emission at DCEN.

Question 2: In GTAW, what is the primary function of the post-flow shielding gas after the arc is extinguished?

Cool the tungsten electrode and protect the solidifying weld pool and hot tungsten from oxidation (Correct answer)
Preheat the base metal for the next pass
Remove spatter from the weld area
Increase arc voltage for better penetration

Correct answer: Cool the tungsten electrode and protect the solidifying weld pool and hot tungsten from oxidation

Post-flow shielding gas protects both the hot tungsten electrode and the solidifying weld pool from atmospheric contamination (oxidation and nitridation) until they cool below the temperature at which oxidation occurs.

After the GTAW arc is stopped, both the tungsten electrode and the weld pool remain at extremely high temperatures that make them susceptible to oxidation and nitridation from atmospheric oxygen and nitrogen. Post-flow gas (typically 5–15 seconds depending on amperage) maintains an inert shield over these hot surfaces until they cool sufficiently. Inadequate post-flow results in tungsten contamination (blue/black discoloration) and weld pool oxidation, both of which compromise weld quality and increase rework.

Question 3: Which shielding gas is most widely used for GTAW on carbon and low-alloy steels?

100% Argon (Correct answer)
100% Helium
75% Argon / 25% CO2
Argon-Hydrogen mixtures

Correct answer: 100% Argon

100% Argon is the most widely used shielding gas for GTAW on carbon and low-alloy steels because it provides stable arc characteristics and adequate shielding at a lower cost compared to helium.

Argon is the standard shielding gas for most GTAW applications due to its good arc stability, effective shielding, lower ionization potential (easier arc starting), and lower cost compared to helium. Helium produces a hotter arc and higher travel speeds but is more expensive and requires higher flow rates. Argon-hydrogen blends are used for austenitic stainless steels to improve heat input and surface cleanliness. CO2-based mixtures are used in GMAW/FCAW, not GTAW.

Question 4: What AWS classification covers filler metals used in GTAW of carbon steel?

AWS A5.18 (Correct answer)
AWS A5.1
AWS A5.9
AWS A5.20

Correct answer: AWS A5.18

AWS A5.18 covers carbon steel filler metals for gas shielded arc welding, including GTAW rods such as ER70S-2 and ER70S-6.

AWS A5.18 specifies requirements for carbon steel electrodes and rods for gas shielded arc welding, covering both GMAW wire and GTAW rods. Common classifications include ER70S-2 (triple-deoxidized, best for rusty or dirty base metals), ER70S-3 (general purpose), and ER70S-6 (high manganese and silicon for GMAW). AWS A5.1 covers shielded metal arc welding (SMAW) electrodes, A5.9 covers stainless steel filler metals, and A5.20 covers flux-cored steel electrodes.

Question 5: In GTAW, what condition is indicated when the tungsten electrode becomes contaminated with base metal (appears gray or has inclusions)?

The electrode dipped into the weld pool (Correct answer)
Insufficient post-flow gas time
Incorrect polarity selection
Excessive travel speed

Correct answer: The electrode dipped into the weld pool

Tungsten contamination with base metal material occurs when the electrode tip makes contact with (dips into) the molten weld pool, transferring base metal into the tungsten and requiring re-preparation of the electrode.

When a GTAW electrode contacts the weld pool, base metal is transferred to the electrode tip, contaminating it. A contaminated electrode causes arc wander, tungsten inclusions in the weld, and unstable arc behavior. The welder must stop, remove the contaminated section of the electrode by breaking or grinding it back to clean tungsten, and re-prepare the tip (point for DCEN, ball for AC) before continuing. This is one of the most common skill-related defects in GTAW.

Question 6: Which GTAW technique is preferred when welding in the overhead position to minimize the risk of weld pool sag?

Reduce amperage and use a smaller weld pool with faster travel speed (Correct answer)
Increase amperage to ensure full fusion
Use a larger diameter electrode
Switch to DCEP polarity for better penetration

Correct answer: Reduce amperage and use a smaller weld pool with faster travel speed

In overhead GTAW, reducing amperage to keep the weld pool small and increasing travel speed minimizes the volume of molten metal that gravity can pull away from the joint, reducing sag.

Overhead welding with GTAW presents the challenge of gravity pulling the molten pool downward (away from the joint). The solution is to reduce heat input by lowering amperage, using a smaller weld pool, and maintaining a faster travel speed to prevent excessive molten metal accumulation. Multiple passes with lower heat input are preferred over single large passes. Increasing amperage would make the problem worse by creating a larger, heavier weld pool more susceptible to sag and drip.

AWS Gas Tungsten Arc Welding (GTAW/TIG) 2

What type of tungsten electrode is most commonly recommended for welding aluminum using AC current in GTAW?