456A TIG (GTAW - Gas Tungsten Arc Welding) 1 — Questions and Answers
Question 1: What type of tungsten electrode is recommended for AC GTAW welding of aluminum?
- Pure tungsten (EWP) or zirconiated tungsten (EWZr) (Correct answer)
- 2% thoriated tungsten (EWTh-2)
- Ceriated tungsten (EWCe-2)
- Lanthanated tungsten (EWLa-1)
Correct answer: Pure tungsten (EWP) or zirconiated tungsten (EWZr)
Pure tungsten (EWP) and zirconiated tungsten (EWZr) are recommended for AC GTAW because they form a rounded, balled tip under AC that provides stable arc characteristics and good cathodic cleaning on aluminum. Thoriated tungsten is used for DC welding and does not ball properly on AC.
Question 2: Why is DCEN (Direct Current Electrode Negative) used for most GTAW applications on steel and stainless steel?
- DCEN concentrates about 70% of the arc heat at the workpiece, providing deep penetration while keeping the tungsten cool (Correct answer)
- DCEN provides cathodic cleaning action to remove oxide layers
- DCEN allows higher wire feed speeds for filler metal addition
- DCEN reduces the risk of tungsten contamination
Correct answer: DCEN concentrates about 70% of the arc heat at the workpiece, providing deep penetration while keeping the tungsten cool
With DCEN, electrons flow from the tungsten electrode to the workpiece. This concentrates approximately 70% of the arc heat at the workpiece (for deep penetration) and only 30% at the electrode (keeping the tungsten from overheating). This is opposite to DCEP, which cleans the surface but overheats the tungsten.
Question 3: What is the purpose of the 'high-frequency (HF) arc start' feature on a GTAW power source?
- Initiates the arc without contact between tungsten and base metal, preventing tungsten contamination (Correct answer)
- Stabilizes the AC arc by re-igniting it at each zero crossing
- Increases the welding current during the start phase to prevent cold starts
- Activates post-flow shielding gas after the arc is extinguished
Correct answer: Initiates the arc without contact between tungsten and base metal, preventing tungsten contamination
High-frequency arc starting superimposes a high-voltage, high-frequency spark on the welding circuit that ionizes the shielding gas between the tungsten and workpiece, allowing arc initiation without contact. This prevents tungsten contamination from touching the base metal and maintains electrode tip geometry. HF is also used continuously in AC GTAW to re-strike the arc at each polarity reversal.
Question 4: What shielding gas is standard for GTAW welding of stainless steel?
- 100% Argon (Correct answer)
- 75% Argon / 25% CO2
- 100% Helium
- 100% CO2
Correct answer: 100% Argon
100% Argon is the standard shielding gas for GTAW on stainless steel and most metals. It provides excellent arc stability, good oxide cleaning in AC mode, minimal reactivity with the weld pool, and good coverage at moderate flow rates. Helium or He/Ar mixtures are sometimes used for deeper penetration but are more expensive.
Question 5: What is 'tungsten inclusion' in GTAW and how is it caused?
- Tungsten particles embedded in the weld metal; caused by the tungsten tip contacting the weld pool or being contaminated (Correct answer)
- Tungsten deposits on the base metal surface from the shielding gas
- Oxidation of the tungsten electrode during welding
- Excessive tungsten electrode consumption during AC welding
Correct answer: Tungsten particles embedded in the weld metal; caused by the tungsten tip contacting the weld pool or being contaminated
Tungsten inclusion occurs when a piece of the tungsten electrode breaks off and becomes embedded in the weld metal. It is caused by: touching the tungsten to the weld pool, excessive current melting the tip, contamination of the tungsten by the base metal, or improper arc starting technique. Tungsten inclusions are rejectable defects and are highly visible on radiographic film.
Question 6: What is the recommended included angle for grinding a pointed tungsten tip for DCEN GTAW on steel?
- 2.5× the electrode diameter length, ground longitudinally to produce a consistent tip (Correct answer)
- Flat (no point) — DCEN requires a flat-ended tungsten
- Blunted to a small flat equal to half the electrode diameter
- Maximum 90° included angle only
Correct answer: 2.5× the electrode diameter length, ground longitudinally to produce a consistent tip
For DCEN GTAW, the tungsten should be ground to a point with the length of the taper equal to approximately 2–2.5× the electrode diameter. Grinding should be done longitudinally (along the length of the electrode) to prevent spiral arc deviation. A consistent taper angle concentrates the arc and provides good penetration.
456A TIG (GTAW - Gas Tungsten Arc Welding) 1
What type of tungsten electrode is recommended for AC GTAW welding of aluminum?