A welder is setting up for GTAW on a 6 mm thick aluminum plate using AC current. They observe that their 2.4 mm diameter, 2% thoriated tungsten electrode is melting back and forming a large, unstable ball. What is the most likely cause of this issue?

Question: A welder is setting up for GTAW on a 6 mm thick aluminum plate using AC current. They observe that their 2.4 mm diameter, 2% thoriated tungsten electrode is melting back and forming a large, unstable ball. What is the most likely cause of this issue?

Answer options: ✅ The electrode current is too high for the electrode diameter and type.

• The shielding gas flow rate is too low, causing atmospheric contamination.
• The AC balance control is set too high, providing excessive cleaning action.
• The electrode extension is too long, causing overheating.

Correct answer: The electrode current is too high for the electrode diameter and type.

Explanation: A 2% thoriated tungsten electrode is primarily designed for DC welding and has a higher melting point than pure tungsten or zirconiated tungsten. When used with AC current at too high amperage, it will overheat and melt back, forming an unideal, large ball unstable for GTAW. High current for the electrode's diameter would be the direct cause.