A heavy equipment technician is setting up a GMAW (MIG) welder to repair a cracked steel frame component on a large excavator. The component is 12 mm thick mild steel. They plan to use solid wire and a mixed gas shielding. Which of the following shielding gas compositions is most appropriate for achieving good penetration and minimal spatter for this application?

Question: A heavy equipment technician is setting up a GMAW (MIG) welder to repair a cracked steel frame component on a large excavator. The component is 12 mm thick mild steel. They plan to use solid wire and a mixed gas shielding. Which of the following shielding gas compositions is most appropriate for achieving good penetration and minimal spatter for this application?

Answer options: ✅ 75% Argon / 25% CO2

• 100% Argon
• 98% Argon / 2% Oxygen
• 100% CO2

Correct answer: 75% Argon / 25% CO2

Explanation: For GMAW on mild steel, especially for thicker sections like 12mm, a blend of 75% Argon / 25% CO2 is a common and effective shielding gas. It provides a good balance of arc stability, penetration, bead appearance, and reduced spatter compared to pure argon or pure CO2. Pure argon does not provide enough penetration for mild steel, while pure CO2 offers extensive penetration but often results in excessive spatter and an unstable arc.