1.When does the zinc layer evaporate?
The zinc layer on the surface of galvanized coils rapidly evaporates and vaporizes at the high arc temperatures (starting at approximately 420°C and exceeding 6000°C at the center of the arc). This can cause:
Porosity: Zinc vapor is trapped in the molten pool and cannot escape before cooling, forming pores.
Spatter: The rapid expansion of zinc vapor destabilizes the droplet transfer, resulting in large particles of spatter.
Lack of fusion and undercutting: Zinc vapor interferes with the arc and blows away the molten metal, resulting in poor weld formation and undercutting.
Health hazards: Volatile zinc oxide (zinc oxide fumes) are harmful to the human body, and a good ventilation and dust removal system is essential.
2.What are the advantages of argon-rich mixtures?
Arc Stability: Argon, as an inert gas, provides a very stable and gentle arc, reducing spatter.
Moderate Penetration: The addition of 15%-25% CO₂ maintains arc stability while increasing arc penetration and preventing incomplete penetration.
Excellent Weld Bead Formation: The weld has a smooth, aesthetically pleasing appearance and minimal undercutting.
Overcoming Zinc Interference: This ratio achieves the optimal balance between weld quality and controlling the effects of zinc vapor.
3.What are the advantages and disadvantages of pure carbon dioxide gas?
Advantages:
Lowest Cost: CO₂ gas is significantly cheaper than argon gas mixtures.
Deep Penetration: The arc has very strong penetrating power.
Disadvantages:
Excessive Spatter: The arc is unstable, resulting in a large amount of spatter and cumbersome post-weld cleanup.
Poor Weld Bead Formation: The weld has a rough, black appearance and lacks clear fish-scale patterns.
Exacerbates Zinc Problems: The intense arc and spatter increase zinc evaporation, making it more likely to cause porosity and undercutting.
Excessive Fume: Generates more welding fume.
4.What are the advantages and disadvantages of ternary mixture?
Advantages:
More stable arc: A trace amount of oxygen further refines the droplets, making the transition smoother.
Excellent weld formation: The weld surface is very smooth with minimal undercutting.
Improved wettability: Improves the bond between the weld and the base metal (wettability), making it particularly effective in overcoming undercutting caused by zinc coatings.
Disadvantages:
Higher cost.
Increased burnout: Oxygen is oxidizing and can burn out some alloying elements.
5.What are the key techniques for welding galvanized coils?
Beveling and Cleaning: At the weld joint, use an angle grinder to remove as much zinc as possible within 20-30 mm of the bevel and both sides. This is the most effective and fundamental way to avoid porosity and undercutting.
Increase Gas Flow: Because zinc vapor can contaminate the shielding gas, it is recommended to increase the gas flow by 20%-30% compared to welding standard steel, for example, to 20-25 L/min, to ensure an effective shielding gas hood.
Adjust Welding Parameters:
Appropriately reduce voltage and current: Reducing heat input can reduce zinc evaporation.
Use a faster welding speed: Avoid holding the arc in one spot for too long, which can burn off excessive zinc.
Use a slightly longer arc length: Keep the arc slightly away from the weld pool to allow zinc vapor to escape.
Torch Angle: Use a backward tilt (drag-torch welding) method to ensure that the shielding gas covers the weld area in advance and helps dissipate zinc vapor in the forward direction.
Ventilate! Ventilate! Ventilate! : Be sure to use a welding fume extractor or work in a well-ventilated environment to avoid inhaling zinc oxide fumes.