1.How do zinc layer characteristics affect the welding quality of SECC galvanized sheet?
Zinc evaporation and smoke: Zinc evaporates violently at high welding temperatures, producing a large amount of zinc vapor, which forms zinc oxide (ZnO) smoke after cooling.
Impact: Leads to unstable molten pool, increased spatter, weld porosity (zinc vapor retention), and shielding of arc/laser.
Deteriorating conditions: Excessive heat input, insufficient shielding gas, and poor ventilation.
Liquid zinc infiltration and embrittlement: Molten zinc penetrates into the weld or heat-affected zone (HAZ) along the grain boundary, causing liquid metal embrittlement (LME).
Impact: Microcracks are generated in the weld or heat-affected zone (especially at stress concentrations).
Passivation film residue: Chromate passivation film (containing Cr⁶⁺ or Cr³⁺) has poor conductivity and is difficult to completely remove.
Impact: Increased contact resistance during resistance welding, resulting in unstable energy in the initial stage of welding; may contaminate the molten pool.
2.How does material and equipment selection affect SECC welding quality?
Welding wire/electrode selection: Arc welding wire: Welding wire containing Si and Al (such as ER70S-6) is recommended to improve the fluidity of the molten pool and reduce pores.
Electrode material: Chromium zirconium copper electrode has better resistance to zinc corrosion than pure copper.
Galvanized layer thickness and uniformity: The thicker the zinc layer, the more intense the evaporation and the more difficult it is to weld (such as double-sided galvanized sheet).
Uneven zinc layer leads to differences in local contact resistance, affecting the consistency of welds.
Surface cleanliness: Oil and dust increase the risk of pores and spatter, and degreasing and cleaning are required before welding.
3.How does the environment affect SECC's welding quality?
Ventilation conditions: Forced exhaust system: Required! Inhalation of zinc fume (zinc oxide) causes "metal fume heat" and contaminates the equipment.
Workpiece assembly accuracy: Too large a gap causes the molten pool to drop (arc welding) or lack of fusion (spot welding). Operation stability: ◦ Deviations in the perpendicularity of the welding gun/electrode to the workpiece change the heat distribution and aggravate the problem of zinc evaporation.
4.What are the common causes of welding defect types?
Spatter: Zinc vapor explosion, current/pressure mismatch, electrode adhesion.
Porosity: Zinc vapor stagnation in the molten pool, insufficient shielding gas, too fast welding speed.
Cracks: Zinc infiltration into grain boundaries, high restraint stress, slow cooling rate.
Incomplete fusion/small weld nugget: Insufficient current, too short time, too high electrode pressure.
Electrode adhesion: Improper electrode material, insufficient grinding, too low pressure.
Porous weld surface: Zinc vapor floats to the weld surface (common in laser welding/arc welding).
5.What are the optimization measures to improve welding quality?
Resistance spot welding: high current + short time + high pressure + large end chromium zirconium copper electrode + automatic grinding system.
Arc welding: short circuit transition + forward tilting welding gun + Ar/CO₂ mixed gas + low heat input welding parameters.
Laser welding: reserved gap + double-sided gas protection + high power density.
General requirements: strict cleaning before welding, forced ventilation, and selection of special welding materials.