1.What is the impact of the "shielding" effect of the zinc layer during welding?
Zinc has a melting point of approximately 420°C, while steel has a melting point of approximately 1500°C.
In resistance spot welding, current needs to be applied through electrodes. The zinc layer has a lower resistivity than the steel substrate and melts before the steel plate. This results in a dispersed current density, making it difficult to form a concentrated weld nugget (weld joint) between the steel plates.
As a result, a larger welding current and a longer welding time are required to form an effective weld joint.
2.What are the consequences of zinc vapor and splashes?
When the steel plate begins to melt (approximately 1500°C), the surrounding zinc has already vaporized (zinc's boiling point is 907°C).
The zinc vapor is trapped in the molten steel, and when the pressure becomes too high, it breaks through the liquid steel film, producing violent spatter.
Results:
Porosity and shrinkage cavities are generated inside the weld joint, significantly reducing the effective load-bearing area and strength of the weld.
Spatter contaminates the electrode and workpiece surfaces.
It damages the zinc layer, leading to a decrease in the corrosion resistance of the weld area.
3.What are the effects of electrode contamination and wear?
Molten zinc undergoes an alloying reaction with the copper electrode, forming a brass alloy that adheres to the electrode surface.
Result: The electrode tip wears and deforms rapidly, requiring frequent re-grinding, reducing production efficiency and leading to unstable solder joint quality.
4.What are the advantages of soldering tin-plated coils?
Excellent Solderability:
Tin has a lower melting point (approximately 232°C) and is softer.
During resistance welding, the tin layer is easily pushed apart by the electrodes, allowing current to pass directly through the high-resistivity steel substrate, thus efficiently forming a concentrated, high-quality weld nugget.
The welding process is stable with almost no spatter.
High Weld Strength:
Due to the stable welding process, the formed weld nugget is full and dense with very few internal defects.
Therefore, the tensile and shear strengths of the weld are very close to or even equal to the weld strength of the base steel plate, and exhibit excellent consistency.
Main Challenges:
Tin tends to adhere to the electrodes, but this problem is much easier to solve compared to the zinc vapor in galvanizing, and can usually be improved through electrode material selection and surface treatment.
5.What are the differences in welding processes?
Galvanized Coil:
Resistance Spot Welding: Requires a high current, slow voltage increase process, and the use of special electrode caps with raised rings to increase pressure and dissipate the zinc layer.
Arc Welding: Requires grinding away the zinc layer in the weld area; otherwise, zinc vapor will cause porosity and cracks in the weld.
Laser Welding: Has good compatibility with galvanized sheets, but also requires addressing the issue of zinc vapor removal.
Bragging: A common method that uses a filler metal with a melting point lower than zinc to connect the parts, avoiding melting of the base material.
Tin-Plated Coil:
Resistance Spot Welding/Roll Welding: A standard and mature process, widely used in can manufacturing, electronic component housings, etc. It offers a wide process window and easy parameter control.