AWS Certified Welding Inspector — FundamentalsWelding ProcessesMedium
A CWI is inspecting a weldment produced by Resistance Spot Welding (RSW) on two overlapping sheets of galvanized steel. The inspection reveals inconsistent nugget size and occasional expulsion. Which of the following is the MOST likely challenge encountered when spot welding galvanized steel?
- AFormation of brittle zinc-iron intermetallic compounds
- BLower electrical resistance of the zinc coating
- CVaporization of the zinc coating at welding temperatures
- DHigher base metal strength requiring increased current
Show answer & explanationAnswer & explanation
Correct answer: C. Vaporization of the zinc coating at welding temperatures
The zinc coating on galvanized steel has a much lower melting and boiling point than steel. During RSW, the zinc vaporizes rapidly, creating pressure between the sheets, which can lead to expulsion (spitting) and interfere with proper nugget formation, resulting in inconsistent weld quality. Special welding schedules are required to accommodate this.
Why the other options are wrong
- A. While zinc-iron intermetallic compounds can form, the primary challenge in RSW for galvanized steel is the rapid vaporization of zinc, leading to expulsion and inconsistent nugget formation.
- B. Zinc coating actually has higher electrical resistance than steel, which can generate more heat at the interface, contributing to rapid vaporization.
- D. Galvanized steel's strength is similar to plain carbon steel; it's the coating that poses issues, not increased strength.
RSW Galvanized Steel Challenges
Resistance Spot Welding of galvanized steel is challenging due to the low boiling point of the zinc coating, which vaporizes rapidly, causing expulsion and inconsistent weld nuggets.
- Zinc boils at ~907°C (1665°F), steel melts at ~1500°C (2700°F).
- Rapid zinc vaporization creates pressure, leading to expulsion.
- Expulsion reduces nugget size and strength.
- Requires modified welding schedules (e.g., higher current, longer weld time, pre-weld current).
- Can cause electrode tip contamination and wear.
Memory trick: Zinc boils fast, makes a mess, expulsion is a stress.