AWS Certified Welding Inspector — FundamentalsWelding ProcessesHard
A CWI is evaluating a weldment produced by Resistance Spot Welding (RSW) on two overlapping sheets of dissimilar metals, specifically galvanized steel to plain carbon steel. The inspector observes consistently undersized nuggets and occasional expulsion. Which of the following parameter adjustments would be the most effective first step to improve weld quality, assuming the electrodes are in good condition?
- AIncrease the electrode force.
- BDecrease the hold time.
- CDecrease the welding current.
- DIncrease the weld time.
Show answer & explanationAnswer & explanation
Correct answer: A. Increase the electrode force.
Galvanized steel has a low-melting-point zinc coating that can interfere with proper nugget formation and cause expulsion. Increasing the electrode force helps to extrude the molten zinc from the faying surfaces and establish better metal-to-metal contact, leading to a larger, more consistent nugget and reducing expulsion.
Why the other options are wrong
- B. Decreasing hold time would reduce the time the electrodes apply pressure after the weld, which could lead to crater cracking or poor forging of the nugget, not addressing the undersized nugget or expulsion during welding.
- C. Decreasing welding current would reduce heat input, further contributing to undersized nuggets and potentially leading to cold welds or lack of fusion.
- D. Increasing weld time would increase heat input, potentially worsening expulsion or leading to excessive nugget size and sheet separation if not carefully controlled, especially with galvanized material.
RSW Galvanized Steel Challenges
Resistance Spot Welding (RSW) of galvanized steel is challenging due to the low-melting-point zinc coating, which can cause expulsion, porosity, and undersized nuggets.
- Zinc coating creates interface resistance.
- Zinc melts and vaporizes, causing expulsion.
- Requires higher electrode force to displace zinc.
- Shorter weld times, higher currents often used.
Memory trick: Zinc is tricky, force it out quick, or your nugget will be sick.