In spot welding, what happens when current passes between the electrodes?

When current passes between the electrodes in spot welding, heat builds up at the joint, fusing the components together. This process is essential for creating a strong bond between the materials being joined. The resistance of the metal at the joint generates heat due to the flow of electrical current, which raises the temperature sufficiently to melt the surfaces and form a weld nugget as the current flows for a short period of time. This localized heating allows for the materials to bond effectively without needing a filler material, and the cooling that follows solidifies the weld.

The other options do not accurately describe what occurs during spot welding. For instance, the notion that surfaces cool quickly contradicts the idea of localized heating; in fact, the intent is to create enough heat to enable fusion, not rapid cooling. The creation of a large amount of slag is more relevant to processes like stick welding or flux-cored arc welding, which use flux or filler materials, whereas spot welding typically does not produce slag because it relies on clean fusion between the surfaces. Lastly, while improper procedures can lead to weak joints, the standard process is designed to strengthen the metal being joined, not weaken it, as long as welding parameters are correctly set.