What forms the weld pool during the welding process?

During the welding process, the weld pool is created by the molten metal at the edges of the joint being fused together. When heat is applied to the metal workpieces, the localized high temperature causes the metal to melt, forming a pool of liquid metal. This molten metal is crucial as it allows for the fusion of the metal components at the joint. Once the weld pool cools down, it solidifies to create a strong bond between the pieces.

The other options do not accurately represent what forms the weld pool. Unwanted impurities do not contribute to forming the weld pool; instead, they can negatively impact weld quality. While cooling air may affect the cooling rate of the weld pool after it has formed, it doesn't play a role in creating the pool itself. The protective slag layer, generated during some welding processes, helps shield the weld pool from contamination but does not constitute the weld pool itself.