When welding metal, on a microscopic level, intense heat is generated at the welding point, causing the metal to melt. As the molten metal cools, it solidifies and fuses the two pieces together. On a subatomic level, the high heat excites the atoms in the metal, causing them to move more rapidly and collide with each other. This process breaks the metallic bonds, allowing the atoms to rearrange and form new bonds as the metal solidifies. Welding involves complex interactions between atoms and their bonds, leading to the creation of a strong joint between the metal pieces.