Describe the epitaxial growth phenomenon observed during weld metal solidification and its significance in determining the initial grain structure of the weld.

Epitaxial growth is a solidification phenomenon observed during welding where the solidifying weld metal crystals adopt the same crystallographic orientation as the pre-existing base metal grains they contact at the fusion boundary. The fusion boundary is the interface between the molten weld pool and the solid base metal. As the molten weld metal cools and begins to solidify, the solid base metal grains at this boundary act as templates or 'seeds'. Rather than forming new, independent crystal nuclei, the atoms in the molten metal preferentially arrange themselves to extend the crystal lattice of the adjacent solid base metal grains. This continuation occurs because it requi....