Thermally Activated Diffusion

In short: When welding, heat allows the atoms in the metal to migrate. This is important for the weld pool to mix well and for alloying elements to be evenly distributed. If you weld too cold, everything mixes poorly and you get an inhomogeneous structure. If it gets too hot, diffusion runs too fast, which can lead to pores and cracks in the weld metal. So make sure you have a clean, controlled heat input and don't let the part cool down too quickly. Then diffusion will be fine and the structure will be clean.