In Figure 2, there are holes in the crystal lattice called vacancies. The number and location of these openings is affected by many of the same factors that distort the lattice. Atoms at temperatures above absolute zero are in constant vibration. This motion increases as the temperature (amount of energy) in the atom is increased. If an atom has sufficient energy it can jump from its current place in the lattice into one of the  vacancies or even swap places with one of its neighbors. Due to this motion, a certain number of the atoms in a piece of metal are constantly moving around. As temperature increases, the number of atoms changing position in the lattice and the frequency of this motion increases until the melting point of the metal is reached. At this point all of the atoms are now moving about and the crystal structure ceases to exist.

However, well before the melting point is reached, if two

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Figure 3: Two surfaces in intimate contact

Figure 4: Heating promotes diffusion across surfaces

metal surfaces are placed in intimate contact with each other the atoms from one surface will diffuse into the other, and new crystal structures will grow across the surfaces creating a molecular level bond.