As we mentioned earlier, at any given temperature, the metal atoms are at the strictly defined distance from each other. In other words, they carry in themselves a certain internal energy. Since the heat is one kind of energy, the internal energy of the metal increases with increasing temperature. This additional energy causes the increased amplitude of atomic vibrations, which in turn increases the interatomic distance. We can visually observe the effects of this additional energy, since as individual atoms move apart, the overall dimensions of the metal part increase. On the contrary, any decrease in the temperature of the metal leads to the fact that the atoms approach each other. In this case, the compression of the metal is observed.
With the additional heating of the metal, the amplitude of the vibration of atoms continues to increase, which leads to the increase in the interatomic distance, and, correspondingly, to the increase in the dimensions of the metal part. This process continues until the certain point, when the interatomic distance increases so much that the force of mutual attraction of the atoms becomes insufficient to preserve the certain structure of atoms. In this case, the solid metal becomes liquid. The temperature at which this change occurs is called the melting point. The further heating eventually results in the liquid passing to the gaseous state. This last transformation occurs at the temperature known as the evaporation temperature.
The solid metal carries the minimal amount of internal energy and is characterized by the minimum interatomic distance. The liquid metal, which is called «amorphous», or devoid of structure, carries the greater amount of internal energy and is characterized by the large interatomic distance. The metal in the gaseous state is characterized by the greatest amount of the internal energy, the maximum interatomic distance, and also has no structure.
Obviously, when welding and cutting the metal, the heat is transferred. The heating promotes the expansion of the metal. If we considered cases of the uniform heating of the metal, we could measure the change in the length or the overall size of the metal part as it was heated. Any metal alloy has the certain coefficient of the thermal expansion. It means that there is a definite numerical value that characterizes the expansion of the metal in the case of the increase in its temperature by the predetermined value.