Molecules of solid are in fixed position in the crystalline structure. They are held together by intermolecular forces. This constraint makes them to vibrate about their mean equilibrium position. Aluminum is a solid. As the temperature of the solid Aluminium increases the atoms acquire greater kinetic energy and are able to vibrate farther apart from their mean position, thus weakening the inter molecular forces binding them together. At a certain maximum temperature (660°C)the molecules have maximum kinetic energy.
This maximum temperature is the melting point of the solid. Further heat supplied to the substance does not increase the kinetic energy of molecules. Instead the heat energy is used to break down the intermolecular forces binding the molecules together in the solid. The molecules are then free and could slide over one another. The solid then melts at this maximum temperature. Further heating only goes to break more bonds while the temperature remains constant. The heat energy absorbed during this process is known as the latent heat of fusion.
