Copper and aluminum contract and expand differently: the overheating of aluminum conductors can occur at receptacles, switches, pigtails and splices, with the result being either electrical failure, or fire.
When you subject aluminum conductors to constant stress, the aluminum is inclined to creep away from the stressed area. As the temperature increases so too does the rate of creep, and this creep will continue until the stress is the same as the ultimate strength of the aluminum itself. So, if you have an aluminum conductor wrapped around the screw terminal of a receptacle or switch, the aluminum will tend to creep away from the screw, thus creating a high resistance termination.
Aluminum oxide is the fine film that begins to form on the aluminum’s surface within a few short seconds from the bare metal being exposed to the air. Different to copper oxides (which are conductive) aluminum oxides are not conductive and have high dialectic strength. A good aluminum termination means that these oxides are removed during the termination process, thus assisting in preventing the formation of new oxides.