Aluminum and copper both oxidize in air. Copper oxidation is a conductor, aluminum oxidation is an insulator. Aluminum flows easily when hot, copper does not. Aluminum connections in air will oxidize, then get hot and the wire will flow away a tiny bit. Then when it cools there is room for more oxidation and the cycle continues till it gets too hot. Connections should be clean and greased to prevent air from contacting the wire at the connection. Special (expensive) wire nuts have grease inside and a skirt to reduce air infiltration and to keep grease inside. At fixture screws of AL approved devices, grease the wire, tighten the screw, then make sure grease covers it so no air gets in. Done properly with the correct materials, you will not have problems.