Effects of friction stir welding on microstructure of 7075 aluminum

7075 Al plate has been successfully joined by friction stir techniques. Unlike fusion welding, this is a solid-state process with no evidence of melting. The weld is characterized by a recrystallized nugget having a 2-4 mu m grain size. The dislocation density in the nugget is lowered from that in the parent metal; strengthening precipitates appear to have been solutionized during the welding process, with the larger ones re-precipitating on cooling. The process thus provides a method for joining an unweldable aluminum alloy without introducing a cast microstructure.