A cast 7050 friction stir weld with scandium: microstructure, corrosion and environmental assisted cracking

Microstructure, corrosion and environmental cracking behavior of friction stir welded cast aluminum 7050 ingots alloyed with scandium additions was investigated. An over-aged (T7451) and a homogenization (24 h/475 °C) temper were applied to the as-cast plates before friction stir welding, while a post-weld heat treatment was applied to all welds to verify the changes in the corrosion behavior. It was found that the as-cast and heat treated friction stir weld samples exhibited a fully recrystallized equiaxed grain microstructure. The scandium did not significantly dissolve in any of the phases present and remained homogeneously distributed within the matrix. The as-cast friction stir weld microstructure exhibited apart from the nugget region coarse grain boundary phases, wide precipitate-free zones and coarse intragranular precipitates. The post-weld heat treatment (1 h/480 °C–1 h/100 °C boiling water and quench) increased the tensile strength of the as-cast weld, but decreased the strength of the heat treated welds. The heat treatment of the as-cast samples to an overaging (T7451) and homogenization (24 h/475 °C) temper increased the general corrosion susceptibility of the friction stir welds.