Abstract: Evolution of the metallographic structure, recrystallization, second phase, aging precipitated phase, precipitation free zone and microhardness of the 7050-T7451 aluminum alloy thick plate with different thickness layers along the thickness direction were studied by means of metallographic microscopy, field emission scanning electron microscopy, transmission electron microscopy and hardness meter. The results show that the 80 mm thick 7050 aluminum alloy plate has different degrees of recrystallization during heat treatment. The average grain size and area fraction of recrystallization from the surface to the center show a trend of first decreasing and then increasing along the thickness direction, and the recrystallization area fraction of each thickness layer is less than 5%, while the average size of second phase increases gradually, the area fraction is smaller in the surface layer and relatively larger in the center. The density of intragranular precipitates is basically consistent in each thickness layer, but the precipitates at grain boundaries are more discontinuous and larger in size in the center compared to other thickness layers, while the spacing and size of the surface layer are smaller. In addition, there are obvious precipitate free zones(PFZ) in different thickness layers, and their widths vary with the size of precipitates at grain boundaries. The width of the precipitate free zone in the center is relatively wide, about 94 nm. The hardness values of each thickness layer show a trend of first increasing and then decreasing from the surface to the center, and are related to the degree of recrystallization and metastable η′ phase.

Key words: 7050-T7451 alloy thick plate, microstructure, recrystallization, precipitate free zone, microhardness