Microstructural characteristics of 7449 aluminum alloy in rolling and isothermal treatment conditions

The direct isothermal treatment methodology was predominantly intended to materialize the preparation of semi-solid billets (SSB). For hot-rolled 7449 aluminum alloy (7449 AA), a comparison was conducted to dig into how isothermal temperature and holding time impact microstructure evolution and coarsening rate. The results showed that heating the alloy at 610 °C for 20 min transformed its initial lath-shaped grains and low-angle grain boundaries (LAGBs) into nearly spherical equiaxed grains and high-angle grain boundaries (HAGBs), and the microstructure shifted from preferred to random orientation. Post-isothermal treatment, the alloy’s texture changed from a deformation-recrystallization mixture to a recrystallization-dominated one. Rising temperature converted deformed grains to recrystallized ones and reduced texture intensity gradually. Average grain size and shape factor both increased with higher temperatures and longer holding times. Heating at 595 °C for 20 min caused insoluble precipitates and fine second phases to dissolve into the α-Al matrix, with the loss of grain boundary (GB) pinning effects, the grains underwent coarsening. At GBs, a substantial Cu-rich liquid phase formed accordingly, while fine within-grain dot-like Cu-rich liquid phases dispersively formed unevenly. Concurrently, MgZn 2 precipitates were formed and dispersively distributed throughout the 7449 AA. An excessively high holding temperature can induce the formation of dendrites at GBs, and the number of dendrites increases gradually with the prolongation of holding time. Grain growth and spheroidization were governed by the combined mechanism of coarsening and Ostwald ripening.