Recrystallization and microstructure evolution of hot extruded 7075 aluminum alloy during semi-solid isothermal treatment

Abstract

In this paper, semi-solid isothermal treatment of hot extruded aluminum alloy (SSITHEAA) method was used to fabricate semi-solid billets of 7075 aluminum alloys. It contains continuous temperature rising stage and temperature holding stage during semi-solid isothermal treatment. The effect of heating temperature from 480 °C to 580 °C on recrystallization and texture of hot extruded 7075 Al alloy was investigated in the continuous temperature rising stage. The effects of isothermal temperature and soaking time on microstructure evolution of 7075 Al alloy were investigated in the temperature range of 580 °C to 615 °C for 5–25 min. The results indicated that recrystallization occurred in the continuous temperature rising stage, and the recrystallization degree increased with the increase of heating temperature. When heating temperature increased to 580 °C, the recrystallization was essentially completed, and the elongated grains were completely transformed into equiaxed grains. The isothermal temperature and soaking time had significant effects on average grain size and shape factor of 7075 Al alloy semi-solid billets. The results showed that the optimum condition was in the isothermal heating temperature range of 600 °C to 610 °C for 10–15 min. The coarsening rate of 7075 Al alloy increased with the increase of isothermal temperature from 580 °C to 610 °C, and decreased at 615 °C. The intermediate phases and eutectic phases were observed at the grain boundaries of 7075 Al alloy during semi-solid isothermal treatment.