Effect of temperature and strain rate on the grain structure during the multi-directional forging of the Al-Zn-Mg-Cu alloy

Multi-direction forging (MDF) at strain rates of 10(-3) s(-1), 5 x 10(-4) s(-1) and at temperatures ranging from 300 degrees C to 400 degrees C were performed in the Al-Zn-Mg-Cu alloy. The grain refinement effect was controlled by dynamic recrystallization mechanism. The continuous dynamic recrystallization (CDRX) mechanism dominated to form fine recrystallized grains in the grain interiors at 10(-3) s(-1) and at temperatures ranging from 300 degrees C to 360 degrees C or at strain rate 5 x 10(-4)s(-1) and at temperatures ranging from 300 degrees C to 320 degrees C, respectively. With increasing the total strain, the initial coarse grains were gradually replaced by the fine recrystallized grains. Based on the CDRX mechanism, it can be stated that an increase in the MDF temperature or a decrease in the strain rate resulted in accelerating the occurrence of the grain refinement effect. Further, the discontinuous dynamic recrystallization (DDRX) mechanism was considered to be the main recrystallization mechanism when MDF was performed at high temperatures or while using low strain rates, resulting in the formation of recrystallized grains at the grain boundaries. Several coarse grains were remained even after the total strain was increased to 7. Further, grain refinement was not enhanced when the total strain was increased because of the formation of new recrystallized grains, and the growth of the pre-existed recrystallized grains occurred at the same time based on the DDRX mechanism.