Examining the effect of second phase particles on recrystallization and grain refinement of Al-Zn-Mg-Cu alloy via coupling of over-aging and annealing treatments

Abstract

The grain refinement of Al-Zn-Mg-Cu alloys is essential for the superplasticity forming of these alloys. In the present work, the influences of MgZn₂ particles on the recrystallization and final grain size of Al-5.80Zn-2.10 Mg-1.60Cu-0.2Cr alloy under various over-aging and annealing processes were systematically studied. It was revealed that over-aging at 400 °C effectively promotes the coarsening of MgZn₂ particles, facilitating the activation of particle simulated nucleation (PSN) effect, and resulting in equiaxed fine grains. The annealing temperature (400 °C and 480 °C) has no clear influence on the grain sizes of the sample as the recrystallized grain size is primary depend on the volume fraction and size of the MgZn₂ particles. The average size for PSN effect is measured as about 200 nm, much smaller than previous literatures. In contrast, over-aging at 260 °C even for sufficient time has a minimal effect on the coarsening of MgZn₂ particles. The activation of strain-induced boundary migration (SIBM) gives rise to the formation of elongated coarse grains. The grain size resulting from the SIBM mechanism is highly sensitive to the annealing temperature. The sample over-aging at 400 °C for 14 h and annealing at 480 °C for 120 s is selected as the optimum process as it produces an equiaxed fine grains with average size of 10.4 ± 3.5 μm. These results can provide key information for grain refinement of Al-Zn-Mg-Cu alloys for superplasticity forming.