Comparison of microstructural evolution of 7075 aluminum alloy fabricated by SIMA and RAP

Semisolid billets of 7075 aluminum alloy were fabricated by strain induced melting activated (SIMA) and recrystallization and partial remelting (RAP). Effect of isothermal temperature and soaking time on microstructure of fabricated 7075 aluminum alloys was analyzed. The results show that average grain sizes of 7075 aluminum alloys fabricated by SIMA and RAP soaked for 20 min firstly increased and then decreased with increasing isothermal temperature. As to SIMA, when isothermal temperature increased from 580 °C to 610 °C, grain size increased and an isothermal temperature above 610 °C led to a decrease of solid grain size. For RAP, increasing isothermal temperatures from 580 °C to 600 °C led to an increase of grain size and further increase led to a decrease. Larger spheroidal grains, lower spheroidization temperature and better spheroidization effect were obtained in the RAP microstructure in comparison to the SIMA microstructure. Coarsening of grains occurred in the microstructure while extending soaking time at 590 °C and 600 °C. Increasing isothermal temperature reduced spheroidization time. An optimal soaking time of 20 min is required for RAP and SIMA to fabricate high-quality semisolid slurry. Coalescence of grains and Ostwald ripening operated simultaneously in coarsening of grains. The coarsening rate constants are 316 μm³ s⁻¹ and 402 μm³ s⁻¹ respectively in the SIMA microstructure fabricated at 590 °C and 600 °C. The coarsening rate constants of 329 μm³ s⁻¹ and 393 μm³ s⁻¹ were achieved in the RAP microstructure. The RAP microstructure exhibited a slightly higher coarsening rate at 590 °C, but a slightly lower coarsening rate at 600 °C as compared to SIMA.