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 degrees C to 610 degrees C, grain size increased and an isothermal temperature above 610 degrees C led to a decrease of solid grain size. For RAP, increasing isothermal temperatures from 580 degrees C to 600 degrees 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 degrees C and 600 degrees 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 mu m(3) s(-1) and 402 mu m(3) s(-1) respectively in the SIMA microstructure fabricated at 590 degrees C and 600 degrees C. The coarsening rate constants of 329 mu m(3) s(-1) and 393 mu m(3) s(-1) were achieved in the RAP microstructure. The RAP microstructure exhibited a slightly higher coarsening rate at 590 degrees C, but a slightly lower coarsening rate at 600 degrees C as compared to SIMA. (C) 2016 Elsevier B.V. All rights reserved.