Effects of Temperature and Time on Microstructural Evolution of Semisolid 5A06 Aluminum Alloy: Preparation of Semisolid Billets in Ellipsoid Solid Phase

Abstract

To shorten the semisolid billet process and reduce its cost, the effects of isothermal temperature and soaking time on the microstructural evolution and coarsening rate during directly heating room temperature hot-rolled 5A06 aluminum alloy to semisolid temperature were investigated. Samples cut from the hot-rolled plates were heated to semisolid temperatures ranging from 580 to 620 °C, and soaking for 5-30 min. The microstructure of samples was characterized by optical microscopy, scanning electron microscopy with energy-dispersive x-ray spectroscopy, and Image-Pro Plus software. The results showed that the microstructure contained ellipsoidal solid grains with the quantitative parameters of the average grain size of 143 µm and averaged shape factor of 0.55 under an optimal condition of 615 °C for 20 min. Recrystallization occurred within 5 min of holding time, increasing temperature can promote recrystallization, which is beneficial to spheroidization of solid phase, and it will also accelerate melting and the growth of solid particles. Extending soaking time, the liquid phase increased gradually and the change of solid particles was complicated. Abnormal accumulation of liquid phase was found, which will lead to the formation of irregular structure. The reasonable coarsening rate constants ranged from 1319 to 3454 μm3 s−1 under the desired treatment parameters. The segregation of Mg occurred at the grain boundaries, as well as around the inclusions within the grains.