RUE-based semi-solid processing: Microstructure evolution and effective parameters

Highlights: • RUE was used for semi-solid processing of AA7075 for the first time. • The microstructure prepared by new method is better than conventional SIMA. • The inhomogeneity in strain distribution affects thixotropic features. • Increasing RUE cycles accelerates recrystallization and spheroidization. In this paper, repetitive upsetting-extrusion (RUE) was used for the semi-solid processing of 7075 Al alloy for the first time. Specimens of 7075 alloy were subjected to different RUE cycles at 250 °C, and the semi-solid treatment was carried out at a range of 580 to 620 °C for 2–35 min. The results indicated that the semi-solid slurry's microstructure is fine and spheroidal, and is superior to that produced by the conventional strain induced melt activation (SIMA) process. The effect of inhomogeneous strain distribution across the cross section of the pre-deformed sample on the thixotropic microstructural features was investigated using the finite element method (FEM). Increasing the number of RUE cycles accelerates the recrystallization rate, reduces the average grain size and improves the degree of spheroidization. The Lifshitz–Slyozov–Wagner (LSW) theory was used to describe the coarsening process of the semi-solid slurry in the new RUE-based modified SIMA process. The effect of the number of RUE cycles on the coarsening kinetics of the solid particles was discussed.