The Effect of Multidirectional Forging on the Microstructure and Superplasticity of the Al–Mg–Si–Cu System Alloys with Different Contents of Mg and Si

Abstract

This study focuses on investigating the effect of Mg and Si content in different ratios on the microstructure evolution and superplasticity after processing by multidirectional isothermal forging of the Al–Mg–Si–Cu based alloys with additions of Fe and Ni forming large particles of phases of crystallization origin and dispersoid-forming elements Sc and Zr. The study examined alloys with the following Mg/Si content: 1.2/0.4 (3), 1.2/0.7 (1.7), and 2.0/0.7 (2.8) (wt %). The alloys underwent six cycles of isothermal multidirectional forging (MDF) at a temperature of 325°C with cumulative deformation of up to ∑ε = 14.4. During the MDF process, fragmentation occurred in particles of Mg₂Si and Al₉FeNi eutectic phases, resulting in the formation of particles with sizes of 0.6–0.7 and 1.2–1.5 µm, respectively. Due to particle stimulated nucleation mechanism and suppressed grain growth by nanoscale dispersoids the finegrained structure with a mean grain size of 2 μm was formed in alloys. An increase in Si concentration at constant Mg content, as well as an increase in Mg and Si concentration at a close Mg/Si ratio leads to an increase in fraction of recrystallized grains and a decrease in grain size after MDF. The alloy with the lowest Mg and Si content has shown the highest elongation under superplastic deformation, which is explained by the reduced fraction of Mg₂Si phase particles of crystallization origin.