Unravelling dynamic recrystallisation behaviour in AA7050 under hot forging conditions

Understanding dynamic recrystallisation mechanism has significant scientific and practical importance for controlling the microstructure of AA7050 alloy during hot forging process. In this study, interrupted hot compression was conducted using Gleeble to the strain levels of 0.03, 0.15, 0.40 and 0.58 respectively, at 420 °C and strain rate of 0.05 s⁻¹. The grain orientation, geometrically necessary dislocations (GND) density, low-angle grain boundary (LAGB) and high-angle grain boundary (HAGB) at various strain levels were characterised by electron backscatter diffraction (EBSD) and their evolutions as a function of strain were analysed qualitatively and quantitatively. The correlations of GND density, LAGB and HAGB with strain and stress were examined, and the evolution processes of these variables at grain scale were revealed. It was found that dislocations were firstly formed in some of the grains when the deformation started, and the saturation point for the average GND density was 13.5 in log₁₀ scale at the strain of 0.15. Some dislocations transformed into LAGB which could transform to HAGBs and form recrystallised grains in further deformation, demonstrating continuous dynamic recrystallisation (CDRX) behaviour. During this process, the areas of LAGB and HAGB per unit volume increased from 1.2 × 10⁵ to 1.9 × 10⁵ m⁻¹ and from 2.8 × 10⁴ to 7.1 × 10⁴ m⁻¹ respectively, meanwhile the misorientation angles of LAGB and HAGB did not vary markedly with increasing strain level. Based on the direct observation and the statistical analysis, the detailed dynamic recrystallisation mechanisms have been proposed and discussed.