Abstract

Friction stir processing is one of the severe plastic deformation methods. Static grain growth study during friction stir processing does not receive enough attention compared to dynamic recrystallization study. Thus, in this report on static grain growth kinetics of Mg-Li alloy, an ultralight Mg-8.41Li-1.80Al-1.77Zn alloy has been fabricated by rolling, friction stir processing, and annealing. Microstructural examination of the nugget zone in the annealed state revealed that the grain growth rates at 523 and 573K are much lower than the growth rate at 623K. In the meantime, grain growth prior to 30min is not obvious, but grains grow obviously with the increase in time after 30min. The nugget zone grain growth kinetics equation abided by parabolic relation based on the linear fitting of the experimental grain sizes. The grain growth activation energy was 176.191kJ/mol, higher than the lattice diffusion activation energy of magnesium, 135kJ/mol. Probable cause is that the second phase particles increase the difficulty of thermal activation and raise the activation energy. The calculation error between theoretical grain growth model and experimental grain growth model is two orders of magnitude because of the use of an effective diffusivity. Hence, accurate theoretical model for static grain growth remains to be established in the future. This indirectly demonstrates the scientific meaning and value of our established experimental parabolic growth model.

Keywords: Magnesium-lithium alloy; Friction stir processing; Annealing; Static grain growth; Microstructure