Recrystallization in AZ31 Magnesium Alloy during Hot Deformation

Abstract

In this study, isothermal torsion tests were carried out on magnesium AZ31B alloy under constant strain rate conditions, in the range of 250 to 400oC at 0.01, 0.1, and 1.0 s-1. Alloy flow stress dependence on strain rate and temperature can be described by a power law with activation energy of 130 kJ/mol. Microstructural examination of hot deformed samples shows very fine recrystallized grains decorating grain boundaries of larger gains in the form of a necklace. These fine grains are produced by dynamic recrystallization at the grain boundaries of original grains. Microstructure evolution, based on samples quenched at different strain levels, indicates that increasing deformation strain has little effect on recrystallized grain size but widens the recrystallized region, with full recrystallization achieved at a certain high strain level. Recrystallized grain size increases with increasing deformation temperature and strain rate. The latter suggests recrystallization in AZ31 to be essentially a time dependant phenomenon.

Microstructure and Texture Evolution in Hot Rolled and Annealed Magnesium Alloy TRC AZ31

“…The distributions are not random and exhibit three distinct maxima. The first maxima stands out near low misorientation and has been assigned in the literature to induced subgrain boundaries [17]. The last maxima can be attributed to deformation introduced during the sample preparation.…”

“…It is evident in this sample that dynamic recrystallization is partial, i.e. the fraction of recrystallized grains remains very small with no significant grain refinement [16,17]. From Figures 3a to 3d, we can depict that an increase of the reduction ratio led to an increase of the overall recrystallized area.…”

Tool design and stir zone grain size in AZ31 friction stir spot welds

“…Provided that negligible grain growth occurs during the cooling period, it could be expected that the average grain sizes found in the stir zones of AZ31 spot welds were mainly determined by the high temperature deformation conditions imposed by the rotating tool. In this connection, a previous work by Essadiqi et al 27 confirmed that the recrystallisation process in AZ31 alloy is complete when the deformation temperature exceeds 250uC and the strain rate exceeds 1 s 21 and that the final grain size will increase with increasing temperature.…”

Hot working and crystallographic texture analysis of magnesium AZ alloys

“…The superplastic behaviour is attributed to the bimodal grain structures where the large grains contribute to the strain hardening of the alloy and the small grains provide high strain rate sensitivity due to diffusion controlled process. Torsion studies performed on the alloy by Spigarelli et al 30 and Essadiqi et al 18 suggest DRX behaviour and necklace formation followed by reduction in grain size at temperatures 250-400uC and strain rates up to 1 s 21 . Spigarelli et al evaluated the value of activation energy of deformation as 155 kJ mol 21 based on torsion tests.…”

“…8 If the basal plane is perpendicular or almost parallel to the loading axis, the shear stress falls to very low value and a large force is needed to initiate slip. At high temperatures (.225uC), the pyramidal planes 10-11 È É get active as slip planes with [11][12][13][14][15][16][17][18][19][20] Â Ã as the direction of slip. The 10 -11 È É are the second most closely packed structures after the basal plane.…”