Effect of the reduction ratio per pass on the microstructure of a hot-rolled AZ31 magnesium alloy sheet

The effect of the reduction ratio per pass on the microstructural evolution of an AZ31 gravity cast plate during hot rolling was evaluated systematically. {10(1) over bar 2} extension twinning was observed in large grains after an earlier rolling pass, but it disappeared upon a subsequent rolling pass due to the reorientation of the grains in an unfavorable direction caused by repetitive rolling. The finer grains were newly formed at the boundaries of the original large grains and twin boundaries by dynamic recrystallization. The grain size distribution was changed from a bimodal distribution upon a small total reduction to a homogeneous distribution upon a large total reduction due to the formation of finer grains that continued at the grain boundaries of large grains by repetitive hot rolling. The size and distribution of the grains were affected not only by the reduction ratio per pass but also by the total reduction (number of rolling passes).