Effects of hot rolling and inter-stage annealing on the microstructure and texture evolution in a partially homogenized AZ91 magnesium alloy

The evolution of microstructure during hot rolling and inter-stage annealing of a partially homogenized AZ91 magnesium alloy was investigated by microscopic examination and micro-texture analyzing. The effects of key processing parameters including rolling temperature, rolling speed, number of rolling passes and inter-stage annealing treatment on the deformation mechanism and recrystallization behavior of the experimental alloy were studied. At lower rolling temperature, the plastic strain applied during rolling was accommodated by the formation of twin and shear bands whereas at higher temperatures non-basal slip systems were activated resulting in the suppression of twinning and, hence, slip became the dominant deformation mechanism in such rolling conditions. The process facilitated an effective grain refinement by both dynamic and static recrystallization phenomena with an increase in the number of rolling passes and conducting inter-pass annealing stages leading to a reduction of average grain size to below 10 μm. Moreover, optical microscopic examinations for all the rolling conditions demonstrated that twin DRX was the major dynamic recrystallization mechanism at the beginning of the rolling process due to the large initial grain size. However, new grains were found to nucleate preferably at grain boundaries as the average grain size decreased during the hot rolling process. Micro-texture analysis for all the processing conditions showed a relatively strong basal texture produced after the first pass of rolling, which was then weakened during the subsequent annealing and rolling stages.