Changes in microstructure and hardness of rheocast AZ91HP magnesium alloy with stirring conditions

The microstructure of rheocast magnesium alloys is similar to the microstructure of inverse composite materials consisting of soft primary particles distributed in a hard matrix. The size and shape of the primary particles are affected by processing variables such as stirring temperature (TS), stirring rate (VS) and stirring time (tS). The primary particle size decreased with increasing the stirring temperature and stirring rate and decreasing the stirring time. The size distribution of the primary particles is more uniform with increasing the stirring temperature and stirring rate. The shape of the primary particles changes from dendrite to rosette or spherical shape due to stirring. The lower stirring temperature and higher stirring rate and stirring time made the primary solid particles more spherical. The primary particle size as well as solid volume fraction affected the hardness. The hardness of rheocast AZ91HP can be expressed empirically as follows; Hv=64(1−fs)+[44+190d−1/2]fs.