Effect of Annealing on Microstructure and Properties for Semi-Solid Magnesium Strips

The experiment of the roll-casting for semi-solid AZ91D magnesium was carried out on the equipment made by ourselves in our laboratory. Parameters of annealing process and the effect of annealing temperature on structure and hardness of semi-solid magnesium were investigated. It was shown that annealing process can improve the structure and properties of roll-casting strips for semi-solid magnesium. In addition, it can relieve the eutectic structure segregation and make β-Mg17Al12 phase precipitate from primary α-Mg grain, which can cause dispersion strengthening process when the annealing temperature is about 420°C. With the increasing of annealing temperature, semi-solid primary α-Mg grain boundary become obscure gradually, grain growth is indistinct and β-Mg17Al12 phase is precipitated from the inner α-Mg grain. However, the aberrated eutectic α-Mg grain for non-equilibrium solidification increase continuously and separate gradually from β-Mg17Al12 phase, which makes the β-Mg17Al12 phase connect together and distribute on the α-Mg grain boundary closely approaching the equilibrium solidification structure. Hardness in eutectic structure area is higher than primary α-Mg grain. With the increasing of annealing temperature, hardness in eutectic structure area will increase and then decrease, when the temperature is over 400°C, the hardness will increase again, but hardness of primary α-Mg grain nearly unchanged. Keywords: casting-rolling; anneal; semi-solid; magnesium alloy; microstructure