Abstract: The effects of isothermal temperature and holding time on microstructure evolution and forming of AZ91D magnesium alloy in semi-solid state were studied. The results show that the semi-solid isothermal heat-treatment can turn the dendritic structure of AZ91D magnesium alloy by permanent mould cast into spheroids. The process is as following: during the increasing of temperature, the partialγphase at grain interface of eutectic structure is dissolved first; then with the further increasing of temperature, the residualγ phase and the δ phase begin to remelt successively and theδ phase gradually evolves into spheroidal shape during the holding. The higher the isothermal temperature, the faster the semi-solid remelting of samples and the δ phase evolution process. When the isothermal temperature is much higher or the holding time is much longer, the samples are susceptible to deformation. And the spheroidal structure is also susceptible to grow up. The best semi-solid structure for semi-solid-forming can be obtained at about 570℃ and the holding time about 30~45min, or about 580℃ and the holding time about 15~25min.
Effects of isothermal heat-treatment on microstructure evolution and formability of AZ91D magnesium alloy in semi-solid state
Abstract:
The effects of isothermal temperature and holding time on microstructure evolution and forming of AZ91D magnesium alloy in semi-solid state were studied. The results show that the semi-solid isothermal heat-treatment can turn the dendritic structure of AZ91D magnesium alloy by permanent mould cast into spheroids. The process is as following: during the increasing of temperature, the partial γ phase at grain interface of eutectic structure is dissolved first; then with the further increasing of temperature, the residual γ phase and the δ phase begin to remelt successively and the δ phase gradually evolves into spheroidal shape during the holding. The higher the isothermal temperature, the faster the semi-solid remelting of samples and the δ phase evolution process. When the isothermal temperature is much higher or the holding time is much longer, the samples are susceptible to deformation. And the spheroidal structure is also susceptible to grow up. The best semi-solid structure for semi-solid-forming can be obtained at about 570 ℃ and the holding time about 30~45 min, or about 580 ℃ and the holding time about 15~25 min.