Microstructure and properties of Mg-5Al-1.5Ca-0.4Zn based alloy by semi-solid isothermal heat-treatment-die-pressure-casting process

The microstructure and properties of Mg-5Al-1.5Ca-0.4Zn-0.2Mn-Sr-Ti magnesium alloy by semi-solid isothermal heat-treatment-die-pressure-casting process were studied. It is found that there are three main phases formed in the experimental alloy, i.e., α(Mg), (α(Mg)+ Al₂Ca) eutectic and Mg₁₇Al₁₂. When the alloy modified by Sr is heat-treated at 585°C, the primary α(Mg) phase divorces into lumps. Then the lumps evolve into spheroids gradually with isothermal time increasing. After treated for 25 min, the well-distributed spheroidic semisolid-structure with the average grain diameter of 40 μm is formed, then the spheroids agglomerate and coalesce. It is also found that the room temperature mechanical properties are improved greatly by semi-solid isothermal heat-treatment-die-pressure casting process. The elevated temperature strength of experimental alloy at 200°C is up to 140 MPa, better apparently than that of AZ91 die-casting alloy. Homogeneous equiaxied fine grains with the average grain diameter of 25 μm are obtained in experimental alloy by the semi-solid isothermal heat-treatment-die-pressure casting process.