Microstructures and Casting Defects of Magnesium Alloy Made By A New Type of Semisolid Injection Process

We have developed a new type of semisolid injection process that allows magnesium alloys to be formed in high material yields approximating 90%. In this process, generic magnesium billets are heated into their semisolid temperature range in an injection cylinder, without cover gas, and then the material is injected into a mold. In this study, several billets were precision- heated in the cylinder to obtain a desired fraction solid. Plate specimens were produced by injecting the material at different injection speeds. Microstructures were observed by optical microscopy, and casting defects were detected on an X-ray computed tomography scanner. As injection speed was increased, the size and shape of alpha-Mg solid particles became smaller and more spherical, and the defect volume fraction increased. In contrast, as the fraction solid was increased, the defect volume fraction decreased. Spheroidization and miniaturization of solid particles were attributed to shear stress at the nozzle, and defects were affected by viscosity.