Evaluation of Thermal Stability of Mold Materials for Magnesium Investment Casting

The relative thermal stability of mold materials against AZ91D magnesium alloy melt was evaluated. Special attention has been paid to obtain the applicable investment casting condition for conventionally used ZrSiO4 and Al2O3 molds, to develop a stable CaZrO3 mold, and to evaluate the reaction of AZ91D magnesium alloy melt with a thermodynamically stable CaO mold. The results showed that the castings in the ZrSiO4, Al2O3, and CaO molds had clear reaction zones when the mold temperature was over 673 K, but below it no reaction occurred except for the CaO mold. The relative thermal stability of the molds examined can be graded in the order of increasing thermal stability in the following sequence; CaO→ZrSiO4→Al2O3→CaZrO3. Although the CaZrO3 mold is promising for magnesium investment casting, it is worth noting that the colloidal silica bonded ZrSiO4 and Al2O3 molds could be used, provided that careful melting and casting environments were taken into consideration.