The microstructure of AZ91D magnesium alloy in low pressure lost foam casting was observed by OM（optical microscope）

SEM（scanning electron microscope） and EPMA（electron probe micro-analysis）

which was compared with that of ones in other casting methods.The results reveal that the micro-structure of AZ91D magnesium alloy in low pressure lost foam casting is composed of primary AAAAAAAA-Mg phase and divorced eutectic β-Mg17Al12 phase with discontinuous skeleton shape distributed along grain boundary.A little amount of secondary precipitates β-Mg17 Al12 phase is precipitated from near eutectic α-Al-rich zone.Major Al is distributed along grain boundary to create the β-Mg17 Al12 phase

and minor Al is dissolved into the α-Mg matrix.Zn and Si elements were dispersively distributed in a-Mg and β-Mg17 Al12 phase and Mn elements are aggregated on the grain boundary.The microstructure of AZ91D magnesium alloy in low pressure lost foam casting is superior to that in sand casting and in gravity lost foam casting

and is equivalent to that in permanent mold.In addition

the improvement of elongation is attributed to better feeding effects in low pressure improving the compactness of microstructure.