Effects of vacuum degree on the oxidative combustion and volatilization level of liquid AZ91D magnesium alloy were investigated.The AZ91D alloy samples in different vacuum conditions were analyzed by EDS

and surface morphology of the protective film was observed.The results show that vacuum degree of 50 kPa is a critical value for counter-pressure casting AZ91D magnesium alloy.When the vacuum is below 50 kPa

oxidation combustion phenomenon occurs on surface of liquid AZ91D magnesium alloy

and with the decrease of the vacuum degree

magnesium content is declined sharply

resulting in the more serious volatilization of the liquid magnesium.When vacuum degree is more than 50 kPa

magnesium content in the sample is near to standard content

and surface morphology exhibits more and more continuous and sound

inhibiting greatly magnesium combustion phenomena.There exists holes and crack in surface morphology with vacuum degree below 50 kPa.A mathematical model between magenisum contents and vacuum degree was presented

which could provide a guide to counter-pressure AZ91D magnesium alloy.Optimized vacuum degree for counter-pressure casting AZ91D magnesium alloy is 50 kPa.