Molten magnesium alloys oxidizes rapidly in contact with air and surface oxide layer forms over surface of melt immediately. Due to high rate of oxidation, the molten of magnesium alloys are sensitive to surface turbulence during casting. The influence of casting defects on the mechanical properties of aluminum castings has been studied by a number of researchers, with the common conclusion that oxide films play a major role in the formation of these defects. Campbell shows there is a critical velocity for any of molten metals that over it, surface turbulence is occurred. Experimental work consisted of the casting of plates, 10 mm thick, of AZ91 in resin – bonded sand moulds. The metal was introduced into the mould cavity at different mold entry velocities by changing the dimensions of a bottom-gated filling system. Video recording of the filling of glass-fronted molds was a valuable technique to see the filling behavior of the alloy and the mould entry velocity of flow through the gates were measured. The models were used to investigate the liquid metal flow behavior at different in gate velocities. Distribution of macro defects including gas porosity and shrinkage area indicated in x-ray radiography. Three point bending test carried out for determine of mechanical properties in horizontal direction in different velocities. Some of samples prepared for scanning electron microscopy (SEM) fractography. In addition, energy dispersive X-ray (EDX) was applied on fractured bend test specimens. Results indicate that In-gate velocity affected on distribution of properties and performance of castings. In 10-mm plates, smooth, layer- by – layer filling is seen at velocities up to about 300 mm/s, at 500 mm/s, the mushroom morphology of entry of the melt is well developed. Vertical jet geometry of filling is observed at velocities higher than 1000 mm/s. In lower velocities, surface turbulence is decreased and the entrainment is lower, so superior properties are expected. Scanning electron fractograph showing dendrites in shrinkage pores in association with oxide films and EDX microanalysis of oxide film on fracture surface, revealing high oxygen and magnesium peaks. Due to be quiescent of melt in further from gate, mechanical properties will be improved.
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