Oxide bifilms (double oxide films) are one of the most important defects that influence the mechanical properties of cast aluminum alloys. To investigate the formation of oxide bifilms, castings with different heights of runner were examined. SEM and EDX analysis were used to study the presence of oxide bifilms and fracture surface of tensile test samples. The fracture surface of the tensile test bars that were cast with lower height of runner revealed no oxide bifilm and features of ductile fracture mechanisms. On the other hand, the test bars that were cast with a higher height of runner showed a brittle fracture with areas covered by oxide bifilms. The results of tensile test were analyzed by using Weibull two-parameter statistical analysis. Also the quality indexes of the samples were evaluated using the following equation: Qi (quality index) = UTS(MPa) + 150 log (EL%) The results clearly showed that, with increasing the height of runner, the average mechanical properties, the value of Weibull module and quality index decreased significantly. Finally, the formation and entrapment of the oxide bifilms was explained through the concept of critical melt velocity.
