The mold filling behavior of gradual expansion structure in low pressure casting was studied by two phase flow model using the Volume of Fluid method,and was verified by water simulation with a Plexiglas mold.To get smooth mold filling process and provide a guide for the pressurizing speed design in the producing practice,the mathematical model with the pressurizing speed,expansion angle and height of the gradual expansion structure was established.For validation experiments,ZL205A alloy castings were produced under two different pressurizing speeds.Weibull probability plots were used to assess the fracture mechanisms under different pressurizing speeds.Mechanical properties of ZL205A alloy were applied to assess the entrainment of oxide film.The results show that the filling process of a gradual expansion structure in a low pressure casting can be divided into the spreading stage and filling stage by gate velocity.The gate velocity continues to increase in the gradual expansion structure,and increases with the increase of pressurizing speed or expansion angle.Under the effect of the falling fluid raised by the jet flow along the sidewall,the fluid velocity decreases in the jet zone from ingate to free surface.As such,oxide film entrainment does not occur when the gate velocity is greater than the critical velocity,andthe gate velocity no longer reflects the real state of the free surface.The scatter of the mechanical properties is strongly affected by the entrainment of oxide films.