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T he liquid aluminum alloy surface is always covered with a layer of oxide fi lm. If the surface oxide fi lm is broken, a new thin oxide fi lm will form immediately, then it will become thicker. However, the broken oxide fi lm can be entrained in the bulk liquid by the surface turbulence, and some large thick oxide fi lms are easily detected by non-destructive testing and microscopy in the finished castings [1] . The surface oxide film can be folded to double layers in the entrainment process, and the bonding between the opposed interfaces is negligible, causing the defect to then resemble and act as a crack [2] . Recent studies announced that the double oxide film defects, entrained during the filling process when the velocity of liquid aluminum is greater than a critical gating velocity, are the main reason for the failure of aluminum castings [3][4][5] . Runyoro [6] and Bahreinian [7] indicated the critical velocity is 0.5 m·s -1 in the bottom gating system. Abstract:The mold fi lling behavior of gradual expansion structure in low pressure casting was studied by two phase fl ow model using the Volume of Fluid method, and was verifi ed by water simulation with a Plexiglas mold. To get smooth mold fi lling 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 fi lm. The results show that the fi lling process of a gradual expansion structure in a low pressure casting can be divided into the spreading stage and fi lling 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 fl uid raised by the jet fl ow along the sidewall, the fl uid velocity decreases in the jet zone from ingate to free surface. As such, oxide fi lm entrainment does not occur when the gate velocity is greater than the critical velocity, andthe gate velocity no longer refl ects the real state of the free surface. The scatter of the mechanical properties is strongly affected by the entrainment of oxide fi lms.Key words: low pressure casting; two phase fl ow; water simulation; oxide fi lm; ZL205A alloy Low pressure casting (LPC) is regarded as an eff ective solution to avoid the entrainment of oxide film [8] .However, Liu et al. [9] demonstrated that the oxide fi lm could be entrained during the fi lling process of a sudden expansion structure at an unreasonable pressurizing speed in LPC process, which is also the case of sudden contraction structure in LPC [10] . It is obvious that the fi lling behavior and inclusions in LPC are infl uenced no...

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