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Air entrainment during mold filling is a major source of oxide inclusion formation in metal casting. A model was recently developed by the authors to predict the volumetric air entrainment during pouring of metal castings. In the course of validating the model with experimental data for plunging liquid jets, it was shown that the air entrainment rate during mold filling depends fundamentally on the velocity and diameter of the jet formed by the pouring stream. In this study, the effect of more complex pouring conditions and gating system design on air entrainment is examined. Simulations are performed investigating the air entrainment characteristics of castings filled without a gating system, and with bottom-gated and side-gated filling systems. Results indicate that reducing the head height and pouring time, and the addition of a nozzle extension significantly reduces the air entrainment. In addition, using an offset pouring basin with a stopper and pressurizing the gating system further reduces the volume of entrained air. Simulation results also show that the generation of vortex flows inside the filling system is beneficial in reducing free surface turbulence, which results in less air entrainment and oxide inclusion formation during mold filling.

Oxide inclusions form during pouring of metal castings as a result of air entrainment. Recently, a model was developed by the authors to predict the volumetric air entrainment during pouring. It was found that the velocity, diameter, and turbulence intensity of the liquid stream affect the air entrainment rate during pouring. In this study, the developed air entrainment model is validated with water modeling experiments. In the water modeling studies, water was poured using a bottom pour ladle. The effects of nozzle opening, head height, nozzle diameter, and nozzle extension are simulated. The predictions compare favorably with the experimental measurements. Results indicate that low head height and short pouring time have a beneficial effect on reducing the air entrainment during pouring. In addition, a fully open nozzle and the use of a nozzle extension further reduce the amount of entrained air.

Dross is one of the most challenging quality issues in high integrity ductile iron castings. The formation of dross is linked to the applied metallurgy, metal treatment, process control and gating/rigging design. One of the primary concerns in this regard is entrainment of air into the melt during filling of the casting. Today, avoiding and controlling dross relies on the practical experience and process know-how of foundry experts. A quantitative understanding of the formation mechanisms or the prediction of final amounts and locations of dross in castings is not available. In this paper a computational model is developed for predicting the formation, motion and final location of dross inclusions during pouring of ductile iron castings. The focus is on the important mechanism of dross generation due to air entrainment during filling of the mold. The model predicts the local air entrainment rate as a function of the turbulent kinetic energy of the liquid metal and the normal velocity gradient of the metal at the liquid metal-air interface. The dross inclusions resulting from the exposure to the entrained air are transported with the melt flow under the combined influences of drag and buoyancy, and captured by the solidifying casting structure. The model is implemented in a commercial casting simulation software code. Ductile iron casting experiments are conducted to validate the model using castings having a variety of gating systems, section thicknesses, and surface orientations. Dross is measured by serial sectioning of the solidified castings. Good agreement between measured and predicted dross amounts and locations is obtained. The results reveal how gating system design affects dross formation. Clean ductile iron castings can be obtained by minimizing air entrainment during pouring. The present model allows foundries to evaluate different gating designs and pouring parameters before the first casting is poured. The paper also discusses open issues and practical challenges in quantifying the amounts of dross as a function of the entire processing route of a casting.

Oxide inclusions are among the most commonly reported defects in ferrous and non-ferrous castings. They affect the surface quality, machinability, and mechanical performance of a cast part. Air entrainment during mold filling is the main source of the oxygen that is consumed in inclusion formation. A quantitative understanding of the formation mechanisms or the prediction of final amounts and locations of oxide inclusions in metal castings is not available. Ductile iron experiments are conducted to study the formation of oxide inclusions during pouring. Oxide inclusions are measured by serial sectioning of the solidified castings. The effect of different gating systems, section thicknesses, and surface orientations on the inclusion formation and final distribution is studied. In addition, a computational model is developed for predicting the formation, motion and final location of oxide inclusions during pouring of metal castings, with the focus on the important mechanism of generation of oxide inclusions due to air entrainment during mold filling. The developed model calculates the local air entrainment rate as a function of the turbulent kinetic energy and the magnitude of the normal velocity gradient of the liquid metal at the liquid-air interface. The turbulent kinetic energy is estimated from the sum of the squares of the fluctuating velocity components relative to a spatially averaged mean velocity. The air entrainment model is implemented in a casting simulation software and validated by comparing its predictions to experimental air entrainment measurements for a circular water jet plunging into a quiescent pool. The liquid velocity, diameter and the turbulence intensity dependence is determined by a single entrainment coefficient. Oxide inclusions are then generated at the liquid-air interface, transported with the melt flow under the combined influences of drag and buoyancy, and captured by the solidifying casting surface. The developed model provides a powerful technique for predicting the oxide inclusion formation and final location.