On Gas/Air Porosity in Pressure Die Casting

Multifunctional characteristics of die casting parts are significantly compromised by the presence of voids; a substantial decrease in the elastic moduli and thermal conductivity results. Gas/air inclusions constitutes a large part of the total porosity. To reduce the porosity due to gas/air entrainment, several methods can be applied to remove the residual air in the die. In some cases these methods yield good results, i.e. low porosity, while in other cases the results are not satisfactory. The purpose of this work is to analyze the main mechanisms that control gas/air porosity and to examine the mixing process in the die cavity.

Analyses of vacuum and atmospheric venting indicate that there is a critical vent area below which the ventilation is poor and above which the resistance to the air flow is minimal. Models yield simple equations to calculate the required area which is a function of the duct resistance, the evacuated volume, and the filling time. These results should be useful to the design engineer and for numerical simulations of the cavity filling which takes into account the compressibility of the gas flow out.

In the Pore Free Technique, oxygen is introduced into the die to react with the liquid metal. The vacuum created by the reaction reduces the porosity. Differing results obtained with this method can be explained by a model based on the conservation laws. The model indicates that there is a critical dimensionless parameter which describes the border between successful and unsuccessful operation.

The liquid metal enters the die cavity in many designs as a wall jet. Experiments demonstrate that the jet has at least two main flow patterns: continuous flow and spray flow. High speed photography was used to obtain qualitative and quantitative information