Impregnation problem of gas tight aluminum die cast parts

One of the most important characteristics of the quality of die cast aluminum alloy castings is the degree of porosity of the casting, which can usually be determined with satisfactory accuracy by density measurement. In case of increased quality requirements, e.g. gas tight, it is not sufficient to know the total porosity level, since small volume of porous voids and channels within the casting can also cause leaking problems.

The most important and widely used technology for sealing the porosity channels is the vacuum impregnation technology. Manufacturers experience has shown that leakage can occur after impregnation of carefully prepared quality castings due to the presence of a through cavity system with critical size and/or shape for impregnation.

One of the possible reason for the low impregnation efficiency of these narrow porosity channels can be the insufficient pressure level in the vacuuming stage in the cavity system causing the leakage, i.e. the vacuum level may not reach a desirable level before the impregnation stage. This is basically because the permeability of the small cross-sectional channels connecting the porosity cavities is not sufficient to reach the desired vacuum level in the time period available. The estimated geometries of the cavity system in the castings are around the limits where the vacuuming efficiency can vary drastically. In this respect, the role of the diameter of the channels connecting the cavities, which influences the volume flow rate at a given pressure, should be highlighted.

The effectiveness of vacuum treatment is determined by the geometric characteristics of the cavity system, in addition to the vacuum level applied and the duration of the treatment. The paper analyses the relationship between the pressure conditions in the complex cavity system and the expected impregnation efficiency.