Determination of the critical fraction of solid during the solidification of a PM-cast aluminium alloy

During the solidification of aluminium alloys, a flow of the interdendritic liquid through the mushy zone plays an important role in the feedability and shrinkage-driven porosity formation. Therefore, for almost all the studies about casting modelling, a good knowledge of the mushy-zone behaviour is necessary. In this study, the limit of the mushy-zone permeability for permanent-mould (PM)-cast aluminium alloys was investigated. A novel experimental technique was developed to quantify the value of the critical fraction of solid (CFS) at which the interdendritic feeding stops. Grain-refined and non-grain-refined aluminium alloys were used to evaluate the effect of the grain size on the CFS limit of both the non-grain-refined and fully grain-refined alloys. Three different initial mould temperatures were used to find if the mould temperature has any effect on the feeding of a casting. Alloys of varying solidification intervals were also used to determine the critical fraction of solid (CFS) for the feeding limit of solidifying castings. The results showed that the CFS values for solidifying aluminium alloys may vary from 34 % to 57 % depending on the casting conditions. The results also showed that for A356 aluminium-alloy castings the CFS value is not an alloy property, yet it significantly depends on the parameters such as the grain refinement and the modification of eutectic silicon.