On the effect of oxide level in gravity cast A356 aluminium alloy

Porosity formation in aluminium alloys is strongly dependent on melt cleanliness (oxide level) and its hydrogen content. Porosity phenomenon in aluminium alloys is extremely important in case of critically stressed components such as in aerospace applications. In order to investigate the effect of varying oxide levels (by the addition of anodized plates and oxide rods), series of reproducible castings were produced at a constant hydrogen level, i.e. 0.2 ml/100g melt under carefully controlled conditions. The ALSPEK* H probe was used for the measurement of the hydrogen level. The Porous Disc Filtration Apparatus (PoDFA) and Reduced Pressure Test (RPT) were performed in parallel with the casting trials for the melt quality assessment. It has been found that the presence and distribution of the defects is strongly influenced by the melt oxide level. Analysis of fracture surfaces by Scanning Electron Microscope (SEM) shows that the oxides are associated with the pores. The bi-film index and melt inclusions content (mm2/kg) based on the RPT and PoDFA tests, proved to be increased with the oxide additions.