Entrained defects and mechanical properties of aluminium castings

The presence of entrained double oxide films, known as bifilms, has been identified as a contributing factor to the variability in mechanical properties observed in aluminium castings. These bifilms consist of folded-over oxide films containing gas-filled crevices and are formed due to turbulence on the liquid metal's surface during handling and pouring. Additionally, it has been suggested that hydrogen dissolved in the aluminium melt can permeate these defects, causing them to expand and leading to the formation of hydrogen porosity. This, in turn, exacerbates the detrimental effects on the mechanical properties of the castings.

In this study, the ultimate tensile strength (UTS) and percentage elongation of sand cast bars were compared under various casting conditions. These parameters were chosen as indicators of casting reliability, which was expected to be influenced by the presence of oxide films. The results indicated that incorporating filters in the gating system and reducing the runner height led to a noticeable improvement in tensile strength and elongation. This improvement was attributed to enhanced mold filling conditions, which reduced the likelihood of oxide film entrainment.

The findings of this research provide valuable insights into the factors that affect the properties of light metal alloy castings. By understanding these influences, it becomes possible to develop improved practices that result in healthier castings with enhanced mechanical properties.