Double oxide film defects in Al castings and the effect of different element additions

Double oxide film defects consist of doubled-over oxide films containing a gas-filled crevice, and are reported to cause both reductions in mechanical properties and increases in the scatter of properties in Al alloy castings. However, the gas entrapped in a double oxide film defect during its formation may be consumed by reaction with the surrounding melt. The defect might then be closed and its harmful effects might be reduced. In the experiments reported here, an air bubble was trapped inside an A1 melt for up to 1 hour. The change in the volume of the bubble was determined and the oxide film created was investigated using SEM/EDX. The experiment was conducted with additions of Ti, Zr, Mo, Hf, Sc to commercial pure aluminum and 2L99 Al alloy, and it was found that the Mo addition affected the formation of the oxide layer and might therefore accelerate the consumption of the entrapped gas. Tensile testing of sand-cast 2L99 alloy with an addition of Mo suggested that with the Mo addition the Weibull modulus for Ultimate Tensile Strength was increased. Investigation of the fracture surfaces of the test bars suggested mechanisms of how this addition may affect double oxide film defects.