Double oxide film defects in Ni-based super-alloy castings

The typical defects of castings are mainly inclusions, porosity, cracks and mis-run. However, a new class of defects has been recently discovered known as double oxide film defects. Campbell [1] described the concept of an entrained double oxide film and its deleterious effects on the properties of an aluminium casting. Every time the surface of the metal folds upon itself, e.g. by the action of a breaking wave, the surface oxide film becomes entrained in the bulk liquid. This occurs as a doubled-over oxide film in which the internal surfaces are not bonded, but have a layer of air trapped between the two surfaces (Fig. 1), and leads to a crack in the solidified casting. liquid metal during casting. It has also been proposed that they can open and expand due to a variety of driving forces, such as the precipitation of hydrogen into the atmosphere of the double oxide film defect, and the shrinkage of the liquid metal during solidification, large grain size encouraging extensive rafts of dendrites to straighten the films by a dendrite pushing action . Double oxide films have hardly been observed in vacuumcast materials. There is, so far, not enough evidence to describe the characteristics of oxide films in vacuum-cast materials. The only previous work about oxide films in vacuum-cast Ni-based alloys was described by Rashid and Campbell in 2004 . This study has added to the evidence that oxide film defects do exist in studied Ni-based super-alloy in certain vacuum casting conditions.