Interactions of primary intermetallic compound particles and double oxide films in liquid aluminum alloys

One of the main challenges in processing aluminum alloys in the liquid state is the avoidance of the formation of double oxide films and their incorporation into the final parts. Bifilms in liquid alloys can act as heterogeneous nucleation sites for primary intermetallic compounds (IMCs), which phenomenon can be utilized for their aided sedimentation. In this study, TiAlSi IMCs were used for the sedimentation treatment of a liquid Al–Si–Mg–Cu alloy. The interactions between IMCs and bifilms were studied by computed tomography (CT) and scanning electron microscopy (SEM), while the changes in melt quality were assessed by the CT analysis of reduced pressure test (RPT) samples. Additionally, the grain refinement effect of Ti-addition was studied by grain size measurements. It was found that (Al, Si)3Ti particles became attached to bifilms, and their simultaneous sedimentation resulted in lowered porosity. However, the attachment of IMCs to the surface oxide layer caused Ti-macrosegregation in the top region of the melt. The attachment of (Al, Si)3Ti intermetallic particles is caused by the heterogeneous nucleation of IMCs on oxide films and the partial engulfment of oxide film segments during the growth of the (Al, Si)3Ti particles. Investigations of the pore surfaces support the theory of bifilm-initiated pore formation.