Research on the formation mechanism and fracture behavior of bifilm defects

Casting performance is seriously jeopardized by metallurgical defects, especially the long-neglected bifilm defects that have been found in various alloys. The theory of entrainment formation of bifilm defects has been commonly accepted, but their visualization of the formation process and fracture behavior are absent to date, which is explored in the present work. The vivid numerical simulation results showed that the rapid rise of the liquid level leads to violent melt fluctuation, which results in the repeated folding-entrainment-detachment process of the oxide film on the melt surface. Different motion trajectories would be produced by the bifilm defects based on their composition. The observation of metallographic samples confirmed the corresponding results. Meanwhile, the fracture behavior of bifilm defects can be studied using optical in situ tensile. The results show that the bifilm defects are the source of cracks, and the connection of their influence domains causes the cracks to expand laterally. These results will shed new light on the understanding of the formation and fracture process of bifilm defects and improve the defect control of castings.