Eqs. (10), (11) or (12) represents

Eqs. (10), (11) or (12) represents the criterion proposed for crack initiation. Sites for crack initiation are likely already available. Coniglio and Cross [7] indicated that liquid metals may contain micro-sized pores that cannot escape during processing because of their limited buoyancy [2]. They also indicated that metastable pore nuclei may also take the form of folded oxide films or gas trapped at the apex of oxides. Folded oxide films are introduced unintentionally into the melt in both shape casting and directchill casting of Al alloys when the melt is poured into the mold [2]. The oxide films form on the surfaces of the liquid stream and the liquid pool and are carried into the inside of the liquid metal by the action of pouring. Al has a very strong tendency to oxidize, thus oxide films form immediately once liquid Al is exposed to air. Consequently, plenty of oxide films can be expected to be present inside liquid and solid Al alloys. In welding, the oxide films already present in a solid Al alloy can go into in the weld pool. No pores or oxide films are needed, however, if crack initiation occurs at the surface of the weld or casting