Quantification of Entrainment Damage in A356 Aluminum Alloy Castings

Aluminum melts sustain damage to its quality when surface oxides are entrained into bulk liquid. To quantify the extent of entrainment damage, four castings were produced in reduced pressure test (RPT) apparatus; one machined out of the continuously cast ingot, and the other three by pouring the metal from a height of 25, 37.5, and 150 mm. Analyses of RPT samples via micro-computer tomography (μ-CT) scanning demonstrated that the number and volume fraction of pores in aluminum casting increased with increasing velocity of the liquid. However, average and maximum pore sizes first increased with velocity, then decreased, indicating that additional kinetic energy of the metal breaks entrained bifilms. The investigation of the cross sections of RPT specimens via scanning electron microscopy and X-ray maps showed that oxygen was present inside, around, and between the pores, providing further evidence that bifilms act as initiation sites for pores. Moreover, there were unopened bifilms near pores, suggesting that entrainment damage is more extensive than what can be ascertained by the pore size measurements.