Reduced Pressure Test Verification of Healing of Double Oxide Film Defects in Al-Mg Alloys

Formation of bonding between two layers of double oxide film (bifilm) defects (dubbed healing) held in Al alloys containing 0.3, 0.7, and 2 wt pct Mg was investigated by means of reduced pressure test (RPT) technique. The liquid metal was poured from a height into a crucible to introduce oxide defects into the melt. RPT samples were then taken from the melt, which were characterized by determining their porosity parameters and examining the internal surfaces of the pores that were formed in them by scanning electron microscopy. The results showed that the oxide defects started to heal in Al melts containing 0.3 and 0.7 wt pct Mg in about 300 and 90 minutes, respectively. Longer healing time in the 0.3 wt pct Mg melt was attributed to the lower rate of transformations in the oxide layers. No healing was observed in the melt containing 2 wt pct Mg, which was attributed to the high concentration of dissolved H in the melt which kept the two layers of the defects separate from each other during the phase transformations.