Initial study of void formation during aluminum solidification in reduced gravity

Void formation due to volumetric shrinkage during Al solidification was observed in real time using a radiographic viewing system in normal and reduced gravity. An end-chill directional solidification furnace with water quench was developed to solidify 99.9999% pure Al samples during the approx 16 s of reduced gravity ( plus/minus 0.02 g) achieved by flying an aircraft through a parabolic trajectory. The Al was contained in a vacuum sealed, rectangular, pyrolytic boron nitride crucible. An ullage space was present during each test. Void formation was recorded for two cases: a nonwetting system; and a wetting system where wetting occurs between the Al and the crucible lid. The void formation in the nonwetting case is similar in normal and reduced gravity, with a single vapor cavity forming at the top of the crucible. In the wetting case in reduced gravity, surface tension causes two voids to form in the top corners of the crucible, but in normal gravity only one large void forms across the top.