The unidirectional solidification of Al-4 Wt pct Cu ingots in microgravity

Three Al-4 wt pct Cu alloy ingots, 10 mm in diameter and 25-mm long, were unidirectionally solidified in microgravity during the flight of a sounding rocket, with solidification rates of about 1.6 × 10−4 m/s and temperature gradients of about 2600 K/m. The apparatus was comprised of three muffle furnaces, which melted the ingots prior to the launch of the rocket. Unidirectional solidification of the ingots was accomplished by chill plates attached to the furnaces, which were withdrawn from the ingots during the microgravity portion of the flight, bringing the chill plates into contact with the bases of the capsules containing the ingots. Solidification was complete in less than 4 minutes. For comparison, several ground-based ingots were solidified in unit gravity under similar conditions. Metallographic analysis of the solidified ingots showed that the macrostructures of the unit-gravity and microgravity ingots were similar, all exhibiting columnar grains. However, the microstructures were significantly different, with the microgravity ingots exhibiting primary dendrite spacings about 40 pct larger than the unit-gravity ingots and secondary dendrite arm spacings about 85 pct larger. The larger dendrite spacings for the ingots solidified in microgravity are explained by lower dendrite growth velocities. The absence of convective mixing in the microgravity ingots slightly increased temperature gradients in the liquid portion of the alloy during solidification, which resulted in decreased growth velocities.