Relation between cooling rates and microstructures in gravity-die-cast AZ91D disks

In-die temperature measurements during the casting of 3.5- and 11.7-mm-thic k disks in metal dies at 100 degrees C, 200 degrees C, and 400 degrees C en ables calculation of the heat flux from the casting to the die as a functio n of the casting parameters. Using the flux so determined, the temperature field in the plates could be calculated in agreement with measured values. Increasing preheating temperatures reduced the heat flux, which extended th e local solidification time and coarsened the structure. Directional solidi fication, from the wall of the die to the center of the disk, took place on ly when casting a thin disk in a die at 400 degrees C, because of the combi nation of low undercooling (which prevents homogeneous nucleation) and a su fficiently steep temperature gradient at the solidification front to ensure directional growth. A relatively long residence time at the high temperatu re, as occurs during casting in a die at 400 degrees C, leads to a more-uni form distribution of the Al, due to diffusion in the solid, and prevents th e divorced eutectic, distinct from that obtained at slow solidification, of the AZ91D ingot.