Aluminum castings freeze by three different methods. In pure aluminum, shrinkage occurs as a deep pipe or at the centerline of the casting. Solidification of alloy #295, 94% aluminum, 5% copper, 1% silicon begins at the wall but progresses quickly to the center of the casting. Fine grains form randomly in the center of the casting and freezing continues in a mushy state.
Illustration Courtesy Chastain Publishing