Chapter 7 - Solidification shrinkage

Abstract

This chapter discusses the problems related to the contractions, associated with freezing, for a number of metals. Some of these problems include the requirement for: "feeding"—any process that will allow for the compensation of solidification contraction by the movement of either liquid or solid, and for the "shrinkage porosity"—which is the result of failure of feeding to operate effectively. The densest solids are those that have cubic close packed (face-centred-cubic (fcc), and hexagonal close-packed (hcp)) symmetry. Thus the greatest values for contraction on solidification are for such metals. The solidification shrinkage for the less closely packed body-centred cubic (bcc) lattice is in the range 2-3.2%. Other materials that are less dense in the solid state contract by even smaller amounts on freezing. There are a number of situations in which the pressure in the solidifying casting can fall to low or negative values. This hydrostatic stress is a driving force for the formation of shrinkage porosity. However, at the same time, the pressure gradient between the outside and the inside of the casting is also the driving force for the various feeding mechanisms that help to reduce porosity.