FREEZING RANGE EFFECT ON SHELL GROWTH INSTABILITY DURING ALLOY SOLIDIFICATION

During metal casting, nonuniform thermal strains due to small spatial perturbations in the cooling profile may lead to a condition of growth instability. This condition is one where the metal shell thickness be comes highly nonuniform due to the formation of microscopic air gaps a long the mold surface. Documented experimental work with iron-carbon a nd aluminum alloy systems provides qualitative evidence that the freez ing range width has an important influence on the macromorphology of t he freezing of the freezing front and hence the shell thickness. In th is paper, we develop a thermomechanical model which points to a possib le mechanism for growth instability during the solidification of a thr ee-phase system where a small, spatially periodic cooling profile is s uperposed onto uniform cooling. This extends earlier work on two-phase (pure metal) systems. Temperature fields and shell growth are first c alculated using perturbation theory, and then the associated thermal s tresses and strains are determined from a hypoelastic constitutive law . The evolution of the shell/mold contact pressure beneath a shell thi ckness minimum is examined as a function of the freezing range width, and the onset of growth instability is assumed to occur as the contact pressure drops to zero indicating possible air gap nucleation. Under fixed cooling conditions, instability increases with freezing range wi dth at small liquid metal pressures, decreases with freezing range wid th at somewhat large liquid metal pressures, and disappears at still l arger pressures.