Estudo do efeito termodinâmico da pressão na solidificação de ligas de alumínio

Abstract

The idealization of the use of die casting pressure was originally introduced in a British patent in 1819 and cited in a report in 1878 however the first scientific experiment on the subject was not conducted in until 1931, since then this practice has developed rapidly around the world. Among the articles published on die casting under pressure are reported several effects which some are beneficial and can be explored by industry such as grain refining, macrostructure rearrangement, microstructure refining, porosity elimination, and other effects that damage the properties of the casting, and therefore should be minimized or eliminated as a shrinkage and segregation defect. However, the vast majority of published works on die casting under pressure does not provide a satisfactory explanation for these effects but are limited to their description followed by a qualitative and superficial explanation. This paper aims to formulate a qualitative and quantitative theoretical model that describes the influence of pressure on solidification in order to explain the effects reported in the literature. In order to do it so the effect of pressure was accounted on Gibbs free energy, which is the thermodynamic potential used to describe the classic theory of solidification of metallic materials. From this model it was possible to demonstrate that the pressure produces an effect equivalent to undercooling during grain and crystal nucleation, which produces a decrease in activation energy stimulating nucleation and, therefore, refining the structures. It was also shown that pressure exerts an effect equivalent to the temperature gradient with regarding stability of the solid/liquid interface, discouraging dendrite formation in columnar growth, and stimulating dendrite formation in equiaxial growth.