Model for prediction of shrinkage defects in long and short freezing range materials

Abstract

The present paper presents a three-dimensional model that is capable of predicting the pressure drop due to shrinkage. The model is based on fluid flow calculation considering a drag force (Darcy term) that is function of fraction solid and on the external surface resistance to move. It considers external defects, in the form of surface depressions, and internal shrinkage defects. By coupling the feed flow, pressure and solidification calculation for predicting shrinkage defects, a complete methodology that enables the possibility to show dynamic shrinkage geometry in the casting and feeding system is presented. To put in evidence the difference morphology of these defects, the behaviour between long and short freezing range aluminium alloys was compared. The model is used to calculate the shrinkage in a simple casting which is composed by a feeding system and blocks with different necks. Two kinds of aluminium alloys, a long freezing AlSi7 and a short freezing AlSi12 are used. The results show internal and outside shrinkage defects depending on the freezing range of the metal. Short freezing range results mainly in internal shrinkage whereas the long freezing range shows more external shrinkage. The expected shrinkage features are well described by the present model.