Modeling of microporosity formation in A356 aluminum alloy casting

Abstract

A numerical model for predicting microporosity formation in aluminum castings has been developed, which describes the redistribution of hydrogen between solid and liquid phases, the transport of hydrogen in liquid by diffusion, and Darcy flow in the mushy zone. For simulating the nucleation of hydrogen pores, the initial pore radius is assumed to be a function of the secondary dendrite arm spacing, whereas pore growth is based on the assumption that hydrogen activity within the pore and the liquid are in equilibrium. One of the key features of the model is that it uses a two-stage approach for porosity prediction. In the first stage, the volume fraction of porosity is calculated based on the reduced pressure, whereas, in the second stage, at fractions solid greater than the liquid encapsulation point, the fraction porosity is calculated based on the volume of liquid trapped within the continuous solid network, which is estimated using a correlation based on the Niyama parameter. The porosity model is used in conjunction with a thermal model solved using the commercial finite-element package ABAQUS. The parameters influencing the formation of microporosity are discussed including a means to describe the supersaturation of hydrogen necessary for pore nucleation. The model has been applied to examine the evolution of porosity in a series of experimental samples cast using unmodified A356 in which the initial hydrogen content was varied from 0.048 to 0.137 (cc/100 g). A comparison between the model predictions and the experimental measurements indicates good agreement in terms of the variation in porosity with distance from the chill and the variation resulting from initial hydrogen content.