| Abstract |
The main phases that are present in A356 aluminum alloy castings are the primary aluminum and eutectic silicon phases. It is the morphology of these phases, together with the microporosity, that determines the mechanical properties, notably the fatigue life of structural aluminum alloy castings. As part of a program to develop optimized tooling for the design of the casting process for structural A356 aluminum alloy components, models have been developed and used for predicting phase fractions, microstructural length scales, and fraction microporosity. Thermophysical properties needed for the numerical simulation of fluid flow, heat transfer, solidification, and solidification shrinkage have been measured. The permeability of interdendritic liquid in the mushy zone has been evaluated experimentally. This report documents all aspects of the development of the models for the prediction of microstructural length scales and fraction microporosity. The length scales are the primary dendrite size, secondary dendrite arm spacing and cell spacing for the primary aluminum phase, and the particle/rod length, diameter and spacing for the silicon phase. The microstructure models predict phase evolution during solidification and the final length scales after solidification, and consider the mechanisms governing the growth of the primary aluminum and silicon phases. A comprehensive methodology taking into account solidification, shrinkage-driven interdendritic fluid flow, hydrogen precipitation, and porosity evolution has been developed for the prediction of microporosity fraction. The predictions are validated by comparison with independent experimental measurements by other researchers and with data from the literature. The models are implemented in a computational framework consistent with those of commercial casting codes, allowing them to be easily incorporated in commercial casting simulation software. |
