Microporosity Simulation in Aluminum Castings Using an Integrated Pore Growth and Interdendritic Flow Model
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Microporosity Simulation in Aluminum Castings Using an Integrated Pore Growth and Interdendritic Flow Model GERALD BACKER and Q.G. WANG A new computational model for predicting microporosity in aluminum alloys is described. The model was calibrated against literature data for binary Al-7 pct Si alloys, and subsequently applied to a chill plate test casting in A356 alloy and to an engine block in 319 alloy. The new model allows spherical micropores to nucleate and grow by hydrogen diffusion from a material volume surrounding the pores. This differs from a conventional interdendritic flow computational model for calculating porosity that assumes spherical pores have a diameter proportional to the secondary dendrite arm spacing (SDAS). The new integrated pore growth and interdendritic flow model predicts larger pore diameters and a volume fraction of microporosity that is in better agreement with experimental observations than the interdendritic flow model. I.
INTRODUCTION
ALUMINUM castings are used in a wide variety of commercial applications because of their high strength to weight ratio, good corrosion resistance, and relatively low raw material cost. While cost competitive with other manufacturing methods, the casting process can introduce defects into the materials that significantly reduce fatigue and other mechanical properties. A prevalent defect in cast aluminum is microporosity, often in combination with oxides. Predicting the occurrence of such defects prior to establishing a manufacturing process would be of significant value, as design and manufacturing alternatives could be explored that would lead to improved reliability in the cast product. Mathematical modeling of casting processes is now highly advanced, with commercial programs available to predict mold filling and solidification behavior for virtually any casting process. Predictions of trapped air and cold shuts from mold filling codes and macroporosity from solidification codes are readily available, with accuracy dependent on the fidelity of the numerical method and boundary conditions applied. However, the prediction of microporosity is less advanced. State-of-the-art commercial codes use interdendritic flow models that assume pores are sized in a manner that is proportional to secondary dendrite arm spacing (SDAS). However, individual pores are often much larger than the secondary dendrite arm spacing, and it is the size of the largest pores that has the greatest effect on fatigue properties.[1] In this report, a mathematical model is developed that predicts both the volume fraction of microporosity and its size more accurately than an interdendritic flow model.
GERALD BACKER, Contract Engineer, and Q.G. WANG, Senior Materials Engineer, are with the Advanced Materials Group, GM Powertrain, Pontiac, MI 48340. Contact e-mail: [email protected] This article is based on a presentation made in the symposium ‘‘Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,’’ which occurred March 12–16, 2006, during the TMS Spring Mee
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