Quantification of Microsegregation in Cast Al-Si-Cu Alloys
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NEAR-NET shape casting of hypoeutectic aluminum alloys, such as 319 and A356, allows low-cost production of complex automotive and aerospace parts. However, the mechanical integrity of these lightweight cast components is often limited by the presence of microporosity and embrittling secondary phases (e.g., Reference 1), whose formation is intimately linked to interdendritic segregation. A thorough understanding of the solute partitioning and buildup of microsegregation within the liquid mush is vital if these defects are to be controlled effectively. Similarly, the effectiveness of heat treatment schedules, and hence the final yield strength of the alloy, is also dependent upon the extent of as-cast segregation. Fortunately, a wide range of microsegregation models exist to aid our understanding. The Scheil model[2] has been widely used for its simplicity despite its very restricting assumptions (infinite diffusion in liquid and no diffusion in solid). Brody and Flemings[3] and Clyne and Kurz[4] have both extended the analytical formulation of Scheil by incorporating a finite value for the solid diffusion coefficient. These microsegregation laws have been further modified by various authors to consider other phenomena during solidification such as coarsening (e.g., Reference 5). Likewise, microsegregaM. GANESAN, Postgraduate Student, L. THUINET, Research Associate, D. DYE, Lecturer, and P. D. LEE, Professor, are with the Department of Materials, Imperial College, London SW7 2AZ, UK. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled ‘‘Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,’’ which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee. Article published online July 18, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
tion models based on the assumption of a parabolic solute profile in the solid[6] are available, because the formulation of the back-diffusion term is in this case very simple. Combeau et al.[7] suggested generalizing this approach by approximating the solute profile in solid by a power-law or a polynomial function. However, for some multicomponent and multiphase alloys, it might be necessary to completely solve the diffusion equation or to determine the precise thermodynamic quantities by means of a thermodynamic software, which requires a fully numerical approach. ThermoCalc[8] or PANDAT,[9] both based on the CALPHAD methodology, have been directly coupled to a diffusion model for the study of aluminum alloys.[10–13] Alternative methods of coupling, such as the tabular approach, have been tested by several authors to improve calculation time efficiency for multicomponent and multiphase alloys.[14,15,16] Simulation of realistic dend
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