Papers by Keyword: Micro-Macro Model

Abstract: Shrinkage porosity is often found in Spheroidal graphite iron (S. G. Iron) castings because of the mushy zone and special volume change during their solidification. Although the volume expansion is very important to the shrinkage porosity simulation of S.G. Iron castings, conventional methods for predicting the porosity defects do not consider it. A Series of macro-micro models such as macro heat transfer calculation and microstructure formation simulation are proposed to simulate the solidification of S. G. Iron castings. The nucleation and growth models are employed to calculate the accurate latent heat and volume change especially graphite expansion during the solidification. The pressure induced by graphite expansion is introduced as a parameter to predict the shrinkage porosity and a new shrinkage porosity criterion is developed. Cooling curves and solid fraction of each phase are compared with experimental castings. At the same time, the porosity area ratio of castings is compared with the results calculated by several porosity criterions. The results show that the new shrinkage porosity simulation criterion of S. G. Iron castings based on macro-micro models is accurate on shrinkage porosity shape, size and distribution simulation.

Abstract: Direct simulations of solidification processes that account for all space and time scales are often beyond the reach of current computational power. To overcome this limitation micromacro approaches that incorporate the effects of small-scale phenomena into large-scale process models have been developed. An important small-scale solidification phenomenon is microsegregation —the redistribution of rejected solute components at the scale of the solid crystal morphology. This paper outlines a general microsegregation model that not only accounts for many of the critical small-scale phenomena in alloy solidification but also well suited as the micro component of a micro-macro model of metal casting. In the development of this microsegregation model, particular emphasis and testing is placed on alternative treatments of modeling the microscale solute diffusion in the solid phase—the so-called “back-diffusion.”