Modeling of equiaxed microstructure formation in casting
- PDF / 1,530,974 Bytes
- 12 Pages / 590.28 x 785 pts Page_size
- 10 Downloads / 302 Views
I. I N T R O D U C T I O N M I C R O S T R U C T U R E formation during the solidification of alloys is of prime importance for the control of the properties and quality of cast products. In order to predict the properties and the soundness of a casting, empirical methods or trial approaches have been used over the last decades. However, due to the complex interactions occurring during solidification, these methods have a rather limited use and hardly can be extended to other solidification conditions. Furthermore, they usually give little insight into the basic mechanisms of solidification. This is particularly the case in equiaxed microstructure formation where nucleation, growth kinetics, solute diffusion, and grain interactions have to be considered simultaneously with heat diffusion. For these reasons, the modeling of solidification using powerful numerical techniques and consideration of the basic mechanisms of microstructure formation are becoming important fields of development. Since the pioneering work of Murray f~] in 1959 concerning heat flow calculations, most of the effort expended in modeling of casting processes has been focused on the macroscopic scale. Energy, mass, momentum, and/or solute continuity equations have been used to calculate temperature fields, [2] mold filling, [31 convection, {4}or macrosegregation. [5] Developments also have been directed toward geometric modeling to handle complex two- or three-dimensional geometries. Macroscopic calculations provide interesting information about macro-defects due to insufficient feeding, holes, hot spots, hot tearing, and macroporosity, as well as columnar microstructural parameters deduced from the isotherms. Ph. THI~VOZ and J.L. DESBIOLLES, Staff Members, and M. RAPPAZ, Group Leader, are with the l~'cole Polytechnique F6d6rale de Lausanne, D6partement des Mat6riaux, Laboratoire de M6tallurgie Physique, 34 Chemin de Bellerive, CH-1007 Lausanne, Switzerland. Manuscript submitted February 15, 1988. METAI.I.URGICAL TRANSACTIONS A
However, these heat flow calculations, using simplified models to handle the phase change, cannot predict the microstructural parameters of equiaxed structures, such as grain size, eutectic or dendritic spacing, volume fraction of each phase, or solidification morphology. The first attempt to couple the microscopic aspects of solidification with the continuity equation of energy was made in 1966 by Oldfield t61 for gray cast iron. As a matter of fact, most of the work done in this area since 1966 deals with eutectic alloys. 17"891This case, indeed, is much simpler than that of dendritic alloys since the eutectic grains are fully solid. Maxwell and Hellawell {l~ have considered the various mechanisms of nucleation and growth of equiaxed dendritic grains in order to analyze the cooling curve, near to recalescence, of a small ingot of uniform temperature. However, they performed their analysis by making the assumption that the dendritic grain was a solid sphere. Dustin and Kurz {~'] relaxed this assumption by setting the int
Data Loading...
