Microstructural and compositional transients during accelerated directional solidification of Al-4.5 wt pct Cu
- PDF / 1,352,608 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 20 Downloads / 221 Views
I.
INTRODUCTION
Directional solidification (DS) experiments, with detailed chemical and structural analyses of the resulting ingots, have been conducted for many years in order to understand and improve the properties and quality of castings and ingots. Directional solidification is a powerful technique enabling independent control of the two most important parameters controlling microstructure formation: the thermal gradient (G) and the solidification velocity (Vs). Experiments are typically performed under nominally constant G and Vs conditions after an initial transient, during which time the velocity of the dendritic mushy zone accelerates to match that of the imposed crucible velocity (Vc). However, most engineering alloys freeze under transient thermal conditions due to changes in section size and geometry. Thus, it is expected that the interface velocity, the microstructure, and the final casting properties change with time and position in the solidified component. A growing number of experimental and theoretical investigations into the problems of unsteady-state growth conditions are appearing in the literature of solidification. Tiller et al.[1] assumed a planar interface and made an early quantitative analysis of the redistribution of solute for RONG-JIUNN SU, formerly Graduate Research Assistant with the Materials Engineering Program, Mechanical Engineering Department, Auburn University, is Research Engineer with Dura Tek, Inc., Hsinchu, Taiwan. RUEL A. OVERFELT, Associate Professor, is with the Materials Engineering Program, Mechanical Engineering Department, Auburn University, Auburn, AL 36849. WARTAN A. JEMIAN, formerly Professor with the Materials Engineering Program, Mechanical Engineering Department, Auburn University, is retired. Manuscript submitted October 17, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
steady-state conditions and suddenly changed velocity conditions. The response of a nickel-based g/g'-d eutectic alloy to abrupt increases in the velocity imposed on the crucible was examined by Farag et al.[2] Fu et al.,[3] interested in the response of MnBi-Bi eutectic materials to transient growth conditions caused by melt oscillations, experimentally determined the solidification rate and compared it to their theoretical prediction. Wang et al.[4] investigated the planar growth of Ge under microgravity and concluded that the melt system never did reach steady state. The experimental data of Wang et al. were theoretically predicted by Saitou and Hirata[5] using a numerical model as well as a simplified analytical model. Saitou and Hirata also showed that higher thermal gradients should yield a faster response of the solid-liquid interface to changes in thermal conditions. The adjustment of solidification microstructures due to sudden changes in growth conditions is also of interest. Wilcox and Regel[6] noted that the eutectic spacing of MnBi eutectic alloys always corresponds to the instantaneous growth rate during transient experiments, i.e., the eutectic spacing could adapt at least
Data Loading...
