Formation and suppression of channels during upward solidification of a binary mixture
- PDF / 2,531,902 Bytes
- 11 Pages / 613 x 788.28 pts Page_size
- 42 Downloads / 234 Views
I.
INTRODUCTION
W H E N an alloy melt is solidified by cooling from one wall of the mold, a homogeneous casting is produced with columnar grains aligned in the primary growth direction and with few transverse grain boundaries formed. The material produced by this process of unidirectional solidification has improved thermal fatigue strength and increased creep rupture strain, and is used in the manufacture of products such as high-performance gas turbine blades. However, these castings may be characterized by large-scale inhomogeneities in the form of longitudinal solute-rich imperfections (freckles) caused by compositional convection in the melt and in the mushy zone during solidification. Although much attention has been devoted to the study of this defect, the precise mechanisms of when and how the parent channels form in the mushy zone are still not clear. Unidirectional solidification has been investigated experimentally in aqueous salt solutions and organic systems, in addition to metallic systems. The aqueous salt system is preferred for experimentation in view of its translucency, similarity to liquid metal solidification in terms of dendritic growth ill due to its tow entropy of fusion, availability of thermophysical property data, and ease of experimentation due to its low melting temperature and small latent heat. In some of the first such experiments conducted by Copley et al.,fzl a 30 wt pct NH4C1-H20 solution was solidified upward in a cylindrical mold. They found that channels were caused by upward-flowing liquid jets that eroded the mushy zone and led to localized segregation. The erosion by the jets SURESH V. GARIMELLA, Cray-Research Associate Professor, and LEI Z. SCHLITZ, Graduate Research Assistant, are with the Department of Mechanical Engineering, University of WisconsinMilwaukee, Milwaukee, WI 53201. JAMES P. McNULTY, formerly Graduate Student, Materials Department, University of WisconsinMilwaukee, is with Castalloy Corporation, Milwaukee, WI 53187. Manuscript submitted July 15, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
was shown to decrease as the imposed temperature gradient and growth velocity were increased. It was further shown that the tendency to channel is greatest in alloys with a large density difference between the primary phase and the melt, high thermal diffusivity, and low solute diffusivity and viscosity. Other visualization studies of solidification in aqueous solutionst3,41 attributed macrosegregation and the formation of channel-type segregates to gravity-induced interdendritic flows. Chen and Chen ISj discussed the differences between finger- and plume-type convection in upward solidification in a 26 pct NH4C1 solution for a range of cold-plate temperatures; the tendency for plume formation decreased as the cold-plate temperature was increased. Based on estimates of the porosity and permeability of the mushy zone, Rayleigh numbers for the onset of plume convection were predicted. Magirl and Incropera 16J also visualized salt fingers and plumes during the solidifi
Data Loading...
