solidification of an NH4 from the top.19
It was observed that the convective cells Cl-H2
are characterized by zones of up-flow and down-flow, re- sulting in the development of a non-planar interface.
In the present work due to chilling, the temperature of the liquid metal in the vicinity of interfacial region decreases. The liquid metal away from the chill has a higher tem- perature. Hence within the same liquid metal contained in the crucible, warm and hot regions can be expected. This causes the up flow and down flow motions of hot and warm fluids respectively. The location of trough on the casting surface corresponds to the region of down flow of warm metal and the location of the crest corresponds to the region of up flow of hot metal. The downward motion of the warm metal enhances the solidification rate at the interface leading to formation of a trough. In a similar manner a crest is formed at the region of the interface experiencing the up flow of the hot metal from the bot- tom. The solidification is retarded near the zones where the liquid metal rises towards the interface. On the other hand, the solidification is enhanced at locations where the trough formed. The convective flow of the metal stream within the liquid metal below the solidified shell results in the formation of a wavy interface.
Conclusions
Based on the results and discussion, the following conclu- sions were drawn: 1. Higher cooling rates were obtained with aluminum chill as compared to the graphite chill although the thermal diffusivities of both chills are similar. The higher heat diffusivity of the aluminum chill re- sulted in higher flux transients. The transformation from one-dimensional to two-dimensional heat transfer occurred only in the case of graphite chill. This feature was absent in aluminum chills owing to its higher heat diffusivity.
O eutectic solution cooled
2. The peak heat flux obtained with aluminum chill was about 34% higher compared to the graphite chill. The plot of normalized heat flux vs. normal- ized chill surface temperature indicated that the peak in the heat flux transients occurred when the chill temperature reached 50% of the chill satura- tion temperature.
3. The results of chill/water and chill/chill heat trans- fer experiments clearly suggested that the peak in the heat flux transients obtained during solidifica- tion against instrumented chills was not an artifact of the experiment or inverse heat conduction math- ematical model.
4. The macrograph of the casting surface in contact with the chill revealed the occurrence of pre-den- dritic contact points as well as the formation of crests and troughs. A mechanism based on convec- tion within the liquid metal below the solid shell is proposed to account for the formation of wavy casting surface.
acknowledgements
One of the authors (KNP) thanks Defence Research De- velopment Organization (DRDO), Government of India, New Delhi for funding this project work. Authors are grateful to Dr. Bill Griffiths, Senior Lecturer, School of Metallurgy and Materials, University of Birmingham, United Kingdom for his valuable suggestions and useful inputs to this paper.
Figure 11. Macrograph of the casting surface at higher magnification.
Figure 10. Macrograph of the casting surface showing crest formation.
68
Figure 12. Schematic representation of formation of wavy interface on the casting surface in contact with the chill.
International Journal of Metalcasting/Fall 2011
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