For solidification against chills, the heat transfer coeffi- cients (q/∆T) corresponding to the peak heat flux are cal- culated assuming that the occurrence of the peak is due to the formation of a stable solid shell. The casting surface temperature corresponding to the peak in the heat flux tran- sients was taken at the eutectic temperature for 413 alloy. The chill surface temperature is directly obtained from the inverse analysis. Accordingly, the heat transfer coefficients corresponding to the peak heat flux for aluminium and graphite chills were determined and found to be 6494W/ m2
K and 5220W/m2 K respectively.
To examine the effect of convective heat transfer within the liquid metal on the casting surface profile, two types of experiments were conducted. In the first set of experi- ments, the chill was in contact with the liquid metal for a limited period of time (about 20 seconds) and the chill was immediately withdrawn. A thin solidified shell was found to be in contact with the chill surface upon removal of the chill from the casting. The examination of the surface of solidified metal, which was in contact with the surface of the chill, shows an uneven surface texture. The macrostruc- ture of the solidified metal clearly shows contact points where the liquid metal initially comes in contact with the asperities of the chill surfaces (Fig. 9). These locations are considered as predendritic contact points where nucleation starts. Griffiths and Kayikci showed the presence of pre- dendritic contact areas during solidification of Al-4.5% Cu alloy solidifying on a copper substrate.18
The second set of experiments involves complete solidifi- cation of liquid metal in the crucible against the chill. The examination of the casting surface reveals the formation of wavy interface as shown in Figures 10 and 11. The casting surface in contact with the graphite chill showed formation of crest and troughs. The formation of this type of wavy interface is attributed to re-melting of the solidi- fied skin because of convection within the liquid metal. A schematic representation of the mechanism of formation of wavy interface on the casting surface in contact with the graphite chill is shown in Figure 12. Natural convec-
tion is the bulk movement of the liquid under the driving force of density differences in the liquid metal. When the Raleigh number (Ra=
Gr.Pr) is below the critical value of 1708, heat transfer takes place primarily by conduc- tion. When it exceeds the value of 1708, the heat transfer takes place by natural convection. At the initial stages as and when the liquid metal comes in contact with the chill surface, a thin solidified skin is formed. Calculations suggested that the threshold value of 1708 is exceeded in the present work, leading to natural convection within the liquid metal. Thickness of initial solidified shell may not be uniform at locations where the metal comes in contact with the asperities of the chill surface. A thicker solidified shell can be expected at these points compared to other lo- cations. The liquid metal may not be able to come in con- forming contact with the troughs on the chill surface due to surface tension of the liquid aluminum and absence of the gravity effect due to downward solidification against the top chill. Two types of flow (UP and DOWN) are con- sidered here. The UP flow and DOWN flow motions of hot and warm fluid leads to a wavy interface growth. Ku- mar et al. observed Rayleigh–Benard convection during
Figure 8. Heat flux transients for chill-chill experiments.
Figure 7. Heat flux transients for chill-water experiments. International Journal of Metalcasting/Fall 2011
Figure 9. Macrostructure of A413 alloy solidified against graphite chill.
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