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Heat Flux transients and Casting surFaCe MaCro-ProFile during downward solidiFiCation oF al-12% si alloy against CHills


K. Prabhu, K. Sharath and G. Ramesh Department of Metallurgical and Materials Engineering National Institute of Technology Karnataka, Surathkal, Mangalore, India


Copyright © 2011 American Foundry Society abstract


Heat flux transients were estimated during downward solidification of Al-12% Si alloy (A413) against aluminum and graphite chills. The thermal plot of graphite chill indicated one-dimensional heat flow in the initial stages which then changes to two-dimensional heat transfer. The heat transfer becomes one-dimensional again during the final stages of solidification. In aluminum chill, heat flow was nearly one- dimensional. Experiments were designed to verify whether the peak heat flux is an artifact of the experiment. The results


introduction


Modelers of casting production require accurate interfa- cial heat transfer coefficients for realistic models of the temperature field during both the mold filling stage and the solidification of the casting.1-4


Modeling temperature


losses during mold filling is widely used to try to predict casting misrun, where the cast alloy freezes before fill- ing the mold cavity, and to predict the heat lost by the alloy during the filling process and hence the correct temperature distribution with which to begin subsequent solidification modeling. Modeling of the solidification process itself is important in order to predict solidifica- tion shrinkage defects and to improve casting yields but is also gaining in importance as microstructural models become more sophisticated and require more accurately calculated temperature fields.


The measurement of heat transfer coefficients generally involves the use of experimental set-ups designed to bring about unidirectional solidification and which use an in- verse solution to the one-dimensional heat conduction equation. In these experimental set-ups, the temperature distribution is significantly affected by the fluid flow as- sociated with mold filling and natural convection currents caused by temperature gradients. In the microgravity en- vironment, such a problem does not arise, and here, mea- sured heat transfer coefficients represent the actual condi- tions existing at the interface between the metal and the substrate.


clearly showed that the occurrence of the peak in the heat flux transients is not an artifact of the inverse model or the experimental technique. The macro-profile of the casting surface in contact with the chill revealed the occurrence of crests and troughs. A mechanism based on the convection within the liquid metal below the solid shell was proposed to account for the formation of wavy casting surface.


Keywords: casting, solidification, heat flux, convection


Casting solidification modelers require heat transfer coef- ficients for two reasons. One is to model solidification and the possible location of shrinkage defects. In this case the measured heat transfer coefficients under 1g environment are quite suitable for applying to a situation where the sol- id casting surface is in (imperfect) contact with the mold wall. In the second case, models of heat transfer during mold filling are being used to try to predict casting misruns, where the cast alloy freezes before filling the mold cavity, and to predict the heat lost by the alloy during the filling process and hence the correct temperature distribution with which to begin the subsequent solidification modelling.5


In


this case, the approach described above probably does not give accurate measurements of the heat transfer coefficient owing to the effect of fluid flow. The research is aimed at understanding and quantifying the interfacial heat transfer coefficient that applies during the filling of a mold with a liquid alloy under normal gravity conditions and its fea- sibility under microgravity environment. By carrying out these measurements in a microgravity environment, fluid flow processes caused by pouring or by natural convection, which would reduce the accuracy of a terrestrial measure- ment, would be avoided. However, duplicate experiments should be carried out in normal gravity conditions to assess the actual effect of natural convection on the accuracy of the measured heat transfer coefficients in each case. The values of the heat transfer coefficients thus obtained would be of direct use to casting solidification modelers attempt- ing to determine the heat lost during filling of the mold but are currently hampered by a lack of this information. An


International Journal of Metalcasting/Fall 2011


63


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