Effect of Superheat, Mold, and Casting Materials on the Metal/Mold Interfacial Heat Transfer During Solidification in Graphite-Lined Permanent Molds K. Narayan Prabhu and K.M. Suresha (Submitted October 21, 2002; in revised form March 21, 2004) Heat transfer during the solidification of an Al-Cu-Si alloy (LM4) and commercial pure tin in single steel, graphite, and graphite-lined metallic (composite) molds was investigated. Experiments were carried out at three different superheats. In the case of composite molds, the effect of the thickness of the graphite lining and the outer wall on heat transfer was studied. Temperatures at known locations inside the mold and casting were used to solve the Fourier heat conduction equation inversely to yield the casting/mold inter- facial heat flux transients. Increased melt superheats and higher thermal conductivity of the mold material led to an increase in the peak heat flux at the metal/mold interface. Factorial experiments indicated that the mold material had a significant effect on the peak heat flux at the 5% level of significance. The ratio of graphite lining to outer steel wall and superheat had a significant effect on the peak heat flux in significance range varying between 5 and 25%. A heat flux model was proposed to estimate the maximum heat flux transients at different superheat levels of 25 to 75 °C for any metal/mold combinations having a thermal diffusivity ratio ( R ) varying between 0.25 and 6.96. The heat flow models could be used to estimate interfacial heat flux transients from the thermophysical properties of the mold and cast materials and the melt superheat. Metallographic analysis indicated finer microstructures for castings poured at increased melt superheats and cast in high-thermal diffusivity molds. Keywords: composite mold, graphite lining, interfacial heat flux, superheat, thermal diffusivity 1. Introduction The use of chills during the freezing of long-freezing-range aluminum alloys enhances the rate of heat transfer from the casting to the chill material and promotes directional solidifi- cation. [1] Graphite has good thermal conductivity, and the use of graphite as a mold/chill material results in the rapid solidi- fication of the alloy being cast and imparts a dense smooth surface to the cast product. It is also possible to cast alloys with higher melting points such as titanium and copper in permanent graphite molds. [2] In addition, graphite acts as a lubricant, and, hence, graphite molds normally do not require frequent mold coating as is the case with ferrous dies. Graphite mold/chill material prevents premature solidification and does not wrap or distort during casting, since it has a low coefficient of thermal expansion compared with ferrous dies. [3] In summary, the use of a graphite lining for a metallic mold has the following sig- nificant advantages. [4] • Graphite provides a nonwetting surface for the casting and serves as a reservoir for the casting lubricant. Since it experiences little wear, it reduces mold maintenance, lead- ing to longer mold life. • Water cooling of the outer die provides good heat transfer rates. • Graphite is, however, a brittle material, and in composite molds the presence of an outer metallic layer enables the graphite lining to resist mechanical shock during handling. • Graphite facilitates the easy replacement of the lining and, hence, leads to lower maintenance costs. Furthermore, graphite has a low coefficient of thermal ex- pansion (4.65 × 10 -6 /°C), and this results in high thermal shock resistance. Graphite-lined composite molds are currently being explored for the direct-chill and continuous casting of non- ferrous alloys, for the casting of titanium and titanium-based alloys, for the gravity and die casting of aluminum- and zinc- based alloys, for the up-casting of brass and copper, and for the casting of rail wheels. [5,6] Sully [7] studied the effect of casting size, casting alloy, mold material, and mold geometry on the heat transfer coefficient and concluded that the mold geometry affects the heat transfer coefficient significantly, while the mold material and casting alloy have only a small effect on it. Further, the size of the casting controls the temporal variation of the heat transfer co- efficient. While the casting surface temperature has a large effect on heat transfer coefficient, the mold temperature has little effect on it. Nishida et al. [8] determined the heat transfer coefficient for pure aluminum and an Al-13.2Si alloy that were cast into graphite-coated molds with about 0.01 mm of graph- ite. It was found that the level of mold constraint had a large effect on the heat transfer coefficient, while alloy composition had only a minor effect. Strezov and Herbertson [9] studied the heat transfer charac- teristics between a copper substrate embedded in a flat wedge K. Narayan Prabhu and K.M. Suresha, Department of Metallurgical & Materials Engineering, National Institute of Technology Karnataka, Surathkal, P.O. Srinivasnagar 575 025, India. Contact e-mail: prabhukn_2002@yahoo.co.in. JMEPEG (2004) 13:619-626 ©ASM International DOI: 10.1361/10599490420647 1059-9495/$19.00 Journal of Materials Engineering and Performance Volume 13(5) October 2004—619