Metal-Mold Interfacial Heat Transfer During the Solidification of Metal Castings.

The present work identifies the mechanisms of heat transfer at a metal-mold interface during the solidification of a metal casting. It is proposed that in the absence of externally applied pressure such as in die castings, the interface between the metal and the mold or chill may stay in nonconforming contact along isolated asperities, or may be completely separated by an interfacial clearance gap. These two boundary conditions have been studied experimentally using two types of idealized castings which differed in the tendency for gap formation but resembled each other with respect to the flow of heat in one dimension. The casting design for studying heat transfer across an interfacial gap consisted of a top chill and a bottom gate. Three independent methods were applied for the study of transient metal-chill boundary conditions: (1) numerical solution of the inverse heat conduction problem, (2) measurement of the variation of interfacial gap size with time, and (3) detection of electrical continuity across the metal-chill interface. It was demonstrated that the sudden onset of gap growth can be detected by abrupt changes in these measurements. For the case of a contacting interface, the casting design utilized a bottom chill and a top gate. Only methods (1) and (3) above were applied due to the expectation of a contacting interface. Solution of the inverse heat conduction problem showed that the interfacial heat transfer coefficient exhibited an initial rapid drop but stayed relatively constant during the later stages of solidification. Finally, the influence of interfacial heat transfer on solidification time was demonstrated by simulation of castings of aluminum against metallic molds of copper or steel, as well as dry s and mold. The results illustrate the applicability and limitation of existing simplified treatments, such as Chvorinov's rule, on the prediction of solidification time.Ph.D.Engineering, Materials scienceUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/160532/1/8512427.pd