Finding boundary conditions: A coupling strategy for the modeling of metal casting processes: Part I. Experimental study
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I. INTRODUCTION
A moderate change in the boundary conditions imposed at the metal-mold interface in many casting simulations can severely affect the validity of the resulting numerical predictions. This is especially evident with temperature boundary conditions defined by heat transfer coefficients for a number of reasons. First, temperature boundary conditions are inherently transient during most commercial casting processes. Heat transfer coefficients are initially high and tend to drop off at lower temperatures. Second, not one, but several mechanisms of heat transfer occur at the metal-mold interface during solidification and cooling, some of which may or may not be present at any given time. In addition, the specification of a temperature boundary condition is highly dependent on the casting and mold configuration. For instance, certain metal-mold interfaces will remain in contact, while others will develop extensive air gaps depending on their orientation with respect to gravity. Finally, heat transfer coefficients are very sensitive to both mold and metal materials and the surface characteristics of each. If one wishes to minimize these errors, the next step toward unification of the complex phenomena associated with modeling casting processes is the coupling of boundary conditions among all related governing equations in a given system. The proper definition of temperature boundary conditions hinges on MICHAEL TROVANT, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto, is Process Engineer, HATCH ASSOCIATES, Mississauga, ON, Canada L5K 2R7. STAVROS ARGYROPOULOS (To whom correspondence should be addressed. E-mail: [email protected]), Professor, is with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada M5S 3E4. Manuscript submitted January 8, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
determining the effect on boundary conditions of certain process variables dictated by coupled governing phenomena. Accurate heat transfer coefficients are historically difficult to obtain experimentally for all points on the metal-mold interface, especially when the influence of thermal contraction is acknowledged. The main focus of this study is an attempt to remedy some of the pitfalls associated with the specification of heat transfer boundary conditions at the outer boundary of the metal. The ultimate goal is to allow the modeler to estimate the effect of changes in the heat loss at the metal-mold interface, and ultimately to allow the time-dependent heat transfer coefficient for a particular metal-mold system to be assessed via a numerical correlation, without resorting to a specific experiment. To achieve this goal, the processes that take place at the metalmold interface need to be examined. More specifically, the transient mechanisms of heat transfer at the interface and the formation of the air gap between metal and mold must be understood. Part I of this study will focus on the following experimental issues: (1) to de
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