Determination of Interfacial Heat Transfer Coefficients in Investment Casting
Keith A. Woodbury, Michal Pohanka, Jonathan Woolley, and Thomas S. Piwonka
49th Annual Technical Meeting of the Investment Casting Institute
Orlando, Florida, 2001
ABSTRACT
Interfacial heat transfer coefficients between solidifying metal and mold for investment cast materials are not well known because they are difficult to obtain. Better knowledge of these coefficients will lead to improved computational models which, in turn, will result in improved mold design, sounder castings, reduced scrap and increased customer satisfaction.
In this paper we present a methodology for determining the interfacial heat transfer coefficients during investment casting of aluminum alloys. The technique is based on inverse methods, which rely on experimentally obtained temperature histories in the mold shell and the solidifying metal to "back out" the desired thermal resistance. The sensitivity analysis shows that the location of the thermocouples in the mold wall is critical to the success of the inverse calculation and that these locations must be known within 0.5 mm.
A simple cylindrical casting is used to demonstrate the method and provide data for the determination of the heat transfer coefficients. The casting is a cylinder one inch in diameter and 12 inches in length. Data are taken near the tip of the cylinder and near the midsection. Thermocouples are placed in the casting, between the first and second dipcoats, and at a position deeper in the shell between the third and fourth coats. The specific solidification process of interest is the "active cooling" process, in which the mold, once filled with molten metal, is immersed in a bath of a cooling medium, such as a fluidized bed, or bath of liquid metal having a lower melting point that the alloy, to the control solidification. Interfacial heat transfer coefficients are determined between the alloy and the mold, and between the mold and the immersion medium.
The paper will discuss the experimental methods used to gather the data, as well as the analytical and computational techniques used to determine the interfacial heat transfer coefficients.
