1 Copyright © 2002by ASME Proceedings of IMECE 2002: International Mechanical Engineering Congress and Exposition November, 2002, New Orleans, LA 2-20-5-1 OBTAINING TEMPERATURE DEPENDENT THERMAL PROPERTIES OF INVESTMENT CASTING MOLD Michal Pohanka Institute of Aerospace Engineering Faculty of Mechanical Engineering Brno University of Technology Technická 2, 616 69 Brno, Czech Republic pohanka@mat.fme.vutbr.cz Keith A. Woodbury * Mechanical Engineering Box 870276 / 290 Hardaway Hall, University of Alabama, Tuscaloosa, AL 35487, USA woodbury@me.ua.edu Jonathan Woolley Mechanical Engineering Box 870276 / 290 Hardaway Hall, University of Alabama, Tuscaloosa, AL 35487, USA * Corresponding author ABSTRACT An experiment is conducted to determine the temperature dependent thermal properties (k, ρc p ) of a fused silica shell commonly used as a mold material for investment castings. The mold is constructed by building up alternating layers of binder and silica. Different binder and silica are used for the inner layer, resulting in a thin region with different thermal properties that the rest of the shell. A search algorithm based on the simplex method is used to determine the thermal properties of both kinds of layers by finding the minimum of the error between measured and computed temperatures. Two approaches are used to find the thermal conductivity: steady- state and dynamic. INTRODUCTION Precision casting of an aluminum alloy is common but for some alloys difficulties are encountered. During the solidification process microporosity can occur inside the alloy. The microporosity reduces the quality of the alloy and loss of strength or even cracking can occur. This happens especially with alloys that solidify across a wide temperature range. To reduce the incidence of microporosity during casting of such alloys, a controlled solidification process is desired. Control of the solidification process might be achieved by immersion of the mold into a bath of liquid metal with a lower melting point than the aluminum. This will provide a constant cooling condition on the exterior of the mold. The metal bath can have different temperatures, and of course a higher temperature slows down the solidification process. This solidification process can be optimized if the computational model is known. The computational model requires accurate knowledge of thermal material properties of the mold. Because of the wide solidifying temperature range, the material properties must be found as a function of temperature over this range. In addition, the structure of the mold is not uniform. The inner layer where the mold is in contact with the alloy is prepared differently in order to ensure a smooth surface of the alloy. On the contrary, the rest of the mold serves as a backup part that ensures that the mold does not crack during the casting process. These two parts of the mold have different thermal properties. The remainder of this paper discusses the experiment and data analysis to determine the thermal properties of these two layers. The experiment has two facets (steady and unsteady) and will be described next. In the analysis section, two methods are discussed for determining the thermal conductivity of the material based on the steady and unsteady portions of the experiment. The volumetric heat capacity is found from the unsteady portion of the experiment only. The paper closes with a presentation and discussion of the results. NOMENCLATURE Normal c p specific heat at constant pressure, J/kg-K k thermal conductivity, W/m-K k 1 thermal conductivity of the outer mold, W/m-K k 2 thermal conductivity of fine layer the mold, W/m-K