Improved Quality and Economics of Investment Castings by Liquid Metal Cooling: The Selection of Cooling Media

A new directional solidification technique promising a more economic production of large blades is the Liquid-Metal-Cooling (LMC) process. The advantages of this technique are summarized in the paper. Direct temperature measurements as well as microstructural comparisons point towards an increase of the thermal gradient by a factor of two for large industrial turbine blades. This is at the present state of technical development, i.e. further improvements are expected eventually. One of the most important factors in optimizing LMC is the choice of the cooling medium. In this paper tin and aluminium are compared. The advantages and disadvantages of these two cooling media are discussed in detail. Special attention is directed to the heat transfer potential and the possible dissolution of the cast component in the cooling bath in case of inadvertant contact. In addition information on the effect of tin as an intentional alloying element is given. Although tin looks rather favourable with respect to the points discussed, the long term performance on an industrial scale as compared to aluminium remains to be seen. Introduction Higher demands on turbine blades in modern industrial gas turbines (IGT’s) have led to increasing use of directional solidification. The trend towards larger components and more complex alloy compositions reveales the limitations of the conventional Bridgman-technique (High-Rate-Solidification technique, FIRS). Due to the limited temperature gradient only low withdrawal rates can be applied. An economic production based on radiation cooling becomes therefore quite difficult [l]. A promising alternative is the Liquid-Metal-Cooling (LMC) process [2,3]. LMC is expected to provide important improvements and may play a significant role in the industry in the near future [4]. The main advantages of the LMC technique have been well demonstrated for small scale, laboratory test pieces [Sj. The higher thermal gradient and the increased solidification rate results in a finer microstructure as compared to the conventional Bridgmantechnique. This reduces significantly the solution heat treatment soak period. Superalloys 2ooo Edited by T.M. Pollock, R.D. Kissinger, R.R. Bowman, K.A. Green, M. McLean, S. Olson, and J.J. Schina TMS Ghe Minerals, Metals &Materials Society), 2OtXt