An analytical model for optimal directional solidification using liquid metal cooling
- PDF / 288,789 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 79 Downloads / 210 Views
INTRODUCTION
DIRECTIONAL solidification relying on radiation cooling has been used for approximately 3 decades to produce turbine blades for the most demanding high-temperature applications. During this period, significant advances in processing technology have been made allowing the production of larger, more complex blades at a lower scrap rate. Although the field has reached an advanced state of maturity, a number of problems still remain, particularly in the casting of larger turbine blades. These problems include (1) the maintenance of a sharp and well-oriented temperature gradient at the solidification interface, (2) reactions between the melt and the ceramic shell, and (3) deformation and fracture of the ceramic shell. Conventional directional solidification relies on heat transfer by radiation through a vacuum, a mechanism which is relatively inefficient. The limited heat transfer results in low longitudinal temperature gradients, which, in turn, require that low withdrawal rates be used so as to maintain the desired columnar or singlecrystal grain structure. Consequently, the throughput rate is reduced and the time increases in which shell/melt reactions and shell creep can occur. In conventional directional solidification, the rate of cooling is also strongly view factor dependent, which can lead to uneven cooling, particularly in the casting of turbine blade clusters. In practice, the spacing between turbine blade castings must be kept large in order to guarantee even cooling for every casting. This decreases the number of blades that can be cast per run and increases the cost per blade accordingly. Due to the limitations of conventional directional solidification, there is increasing interest in using liquid metal cooling for the directional solidification of turbine blades. In the liquid metal cooling (LMC) process, castings are withdrawn from a hot zone through a baffle and are simultaneously immersed into a bath of liquid cooling metal. The liquid metal bath removes heat from the casting by conduction, and cools the casting approximately 3 times T.J. FITZGERALD, formerly with the Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Germany, is an Advanced Engineer with the Westinghouse Electric Corporation, Combustion Turbine Engineering, Orlando, FL 32826-2399, USA. R.F. SINGER, Professor and Institute Director, (Institute WTM) is with the University of Erlangen-Nuremberg, D-91058 Erlangen, Germany. Manuscript submitted March 21, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
more effectively than with radiation cooling.[1] The improved heat transfer has two main effects: (1) it sharpens the temperature gradient at the solidification interface and (2) it allows the withdrawal rate to be increased.[1–4] The LMC technology should allow turbine blade clusters to be cooled more evenly. Cooling metal can freely flow between castings, which should allow castings to be packed at a higher density within clusters. Intense work on LMC has been carried out recently at a number
Data Loading...
