LMC (Liquid-Metal-Cooling) is a new technology where heat is extracted by convection and conduction by the assist of liquid metal coolant. The improved heat transfer increases the thermal gradient at the solidification front. Therefore, it becomes possible to increase the withdrawal rate and to cast larger single crystal or directionally solidified components.
The thermal field and the grain structure that obtained by the LMC process, have been simulated in three dimensions using a cellular automaton (CA) coupled with finite-element (CAFE) model in this thesis. Then the simulation results are compared with the experimental results.
It was found that the measured and the calculated temperature agreed well above 1200C during the directional solidification of single crystal castings. According to the simulation results, the cooling rate produced by LMC process is far higher than that obtained by HRS process, and the cooling rate increases with increasing the withdrawal rate. The solidification interface changes from convex to concave with increasing the withdrawal rate. At higher withdrawal rate, the thermal gradient decreases to a large extent at the edge of the platform, which gives the possibility for the occurrence of new grain nucleation in front of the crystal growth interface. In order to obtain single crystal blade, the withdrawal rate must be controlled carefully so that the solidification interface is flat or slightly convex, and the solidification of the platform proceeds gradually from inside to outside.
The microstructure simulation results of the directionally solidified industrial gas turbine blade agreed with the experimental results. The correlation between the shape of the S/L interface and the resulting grain structure has been clearly demonstrated. When the withdrawal rate is high, porosities were observed at the platform. Equiaxed grains formed at the bottom of the air foil section due to the concave S/L interface. When the withdrawal rate is low, S/L interface is relatively flat. A fairly well-defined <001> texture developed. The area of porosities at platform apparently decreased and the location of defect moved outward to the edge of the platform. No equiaxed grains were found at the air foil section.
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