An Investigation of Heat Flow in Unidirectional Solidification of Vacuum-Cast Airfoils

Abstract

In order to better understand the thermal events occurring during the unidirectional solidification of Mar M-200 (proprietary alloy of Martin-Marietta, Inc.) alloy turbine blades, an analytical thermal model was developed which permitted simulation of the solidification process. A representative turbine blade was divided into isothermal nodes, and an electrical analog thermal model was constructed and solved using a digital computer. The technique was verified by comparing calculated and measured external mold surface temperatures during solidification. The analytical thermal model showed that thermal gradients in the liquid–solid growth zone are high near the chill, and decrease as the distance from the chill increases. Temperature profiles were converted into solidification profiles from experimental data which showed the percent solid as a function of temperature in the solidification zone. The curves permit a qualitative explanation of the occasional occurrence of segregation and hot cracking in this alloy, and indicate that precautions must be taken in the solidification process to avoid these phenomena.