Influence of cooling rate on the solidification behavior and microstructure of IN738LC superalloy

To optimize the casting procedure, the effect of zircon and alumina ceramic molds on the microstructure, macrostructure and the solidification behavior of IN738LC superalloy in an investment casting process were thoroughly studied. In this regard, the experimental cooling curves, secondary dendrite arm spacing (SDAS) and the grain structure of the solidified IN738LC superalloy in both ceramic molds were compared with the values derived from simulation through ProCAST software and 3-D cellular automaton-finite element (CAFE) model. Furthermore a model was developed to predict the size of SDAS versus solidification time for both alumina and zircon molds. The results showed that the grain size and γ/γ′ eutectic islands developed in the zircon mold were larger than those formed in the alumina mold, thereby revealing a good agreement with the simulation results. Also, metallographic evaluations showed that in IN738LC solidified in the alumina mold, the SDAS gradient was about 10 μm.cm-1 from the surface to the center of sample, marking a disadvantage for this type of mold, while in the zircon mold, the SDAS gradient was about 1.5 μm cm−1 in the similar direction. Quantitative analysis of SEM images also showed that during solidification, with an increase in the cooling rates from about 0.4 °C s−1 to 1.1 °C s−1, the size of γ′ precipitates decreased from about 500 nm to 290 nm, while their volume fraction during heat treatment increased from 46% to 53%. Based on more desirable microstructural features derived, alumina mold is found to be a more appropriate option for investment casting of IN738LC superalloy, compared to its zircon counterpart.