Effect of cooling rate on the phase transformation and stability of the mushy zone during the solidification of Waspaloy

Abstract

Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) were used to study the microstructure, microsegregation, and fluid flow tendency of the superalloy Waspaloy in the mushy zone, which had been solidified at different cooling rates. The investigation was accompanied with the calculation of Rayleigh numbers. It is found that Ti is the main segregating element and the content of Ti is the highest in the final liquid at the cooling rates of 3–6°C/min. The eta phase (η) precipitate presented in the residual liquid at the cooling rates higher than 6 °C/min is responsible for the fluctuations in the curves of Ti content. The dendrite arm spacing is found to markedly decrease with the increase of cooling rate. The maximum relative Rayleigh number occurs at 10–20°C below the liquidus temperature at a cooling rate of 1°C/min, where the mushy zone is most unstable and fluid flow is most prone to occur.

Keywords

Waspaloy / cooling rate / solidification / segregation / stability / mushy zone

References

[1] Mitchell A. Solidification in remelting processes. Mater. Sci. Eng. A, 2005, 413—414, 10.

[2] Yang W.H., Chen W., Chang K.M., et al. Segregation and solid evolution during the solidification of niobium-containing superalloys. Superalloys 2000, 2000 Warrendale, TMS, 75.

[3] Ellott A.J., Tin S., King W.T., et al. Directional solidification of large superalloy castings with radiation and liquid-metal cooling: A comparative assessment. Mater. Metall. Trans. A, 2004, 35, 3221.

[4] Wang L., Zhong B.Y., Dong J.X. Segregation and liquid density variation during solidification of Waspaloy alloy. Rare Met. Mater. Eng., 2007, 36(12): 2104.

[5] Zhong B.Y., Wang L., Zhang M.C., et al. Effect of cooling rate on the segregation and liquid density of Waspaloy alloy during solidification process. J. Univ. Sci. Technol. Beijing, 2008, 30(7): 760.

[6] Wang L., Li C.Q., Dong J.X., et al. An investigation of microsegregation and liquid density redistribution during solidification of Inconel718. Chem. Eng. Commun., 2009, 196(6): 754.

[7] Wang L., Dong J.X., Li C.Q. Effect of cooling rate on the segregation and Rayleigh number of IN718 during solidification. J. Univ. Sci. Technol. Beijing, 2007, 29(12): 1222.

[8] Wang L., Dong J.X., Liu L., et al. Solute redistribution and the Rayleigh number in the mushy zone during directional solidification of Inconel 718. China Foundry, 2009, 6(3): 208.

[9] Yang W.H., de Barbadillo J.J., Morita K., et al. A freckle criterion for the solidification of superalloys with a tilted solidification front. JOM, 2004, 56(9): 56.

[10] Long Z., Liu X., Yang W. Thermodynamic assessment of liquid composition change during solidification and its effect on freckle formation in superalloys. Mater. Sci. Eng. A, 2004, 386, 254.

[11] Aubertin P., Wang T., Cockcroft S.L. Freckle formation and freckle criterion in Superalloy castings. Metall. Mater. Trans. B, 2000, 31(8): 801.

[12] Aubertin P., Mithchell A. Liquid density inversions during the solidification of superalloys and their relationship to freckle formation in castings. Superalloys 1996, 1996 Warrendale, TMS, 443.

[13] Ramirez J.C., Beckermann C. Evaluation of a Rayleigh-number-based freckle criterion for Pb-Sn alloys and Ni-base superalloys. Mater. Metall. Trans. A, 2003, 34, 1525.

[14] Li L.C. Micro-structural Development and Segregation Effects in Directionally Solidified Ni-based Superalloy, 2002 Auburn, Auburn University, 123.

[15] Sung P.K., Poirier D.R., McBride E. Estimating densities of liquid transition-metals and Ni-base superalloys. Mater. Sci. Eng. A, 1997, 231, 189.