Influence of solidification variables on the cast microstructure and porosity in directionally solidified Mar-M247

The solidification microstructure is critical in determining the amount and distribution of porosity that develops during the freezing of castings. As the solidification velocity V_s increases, the microstructural length scales (primary and secondary dendrite arm spacings) decrease; thus (1) the mushy zone permeability would be expected to decrease; and (2) nucleated pores would be increasingly isolated. Although the first effect would tend to increase the observed porosity, the second effect would tend to decrease the porosity. As solidification velocity decreases, the cooling rate decreases allowing additional time for carbide growth. Large carbide size may block feeding in the intercellular spaces increasing observed porosity. To better understand these competitive mechanisms, a series of controlled unidirectional experiments were performed on bars of nickel-base superalloy Mar-M247. Samples were produced with constant dendrite arm spacing throughout an extended length of each cast bar. The axial thermal gradient and withdrawal velocity imposed on each casting were varied between castings to produce a range of microstructures from aligned cellular dendritic to aligned dendritic to misaligned dendritic. Macrosegregation effects along the lengths of the bars were evaluated and the resultant impact upon the density along the lengths of each casting was also characterized. The density measurements were found to be very sensitive to both (1) compositional macrosegregation in these castings and (2) internal porosity. Statistical analyses of microporosity in castings were based upon metallographic measurements. The development of microporosity in the unidirectionally solidified castings is shown to be dependent upon the hydrogen gas content of the samples and the imposed solidification velocity through the sample's cast microstructures. An optimum intermediate withdrawal velocity of 0.005–0.01 cm/s was found, which led to closely spaced dendrite arms, a large number of very small pores and a minimum total porosity level. Lower velocities lead to increased porosity from larger pores whereas higher velocities lead to macroporosity due to centerline shrinkage.