Carbide formation in a nickel-based superalloy during electron beam solid freeform fabrication

Abstract

The Electron Beam Solid Freeform Fabrication process involves the use of an electron beam to make near-net-shape metal parts without the need for tooling. Material in wire form is fed into a melt pool maintained on the surface of the part by the electron beam and a positioning system causes the deposition to occur in a line-by-line, layer-by-layer fashion. Solidification occurs at a high rate, forming a fine dendritic microstructure and fine dispersion of primary carbides. This structure is believed to be optimal for the manufacture and safe use of certain nickel-base superalloy parts, notably turbine disks. The growth of carbide particles from the liquid during EBSFF processing of Alloy 718 has been modeled assuming diffusion control and isolated spherical carbides. The driving force for growth is assumed to increase in a linear manner throughout the temperature range of carbide precipitation. The model predicts the maximum carbide size as a function of EBSFF operating parameters and the alloy niobium and carbon levels. For the material and conditions used experimentally in this work, the model predicts a maximum diameter of approximately I .0 [mu]m. The maximum carbide size will become an important determining factor for turbine disk performance when oxide and nitride inclusions have been eliminated through improved melt practices. To illustrate this, the low-cycle fatigue life as a function of carbide size for a standard specimen geometry was calculated. Extraction replica transmission electron microscopy of EBSFF samples identified carbides in the 300-600 nm range, consistent with a population having the predicted maximum size. Another dispersion of carbides larger than 3 [mu]m was also observed in the EBSFF samples. These are believed to be original carbides that survived the EBSFF thermal cycle without completely dissolving. More thorough dissolution can probably be obtained with EBSFF process modifications. Control material from a conventional vacuum arc remelted ingot with similar composition was also examined and plate-like carbides up to 40 [mu]m in length were noted. This is an indication of the enormous potential of the EBSFF process to refine the carbide morphology and size distribution without the need for a reduction in carbon content.