Carbide formation in alloy 718 during electron-beam solid freeform fabrication

In electron-beam solid freeform fabrication (EBSFF), material in wire form is fed into a melt pool maintained on the surface of the part by the electron beam, and controlled rapid movement of the part deposits metal selectively in a layer-by-layer fashion. Solidification occurs at a high rate, forming a fine dispersion of primary carbides in Alloy 718. Growth of the carbide particles has been modeled, assuming diffusion control and a linear change in the driving force for precipitation with temperature. The model predicts the maximum carbide size as a function of EBSFF operating parameters and alloy composition. For the material and conditions used experimentally in this work, the model predicts a maximum diameter of approximately 1.0 mum. Extraction-replica transmission electron microscopy of EBSFF samples identified carbides in the 300 to 600 nm range, consistent with a population having the predicted maximum size. Control material from a conventional vacuum-arc remelted ingot was also examined, and platelike carbides up to 40 mum in length were noted. This is an indication of the potential of EBSFF to refine the carbide morphology and size distribution.