Browsing by Subject "Superalloy"

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A method for the characterization of white spots in vacuum-arc remelted superalloys
(2011-12) Viosca, Alan Lee; Taleff, Eric M.; Beaman, Joseph J.
Vacuum-Arc Remelting (VAR) is an important process for manufacturing Ti- and Ni-based superalloys. Currently, the sources and mechanisms behind microstructural anomalies produced in VAR superalloy ingots are not well understood. In order to help understand formation processes, a method of characterizing specific anomalies in VAR ingots is desired. This paper presents a method of characterizing the composition and morphology of anomalies in VAR alloy ingots using a combination of serial sectioning and X-ray fluorescence (XRF) energy dispersive spectroscopy (EDS) techniques. This process is demonstrated on a dirty white spot from an Alloy 718 sample. The white spot of interest was serial polished and 2-D XRF EDS maps were acquired at each polish depth. The EDS maps were then stacked to form a 3-D representation of the white spot. In addition, SEM and optical microscopy techniques were used to further characterize the composition and morphology of the dirty white spot. The dirty white spot is composed of both Ti-enriched and Nb-depleted regions. The 2-D EDS maps acquired with the XRF equipment provided adequate contrast for creating a 3-D representation of the Ti-rich region of the dirty white spot. However, contrast was not sufficient to create a 3-D representation of the Nb-depleted region. The XRF EDS equipment combined with SEM and optical microscopy techniques provided valuable information about the morphology and composition of the Alloy 718 dirty white spot. It is concluded that this dirty white spot was produced by fall-in from either the crown or shelf regions during the VAR process.
Reconstruction of solidification history from cast microstructure in remelted nickel alloy 718
(2017-09-12) Ivanoff, Thomas Alexander; Taleff, Eric M.; Kovar, Desiderio; Bourell, David; Engelhardt, Michael; Watt, Trevor
Digital image analysis techniques were developed to autonomously characterize dendritic solidification microstructures and estimate melt pool profiles and solidification rates in remelted nickel alloy 718 ingots. Automated macrophotography was used to image dendritic microstructures in etched ingot cross-sections and create large image montages. Two analysis techniques, particle identification and two-point correlation function analysis, were developed to measure primary dendrite arm orientation and secondary dendrite arm spacing from these digital image montages. Particle identification techniques identified individual primary dendrite arms from the montage images. Primary dendrite arm orientations were measured from the geometry and location of the identified particles. A peak-counting technique was then implemented to measure secondary dendrite arm spacing after primary dendrite arms were identified. Two-point correlation functions were used to measure average primary dendrite arm orientations and secondary dendrite arm spacings from controlled image areas. Fourier analysis was then used to measure the primary dendrite arm orientation from the two-point correlation function. A peak-counting technique was used to measure secondary dendrite arm spacing after primary dendrite arm orientation was measured. The results produced using both analysis techniques were used to estimate melt pool profiles and solidification rates in a remelted alloy 718 ingot. Melt pool profile and solidification rate histories were calculated from primary dendrite arm orientations and secondary dendrite arm spacings, respectively. The techniques developed in this dissertation provide new technology and data needed by industry to validate computational process models of remelting processes such as electro-slag remelting (ESR) and vacuum-arc remelting (VAR).
Single-Layer Deposits of Nickel Base Superalloy by Means of Selective Laser Sintering
(2002) Ramos, Jorge A.; Murphy, Jeremy; Lappo, Karmen; Wood, Kristin; Bourell, David L.; Beaman, Joseph J.
Single layer deposits consisting of Mar-M 247 powder were consolidated on Alloy 718 polycrystalline flat sheets by means of the Selective Laser Melting (SLM) technique. The deposition process consisted in selectively fusing a powder bed precompacted using of an insulating non-wettable mask trough. The mask-powder-substrate arrangement was pre-heated inside a processing chamber under high vacuum. An inertreducing atmosphere was applied, and a focused Nd:YAG laser beam was then raster scanned at high speed along the contour of the trough, providing energy to induce a melting front to propagate along the powder as well as over the surface of the plate. As the laser beam moved forward a solidification front was left behind with a metallurgical bond between the substrate and the deposited layer. Optical microscopy revealed that epitaxial growth occurred in the [001] direction from the melted substrate to about half the height of the deposits. Above that height, the [001] dendritic front bowed towards the [100] direction. A dendritic-equiaxed transition appeared near the free surface of the deposit. These results indicate that the SLM technique could have potential application for growing single crystal structures as well as in repairing damaged or worn turbine blades.