Reconstruction of solidification history from cast microstructure in remelted nickel alloy 718

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).