This article describes a comprehensive microstructural characterization of an inertia friction welded joint between nickel-based superalloys 720Li and IN718. The investigation has been carried out on both as-welded and postweld heat-treated conditions. The detailed metallographic analysis has enabled the relation of hardness profiles across inertia-welded alloy 720Li to IN718 and morphological changes of the precipitates present. The work demonstrates that inertia friction welding (IFW) 720Li to IN718 results in a weld free of micropores and microcracks and no significant chemical migration across the weld line. However, substantial differences in terms of grain structure and precipitation phase distribution variations are observed on each side of the dissimilar weld. The high c¢ volume fraction alloy 720Li exhibits a wider heat-affected zone than the mainly c¢¢ strengthened IN718. Alloy 720Li displays only a small hardness trough near the weld line in the as-welded condition due to the depletion of c¢, while c †-strengthened IN718 shows a soft precipitation-free weld region. Postweld heat treatment (PWHT) of the dissimilar weld at 760°C, a typical annealing temperature for alloy 720Li, results in an overmatch of the heat-affected zone in both sides of the weld. The comparison of the as-welded and postweld heat-treated condition also reveals that IN718 is in an overaged condition after the stress relief treatment.
A potential problem in applying the laser direct metal deposition (LDMD) technique to the fabrication of superalloys is the possibility of an inconsistent microstructure and gamma-prime constituency throughout a component. Understanding the causes for these inconsistencies is a crucial step towards eliminating it and facilitating widespread application of the technique. This article uses thermocouple and pyrometric thermal monitoring of the LDMD process and optical metallographic, scanning electron microscope, and electron backscattered diffraction analyses of components built from Waspaloy to correlate LDMD process parameters and final part microstructural characteristics. Temperatures in thin wall structures show a good match to classical Rosenthal heat flow models. The Waspaloy grain morphology and orientation are found to be sensitive to LDMD power and powder mass flow rate parameters, with columnar grains forming preferentially at lower powder mass flow rates. Results cannot be explained purely in terms of established maps that relate microstructure to temperature gradient at the solidification front and its velocity. This leads to the conclusion that intra melt pool factors such as local fluctuations in temperature gradients and changes in nucleation density are significant.
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