As-cast Structure and Homogenization of Nickel-based Superalloys for Aerospace Applications

For jet engine applications, where both high-temperature strength and oxidation resistance are required, nickel-based superalloys have been the benchmark material for decades and are unlikely to be replaced soon.

The finished components are the result of multiple processing steps, starting with the primary melting itself and remelting of the electrode. These remelted ingots are subsequently forged into forging billets of various dimensions, which then serve as the starting material for the actual forging of the component. Stringent requirements are imposed on the billets regarding the chemical homogeneity and uniform distribution of precipitates which could not be met by starting with an as-cast ingot due to its significant segregations and brittle phases formed during solidification.

For this reason, remelted ingots are typically subjected to a homogenization treatment. These heat treatments, some of which last for days, must be precisely tailored to the specific alloy to dissolve undesired segregations and precipitates while avoiding incipient melting, which would be detrimental in the subsequent forging steps.

The present study focuses on the homogenization of ingots made of nickel-based superalloys on an industrial scale. We investigated the solidification behavior of ingots with diameters of up to 600 mm for well-established superalloys such as IN718 (UNS N07718), Waspaloy (UNS N07001) and Udimet 720 (UNS N07720) by also considering the big differences between the inner and the outer parts of the ingots. Emphasis is placed on the feasibility of such investigations in an industrial environment, highlighting the challenges associated with experiments on multi-ton ingots—such as sample extraction—and showing possible solutions. Furthermore, the response of these alloys to homogenization treatments at different temperatures and durations is documented. In this context, methods are shown to mimic the large-scale homogenization heat treatments in the laboratory and evaluating the effectiveness of the homogenizations using Energy Dispersive X-ray Spectroscopy.