Precipitation of γ′ in Two γ-γ′ Ni-Based Superalloys During the Solvus Transition Stage of Ingot to Billet Conversion: Effects on γ Grain Structure and Implications for Open Die Forging

Udimet®720Li andAD730 superalloy AD730®AD730® are γ-γ′ Ni-based superalloysNi- based superalloys manufactured through casting and wroughtWrought processingProcessing, i.e., using ingot-to-billet conversionBillet conversion (UDIMET is a registered trademark of Special Metals Corporation. AD730 is a registered trademark of Aubert & Duval). These alloys are intended for use in safety critical aeroengine components, and there are strict requirements on the microstructural characteristics they must achieve at the end of the conversion process. Conventional ingot-to-billet conversionBillet conversion is an expensive and complex process, requiring multiple open-die forgingForging operations and reheating steps to achieve a homogeneous microstructureMicrostructure. A main goal of this conversion process is to refine the as-cast grain γ structure, which comprises grains centimetres in size, down to a grain sizeGrain size of approximately 20 μm. The present work studies the microstructural evolutionMicrostructural evolution of Udimet 720Li and AD730 billet material at the “solvus transition” stage of the conversion process, i.e., the controlled cooling through the solvus temperature. The formation of γ′ prime precipitatesPrecipitates during controlled cooling from supersolvus temperatures, and their interaction with the γ grain structure is the focus of this work. It is shown that the size and morphology of the γ grains in both alloys are significantly affected by discontinuous precipitationDiscontinuous precipitation during cooling through the solvus temperature, and a method to exploit this during industrial ingot-to-billet conversionBillet conversion is suggested.