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Large-scale preforms with microcrystalline (MC) structures are required for superplastic (SP) roll forming of turbine components. The MC type microstructures are very important for alloy 718 because its flow stress is higher than that for other nickel base alloys, even under optimum SP conditions. The high flow stress represents a significant challenge for efficient net shape forming of alloy 718 components. This paper will describe the development of regimes for generating MC structures in large-scale preforms of alloy 718. It will be shown that large-scale alloy 718 preforms (up to 150 kg) can be produced with a grain size of 2-7 pm by isothermal deformation under the correct temperature and strain rate conditions. Complex-shaped rings and disks of alloy 718 have also been manufactured by roll forming under SP conditions. The microstructure and mechanical properties of forgings and roll formed parts after thermomechanical processing will be described.

Production of Large-Scale Microcrystalline Forgings for Roll Forming of Axially Symmetric Alloy 718 Components