Pulse Current Assisted Heat Treatment for Impeller Disks

Advanced turboshaft engine impeller disks work at a high rotational speed at elevated temperatures which vary from the disk rim to its bore. Due to contact with combustion gases, the rim possesses a significantly greater temperature than the bore. This temperature gradient implies different requirements for the mechanical properties of the material in the rim and bore areas. A nickel based superalloy at the rim should have coarse grain microstructure to provide high creep resistance, while in the bore the microstructure needs to be fine to ensure high tensile properties. In order to provide such a dual microstructure, a technique which is known in powder metallurgy as plasma assisted treatment will be used. High current pulses with 40-60 Hz frequency will be applied to the impeller disk rim only, while the disk bore will be forcedly cooled. Micro plasma discharges on all the flaws inside the impeller disk rim facilitate mass transfer, motion of grain boundaries and grain growth, while the microstructure of the bore remains intact. A significant pulse current assisted grain growth in a high temperature superalloy has already been demonstrated experimentally.