Taguchi Design for Heat Treatment of Rene 65 Components

Rene 65 is a nickel-based superalloy used in aerospace components such as turbine blades and disks. The microstructure in the as-received condition of the superalloy consists of ~ 40% volume fraction of gamma prime precipitates, which gives such a high strength that thermomechanical processing is problematic. The goal of this study was to improve the processability of Rene 65 by developing a heat treatment to lower the strength through changes in the size distribution and volume fraction of those precipitates. Gamma prime in this alloy is observed in three sizes, ranging from a few μm to tens of nm. For the design of the heat treatments, Taguchi’s L8 matrix design of experiments was used. The four factors that are examined are cooling rate, hold temperature, hold time and cooling method to room temperature. The levels of the factors were two (high and low) with replication. Microstructures were characterized by scanning electron microscopy and mechanical properties by Vickers microhardness testing. Regression analysis on the results revealed that the most significant factor for this design is hold temperature. The softest sample and the hardest sample have a significant difference microstructurally, with the latter having a trimodal distribution of precipitates which is believed to cause the strength.