Understanding forgeability in highly reinforced cast-and-wrought superalloys

Abstract

A physical mechanism is presented to describe the relationship between processing parameters, microstructural evolution, and the occurrence of surface cracking in the hot die forging of highly γ′-reinforced cast-and-wrought nickel-based superalloys. The theory postulates that surface cracking is controlled by the dissolution of the secondary γ′ phase and its subsequent reprecipitation as very fine precipitates upon cooling. The commercial alloy Udimet 720 is used as a benchmark to explore this mechanism. A novel testing method is devised to assess forgeability and surface cracking through laboratory-scale tests on a Gleeble thermo-mechanical simulator. The method mimics the die chilling effect characteristic of hot die forging. FEM simulations are employed to obtain full-scale forging data to feed this method, and thermal analysis and electron microscopy observations are used to map the relevant solvus temperatures. Remarkably, samples heated up to higher initial temperatures prior to undergoing die chilling showed more extensive surface cracking than those heated up to lower temperatures. This challenges the notion commonly accepted in industry that these alloys ought to be forged at temperatures below but close to the γ′ solvus. Post-deformation samples are examined via electron microscopy, energy dispersive X-ray microanalysis, and electron backscatter diffraction to confirm the hypothesis and explore deformation and fracture mechanisms. In addition, the investigation is extended by the said techniques to a second highly γ′-reinforced alloy – René 65 – for validation purposes. The results indicate that – along with the local mechanical conditions of the forging – die-chilling effects and forging temperatures are paramount in controlling surface cracking, as the key process variables governing the distribution of γ′. These findings and the novel forging simulation technique pave the way towards designing superalloys with improved processability and excellent performance.