Enhancing hot tooling to forge high-γ ′ superalloys

Abstract

High-γ ′ nickel superalloys are increasingly employed in aircraft turbines for their remarkable high-temperature properties to enhance engine efficiency and reduce aviation emissions. These alloys are forged at temperatures approaching 1100 °C and require careful selection of process parameters to eliminate severe billet cracking and the application of proper die materials to meet the elevated temperature requirements. This research aims at appraising the existing hot die material technology to facilitate the effective hot forging of high-γ ′ alloys like Udimet 720 with increased efficiency of the forgings (in terms of the number of forged parts and minimised cracking per part). Widely applied hot tooling materials like hot work tool steels or Molybdenum alloys are severely restricted above 500 °C. The recent considerations of IN718 hot dies for Ti forgings discouraged the use of the alloy above 650 °C. This research investigates the application of low-γ ′ alloys as hot die materials for their acceptable elevated temperature capabilities and cost effectiveness compared to high-γ ′ alloys. Finite element modelling (FEM) was performed to determine optimal range of die temperatures and stresses to handle the pre-heated billets above 1060 °C for the current requirement. For the suitable die core temperature of 750 °C, the predicted ranges of temperatures were between 700 – 900 °C and stresses were between 200 – 750 MPa at various locations of the dies. Machine learning models were developed to predict optimal alloy composition, ensuring the desired short-term and long-term mechanical performances of a die. From a pool of commercially available alloys, Haynes 282 and VDM 780 were identified for their close resemblance to the predicted composition in chemistry and performance trends within the required temperature range. The alloys were subjected to a series of thermomechanical tests to replicate the complex forging cycles at the laboratory scale. The alloys demonstrated high compressive proof stresses between 700 – 900 °C, surpassing FEM predictions. VDM 780 exhibited superior proof stresses compared to H282, particularly at lower temperatures due to its high, fine bimodal γ ′ fraction. Furthermore, both the alloys displayed good thermal stability upon isothermal ageing at 700 °C. H282 maintained stability in microstructure and strength even above 800 °C, while VDM 780 experienced weakening due to dissolution and coarsening of γ ′ and a simultaneous δ precipitation. Creep testing conducted from 700 – 900 °C under stresses ranging from 250 – 750MPa revealed that VDM 780 outperformed H282 up to 850 °C, whilst H282 was more creep resistant and maintained microstructural stability even at higher temperatures exceeding 850°C. EBSD misorientation mapping provided a qualitative and a quantitative measure of the spread of creep deformation in the alloys, aligning with their macroscopic strain and highlighted the regions of stress initiations as grain boundaries, and grain boundary and intragranular carbides. Linear slip was observed at lower temperatures with secondary slip systems being activated at 800 °C and a more diffuse slip noticeable at 900 °C. TEM analysis further elucidated the dominant creep deformation mechanisms in the alloys. Stacking fault shear was prevalent at lower temperatures and Orowan looping took precedence at higher temperatures. Despite differences in chemistries and macroscopic strains, similar mechanisms were observed in the alloys under identical test conditions due to comparable mean γ ′ sizes and area percentages. The findings concluded that VDM 780 demonstrated better suitability for high-stress applications below 850 °C, while H282 exhibited superior performance at temperatures up to 875 °C for prolonged exposure times under lower stresses. However, both alloys were found unsuitable at 900 °C. The performance of these alloys was closely tied to the kinetics of γ ′, with the reduced pace of coarsening and dissolution of the phase proving to be crucial between 700 – 900 °C, which was further influenced by the alloy chemistry. Additive manufacturing (AM) presented the viable route of fabricating hybrid dies, combining economic advantages with the capability of hot forging high-γ ′ superalloys. AM trials using H282 and VDM 780 yielded successful results, with the printed specimens displaying performance trends consistent with wrought samples in both short-term and long-term testing. These insights hold potential for future efforts aimed at enhancing the forgeability of advanced turbine disc alloys.