The strong need for higher efficiency, reduced CO2, NOX emissions, weight and noise reduction in aircraft engines leads to a demand of innovative materials with optimised mechanical and physical properties. The special design of new generation geared turbofan aircraft engines with their faster rotating LPT leads to higher temperatures in the turbine, casing and engine mount and thus requires parts with increased high temperature properties. High temperature strength means in most cases bad forgeability and weldability as well as combined with high toughness challenging machinability. Thus, beside of new designs the production processes have to be altered to get high quality parts.
The overall goal of this project was an improved understanding of thermomechanical processing and its effect on residual stresses and distortion as well as microstructure and mechanical properties of forgings used for improved temperature exhaust cases.
The proposed project consortium had significant experience with regard to nickel base superalloys with higher temperature capability than Inconel718 like Udimet720, Waspaloy, Allvac718Plus, RENE65 and Haynes282. Together with the know-how on residual stress simulation and measurement established in several projects since 2001 a successful realisation of this project was possible.
Beside project management according IPMA standards six further work packages had been defined. One for radial forging and one for closed die forging was used to optimise thermomechanical processes based on simulation and to produce demonstrator parts. Open die forgings were used for residual stress and microstructure investigations. Material data for finite element simulation and residual stress modelling were generated in one work package and verified together with the customer in an other. Residual stress modelling was verified by neutron diffraction measurements and other methods in the final work package.
The strong need for higher efficiency, reduced CO2 and NOX emissions, weight and noise reduction in aircraft engines leads to a demand of innovative materials with optimized mechanical and physical properties. The new generation of aircraft engines with their geared turbofan concept incorporate these mentioned demands. The special design of those engines with their faster rotating low pressure turbine leads to higher temperatures in some areas of the turbine, casing and engine mount and therefore requires parts with increased high temperature properties. These high temperature properties are with regard to processability a disadvantage. High temperature strength means in most cases bad forgeability and weldability as well as combined with high toughness challenging machinability. Thus, beside of new parts the production processes have to be altered in order to get best quality with optimized costs.
The project partner Bohler Edelstahl GmbH & Co KG (hereafter: BEG) produces among others billets as well as milled semi-final products made of different double or triple melted nickel base alloys. The recent investment in the world biggest rotary forging machine offers the possibility to produce billet material in less steps and additionally higher yield. Therefore two promising materials usable for higher temperature casing and mount parts have been chosen for conversion trials. It was possible to produce both materials according to a draft spec. with a high yield rate.
Böhler Schmiedetechnik GmbH & Co KG (hereafter: BSTG) as specialist in thermomechanical processing of these materials signs responsible for the design, production and testing of open and closed die forgings. The influence of different forging and heat treatment parameters have been evaluated in accordance with requirements defined by the customer. Microstructure, tensile, stress rupture and low cycle fatigue properties have been investigated as a function of processing parameters.
Due to the narrow forging windows of nickel base alloys and the high mechanical properties necessary in the final product the knowledge of micro- and nano-structural changes during themomechanical processing as well as residual stresses introduced due to thermal gradients are of high importance. One possibility to estimate these properties can be achieved by using finite element (FE) simulation. Therefor the project partner Lehrstuhl für Werkstoffkunde und Werkstoffmechanik TU München (hereafter: WKM) had generated material data in order to guarantee an accurate finite element modelling of the processes. In addition experiments to investigate the influence of thermomechanical processing on microstructure and residual stresses had been performed. Finally the FE models had been validated using full scale demonstrator parts.
As a matter of this the main goal of an improved high efficient design and production process of thermomechanical processed jet engine components out of alloys with higher temperature capability had been achieved.