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Integration of a property simulation tool for virtual design and manufacturing of forged disks for aero engine applications

Periodic Reporting for period 3 - PROSIT (Integration of a property simulation tool for virtual design and manufacturing of forged disks for aero engine applications)

Reporting period: 2019-08-01 to 2020-07-31

The main objective of PROSIT is the development and improvement of an integrated simulation chain for the prediction of material properties after thermo-mechanical processing of direct aged alloy 718 turbine disks. This simulation chain should enable the calculation of the final microstructure and its effect on the mechanical properties. The idea is that the material models should also consider the effect caused by local inhomogeneity in the billet. The output of the simulation chain should be the prediction and quantification of the local microstructure of the whole turbine disk and the corresponding local mechanical properties.
In this project, the effect caused by billet inhomogeneity is investigated especially when it comes to circumferential variation of the yield strength in a rotationally symmetric disc. Another critical issue is the effect on mechanical properties caused by non-uniform (e.g. duplex) microstructure that was investigated in this project. The improved simulation chain will enhance the development process of new parts in respect to time, cost and quality. This allows a further optimization of the forgings in respect of weight, efficiency and CO2 reduction of modern aircrafts.
The main focus in this project is the improvement of the simulation chain in regards to applicability and the quality of material models for the calculation of mechanical properties. The variation of the yield strength in circumferential direction of a disc is a phenomenon discovered in a previous Clean Sky project INTFOP, where the metallurgical reason behind the variation is unclear. Therefore, several effects, including the billet processing, were investigated in this project. The billet inhomogeneity is implemented into the simulation chain and the effect on the mechanical properties of the final disk is analyzed by comparison between simulation and actual forgings. Furthermore, the effect of duplex microstructure on mechanical properties is investigated using simulation and testing. A suiting material model for the calculation of duplex grain structures has to be developed and integrated in the simulation chain. Finally, the enhanced modelling and simulation approach was optimized in regards to prediction of mechanical properties of turbine disks with focus on the direct age effect.
The emphasis in the first half of the project was the assessment of the simulation chain, the simulation of the billet processing, the analysis of the circumferential yield strength variations, the implementation of the grain class model for the simulation of duplex grain structures as well as the design and forging of duplex discs.
During the assessment, the simulation chain was applied for the design of new direct aged forgings showing reasonable results for the calculation of the yield strength and the low cycle fatigue limit. The simulation of the billet processing was assessed with the emphasis on the material models for recrystallization during radial forging. It was shown that the used recrystallization model is insufficient to describe the occurring mechanisms during radial forging in the simulation. However, the simulated local strain, strain rate and temperature data was used in the subsequent simulation chain.
In order to analyze the circumferential variation high-resolution measurements were performed and fundamental material mechanisms were investigated during the whole project (all three report periods). Basic differences between the direct aged condition and standard aged condition were analyzed in detail. Systematic effects involving dislocation structures, precipitation kinetics and the interaction between dislocations and precipitates caused by a multi-blow forging were observed and used to develop a hypothesis on the effects causing circumferential yield strength variations. This knowledge is of scientific interest and very valuable for BSTG as a forging supplier of critical parts. The results were published and presented at scientific conferences.
Furthermore a grain class model to describe the duplex microstructure was implemented in the simulation tool. Due to an incompatibility of the model with the simulation tool caused by interpolation after remeshing the model could not be used. An alternate modelling approach was therefore developed using the meta-dynamic recrystallization to predict regions with non-homogeneous grain structures.
In the second reporting period this model was used to design the processing route of a turbine disk. In order to validate the developed model the forged disks were cut up and microstructure as well as mechanical properties were investigated. It could be shown that the model is only suited for a qualitative prediction of the microstructure. In order to be able to make quantitative predictions, the originally planned grain class model was developed as well in a parallel project.
During the third reporting period the grain class model was parametrized and implemented into the automated simulation chain. Further turbine disks were designed, forged and fully investigated to validate both developed models. By the end of the project, it could be shown that both models are suited for the prediction of mechanical properties. The first model can give an overview qualitatively of the regions that are endangered of the occurrence of non-uniform grain structures. The second model is also able to quantify the microstructure (e.g. grain size, fractions). The use of the models is a very powerful tool to improve the production route of turbine disks and to make them even safer in use. Therefore, the models will be used at BSTG for the design of turbine disks. The modeling and simulation results were published and presented at scientific conferences.
The final results of the project PROSIT provide a better understanding of material effects during billet processing and closed die forging of direct aged alloy 718 turbine discs. Furthermore, an automated modelling and simulation chain was developed which is capable of considering non-uniform grain structures and an optimized forging window for a distinct direct age effect. The simulation aided study of the billet processing provides an excellent opportunity to implement a specific inhomogeneity of the billet in the design of forgings. The simulation chain could be improved and extended providing information on duplex microstructure and the contribution of non-homogeneous microstructure on mechanical properties.
The automated simulation chain was established in order to simplify the usability of the simulation for the design of new disc forgings as well as to make process variation studies without additional personal resources possible. The process variation study can be applied to analyze the stability of the developed process in terms of sensibility to process/parameter variations. Furthermore, process variation studies can be used to identify an optimal combination of manufacturing parameters leading to optimized material conditions and mechanical properties.
The better understanding of the material mechanisms during the direct age process will be used to further improve the modelling approach leading to an even better quality and reliability of the simulation. It can be expected that the design process of new forgings will greatly benefit from the improved simulation chain in terms of time, costs and quality leading to sustainable economic advantage in the aerospace industry.
Automated simulation chain for the optimization of mechanical properties of aerospace turbine discs