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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - PROSIT (Integration of a property simulation tool for virtual design and manufacturing of forged disks for aero engine applications)

Teaser

Summary

The objective of PROSIT is the improvement of an integrated simulation chain under consideration of the effect caused by local inhomogeneity after billet processing and non-homogeneous microstructure on the material properties after the thermo-mechanical processing of direct aged alloy 718 aerospace turbine discs. The microstructure inhomogeneity after billet processing is believed to have an effect on the material properties even of the final disc. In this project the effect caused by billet processing 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 duplex microstructure that needs to be investigated in this project as well. 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.
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. The main reason behind the variation is assumed to be the highly asymmetric billet processing. As such the billet processing is investigated in this project in term of local inhomogeneity of the microstructure (grain size distribution as well as size, morphology and area fraction of precipitates). The billet inhomogeneity is implemented in 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. Therefore 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 should be optimized in regards to prediction of mechanical properties of turbine disks with focus on the direct age effect.

Work performed

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 â€“ especially all relevant strengthening mechanisms and the direct age effect â€“ and the low cycle fatigue limit, hence, proving the applicability of the simulation chain for the calculation of mechanical properties. Furthermore the simulation of the billet processing was assessed with the emphasis on the material models for recrystallization during radial forging. The assessment showed that the currently used recrystallization model is insufficient to describe the occurring mechanisms during radial forging in the simulation. Therefore the simulation based calculation of the grain size canâ€™t be used for the course of this project. However, the simulated local strain, strain rate and temperature data will be used in the subsequent simulation chain.
In order to analyse the circumferential variation high resolution measurements were performed and fundamental material mechanisms were investigated. Basic differences between the direct aged condition and standard aged condition were analysed 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.
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 â€“ especially after multiple forging operations â€“ the model canâ€™t be used. An alternate modelling approach was therefore developed using the velocity of meta-dynamic recrystallization to calculate regions that include non-homogeneous grain structures.This modelling concept was used to design a forging process for pancakes with a distinct duplex grain size region. Based on the results of the pancakes the effect of duplex grain regions on mechanical properties will be quantified and implemented in adequate models.
In the second reporting period the new model was used to design the processing route of a turbine disk. Some turbine disks were then forged according to the simulation results. In order to validate the developed model the 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 (e.g. non-uniform grain structure). In order to be able to make quantitative predictions, the originally planned model will be implemented into the simulation chain during the last period of the project. This model will then be validated on the basis of forged turbine disks.

Final results

The final results of the project PROSIT will 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 will be developed which is capable of considering duplex 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. Due to shortcomings of the currently used model the billet inhomogeneity will be implemented in the simulation chain except for the grain size variation. The simulation chain will 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 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.