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Development of an advanced design and production process of High Temperature Ni-based Alloy Forgings

Final Report Summary - HITNIFO (Development of an advanced design and production process of High Temperature Ni-based Alloy Forgings)

Executive Summary:
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) has generated material data in order to guarantee an accurate finite element modeling of the processes. In addition experiments to investigate the influence of thermomechanical processing on microstructure and residual stresses have been performed. Finally the FE models have 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 has been achieved.
Project Context and Objectives:
Work package 1:
Successful project management, controlling of schedule, resources, costs, risks and
Work package 2:
Production of billets with better microstructure and mechanical properties and consequently better NDT capability and further processability. Purchasing of prematerial. Planning of convertion trials.
GFM forging of two different alloys. Ultrasonic testing of the forged billet material. Microstructure and mechanical testing of the converted billet material.

Work package 3:
Purchasing of prematerial. Design and modeling of open die forging process. Testing of material forgeability on easy geometries. Evaluation of process influences on microstructure and mechanical properties. Production of trial material for residual stress measurement and verification of simulation results.

Work package 4:
Purchasing of prematerial. Design and modeling of closed die forging. Design of an efficient thermomechanical process to produce demonstrator parts. Production of demonstrator parts with optimized residual stress state and dimensional quality. Production of demonstrator parts with conforming microstructural and mechanical properties.

Work package 5:
Generation of material data for elasto-plastic and plastic finite element simulation. Preparation of specimens. Verification of existing data. Literature recherché. Measuring of flow stress data. Measurement of thermophysical data. Measuring of elastic data. Testing and verifying of new material data.

Work package 6:
Simulation of residual stresses in simple geometries and trial forgings.
A clear and efficient simulation and data transfer process.
Work package 7:
Verification of residual stress simulation using advanced measurement techniques. Planning of neutron measurement. Residual stress measurement by neutron scattering. Residual stress measurement using other methods.

Project Results:
Work package 2:
A RCS 10’’ billet has been ordered by the producer and received in June 2012. A production plan as well as a testing plan (for mechanical properties, macro- and microstructure and NDT) was created for two nickel base alloys.
The thermomechanical process and testing was performed according to the deliverable 2.2 “Billet Conversion TestingPlan”. The achieved results were presented and discussed at the project meeting on 25th of September 2012.
All agreed steps/work were finished end of September 2012 and the material was delivered for closed die forging trials in October 2012. (Deliverable 2.4)

Work package 3:
Prematerial for a first open die forging trial has been ordered and received in April 2012
A DOE for the open die trial as well as a testing plan for mechanical properties and microstructure has been defined (Deliverable 3.3).
8 open parts have been forged according DOE but where subsequently scraped by mistake.
A new extended trial on 11 parts has been performed according updated DOE. All forgings have been tested and the results have been evaluated.
Annealing trials to investigate change of microstructural properties with time and temperature have been performed on two different prematerial conditions.
As a result of these trials it was possible to identify the main influence parameters on mechanical properties like yield strength, elongation and stress rupture, thus an optimized thermomechanical processing becomes possible.
One part has been used for verification of residual stress simulation.

Work package 4:
A risk assessment of the production of closed die forgings out of a new alloy has been generated. Standard 6“ prematerial for closed die forging trials has been ordered and received in April 2012. Additionally GFM forged 6” Material has been delivered to BSTG (see WP2).
Closed die forging processes have been designed and simulated in accordance to GKN drawings and specifications. (Deliverable 4.3)
A process sheet as well as a testing plan for the first closed die forging trial has been defined together with GKN.
Dies for the forging process have been produced.
A first trial lot of 12 parts in two alloys with a variation in the thermomechanical process has been produced and tested according defined DOE.
As a matter of the results one alloy has been chosen and further trials using the same die design as well as trials with other geometries have been defined to further optimize the mechanical properties of the parts. Therefore a cost neutral extension of the project had to be planed.
All parts have been produced and tested according to plan. (Deliverable 4.5)
Three parts have been forwarded to the partner for residual stress measurement.
The influence of the production process parameters (Temperatures, strokes, cooling media, etc.) on the microstructure and mechanical properties is in accordance to the results of WP3.
Based on these results a further optimization of the coming serial production is possible.

Work package 5:
A recherché of literature on materials data has been performed.
Rastegaev specimens for measurements of flow stress data have been machined at BSTG.
Flow stress measurements at 7 different temperatures and 2 different strain rates have been performed at IBF Aachen. Measurement of thermophysical material parameters (heat capacity and thermal conductivity) were carried out at IWM Aachen. A correction of the recorded flow curves (IBF Aachen) was accomplished to compensate the effects of sample heating due to plastic deformation to obtain a set of isothermal (corrected) flow curves.
Metallographic investigations of the deformed Rastegaev specimens have been made at WKM to assess the microstructural state. Special focus was laid on the grain size distribution to discuss the recrystallization behavior (in dependence of temperature and strain rate), which can be applied (by BSTG) to adjust the parameters of models describing the mircrostructure evolution.
The flow curve set and the thermophysical data completed the already existing material data and provide the basis of all FE-simulations of the residual stress evolution within WP6. (Deliverable 5.6)
A verification of the material data was performed by comparison of the simulations results with the experimental results processed in WP7. These results have been reported at the project meetings and the Master Thesis of J. Seidl.

Work package 6:
Finite element models for an efficient simulation of residual stress development in simple substitute geometries have been set up and tested at WKM.
Using these models a parametric study has been performed to identify appropriate thermal boundary conditions by comparison to experimental results (WP7).
These models have been also applied to identify the measurement positions (hole drilling method, neutron diffraction) as well as to set up the dissectioning method (WP 7).
Simulations with different mesh densities have been performed to estimate the residual stress distributions in components of complex geometry. To assess the quality of these simulation results a comparison to the experimental results (neutron diffraction, WP7) has been performed.
A comparison of residual stress simulation results based on simple substitute geometries with results based on real component geometries has been performed and discussed. These results have been reported at the project meetings and the Master Thesis of J. Seidl.
A comparison of residual stress simulation results based on different FEM-codes used at university and companies has been performed.
As a result of this a simulation guideline has been prepared for the production company in order to avoid mistakes in the simulation results. (Deliverable 6.7)
A first draft for data transfer of residual stress simulation results to the customer has been prepared, further testing and optimization has to be done after project finalization.

Work package 7:
Planning of 4 point bending tests with ex-situ neutron diffraction and subsequent residual stress relaxation experiments. First trials of measurements using IN718 substitute specimens have been performed at 02-2012.
First neutron diffraction experiments have been conducted to investigate the phase composition of the alloy 1 at different stages of the standard heat treatment.
Planning of in-situ neutron diffraction measurements during tensile loading with proposal at PSI.
A proposal for in-situ measurements at the instrument POLDI at the Paul-Scherer institute (CH) has been worked out.
In-situ measurements have been performed in June 2012 at the instrument POLDI (PSI, CH).
A proposal for grating-interferometry measurements using alloy 1 at the PSI has been worked out, concerning the observation of carbides in this alloy using the described technique.
Specimen preparation for further 4 point bending tests (ex- and in-situ neutron diffraction measurements) will be machined as soon as the alloy 1 material is available in appropriate size.

A proposal for the measurement of the residual stress distribution in a forged component of HAYNES 282 alloy at the materials research diffractometer STRESS- SPEC at the high-flux neutron source FRMII (TUM) has been prepared and submitted in July 2013. In this course one main aspect addresses also to the homogeneity and anisotropy of the reference lattice parameters, providing important knowledge for future residual stress measurements via neutron diffraction on components of this Ni-base alloy. The stress measurements via neutron diffraction were performed at critical positions through the sample thickness, which were identified with the help of residual stress simulations (WP6).
Complementary stress measurements on the component and forging trials (chiabatta forging) were carried out using the hole drilling method and the dissectioning method via WEDM (wire-electrical discharche machining). Both, planning of the tests and the evaluation and discussion of the experimental data was based on the simulation results achieved within WP6. These results have been reported at the project meetings and the Master Thesis of J. Seidl.

Potential Impact:
The main impact of the project is the possibility to use a new material with higher temperature capabilities for aircraft engine parts. Due to that fact engines with higher exhaust temperature and therefore higher efficiency can be realized. Subsequently a reduction of weight of the engine and as a consequence the aircraft becomes possible. The results of this project will be used for future design of forded parts out of this alloy with low risk of deviating properties or geometrical problems due to residual stresses.
List of Websites:
Martin Stockinger, Böhler Schmiedetechnik GmbH & Co KG, 8605 Kapfenberg, Austria