Rolled-Only IN718 Ring-shaped Components

Final Report Summary - RORC (Rolled-Only IN718 Ring-shaped Components)

Executive Summary:
The objective of this project was to develop and validate a robust and reliable ring-rolling-only route for manufacturing IN718 alloy near-net-shaped rotating aero-engine disk components, RORC (e.g. low-pressure turbine disks). The material and mechanical response of the RORC should comply with the current microstructural and mechanical requirements demanded for close-die forged disks with an economical advantage (more efficient use of raw material and lower manufacturing costs). The project represents the collaborative effort of an experienced company specialised in ring-rolling of superalloys, Forgital (Italy), and a technological research centre, Ceit-IK4 (Spain), with great competence in thermo-mechanical processes analysis and modelling. The project responds to a Clean Sky Call for Proposals, Integrated Technology Demonstrator SAGE 3, contract nº 32597. The 36-month project started the 1st March 2013.
Development of an advanced predictive ring-rolling modelling tool was a pre-requisite for the success of the project; laboratory experiments were to be performed for developing a predictive constitutive description of the behaviour of the alloy in conditions reproducing actual ring-rolling operation sequences. From the industrial part, 5 full-size rings have been produced by rolling alone and tested back-to back with geometrically similar close-die forged (CDF) rings, as demonstrators of the competitiveness of the new process route. The results of the project highlight the potential and limits of the current ring-rolling technology and provide clues for its further improvement.
The main final result is to have demonstrated mastering of processing of IN718 turbine disks by ring-rolling alone with properties, if not yet similar, comparable to those of disks obtained by using the current manufacturing technology (close-die forging) and with significant economic advantages. Another result from the project is the development of new know-how and new engineering knowledge valid for suggesting design modifications of current mills capable of rolling valid disks well beyond the capabilities of the current largest available mills; this issue will be critical in order to cope with the requirements of future large-diameter turbo-engines according to current trends.

Project Context and Objectives:
Project context and objectives
The objective of this project was to develop and validate a robust and reliable ring-rolling-only route for manufacturing IN718 alloy near-net-shaped rotating aero-engine disk components, RORC (e.g. low-pressure turbine disks). The material and mechanical response of the RORC should comply with the current microstructural and mechanical requirements demanded for close-die forged disks with an economical advantage (more efficient use of raw material and lower manufacturing costs). The project represents the collaborative effort of an experienced company specialised in ring-rolling of superalloys, Forgital, and a technological research centre, Ceit-IK4, with great competence in thermo-mechanical processes analysis and modelling. The project responds to a Clean Sky Call for Proposals, Integrated Technology Demonstrator SAGE 3, contract nº 32597. The 36-month project started the 1st March 2013.
Development of an advanced predictive ring-rolling modelling tool was a pre-requisite for the success of the project; laboratory experiments were to be performed for developing a predictive constitutive description of the behaviour of the alloy in conditions reproducing actual ring-rolling operation sequences. From the industrial part, full-size rings were to be produced by rolling alone and tested back-to back with geometrically similar close-die forged (CDF) rings as demonstrators of the competitiveness of the new process route. The results of the project highlight the limits of the current ring-rolling technology and provide clues for its further improvement.

Partners
Ceit-IK4 Technology Centre (San Sebastian, Spain).
Ceit-IK4 is a private non-profit multidisciplinary research centre associated to TECNUN, the School of Engineering of the University of Navarra (Spain). It is located in San Sebastian (Basque Country). Its mission is to provide the industry with services through the development of medium-long term technical research projects and to form young researchers and PhD students. Ceit-IK4 is founder member of IK4, a research alliance comprising 9 local applied research institutions with a total turnover of 103 M€ and involving 1430 persons.
Ceit-IK4 has a staff of 276 employees and 56 scholarship holders (332 total personnel) and an annual budget over 14.7 M€. Among them, 116 have a PhD degree and currently 132 are PhD students (all figures are from 2011). The personnel of the Department of Materials (involved in this project) amounts to 91 persons; 67 of them belong to the staff. The Materials Department is equipped with state-of-the art facilities that cover most of the testing techniques required for experimental metallurgical research (high resolution TEM, SEM and FIB microscopies, X-ray diffraction, dilatometry, calorimetry, several techniques for chemical analyses, several techniques for mechanical testing at different temperatures, including torsion, plane strain compression at high temperature, HCF, LCF and TMF fatigue and fracture mechanics, creep, thermal treatments, powder consolidation techniques), as well as different commercial or in-house developed software for modelling and simulation of materials and processes. The Materials Department has done research for or in collaboration with several European aero-engine companies (e.g. Rolls-Royce, SNECMA, MTU, ITP). Ceit-IK4 is coordinator of RORC project and it is in charge of virtual modelling the process of ring rolling on the basis of experimentally-based constitutive descriptions of superalloy behaviour.
Forgital (Velo d’Astico, Italy).
Forgital Italy produces rings and forgings in Nickel-based superalloys, titanium, aluminum, steel and stainless steel up to 7.0 meters in diameter and 35 tons. Large rings for fan cases, LPT cases, compressor cases, rotating seals, and other cylindrical parts for turbine and compressor applications as well as nacelle and mount rings. Forgital will make available for the project trials two large Wagner RAW radial-axial ring mills with maximum separating radial/axial forces in tons of 630/500 and 800/600 and diameter-height maximum capabilities far in excess for the application (7 m-1.4 m, respectively). The main task of Forgital in RORC is the selection of full-scale industrial conditions and the production of two sets of disks of IN718 alloy by a ring-rolling route that excludes close-die forging.
Forgital Group is a partner for all the major OEMs in the aero-engine sector for their most important projects on the main aero-engines now being produced or at the design stage. As a selected supplier, the Group works exclusively with the subcontractors qualified by the OEMs and holds approvals for all the Aerospace OEMs as well as AS/EN 9100, ISO/EN 9001, NADCAP, OSHAS 18001, ISO 14001 and other third party accreditations. Forgital is a supplier of seamless rolled rings and other forged parts for GEAE, PWA, Rolls-Royce, Snecma, Techspace, ITP, MTU and Avio as well as major land based turbine and nuclear turbine manufacturers. Forgital employs more than 1000 employees in the group. Forgital Italy alone produces 100,000 tons of steel, stainless steel, nickel-based, titanium and aluminum seamless rings each rear.

Project Results:
Work performed in the project
1. A study of the state-of-the-art of the ring-rolling process of superalloys, the available capabilities of existing industrial mills and their limits, and a decision on the full-scale parts to be manufactured during the project.
2. Id., a bibliographical review of the numerical simulations of ring-rolling of superalloys and of constitutive descriptions of the IN718 behaviour in the hot forming conditions expected in ring-rolling.
3. Procurement of certified raw material in shape and quantity sufficient for the planned project tasks.
4. Laboratory tests in a wide range of temperatures and strain rates and their analysis for obtaining microstructure-informed constitutive equations of the IN718 alloy.
5. Implementation of the constitutive equations in a commercial FEM code and simulation of the ring rolling with the geometry and conditions of the operations performed by the industrial partner.
6. Manufacturing by rolling-alone, heat treatment and delivery of a first set of three full-size rings with rectangular section.
7. Characterization of the properties and structure of the first set of rings. The mechanical properties have been measured by a NADCAP-approved testing laboratory independent from the partners.
8. Laboratory tests of the 718 alloy for assessing the effects of non-monotonic deformation (interpass static softening, effect of passes with partial strain reversal).
9. Manufacturing by rolling-alone of a second set of full-size rings. Two double rings with a profiled section. Processing parameters decided with the help of the simulation tool developed in this project.
10. Detailed characterization of the microstructure and mechanical properties of the two profiled rings (microstructural study by the two partners and subcontracted NADCAP-approved NDT inspection and mechanical tests).
11. A comparison of measured structural data of the profiled rings with the predictions from the numerical simulations.
12. A critical appraisal of the results of the mechanical tests of the rings, with special emphasis on their anisotropy, in comparison with available data and experience on the properties and anisotropy of CDF (close-die forged) disks.
13. An exercise of application of the results of RORC project for re-defining the current experience-based process window for ring-rolling the 718 alloy.
14. Id., for improving the capabilities of currently available ring-rolling mills in order to go beyond the achievements of this project.

Main results achieved in the project
1. A first set of 3 full-size rings delivered for characterization.
2. A new constitutive description of the thermomechanical behaviour of IN718 extracted from a large series of high-temperature torsion tests analysed with physically-based equations. The new set of equations allows for calculating the mechanical response in the hot working range used in ring-rolling and simultaneously provides with some information on the concomitant structural evolution.
3. Numerical (FEM) predictions of the ring rolling process making use of the new description of constitutive behaviour have been confronted to the industrial results with satisfactory agreement (evolution of geometrical parameters of the part, forces in the mill, temperatures, etc.). We dispose now of a reliable tool for virtual ring-rolling process design.
4. Detailed results from mechanical properties and their anisotropy of the first set of rectangular rings made available for analysis and comparison with numerical predictions.
5. Processing and delivery of two full-size profiled double rings delivered for NDT and structural and mechanical characterization. The profiles have been adapted for two real geometries of low-pressure turbine disks.
6. A study of effects of non-monotonic strain paths in the constitutive behaviour of 718 alloy.
7. A local “structural fibering index” susceptible of both, experimental metallographic measurement and numerical prediction by FEM simulations has been devised and validated. It is based on assuming (with bibliographic and our own results support) that |-phase is the main contributor to the anisotropy of ductility, toughness and fatigue properties of disks forged or rolled below the |-solvus temperature of 718 alloy. An excellent correlation has been found between the predicted and measured fibering index.
8. Detailed structural and mechanical properties in different locations, orientations and sections of the two profiled rings and have been extensively measured.
9. A good correlation between the anisotropy of mechanical properties and the proposed “fibering index” has been demonstrated. This proves the possibility of using confidently the predictions of FEM simulations for the future development of ring-rolling strategies and their optimized process windows with great technical and economic disks advantage relative to the current empirically-based rules of operation.
10. A comparison of the mechanical properties of the RORC rings with those available from CDF disks shows that, although being above the nominal thresholds for acceptability, they possess an anisotropy that cannot yet match the limited anisotropy imparted by CDF; i.e. the current technology of ring-rolling does not allow to produce rings that would be directly accepted for machining LPT disks according to the current aeronautical design standards.
11. The last conclusion points to further study the ring-rolling process by making use of the simulation tool developed in this project. Preliminary virtual explorations during the project already provide some suggestions for enlarging the capabilities of current rolling mills in order to close the quality gap between the ring-rolled and CDF disks.

Summary of results and their potential impact and use
The main final result is to have demonstrated mastering of processing of IN718 turbine disks by ring-rolling alone with properties, if not yet similar, comparable to those of disks obtained by using the current manufacturing technology (close-die forging) and with significant economic advantages. Another result from the project is the development of new know-how and new engineering knowledge valid for suggesting design modifications of current mills capable of rolling valid disks well beyond the capabilities of the current largest available mills; this issue will be critical in order to cope with the requirements of future large-diameter turbo-engines according to current trends.
Forgital intends to exploit the ring rolling-only production route for turbine discs to gain a market share in the Aero-engine turbine disc business; a gain of 10% of the current market share represents about 30 Million Euros annually. The project results can improve European competitiveness by allowing the EU to, in the future, manufacture LPT discs directly by ring rolling only technology and therefore reduce costs of said LPT discs and increase the number of potential suppliers compared to conventional closed-die manufacturing.

Potential Impact:
1. The main final result of the RORC project is to have demonstrated mastering of processing of IN718 turbine disks by ring-rolling alone with properties, if not yet similar, comparable to those of disks obtained by using the current manufacturing technology (close-die forging) and with significant economic advantages.

The innovation of the project is to have developed a fundamentally-guided near-net-shape rolling-only process of aero-engine turbine disks by starting from an upset preform, eliminating any close-die forging step from the process. Manufacturing by contoured ring-rolling can be considered a near-net-shape manufacturing process of disks. Tools are simpler and of more universal use than the tools needed for close-die forging (specific for each ring) and the shape of the desired disk cross section can be approximated with little requirements of final machining.

2. The economical benefits of the RORC route come from the elimination of the final forging step and its tooling, a smaller raw material use and a lowering of the machining costs because of a better approximation to the shape of the radial section of the final part (rolling of contoured rings). The volume of large and very large civil aero engines continues to increase as the size of aircraft and engine efficiency increase.

3. Forgital intends to exploit the ring rolling-only production route for turbine discs to gain a market share in the Aero-engine turbine disc business; a gain of 10% of the current market share represents about 30 Million Euros annually. The project results can improve European competitiveness by allowing the EU to, in the future, manufacture LPT discs directly by ring rolling only technology and therefore reduce costs of said LPT discs and increase the number of potential suppliers compared to conventional closed-die manufacturing.

4. Another result from the project is the development of new know-how and new engineering knowledge valid for suggesting design modifications of current mills capable of rolling valid disks well beyond the capabilities of the current largest available mills; this issue will be critical in order to cope with the requirements of future large-diameter turbo-engines according to current trends.

5. The final closed-die forging step currently required for rotary turbine components stems from concerns about the ability of ring-rolling to assure an acceptable microstructure, good enough average properties, narrow anisotropy of properties and good reproducibility. When compared with the microstructure and properties of forged disks, the RORC material has shown acceptable properties (above the nominal threshold for acceptance) but larger anisotropy and thus, larger dispersion of the properties in the disk section. Such properties are not yet acceptable for disk manufacture by using the current design criteria. Further refinements are still necessary for reching such situation; alternatively, the rolled-only rings could be used for machining disks if dedicated design modifications were devised, adapted to their properties.

6. Moreover, our development of the RORC simulation tool for virtual design of ring-rolling operations can also be used for optimizing the rolling of large rings used in other, non-aeronautical industrial sectors: i.e. steel rings for wind-mill energy or oil energy sector.

List of Websites:
http://www.tecnun.es/rorc/

Final Report Summary - RORC (Rolled-Only IN718 Ring-shaped Components)

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