Final Report Summary - HI-POTENTIAL (Higly Innovative Isothermal Forging of Gamma TIAL Alloy for LPT blades)
Executive Summary:
The proposal consists in developing an alternative solution to the existing LPT blade on the Geared Turbo Fan made of cast nickel based alloy by transcending the work fundamentally performed on the TNM Gamma TiAl alloy. This proposal is beyond the state of the art and is based on innovative hot-die forging process, advanced tooling devices, specific heat treatments and an optimized near net shape geometry. Bringing together these conditions is mandatory to meet the engine manufacturer requirements in terms of microstructure and associated mechanical properties but also to target the cost challenge in the scope of a mass production activity.
Following RTD and Demonstration activities are to be performed through the project :
- Development of hot-die forging process parameters
- Development of innovative toolings for hot-die forging process
- Development of optimized billet and blade designs for hot-die forging process
- Test hardware manufacturing
Project Context and Objectives:
Environment is nowadays a major preoccupation in the world, especially in developed countries particularly concerning worldwide climate change. Thus transportation must fulfill stronger and stronger requirements to minimize the environmental impact on earth. The aircraft industry must find a breakthrough to meet this challenge. As a consequence, engine manufacturers must develop new technology to decrease fuel consumption (airline’s operating cost), reduce noise and minimize CO2 emissions. At the same time the overall objectives of reliability and maintenance cost will continue to be amongst the most important focus areas and can not be compromised.
Extensive work has been conducted since the 80’s on γ-TiAl material. It is used in turbochargers for high-tech cars in the automotive industry and formerly for valves in Formula 1 before being banned by the FIA regulations. In the aerospace industry, after all the research effort, the material has been selected for LPT blades in GEnx engine. The selected alloy is the Ti-48Al-2Cr-2Nb alloy from the first generation. Parts are manufactured from casting. New γ-TiAl alloys were developed with high niobium content to increase useable strength and operating temperature and to allow forging process. TNM alloy (Ti-43,5Al-4Nb-1Mo-0.1B) developed in Austria and Germany is part of this class of alloy and can offer higher relative properties compared to Ti-48Al-2Cr-2Nb and Inconel 718.
For GTF demonstrator as part of SAGE platform, MTU has selected the TNM alloy for LPT. The alloy offers good processing capabilities. It was shown that it is fundamentally feasible to forge TNM γ-TiAl pancakes by isothermal forging process. However the technical and economical feasibility of the forging process must be proven and the window control of the parameters must be investigated in order to control microstructure and associated mechanical properties with consistence.
Blades for aero engines made of Ti and Ni based alloys are commonly forged on screw presses by using steel die materials preheated in the range of 200 to 400°C. The isothermal forging process is used mainly by aerospace forging companies to forge specific disk alloys and is mostly conducted under specific atmosphere or vacuum. This results usually in low productivity not compatible with the cost target for LPT blades. Forges de Bologne proposes to develop a new process using a tailored device to improve the efficiency of the isothermal forging process by operating under air condition on a hydraulic press. Specific die design and die material such as nickel based alloys or ceramic will be used. The technological choice will be conducted to minimize the operating cost of the equipment (productivity and die life with regard to die manufacturing).
TNM alloys have improved processing capabilities thanks to β2 phase. The forging process can be conducted either below or above the α-transus when β2 phase is maximum. The forging feasibility was proven by tests conducted on TNM alloy on an industrial press at Forges de Bologne. Blades are usually net shape or near net shape for common products. In the case of the γ-TiAl blades, those will be near net shape forged in order to minimize the cost of input material. The degree of isothermal/hot-die near net shape forging will be a compromise between cost of input material, die life and number of forging step to optimize the global costs.
After the forging process, an innovative heat treatment will be conducted in order to meet required microstructure and related properties for GTF application. The heat treatment has to be done in a very narrow temperature range that is not state of the art to meet the requirements of the engine manufacturer. Due to the nature of the phase diagram, a tight control of the temperature is required to achieve the desired phase distribution. For this alloy, the chosen furnace must have a tighter class, specific thermocouples adapted to this temperature values. NADCAP approval is also a requirement for Aerospace.
As a result of the development program on the hot-die/isothermal forging of TNM LPT blades, extensive exchanges with the aero engine manufacturer will permit to develop the technology to target the cost challenge and the necessary knowledge on the TNM γ-TiAl alloy to prepare mass production activity. The work plan that is divided into work packages covers the activity within the project. Those are planned to follow the logic of the project.
First, the work consists of the development of optimized processing parameters to achieve the material requirements of the engine manufacturer and for the proposed forging technology. It defines the material specification, the mechanical testing procedures. It provides material data for forging process modeling. Microstructures after deformation and heat treatment but also mechanical properties (tensile and creep) are investigated to generate data. The appropriate manufacturing route (forging + heat treatment) for blade forging is chosen on this basis.
The material is procured according to a specification defined in agreement with the aero engine manufacturer. Billet dimensions (diameter and length) are chosen to have dimension capability of testing specimens after forging. Material source selection is agreed with the engine manufacturer.
Regarding mechanical testing to assess material property, the specimen geometry, dimension and manufacturing but also the testing lab and procedure are agreed with the engine manufacturer. The development of the forging parameters is conducted at 2 scales both at the laboratory and at the industrial level. First, forging parameters are investigated at the laboratory. The work is subcontracted at Mines ParisTech, CEMEF laboratory based in Sophia Antipolis, France. Flow curves (stress-strain curves) are determined at different temperatures and strain rates. The aim is to test material behavior for different processing conditions and generate the data to be used for forging process modeling at FdB and then assess forging route. At the same time, the microstructure changes are investigated on SEM in term of phase percentage and distribution before and after the deformation process and also after heat treatment. This will give first basis of phase contribution in material behavior at forging conditions and also on how microstructure evolves after heat-treatment depending on the forging conditions.
Tests are also conducted at the industrial level. Pancake forging conditions are defined on the basis of the parameters defined at the laboratory scale. The aim is to confirm the microstructures obtained at the laboratory and conduct testing campaign on the forged material. Modeling of the pancake forging is conducted at FdB with the data coming from the laboratory. It gives the local strain and is used to define microstructure or specimen locations that are to be agreed with the engine manufacturer. Microstructures are investigated before and after heat treatment on SEM. Achieved microstructures are criteria for process parameter selection.
The heat treatment parameters are discussed with the engine manufacturer to achieve fully recrystallized microstructure. This will be investigated in relation with the local strain put on the parts during the forging. Mechanical tests are also conducted on heat-treated parts. It consists of tensile tests at room temperature, tensile tests at elevated temperature and creep tests. The obtained mechanical properties are compared with the engine manufacturer database. This is criteria to validate the chosen route. Achieved microstructures and mechanical properties give a map of the processing conditions. If data are not in accordance with the engine manufacturer requirements, new trials are defined and the same testing procedure is conducted. Such loop is carried out until the requirements are met. The optimum forging parameters are selected from this study and the recommendations and the expertise apply in the process design of LPT blades.
In second place, forging tools have to be evaluated from different materials (nickel base, ceramic). The selection of the appropriate material to forge in hot-die/isothermal condition under air is conducted with regard to criteria such as strength, durability (oxidation, corrosion and embrittlement) at high temperature but also material cost, material affordability, material reparability and die manufacturing cost. This selection is also conducted in relation with the temperature allowing proper hot-die/isothermal forging material processing under air, which is a significant step forward. The die material is also assessed with pancake forging. It results in the selection of die material active part. The choice may be argued after first set manufacturing. To make the test, appropriate die holder is manufactured; it is an adaptation of the current technology that was used for specific testing in the past at FdB. A specific die holder more relevant for mass production is designed and built in order to optimize the manufacturing costs of the hot-die/isothermal forging process.
With the results obtained from the development of forging parameters and forging tool selection, the LPT blade process can be designed and blade properties can be assessed. It consists of the near net shape forging part modeling in CAD based on the engine manufacturer blade design. It is a key point to get the model as early as possible otherwise the schedule to manufacture the blades may be affected. Billet geometries are tested by modeling. It is used to assess material flows, local strain in the parts. The work may require several tests to assess and select the billet geometry. To validate the billet design, forging tests have been performed. When satisfactory, parts are characterized after heat treatment to assess microstructure and mechanical properties in order to meet the specification of the engine manufacturer for GTF application. In case of not satisfactory, process tuning is conducted and new trials are carried out. The engine test hardware manufacturing is allowed only when the results are satisfactory.
Then the next work consists of the LPT blade test hardware manufacturing. Material is procured according to the criteria defined in the previous work packages. The set of part is produced according to parameters defined during the blade process set up. It consists of a batch of 80 to 90 parts for the engine test. The part are forged ideally in the industrial condition and heat treated according to the defined best practice. Several batches are manufactured, batch size depending on material batch size and heat treatment furnace capacity. Engine test hardware is delivered on continuous basis for machining. The first batch is fully investigated (microstructure and mechanical tests) in accordance with the agreed methods and tests of the engine manufacturer specifications. Microstructure at defined location will be investigated on the other batches to assess the conformity. Machining of the parts may be considered as an option. However this task is not taken into account in the budget and is subject to agreement with the engine manufacturer.
Then work will consist of the finalization of the Method of Manufacturing taking into account the global cost approach to transfer the parameters from prototype to mass production (and not limiting it to lab scale). This task consists of the discussion of the manufacturing choice issues. Other billet quality (surface, source) can be also a point of discussion.
Project Results:
Work Package 1 : Developement of forging parameters
Task 1.1 Material specification + Procurement: Forges de Bologne has written a forging stock specification for ɣ-TiAl TNM billets in order to tighten the required ranges of each element of the chemical composition of the alloy and to add the requirements concerning the surface state of the billets. In fact, we need a surface free from defects that could be crack initiation sites during forging process.
Task 1.2 Testing procedure: Forges de Bologne has written the testing procedure that will be used for process qualification. It concerns the specimen geometry, the mechanical properties requirements for tensile and creep tests and the list of approved laboratories for mechanical testing.
Task 1.3 Material flow curves and pancake simulation: flow curves of TNM alloy were determined by compression tests at different temperatures and strain rates. These flow curves were implemented in the modeling software in order to simulate pancake and blade forging. These mechanical tests gave us a first indication on the forging window for TNM alloy. In fact, all the specimens tested below the disordering temperature of beta phase showed an important brittleness of the material.
Task 1.4 Pancake forging + Heat Treatment: pancakes were forged with different thermo-mechanical parameters in order to find the forging window that will permit to avoid cracks and to reach the required microstructure and mechanical properties after the heat treatment. The first heat treatment step at high temperature was performed by a subcontractor and Forges de Bologne and the second step which is performed at a more conventional temperature was performed by Forges de Bologne.
Task 1.5 Microstructure evolution: in a first step, the evolution of the microstructure as a function of the temperature was studied. The aim is to determine the phase fractions and morphologies at different temperatures in order to tailor the post-forging heat treatment which will permit to reach the required mechanical properties. The first heat-treatments tested gave us an indication on the temperature window below ɣ-solvus to reach the required phase fractions. The rest of the work concerning microstructure evolution was performed on forged pancakes to be as near as possible from the real thermo-mechanical parameters which will be applied for the LPT blades. Samples extracted from the pancakes were prepared for microscope observation in order to assess the phase fractions and the grain sizes. Usually, we use SEM for the analysis of ɣ-TiAl microstructure but Forges de Bologne has set up in its laboratory a method of metallographic preparation to observe TNM microstructure also with optical microscope, which reduced significantly the cost of the studies.
Task 1.6 Mechanical properties: tensile and creep tests were performed on specimens extracted from the pancakes. The results indicate that we have good creep properties but insufficient tensile properties also for the pancakes with the highest deformation levels. This result is probably due to an insufficient deformation: it is recommended to apply a deformation above 50%.
Work Package 2 : Development of tooling
FdB decided to continue the project with hot-die forging process rather than isothermal forging process. Because of this decision, the task 2.5 was redone with an adapted design of die holders for hot-die forging process.
Task 2.1 Die material specification + procurement: bars of nickel based alloy were ordered to manufacture the dies for hot-die forging process. A very usual grade was chosen for this application. The material specification used for procurement is an existing specification of an aerospace engine manufacturer.
Task 2.2 Die material mechanical testing: the chosen grade of nickel based alloy was tested by performing mechanical tests on specimens extracted from forged and heat treated blocks. The temperature of tensile tests was the same than the maximum heating temperature of the dies during hot-die forging process. Above this temperature, the dissolution of the hardening phase occurs and we have a loss of mechanical properties.
Task 2.3 Die manufacturing: the bars in nickel based alloy were forged, heat treated and machined to obtain the upper and the lower dies.
Task 2.4 Die material testing in processing conditions: the dies were tested during the first blade forging trials. The first trials showed an abrasive wear and a local plastic deformation of the material. For test hardware manufacturing, we planned to use a special coating for wear resistance.
Task 2.5 Die holder design for testing conditions: one of the main issues was the choice of the appropriate materials for the die holders and each part of the die stack assembly. In fact, a combination of high-temperature materials is necessary because of the conduction of heat into the die assembly. Depending on operating temperature, a variety of nickel based superalloys may be employed as holders and bolsters material. This task was redone to design die holders for hot-die forging process rather than for isothermal forging process.
Task 2.6 Die holder manufacturing for testing conditions: die holders specially adapted for hot-die forging process were manufactured.
Task 2.7 Die holder design for blade isothermal process: same design than task 2.5.
Task 2.8 Die holder manufacturing for blade isothermal process: as we decided to manufacture the test hardware of WP4 by hot-die forging process, the die holders would be the same than those used for blade forging trials of WP3.
Work Package 3 : Development of blade processing design
Task 3.1 CAD (die design and billet geometry): the die is designed starting from the as-forged blade geometry. We know exactly the input weight to obtain the final part and consequently we can choose the appropriate billet geometry (diameter and length).
The task 3.2 Forging modeling: the aim is to assess the correct filling of the die engraving and to see if the thermo-mechanical parameters in the blade will permit to reach the required microstructure after the two-step heat treatment. For example we have to apply a sufficient deformation in the part in order to induce a total recrystallization during the heat treatment.
Task 3.3 Forging trial: three forging campaigns were performed. The main issue of this task is to avoid material damage because of the brittleness of the material. A significant improvement of the results was obtained after each campaign. During the first one, the blades broke, in the second one we had an important damage with deep cracks and in the third one we had only a kind of crack network at the surface of the blades.
Task 3.4 Heat-treatment: blades which “have seen” different forging and heat treatment parameters have been characterized in terms of microstructure and mechanical properties to find the optimized process parameters.
Task 3.5 Microstructure: a metallographic investigation was performed on the blades. The observed result is a fine and homogeneous microstructure with globular ɣ and β grains and (α2+β) lamellar colony with globular shape and fine lamellar spacing. However, some inhomogeneities consisting in clusters of ɣ-phase were found. In as-forged microstructure, the heterogeneities are not broken by the deformation but we can see the effect of forging: a « macro scale effect » which is a grain flow. This elongated morphology is found in highly deformed areas. We have also a « micro scale effect » on the heterogeneities: we can see a fragmentation of coarse ɣ-lamellae in small globular ɣ-grains. The heterogeneities are sometimes still present in the heat-treated microstructure. They are composed of globular ɣ-grains and we find the same morphologies: elongated heterogeneities and clusters.
Task 3.6 Mechanical properties: tensile tests at room temperature, hot tensile tests and creep tests were performed. Tensile strength and 0,2% proof stress are acceptable at room temperature but the elongation was insufficient. At a higher temperature, we obtained better results in terms of elongation. Creep tests at high temperature showed excellent results.
Work Package 4 : Test hardware manufacturing
Task 4.1 Material procurement: the billets for test hardware have been delivered to Forges de Bologne in three lots for a total of around 100 billets.
Task 4.2 Blade forging: the blades for test hardware were manufactured by closed-die forging process.
Task 4.3 Heat treatment: the blades of test hardware were heat-treated by Forges de Bologne after many tests on forged material to choose the appropriate temperature.
Task 4.4 Characterization: mechanical testings and metallographic examinations have been performed by the project partner for FAI qualification.
Work Package 5 : Finalization of method of manufacturing
The management of HIPOTENTIAL project was mainly based on conference calls gathering Forges de Bologne and our project partner. Forges de Bologne is was composed from the Head of R&D Department who is the Project Coordinator, the Metallurgy Engineer who is the Technical Coordinator and the Sales Manager. The aim of these conferences is to discuss the progress for each Work Package. The conference calls passed from a monthly basis to a weekly basis in the last months. Here are some examples of topics discussed during the conference calls:
- Forging and heat treatment parameters for Test hardware
- The method to choose the heat-treatment temperature
- Dimensional control of the forgings
The Coordination Memo (CoMemo) is the second way of communication between Forges de Bologne and the project partner. After each conference call, Forges de Bologne sends to all the participants a CoMemo summarizing the discussion. To formalize a demand expressed toward Forges de Bologne concerning a technical issue (heat treatment parameters, mechanical testing...) the project partner sends to Forges de Bologne a CoMemo to share information.
Potential Impact:
The technology applied today on the LPT section of engines is Ni-based castings. Outside Europe, the technology applied today for γ-TiAl manufacturing is casting with an alloy having lower mechanical properties compared with the TNM material. The development of the γ-TiAl forging technology will bring more benefit. It will contribute to the elevation of the European competitiveness with regard to today’s challenges of the aerospace market such as fuel burn, noise reduction, improved maintenance costs and global reliability.
The application of a forged blade in TiAl on the LPT section of the Geared Turbo Fan will customize the yield of the engine and then the fuel efficiency and the environmental performance of it. This application will open new markets for FdB because the forged blade will replace existing castings. It will support MTU into the introduction of the GTF on the various aircraft markets such as Large Single Aisle Re-Engining for commercial airplanes and business jets like MRJ and BA C-Series.
The introduction on the market is planned between mid-2013 and end 2015 for these engines. With the growing demand for Large Single Aisle aircraft (representing more than 70% of the Single Air Segment) and additional environmental challenges, it is FdB’s strategy to join MTU in the promotion of its GTF on the market. A further step will be the GTF’s contribution under MTU’s Clean Air Engine program to the compliance with the ACARE targets meaning more safety, more quality and affordability and an improved air transport efficiency.
List of Websites:
No website available
The proposal consists in developing an alternative solution to the existing LPT blade on the Geared Turbo Fan made of cast nickel based alloy by transcending the work fundamentally performed on the TNM Gamma TiAl alloy. This proposal is beyond the state of the art and is based on innovative hot-die forging process, advanced tooling devices, specific heat treatments and an optimized near net shape geometry. Bringing together these conditions is mandatory to meet the engine manufacturer requirements in terms of microstructure and associated mechanical properties but also to target the cost challenge in the scope of a mass production activity.
Following RTD and Demonstration activities are to be performed through the project :
- Development of hot-die forging process parameters
- Development of innovative toolings for hot-die forging process
- Development of optimized billet and blade designs for hot-die forging process
- Test hardware manufacturing
Project Context and Objectives:
Environment is nowadays a major preoccupation in the world, especially in developed countries particularly concerning worldwide climate change. Thus transportation must fulfill stronger and stronger requirements to minimize the environmental impact on earth. The aircraft industry must find a breakthrough to meet this challenge. As a consequence, engine manufacturers must develop new technology to decrease fuel consumption (airline’s operating cost), reduce noise and minimize CO2 emissions. At the same time the overall objectives of reliability and maintenance cost will continue to be amongst the most important focus areas and can not be compromised.
Extensive work has been conducted since the 80’s on γ-TiAl material. It is used in turbochargers for high-tech cars in the automotive industry and formerly for valves in Formula 1 before being banned by the FIA regulations. In the aerospace industry, after all the research effort, the material has been selected for LPT blades in GEnx engine. The selected alloy is the Ti-48Al-2Cr-2Nb alloy from the first generation. Parts are manufactured from casting. New γ-TiAl alloys were developed with high niobium content to increase useable strength and operating temperature and to allow forging process. TNM alloy (Ti-43,5Al-4Nb-1Mo-0.1B) developed in Austria and Germany is part of this class of alloy and can offer higher relative properties compared to Ti-48Al-2Cr-2Nb and Inconel 718.
For GTF demonstrator as part of SAGE platform, MTU has selected the TNM alloy for LPT. The alloy offers good processing capabilities. It was shown that it is fundamentally feasible to forge TNM γ-TiAl pancakes by isothermal forging process. However the technical and economical feasibility of the forging process must be proven and the window control of the parameters must be investigated in order to control microstructure and associated mechanical properties with consistence.
Blades for aero engines made of Ti and Ni based alloys are commonly forged on screw presses by using steel die materials preheated in the range of 200 to 400°C. The isothermal forging process is used mainly by aerospace forging companies to forge specific disk alloys and is mostly conducted under specific atmosphere or vacuum. This results usually in low productivity not compatible with the cost target for LPT blades. Forges de Bologne proposes to develop a new process using a tailored device to improve the efficiency of the isothermal forging process by operating under air condition on a hydraulic press. Specific die design and die material such as nickel based alloys or ceramic will be used. The technological choice will be conducted to minimize the operating cost of the equipment (productivity and die life with regard to die manufacturing).
TNM alloys have improved processing capabilities thanks to β2 phase. The forging process can be conducted either below or above the α-transus when β2 phase is maximum. The forging feasibility was proven by tests conducted on TNM alloy on an industrial press at Forges de Bologne. Blades are usually net shape or near net shape for common products. In the case of the γ-TiAl blades, those will be near net shape forged in order to minimize the cost of input material. The degree of isothermal/hot-die near net shape forging will be a compromise between cost of input material, die life and number of forging step to optimize the global costs.
After the forging process, an innovative heat treatment will be conducted in order to meet required microstructure and related properties for GTF application. The heat treatment has to be done in a very narrow temperature range that is not state of the art to meet the requirements of the engine manufacturer. Due to the nature of the phase diagram, a tight control of the temperature is required to achieve the desired phase distribution. For this alloy, the chosen furnace must have a tighter class, specific thermocouples adapted to this temperature values. NADCAP approval is also a requirement for Aerospace.
As a result of the development program on the hot-die/isothermal forging of TNM LPT blades, extensive exchanges with the aero engine manufacturer will permit to develop the technology to target the cost challenge and the necessary knowledge on the TNM γ-TiAl alloy to prepare mass production activity. The work plan that is divided into work packages covers the activity within the project. Those are planned to follow the logic of the project.
First, the work consists of the development of optimized processing parameters to achieve the material requirements of the engine manufacturer and for the proposed forging technology. It defines the material specification, the mechanical testing procedures. It provides material data for forging process modeling. Microstructures after deformation and heat treatment but also mechanical properties (tensile and creep) are investigated to generate data. The appropriate manufacturing route (forging + heat treatment) for blade forging is chosen on this basis.
The material is procured according to a specification defined in agreement with the aero engine manufacturer. Billet dimensions (diameter and length) are chosen to have dimension capability of testing specimens after forging. Material source selection is agreed with the engine manufacturer.
Regarding mechanical testing to assess material property, the specimen geometry, dimension and manufacturing but also the testing lab and procedure are agreed with the engine manufacturer. The development of the forging parameters is conducted at 2 scales both at the laboratory and at the industrial level. First, forging parameters are investigated at the laboratory. The work is subcontracted at Mines ParisTech, CEMEF laboratory based in Sophia Antipolis, France. Flow curves (stress-strain curves) are determined at different temperatures and strain rates. The aim is to test material behavior for different processing conditions and generate the data to be used for forging process modeling at FdB and then assess forging route. At the same time, the microstructure changes are investigated on SEM in term of phase percentage and distribution before and after the deformation process and also after heat treatment. This will give first basis of phase contribution in material behavior at forging conditions and also on how microstructure evolves after heat-treatment depending on the forging conditions.
Tests are also conducted at the industrial level. Pancake forging conditions are defined on the basis of the parameters defined at the laboratory scale. The aim is to confirm the microstructures obtained at the laboratory and conduct testing campaign on the forged material. Modeling of the pancake forging is conducted at FdB with the data coming from the laboratory. It gives the local strain and is used to define microstructure or specimen locations that are to be agreed with the engine manufacturer. Microstructures are investigated before and after heat treatment on SEM. Achieved microstructures are criteria for process parameter selection.
The heat treatment parameters are discussed with the engine manufacturer to achieve fully recrystallized microstructure. This will be investigated in relation with the local strain put on the parts during the forging. Mechanical tests are also conducted on heat-treated parts. It consists of tensile tests at room temperature, tensile tests at elevated temperature and creep tests. The obtained mechanical properties are compared with the engine manufacturer database. This is criteria to validate the chosen route. Achieved microstructures and mechanical properties give a map of the processing conditions. If data are not in accordance with the engine manufacturer requirements, new trials are defined and the same testing procedure is conducted. Such loop is carried out until the requirements are met. The optimum forging parameters are selected from this study and the recommendations and the expertise apply in the process design of LPT blades.
In second place, forging tools have to be evaluated from different materials (nickel base, ceramic). The selection of the appropriate material to forge in hot-die/isothermal condition under air is conducted with regard to criteria such as strength, durability (oxidation, corrosion and embrittlement) at high temperature but also material cost, material affordability, material reparability and die manufacturing cost. This selection is also conducted in relation with the temperature allowing proper hot-die/isothermal forging material processing under air, which is a significant step forward. The die material is also assessed with pancake forging. It results in the selection of die material active part. The choice may be argued after first set manufacturing. To make the test, appropriate die holder is manufactured; it is an adaptation of the current technology that was used for specific testing in the past at FdB. A specific die holder more relevant for mass production is designed and built in order to optimize the manufacturing costs of the hot-die/isothermal forging process.
With the results obtained from the development of forging parameters and forging tool selection, the LPT blade process can be designed and blade properties can be assessed. It consists of the near net shape forging part modeling in CAD based on the engine manufacturer blade design. It is a key point to get the model as early as possible otherwise the schedule to manufacture the blades may be affected. Billet geometries are tested by modeling. It is used to assess material flows, local strain in the parts. The work may require several tests to assess and select the billet geometry. To validate the billet design, forging tests have been performed. When satisfactory, parts are characterized after heat treatment to assess microstructure and mechanical properties in order to meet the specification of the engine manufacturer for GTF application. In case of not satisfactory, process tuning is conducted and new trials are carried out. The engine test hardware manufacturing is allowed only when the results are satisfactory.
Then the next work consists of the LPT blade test hardware manufacturing. Material is procured according to the criteria defined in the previous work packages. The set of part is produced according to parameters defined during the blade process set up. It consists of a batch of 80 to 90 parts for the engine test. The part are forged ideally in the industrial condition and heat treated according to the defined best practice. Several batches are manufactured, batch size depending on material batch size and heat treatment furnace capacity. Engine test hardware is delivered on continuous basis for machining. The first batch is fully investigated (microstructure and mechanical tests) in accordance with the agreed methods and tests of the engine manufacturer specifications. Microstructure at defined location will be investigated on the other batches to assess the conformity. Machining of the parts may be considered as an option. However this task is not taken into account in the budget and is subject to agreement with the engine manufacturer.
Then work will consist of the finalization of the Method of Manufacturing taking into account the global cost approach to transfer the parameters from prototype to mass production (and not limiting it to lab scale). This task consists of the discussion of the manufacturing choice issues. Other billet quality (surface, source) can be also a point of discussion.
Project Results:
Work Package 1 : Developement of forging parameters
Task 1.1 Material specification + Procurement: Forges de Bologne has written a forging stock specification for ɣ-TiAl TNM billets in order to tighten the required ranges of each element of the chemical composition of the alloy and to add the requirements concerning the surface state of the billets. In fact, we need a surface free from defects that could be crack initiation sites during forging process.
Task 1.2 Testing procedure: Forges de Bologne has written the testing procedure that will be used for process qualification. It concerns the specimen geometry, the mechanical properties requirements for tensile and creep tests and the list of approved laboratories for mechanical testing.
Task 1.3 Material flow curves and pancake simulation: flow curves of TNM alloy were determined by compression tests at different temperatures and strain rates. These flow curves were implemented in the modeling software in order to simulate pancake and blade forging. These mechanical tests gave us a first indication on the forging window for TNM alloy. In fact, all the specimens tested below the disordering temperature of beta phase showed an important brittleness of the material.
Task 1.4 Pancake forging + Heat Treatment: pancakes were forged with different thermo-mechanical parameters in order to find the forging window that will permit to avoid cracks and to reach the required microstructure and mechanical properties after the heat treatment. The first heat treatment step at high temperature was performed by a subcontractor and Forges de Bologne and the second step which is performed at a more conventional temperature was performed by Forges de Bologne.
Task 1.5 Microstructure evolution: in a first step, the evolution of the microstructure as a function of the temperature was studied. The aim is to determine the phase fractions and morphologies at different temperatures in order to tailor the post-forging heat treatment which will permit to reach the required mechanical properties. The first heat-treatments tested gave us an indication on the temperature window below ɣ-solvus to reach the required phase fractions. The rest of the work concerning microstructure evolution was performed on forged pancakes to be as near as possible from the real thermo-mechanical parameters which will be applied for the LPT blades. Samples extracted from the pancakes were prepared for microscope observation in order to assess the phase fractions and the grain sizes. Usually, we use SEM for the analysis of ɣ-TiAl microstructure but Forges de Bologne has set up in its laboratory a method of metallographic preparation to observe TNM microstructure also with optical microscope, which reduced significantly the cost of the studies.
Task 1.6 Mechanical properties: tensile and creep tests were performed on specimens extracted from the pancakes. The results indicate that we have good creep properties but insufficient tensile properties also for the pancakes with the highest deformation levels. This result is probably due to an insufficient deformation: it is recommended to apply a deformation above 50%.
Work Package 2 : Development of tooling
FdB decided to continue the project with hot-die forging process rather than isothermal forging process. Because of this decision, the task 2.5 was redone with an adapted design of die holders for hot-die forging process.
Task 2.1 Die material specification + procurement: bars of nickel based alloy were ordered to manufacture the dies for hot-die forging process. A very usual grade was chosen for this application. The material specification used for procurement is an existing specification of an aerospace engine manufacturer.
Task 2.2 Die material mechanical testing: the chosen grade of nickel based alloy was tested by performing mechanical tests on specimens extracted from forged and heat treated blocks. The temperature of tensile tests was the same than the maximum heating temperature of the dies during hot-die forging process. Above this temperature, the dissolution of the hardening phase occurs and we have a loss of mechanical properties.
Task 2.3 Die manufacturing: the bars in nickel based alloy were forged, heat treated and machined to obtain the upper and the lower dies.
Task 2.4 Die material testing in processing conditions: the dies were tested during the first blade forging trials. The first trials showed an abrasive wear and a local plastic deformation of the material. For test hardware manufacturing, we planned to use a special coating for wear resistance.
Task 2.5 Die holder design for testing conditions: one of the main issues was the choice of the appropriate materials for the die holders and each part of the die stack assembly. In fact, a combination of high-temperature materials is necessary because of the conduction of heat into the die assembly. Depending on operating temperature, a variety of nickel based superalloys may be employed as holders and bolsters material. This task was redone to design die holders for hot-die forging process rather than for isothermal forging process.
Task 2.6 Die holder manufacturing for testing conditions: die holders specially adapted for hot-die forging process were manufactured.
Task 2.7 Die holder design for blade isothermal process: same design than task 2.5.
Task 2.8 Die holder manufacturing for blade isothermal process: as we decided to manufacture the test hardware of WP4 by hot-die forging process, the die holders would be the same than those used for blade forging trials of WP3.
Work Package 3 : Development of blade processing design
Task 3.1 CAD (die design and billet geometry): the die is designed starting from the as-forged blade geometry. We know exactly the input weight to obtain the final part and consequently we can choose the appropriate billet geometry (diameter and length).
The task 3.2 Forging modeling: the aim is to assess the correct filling of the die engraving and to see if the thermo-mechanical parameters in the blade will permit to reach the required microstructure after the two-step heat treatment. For example we have to apply a sufficient deformation in the part in order to induce a total recrystallization during the heat treatment.
Task 3.3 Forging trial: three forging campaigns were performed. The main issue of this task is to avoid material damage because of the brittleness of the material. A significant improvement of the results was obtained after each campaign. During the first one, the blades broke, in the second one we had an important damage with deep cracks and in the third one we had only a kind of crack network at the surface of the blades.
Task 3.4 Heat-treatment: blades which “have seen” different forging and heat treatment parameters have been characterized in terms of microstructure and mechanical properties to find the optimized process parameters.
Task 3.5 Microstructure: a metallographic investigation was performed on the blades. The observed result is a fine and homogeneous microstructure with globular ɣ and β grains and (α2+β) lamellar colony with globular shape and fine lamellar spacing. However, some inhomogeneities consisting in clusters of ɣ-phase were found. In as-forged microstructure, the heterogeneities are not broken by the deformation but we can see the effect of forging: a « macro scale effect » which is a grain flow. This elongated morphology is found in highly deformed areas. We have also a « micro scale effect » on the heterogeneities: we can see a fragmentation of coarse ɣ-lamellae in small globular ɣ-grains. The heterogeneities are sometimes still present in the heat-treated microstructure. They are composed of globular ɣ-grains and we find the same morphologies: elongated heterogeneities and clusters.
Task 3.6 Mechanical properties: tensile tests at room temperature, hot tensile tests and creep tests were performed. Tensile strength and 0,2% proof stress are acceptable at room temperature but the elongation was insufficient. At a higher temperature, we obtained better results in terms of elongation. Creep tests at high temperature showed excellent results.
Work Package 4 : Test hardware manufacturing
Task 4.1 Material procurement: the billets for test hardware have been delivered to Forges de Bologne in three lots for a total of around 100 billets.
Task 4.2 Blade forging: the blades for test hardware were manufactured by closed-die forging process.
Task 4.3 Heat treatment: the blades of test hardware were heat-treated by Forges de Bologne after many tests on forged material to choose the appropriate temperature.
Task 4.4 Characterization: mechanical testings and metallographic examinations have been performed by the project partner for FAI qualification.
Work Package 5 : Finalization of method of manufacturing
The management of HIPOTENTIAL project was mainly based on conference calls gathering Forges de Bologne and our project partner. Forges de Bologne is was composed from the Head of R&D Department who is the Project Coordinator, the Metallurgy Engineer who is the Technical Coordinator and the Sales Manager. The aim of these conferences is to discuss the progress for each Work Package. The conference calls passed from a monthly basis to a weekly basis in the last months. Here are some examples of topics discussed during the conference calls:
- Forging and heat treatment parameters for Test hardware
- The method to choose the heat-treatment temperature
- Dimensional control of the forgings
The Coordination Memo (CoMemo) is the second way of communication between Forges de Bologne and the project partner. After each conference call, Forges de Bologne sends to all the participants a CoMemo summarizing the discussion. To formalize a demand expressed toward Forges de Bologne concerning a technical issue (heat treatment parameters, mechanical testing...) the project partner sends to Forges de Bologne a CoMemo to share information.
Potential Impact:
The technology applied today on the LPT section of engines is Ni-based castings. Outside Europe, the technology applied today for γ-TiAl manufacturing is casting with an alloy having lower mechanical properties compared with the TNM material. The development of the γ-TiAl forging technology will bring more benefit. It will contribute to the elevation of the European competitiveness with regard to today’s challenges of the aerospace market such as fuel burn, noise reduction, improved maintenance costs and global reliability.
The application of a forged blade in TiAl on the LPT section of the Geared Turbo Fan will customize the yield of the engine and then the fuel efficiency and the environmental performance of it. This application will open new markets for FdB because the forged blade will replace existing castings. It will support MTU into the introduction of the GTF on the various aircraft markets such as Large Single Aisle Re-Engining for commercial airplanes and business jets like MRJ and BA C-Series.
The introduction on the market is planned between mid-2013 and end 2015 for these engines. With the growing demand for Large Single Aisle aircraft (representing more than 70% of the Single Air Segment) and additional environmental challenges, it is FdB’s strategy to join MTU in the promotion of its GTF on the market. A further step will be the GTF’s contribution under MTU’s Clean Air Engine program to the compliance with the ACARE targets meaning more safety, more quality and affordability and an improved air transport efficiency.
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