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Powder prospects ‘set for takeoff’ as production methods change
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Powder prospects 'set for takeoff' as production methods change New material production processes such as FFC and improved control and prediction of powder consolidation offer great potential for PM manufactured components in gas turbine engines and other areas outside the aerospace sector, says a team from Rolls Royce...
N
ear-shape and net-shape titanium alloy powder technology has many potential gas turbine applications with potential benefits that include improved mechanical properties, cheaper components, reduced life cycle costs, reduced environmental concerns, and the possibility of using difficult materials for complex shapes (Figure 1). Production of aerospace parts by conventional ingot metallurgy often results in low material yield and can be less than 10 per cent for complex parts when assessed using the percentage yield from raw material used for the starting ingot. Further improvements in process yield are becoming increasingly difficult as a result of numerous intrinsic mandatory stages such as surface dressing to remove alpha case on Ti alloys. Near net shape processes have been used for some time by Rolls-Royce and show considerable potential for more complex and demanding hybrid components which also exploit the key benefits of traditional ingot metallurgy production routes. Examples of this include produc-
tion of flanges and bosses on casings, and aerofoil leading edge and tip repairs. The introduction of new processes and hybrid structures of this type have to be evaluated and controlled in order to ensure component loading regimes, material properties and component behaviour are fit for purpose. Modern gas turbine engines are expected to achieve increasingly higher levels of fuel economy, reduced NOx emissions and noise, with reduced module weight to meet the demand for reduced acquisition and life cycle costs in civil aviation. These challenges
necessitate higher overall pressure ratios, and higher compressor discharge and turbine entry temperatures leading to the need for disc rotors in the HP compressor and turbine to accommodate higher temperatures and stresses. This places significant demands on highstrength nickel alloys that are used for these critical components, particularly as loss of integrity can threaten the safety of the aircraft and passengers. To this end, Rolls-Royce has developed the alloy known as RR1000 to replace alloy 720Li, which is the best of the current list of nickel disc alloys available to the
The author THIS article is based on The Development of Powder Consolidated Components for the Gas Turbine Engine, a paper by Wayne Voice, Mark Hardy and David Rugg of Rolls-Royce plc, Derby, UK, presented at the PM 2004 World Congress in Vienna.
8
MPR April 2005
Figure 1. The Trent seies of three-shaft aero9 engines represent advanced co-evolution of materials and design methods.
0026-0657/05 ©2005 Elsevier Ltd. All rights reserved.
Figure 2. Rolls Royce engineers and a ‘ship set’ of Trent 500 engines destined for an Airbus A340-500.
company. Great emphasis is placed on powder processing and subsequent heat treatment which have a profound effect on the mechanical properties and the material cleanliness of the alloy component. A rebirth in interest in net shape powder technology has occurred since the turn of the 21st Century. One of the factors responsible is the remarkable achievement in dimensional accuracy made by such groups as LNT of Moscow that are now able to model consolidation of powders sufficiently well to allow net shape manufacture of blisks, shrouded impellors and casings, (Figure 2). While current applications are mainly limited to rocket motors, the potential is clear for the manufacture of complex components in high-performance applications which would otherwise be difficult to produce via a forged or fabricated route. Dimensionally accurate structures have also been demonstrated by laser powder deposition, allowing its use for
metal-powder.net
construction of complex 3-D geometries. Close interaction between the original equipment manufacturer and the manufacturing technology provider is vital in this area in order to focus the work on the most fruitful components for low-risk but high-commercial-benefit applications. From an industrial context the work of establishments like the AeroMet Corporation illustrates the range of component sizes and volumes of titanium powder deposit that are being explored commercially. An additional driver in powder-based manufacturing techniques stems from the promise of low-cost alloy powder through the development of new chemical and electro-chemical processes such as the electrolytic de-oxidation (known as EDO or FFC) process. It is recognised that the most attractive product would substitute into the manufacturing process as far downstream as possible. The direct economic manufacture of powders potentially opens the market for titanium alloys in non-aerospace applica-
tions which is important to minimise time to market and to increase total volume of product. This approach would also be benefit aerospace by providing an established and robust manufacturing supply chain where detailed cost analysis and potential for production "issues" are well understood. There may also be significant technological advances in terms of powder size distributions, morphology and chemistry that dictate many important factors in net-shape manufacture; powder flow characteristics determine mould fill and therefore what section size and part complexity is possible. Powder packing density determines the shrinkage required to achieve full density in the finished part. Previous work on powder based titanium routes has centred on rotating electrode (PREP), gas jet atomisation and hydride/dehydride as manufacturing sources. Each of these techniques has a characteristic product size distribution and morphology dictated by the physics of the process. An interesting
April 2005 MPR
9
Figure 3. The morphology of titanium powder produced by electrolytic reduction.
Table I. Compositions of RR1000 and U720 in Weight Percent. Alloy Co Cr Mo Ti Al Ta W Hf C RR1000 18.5 15 5 3.6 3 2 0.5 0.03
B Zr Ni 0.02 0.06 Bal.
U720
0.02 0.04 Bal
16
15
3
5
2.5 -
1
-
0.02
designing against fatigue failure and a good understanding of fatigue scatter and the "stack up" of contributing factors is vital. Progression towards more complex components such as blisks accentuates the requirement for aerofoil repair given the cost associated of blisk replacement. An accurate, less intrusive repair technique might be through direct laser deposition as developed by Loretto and Wu at Birmingham IRC. Although billets for large-scale forgings of alloy 720Li are being produced via the cast and wrought route, powder metallurgy is being considered for increasingly complex nickel disc alloys, such as RR1000 (See Table 1 and Figure 4), to avoid the risk of elemental segregation and unacceptable levels of carbonitride stringers. The composition of RR1000 has been designed with the minimum volume fraction of γ' to achieve the required levels of strength and damage tolerance. Specific quantities of chromium, cobalt and hafnium, and the ability to decorate grain boundaries with precise amounts of M23C6, M6C and MC are considered critical for good crack growth resistance at elevated temperature, particularly under creep fatigue conditions. The levels of aluminium, chromium and molybdenum have also been manipulated to avoid excessive TCP formation at the projected time at peak temperature in service. It is recognised that the use of powder processing will increase the price of billet material above the level for cast and wrought 720Li, and requires specific methods for probabilistic design/lifing and to ensure that a consistent level of cleanliness is maintained in production
aspect of electrolytic production is the possible existence, and manipulation, of a very wide range of production parameters to produce the “ideal” powder for a given application type. Of particular interest is the possibility of producing very fine powders that are currently very expensive given that they represent the tail end of the distribution curve for gasatomised product. A very fine mesh size would allow control of the size distribution of some sources of particulate defect which could be exploited in terms of increased component life or allowable stress level. Another benefit of metal oxide powder reduction is the potential for exploration of alloy systems, otherwise not possible.
This arises from the solid-state nature of the reaction, so that alloy systems previously rejected for reasons such as segregation could now be commercially exploited. Prospective candidates include the Ti-Mn, Ti-Ni and Ti-Fe beta eutectoid systems that could provide low-cost, age-hardenable and metastable beta alloys. It is well established that repair/life extension of aero-engine components is a prerequisite for minimised life cycle costs. Aerofoils represent a good example where economic repair schemes are possible, either at the leading edge due to foreign object damage or at the blade tip. Fan and compressor aerofoils present some unique difficulties in terms of
Figure 4. An intermediate pressure turbine disc forged from RR1000.
Figure 5. The influence of solution treatment temperature on the room temperature proof and ultimate tensile strength of fully heat treated RR1000. Fan air cooling was used to quench large-scale forgings after solution treatment. Also shown is an optical micrograph of RR1000 after near solvus treatment at 1125°C to page 12 give a grain size of ASTM 12 (6µm).
10
MPR April 2005
metal-powder.net
page 10
Figure 6. Metal as poetry? The graceful symmetry of aero engine turbine blades could soon be enhanced by the strength of PM technology.
material. While the powder route is currently preferred, it is considered that large diameter billet can be produced from cast and wrought processing but is unlikely to offer a cost saving over the powder product. It is well documented that the presence of inclusions, which are picked up during powder processing, can severely limit the fatigue performance of an alloy. Over the last decade, the cleanliness of argon gas-atomised powder has improved considerably through the elimination of organic seals and the use of progressively finer sieve sizes to screen out non-metallic inclusions. This has been made affordable by the development of nozzles for high yield powder production. Following consolidation of canned powder by hot isostatic pressing or hot compaction, it is the industry practice to extrude the cans to a reduction ratio of 5.5:1 to break down any prior particle boundaries and to produce a fine grain
12
MPR April 2005
size that is typically smaller than ASTM 12 (6 µm). The RR1000 data in Figure 5 indicate that a relatively small change in average grain size, under creep-fatigue conditions, can result in significant changes in crack growth rate. The fine structure is also ideal for inspection of the billet and subsequent superplastic deformation on isothermal forging. As lower strain rates are used during deformation compared to the hot die route, isothermal forging currently offers greater control of structure and the ability to forge quite complex, "nearnet" shapes that enable significant reductions in material input weight and machining time. Sub-solvus solution heat treatment is applied to RR1000 to give a stronger material compared to super-solvus treatment, (Figure 6). There is an incentive to minimise subsequent quenching stresses in powder-processed alloys due to the occurrence of ceramic inclusions. Although the majority of these inclusions
are small, they must remain uncracked if the finished component is to realise the full fatigue life potential of the alloy. Evidence from cyclic rig tests on subscale powder discs has shown that having considered quenching residual stresses after ageing, discs produced from fan aircooled forgings exhibit a greater fatigue life than those machined from oilquenched forgings. For this and other reasons, RR1000 forgings are fan aircooled after solution treatment. However, as the strength of the alloy is not fully optimised, the design envelope for discs may increase to provide the required tolerance to counter high stresses that may arise from unforeseen shaft over-speed events. The current combination of technical and commercial drivers demand significant advances in component reliability and reduced life cycle costs. Together with the shortening of timescales associated with new product development and production lead times, this is proving to be fertile ground for net shape technologies and innovative thinking on powder. It is also probable that utilisation of metal deposition techniques for repair will expand significantly in the near term. Production of pre-alloyed powder via electrolytic reduction of mixed oxide powders offers the potential of both widespread commercial use of net shape HIP and new alloys. However, it is probable that the most economically attractive market for the manufacturing supply chain lies outside the aerospace industry. The composition of RR1000 has been designed with the minimum volume fraction of γ' to achieve the required levels of strength and damage tolerance. The acceptable balance between tensile strength and crack growth resistance with a 25°C increase in temperature capability has been established by forging consolidated powder billet coupled with advanced fan air-cooling from near-solvus solution heat treatment and additional 30°C improvements may be possible.
Footnote All images in this feature are reproduced with the permission of RollsRoyce plc, copyright © Rolls-Royce plc 2004.
metal-powder.net
Powder prospects 'set for takeoff' as production methods change New material production processes such as FFC and improved control and prediction of powder consolidation offer great potential for PM manufactured components in gas turbine engines and other areas outside the aerospace sector, says a team from Rolls Royce...
N
ear-shape and net-shape titanium alloy powder technology has many potential gas turbine applications with potential benefits that include improved mechanical properties, cheaper components, reduced life cycle costs, reduced environmental concerns, and the possibility of using difficult materials for complex shapes (Figure 1). Production of aerospace parts by conventional ingot metallurgy often results in low material yield and can be less than 10 per cent for complex parts when assessed using the percentage yield from raw material used for the starting ingot. Further improvements in process yield are becoming increasingly difficult as a result of numerous intrinsic mandatory stages such as surface dressing to remove alpha case on Ti alloys. Near net shape processes have been used for some time by Rolls-Royce and show considerable potential for more complex and demanding hybrid components which also exploit the key benefits of traditional ingot metallurgy production routes. Examples of this include produc-
tion of flanges and bosses on casings, and aerofoil leading edge and tip repairs. The introduction of new processes and hybrid structures of this type have to be evaluated and controlled in order to ensure component loading regimes, material properties and component behaviour are fit for purpose. Modern gas turbine engines are expected to achieve increasingly higher levels of fuel economy, reduced NOx emissions and noise, with reduced module weight to meet the demand for reduced acquisition and life cycle costs in civil aviation. These challenges
necessitate higher overall pressure ratios, and higher compressor discharge and turbine entry temperatures leading to the need for disc rotors in the HP compressor and turbine to accommodate higher temperatures and stresses. This places significant demands on highstrength nickel alloys that are used for these critical components, particularly as loss of integrity can threaten the safety of the aircraft and passengers. To this end, Rolls-Royce has developed the alloy known as RR1000 to replace alloy 720Li, which is the best of the current list of nickel disc alloys available to the
The author THIS article is based on The Development of Powder Consolidated Components for the Gas Turbine Engine, a paper by Wayne Voice, Mark Hardy and David Rugg of Rolls-Royce plc, Derby, UK, presented at the PM 2004 World Congress in Vienna.
8
MPR April 2005
Figure 1. The Trent seies of three-shaft aero9 engines represent advanced co-evolution of materials and design methods.
0026-0657/05 ©2005 Elsevier Ltd. All rights reserved.
Figure 2. Rolls Royce engineers and a ‘ship set’ of Trent 500 engines destined for an Airbus A340-500.
company. Great emphasis is placed on powder processing and subsequent heat treatment which have a profound effect on the mechanical properties and the material cleanliness of the alloy component. A rebirth in interest in net shape powder technology has occurred since the turn of the 21st Century. One of the factors responsible is the remarkable achievement in dimensional accuracy made by such groups as LNT of Moscow that are now able to model consolidation of powders sufficiently well to allow net shape manufacture of blisks, shrouded impellors and casings, (Figure 2). While current applications are mainly limited to rocket motors, the potential is clear for the manufacture of complex components in high-performance applications which would otherwise be difficult to produce via a forged or fabricated route. Dimensionally accurate structures have also been demonstrated by laser powder deposition, allowing its use for
metal-powder.net
construction of complex 3-D geometries. Close interaction between the original equipment manufacturer and the manufacturing technology provider is vital in this area in order to focus the work on the most fruitful components for low-risk but high-commercial-benefit applications. From an industrial context the work of establishments like the AeroMet Corporation illustrates the range of component sizes and volumes of titanium powder deposit that are being explored commercially. An additional driver in powder-based manufacturing techniques stems from the promise of low-cost alloy powder through the development of new chemical and electro-chemical processes such as the electrolytic de-oxidation (known as EDO or FFC) process. It is recognised that the most attractive product would substitute into the manufacturing process as far downstream as possible. The direct economic manufacture of powders potentially opens the market for titanium alloys in non-aerospace applica-
tions which is important to minimise time to market and to increase total volume of product. This approach would also be benefit aerospace by providing an established and robust manufacturing supply chain where detailed cost analysis and potential for production "issues" are well understood. There may also be significant technological advances in terms of powder size distributions, morphology and chemistry that dictate many important factors in net-shape manufacture; powder flow characteristics determine mould fill and therefore what section size and part complexity is possible. Powder packing density determines the shrinkage required to achieve full density in the finished part. Previous work on powder based titanium routes has centred on rotating electrode (PREP), gas jet atomisation and hydride/dehydride as manufacturing sources. Each of these techniques has a characteristic product size distribution and morphology dictated by the physics of the process. An interesting
April 2005 MPR
9
Figure 3. The morphology of titanium powder produced by electrolytic reduction.
Table I. Compositions of RR1000 and U720 in Weight Percent. Alloy Co Cr Mo Ti Al Ta W Hf C RR1000 18.5 15 5 3.6 3 2 0.5 0.03
B Zr Ni 0.02 0.06 Bal.
U720
0.02 0.04 Bal
16
15
3
5
2.5 -
1
-
0.02
designing against fatigue failure and a good understanding of fatigue scatter and the "stack up" of contributing factors is vital. Progression towards more complex components such as blisks accentuates the requirement for aerofoil repair given the cost associated of blisk replacement. An accurate, less intrusive repair technique might be through direct laser deposition as developed by Loretto and Wu at Birmingham IRC. Although billets for large-scale forgings of alloy 720Li are being produced via the cast and wrought route, powder metallurgy is being considered for increasingly complex nickel disc alloys, such as RR1000 (See Table 1 and Figure 4), to avoid the risk of elemental segregation and unacceptable levels of carbonitride stringers. The composition of RR1000 has been designed with the minimum volume fraction of γ' to achieve the required levels of strength and damage tolerance. Specific quantities of chromium, cobalt and hafnium, and the ability to decorate grain boundaries with precise amounts of M23C6, M6C and MC are considered critical for good crack growth resistance at elevated temperature, particularly under creep fatigue conditions. The levels of aluminium, chromium and molybdenum have also been manipulated to avoid excessive TCP formation at the projected time at peak temperature in service. It is recognised that the use of powder processing will increase the price of billet material above the level for cast and wrought 720Li, and requires specific methods for probabilistic design/lifing and to ensure that a consistent level of cleanliness is maintained in production
aspect of electrolytic production is the possible existence, and manipulation, of a very wide range of production parameters to produce the “ideal” powder for a given application type. Of particular interest is the possibility of producing very fine powders that are currently very expensive given that they represent the tail end of the distribution curve for gasatomised product. A very fine mesh size would allow control of the size distribution of some sources of particulate defect which could be exploited in terms of increased component life or allowable stress level. Another benefit of metal oxide powder reduction is the potential for exploration of alloy systems, otherwise not possible.
This arises from the solid-state nature of the reaction, so that alloy systems previously rejected for reasons such as segregation could now be commercially exploited. Prospective candidates include the Ti-Mn, Ti-Ni and Ti-Fe beta eutectoid systems that could provide low-cost, age-hardenable and metastable beta alloys. It is well established that repair/life extension of aero-engine components is a prerequisite for minimised life cycle costs. Aerofoils represent a good example where economic repair schemes are possible, either at the leading edge due to foreign object damage or at the blade tip. Fan and compressor aerofoils present some unique difficulties in terms of
Figure 4. An intermediate pressure turbine disc forged from RR1000.
Figure 5. The influence of solution treatment temperature on the room temperature proof and ultimate tensile strength of fully heat treated RR1000. Fan air cooling was used to quench large-scale forgings after solution treatment. Also shown is an optical micrograph of RR1000 after near solvus treatment at 1125°C to page 12 give a grain size of ASTM 12 (6µm).
10
MPR April 2005
metal-powder.net
page 10
Figure 6. Metal as poetry? The graceful symmetry of aero engine turbine blades could soon be enhanced by the strength of PM technology.
material. While the powder route is currently preferred, it is considered that large diameter billet can be produced from cast and wrought processing but is unlikely to offer a cost saving over the powder product. It is well documented that the presence of inclusions, which are picked up during powder processing, can severely limit the fatigue performance of an alloy. Over the last decade, the cleanliness of argon gas-atomised powder has improved considerably through the elimination of organic seals and the use of progressively finer sieve sizes to screen out non-metallic inclusions. This has been made affordable by the development of nozzles for high yield powder production. Following consolidation of canned powder by hot isostatic pressing or hot compaction, it is the industry practice to extrude the cans to a reduction ratio of 5.5:1 to break down any prior particle boundaries and to produce a fine grain
12
MPR April 2005
size that is typically smaller than ASTM 12 (6 µm). The RR1000 data in Figure 5 indicate that a relatively small change in average grain size, under creep-fatigue conditions, can result in significant changes in crack growth rate. The fine structure is also ideal for inspection of the billet and subsequent superplastic deformation on isothermal forging. As lower strain rates are used during deformation compared to the hot die route, isothermal forging currently offers greater control of structure and the ability to forge quite complex, "nearnet" shapes that enable significant reductions in material input weight and machining time. Sub-solvus solution heat treatment is applied to RR1000 to give a stronger material compared to super-solvus treatment, (Figure 6). There is an incentive to minimise subsequent quenching stresses in powder-processed alloys due to the occurrence of ceramic inclusions. Although the majority of these inclusions
are small, they must remain uncracked if the finished component is to realise the full fatigue life potential of the alloy. Evidence from cyclic rig tests on subscale powder discs has shown that having considered quenching residual stresses after ageing, discs produced from fan aircooled forgings exhibit a greater fatigue life than those machined from oilquenched forgings. For this and other reasons, RR1000 forgings are fan aircooled after solution treatment. However, as the strength of the alloy is not fully optimised, the design envelope for discs may increase to provide the required tolerance to counter high stresses that may arise from unforeseen shaft over-speed events. The current combination of technical and commercial drivers demand significant advances in component reliability and reduced life cycle costs. Together with the shortening of timescales associated with new product development and production lead times, this is proving to be fertile ground for net shape technologies and innovative thinking on powder. It is also probable that utilisation of metal deposition techniques for repair will expand significantly in the near term. Production of pre-alloyed powder via electrolytic reduction of mixed oxide powders offers the potential of both widespread commercial use of net shape HIP and new alloys. However, it is probable that the most economically attractive market for the manufacturing supply chain lies outside the aerospace industry. The composition of RR1000 has been designed with the minimum volume fraction of γ' to achieve the required levels of strength and damage tolerance. The acceptable balance between tensile strength and crack growth resistance with a 25°C increase in temperature capability has been established by forging consolidated powder billet coupled with advanced fan air-cooling from near-solvus solution heat treatment and additional 30°C improvements may be possible.
Footnote All images in this feature are reproduced with the permission of RollsRoyce plc, copyright © Rolls-Royce plc 2004.
metal-powder.net