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Nuclear fusion ‘a future option for MIM tungsten’
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technical trends
Nuclear fusion 'a future option for MIM tungsten' The characteristics of hardmetals and refractory materials make them clear candidates for use in 'hot' technologies such as power generation by nuclear fusion. Ken Brookes continues his coverage of a Florida conference that set out some cutting edge applications... wo technologies that could be regarded as cutting edge in terms of manufacturing technique and ultimate application were dealt with during a discourse on a possible role for tungsten in nuclear fusion by Berthold Zeep of the Forschungszentrum Karlsruhe Institute for Materials Research in Karlsruhe, Germany. His paper was entitled Metal injection moulding of microstructured tungsten components for fusion applications.
T
THIS IS the second part of coverage of the 2006 International Conference on Tungsten, Refractory & Hardmetals VI, sponsored jointly by the Metal Powder Industries Federation, APMI International and the Refractory Metals Association. Papers included are those where I attended the actual presentation, which means that some excellent contributions will necessarily be omitted from this series. Those which are missed, including some papers which were not even presented at the conference - perhaps due to health or transportation problems - are nevertheless included in the electronically published proceedings. This CD, produced in a creditably short time, can be purchased from the MPIF in Princeton, New Jersey. The contributions reviewed here were chosen for their general interest, rather than for any particular theme. Interestingly, though, the first and last papers illustrate dramatically different methods of sintering tungsten components, the first employing a high-temperature furnace at 2500ºC and the second a furnace limited to 1600ºC and requiring either copper infiltration or additional alloying to provide a liquid phase.
Ken Brookes
22
MPR May 2006
For future fusion power plants, the Forschungszentrum Karlsruhe is designing a helium-cooled divertor (Figure 1) for the European Power Plant Conceptual Study (PPCS). The main functions of a divertor are to remove fusion reaction "ash" (alpha particles), unburnt fuel and eroded particles, and not least about 15 per cent of the thermal power from the reactor. The divertor is one of the plasma-facing components and one of the most critical parts in the plant, since it determines the amount of impurities in the plasma and must withstand thermal loads of up to 10 MW/m². Though even tungsten could hardly withstand the millions of degrees involved in fusion reactions, it is considered the most promising material to withstand the extreme environment because of its high melting point, high thermal conductivity and relatively low thermal expansion. In addition, it is 'lowactivating', possesses high hardness, good resistance to sputtering and erosion and is suitable for use as a thermal shield.
Nevertheless, further alloy development will probably be necessary to fulfil the exacting material requirements. Two possible cooling strategies are being investigated, a multiple jet design, based on direct jet-to-wall impingement cooling, and convective cooling using passive heat transfer components, so-called slot arrays to increase the cooling surface and optimise heat transfer capacity. Each concept requires a large number of complex shaped tungsten parts. The properties of tungsten made it difficult to manufacture such parts on a large scale, for example 300 000 slot arrays per divertor. A suitable production method had to be developed, the most promising of which appeared to be powder injection moulding, the subject of the research described in this paper. Initial experiments were carried out in a Brabender measuring mixer providing a batch volume of 50ml. The rheological properties of the feedstocks were proportional to the mixing torque. By keeping the amount of feedstock in the mixing
Figure 1. Divertor design and required tungsten components.
0026-0657/06 ©2006 Elsevier Ltd. All rights reserved.
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chamber constant, viscosities of different experimental feedstocks could be estimated and compared directly. Milling was carried out in an Atritor multirotor cell mill. A Battenfeld Microsystem 50 (MS50) injection-moulding machine was used for initial feedstock tests, the plastification unit with 14mm screw allowing the processing of small feedstock batches prepared in the measuring mixer. The MS50 has an injection module especially designed for micro-injection moulding. For optimised process control, the units for plastification, dosage and injection were separated. For scale-up experiments, feedstock was produced on a Leistritz ZSE 27 twinscrew extruder. Gravimetric dosage systems from Brabender Technologie were applied for feeding the extruder. Replication experiments on macrosized samples were carried out on a Ferromatik K50S screw injection-moulding machine. The injection-moulded samples were sintered at 2500°C in H2 in a Plansee Metall furnace. Further densification experiments were carried out in a Dieffenbacher HIP3000. Sinter densities were measured with a Micromeritics helium pycnometer. A major challenge was to develop a tungsten feedstock for powder injection moulding. Grain size had to be as small as possible to provide good design accuracy in the final component as well as high sinter activity in the powder. Commercially available tungsten powders are usually highly agglomerated. Depending on the reduction parameters, different types of agglomeration appear. Very fine powders with grain size below 1μm build sponge-like agglomerates called pseudomorphs which stick together in a more or less loose manner. Larger grain sizes result in agglomerates of individually grown crystals. In both cases, deagglomeration tends to be insufficient in a measurement mixer or a twin-screw extruder. One referenced source showed that in W-Ni-Fe feedstocks employing tungsten powder with a median grain size of 16.5μm, deagglomeration of the powder down to 8.9?m was sufficient to reduce feedstock viscosity and increase the achievable solid loading. With powders of grain size <1μm, it was found that milling generally had no or even a nega-
metal-powder.net
Figure 2. Torque diagrams of feedstocks applying as received and premilled tungsten powders with grain sizes of 0.7?m and 45 vol per cent of solid load (left) and a grain size of 3.0?m and 50 vol per cent solid load (right). Mixing was performed at 125°C and 60rev/min for 1h. Agglomerates built of individual crystals could be broken down by applying an appropriate milling method (eg multirotor cell mill), whereas the sponge-like agglomerates built by particles <1?m are only broken down to smaller sponge pieces.
tive effect on the 'compoundability' of the feedstock, whereas deagglomeration of a powder with grain size of 3μm significantly improved the compoundability (Figure 2). A premilled tungsten powder with D50=2.75?m was found to be most suitable for further experiments on binder optimisation. A wax/thermoplastic-based binder system was developed to fulfil the special demands of high green stability of the feedstock and green density of approximately 11g/cm³. Feedstocks with solid loads between 50 and 64 vol per cent were produced and tested in injection moulding experiments. For injection moulding of microstructured parts, a solid load of 55 vol per cent was optimal. Processable temperature range was limited by the melting points of the binder ingredients at the lower end and by thermal degradation of the binder as well as tungsten powder oxidation at the upper end. Within a temperature range of 125°C to 180°C, microstructured gear cases with
outer diameter of 8.8mm, 37 teeth and a module of 0.216, and individual micro gear wheels with outer diameter 850μm and 9 teeth, were replicated by injection moulding, using the feedstock with solid load of 55 vol per cent. In both cases, complete mould-filling and part removal were made possible by an adapted injectionmoulding process (Figure 3). Tungsten testpieces for tensile strength, yield strength and fracture toughness had been injection moulded (Figure 4) and were in process of sintering at the time this paper was presented. To replicate a fusion-relevant demonstration sample by powder injection moulding, a mould insert for a passive heat-transfer 'strait slot array' with 24 slots was manufactured by micromilling in brass and integrated into an existing tool. Due to the tool design having the cavity on the runner side, ejectors could not be employed with this mould. Experiments showed that the lack of cavity ejectors caused either slightly insufficient mould filling at the slot tips
Figure 3. Green demonstrators for injection-moulded micro parts and microstructured parts made of a feedstock with 55 vol per cent solids (left: micro gear wheel with outer diameter of 850μm and 9 teeth; right: gear case with outer diameter of 8.8mm, 37 teeth and module 0.216).
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Figure 5. Injection-moulded green body of the passive heat-transfer-promoting 'strait slot array' (left) showing good demoulding behaviour but insufficient mould-filling at the slot tips (right).
Figure 6. Sintered (left) and HIPed (right) samples prepared by injection moulding, using a powder with D50=2.75μm.
(Figure 5) or inefficient part removal. An improved injection-moulding tool would be used in future experiments. In MIM, inadequate debinding leads to distortion, fractures, bubble formation and
other defects. The author used a two-step debinding procedure, starting with solvent debinding of the wax component, followed by heating to remove residual binder. After solvent debinding for at least 72 hours at
room temperature, carbon content was less than 0.003 wt per cent. Heating the solvent to 50°C reduced solvent debinding time to approximately two hours, with a final carbon content of 5ppm. After thermal analysis of test heating cycles, a heating rate of 2K/min and holding times at 80°C and 140°C were set. Further optimisation was in progress. Parts were sintered in dry hydrogen at 2500°C for two hours. This resulted in a grain size of approximately 20μm and a final density of 96 per cent, measured by He psychometric analyses. To improve sintered density, the sintered components were HIPed. In the first HIP trials, massive grain coarsening up to 100μm was observed (Figure 6). Extensive grain growth during HIPing was suppressed by combining a temperature of 1800°C with a high pressure of 2800 bar for 30 minutes. Further optimisation of HIP parameters for higher density and greater suppression of grain growth was in progress. Further experiments are planned for tungsten alloys such as W/1La and W/Ni/Fe. Fracture test specimens made from these alloys will be tested and compared with pure tungsten samples prepared by MIM.
How (not) to join steel and carbide Failure can have its uses if we learn lessons from it... sense, this investigation was a failure, since it failed to attain its hoped-for objectives. Yet, by documenting all the procedures in great detail, it succeeded in showing a direction to be avoided and provided useful pointers for follow-up research. Though perhaps misnamed, the well-produced paper by Animesh Bose (Materials Processing Inc, Fort Worth, Texas) entitled Joining of hardmetal with steel was therefore of considerable interest. I found the choice of steels particularly interesting, since maraging steels were chosen by the investigators, and I wrote one of the first data publications on these high-strength alloys when they were invented by Inco around 40 years ago. In this paper, Imperial units were used throughout, with no conversions given.
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N ONE
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MPR May 2006
Joining of dissimilar materials has become increasingly important defencerelated applications (Bose's sponsor was the United States Air Force). Additionally, this technical area is of great importance as it has commercial applications in the area of cutting tools and wear-resistant composites for the oil and gas industry. Two different grades of steels were examined during this investigation, an ultra-high strength and high fracture toughness maraging steel (precipitationhardened martensitic stainless steel) and the low-carbon AISI 1018 steel having low strength but high impact resistance. Since maraging steels harden by precipitation of intermetallic compounds, the hardening is independent of cooling rate and through 'thick-section' hardness is achievable with little or no distortion.
The other problem commonly associated with conventional high-strength steels based on carbon is the problem of decarburisation. Since this alloy does not require carbon for hardness, decarburisation is not an issue with maraging steels. Typically, these steels attain strength through an aging treatment carried out at 480ºC for three to 12 hours depending on the desired strength level. There are several grades of maraging steel, typically designated by their nickel content and the yield strength expressed in ksi (103lbf/in2). For example, an 18Ni(250) is a maraging steel that has 18 per cent Ni and attains a strength of 250 ksi. It is interesting that the cobalt and titanium contents gradually increase as the strength of the alloy is increased. The 18Ni (350) alloy can be classified as an
metal-powder.net
23/05/2006
17:06
Page 22
technical trends
Nuclear fusion 'a future option for MIM tungsten' The characteristics of hardmetals and refractory materials make them clear candidates for use in 'hot' technologies such as power generation by nuclear fusion. Ken Brookes continues his coverage of a Florida conference that set out some cutting edge applications... wo technologies that could be regarded as cutting edge in terms of manufacturing technique and ultimate application were dealt with during a discourse on a possible role for tungsten in nuclear fusion by Berthold Zeep of the Forschungszentrum Karlsruhe Institute for Materials Research in Karlsruhe, Germany. His paper was entitled Metal injection moulding of microstructured tungsten components for fusion applications.
T
THIS IS the second part of coverage of the 2006 International Conference on Tungsten, Refractory & Hardmetals VI, sponsored jointly by the Metal Powder Industries Federation, APMI International and the Refractory Metals Association. Papers included are those where I attended the actual presentation, which means that some excellent contributions will necessarily be omitted from this series. Those which are missed, including some papers which were not even presented at the conference - perhaps due to health or transportation problems - are nevertheless included in the electronically published proceedings. This CD, produced in a creditably short time, can be purchased from the MPIF in Princeton, New Jersey. The contributions reviewed here were chosen for their general interest, rather than for any particular theme. Interestingly, though, the first and last papers illustrate dramatically different methods of sintering tungsten components, the first employing a high-temperature furnace at 2500ºC and the second a furnace limited to 1600ºC and requiring either copper infiltration or additional alloying to provide a liquid phase.
Ken Brookes
22
MPR May 2006
For future fusion power plants, the Forschungszentrum Karlsruhe is designing a helium-cooled divertor (Figure 1) for the European Power Plant Conceptual Study (PPCS). The main functions of a divertor are to remove fusion reaction "ash" (alpha particles), unburnt fuel and eroded particles, and not least about 15 per cent of the thermal power from the reactor. The divertor is one of the plasma-facing components and one of the most critical parts in the plant, since it determines the amount of impurities in the plasma and must withstand thermal loads of up to 10 MW/m². Though even tungsten could hardly withstand the millions of degrees involved in fusion reactions, it is considered the most promising material to withstand the extreme environment because of its high melting point, high thermal conductivity and relatively low thermal expansion. In addition, it is 'lowactivating', possesses high hardness, good resistance to sputtering and erosion and is suitable for use as a thermal shield.
Nevertheless, further alloy development will probably be necessary to fulfil the exacting material requirements. Two possible cooling strategies are being investigated, a multiple jet design, based on direct jet-to-wall impingement cooling, and convective cooling using passive heat transfer components, so-called slot arrays to increase the cooling surface and optimise heat transfer capacity. Each concept requires a large number of complex shaped tungsten parts. The properties of tungsten made it difficult to manufacture such parts on a large scale, for example 300 000 slot arrays per divertor. A suitable production method had to be developed, the most promising of which appeared to be powder injection moulding, the subject of the research described in this paper. Initial experiments were carried out in a Brabender measuring mixer providing a batch volume of 50ml. The rheological properties of the feedstocks were proportional to the mixing torque. By keeping the amount of feedstock in the mixing
Figure 1. Divertor design and required tungsten components.
0026-0657/06 ©2006 Elsevier Ltd. All rights reserved.
MPR_May_22_30.qxd
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Page 23
chamber constant, viscosities of different experimental feedstocks could be estimated and compared directly. Milling was carried out in an Atritor multirotor cell mill. A Battenfeld Microsystem 50 (MS50) injection-moulding machine was used for initial feedstock tests, the plastification unit with 14mm screw allowing the processing of small feedstock batches prepared in the measuring mixer. The MS50 has an injection module especially designed for micro-injection moulding. For optimised process control, the units for plastification, dosage and injection were separated. For scale-up experiments, feedstock was produced on a Leistritz ZSE 27 twinscrew extruder. Gravimetric dosage systems from Brabender Technologie were applied for feeding the extruder. Replication experiments on macrosized samples were carried out on a Ferromatik K50S screw injection-moulding machine. The injection-moulded samples were sintered at 2500°C in H2 in a Plansee Metall furnace. Further densification experiments were carried out in a Dieffenbacher HIP3000. Sinter densities were measured with a Micromeritics helium pycnometer. A major challenge was to develop a tungsten feedstock for powder injection moulding. Grain size had to be as small as possible to provide good design accuracy in the final component as well as high sinter activity in the powder. Commercially available tungsten powders are usually highly agglomerated. Depending on the reduction parameters, different types of agglomeration appear. Very fine powders with grain size below 1μm build sponge-like agglomerates called pseudomorphs which stick together in a more or less loose manner. Larger grain sizes result in agglomerates of individually grown crystals. In both cases, deagglomeration tends to be insufficient in a measurement mixer or a twin-screw extruder. One referenced source showed that in W-Ni-Fe feedstocks employing tungsten powder with a median grain size of 16.5μm, deagglomeration of the powder down to 8.9?m was sufficient to reduce feedstock viscosity and increase the achievable solid loading. With powders of grain size <1μm, it was found that milling generally had no or even a nega-
metal-powder.net
Figure 2. Torque diagrams of feedstocks applying as received and premilled tungsten powders with grain sizes of 0.7?m and 45 vol per cent of solid load (left) and a grain size of 3.0?m and 50 vol per cent solid load (right). Mixing was performed at 125°C and 60rev/min for 1h. Agglomerates built of individual crystals could be broken down by applying an appropriate milling method (eg multirotor cell mill), whereas the sponge-like agglomerates built by particles <1?m are only broken down to smaller sponge pieces.
tive effect on the 'compoundability' of the feedstock, whereas deagglomeration of a powder with grain size of 3μm significantly improved the compoundability (Figure 2). A premilled tungsten powder with D50=2.75?m was found to be most suitable for further experiments on binder optimisation. A wax/thermoplastic-based binder system was developed to fulfil the special demands of high green stability of the feedstock and green density of approximately 11g/cm³. Feedstocks with solid loads between 50 and 64 vol per cent were produced and tested in injection moulding experiments. For injection moulding of microstructured parts, a solid load of 55 vol per cent was optimal. Processable temperature range was limited by the melting points of the binder ingredients at the lower end and by thermal degradation of the binder as well as tungsten powder oxidation at the upper end. Within a temperature range of 125°C to 180°C, microstructured gear cases with
outer diameter of 8.8mm, 37 teeth and a module of 0.216, and individual micro gear wheels with outer diameter 850μm and 9 teeth, were replicated by injection moulding, using the feedstock with solid load of 55 vol per cent. In both cases, complete mould-filling and part removal were made possible by an adapted injectionmoulding process (Figure 3). Tungsten testpieces for tensile strength, yield strength and fracture toughness had been injection moulded (Figure 4) and were in process of sintering at the time this paper was presented. To replicate a fusion-relevant demonstration sample by powder injection moulding, a mould insert for a passive heat-transfer 'strait slot array' with 24 slots was manufactured by micromilling in brass and integrated into an existing tool. Due to the tool design having the cavity on the runner side, ejectors could not be employed with this mould. Experiments showed that the lack of cavity ejectors caused either slightly insufficient mould filling at the slot tips
Figure 3. Green demonstrators for injection-moulded micro parts and microstructured parts made of a feedstock with 55 vol per cent solids (left: micro gear wheel with outer diameter of 850μm and 9 teeth; right: gear case with outer diameter of 8.8mm, 37 teeth and module 0.216).
May 2006 MPR
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Figure 5. Injection-moulded green body of the passive heat-transfer-promoting 'strait slot array' (left) showing good demoulding behaviour but insufficient mould-filling at the slot tips (right).
Figure 6. Sintered (left) and HIPed (right) samples prepared by injection moulding, using a powder with D50=2.75μm.
(Figure 5) or inefficient part removal. An improved injection-moulding tool would be used in future experiments. In MIM, inadequate debinding leads to distortion, fractures, bubble formation and
other defects. The author used a two-step debinding procedure, starting with solvent debinding of the wax component, followed by heating to remove residual binder. After solvent debinding for at least 72 hours at
room temperature, carbon content was less than 0.003 wt per cent. Heating the solvent to 50°C reduced solvent debinding time to approximately two hours, with a final carbon content of 5ppm. After thermal analysis of test heating cycles, a heating rate of 2K/min and holding times at 80°C and 140°C were set. Further optimisation was in progress. Parts were sintered in dry hydrogen at 2500°C for two hours. This resulted in a grain size of approximately 20μm and a final density of 96 per cent, measured by He psychometric analyses. To improve sintered density, the sintered components were HIPed. In the first HIP trials, massive grain coarsening up to 100μm was observed (Figure 6). Extensive grain growth during HIPing was suppressed by combining a temperature of 1800°C with a high pressure of 2800 bar for 30 minutes. Further optimisation of HIP parameters for higher density and greater suppression of grain growth was in progress. Further experiments are planned for tungsten alloys such as W/1La and W/Ni/Fe. Fracture test specimens made from these alloys will be tested and compared with pure tungsten samples prepared by MIM.
How (not) to join steel and carbide Failure can have its uses if we learn lessons from it... sense, this investigation was a failure, since it failed to attain its hoped-for objectives. Yet, by documenting all the procedures in great detail, it succeeded in showing a direction to be avoided and provided useful pointers for follow-up research. Though perhaps misnamed, the well-produced paper by Animesh Bose (Materials Processing Inc, Fort Worth, Texas) entitled Joining of hardmetal with steel was therefore of considerable interest. I found the choice of steels particularly interesting, since maraging steels were chosen by the investigators, and I wrote one of the first data publications on these high-strength alloys when they were invented by Inco around 40 years ago. In this paper, Imperial units were used throughout, with no conversions given.
I
N ONE
24
MPR May 2006
Joining of dissimilar materials has become increasingly important defencerelated applications (Bose's sponsor was the United States Air Force). Additionally, this technical area is of great importance as it has commercial applications in the area of cutting tools and wear-resistant composites for the oil and gas industry. Two different grades of steels were examined during this investigation, an ultra-high strength and high fracture toughness maraging steel (precipitationhardened martensitic stainless steel) and the low-carbon AISI 1018 steel having low strength but high impact resistance. Since maraging steels harden by precipitation of intermetallic compounds, the hardening is independent of cooling rate and through 'thick-section' hardness is achievable with little or no distortion.
The other problem commonly associated with conventional high-strength steels based on carbon is the problem of decarburisation. Since this alloy does not require carbon for hardness, decarburisation is not an issue with maraging steels. Typically, these steels attain strength through an aging treatment carried out at 480ºC for three to 12 hours depending on the desired strength level. There are several grades of maraging steel, typically designated by their nickel content and the yield strength expressed in ksi (103lbf/in2). For example, an 18Ni(250) is a maraging steel that has 18 per cent Ni and attains a strength of 250 ksi. It is interesting that the cobalt and titanium contents gradually increase as the strength of the alloy is increased. The 18Ni (350) alloy can be classified as an
metal-powder.net