Prototype Rhenium Component for Stirling Engine Power Conversion Todd Leonhardt1 and Frank Ritzert2 1
Rhenium Alloys, Inc., Elyria, OH 44036 NASA Glenn Research Center, Cleveland, Ohio 44135 440-365-7388, fax 440-3669831,
[email protected] 2
Abstract. The Stirling engine power conversion concept is a candidate to provide electrical power for deep space missions. A key element for qualifying potential flight hardware is the long-term durability assessment for critical hot section components of the power converter. One such critical component is the power converter heater head, which is a high-temperature pressure vessel that transfers heat to the working gas medium of the converter. Rhenium is a candidate material for the heater head application because of its high melting point (3453 K), high elastic modulus (420 GPa), high yield and ultimate tensile strengths at both ambient and elevated temperatures, excellent ductility, and exceptional creep properties. Rhenium is also attractive due to the potential of near-net-shape (NNS) manufacturing techniques that allow components to be produced using less material, which lowers the overall cost of the component. The objective of this research was to demonstrate the manufacturing method using rhenium for this high-temperature power conversion application to provide space power system designers with generally applicable technology for future applications.
INTRODUCTION The Department of Energy (DOE), Lockheed Martin, Stirling Technology Company (STC), and NASA Glenn Research Center (NASA GRC) are now developing a Stirling Radioisotope Generator (SRG) as a high-efficiency alternative to Radioisotope Thermoelectric Generators (RTG’s) for NASA space science missions (Fig. 1). The SRG is being developed for multi-mission use, including providing electric power for unmanned Mars rovers and deep space missions. An attractive feature of the SRG system is that its efficiency of 20-25% would reduce the required amount of radioisotope by a factor of three or more compared to RTG’s. This significantly reduces radioisotope cost, radiological payload, system cost, and provides efficient use of the scarce domestic supply of radioisotope resources. A follow-on task to this highly focused SRG flight program is a NASA in-house effort intended to develop an even higher efficiency, lower mass Stirling converter for use with a radioisotope, reactor, or solar concentrator heat source. One of the goals for this upgraded version of the SRG is to achieve improved performance by increasing the Carnot efficiency of the converter. To achieve this improvement, it is necessary to maximize the hot-end temperature and the pressure of the working gas. While increasing both the temperature and pressure of the working fluid will place an increased burden on all components of the converter, the heater head will be the most severely tested. The current SRG heater head is made from the nickel base superalloy 718 and operates at 650°C, which is this alloy’s maximum use-temperature. Further increases in temperature will require the use of a more advanced high-temperature superalloys, ceramics, or refractory metal alloys. More advanced superalloys have the potential of increasing the operating temperature to perhaps 1123K, while refractory materials (metals and ceramics) may allow temperatures to be as high as 1398K. As part of the overall advanced Stirling program, a materials assessment of potential high– temperature candidates was undertaken at NASA GRC.
For space-based applications it is envisioned that Stirling converters will operate continuously once the radioisotope fuel has been loaded. The absence of repeated start-up and cool-down cycles greatly reduces the overall burden placed on the material system. In particular, because the reverse plasticity requirement is intended to insure that plasticity doesn’t occur during shutdown, this material property criterion can be ignored during the screening process. Similarly, fatigue damage and creep-fatigue are also not considered significant since the internal gas pressure does not vary significantly, and the resulting heater head wall stress will vary by only +/- 10% of the mean stress. Referring to a
classical mean stress diagram (Modified Goodman, for example), and locating this service operational point on the normalized mean stress diagram, it is found to lie so far below the Modified Goodman boundary (that represents the lowest possible fatigue strength) that it is clear that fatigue is not an issue. Therefore, for a continuously operating converter, creep and creep-related mechanisms are the only dominant failure mechanisms, and thus was the mechanical property used to quickly screen a wide variety of materials. Of course, other factors, in addition to failure mode considerations, will also dictate whether a particular material is suitable for use as a heater head. The following is a generic list of screening criteria used to screen the candidate materials: 1) creep properties, 2) fabricability, 3) helium gas containment, 4) long-term stability/compatibility, 5) ability to form a hermetical close-out seal, and 6) ductility/toughness (to assist in fabrication, handling, and resistance to foreign object damage). In addition to these issues, each particular material system has its own unique characteristics that must be addressed as well.
ASTAR-811C and Rhenium were chosen for in-depth study for the advanced Stirling heater head program. W-Re alloys could provide a worthwhile second-tier study along with, possibly, the Nb-based alloys. Long-term creep testing of all alloys is required and their ductility or brittleness after welding must be investigated. Other aspects such as weldability, microstructural stability, and environmental issues will also be addressed. The results of all refractory alloy research will provide space power system designers with information and options for future applications supporting ambitious space-related missions.
FIGURE 1. Stirling Radioisotope Generator (SRG).
A Stirling engine power conversion concept is a candidate to provide electrical power for deep space missions. A key element for qualifying potential flight hardware is long-term durability assessment for critical hot section components of the power converter. One such critical component is the power converter heater head. The heater head is a hightemperature pressure vessel that transfers heat to the working gas medium of the converter, which is typically helium. An efficient heater head design is the result of balancing the divergent requirements of, on one hand, thin walls for increased heat transfer, and on the other hand, thick walls to lower the wall stresses and thus improve creep resistance/durability. The development of a higher efficiency, lower mass Stirling converter for use with a radioisotope, reactor, or solar concentrator heat source will improve performance by increasing the Carnot efficiency
of the converter. To achieve this improvement, it is necessary to maximize the hot-end temperature and the pressure of the working gas. While increasing both the temperature and pressure of the working fluid will place an increased burden on all components of the converter, the heater head will be the most severely tested. Research on refractory metals is being conducted in support of advanced space power and propulsion materials requirements. The objective of this research is to develop and characterize new high-temperature power conversion materials to provide space power system designers with generally applicable technology for future applications. The successful refractory metal candidates will meet or exceed design needs for “next generation” Stirling engine heater heat requirements. In particular, alloy performance at temperatures between 1200K and 1366K for 10 years and beyond is desirable. Other primary design criteria include a design stress most likely not to exceed 140 MPa, favorable joining characteristics, long-term thermal stability, and good physical properties. Such targets exceed current heater head design requirements and would take advantage of the strong performance of refractory alloys at high temperatures and stresses.
Rhenium Rhenium is one of the last naturally occurring elements to be discovered by Ida Tacke, Walter Noddak and Otto Berg in 1925. Rhenium is named after Germany’s Rhine River. Only a few milligrams of rhenium were produced in 1927, and the first full gram in 1928. It was not until the 1960’s that rhenium was produced in a full-scale manufacturing operation (Davenport, 1964): (Bryskin, 1991). Rhenium is often referred to as a refractory metal because of its melting point of 3453 K, but it is not a true refractory metal because the crystal structure is hexagonal close-packed, and rhenium does not form a carbide. Rhenium is unique because of its high elastic modulus (420 GPa), and has a work hardening rate of (n = 0.5) (Lukens, 2001). Furthermore, rhenium also possesses high electrical resistance across a wide temperature range (Bryskin, 1991); (Grobstein, 1990). Additionally, powder metallurgy rhenium has consistently provided high yield and ultimate tensile strengths at both ambient and elevated temperatures, while maintaining excellent ductility Table 1 (Biaglow 1998). Rhenium also has excellent creep and low-cycle fatigue properties required by demanding high-temperature applications (Chazen, 1995); (Chazen, 1998); (Biaglow, 1998); (Leonhardt, 1998). TABLE 1. Tensile Properties of Hot Isostatic Pressed Rhenium SAMPLE NUMBER
TEST ANNEALING TEMP (K) TEMP (K) TIME
0.2% YIELD ULTIMATE ELASTIC STRENGTH STRENGTH MODULUS, (MPa) (MPa) (GPa) 236.5
910.8
STRAIN TO FAILURE RATIO (%)
407.5
17.2
915.6
428.9
18.5
1017.8
301.86
ELONGATION (%)
HIPED-1
294
1922/0.5
HIPED-2
294
1922/0.5
232.4
HIPED-3
294
1922/0.5
414.39
HIPED-4
294
1922/0.5
422.87
1047.62
265.25
30
HIPED-5
294
1922/0.5
610.2
1067.6
311
26
25
HIPED-6
294
1922/0.5
506.8
997.6
479
HIPED-7
1088
1922/0.5
254.4
561.9
186.2
26
23.9
HIPED-8
1088
1922/0.5
264.1
497.8
193.7
12.3
HIPED-9
1088
1922/0.5
337.85
496.44
328.89
HIPED-10
1088
1922/0.5
375.85
552.28
326.13
35
HIPED-11
1367
1922/0.5
274
285.58
33.71
7.2
HIPED-12
1367
1922/0.5
264.5
271.03
30.46
HIPED-13
1644
1922/0.5
179.9
215.8
145.5
2.28
HIPED-14 HIPED-15
1644 1922
1922/0.5 1922/0.5
191 237.19
251.6 249.59
137.2 36.54
4.49
HIPED-16
1922
1922/0.5
175.13
198.57
59.29
9
HIPED-17
2422
1922/0.5
77.22
79.98
7.58
10
HIPED-18
2422
1922/0.5
77.22
83.43
10.34
10
HIPED-20
2506
1922/0.5
54.5
56.2
13.71
19.2
14
8.8
14
Manufacturing To meet the goals of Stirling converter for use with a radioisotope reactor, a heater head prototype was manufactured by near net shape powder metallurgy processing. The NASA advanced stirling heater head program obtained one traditional powder metallurgy rhenium rod and one near net shaped (NNS) header head to examine the long-term creep properties for deep space missions. The rhenium is being examined as a possible candidate material because of it inherent high temperature properties. The powder metallurgy rhenium rod, NNS shape heater head, and the tantalum alloys ASTAR-811C rod are being evaluated to see which candidate material performs better for the specific application. The goal is to produce a cost effective material, which is best suited as a high-temperature pressure vessel. An efficient heater head design is the result of balancing the divergent requirements of, on one hand, thin walls for increased heat transfer, and on the other hand, thick walls to lower the wall stresses and thus improve creep resistance/durability. These goals may be met by using one of the candidate material because their strengths. To meet the goals a prototype header head manufactured via a near net shape process was employed to meet the cost and manufactability requirements.
The method of manufacturing NNS rhenium components was developed under a NASA Phase II SBIR (Leonhardt, 2000). The NNS manufacturing method was developed to produce rhenium high performance liquid apogee engines. The goal of the program was to significantly reduce the cost which in turn pushed to reduce the quantity of rhenium required to produce rhenium chambers. The same method discovered under the SBIR program was employed to manufacture the prototype (SGR) heater head.
To minimize the cost to NASA advanced Stirling heater head program, NASA supplied rhenium sheet to be reclaimed to ammonium perrhenate (APR) to produce rhenium powder. Ammonium perrhenate is the starting material to produce rhenium metal powder. Ammonium perrhenate is 69.4% rhenium containing compound Figure 2. The conversion is around 90% efficient for reclaiming rhenium solid to rhenium powder. By using reclaimed rhenium, the cost was reduced without sacrificing purity or properties. After purification, the rhenium powder produced is 99.99% pure Figure 3. This method reduces to overall cost to the customer without compromising purity or properties. Both the rhenium rod and the NNS prototype heater head were manufactured with reconstituted rhenium sheet supplied by NASA GRC.
FIGURE 2. Ammonium Perrhenate 69.4% Rhenium.
FIGURE 3. –200 Mesh Rhenium Metal Powder.
The process of producing a NNS rhenium component has several steps starting with packing rhenium powder into a cold isostatic pressing mold. To increase compaction, powder is poured into a vibrating mold to increase packing density of powder. By compacting the powder with vibration or tapping, the apparent density increases from 1.84 g/cc to 3.03 g/cc. The cold isostatic pressing (CIP) method used for NNS processing is referred as the wet-bag process as shown in Fig 4 and 5 (Price, 1997). The cold isostatic press compacts rhenium powder at 400 MPa with hydrostatic
pressure Figure 5. At this pressure, the powder compact achieves excellent green strength and the desired complex shape due to the fixed mandrel Figure 6. After pressing, the NNS consolidated powder compact was removed from the CIP tooling Figure 4. This also includes the mandrel. Then the NNS component was pre-sintered at 0.5 of the melting point, so powder compact has enough strength to be handled part. Pre-sintering was performed in a pusher furnace for 16 hours under a hydrogen atmosphere.
Sintering is a high temperature consolidation process to promote solid-state diffusion, and this is typically done at 0.75 of the melting point of rhenium. Sintering was performed in a hydrogen atmosphere at 2700K for 5 hours. After sintering, container-less, hot isostatic pressing was employed to increase density and move the porosity into the grains Fig 7. To obtain the final dimensions and to increase the density, hot isostatic pressing parameter was performed at 2100K for 4 hours at 200 MPa of argon pressure. The final density of the manufactured heater head was 20.82 g/cc obtained by immersion in water.
FIGURE 4. Cold Isostatic Pressing Tooling and Fixed Mandrel.
FIGURE 6. On the Left an Un-Machined NNS Header Head, and on the Right an As Pressed NNS Heater Head.
FIGURE 5. 420 Mpa Cold Isostatic Press.
FIGURE 7. Microstructure of the Hot Isostatic Pressed Rhenium. Porosity is White, and the Grains are Gray Contrast Due to Image Produced by Polarized Light.
As shown in figure 6, the NNS header head shrinks approximately 30% in the vertical direction while 15% in the horizontal direction after thermal processing. The NNS shape part is slightly longer than is required. This is to allow for any break away, as shown in figure 6. Break away is a common occurrence in processing NNS powder metallurgy
parts. As shown in figure 7, the microstructure of the NNS shaped part has a fine grain size after hot isostatic pressing. This is typical rhenium microstructure obtained by hot isostatic pressing, the porosity is internal to grain structure, and the porosity has no effect of the properties of rhenium. As shown in figure 8, the NNS heater head was sectioned along its length to examine the wall thickness variation, and to further remove mechanical test bar to compare rhenium rods to NNS components.
CONCLUSION
FIGURE 8. Sectioned Heater Head.
Historically, near-net shape powder metallurgy rhenium components reduce manufacturing cost of the rhenium component by 25%, and reduce the manufacturing time by 30-40%. The quantity of rhenium metal powder used to produce a rhenium component is reduced by approximately 70% and the subsequent reduction in machining schedule and costs is nearly 50%. In the case of the NNS heater head only 2500 grams of rhenium were used compared if it were made from a solid piece of rhenium. The solid piece would weigh almost 20 Kg. This is a savings of 87% on raw material. The sold heater head would require significant machining via electric discharge machine. The NNS heater head would require minimal diamond grinding to finish the part to size. Since NASA GRC provided the rhenium sheet for reconstitution to rhenium powder, the cost saving to the program to manufacture the rhenium rod and the NNS heater head was over $14,000.
The future generation Stirling engine heater head will operate between 1200K and 1400K for 10-15 years with stress levels around 140 MPa. The material must be able to be joined with excellent long-term thermal stability, and good creep properties. Such targets properties exceed current information bases on refractory metals at high temperatures and stresses. The tantalum alloy ASTAR-811C and rhenium are the best candidates for this long–term application, but further research is needed to determine the overall best candidate based on ease and cost of manufacturing with required properties.
ACKNOWLEDGMENTS The authors would like to acknowledge the significant contribution made by James Downs for his assistance in producing the NNS header head, and Clifford Guthman for his assistance on the preparation of this paper,
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