Ivan E. Locci

University of Toledo, Toledo, Ohio 

Cheryl L. Bowman 

Glenn Research Center, Cleveland, Ohio

A Compendium of Brazed Microstructures For 

Fission Power Systems Applications

NASA/TM—2012-217623

July 2012
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Ivan E. Locci

University of Toledo, Toledo, Ohio 

Cheryl L. Bowman 

Glenn Research Center, Cleveland, Ohio

A Compendium of Brazed Microstructures For 

Fission Power Systems Applications

NASA/TM—2012-217623

July 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

5th International Brazing and Soldering Conference (IBSC)

sponsored by the American Welding Society

Las Vegas, Nevada, April 22–25, 2012
Acknowledgments

This work was performed for the NASA Exploration Technology Development Project and the Fission Power Systems Project. 

The authors gratefully acknowledge the excellent technical assistance of Joy Buehler in the metallographic lab, Terry McCue 

and David Hull in the microscopy lab, Mark Jaster, John Juhas and Adrienne Veverka in the furnace room, Timothy Ubienski, 

Joseph Lavelle, Aldo Panzanella (RIP), Anthony Kapucinski in the specimen machining group and Randy Bowman for excellent 

technical discussions.

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Level of Review: This material has been technically reviewed by technical management. 

This report contains preliminary findings, 

subject to revision as analysis proceeds.
NASA/TM—2012-217623 1 

A Compendium of Brazed Microstructures For 

Fission Power Systems Applications 

Ivan E. Locci 

University of Toledo 

Toledo, Ohio 43606 

Cheryl L. Bowman 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

NASA has been supporting design studies and technology 

development for fission-based power systems that could 

provide power to an outpost on the Moon, Mars, or an 

asteroid. Technology development efforts have included 

fabrication and evaluation of components used in a Stirling 

engine power conversion system. This investigation is part of 

the development of several braze joints crucial for the heat 

exchanger transfer path from a hot-side heat exchanger to a 

Stirling engine heat acceptor. Dissimilar metal joints are 

required to impart both mechanical strength and thermal path 

integrity for a heater head of interest. Preliminary design work 

for the heat exchanger involved joints between low carbon 

stainless steel to Inconel 718, where the 316L stainless steel 

would contain flowing liquid metal NaK while Inconel 718, a 

stronger alloy, would be used as structural reinforcement. This 

paper addressed the long-term microstructural stability of 

various braze alloys used to join 316L stainless steel heater 

head to the high conductivity oxygen-free copper acceptor to 

ensure the endurance of the critical metallic components of 

this sophisticated heat exchanger. The bonding of the 316L 

stainless steel heater head material to a copper heat acceptor is 

required to increase the heat-transfer surface area in contact 

with flowing He, which is the Stirling engine working fluid. 

Introduction 

NASA is maintaining the option for fission surface power 

for the Moon, Mars and beyond by developing and 

demonstrating technology for affordable fission surface power 

systems (FSPS). The generation of stable and continuous 

power is critical in the exploration of the surfaces of any 

planet or even an asteroid. Nuclear power is an important 

option, especially for locations in the solar system where 

sunlight is limited in availability or intensity. The baseline 

power conversion system technology chosen for the FSPS 

concept is Stirling dynamic power conversion (Refs. 1, 2, and 

3). Stirling power conversion underwent extensive 

development for space mission applications during the 1980s. 

The Space Power Demonstration Engine, consisting of two 

thermodynamically-coupled 12.5 kWe Stirling engines, was 

built to demonstrate 25 kWe Stirling conversion for use with 

the SP-100 space power reactor. 

A cornerstone of the design architecture for an affordable 

FSPS was the decision to constrain the reactor coolant outlet 

temperature to less than 900 K (627 C) (Ref. 4). This 

represents a relatively low temperature compared to 1375 K 

(1100 C) for the SP-100 reactor designed during the 1980s. 

Constraining the reactor operating temperature allows the use 

of stainless steel and other non-refractory materials for the 

power system structure. Under the sponsorship of NASA, 

Sunpower, Inc. began the design and development of the 

Power Conversion Unit (PCU) for the fission power 

Technology Demonstration Unit (TDU) in April 2009, and is 

on schedule for delivery of a 12 kWe PCU consisting of two 

thermodynamically-coupled 6 kWe Stirling engines in 2012 

(Ref. 5). A preliminary graphic of the PCU is shown in 

Figure 1 (Ref. 2). Dissimilar metal joints are required to impart 

both mechanical strength and thermal path integrity for the 

Stirling power conversion system hot-side heat exchanger 

(HX) components for a Fission Power System (FPS). Brazing 

is frequently a good approach for joining dissimilar metals in 

net-shape or near-net-shape configurations. Prototype designs 

include joints between 316L stainless steel, IN718, and copper 

in various combinations. A GRC-led research task was 

undertaken to evaluate and develop brazing methods to join 

various prototypical designs using nickel-, gold- and copperbased braze materials. Microstructural evaluation of the 

brazed joints was used to down-select braze alloys and 

methods for each material combination. This paper reports the 

results of electron microscopy (EM) and energy dispersive 

spectroscopy (EDS) microstructural evaluations, used to 

explore the varying degrees of chemical interactions of the 

brazed and the long-term-aged brazed joints, between 316L 

stainless steel (SS316L) and high conductivity oxygen-free 

copper (OFHC Cu). The long-term durability exposure 

consisted of heat treating the brazed samples at 823 K 

(550 C) for 1000 hr in vacuum. A successful dissimilar-metal 

joint in this component application must provide mechanical 
NASA/TM—2012-217623 2 

Figure 1.—TDU power conversion unit design (Sunpower, Inc.). 

integrity, thermal conductivity, and stability for the expected 

mission life at elevated temperatures. Promising joints have 

been produced and a “toolbox” of options is described to 

respond to numerous potential designs or material selection 

options in the heat exchanger assembly. 

Experimental Procedure 

The braze alloys selected were originally identified to 

produce a physically sound joint. The various braze alloys 

evaluated followed the recommended literature and 

manufacturers’ practices (Refs. 6 and 7). Two similar nickelbased brazes produced by Sulzer Metco (Braze Materials 

Guide, 2006) were used to investigate the effect of the 

presence of boron and the possible formation of borides in the 

joining systems. The presence of boron (B) in AMDRY 936 

braze reduces the liquidus temperature to 1261 K, about 22

lower than AMDRY 930 to which B is not added. Both braze 

alloys have successfully been used in the past to join 

superalloys, stainless steels in complex assemblies, 

honeycomb, heat exchangers and wire screens. Technical 

interactions with the Morgan Advanced Ceramics, Westgo 

Metals Division resulted in the recommendation of the 35% 

gold – 65% copper braze (in wt.%). Since the liquidus 

temperature of the 35% Au – 65% Cu braze is approximately 

1283 K, a second group of Au-Cu brazes (40% Au – 60% Cu 

and 50 % Au - 50 % Cu) with lower liquidus temperatures 

(1273 and 1243 K, respectively) were also included in the 

study. The gold-nickel braze alloy Nioro, successfully used to 

join SS316L to Inconel 718, was added to the study. A 

summary of these commercially available braze alloys used to 

join the SS316L with an approximate nominal composition in 

wt.% of Fe-12Ni-17Cr-2.5Mo-2Mn-0.035max C and the 

OFHC Cu in the heat exchanger is presented in Table 1. 

TABLE 1.—Ni- AND AuCu-BASED BRAZE ALLOYS USED IN 

THIS STUDY TO JOIN 316L STAINLESS STEEL TO OFHC 

COPPER 

 Alloy/braze Composition, 

wt.% 

Form Braze 

temperature,

K (C) 

Ni-based AMDRY 936 Ni-19Mn-4Cu-6Si-1B0.03Re Paste 1270 (997) 

AMDRY 930 Ni-22Mn-5Cu-7Si Tape 1293 (1020) 

Au-based

Nioro 82Au-19Ni Foil (0.001 in.) 1253 (980) 

35%Au-65%Cu Au34.70-Cu65.30 Foil (0.001 in.) 1293 (1020) 

40%Au-60%Cu 39.58Au-60.42Cu Foil (0.001 in.) 1283 (1010) 

50%Au-50%Cu 49.67Au-50.33Cu Foil (0.001 in.) 1253 (980) 

The experimental brazing trials were done in vacuum at 

approximately 6×10–3 Pa at the indicated braze temperatures 

shown in Table 1. The brazing hold time was approximately 

10 min and a small load was applied on top of the joining 

samples. Optical microcopy revealed that in most bonded 

couples a continuous bond was produced indicating that the 

copper and stainless steel alloys were wetted by the braze 

alloys studied with only one exception, the Nioro braze alloy, 

as described later. Occasionally, small regions with 

discontinuous porosity were detected in the metallographic 

cross sections. Despite the good bonding with both sides, 

careful metallurgical evaluation of the joints revealed that 

varying degrees of chemical and diffusion reactions occurred 

at or near the joint interfaces. Some of these interactions, such 

as second phase precipitation, could affect the long-term 

structural stability of the stainless steel or copper brazed 

materials. This paper describes the brazing options explored to 

join SS316L to OFHC Cu, the resultant microstructure 

produced and the microstructure stability of the brazed joints 

after exposure for 1000 hr at 823 K (550 C) in vacuum. The 

group of brazed samples that were exposed over 1 month 

(1000 hr) in vacuum at 823 K (550 C) is exhibited in 

Figure 2. The samples were metallographically mounted, 

polished and observed in optical and electron microscopes. 

Chemical analysis, by EDS, of the main features observed 

near the braze/alloy interface was used to identify the 

chemistry of new phases that may have formed during the 

brazing and aging process. 

Results and Discussion 

Ni-Based Brazes 

The first two brazes presented in Table 1, AMDRY 936 and 

AMDRY 930, are very similar in composition with the 

exception of the presence of boron in AMDRY 936. They both 

contain similar levels of manganese (Mn), silicon (Si) and 

comparable amounts of copper (Cu). Figure 3 shows optical 

micrographs of the brazed SS316L to Cu with AMDRY 936 

(Figure 3a) and AMDRY 930 (Figure 3b). Good braze coverage 

and bonding was achieved with both alloys. Figure 4a and b 

show field emission scanning electron microscopy 
NASA/TM—2012-217623 3 

Figure 2.—Group of brazed samples aged at 823 K (550 oC) 

for 1000 hr in vacuum. 

Figure 3.—Optical micrographs of 316L stainless steel brazed 

to copper with (a) AMDRY 936 and (b) AMDRY 930. 

Figure 4.—FESEM micrographs of 316L stainless steel brazed 

to copper with (a) AMDRY 936 and (b) AMDRY 930. Eutectic 

phases formed at the center. Inserts in Figure 4a show (A) 

formation of chrome borides (CrB2) and (B) Manganesecopper (Mn-Cu) enriched particulates. No second phase 

precipitation occurred near the stainless steel / AMDRY 930 

braze interface. 

(FESEM) cross section images of the features observed in the 

as-brazed SS316L to Cu samples with AMDRY 936 and 

AMDRY 930, respectively. Both alloys tend to form a 

eutectic-like microstructure in the center of the brazed regions, 

with one phase, darker in contrast (Figure 4a and b), enriched 

in Mn and Si with Ni, while the other, lighter in contrast, is 

enriched Cu with Ni with lower levels of Si and Mn. One of 

the main concerns with the presence of B in alloy AMDRY 

936 is the possibility of B diffusing into the stainless steel and 

combining with iron (Fe) or chrome (Cr) to form some type of 

boride. This would deplete the stainless steel alloy of critical 

elements in regions near the braze, which could result in 

changes in the corrosion resistance and general properties of 

the alloy. Figure 4a (insert A) reveals the SS316L grain 

boundaries near the interface which are visible due to the 

presence of boride precipitates decorating the boundaries. 

A

B
NASA/TM—2012-217623 4 

Evidence of Cr tied with B at the grain boundaries near the 

brazing interface was confirmed by electron microprobe 

analysis (Figure 5). Figure 5b and c show microprobe 

wavelength dispersive spectroscopy (WDS) quantitative maps 

of Cr and B in the region shown in Figure 5a. Boron was one 

of the elements contained in the braze alloy AMDRY 936, 

while Cr was available from the stainless steel alloy. The 

diffusion reaction that resulted in the formation of the borides 

seems to extend ~ 30 m into the stainless steel side and 

occurs mainly at the stainless steel/braze interface in the asbrazed condition. The use of this braze alloy raises the concern 

that segregation of the boride precipitates could extend further 

in the SS316L alloy during long-term exposures. In contrast, 

the braze alloy AMDRY 930 that does not contain B, resulted 

in no second phase precipitation at or near the stainless 

steel/AMDRY 930 braze interface (Figure 4b). However, as 

mentioned earlier, AMDRY 930 requires a much higher 

brazing temperature than AMDRY 936. Multiple Mn-Cu rich 

particulates and strings were observed near the braze/copper 

interface in both braze alloys investigated, as revealed in the 

insert B of Figure 4a for AMDRY 936 and Figure 4b for 

AMDRY 930. Figure 6 shows a typical EDS spectrum of the 

Mn-Cu particulates present on the copper side of the couple 

brazed with AMDRY 930. The Mn is present in the braze 

alloy, and since copper and manganese have complete 

solubility in each other, diffusion of Mn during the brazing 

process must have occurred to form the fine Mn-Cu 

particulates inside the Cu matrix alloy. Figure 7 contrasts the 

extent of the Mn-Cu particulates found on the copper side in 

the as-brazed versus the 823 K (550 C), 1000 hr aged 

condition. The aging exposure had more than doubled the MnCu formation depth of approximately 40 to 100 m as 

measured from the braze/copper interface into the copper side. 

The aging at 823 K (550 C) for 1000 hr of the SS316L to 

Cu joint brazed with AMDRY 930 resulted in several other 

unique microstructures. Figure 8a and b show low and high 

magnification FESEM images of samples in the brazed plus 

aged condition. Besides the common eutectic microstructure 

observed in the middle of the brazed section for both asbrazed and aged conditions, fine platelets have also formed in 

the vicinity of either SS316L or Cu sides during the aging 

treatment. These platelets were not present in the as-brazed 

microstructure. Clearly some elements have a sufficient 

driving force to diffuse and combine into more stable 

compounds. As-shown in Figure 8, Spectrum EDS-A 

corresponds to a Cr (Mo, Ni) – silicide phase that seems to 

have formed near the interface boundary between the SS316L 

and the brazed region. Spectrum EDS-B indicates that the 

platelet precipitates are mostly enriched in Ni, Si and Mn, 

while Spectra EDS-C and EDS-D correspond to the 

composition typically observed in the eutectic region. One is 

enriched in Ni, Si and Mn (dark region) and the other is 

enriched with Cu, Ni and Mn (bright region). 

Figure 5.—Brazed SS316L to Cu with AMDRY 936 (a) Electron 

microprobe image and (b) and (c) Wavelength dispersive 

Spectroscopy (WDS) quantitative maps for Cr and B, 

respectively. 
NASA/TM—2012-217623 5 

Figure 6.—Typical EDS spectrum of the fine Mn-Cu particulates 

formed on the Cu side of the as-brazed SS316L/Cu with 

AMDRY 930. 

Figure 7.—FESEM Micrographs of the AM936 braze/copper 

interfaces in (a) as-brazed and (b) aged to 823 K (550 C) 

for 1000 hr. Aging exposure resulted in an expansion of the 

region with Mn-Cu particulates. 
NASA/TM—2012-217623 6 

 

Figure 8.—(a) Low and (b) high magnification FESEM micrographs for sample aged for 1000 hr at 823 K 

(550 C). Brazing alloy used is AMDRY 930. EDS analyses of second phases present next to 

SS/braze interface eutectic region. 

Fe

Fe

Fe
NASA/TM—2012-217623 7 

Figure 9.—Platelets formed inside the AMDRY 930 brazed 

section, near the braze/Cu interface showing similar 

composition as the one near the SS/raze interface EDS-B 

of Figure 8. 

The platelets formed inside the braze section, near the 

Cu/braze interface have similar composition as the one found 

near the stainless steel/braze interface and are presented in 

Figure 9. The EDS-E spectrum confirms that the platelets are 

again enriched in Ni, Si, Mn, and Cu. The Cu found came 

from the surrounding matrix. 

Au-Based Brazes 

A group of gold-copper (Cu/Au) brazes and an additional 

gold-nickel (Nioro) braze were explored as an option to join 

the stainless steel component to the copper. The Cu/Au braze 

alloys were selected to minimize possible reactions with the 

components to be joined and also because their different Au to 

Cu composition levels control their melting points and 

therefore, their brazing temperatures. Figure 10 shows optical 

micrographs of the three Au-Cu braze alloys. Each braze 

wetted and spread evenly on both alloys, which resulted in 

good bonds. 

Figure 10.—Optical micrographs of stainless steel brazed to 

copper using gold-copper braze alloys showing relatively 

good bonding. 

Figure 11 shows optical micrographs of the brazed alloys after 

the 1000 hr aging heat treatment at 823 K (550 C). Although 

difficult to resolve in the optical micrographs, some increased 

reactions have occurred at the braze/copper side interface. 

Figure 12 shows low and high magnification FESEM images 

of the two SS316L and Cu alloys brazed with the 50% Au – 

50% Cu braze alloy. The high magnification 

image shows clearly the particulates that have precipitated 

near the braze/copper interface. EDS analyses (Figure 13) of 

the region selected in Figure 12, indicate that the precipitates 

observed on the Cu side are enriched in Fe (with some Cr) in 

addition to Cu and Au. Diffusion of Fe and Cr from the 

stainless side is, therefore, possible in this system through the 

Au-Cu braze. Similar features were observed in the other two 

Fe 
NASA/TM—2012-217623 8 

Figure 11.—Optical micrographs of stainless steel brazed 

to copper using gold-copper braze alloys after an 

exposure for 1000 hr at 550 oC showing some reactions 

at the braze/copper interface. 

Figure 12.—Low and high magnification FESEM images of 

stainless steel brazed to Cu using a 50 Au – 50 Cu braze 

alloy after the aging heat treatment at 550 oC showing the 

formation of secondary phases at the braze/Cu interface. 

Au – Cu braze compositions. The extent of this reaction is 

limited to approximately 30 m deep in the copper side, even 

after aging for 1000 hr. 

The Nioro braze resulted in limited wetting of the copper 

surface and discontinuous bonding. Figure 14 shows an 

example of the poor braze bond observed when Nioro was 

used to bond SS316L to OFHC copper. Nioro was removed 

from the list of potential braze alloy candidates for SS316L to 

Cu bonding. 
NASA/TM—2012-217623 9 

Figure 13.—Energy dispersive spectroscopy (EDS) spectra of 

main particulates and matrix detected in the selected region 

shown in Figure 12. Most precipitates are enriched in Fe 

(with some Cr) in addition to the presence of Cu and Au.

Figure 14.—Stainless steel brazed to copper using Nioro braze 

alloy showing poor bonding. 

Summary 

NASA GRC, collaboratively with industry, examined both 

paste and foil brazing materials to develop procedures, and 

brazing methods to join various samples and prototypical 

Stirling heat exchanger concentric ring configurations. Careful 

metallurgical evaluation of brazed joints between 316L 

stainless and copper had revealed varying degrees of chemical 

interactions in the as-brazed condition and after the long-term 

aging exposure. The microstructure stability and chemical 

interactions of the various brazes with the component alloys 

were explored by electron microscopy and energy dispersive 

spectroscopy after brazing and aging. The Ni-based brazes, 

with and without B, and the three gold-copper brazes were 

down-selected for the aging study at 823 K (550 C). Despite 

some chemical interaction, the present study suggests that 

these alloys are adequate for joining 316L stainless steel to 

copper for use at 823 K (550 C) for a non-load-carrying 

component. Conversely, the poor wetting and bonding of the 

Nioro braze to copper resulted in its elimination as a candidate 

braze alloy to join SS316L to Cu. 

Table 2 summarizes the findings and recommendations after 

the long-term aging study. All of the alloys have, up to some 

degree, reacted with the surrounding matrices and formed 

secondary small phases. These precipitates, or secondary 

phase formations, do not seem to create a major concern at the 

823 K (550 C) application temperature. In the case of 

AMDRY 936, the B reaction with Cr zone did not expand 

beyond 30 m from the alloy/braze interface after the 550 C 

exposure for 1000 hr. Due to the relatively limited reaction, 

this SS316L/AMDRY936/Cu combination is deemed 

acceptable and comparable to AMDRY 930. However, the 

presence of B might be of concern if the brazed alloys are to 

O

Cu
NASA/TM—2012-217623 10 

TABLE 2.—POST-BRAZE OBSERVATIONS AFTER AGING EXPOSURE AT 823 K (550 C) FOR 1000 hr 

Metal 1 Metal 2 Braze alloy Wetting Phase formation with 

Metal 1 

Phase formation with 

Metal 2 

Compatibility with base materials Recommended 

SS316L Cu AMDRY 936 Good Cr-Boride at G. Bds Mn-Cu Comparable to as-brazed microstructure Acceptable 

SS316L Cu AMDRY 930 Good Minimal Mn-Cu Formation of platelets in brazed section Acceptable 

SS316L Cu Nioro Poor ------------------------ ---------------- ----------------------------------------------- No 

SS316L Cu 35Au-65Cu Good Minimal Fe(Cr) Cu, Au Diffusion of Cr, Fe to Cu Acceptable 

SS316L Cu 40Au-60Cu Good Minimal Fe(Cr) Cu, Au Diffusion of Cr, Fe to Cu Acceptable 

SS316L Cu 50Au-50Cu Good Minimal Fe(Cr) Cu, Au Diffusion of Cr, Fe to Cu Acceptable 

be exposed to a corrosive fluid or to significant radiation 

fluence. Higher use-temperatures beyond those used in this 

study, could result in more extensive inter-diffusion of some 

key elements found in the stainless steel or braze alloys and, 

therefore, the usefulness of these alloy systems would have to 

be reassessed. 

Conclusions 

All brazing alloys based on Ni or Au-Cu have, to some 

degree, reacted with the surrounding matrices and formed 

secondary small phases after the long-term aging exposure. 

However, these reactions are limited and not a major concern 

for an application at 823 K (550 C). The Nioro braze alloy is 

the only braze that is not recommended due to its poor wetting 

of the copper substrate. 

References 

1. Mason, L. S. and Poston, D. I., “A Summary of NASA 

Architecture Studies Utilizing Fission Surface Power 

Technology,” NASA/TM—2011-216819, April 2011. 

2. Palac, D. T., “Scotty, I Need More Power – The Fission 

System Gateway Abundant Power for Exploration,” 

Nuclear and Emerging Technologies for Space (NETS2011), Paper 3316, February 2011, also in NASA/TM—

2011-217005, May 2011. 

3. Nuclear and Emerging Technologies for Space (NETS2011), Paper 3316, February 2011, also in NASA/TM—

2011-217005, May 2011.Palac, D. T. et al., “Fission 

Surface Power Technology Development Status,” 

NASA/TM—2010-216249, April 2010. 

4. Fission Surface Power Team, “Fission Surface Power 

System Initial Concept Definition,” NASA/TM—2010-

216772, August 2010. 

5. Wood, J. G. et al., “ The Design and Development of a 12 

kWe Stirling Power Conversion Unit for Fission Power 

Technology Demonstration,” Nuclear and Emerging 

Technologies for Space (NETS-2011), paper 3488, 

February 2011 

6. Locci, I. E. et al., “Development of High Temperature 

Dissimilar Joint Technology for Fission Surface Power 

Systems,” Proceedings of the 4th International Brazing and 

Soldering Conference, pp. 165–175, April 2009. 

7. Locci, I. E. et al., “Material Studies Related to the Use of 

NaK Heat Exchangers Coupled to Stirling Heater Heads,” 

Nuclear and Emerging Technologies for Space (NETS2011), Paper 3389, February 2011, also in NASA/TM—

2011-217001, April 2011. 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

NASA has been supporting design studies and technology development for fission-based power systems that could provide power to an 

outpost on the Moon, Mars, or an asteroid. Technology development efforts have included fabrication and evaluation of components used in 

a Stirling engine power conversion system. This investigation is part of the development of several braze joints crucial for the heat 

exchanger transfer path from a hot-side heat exchanger to a Stirling engine heat acceptor. Dissimilar metal joints are required to impart both 

mechanical strength and thermal path integrity for a heater head of interest. Preliminary design work for the heat exchanger involved joints 

between low carbon stainless steel to Inconel 718, where the 316L stainless steel would contain flowing liquid metal NaK while Inconel 

718, a stronger alloy, would be used as structural reinforcement. This paper addressed the long-term microstructural stability of various 

braze alloys used to join 316L stainless steel heater head to the high conductivity oxygen-free copper acceptor to ensure the endurance of the 

critical metallic components of this sophisticated heat exchanger. The bonding of the 316L stainless steel heater head material to a copper 

heat acceptor is required to increase the heat-transfer surface area in contact with flowing He, which is the Stirling engine working fluid. 

15. SUBJECT TERMS

Stirling engines; Heat exchangers; Stainless steel; Power reactors; Nickel alloys; Brazing; Fabrication; Metal bonding; Metal joints; 

Microstructure 

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