The next generation of power systems for future spacecraft and lunar surface systems will likely have a strong dependence on nuclear power. The design of a space nuclear power plant involves integrating the major subsystems of the reactor, the power-conversion system, and the heat-rejection system. Optimum material choices are very different for each subsystem, with likely choices for the reactor tending toward high-temperature refractory metal alloys, the power-conversion system tending toward intermediate-temperature nickel-based (Ni-based) superalloys, and the heat-rejection system tending toward titanium and carbon/carbon composites. The transitions between these three subsystems are critical points in the system, and thus joining the dissimilar metals is a key factor in determining life and performance. The NASA Glenn Research Center has, therefore, initiated a comprehensive program to evaluate the best technologies to join the dissimilar metals required for future exploration missions.
The joining of refractory metals to Ni-base alloys was considered the most significant challenge, since these joints will likely be exposed for thousands of hours at high temperatures. The refractory alloy candidates are based primarily on molybdenum (Mo), niobium (Nb), tantalum (Ta), or rhenium (Re), whereas the superalloy candidate would most likely be a wrought alloy with a significant history and pedigree. Wrought superalloys are preferred for hot duct applications because of their fabricability and weldability. Both gas- and liquid-cooled joint designs require the ducting to be hermetically sealed, accompanied by good strength at the joint, with long-term thermal stability. Conventional fusion techniques for joining were considered but were rejected since intermetallic phases and cracking are associated with the postweld zones accompanying those techniques. Thus, solid-state methods of joining the dissimilar materials provided the best opportunity to eliminate unwanted phases and cracking at the transition joint.
Joint between Hastelloy X and Mo-47Re. Left: Electrospark deposition. Right: Inertia welding.
In collaboration with the Edison Welding Institute, Glenn researchers selected magnetic pulse welding, electrospark deposition (ESD), and inertia welding to join the refractory metals tantalum (Ta10W and T111) and molybdenum (Mo-47Re) to representative superalloys--both a wrought (Hastelloy X) and a cast (MarM-247) alloy. Inertia welding techniques were successfully developed for MarM-247 with Mo-47Re and for Hastelloy X with both Ta10W and Mo-47Re. ESD methods also were developed for Mar-M247 and Hastelloy X with T111 and Mo-47Re. Examples are shown in the preceding photomicrographs. Difficulties were encountered with efforts to develop magnetic pulse welding, but future modifications to the method appear promising.
A second class of joining process that was considered for dissimilar metals is brazing. Several different refractory metal candidates (Re, Mo-47Re, Nb-1Zr, and ASTAR 811C) were brazed to both Ni201 and Hastelloy X to assess process viability, to evaluate bond integrity, and to examine resultant microstructures at the interface. Candidate filler materials included Palni, Palco, Palniro-4, and copper (Cu) ABA. Preliminary microstructural observations showed that brazing these alloys is feasible, and the followin photomicrograph shows an example of a sound braze joint.
Complex braze interface, Hastelloy X/Palni/ASTAR–811C.
Because space missions could last for decades, it is important to evaluate and develop processes that will promote long-term bonding, strength, and phase stability. Therefore, all the successfully joined materials are scheduled for mechanical strength evaluation as well as for long-term stability testing at appropriate temperatures. Continued evaluation and optimization of solid-state and brazing options are necessary for systems that will rely on the long-term durability of a transition joint.
Glenn contacts: Frank J. Ritzert, 216-433-8199, Frank.Ritzert@nasa.gov; Dr. Cheryl L. Bowman, 216-433-8462, Cheryl.L.Bowman@nasa.gov; and Dr. John Gayda, 216-433-3273, John.Gayda-1@nasa.govLast updated: October 12, 2006
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
