A Compendium of Brazed Microstructures for Fission Power Systems Applications

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 paper addressed the integrity of several dissimilar brazed 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. One possible design for the heat exchanger involves a joint between dissimilar metals, stainless steel and Inconel 718, where the 316L stainless steel could contain flowing liquid metal NaK while Inconel 718, a stronger alloy, is used as structural reinforcement. The heater head material would be bonded also to a copper heat acceptor to increase the heat-transfer surface area in contact with flowing He. This investigation reports on the long-term microstructural stability of various braze alloys used to join these critical metallic components for this sophisticated heat exchanger.