Joining of niobium-1% zirconium to incoloy 909 for possible application in a refractory AMTEC cell wall

The development of a new generation of radioisotope space power system based on AMTEC (Alkali Metal Thermal to Electrical Conversion) technology is presently being designed using a variety of innovative material selections. Due to the increasing stringent anticipated operating parameters of the AMTEC technology, several different refractory superalloy metals are under consideration for application in the fabrication of the cell walls. One material combination which has been proposed consists of utilizing Niobium-1%Zirconium (Nb-1%Zr) for the hot end of the cell wall and Incoloy 909 for the cold end of the cell wall. The proposed use of a dual material cell wall is being considered in order to obtain the highest thermal to electrical efficiency possible for the new radioisotope space power system. The emphasis of this paper centers on investigating potential welding/brazing processing techniques which could be applied in joining these alloys into AMTEC cell walls. This is difficult due to their dissimilar physical and chemical properties. Several different joining techniques were investigated with a Nd-YAG laser welding process showing the most promise.