Brazing of Ta Sheet to W Block Using Various Filler Materials

Tungsten is a primary candidate for the solid target of high intensity Spallation Neutron Source.   However, corrosion resistance of pure tungsten is relatively weak in hot water under irradiation.   Therefore, in order to protect the target from corrosion, we investigated brazing of tantalum sheet to tungsten block using platinum and titanium as a first step.

In this study, we tried brazing of tantalum sheet to tungsten block using Ti alloys, Au alloys and Ag alloys.   Purposes of useage of these alloys are as follows.   First, lowering of brazing temperature is quite favorable to reduce the heat load causing embrittlement of the base metal, particularly tungsten.  Secondly, use of an alloy, not a pure metal, possibly decreases abrupt change of hardness across the interface between the bonded region and the base metal.   Microstructure and element distribution were examined using SEM and EDX.   Hardness change across the interface between the bonded region and the base metal was measured using a micro-Vickers hardness tester.   Joint strength was evaluated using a three-point bend test at room temperature.   Fracture mode was examined using SEM.   As a summary of results, the most promising filler metals were Ti-50%Ni and Ti-35%Ni-25%Nb.   Various kinds of phases were recognized in the bonded region, but the joint indicated 530-670 MPa and a little ductility.