Microstructural characterization of graphite/CuCrZr joints brazed with CuTiH2Ni-based fillers

Brazing of graphite to CuCrZr alloys has been achieved using CuTiH2Ni-based fillers (CuTiH2Ni, CuTiH2Ni+TiC and CuTiH2Ni+B) at 1173 K. Microstructural observation of the joints reveals a favorable interfacial bonding. The joining area of joints with CuTiH2Ni filler is composed of a TiC reaction layer and a filler layer containing Cu-based solid solutions, τ1-Ti(CuxNi1-x)2, Ti2(Cu,Ni) and TiCu intermetallics. For the graphite/CuTiH2Ni+TiC/CuCrZr joints, both TiC particles distribution in the filler layer and liquid filler penetration into the graphite open pores are observed. In the case of joints with CuTiH2Ni+B filler, TiB whiskers synthesized by the in situ reaction of boron with Ti are dispersed uniformly in the filler layer. Compared to CuTiH2Ni filler, the introduced TiC particles and TiB whiskers can work as ceramic reinforcements to strengthen the graphite/CuCrZr joints. Meanwhile, these reinforcements can also provide heterogeneous nucleation sites for the molten filler layer and suppress the grain growth of the precipitated phases, resulting in strengthening the filler layer by grain refinement. The electrical resistivity of the joining area in the joints brazed with CuTiH2Ni-based fillers is relatively low, satisfying the requirement for the applications of carbon commutators.